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VII International Congress on Architectural Envelopes
May 27, 28, 29 2015, San Sebastian-Donostia, Spain
An Experimental and Numerical Simulation Study of an Active
Solar Wall Enhanced with Phase Change Materials
Dionysios I. Kolaitisa,*, Roberto Garay Martinezb, Maria A. Fountia
a School of Mechanical Engineering, National Technical University of Athens, Athens, Greece
b Sustainable Construction Division, Tecnalia, Derio, Spain
*e-mail: dkol@central.ntua.gr
Key words: Solar wall, Trombe wall, Phase change materials, PCM, experimental data, CFD,
simulation, building energy performance, energy savings
Abstract
Solar walls can be used to increase the overall energy efficiency of a building. Phase Change Materials
(PCM) are capable of increasing the effective thermal mass of building elements, thus resulting in
reduced overall energy consumption. Recently, the incorporation of PCM in a solar wall has been
proposed, aiming to increase the total energy efficiency of the system. A PCM-enhanced solar wall
(PCMESW) can be installed in new buildings, as well as in existing buildings, in the frame of an
energy retrofitting process. The main scope of this work is to investigate the thermal behaviour of this
innovative system, using both experimental and numerical simulation techniques. A prototype
PCMESW is installed in Tecnalia's Kubik large-scale test facility and is exposed to dynamically
changing ambient conditions. A broad range of sensors, used to monitor the time-evolution of several
important physical parameters (e.g. air cavity and surface temperature, heat flux and air velocity), is
employed to assess the dynamic response of the PCMESW. A host of additional sensors is used to
accurately describe the time-varying meteorological conditions (e.g. temperature, relative humidity,
wind velocity and direction, incident total and diffuse solar radiation). A range of operational scenarios
is investigated, aiming to evaluate the thermal performance of the system in various conditions. In
addition, a Computational Fluid Dynamics tool is used to numerically investigate the thermal
behaviour of the PCMESW prototype. Predictions of the developing flow- and thermal-field in the
PCMESW's air cavity are validated by means of comparison with the obtained measurements; in
general, good levels of agreement are observed.
1 Introduction
Primary energy use in buildings accounts for approximately 40% of the total annual energy
consumption and CO2 emissions in the European Union (E.U.) [1]. Therefore, energy efficiency of
buildings is increasingly becoming, worldwide, a major energy policy objective. There is a large
Dionysios I. Kolaitis, Roberto Garay Martinez, Julen Astudillo Larraz, Maria A. Founti
variety of construction techniques and materials available that can be used to improve the energy
efficiency of buildings. Solar walls (SW) can be used as passive building elements aiming to decrease
energy consumption for heating and cooling purposes. In addition, incorporation of Phase Change
Materials (PCM) in building elements can be used to increase their thermal inertia. Recently, the use
of PCM to increase the effectiveness of a conventional SW has been proposed [2]; the PCM-enhanced
SW can be installed both in new and existing buildings, as a means to improve the total building
energy efficiency. The main scope of the current work is to investigate the dynamic operational
behaviour of an innovative PCM-enhanced SW, using detailed experimental and numerical simulation
techniques.
1.1 Solar Wall
A Solar Wall (SW) is essentially a thermal system comprising a glazing panel and a high thermal mass
wall, separated by an air cavity. The orientation of the SW is selected to maximise the incident solar
radiation (e.g. Southern orientation in the Northern hemisphere). The temperature of the high thermal
mass wall is increased, thus heating the air in the cavity; the heated air flows upwards, due to thermal
buoyancy (natural convection) or mechanical devices, e.g. fans (forced convection). Air is allowed to
enter and exit the cavity through ventilation openings in its lower and upper side, respectively. The air
ventilation openings can be controlled in order to allow air to enter or exit the cavity either from the
external (ambient) or internal (indoors) boundary of the system. SW are installed aiming to reduce the
overall energy demand for heating and cooling.
1.2 Phase Change Materials
Latent heat storage systems can be used to decrease the overall energy use in buildings; Phase Change
Materials (PCM), exhibiting high values of latent heat of melting (or solidification), are extensively
used in latent heat storage systems. Use of PCM in building elements has been recently proposed,
aiming to increase the indoor thermal comfort (reduction of air temperature peaks, decrease of diurnal
indoor temperature fluctuations), decrease the required energy consumption for heating/cooling
(reduction of the heating and cooling peak loads) and take advantage of off-peak energy savings [3].
Latent heat storage by incorporating PCM in building elements is an attractive way to compensate for
the limited thermal mass of modern lightweight buildings. Aiming to improve the overall building
energy efficiency, PCM can be used in a variety of building elements such as walls (construction units,
insulation materials), glazing and shading devices (glazing systems, solar walls), thermally activated
constructions or even HVAC systems [4].
1.3 PCM-Enhanced Solar Wall
When a conventional SW is enhanced with a PCM the total thermal mass of the system is increased. In
this case, it is possible to reduce the overall volume of the wall, thus resulting in significant advantages
in terms of cost and dead load. A number of experimental and theoretical analyses have been
conducted to investigate the applicability of using PCM in SW systems [3, 5, 6]. In a PCM-Enhanced
Solar Wall (PCMESW), the large sensible thermal mass of a conventional SW is essentially
substituted by the latent heat loads from the PCM phase change process.
An innovative technological concept based on this idea has been proposed in a recent European Patent
[2]; the proposed system can be installed either to new or existing (energy retrofitting) buildings. The
PCMESW system (Figure 1) comprises a double glazing layer, a highly reflective solar protection
“roller blind” system, a high thermal mass wall which incorporates PCM and an insulation layer
located between the PCM-enhanced wall and the indoor environment; an air cavity is formed between
VII International Congress on Architectural Envelopes. May 27, 28, 29 - 2015, Donostia-San Sebastián, Spain
the glazing and the wall. On the lower and upper sides of the cavity, air ventilation openings are
installed that allow air to enter or exit the cavity either from the external (ambient) or internal (indoors)
boundary of the system. Air ventilation is assisted by the use of 4 small electrically driven fans,
installed at the bottom of the air cavity.
This innovative PCMESW system is able to operate in 3 main operational modes; each mode is further
split in 2 sub-modes, one for “daylight” operation and the other for “night-time” conditions. The
general configuration of the PCMESW system in each operational mode is shown in Figure 1. In
operational mode 1, mainly used during the heating season (winter), the PCMESW system operates as
a conventional Trombe wall; air is allowed to enter and exit the system only from the “indoors” side.
During daytime (sub-mode 1-D), the solar protection roller blinds are inactive, thus allowing the
incident solar radiation to heat both the air, and the PCM-enhanced wall. During the night-time (sub-
mode 1-N), the solar protection is activated to mitigate heat losses to the ambient. Operational mode 2,
also used during the heating season (winter), corresponds to a parietodynamic wall. In this case,
ambient air is allowed to enter the system through the bottom inlet vents and, after being heated by the
high thermal mass wall, is directed indoors, through the top outlet vents. Similar to operational mode
1, the roller blinds are inactive during daytime (sub-mode 2-D) and are activated during the night (sub-
mode 2-N). In operational mode 3, commonly used during the cooling season (summer), the
PCMESW system operates as a ventilated façade; air is allowed to enter and exit the system only from
the “ambient” side. In this case, the solar protection “blind” system is always active, thus preventing
the incident solar radiation to reach the high thermal mass wall. In the daytime (sub-mode 3-D), the
ambient air that enters the system is moving upwards, thus cooling the wall by means of natural
ventilation. In the night-time (sub-mode 3-N), the wall releases the stored thermal energy to the
ambient air moving in the cavity, thus reducing the energy demand for cooling.
Figure 1: Schematic layout of the 3 operational modes and 6 sub-modes.
Operation of the PCMESW system is optimized by adapting to the varying meteorological conditions.
Selection of the appropriate operational mode depends on the prevailing ambient conditions and the
desired indoor thermal comfort conditions; changes between operational modes can be made either on
predefined time periods (diurnal and annual) or dynamically, using real-time information provided by
sensors. Operational modes 1 (Trombe wall) and 2 (parietodynamic wall) are used during the heating
season, whereas operational mode 3 (ventilated façade) is used during the cooling season. Selection of
the actual operational mode of the PCMESW system can be made either automatically, using
appropriate control algorithms, or manually. The different operational modes are set by altering the
position of the air ventilation openings (air can enter or exit either to the external (ambient) or the
internal (indoors) environment), activation of the solar protection roller blind system (when activated it
prevents solar heat flux to reach the high thermal mass wall) and activation of the ventilation fans
installed at the bottom of the system. The configuration of each controllable device for every
operational mode and sub-mode available is presented in Table 1.
Dionysios I. Kolaitis, Roberto Garay Martinez, Julen Astudillo Larraz, Maria A. Founti
Table 1: System configuration for the 3 operational modes and 6 sub-modes.
Operation
Trombe wall
Parietodynamic wall
Ventilated Façade
Mode
1
2
3
Sub-Mode
1-D
1-N
2-D
2-N
3-D
3-N
Bottom Vent
In
In
Out
Out
Out
Out
Top Vent
In
In
In
In
Out
Out
Roller blind
Up
Down
Up
Down
Down
Down
Ventilation Fan
On
On
On
On
Off
On
2 Experimental Study
As part of the development of the PCMESW system, a scientific and technical assessment of the
PCMESW concept was carried out, consisting on a full scale test under real atmospheric conditions.
This kind of test provides not only important information of the energy performance of such a system,
but also confidence is gained in the level of integration between different elements in the solar
collector system as a prior step to industrialization of the design. For this purpose, a South-oriented
portion of the building envelope in the experimental research facility Kubik by Tecnalia was modified
to house the PCMESW prototype. Although this paper refers mainly to thermal assessments of the
PCMESW development, manufacturability, installation, wear, accuracy of actuators, suitability of
fans, wiring and other aspects were also tested in the process.
2.1 Layout of the Test Facility
Kubik by Tecnalia is a multi-rise building aimed at realistic testing of building concepts, for which it
provides a fully adaptable environment (internal boundary conditions, HVAC system layout,
adaptation of building envelopes, fully customizable building automation & control). It is located in
Derio, on the Atlantic coast of Spain, which exhibits a Cfb climate based on the Koeppen climate
classification system [7]. The Cfb climate characterizes most of central and West Europe, including
the British Islands, and some locations in the Mediterranean Coast.
Figure 2: View of the PCMESW system installed in the southern façade of the Kubik building (left)
and schematic layout of the installed sensors (right).
The Kubik test facility is designed and operated as a test facility to bridge the gap between laboratory
testing and full scale deployment, and is customized on a case-by case basis to meet the specific needs
VII International Congress on Architectural Envelopes. May 27, 28, 29 - 2015, Donostia-San Sebastián, Spain
of each project. In this particular case, a portion of the south-facing curtain wall system was removed
to house the PCMESW system (Figure 2, left). This system was heavily instrumented to verify the
behaviour of the PCMESW as a passive system (solar thermal collector) and as an adaptive system
(control algorithm). Temperature sensors were located at different heights within the air gap and the
PCM surface of the PCMESW system. These measurements were complemented with the weather
station at the Kubik test facility, additional ambient air temperature and radiant temperature sensors
installed in the façade, hot wire anemometers in the air gap, and indoor measurements (Figure 2,
right).
2.2 Experimental Results
The PCMESW system was monitored during the autumn-winter period of 2013-2014. Within this
period a complete dataset for approximately 1.5 months was collected. In Figure 3, indicative
temperature and solar radiation data are shown. Recorded temperatures at the exposed surface of the
PCM-enhanced wall are found to reach peak values of 50-60ºC, when the ambient air temperature is
far lower (10-20ºC), thus demonstrating the potential of the PCMESW system to assist space heating.
Figure 3: Recorded measurements of temperatures in the PCMESW system (left) and incident solar
radiation (right) over a typical 8-day period.
2.3 Net Output Energy for Space Heating
The net output energy of the PCMESW system, available for space heating, was estimated using the
obtained measurements. Empirical correlations were developed, based on a detailed analysis of the
measured data, to allow estimation of the maximum daily energy output of the system during a period
of one year. As expected, the monthly energy output of the system was found to be highly dependent
on the available solar energy for that particular period. In Figure 4, the estimated monthly energy
output for a typical building located in Bilbao (Spain) is presented. During the winter period
(November-March), an average monthly energy output of 4 kWh/m2 can be achieved. As expected,
larger output energy values for the summer period are obtained; however, this output should be filtered
out, according to the actual heating needs of the building during the summer (cooling) period.
Dionysios I. Kolaitis, Roberto Garay Martinez, Julen Astudillo Larraz, Maria A. Founti
Figure 4: Estimated monthly energy output for space heating, per unit surface area of the PCMESW.
3 Numerical Simulations
An in-depth view of the thermal behaviour of SW in various operational conditions can be obtained by
means of numerical simulations; the obtained results may be used to assist the design process of such
complex thermal systems [8]. In this work, a detailed numerical simulation study is performed, using
the ANSYS-CFX 15.0 Computational Fluid Dynamic (CFD) tool. CFX is a general-purpose CFD tool,
capable of simulating complex turbulent, multi-phase, multi-component and reacting flows. The
performed time-transient simulations allow investigation of the developing flow- and thermal-fields,
aiming to determine the dynamic thermal response of the PCMESW system.
3.1 Simulation Setup
A simplified geometry of the actual force-ventilated PCMESW system, used in the experimental
campaign, is simulated. The system consists (from outdoors to indoors) of a double glazing (6mm and
4mm glazing panels, separated by a 6mm thick enclosed air cavity), a 73mm thick ventilated air cavity
and a 17mm thick PCM layer. The total height of the simulation domain is 1830mm. A 2D non-
uniform Cartesian grid, comprising 51380 elements, is used for the simulations; the gas-phase grid is
refined close to the solid boundaries in order to improve the flow resolution in the region of the
developing boundary layers (Figure 5, left). The total simulation time corresponds to a full 24-hr
period; an adaptive time-step is used, ranging from 15s to 300s. The Reynolds-Averaged Navier-
Stokes formulation is used to describe turbulent flow phenomena; the Shear Stress Transport
turbulence model is employed.
Figure 5: Indicative view of the mesh (left) and time-varying thermal boundary conditions (right).
VII International Congress on Architectural Envelopes. May 27, 28, 29 - 2015, Donostia-San Sebastián, Spain
Measurements of the time-varying ambient temperature and solar incident heat flux during a typical
24-hr period (12 October 2013) are used as thermal boundary conditions (Figure 5, right); these values
are imposed on the outer surface of the external glazing that is directly exposed to the environment.
The PCM layer on the indoor side of the system is assumed to be attached to a well-insulated wall;
adiabatic boundary conditions are used in this case. At the bottom entrance of the air cavity, a constant
volumetric air flow (26.3 l/s) is employed, corresponding to the forced ventilation conditions created
by the 4 fans installed at the air inlet region. A single operational mode (sub-mode 1-D, Trombe wall)
is used for the entire duration of the simulation, following experimental practice. Therefore, ambient
air temperature measurements are also used to estimate the air temperature at the bottom inlet of the
system. The thermal behaviour of the commercial PCM, used in the actual PCMESW system, is
simulated using the effective specific heat methodology [9]; in this case, the specific heat of the
material is artificially modified in the temperature region of the phase change, in order to account for
the PCM’s latent heat of melting/solidification.
3.2 Numerical Results
In Figure 6 (left), CFD predictions for the air cavity and PCM wall surface temperatures are compared
to the respective experimental values, obtained at a height of 1600mm. Numerical results for the PCM
surface temperature are found to be slightly higher than the measured values; these discrepancies are
attributed mainly to the assumptions used to estimate the (unavailable) thermal properties of the
commercial PCM system used in the prototype (e.g. thermal conductivity, emissivity). However, a
very good quantitative agreement is achieved for the air cavity temperature, suggesting that the CFD
code may be effectively used to analyse the characteristics of the flow-field developing in the cavity.
A comparison of the developing velocity and temperature profiles in the air cavity is presented in
Figure 6 (right). The depicted curves correspond to 3 characteristic positions along the height of the
cavity, i.e. 5%, 50% and 95% of the total height, as measured from the bottom inlet; the time instant
corresponds to 14:20. Predictions of air velocity and air temperature in the cavity suggest that the
mean temperature is increased with increasing height; due to the increased thermal mass of the PCM,
higher air velocity and temperature values are observed at the boundary layer developing near the
PCM wall. The almost homogeneous velocity profile at the inlet (5%) is gradually transformed to a
highly distorted profile, where high velocities are observed near the wall. A similar behaviour is
observed in predictions of the temperature profiles. Near the top outlet of the cavity, the air
temperature near the PCM wall is almost 31oC higher than the respective temperature close to the
inner glazing.
Figure 6: Temporal evolution of air cavity and PCM wall surface temperatures (left) and spatial
distribution of air cavity velocities and temperatures at various heights (right).
Dionysios I. Kolaitis, Roberto Garay Martinez, Julen Astudillo Larraz, Maria A. Founti
4 Conclusions
An innovative solar wall, enhanced with phase change materials, has been investigated using both
experimental and numerical simulation techniques. A prototype PCMESW has been installed in a real-
scale testing building and was monitored for 1.5 months in the autumn-winter season. The obtained
measurements indicated that the potential monthly net output thermal energy of the system, during the
winter period in a typical Cfb climate region, may be as high as 4 kWh/m2. Numerical simulations of
the prototype device have been also performed, using a CFD tool. Predictions have been found to
agree reasonably well with the measured values. CFD results allowed an in-depth view of the
developing thermal- and flow-field in the prototype PCMESW.
Acknowledgements
This study has been financially supported by the E.C. in the frame of the FP7 project "MeeFS:
Multifunctional Energy Efficient Façade System for Building Retrofitting" (EeB.NMP.2011-3, Grant
No. 285411).
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