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Renewable and Sustainable Energy Reviews
journal homepage: www.elsevier.com/locate/rser
COOLFACADE: State-of-the-art review and evaluation of solar cooling
technologies on their potential for façade integration
Alejandro Prieto
a,⁎
, Ulrich Knaack
a
, Thomas Auer
b
, Tillmann Klein
a
a
Delft University of Technology, Faculty of Architecture and the Built Environment, Department of Architectural Engineering + Technology, Architectural Façades &
Products Research Group, Julianalaan 134, 2628BL Delft, Netherlands
b
Technical University of Munich, Department of Architecture, Chair of Building Technology and Climate Responsive Design, Arcisstraße 21, 80333 Munich, Germany
ARTICLE INFO
Keywords:
Solar cooling
Façade design
Integrated facades
Product development
Barriers
ABSTRACT
Increasing cooling demands in the built environment call for innovative technical solutions and systems for
application in buildings. Cooling loads represent an important share of the total energy consumption in warm
climates, especially in commercial and office buildings. Moreover, mechanical systems will still be needed in
most cases to cope with cooling loads, even after considering passive cooling strategies in the design of the
building and its façade. Solar cooling technologies present interesting assets, being based on environmentally
friendly cooling processes, driven by solar and thus renewable energy. However, their application in the built
environment remains greatly limited.
This paper assesses several solar cooling technologies in terms of their potential for façade integration; aiming
to promote widespread application in buildings throughout the development of integrated architectural façade
products. The assessment is based on a state-of-the-art review and discussion of key attributes for façade in-
tegration of selected technologies; and a qualitative evaluation of their suitability to respond to main product
related barriers for the integration of building services identified in an earlier work by the authors. The cooling
principles behind the operation of the assessed technologies have been extensively presented in the literature, so
this paper focuses exclusively on key aspects to overcome barriers related to the technical feasibility, physical
integration, durability, performance, and aesthetics of future integrated concepts.
Results show that the suitability of the assessed technologies varies according to each particular barrier.
Hence, no technology currently fits all required aspects. Nonetheless, the use of thermoelectric modules and
compact units based on absorption technologies are regarded as the most promising for the development of
either integral building components, or modular plug & play systems for façade integration. In any case, this is
heavily conditioned to further efforts and explorations in the field to overcome identified challenges and
knowledge gaps.
1. Introduction
Cooling demands in the built environment present a highly relevant
challenge for the design of sustainable buildings and cities. On the one
hand, several studies have attributed around 15–17% of global elec-
tricity consumption to air-conditioning and the refrigeration sector
[1–3]. This demand is expected to increase continuously during the
coming years, following current trends [4,5], due to several factors such
as increasing standards of life, climate change, and affordability of air
conditioning [6]. Yearly sold room size AC units surpassed 100 mill
worldwide on 2014, and they are expected to reach over 1.6 bill by
2050, with yearly sales growing at 10–15% in fast growing developing
countries from warm climates [7]. Moreover, it has been stated that just
Non-OECD Asia will account for more than half of the world's total
increase in energy consumption between 2012 and 2040 [8], which
puts pressure on design guidelines, regulations and further exploitation
of renewable sources of energy.
On the other hand, refrigerants used as working fluids have serious
environmental impact. Chlorofluorocarbons (CFCs) were banned and
hydrochlorofluorocarbons (HCFCs) are being phased out according to
the schedules set by the Montreal Protocol in 1987, due to their impact
on the ozone layer [9]. The most common refrigerants currently used
are hydrofluorocarbons (HFCs), such as R134a, a non-ozone-depleting
substance, but with a global warming potential (GWP) 1430 times that
of CO
2
[10,11]. As a result of the Kigali amendment to the Montreal
Protocol, signed in 2016, the use of these substances will be also phased
https://doi.org/10.1016/j.rser.2018.11.015
Received 16 May 2018; Received in revised form 20 July 2018; Accepted 14 November 2018
⁎
Corresponding author.
E-mail addresses: A.I.Prietohoces@tudelft.nl (A. Prieto), U.Knaack@tudelft.nl (U. Knaack), Thomas.Auer@tum.de (T. Auer), T.Klein@tudelft.nl (T. Klein).
Renewable and Sustainable Energy Reviews 101 (2019) 395–414
1364-0321/ © 2018 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/BY/4.0/).
T
down, over the period of 2019–2036 and 2024–2047 in developed and
developing countries respectively [12]. This milestone means breaking
the vicious circle established by the operation of refrigerants that
contribute to temperature raise in urban areas, in turn increasing
cooling demands and further need for refrigerants.
The current challenge for sustainable cooling in the built environ-
ment is then threefold: there is (a) a need for climate-responsive
building design to decrease cooling demands as much as possible
through the application of passive strategies; while the remaining load
is covered by (b) efficient building systems that not only use renewable
energies as input, but also (c) consider environmentally friendly
working materials and processes. In that regard, solar cooling tech-
nologies have experienced increased interest over the last couple of
decades, being widely recognised as promising alternatives to tradi-
tional vapour compression based refrigeration [13–15]. The main
benefits of these systems are the direct use of solar radiation as a re-
newable energy source, and the use of environmentally friendly
working materials in the cooling process. Nonetheless, building appli-
cation remains mostly limited to demonstration projects and pilot ex-
periences [16,17].
One alternative to promote further application in the built en-
vironment, is the development of multifunctional building components
for architectural design. Working experiences of decentralised services
integration in façade modules, plus the exposed area available for solar
collection, point towards façade integration of solar cooling technolo-
gies as a clear road to develop small-scale, flexible products for wide-
spread application. The potential for solar collection in facades has
been explored through the development of building integrated photo-
voltaics (BIPV) and building integrated solar thermal collectors (BIST),
resulting in guidelines, prototypes and commercialised products
[18–20]. On the other hand, solar driven refrigeration has been de-
scribed and categorised in terms of working principles [21,22], and
evaluated and compared considering performance [23,24] and to a
lesser degree, economic aspects [25,26]. However, besides stand-alone
prototypes and integrated concepts, there is a knowledge gap regarding
guidelines for building application and especially integration possibi-
lities within façade components.
The goal of this paper is to assess the potential for façade integration
of several solar cooling technologies, based on a state-of-the-art review
and discussion of specific attributes, and their capability to respond to
main identified barriers for façade integration of building services. The
assessment focuses on five main solar electric and solar thermal tech-
nologies, based on widespread categorisations: thermoelectric, ab-
sorption, adsorption, solid desiccant, and liquid desiccant cooling
[22,27]. Even though the energy input is a fundamental part of the
system, the assessment will concentrate on the cooling process and the
required components to generate, distribute and deliver the cooling
effect indoors. Hence, façade integration possibilities of PV panels and/
or solar collectors will not be directly addressed. In turn, the outcome of
this assessment is expected to serve as complement to previous and
established research on BIPV and BIST [28–30], providing feedback to
system developers and façade designers for the development of fully
self-sustaining solar cooling façade modules for buildings in warm cli-
mates.
2. Strategy and methods
The assessment of the defined solar cooling technologies was carried
out in two separate stages, sequentially presented in this paper: a state-
of-the-art review of the solar cooling technologies focused on relevant
aspects for façade integration; and the evaluation of these technologies,
based on the addressed aspects, on their potential to overcome pre-
viously identified barriers for façade integration of building services.
The review focuses on façade integration potential, by character-
ising the selected technologies in four main aspects: performance,
component complexity, operation, and development level, providing an
specialised overview of the state-of-the-art of the particular technolo-
gies, for each aspect addressed. A brief description of the considered
aspects and sub-aspects is shown in Table 1. Many sources were con-
sidered for the review, such as peer-reviewed scientific publications,
research reports, patented concepts, and technical info from manu-
facturers and distributors in the case of market-ready products. Façade
integration potential means the integration of small decentralised units,
so the review focused on small-scale developments, ranging up to
20 kW. Larger capacities were discussed as reference if applied, but they
were not explored in detail.
The review does not consider a detailed description of the cooling
principles and processes behind each technology, having been ex-
tensively described on the literature [21,27]. Similarly, economic as-
pects were not explicitly considered in the review. Even though cost is a
relevant issue for the development of integrated concepts, there is
limited amount of information on small-scale concepts, due to their
early stages of research and development. Hence, cost estimations of
integrated concepts may be troublesome. In any case, broad economic
considerations are implicitly considered in the discussed aspects, in
terms of the materials used, complexity of the system, and overall
performance during operation, providing less payback time with a more
efficient solution, compared to a base initial cost.
The potential for façade integration of these technologies was
evaluated based on their prospects to overcome main barriers for the
integration of building services in façade components. These barriers
were identified in a previously published article by the authors [31],
validated and discussed along similar experiences in the topic [32–35].
This previous work was based on a survey addressed to specialised
professionals with practical experience in the development of façade
systems for office buildings. The main outcome was the identification of
the main perceived barriers for integration, through open questions,
during three main product development stages: design, production, and
assembly. Open ended responses by the experts were then categorised
into main process and product related issues, as shown in Fig. 1.
Only product related barriers and subsequent issues are considered
in the evaluation of solar cooling technologies, focusing the discussion
on key characteristics for potential future integrated products, rather
than the logistics, knowledge and coordination required to successfully
produce them. Moreover, process related aspects need to be tackled in
general, to allow further application of all of the assessed technologies,
so these barriers do not provide direct and discernible criteria for
comparison between current solar cooling technologies. Therefore, the
evaluation centres around the identified product related barriers, dis-
cussing and assessing the potential of the different technologies in
overcoming them.
It is worth pointing out that results from the survey showed that
some barriers seem to be more relevant that others, based on the total
amount of mentions. This suggests perceived priorities for current ex-
ploration; however, the assessment considers them separately, ex-
ploring the current state of each technology without attempting an
Table 1
Key aspects considered in the review.
KEY ASPECTS SUB-ASPECTS
PERFORMANCE General reported performance
Reported performance of small scale applications
COMPONENT COMPLEXITY Dimensions – size, volume and weight of systems
and components
Components – number and types of required
components
Connections – types of required connections and
materials involved
OPERATIONAL ISSUES Health, safety and comfort issues
Maintenance requirements
DEVELOPMENT LEVEL Technical maturity
Market/commercial maturity
A. Prieto et al. Renewable and Sustainable Energy Reviews 101 (2019) 395–414
396
overall comparison. Hence, each barrier is discussed separately, while
the barrier-specific integration potential of each technology is illu-
strated and compared by a qualitative score system depicted in Table 2,
along with a brief description of the barriers. The barrier-based as-
sessment of the façade integration potential of the selected technologies
aims to identify specific bottlenecks and propose recommendations to
bring them closer to façade integration, sketching a roadmap for the
future development of solar cooling integrated architectural products.
3. State-of-the-art review of solar cooling technologies on key
aspects for façade integration
Solar cooling technologies are usually categorised according to their
energy input, under either solar electric or solar thermal processes,
hence using electricity from PV panels or heat stored in solar thermal
collectors as the main input for the cooling process [22,27].Table 3
shows the main solar cooling principles and associated common tech-
nologies in existence. Even though the use of a vapour compression heat
pump could be considered under solar electric processes, provided that
is driven by PV panels, it is not considered in the review due to the
environmental hazards of commonly used refrigerants. Similarly,
thermomechanical cooling technologies are not discussed due to the
lack of development and consequent available information compared to
the rest. Therefore, the review and evaluation focus on five solar
cooling technologies regarded as the most promising options for further
development of integrated building components: thermoelectric, ab-
sorption, adsorption, and (solid and liquid) desiccant cooling.
All technologies addressed in the review share general advantages
and disadvantages compared to commonly used vapour compression
systems. The most relevant advantages are the use of renewable energy
as main direct input, either directly supplied as electricity or low-grade
thermal energy; and the use of environmentally friendly working ma-
terials as refrigerants, with no global warming nor ozone depletion
potential. The most important disadvantage is the performance of these
systems in terms of their electrical or thermal efficiency, besides the
technical and commercial maturity of systems and components. Aside
from these, specific advantages and disadvantages inherent to each
technology are presented in Table 4, as an initial overview of possibi-
lities and constraints for widespread application. The specific review of
each technology regarding key aspects for façade integration is carried
out separately, in the following sections.
3.1. Thermoelectric cooling
3.1.1. Performance of cooling systems and integrated concepts
3.1.1.1. General reported performance. The efficiency of a
thermoelectric (TE) module mostly depends on the ability of the base
material to produce thermoelectric power from a temperature
differential (or vice versa). This material property is measured with a
dimensionless figure of merit denominated ZT. Commercially available
common materials such as Bismuth telluride (Bi
2
Te
3
) have ZTs around
1.0–1.2 [36], while it has been reported that it is possible to achieve
COP values between 1.0 and 1.5 for TE HVAC systems using currently
available TE materials of ZT = 1 [15,37]. Furthermore, it has been
stated that TE cooling reaches Carnot efficiencies between 0.1 and 0.15
[38], with a theoretical potential up to 0.37–0.4 assuming reported
developments in material science [13,37,39]. These efficiencies would
mean comparable values to vapour compression technologies (~0.45),
so research on new materials has been a priority in the field. It has been
stated by several authors that achieving a ZT value of 2 [38,40,41], or 3
[15,42–44] would make domestic & commercial HVAC TE systems
competitive and practical for widespread application. More
conservative estimates declare that a ZT = 4.4 would be needed
accounting for losses in the system [13]. In the last decades, new
nanotechnology driven materials with ZTs of 1.5–2.0 and even 2.4 have
been reported [45]; however, these remain experimental and outside of
the market.
3.1.1.2. Reported performance of small scale applications. There are
several experimental TE driven HVAC concepts in the literature, with
reported COP ranging from 0.38 to ~2.00 under diverse operating
conditions. It is relevant to mention that in thermoelectric technology,
there is a trade-off between COP and cooling power, so a balance
between them is usually identified as the optimal operating condition.
Tan & Zhao [46] reported a maximum COP of 1.71 for a TE AC system,
however, the optimum balance was found to achieve a COP of 0.82,
under 5 A and cooling power of 37 W. Cosnier [47] reported COP values
around 1.5–2.0 for an air cooling/heating system, with 50 W per
module under 4 A, maintaining 5 °C difference between hot and cold
sides (10 °C maximum). Shen et al. [48] achieved a COP of 1.77, with
1.2 A and 8.82 W, while Zhao & Tan [40] obtained an average and
maximum COP of 0.8 and 1.22 respectively, for a PCM integrated TE
AC, with a maximum cooling capacity of 210 W corresponding to the
maximum COP value.
Regarding TE cooling integrated façade concepts, COP values range
between 0.5 and 1.8 for the reported experiences shown in Table 5.
Considering these, it could be feasible to estimate a COP of 1.0–1.2 for
performance assessment of building integrated TE cooling systems, al-
though this value needs to be corroborated, taking into account the
required cooling capacity of a designed system for a particular context.
3.1.2. Complexity of systems and components
3.1.2.1. Dimensions – size, volume and weight of systems and
components. Thermoelectric technologies are conceived for small scale
application, such as cooling of electronic equipment or spot AC.
Common dimensions of TE modules are in the range of 40 × 40 mm,
so it is highly suitable to be assembled in a compact and modular
package [37,40,65]. Because of these characteristics, several authors
Fig. 1. Main perceived barriers for façade integration of building services: number of mentions per development stage by respondents.
A. Prieto et al. Renewable and Sustainable Energy Reviews 101 (2019) 395–414
397
have explored building application through prototype TE cooling
façade concepts (Fig. 2)
3.1.2.2. Components – number and types of required components. The
core of a TE HVAC system basically consists of a PV panel, a Peltier
module (Fig. 2), a heatsink for heat rejection, and the connecting wires
for electricity transfer. TE modules are powered by direct current, thus
it is not necessary to consider an inverter as part of the system,
providing a good match with PV panels [15,39,51,66]. Additionally,
heat rejection may be improved by the use of fans, and the system may
be benefited by steady current input by means of integrating a battery
for electricity storage, between the PV panel and the TE module.
[47,62]
3.1.2.3. Connections – types of required connections and materials
involved. This technology does not consider moving parts nor working
fluids in the core refrigerating machine [37,41,48,51,67]. Hence, the
connections between the core components are solved with electrical
wires. Furthermore, being a solid-state technology, cooling
transmission is produced by direct contact between the cold side of
the TE module and the transfer medium (air-water), or directly the
indoor environment via a radiant panel/ceiling [27].
3.1.3. Cooling system operation
3.1.3.1. Health, safety and comfort issues. Thermoelectric technology is
ecologically clean, given that is refrigerant free. Because of that, its
operation does not present particular hazards nor safety concerns
[41,47,48,65,66].
3.1.3.2. Maintenance requirements. A basic TE system only requires
basic electrical maintenance. Moving parts such as fans will require
specific maintenance activities. However, the fact that currently there
are not commercially available TE HVAC systems, means that these
systems are not fully tested on the long-term. Furthermore, PV panels
are commonly guaranteed for around 25 years, so life expectancy has to
be accounted for in cost/performance analysis [68].
3.1.4. Level of development / maturity
3.1.4.1. Technical maturity. The thermoelectric principle was
discovered in the early 1800s (Seeback and Peltier effect), and
cooling technology driven by it has been explored since, for several
applications such as medical and space equipment, electronics, and
household devices such as portable refrigerators and camping gear. The
principles are quite well understood and their use for small scale
applications is well documented. Nonetheless, TE HVAC applications
are still in early R&D development, with effort focused on improving
the efficiency of several concepts by researching at material level
through nanotechnology, or by enhancing the performance of auxiliary
components of the system, such as heat rejection units, PV panels, or
cooling delivery methods [42,69].
3.1.4.2. Market/commercial maturity. Currently there are not
commercially available thermoelectrically driven HVAC systems for
building application. Nevertheless, small scale cooling equipment, such
as recirculating chillers, refrigerators, or spot air-conditioners are
marketed by several companies, with cooling power up to 700 W
(CustomChill [70], Solid State cooling systems [71], Sheetak [72]).
Besides commercialising these products, companies such as Phononic
[73] and Evident Thermoelectrics [36] offer custom made scalable
devices, and even distribute ‘test kits’ to encourage research and
development of new applications for future market possibilities. This
is seen as a promising fact, related to further development and
commercial interest for TE technologies.
Table 2
Main identified barriers for façade integration and score system for the evaluation.
BARRIERS DESCRIPTION SCORE VALUES
o + ++ +++
TECHNICAL FEASIBILITY Overall feasibility to integrate systems into façade modules
based on each cooling principle. This considers functional and
physical constraints for integration
Required components cannot be
integrated in a façade module.
Standalone examples of
compact systems (R&D
needed).
Development of small compact
units and stand-alone facades
Several façade integrated
concepts and tested prototypes
PHYSICAL INTEGRATION Externalities derived from the physical integration of
components and parts, considering compatibility of sub-systems,
modularity, working materials and connections.
Components & working mat.
incompatible with façade
functions.
Complex connections
between operating sub-
systems.
Connections of mild complexity /
compatible components
Modular components with
simple plug&play connections.
DURABILITY &
MAINTENANCE
Durability of components over time, maintenance requirements
and ease to perform repairs or replace components and parts.
Fragile components with costly
and frequent maintenance.
Robust components w/
complicated maintenance.
Robust components with
periodic but simple maintenance.
Robust systems and components
virtually maintenance free.
PERFORMANCE Technical limitations based on past experiences. It also considers
externalities for indoor comfort, derived from the operation of
the systems.
Performance is out of required
range / Hazardous operation.
Isolated in-range experiences
/ Minor externalities
Several cases of high, reliable
performance / Minor
externalities.
High & proven relative
performance, with no
externalities.
AESTHETICS &
AVAILABILITY
Potential to allow for design flexibility and variability, which
may grant freedom for architectural expression and façade
composition.
Operating principles & potential
sizes greatly restrict design
choices.
Some restrictions and isolated
options for façade design
Minor restrictions / various
options for components.
High design flexibility and
variety of options and
components.
A. Prieto et al. Renewable and Sustainable Energy Reviews 101 (2019) 395–414
398
3.2. Absorption cooling
3.2.1. Performance of cooling systems and integrated concepts
3.2.1.1. General reported performance. The performance of absorption
chillers has been extensively reported during the last couple of decades,
with early developments ranging back to the late 70 s and 80 s [74,75].
The decisive difference regarding the performance of available systems
is the number of successive stages where regeneration of the working
pair takes place, defining single-effect, double-effect and triple-effect
absorption chillers. Common reference COP values are 0.6–0.7 (single);
1.1–1.3 (double); and 1.6–1.7 (triple) [15,76]. Although double and
triple effect chillers have markedly higher COP, their application
considers higher complexity on systems and connections involved,
and higher heat input as driver for the system (input temperatures
over 130 °C, compared to 75–100 °C required for single-effect chillers)
[21]. For these reasons, double and especially triple-effect chillers are
constrained to large scale applications, leaving single-effect chillers for
medium sized buildings and potentially decentral applications.
Commercially available single-effect absorption chillers cover a wide
range of cooling capacities, typically ranging from 4.5 to 20,500 kW,
with reported COP from 0.5 to 0.8, depending on their working pair and
sizes [16,21,77–80]. This review focuses on small scale applications,
exploring possibilities for architectural integration. Hence, the mention
of absorption chillers will only consider single-effect technologies from
this point onwards.
3.2.1.2. Reported performance of small scale applications. Most
commercially available chillers cover large scale requirements.
Nonetheless, it is possible to find market-ready units with cooling
capacities around 10–12 kW, with COP values between 0.62 and 0.77
(SolarNext [81], Sonnenklima [82]); and even a smaller unit of 4.5 kW,
commercialised until 2010 by Rotartica, with nominal COP of 0.67 and
reported experimental COP values between 0.58 and 0.66 [83,84].
Small size heat pumps have been consistently designed and prototyped
in the last years. Said et al. tested an ammonia-water heat pump of
5 kW, reporting a COP of 0.6 [85]. Similarly, Franchini et al. designed a
micro-scale chiller of 5 kW nominal cooling capacity. Experimental
results showed 3.25 kW on average with a COP of 0.358 [86]. Besides
considering smaller sizes, several researchers have explored concepts
that integrate heat rejection into the cooling unit, hence devising air
cooled absorption heat pumps, depicted in Table 6. Evidence seems to
show that a COP of 0.6 would be a conservative estimate for the
efficiency of a small scale unit, while a solar COP around 0.2–0.25
could be expected for collector integrated concepts. These values
should potentially increase following further development of current
prototypes.
3.2.2. Complexity of systems and components
3.2.2.1. Dimensions – size, volume and weight of systems and
components. Sizes of common small-scale commercially available
chillers (4.5 kW–17.5 kW) range from 0.85 to 2.3 m
3
, (Yazaki WFC-
SC5, 17.5 kW and EAW Wegracal SE15, 15 kW), with length and depth
as low as 80 × 60 cm and heights from 175 to 220 cm for that given
area (Yazaki WFC-SC5 and SolarNext ACS08). Nominal weight of these
units ranges from 290 kg to 660 kg (Rotartica, 2.5 kW; EAW Wegracal
SE15). Smaller concepts have been explored as ‘micro-scale heat
Table 4
Specific advantages and disadvantages of selected solar cooling technologies.
SOLAR COOLING ADVANTAGES DISADVANTAGES
THERMO ELECTRIC – Solid-state technology, refrigerant free.
– No moving parts in the core system.
– Small size of components comprehend packaging
advantages for product development.
– Quick operation (quickly reaches steady-state conditions).
– Low power/efficiency of current materials. There is a trade-off between reported
efficiency (COP) and cooling power of researched concepts.
– Technology in early R&D stages for HVAC application.
ABSORPTION – Mature technology with high reliability. Current efforts
target cost and complexity.
– Larger COP than other thermally operated technologies.
– Potential solution crystallisation, which could cause irreparable damage, added to
corrosion risk and need to maintain vacuum.
– High upfront costs. Economics become more favourable for larger buildings.
ADSORPTION – Few moving parts and factory sealed units (maintenance
free system)
– Non-toxic, non-flammable working fluid (silica gel/water)
– No crystallisation nor corrosion in inner components
– Large sizes and weight (bulkiness) due to inefficiency of the cycle (expected cooling
capacity).
– Alternating operation and long cycles (intermittent) under simplest mode (1
adsorption bed)
SOLID DESICCANT – Non-flammable and non-corrosive materials
– Easy to clean and low maintenance costs, due to its
operation at almost atmospheric conditions
– Temperature and humidity control separately (sensible and
latent loads)
– Limited performance of materials (adsorption capacity of silica gel is low while
zeolites have low water capacities and higher cost of regeneration)
– Slightly complicated system instalment
– Generally larger in size/shape than conventional systems
LIQUID DESICCANT – High potential indoor air quality, capacity of absorbing
pollutants and bacteria
– Low-pressure drop, for use with low regeneration temp.
– Potential small and compact units by pumping solution.
– Desiccant storage for use when heat source is not available
– All aqueous solutions are highly corrosive (plastic materials must be used)
– Health hazards due to carry-over with supply air stream
– Aqueous salts are subject to crystallisation, and freezing risk.
Table 3
Available cooling technologies based on solar electric and solar thermal processes.
ENERGY INPUT COOLING PRINCIPLE COOLING TECHNOLOGIES
SOLAR ELECTRIC PROCESSES - ELECTRICITY Vapor compression cooling Compression heat pump
Thermoelectric cooling Peltier modules
SOLAR THERMAL PROCESSES - HEAT Sorption cooling Absorption heat pump
Adsorption heat pump
Desiccant cooling Solid desiccant (+ Evaporative cooling)
Liquid desiccant (+ Evaporative cooling)
Thermomechanical cooling Steam ejector system
Stirling engine
Rankine cycle heat pump
A. Prieto et al. Renewable and Sustainable Energy Reviews 101 (2019) 395–414
399
pumps’ (Franchini 2015), with a 2.5 kW LiBr-H
2
O heat pump developed
and commercialised by Purix as the leading example [96]. This chiller
unit measures (L × W × H) 64 × 45 × 135 cm (0.4 m
3
) and weighs
only 75 kg, driven by a thermal collector array of 5 m
2
, while using
decoupled water-based cooling delivery systems in each room. This unit
is regarded as a notable example of the sizing limits inherent to the
principles in play. In general, bulky sizes and weight are stated as
important drawbacks of the technology, casting doubts about the
feasibility and the rationale behind potential decentral applications.
Nonetheless, the flexibility given by the use of liquid working materials
has been also cited as an advantage for the development of compact
machines, compared to solid based adsorption technologies [7,97]. This
Fig. 2. Thermoelectric cooling façade prototype developed by Ibanez-Puy et al. and general thermoelectric cooling principle.
Table 6
Absorption based integrated concepts reported in the review.
REF AUTHORS DESCRIPTION
[87] Castro et al. COP values between 0.37 and 0.68 for a 2 kW LiBr-H
2
O serpentine based heat pump design, to allow for air currents.
[88] Lizarte et al. Air-cooled heat pump of 4.5 kW, coupled with a flat-plate collector array (42.2m
2
) and a storage tank (1.5 m
3
) in a 40 m
2
room in Madrid.
Reported experimental COP was of 0.55–0.62.
[89] Izquierdo et al. Mean daily COP of 1.05, by testing an air-cooled prototype of 7 kW (nominal) coupled with a flat-sheet absorber for solar input, in Madrid.
[90–93] Hallstrom et al. Integrated LiCl-H
2
O absorption system within vacuum tubes collector. Experimental solar COP values of 0.19–0.25 through outdoor tests in
rooftop application in Sweden. Module operates in absorption-regeneration cycles based on solar availability.
[94,95] Avesani, Bonato et al. Integrated LiCl-H
2
O absorption system within vacuum tubes collector for façade application. Simulated solar COP values of 0.27–0.36 and
0.17–0.23 in Stockholm and Rome, with cooling capacities of 1.44 kWh/day per module. Best results in east orientations, regenerating the
solution during the morning and providing cold air during the afternoon (alternate operation in sorption/desorption).
Table 5
Thermoelectric cooling integrated concepts reported in the review.
REF AUTHORS DESCRIPTION
[49,50] Liu et al. PV coupled TE wall systems with COP values of 0.95 and 1.28 for tilted PV at 60° and 90° Calculated final COP of 0.74 for both after accounting for
thermal losses in the system.
[51–53] Luo et al. Instantaneous COP values of 0.7–1.8 for an active building integrated PV panel coupled with a TE wall system in the back of the room.
[54–57] Ibanez-Puy et al. Experimental evaluation of a TE façade mounted on a 1:1 test cell. Façade composed of 16 peltier cells of 51.4W each, working under variable
voltage of 7.2 V and 12 V. Measured COP of 0.66–0.78.
[58,59] Vasquez et al. COP values of 0.56–2.06 through simulations of TE window unit with electrical currents of 6–1.5 A. COP values of 1.01–1.04 under 4 A for best
balance COP/cooling power.
[60–62] Xu et al. Experimental COP of 0.51–1.42 for a single TE unit. Best COP values of 1.31–1.33, considering balance with cooling power, for 8 TE units connected
in series and parallel under 5 V.
[60] Le Pierres et al. Simulation and experimental evaluation of PV+TE modules for air pre-heating and pre-cooling in dwellings. Cooling COP was only higher than 1
for input currents lower than 3 A.
[63] Khire et al. Numerical calculations for different TE configurations. Best combination at COP of 1.529 for a TE active wall concept, with a cooling power of 30 W,
under 1.393 A and 0.741 V.
[64] Irshad et al. TE air duct system coupled with a PV wall for the tropical climate, reporting a COP of 1.15, with a cooling capacity of 517.24 W under 6 A, and an
optimal T° difference of 6.8 °C.
A. Prieto et al. Renewable and Sustainable Energy Reviews 101 (2019) 395–414
400
characteristic has been greatly explored by a group of researchers for
the design of an integrated sorption collector (Fig. 3). This unit has been
developed with compactness and simplicity in mind, being tested for
rooftop applications and even façade integration as the opaque
component of an unitised curtain-wall ensemble [95,98]. The
dimensions of the optimised unit for façade integration were
150 × 40 × 90 cm (0.54 m
3
), for a fully air-based decentralised unit
with all required mechanical components [94], being a promising
alternative for building integration and specifically retrofit
applications.
3.2.2.2. Components – number and types of required
components. Absorption chillers consist of a condenser and
evaporator, like vapour compression systems, but with a heat driven
generator and absorber instead of the electric compressor commonly
used (Fig. 3). This equipment is enclosed in a sealed unit with few
moving parts, namely an integrated pump for the circulation of the
solution in a closed loop [15]. Besides the inner mechanisms and
components, a heat rejection system is needed, as well as input heat
from a solar array or a gas fired boiler. Heat/cold storage is optional,
although it increases the capabilities and flexibility of the overall
system. The common use of cooling towers as heat rejection system
for absorption chillers has been cited as one of the disadvantages for
their application on small scales [7,21,39,99]. Consequently, air-cooled
systems, with integrated heat rejection have been prototyped and tested
by several researchers [87,89,100], striving for simplicity and
compactness. This line of research was pushed even more in the
aforementioned sorption collector unit [95,98], where heat rejection,
cooling generation and solar collector were integrated in a stand-alone
ventilation unit for façade application. This is regarded as the best
available example of integrated modular design based on absorption
technology.
3.2.2.3. Connections – types of required connections and materials
involved. Absorption chillers are factory sealed units, comprising
necessary inner equipment and connections to input and output
circuits. Common commercially available water-based chillers require
connection to the heat source, heat rejection system and the cooling
delivery system through pump driven water pipes. Nonetheless, air-
based integrated designs have been prototyped and tested for stand-
alone operation, only requiring electric input to power low
consumption fans and dampers.
3.2.3. Cooling system operation
3.2.3.1. Health, safety and comfort issues. Health hazards largely
depend on the working fluids used to drive the absorption cycle. The
most common absorbent/refrigerant pairs utilised are lithium bromide/
water (LiBr/H
2
O) and water/ammonia (H
2
O/NH3), respectively.
Additionally, the use of lithium chloride (LiCl) has been explored as
absorbent, with water as refrigerant; but its use is still limited in current
experiences [101]. All fluids are environmentally friendly, but have
certain drawbacks. Ammonia is toxic in high concentrations, causing
immediate irritation in eyes and the respiratory track [76,102]. Lithium
bromide and lithium chloride have low toxicity, but they are corrosive
materials, so proper maintenance must be conducted to ensure correct
operation of the system. Regarding general comfort, silent operation
has been regarded as an advantage compared to conventional systems,
solely depending on pumps to recirculate the solution [7,39].
3.2.3.2. Maintenance requirements. Absorption chillers have minimal
moving parts, so maintenance activities are focused on handling their
working fluids to allow for correct operation. This may imply
complicated maintenance activities to meet optimal operation
requirements. One of the main concerns associated with absorption
chillers is the risk of crystallisation of lithium bromide by temperature
differentials, which may cause irreparable damage [83,103,104].
Hence, measures must be taken to mitigate the risk and prevent this
from happening. Additionally, leakages must be checked periodically to
prevent health hazards, and avoid corrosion, as mentioned before. In
this sense, an added difficulty is that absorption chillers’ operation
relies on having internal vacuum conditions, which need to be
maintained over time, to avoid continuous air leakage into the system
that induces corrosion by reacting with the working fluids
[39,102,103].
3.2.4. Level of development / maturity
3.2.4.1. Technical maturity. Absorption cooling is regarded as a mature
technology with high reliability; being one of the oldest refrigeration
technologies registered [39,86,103,105]. First experiences in the field
go as far back as the 1700s, with water/ammonia chillers first designed
by Ferdinand Carre in 1859 [76,99]. Lithium bromide/water chillers
have been around since mid-1900s, with a first commercial absorption
chiller developed by Carrier in 1945 [106]. The expected performance
of the refrigeration cycle has reached stable and optimised values under
single, double and triple effect operation, so its use is justified in large
applications when waste heat is available. Hence, current challenges for
absorption cooling are directed to allow for widespread application,
simplifying the operation of centralised units, while exploring
alternatives for small scale decentral application. On the one hand,
new working pairs are being tested, such as lithium chloride / water
Fig. 3. Integrated sorption collector & sorption tube technology developed by Avesani, Hallstrom et al., and general absorption cooling principle.
A. Prieto et al. Renewable and Sustainable Energy Reviews 101 (2019) 395–414
401
units, to reach good performances without the risk of crystallisation
[107]. On the other hand, new simpler designs are being explored based
on proven working principles, reducing sizes and weight of units to
lower initial costs. Current examples also consider an integrated and
multifunctional approach, striving for direct heat rejection and less
connections and overall complexity [103,107].
3.2.4.2. Market/commercial maturity. As mentioned, absorption chillers
have been commercialised since the 1940s. Several researchers have
estimated that there are currently between 1000 and 1200 solar assisted
cooling units installed worldwide [7,17], with absorption chillers
accounting for about 80% of the total [39]. It has also been reported
that Asian markets have around 85% of the stock of absorption chillers
with capacities over 350 kW, being by far the largest regional market
followed by Europe [13]. Consequently, the market for large scale
systems, ranging from 100 s to 1000 s kW is dominated by Asian
companies such as Yazaki [108], Hitachi [109]and Broad [110];
followed by large American corporations with vast experience in
refrigeration such as Carrier [111], Trane [112] and York [113]. In
recent years, several European companies have been exploring the
development of small scale units for light commercial and residential
application, either providing small size chillers [114], comprehensive
solar kits [81,96], or integrated designs striving for efficiency [101].
Evidence seems to point out that small size absorption is a developing
niche, so further products should follow in the coming years.
3.3. Adsorption cooling
3.3.1. Performance of cooling systems and integrated concepts
3.3.1.1. General reported performance. The performance of
commercially available adsorption systems has been well documented
by several researchers during the last 15 years, with small changing
COP values from 0.5 to 0.7, and cooling capacities commonly ranging
from 5.5 to 1000 kW [16,21,79,80]. The performance mainly depends
on the heat and mass transfer potential of the utilised adsorbent.
Possible working pairs of adsorbent/adsorbate are activated carbon /
methanol, ethanol or ammonia; zeolites / ethanol or water; or silica gel
/ water. The latter has been found to be the most efficient combination
for AC applications [115], although its performance is still limited to be
a competitive alternative against vapour compression technologies
[116]. In terms of the adsorption process, the fact that basic
operation is intermittent, constrained to adsorption/desorption cycles,
is often cited as a disadvantage [116–118]. This is overcome by using
two adsorption beds, alternating them for continuous operation, but of
course this increases the size of the system. Other factors that have an
impact on the performance of adsorption systems are the length of the
absorption/desorption cycle, temperature of hot water from the solar
source, and the use of heat/cold storage units. It has been found that
longer cycles, allow for larger COP values, with optimum lengths of
10–15 min [119,120]. Similarly, COP has been reported to increase
with higher inlet temperatures [121] and when hot storage is
considered [116,122].
3.3.1.2. Reported performance of small scale applications. Common
commercial applications consider large chiller units, up to 1000 kW.
Nonetheless, in the last years, researchers have been increasingly
interested in the development of small scale adsorption units, mostly
thinking about the residential market [77]. Examples of small capacity
commercially available systems are InvenSor LTC10e, and SorTech
ACS08, with nominal power of 10 kW and 8 kW and nominal COP
values of 0.7 and 0.6 respectively [123,124]. The latter has been tested
and monitored in different European countries as part of an
standardised assembly, within the SolCoolSys project, measuring COP
values between 0.4 and 0.5 [125]. Smaller units have been explored
using different methods by several researchers, as shown in Table 7.
Based on the presented examples, it would be possible to expect a COP
of 0.4 – 0.5 for adsorption units below 5 kW.
3.3.2. Complexity of systems and components
3.3.2.1. Dimensions – size, volume and weight of systems and
components. One of the main disadvantages of adsorption units
usually mentioned in the literature, is their large sizes and weight
compared to their cooling capacity [103,116,117]. The smallest
commercially available chillers (SorTech ACS08, 8 kW nominal
cooling power) are 79 × 106 × 94 cm (LxWxH) with a weight
without water of 265 kg [124]. Smaller prototypes have been
developed, reaching dimensions of 60 × 60 × 100 cm (L × W × H)
for a 2.5 kW chiller under the EU project PolySmart [120] (Fig. 4).
Moreover, small sizes able to fit in the back of a car have been
developed for demonstration purposes of automobile AC applications,
reaching a weight without water of 86 kg [131]. Another explored
alternative to decrease sizes has been the integration of adsorption
systems and solar thermal collectors, to develop so-called adsorption
tubes [118,133,134]. Nonetheless, these experiences are in early R&D
stages.
3.3.2.2. Components – number and types of required components. A basic
solar driven adsorption cooling system needs the adsorption unit, a
solar collector, a heat rejection system, and an hydronic system for
water circulation, with small pumps to control the flow. Additionally,
the use of hot and/or cold storage units could be beneficial to increase
the efficiency of the system and achieve higher cooling capacities at the
beginning and end of the adsorption/desorption cycle [122].
Furthermore, it is necessary to consider a cooling delivery system,
such as fan-coils or water based radiative cooling devices such as
chilled ceilings or beams [27].
3.3.2.3. Connections – types of required connections and materials
involved. Commercially available adsorption units are factory sealed,
so they only require connection to the heat rejection system, the heat
source (solar thermal collector), and the cooling delivery system.
Connections are made through pipes, in closed water circuits driven
by pumps (heat rejection, driving heat, and chiller water circuits). The
main issue then, is preventing leakages throughout the whole system.
3.3.3. Cooling system operation
3.3.3.1. Health, safety and comfort issues. Health and safety hazards
related to adsorption technology highly depend on the materials used as
working pairs. Although all adsorption units are sealed, there could be a
risk of contamination through leakages. Direct exposure to ammonia
mixed with indoor air could cause problems in the respiratory track,
while methanol is regarded as a toxic and highly flammable material
[115,135]. Nevertheless, the most common working pair used in
adsorption chillers is the combination of silica gel and water, which
does not consider any hazard risk to building occupants, being both
non-toxic and non-flammable [15]. Moreover, the fact that silica gel/
water adsorption chillers are based on an environmentally friendly
process, is usually cited as one of the main advantages of adsorption
systems [121,136–139]. Regarding other comfort issues, the fact that
no moving parts are involved mean that noise levels are lower than
conventional cooling systems [116,117,119].
3.3.3.2. Maintenance requirements. Adsorption units are factory sealed
and consider no moving parts [119,140]. Additionally, there is no risk
of crystallisation nor internal corrosion in inner components
[117,121,136], so the basic refrigeration machine is regarded as
virtually maintenance free. Nonetheless, water pipe connections from
and to the adsorption unit must be checked to prevent leakages, while
small pumps needed for water circulation would need basic
maintenance to allow for continuous operation.
A. Prieto et al. Renewable and Sustainable Energy Reviews 101 (2019) 395–414
402
3.3.4. Level of development / maturity
3.3.4.1. Technical maturity. Overall, adsorption chillers are a mature
technology, with several decades of research development. First
adsorption based refrigerating systems appeared in USA around 1920,
while solar driven experiences have been reported since the late 70 s
[136]. Since then, research has been focused on improving the
performance of the units, experimenting with different working pairs
of adsorbent/adsorbate; and lately, on decreasing the size of systems to
allow for easier application in residential buildings. Given that the
performance heavily relies on the working pairs within the process
itself, it is difficult to think that there will be an increase of COP values,
having been optimised up to this point. Hence, future challenges will
keep focusing on decreasing sizes and weight, and achieving shorter
cycle times to allow for a greater array of applications.
3.3.4.2. Market/commercial maturity. Adsorption chillers represent the
second largest market for solar cooling, after absorption chillers. Until
2014, 1200 solar cooling installations had been reported worldwide,
mostly located in Europe [7]. Out of this total, 10–11% are reported to
be adsorption based technologies with cooling capacities ranging from
8 to 1000 kW [2,121]. Among well-known companies commercialising
adsorption chillers, are InvenSor and Fahrenheit (formerly known as
SorTech). The former distributes zeolites/water adsorption chillers in
the 10–105 kW range [123], while the latter commercialises silica gel/
water and zeolites/water chillers from 8 to 50 kW, as a spinoff of
Fraunhofer Institute for Solar Energy (ISE) [141]. It is stated in both
websites that zeolites/water chillers are the next generation (eZea was
the first one to be commercialised in 2015, by Fahrenheit), being
relatively smaller and lighter than conventional silica gel/water units.
3.4. Solid desiccant cooling
3.4.1. Performance of cooling systems and integrated concepts
3.4.1.1. General reported performance. Widespread general assessments
of solid DEC technology give it COP values between 0.5 and 1.0, with
cooling capacities ranging from 6 to 350 kW [21,78–80,142,143].
These values come mostly from several monitoring campaigns carried
out over the last 20 years on demonstration projects throughout
Europe. Thus, solar driven DEC pilot experiences have been designed
and evaluated in Germany [78,80,144], Austria [78,80,145], Spain
[78,146], Portugal [80] and Greece [78]; with cooling capacities from
18 kW to 75 kW and thermal COP values ranging from 0.43 to 0.86.
Additionally, the potential to reach higher COP values under optimised
system configurations has been simulated and experimentally tested. Ge
Fig. 4. Small-scale heat pump and AC unit for car application developed by Bakker et al. and De Boer et al.; and adsorption cooling principle.
Table 7
Small-scale adsorption based concepts reported in the review.
REF AUTHORS DESCRIPTION
[126] Clausse et al. Model to simulate the performance of a 4.6 kW activated carbon/methanol chiller for residential application in Orly, France, obtaining COP
values between 0.12 and 0.6 and an average COP of 0.49.
[127] de Lieto et al. Simulation of a 2 kW silica gel/water chiller in Rio de Janeiro. COP values of 0.43–0.58 using 20 m
2
of solar thermal collectors, which were
found to be enough to provide the necessary input.
[128] Luo et al. Design and construction of a silica gel/water chiller powered by 49.4 m
2
of evacuated tubes collectors. Experimental results of max. cooling
power of 4.96 kW and a peak average COP of 0.324.
[129] Lu et al. Experimental assessment of a silica gel/water prototype, reporting a cooling power of 4.9 and 5.7 kW, with COP values of 0.42 and 0.41,
respectively.
[120,130] Bakker et al. Small scale 2.5kW adsorption heat pump for residential application. Experimental reports of cooling power of nearly 2.5 kW and COP=0.45
for a 10 min operating cycle. Performance also monitored in a real house, reporting a decrease of 25–30% of cooling power compared to lab
results (EU Project PolySmart).
[131,132] De Boer et al. Verde 1.5 kW silica gel/water adsorption AC unit for car applications, driven by waste heat from the engine. Monitoring results on a Fiat Grande
Punto showed a cooling power of 800 W and COP of 0.5–0.6; in contrast to lab results of 2kW and 0.4 respectively.
A. Prieto et al. Renewable and Sustainable Energy Reviews 101 (2019) 395–414
403
et al. obtained a COP of 1.28 for a 101 kW 2-stage silica gel rotary DEC
for a complete floor in Shanghai, with 680 m
2
of vacuum tube collectors
[147]. Similarly, Fong et al. reported a COP of 1.38 for a solid DEC
system driven by 100 m
2
flat-plate air collectors [148]. Besides the
configuration of the system, performance of the cooling cycle relies on
the materials. Commonly used silica gel and zeolites have lower
sorption capacity than liquid desiccants, so research efforts are
focused on advanced materials by combination of silica gel with
other salts. Jia et al. experimentally obtained a COP of 1.28 by
employing compound desiccants, improving the performance of a
silica gel based DEC system by 20–30%; providing evidence of further
potential on this field [149].
3.4.1.2. Reported performance of small scale applications. Smaller
cooling capacities have been explored in an effort to promote the
development of compact units targeting new application niches,
depicted in Table 8. Besides these, some of the most relevant
experiences in the development of small-scale DEC systems for
building integration have been the results and prototypes developed
by SolarInvent under their FREESCOO patent [150]. Working
prototypes for a rooftop unit have been installed and monitored in
Palermo (2.7 kW) and Rome (5.5 kW), obtaining daily thermal COP
values of 1.1 and 1.36 [151–153]. As a logical next step, the developers
are currently working on the design of façade units, with decoupled
solar regeneration modules on the roof (circulating hot water to
regenerate the packed-bed desiccant material). These have not been
tested on the field yet, but preliminary evaluation shows nominal
thermal COP values around 1.25, for maximum cooling power of
2.5 kW [150].
3.4.2. Complexity of systems and components
3.4.2.1. Dimensions – size, volume and weight of systems and
components. One of the main drawbacks of solid desiccant
evaporative cooling systems (solid DEC) is their dimensions, being
larger in size than other solar cooling technologies, especially
considering relative dimensions per cooling capacity [15].
Commercially available DEC systems may occupy an entire room,
with dimensions (LxWxH) starting from 500 × 160 × 180 cm and
weight of 1600 kg (DesiCool®2.2 unit) [160]. This is due to the
different stages needed for air treatment and the accompanying
equipment, and particularly due to the use of desiccant wheels as
common carrier method. A noteworthy effort in size reduction was
conducted by Finocchiaro et al. in the design of their FREESCOO system
[152]. The compact rooftop prototype developed in Palermo considered
2.4 m
2
of PVT surface, and occupied a volume of about 2.5 m
3
,
considering a floor area of 200 × 120 cm (LxW) and a maximum and
minimum height of about 150 and 50 cm respectively (Fig. 5). While
these dimensions are still considerable, they imply a reduction of over
80% of the total volume compared to a currently commercially
available DEC unit, even considering integrated equipment for
desiccant regeneration. Additionally, the developers are currently
exploring potentially smaller sizes for façade integration. The
dimensions of concepts for façade units (without equipment for solar
input) are 200 × 35 × 100 cm (LxWxH), being regarded as a promising
alternative for building integration in the coming years [150].
3.4.2.2. Components – number and types of required components. Solar
driven solid desiccant cooling systems basically consist of desiccant
assisted evaporative coolers as air handling units, comprising a small
number of simple and robust components [154,161,162]. Incoming air
gets in contact with the desiccant, commonly placed in a slowly rotating
wheel (although it could also be stored in adsorbent beds). After
dehumidification, the air is cooled down by means of an evaporative
cooler (direct or indirect), to then be delivered to the room. Exhaust air
gets in contact with heat from the regenerator (solar thermal collectors
with optional heat storage), and then passes through the desiccant
wheel again on its way out, evaporating the previously absorbed water.
Fans are required to drive in and out air streams through separate ducts,
while pumps are required to circulate water in the regeneration and
evaporative cooling loops. Additional components commonly used are
heat exchangers for heat recovery between incoming and outgoing
streams and cooling towers for heat rejection.
3.4.2.3. Connections – types of required connections and materials
involved. The main connections needed for the operation of the
system are between the heat source and the desiccant machine. If
heat is supplied by water based solar collectors, this would mean pipes
in closed water circuits driven by pumps. Alternatively, an air-based flat
plate may be used, directly mixing warm air from the collector with the
outgoing air stream for the regeneration of the desiccant, throughout
air ducts. Accordingly, the system instalment has been judged as
slightly complicated [15,163]. Additionally, a water source and pipes
are needed for the evaporative cooler, besides electric input for
auxiliary equipment such as fans, pumps and the desiccant wheel
rotor [154].
3.4.3. Cooling system operation
3.4.3.1. Health, safety and comfort issues. The use of desiccants to cope
with latent loads potentially leads to better indoor air quality when
compared to common AC technologies, especially in hot-humid
climates. Vapour compression systems cool down the incoming air
below dew point, to drop humidity levels, to then reheat it to desired
temperatures. This process considers condensation, which creates a
suitable environment for microorganisms and mold within the system,
which are avoided under desiccant operation [164]. Additionally,
common materials such as silica gel and zeolites are non-toxic and
non-flammable, working under an entirely environmentally friendly
process [154,165].
3.4.3.2. Maintenance requirements. Solid desiccant systems consist of
Table 8
Small-scale solid desiccant based concepts reported in the review.
REF AUTHORS DESCRIPTION
[154] Goldsworthy & White Mathematical model for a DEC system, obtaining COP values of 0.2–0.7, where 0.4 was found to be the optimal value to achieve a balance
with the system's cooling capacity of about 1.5 kW.
[155] Wang et al. COP values of 0.46–0.49 obtained from the experimental evaluation of a novel self-cooled solid desiccant coated heat exchanger in
Shanghai, with cooling capacities below 714 W.
[156] Kabeel CaCl based DEC system coupled to a porous type solar air heater of 1.2 m
2
in Egypt, achieving cooling capacities of 0.8–1.0 kW and COP
values from 0.65 to 0.9.
[157,158] Ge at al. Design and evaluation of a 5 kW 2-stage DEC using silica gel-LiCl composite desiccant and a flat-plate air collector array of 15 m
2
, reporting
COP values over 1.0.
[159] Alahmer Numerically assessment of a DEC system for car application: rotary desiccant dehumidifier, compact heat exchanger and evaporative cooler,
with regeneration heat from the engine. Cooling capacity of 4.2 kW with thermal COP of 0.7–0.9.
[151–153] Finocchiaro et al. Prototype for integrated DEC +thermal collector unit. 2.7 kW unit monitored in Palermo with a daily thermal COP of 1.1. An 5.5 kW unit
monitored in Rome, with daily thermal COP of 1.36.
A. Prieto et al. Renewable and Sustainable Energy Reviews 101 (2019) 395–414
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simple and robust components, and non-corrosive working materials
[161,166]. Because of this, they are easy to clean and maintain in their
simplest forms. Furthermore, the fact that solid desiccant units operate
at almost atmospheric conditions (there is no need to maintain
vacuum), helps keeping maintenance costs low [163]. Nevertheless,
their operation in combination with evaporative coolers, to handle
sensible loads, increases the complexity of the entire system, adding
water connections that need to be checked for leakages from time to
time [15].
3.4.4. Level of development / maturity
3.4.4.1. Technical maturity. Desiccant AC using a rotary wheel has been
explored for over 50 years, with the first patent being introduced by
Pennington in 1955 [143]. Currently, solid desiccant dehumidification
is a mature technology, used for moisture control in large industrial
sites; while evaporative cooling is regarded as the oldest cooling
technique [166]. However, their combined use was not fully explored
until the turn of the century, promoted by the potential to use low-
grade heat (and particularly solar energy) as the driver of a refrigerant-
free cooling process. Today, solid desiccant cooling installations mount
up to around 7% of all solar driven cooling systems installed
worldwide, being third in numbers behind absorption and adsorption
chillers [39]. These experiences mostly consist of pilot and
demonstration projects carried out by researchers with private or
governmental funding to promote these systems and overcome
present boundaries for mass-application [78]. Current development
efforts are focused on performance issues, researching new advanced
desiccant materials and optimising the configuration of the overall
system; while striving for smaller and compact units for new markets
[143].
3.4.4.2. Market/commercial maturity. Solid desiccant components such
as rotary wheels have been commercialised since the 1980s,
experiencing an important increase in the last decades [15,143].
Companies such as Rotor Source [167] and Proflute [168] have
specialised on desiccant rotors, while companies such as DehuTech
[169] and big corporations like Munters [170] have achieved large
experience on both solid desiccant air dehumidification units, and
evaporative coolers. This combined expertise has resulted on the
development and commercialisation of solid desiccant AC units, such
as DehuTech's DS units and Munters’ DesiCool®units, but solar thermal
has not been fully explored yet for market introduction under
integrated systems. Nonetheless, the FREESCOO system is being
further developed for market release by SolarInvent, a start-up
company created in 2014 explicitly with that goal in mind, offering
customised units for specific applications for the time being [150].
3.5. Liquid desiccant cooling
3.5.1. Performance of cooling systems and integrated concepts
3.5.1.1. General reported performance. Although LDAC systems are
relatively new compared to other solar cooling principles and
technologies, the general assessment is that they may reach higher
potential efficiencies in comparison, backed by several research
projects and standalone experiences. Different overviews on the state
of the technology over the last 15 years have shown COP values circling
1.0, with the potential to go higher [21,78,80]. Standalone experiences
have been designed and tested, with cooling capacities ranging from
0.3 kW [171,172] to around 25 kW [173,174], besides demonstration
projects of 11–30 kW monitored on existing buildings [78,175,176],
with registered COP values between 0.47 and 0.8 under real operating
conditions. The efficiency of a LDAC system mainly relies on the
dehumidification capacity of the desiccant material and its
application, and the cooling performance of the evaporative cooler.
Hence, current efforts deal with the optimisation of each sub-system
separately, testing new materials and performance-based designs; while
at the same time exploring integration potential between them striving
for compact units for mass-market production.
3.5.1.2. Reported performance of small scale applications. Most
experiences on LDAC systems have focused on small capacity ranges
(below 30 kW), benefiting on potentially compact units for residential
application. Relevant examples are shown in Table 9. Thermal COP
values around 0.7 seem to be consistent among small systems,
especially on those below 6 kW, with potential for building
integration. At the same time, maximum COP values around 1.2 were
found, which evidences that there is room for future improvements in
the performance of small scale units.
3.5.2. Complexity of systems and components
3.5.2.1. Dimensions – size, volume and weight of systems and
components. One of the most reported advantages of LDAC systems is
the potentially small and compact sizes that they may reach, being
based on a liquid and easy to handle solution [166,182]. Current efforts
Fig. 5. Compact solid-desiccant prototypes developed by SolarInvent, for rooftop and façade applications; and general cooling principle.
A. Prieto et al. Renewable and Sustainable Energy Reviews 101 (2019) 395–414
405
have focused on designing compact systems to explore new markets
such as residential air-conditioning. Buker et al. [181] and Das & Jain
[180] evaluated the performance of small scale LDAC systems (around
5 kW of cooling capacity), considering dehumidifier units of
90 × 50 × 130 cm and 30 × 60 × 80 cm respectively. Even smaller
units were designed and tested by Chen et al.[171,172] and Elmer
et al. [161], with integration potential in mind. The former designed a
membrane based LDAC, comprised of highly compact modules for
regeneration and dehumidification of 41 × 23 × 21 cm each. Elmer
et al. designed an integrated system, combining a regenerator,
dehumidifier and evaporative intercooler into a single membrane
based heat and mass exchanger (Fig. 6). The entire unit dimensions
were 100 × 42 ×24cm plus split desiccant and water tanks and small
pumps [161]. Similarly, Kozubal et al. developed and evaluated a
membrane based desiccant indirect evaporative system, within a sealed
packaged unit of 60 × 50 × 48 cm to cover a cooling demand of around
3.5 kW [177].
3.5.2.2. Components – number and types of required
components. Basically, a solar driven liquid desiccant cooling system
consists of three main parts: the dehumidifier, the regenerator, and the
sensible cooling machine, commonly an evaporative cooler. The
dehumidifier is where intake air gets in contact with the desiccant,
either directly (applied in packed beds, spray tower, or falling film), or
indirectly (membrane based exchangers). It considers a storage tank for
the desiccant solution, and the contact medium within a
dehumidification chamber. The regenerator consists of a solar
thermal collector array to collect heat to be used to regenerate the
desiccant. The regeneration may take place in a regeneration chamber
where stored heat is applied to the solution, or the desiccant itself may
pass through the collector via permeable pipes. Thermal storage units
are advised under the former operation mode. Lastly, evaporative
cooling may be direct or indirect, adding moisture to the incoming
air or not, respectively. This considers the need for a water tank and a
pump, increasing the complexity of LDAC systems [183], thus the
design of simple and compact evaporative coolers is an essential aspect
in the promotion of LDAC systems.
3.5.2.3. Connections – types of required connections and materials
involved. A common arrangement for LDAC systems requires two
independent circuits of circulating liquid: the first carries the
desiccant solution and goes from the dehumidifier to the regenerator
and back; while the second carries water for the evaporation cooler. An
hydronic system and pumps are needed, besides storage tanks for the
desiccant and water [27]. These circuits come in contact sequentially
with the incoming air flow, which needs to be carried inside through
vents and ducts, driven by fans. Electricity is needed to power these
components, causing parasitic loads of varying importance depending
on the specific design. Among relevant new developments aimed to
simplify these systems, natural convection driven units and
Table 9
Small-scale liquid desiccant based concepts reported in the review.
REF AUTHORS DESCRIPTION
[177] Kozubal et al. Simulation of a desiccant enhanced evaporative cooler in several cities in USA, with cooling capacities of 3.5–14 kW. Estimated cooling energy
savings up to 61% compared to common VC technologies.
[178] Zhang et al. Experimental assessment of a LD unit +2-stage evaporation cooling system, with cooling capacities from 6 to 12 kW. Measured COP values were
0.4–0.6, with an average of 0.56.
[179] Jain et al. Experimental evaluation of an indirect contact LDAC system in tropical climates, obtaining COP values of 0.4–0.8 for cooling capacities between 2.5
and 5.5 kW.
[180] Das & Jain 1.7–5.5 kW LDAC dedicated outdoor air system including a 12.5 m
2
ETC array as input for the regeneration. COP values of 0.25–0.83 were
experimentally obtained.
[181] Buker et al. Experimental thermal COP of 0.73 for a 5.2 kW membrane based LDAC with an indirect evaporative cooler and a BIPVT for regeneration and
electricity generation.
[171,172] Chen et al. Experimental thermal COP of 0.7 for a membrane based LDAC based on modular units for dehumidification and regeneration. Solution concentration
of 36% CaCl
2
was found to be optimal in the 0.34–0.43 kW system
[161] Elmer et al. Novel integrated membrane based LDAC system. Average COP values of 0.72 were experimentally obtained for the 0.57–1.36 kW system, using
CHKO
2
as desiccant material. Maximum reported COP around 1.2.
Fig. 6. Integrated small-scale membrane based LDAC unit developed by Elmer et al. and liquid desiccant general cooling principle.
A. Prieto et al. Renewable and Sustainable Energy Reviews 101 (2019) 395–414
406
photovoltaic electrodyalisis stand out. The former benefit by the
concentration gradient of the solution to generate free motion of the
desiccant without pumps [184]; while photovoltaic electrodyalisis (PV-
ED), uses PV panels instead of thermal collectors for desiccant
regeneration by transporting ions through selective membranes under
the influence of an electrical field [185,186]. Nonetheless, these
technologies are still in early development stages.
3.5.3. Cooling system operation
3.5.3.1. Health, safety and comfort issues. The use of desiccants directly
improves indoor air quality in humid environments, by avoiding
overcooling to cope with latent loads. Condensation derived by
lowering ambient temperature below dew-point creates an
environment suitable for mold and bacteria, present when using
common compressor based HVAC technologies [164]. Additionally,
liquid desiccant materials exhibit the capacity of absorbing pollutants
and bacteria present in the incoming air [15,165]. On the other hand,
one of the main concerns regarding the use of desiccants is the risk of
material carry-over with the supply air stream, being an open cycle air
treatment system [3,166,187]. Liquid desiccants have reported low-
toxicity, but may still be a source for discomfort and a hazard in high
concentrations. Nonetheless, this issue has been solved in latter
experiences, by using semi-permeable membranes as contact barrier
between desiccants and inlet air, allowing heat and moisture transfer
but preventing transfer of the desiccant material, in liquid-to-air
membrane heat exchangers [172,173].
3.5.3.2. Maintenance requirements. The most common liquid desiccant
materials currently available are aqueous halide salts, which have
strong dehumidification capabilities but are highly corrosive
[165,188]. Hence, periodical maintenance is mandatory to assess
possibly damage in the system due to filtration or material carry-over
through the air flow. Among these salts, lithium chloride (LiCl) has low
vapour pressure and more stability, while the regeneration performance
of lithium bromide (LiBr) is better, and calcium chloride (CaCl
2
) has
less absorption ability but is cheaper and easily available. Additionally,
these salts are subject to crystallisation at higher concentrations, which
needs to be checked [15,183]. Alternatively, there are current
explorations of salts of weak organic acids such as potassium formate
(HCOOK/CHKO
2
) and sodium formate (HCOONa), which have low
toxicity and are not corrosive, but have less absorption capacity, so
higher concentrations are needed (50% of CHKO
2
solution
concentration roughly equals the performance of 27% of LiCl) [161].
In any case, these latter materials are not fully explored so there is room
for new developments in the coming years.
3.5.4. Level of development / maturity
3.5.4.1. Technical maturity. Solar driven liquid desiccant systems for
space cooling are still in early R&D stages, with different levels of
development depending on particular applications. First experimental
studies on LD systems go back to the 1950s [189], but interest on these
materials and technologies really sparked in the mid-90s conducing to
an increasing number of experiences over the last 20 years [3,174,190].
The use of liquid desiccants as complement to vapour compression
chillers has been widely explored in the last years, seeking an energy
efficient way to handle latent heat in hot-humid climates, instead of
recurring to overcooling and subsequent heating to control humidity.
This has given room for experiences coupling LD with evaporative
cooling systems, as the best alternative to vapour compression chillers.
Hence, the development of LDAC goes hand in hand with the
development of evaporative cooling systems. The dehumidification
capacity of common LD materials has been largely explored,
designing and testing several application modes; while evaporative
coolers are regarded as a mature technology but have only achieved
limited market penetration [15]. Hence, current development efforts
focus on integration and simplicity on the design of the system to allow
for small capacity LDAC+EC units, while at the same time exploring
efficient ways to use solar heat for regeneration purposes.
3.5.4.2. Market/commercial maturity. Currently, there are no solar
driven LDAC systems commercially available. The most developed
experiences consist of several prototypes and patents, besides a few
demonstration projects for long term monitoring purposes and raising
awareness and interest in the technology. Advantix Systems, an US
based company founded on 2006 received great attention between
2010 and 2013, by manufacturing and commercialising hybrid LDAC
systems (LiCl system coupled with a vapour compressor) for
commercial and industrial buildings, being regarded as pioneers in
the field [191–194]. Nonetheless, there are no signs of the company
after 2014, suggesting that it went out of business. Additionally, Alfa
Laval Kathabar [195], a company specialised on dehumidification and
HVAC, has recently developed large scale LD based dedicated outdoor
air systems using LiCl, also considering a compressor as part of the unit.
In any case, ostensibly R&D efforts are still needed to allow for the
integration of small size units into the market.
4. Evaluation & discussion: potential for façade integration
An assessment of the technologies is presented, discussing their
potential to overcome identified barriers for façade integration of
building services. The evaluation was conducted following the strategy
and rubrics presented in the methods section of the present document
(Table 2), seeking to provide a referential qualitative assessment of the
current state-of-the-art, and specifically how each technology fares re-
garding different relevant aspects for façade integration. Fig. 7 shows
maps for each technology, while barriers are discussed separately.
4.1. Technical feasibility
This refers to the overall practicability of integrating all required
components for cooling in façade modules. Hence, addressing sizes of
the entire system and its components, and their adequate operation in
small scales. First of all, it is relevant to point out that the review
showcased small scale working examples, with cooling capacities below
3 kW for all selected technologies, which, leaving efficiencies aside for
the moment, shows that all of them may operate in the small capacity
range.
Besides the existence of small scale concepts, the most clear proof of
technical feasibility is the development of working integrated façade
prototypes. In this regard, the simplicity of the cooling principle and
sizes of the required components have made thermoelectric cooling the
technology of choice for the development of most façade integrated
concepts found in the literature. These prototypes, even in cases with
underwhelming efficiencies, are regarded as evidence of the feasibility
of integrated concepts for façade applications. Second to thermoelectric
based systems, there are also stand-alone façade concepts based on
absorption and solid desiccant principles. Both technologies are quite
mature (especially absorption) but are commonly employed in larger
scales, considering bulky components. The fact that compact experi-
ences for façade integration are being developed and tested seems
promising for future applications.
On the other hand, although compact systems are being designed
and tested, liquid desiccant systems still need further research and
development to allow for façade integrated concepts. Finally, adsorp-
tion based systems still present certain issues related to the intrinsic
bulkiness of their components, and the need for another conduit for air
supply, being based on closed refrigerating cycles.
4.2. Physical integration
Barriers related to physical integration refer to externalities derived
from the connection of the required components, and the compatibility
A. Prieto et al. Renewable and Sustainable Energy Reviews 101 (2019) 395–414
407
of sub-systems and working materials. In these aspects, technologies
based on solid-state heat transfer, such as thermoelectric cooling, have
clear advantages, being based on simple direct contact between com-
ponents, besides simple electrical connections to the PV array for en-
ergy input. Peltier modules are easy to handle and integrate, although
further exploration is needed in order to develop ready-made building
components to use in architectural designs.
For all other technologies, a basic distinction could be made be-
tween closed cycle and open cycle processes; namely absorption/ad-
sorption, and desiccant cooling respectively. Both absorption and ad-
sorption heat pumps commonly consist of factory sealed units, only
needing connections to the heat source, heat rejection system and
cooling distribution network, usually carried out throughout pipes. Of
course, the fact that refrigerant is carried in a closed cycle, implies that
a heat exchanger is needed for cooling delivery, commonly using fan-
coils in central water-air applications. Nonetheless, these types of
connections are quite common, and easy to solve, dealing with cooling
and ventilation requirements through two separate but complementary
channels. Furthermore, the current research and development of sorp-
tion based integrated concepts, considering a collector array, sorption
heat pump, and decentralised air intake [94]; is pushing the boundaries
on packaged systems, with no further needs than a discrete electric
input for fans, following a plug & play approach.
More complex connections are present in the case of desiccant
technologies, mostly due to the fact that a complementary system is
needed to take care of sensible loads. While desiccants account for la-
tent loads, evaporative cooling is commonly used to provide sensible
cooling. Solid desiccant installations have been judged as slightly
complicated [15,163], while liquid desiccant units present the added
challenge of handling liquid material for dehumidification on a sepa-
rate hydronic circuit, with the associated pumps and storage tanks.
Future applications of liquid desiccant enhanced evaporative cooling
systems largely depend on the simplification and compatibility of their
components. Early experiences of natural convection driven units
[184], membrane based systems [161], and the use of photovoltaic
electrodyalisis for regeneration purposes [186] are steps in the right
direction, but further research & development is still needed to ad-
vocate for the application of liquid desiccant based cooling systems in
buildings.
4.3. Durability & maintenance
This refers to both the durability of required components over time,
and maintenance requirements that have an impact on operational
costs associated to each technology. Following the aforementioned
simplicity associated with the technology, and the lack of moving parts
nor the use of refrigerants; thermoelectric components are regarded as
the most durable, being virtually maintenance free besides basic elec-
trical maintenance. However, their lifetime within building compo-
nents has not been fully tested. Regarding thermal driven technologies,
durability of components and maintenance requirements highly depend
on the working materials used in the cooling process. Liquid desiccant
and absorption cooling rely on liquid dehumidification materials, while
the principles behind solid desiccant and adsorption cooling are based
on the use of solid materials on carrier surfaces.
The most common liquid desiccants currently used are aqueous
halide salts, such as lithium bromide (LiBr) and lithium chloride (LiCl).
These salts are highly corrosive and are subject to crystallisation at high
concentrations, so careful maintenance must be conducted to assure
that there is no corrosion in components nor carry-over to the supply air
stream, assuring optimal operational conditions. This is more trouble-
some for liquid desiccant systems, based on open refrigerant cycles,
although further exploration of indirect contact membrane based sys-
tems could lead to carry-over free products. An alternative option to
overcome these issues is further exploration of other non-corrosive
materials, but experiences are still in early stages.
On the other hand, solid materials such as silica gel and zeolites do
not present any hazard and are corrosion free. This fact, plus the lack of
moving parts in their inner mechanism, makes adsorption heat pumps
virtually maintenance free. Solid desiccant systems, comprising more
complex connections but simple and robust components, require peri-
odic but simple maintenance.
4.4. Performance
The performance of the selected technologies was mainly addressed
in terms of cooling output and efficiency values reported by the re-
viewed experiences, besides considering potential hazardous ex-
ternalities for indoor comfort. The aforementioned hazard risk by carry-
over associated to liquid desiccant materials is the only health concern
worth mentioning, being solved by means of indirect contact with the
air stream. On the other hand, the use of desiccant materials (solid or
liquid) to deal with latent loads may prove beneficial for indoor comfort
by avoiding condensation derived from re-heating the air stream to
comfort temperatures.
The reported cooling power and the coefficient of performance
Fig. 7. Qualitative assessment maps for the facade integration potential of selected solar cooling technologies.
A. Prieto et al. Renewable and Sustainable Energy Reviews 101 (2019) 395–414
408
(COP) of the reviewed systems and prototypes are shown in the graph
below (Fig. 8). The COP refers to the input energy so it is electrical
efficiency in the case of thermoelectric cooling, and thermal COP for
solar thermal driven technologies. Moreover, this only considers the
efficiency of the cooling system for purposes of the comparison, so the
efficiencies of PVs or solar thermal collectors are not accounted for.
Regarding cooling power, only experiences below 10 kW were con-
sidered. Also, the graph shows the range of cooling design capacities for
a single office in different orientations, based on simulations conducted
by the authors in a previous work [196], with dark and light grey
marking the design values for low and high cooling demands respec-
tively.
First of all, it is possible to see that although thermoelectric concepts
may reach high COP values, they are out of range for the cooling power
required for a single office room. Hence, current integrated thermo-
electric concepts do not seem to be able to meet decentral cooling de-
mands, by lack of cooling power, requiring a back-up system. Further
development and testing of larger concepts is needed to fully assess
their in-range efficiencies, keeping in mind that there is a trade-off
between the efficiency of the system and its cooling output.
Discussing thermal driven technologies, possibilities greatly open
up. Closed-cycle systems, and particularly adsorption heat pumps
(N = 36), comprise the majority of reviewed small-scale experiences.
The performance of closed systems seems to be consistent among the
sample, usually reaching thermal COP values of 0.6 and 0.8 for ad-
sorption and absorption systems, respectively. The high amount of
adsorption based experiences follows current research trends aimed to
boost the development of small scale systems for residential applica-
tion, considering several prototypes in the 3–5 kW range. Regarding
absorption, most experiences are higher than 4 kW, with smaller ca-
pacities only tailored to façade integrated purposes in the aforemen-
tioned sorption collector unit [94].
In general, desiccant based cooling systems discussed in the review
comprise lower cooling capacities than closed-cycle systems. However,
this does not seem to present a problem due to their close match with
the required range for the design of cooling systems. Regarding effi-
ciency, these technologies reach higher in-range thermal COP values,
especially in the case of solid desiccants. Nonetheless, more research is
required to ensure reliable results following the maturity achieved by
closed-cycle technologies.
4.5. Aesthetics & availability
Aesthetics is a highly complex topic of discussion not only regarding
façade integration, but in architecture and the arts in the wider sense,
opening different trends of thought that fall out of the scope of the
present research. Hence, it is necessary to constrain its understanding
for the evaluation at hand. Moreover, the external appearance of façade
integrated solar cooling concepts will be heavily, if not totally, influ-
enced by the solar energy conversion system utilised, namely photo-
voltaics or solar thermal collectors. While there are several options to
choose for any given design or preferred appearance, they do not make
any difference for the evaluation of the solar cooling systems driven by
them. Therefore, for the purpose of addressing this aspect in the eva-
luation of the selected solar cooling technologies, the aesthetical po-
tential is understood as the potential to allow for flexibility and varia-
bility in façade design.
Sizes and overall complexity of the components and overall current
systems are regarded as a relevant hindrance for design flexibility,
limiting the application of adsorption and desiccant technologies re-
spectively. The lack of standardised components, or plug & play mod-
ular systems; limits the applicability of these technologies at best to
tailor-made solutions under an integral façade design. The development
of integrated compact solutions are steps in the right direction in order
to develop building products for architects and facade designers to use,
but further efforts need to be conducted in the field. This is also true in
the case of thermoelectric components. Due to their dimensions and low
complexity, they do not greatly restrict design choices, but exploration
and research should eventually conduct to the development of archi-
tectural products, easy to integrate in early stages of façade design,
while ensuring reliable operation.
4.6. General assessment: charting a roadmap for the development of facade
integrated concepts
The qualitative assessment of solar cooling technologies in terms of
their potential for façade integration, clearly shows what is currently
possible, but at the same time serves to identify shortcomings and
bottlenecks related to each technology, if façade integration is the final
goal. Table 10 shows recommendations for further development of all
assessed technologies, drafting a roadmap for future R&D experiences
focusing on key aspects to overcome for façade integration. Further-
more, the most pressing issues to solve per technology are highlighted,
following the lower ranked aspects at the assessment (below ++).
In general, it is quite evident that these technologies are not ready
yet for façade application, with all of them ranking low in ‘aesthetics &
availability’ aspects. Further developments and exploration focused on
the generation of integrated building products, or plug & play compact
systems, are needed for all assessed technologies. At the same time, the
fact that liquid desiccant cooling technologies have been more recently
explored, compared to other thermal driven systems, gives them a
disadvantage in development level and maturity, needing further re-
search in most aspects to be up to date. In any case, the rate of new
developments in the field is seen as highly promising. Although the
conducted assessment only considers current possibilities, it is the au-
thors’ opinion that liquid desiccant cooling technologies have large
unexplored potential, with auspicious opportunities for application in
the built environment.
In the case of technologies that consider solid desiccants as basis of
their operation (adsorption, solid desiccant cooling), the main current
bottlenecks are related to the size of components and generation of
compact integrated systems. Even considering latest developments of
compact desiccant units [150], they still need to be field tested and
thoroughly validated under different working conditions. The technol-
ogies that currently seem closer to commercial façade applications are
thermoelectric cooling and absorption based systems. However, im-
portant bottlenecks still remain regarding performance issues and the
Fig. 8. Cooling power (kW) and coefficient of performance of reported small-
scale solar cooling prototypes.
A. Prieto et al. Renewable and Sustainable Energy Reviews 101 (2019) 395–414
409
development of compact units with durable components and working
materials, respectively.
The assessment above has shown current possibilities and bottle-
necks for façade integration of selected technologies, drafting re-
commendations for further development. However, the discussed topics
also allow to debate façade integration itself, defining different paths
for product development depending on the understanding of integra-
tion and its implications. Authors have proposed the distinction be-
tween ‘integral’ and ‘modular’ construction as two ways to integrate
extra functions into the building envelope. The former considers func-
tions embedded in a multifunctional component, while the latter refers
to different mono-functional parts, connected to form a multifunctional
whole [197]. Following this distinction, there are two clear product
development paths for façade integration: (a) the development of in-
tegral building components for architectural design, and (b) the de-
velopment of modular packaged systems ready to be installed if the
required connections are space are provided.
Current strengths and shortcomings of each assessed solar cooling
technology make them more suitable for either one of these product
development paths, defining potential product types worth exploring.
Based on the review and assessment, the most viable options for the
development of distinct façade integrated products are depicted in
Fig. 9, using a chart for the categorisation of solar cooling technologies
for façade integration purposes, proposed in an earlier work by the
authors [27].
Performance issues aside, thermoelectric cooling technologies are
regarded as the most suitable for developing integral building compo-
nents. The compact dimensions of their basic elements and the sim-
plicity of their connections and operation principles give them an im-
portant advantage for the design of active building components, at
different scales, ranging from window elements [58,60] to entire ra-
diant walls and façade units [53,54]. On the other hand, the sorption
collector integrated with decentral ventilation showcased in the review
[94] is regarded as the best current example for the development of self-
sufficient compact systems under absorption processes. In this case, the
challenge for designers would be how to plan the connection of these
modular packaged systems, while assimilating them into the overall
façade composition. Similarly, future developments on liquid desiccant
units should follow this path, taking advantage of the flexibility given
by the use of liquids as working material.
Finally, a third possible path for product development is in-
corporated in the discussion, as partial façade integration (c). This re-
fers to the integration of certain components of the solar cooling system
in façade units, such as the solid desiccant based facade ventilation
unit, with decoupled solar regeneration (thermal collectors located in
the roof) developed by SolarInvent [150]. Also, even if small-scale
absorption/adsorption chillers remain too big for façade integration,
cooling distribution may be conducted through especially designed
façade elements for central or semi-decentral applications (various
rooms or an entire floor). Furthermore, façade integrated water-based
systems may be alternatively connected to cooling delivery elements far
from the façade, such as cooled ceilings or beams, through an hydronic
system, providing further application possibilities for deep plan build-
ings. Based on current possibilities, solid desiccant cooling and sorption
chillers seem to be apt for partial façade integration. While break-
throughs may come in the future, for the time being, the generation of
self-sustaining solar cooling facades based on these technologies seems
unlikely. Hence, in this case, seems logical to promote façade integra-
tion of certain key components to enhance new application possibilities
based on the comparative strengths of these technologies.
5. Conclusions
This paper sought to discuss the potential for façade integration of
selected solar cooling technologies, based on a state-of-the-art review of
technology-specific attributes and the assessment of their capability to
respond to previously identified barriers for façade integration of
building services. The review focused on small-scale systems and
Table 10
Recommendations for further development of solar cooling technologies, to overcome identified barriers for façade integration.
BARRIERS SOLAR COOLING TECHNOLOGIES
THERMO ELECTRIC ABSORPTION ADSORPTION SOLID DESICCANT LIQUID DESICCANT
TECHNICAL
FEASIBILITY
Prototype tesng and
experimental
measurement of
facade integrated
concepts.
Further exploraon
and development of
compact systems for
façade integraon.
Size reducon of
components and
exploraon of
alternave processes.
Development and
validaon of compact
systems for façade
integraon.
Development and
tesng of compact
units.
PHYSICAL
INTEGRATION
Standardise
connecons and
components for
development of
architectural
products.
Further exploraon
of plug & play
integrated
approaches to system
design.
Exploraon of
integrated systems.
Exploraon of
integrated compact
systems.
Exploraon of
alternave processes
to simplify
connecons and
increase
compability.
DURABILITY &
MAINTENANCE
Tesng of durability
of TE modules
applied in building
components over
me and different
climate condions.
Exploraon of non-
corrosive working
pairs and vacuum
sealed compact
systems.
Tesng of compact
adsorpon systems
over me and
different climate
condions.
Tesng of compact
solid desiccant
systems over me
and different climate
condions.
Exploraon and
tesng of alternave
non-corrosive
materials.
PERFORMANCE
Increase cooling
power of peler
modules, balancing
adequate COP values.
Explore up-scaled
components.
Further development
and tesng of
compact systems
below 3kW.
Increase COP values
of small scale
systems.
Further development
and tesng of
compact systems
below 3kW for
reliability of COP
values.
Further development
and tesng of
compact systems
below 3 kW for
reliability of COP
values.
AESTHETICS &
AVAILABILITY
Development of
architectural
products and
integrated building
components.
Development of plug
& play systems for
façade integraon.
Size reducon of
components for
development of plug
& play systems.
Size reducon and
simplificaon of
connecons for
development of
decentralised
venlaon systems.
Development and
validaon of compact
integrated systems
for future product
development.
A. Prieto et al.
Renewable and Sustainable Energy Reviews 101 (2019) 395–414
410
concepts, exploring current boundaries and development level of
compact units for integration purposes. Moreover, the evaluation
tackled the aforementioned barriers, categorised in five groups of as-
pects to overcome for façade integration: technical feasibility, physical
connections, durability & maintenance, performance; and aesthetics &
availability.
The review showcased examples of small-scale units for all tech-
nologies; and even façade integrated concepts for some of them. On the
other hand, the assessment showed that the suitability of the selected
technologies varies according to each particular barrier for façade in-
tegration. Hence, currently there is no technology that fits all required
aspects. Further research and development are needed for all technol-
ogies to allow for widespread application of integrated concepts, and
future commercialisation of architectural products. Therefore, current
possibilities were mapped, identifying certain bottlenecks and drafting
recommendations for further development, focusing on key aspects to
solve per technology.
Although they are not ready for widespread application yet, the use
of thermoelectric modules and compact units based on absorption
technologies, are regarded as the most promising ones for the devel-
opment of either integral building components, or modular plug & play
systems for façade integration. In the case of thermoelectric cooling
concepts, the main constraint is their comparative performance in terms
of the expected cooling output; requiring the development and testing
of scaled-up components that maintain high efficiencies reported by
systems out of the required range. For absorption units, the main
challenges are the exploration of non-corrosive working materials, and
further development and testing of compact packaged units under a
modular design approach. On a separate note, liquid desiccant cooling
technologies are deemed as potentially promising, based on the rate of
new developments and the flexibility given by liquid working materials.
Nevertheless, they are less mature in comparison to the rest, so general
research is required to explore their potential.
Finally, it is recommended that further explorations on compact
adsorption and solid desiccant cooling systems focus on partial façade
integration. Thus, promoting an alternative development path based on
the strengths and shortcomings associated with the technologies.
Specific challenges are size reduction and simplification of the cooling
processes, however, the integration of only certain components in
façade units mitigates size constraints, while opens new possibilities for
semi-decentral applications and cooling distribution to areas far from
the façade.
The presented assessment and recommendations aim to coordinate
future efforts and explorations in the field, charting paths for the de-
velopment of a variety of architectural products and new building ap-
plications; and specific challenges to overcome. This supports a general
vision for further promotion and widespread integration of renewable
energy sources and environmentally friendly cooling processes in the
built environment; however, this has to be thoroughly combined with
campaigns and measures to reduce our energy demands, and the cli-
mate responsive design of buildings and cities, particularly in warm
climate contexts.
Acknowledgements
This paper is part of the ongoing Ph.D. research project titled
COOLFACADE: Architectural integration of solar cooling technologies
in the building envelope, developed within the Architectural Façades &
Products Research Group (AF&P) of the Department of Architectural
Engineering + Technology, Delft University of Technology (TU Delft).
The research project is being funded through a scholarship granted by
CONICYT, the National Commission for Scientific and Technological
Research of Chile (Resolution N°7484/2013).
Fig. 9. Research and development paths for the generation of distinct architectural products for façade integration.
A. Prieto et al. Renewable and Sustainable Energy Reviews 101 (2019) 395–414
411
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