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Solar Cooling Technologies for Southern Climates
- A System Comparison –
Dirk Pietruschka1, Uli Jakob2, Ursula Eicker1
1 Centre of Applied Research Sustainable Energy Technology - zafh.net, Stuttgart
University of Applied Sciences, Schellingstrasse 24, D-70174 Stuttgart, Germany,
Phone +49 711 8926 2674, dirk.pietruschka@hft-stuttgart.de
2 Solem Consulting, Postfach 2127, D-71370 Weinstadt, Germany
Phone +49 174 4130921, uli.jakob@solem-consulting.com
Abstract
In the present paper different solar thermal cooling systems are compared to a PV
driven and a net connected compression chiller in hot and dry southern climate. The
cooling systems are considered to be applied to a planned innovative office building
in Cairo, Egypt. A single effect absorption chiller with vacuum tube collectors is
analysed as well as a double and a triple effect absorption chiller with higher
concentrating Fresnel collectors. For the PV driven compression chiller system high
efficient mono crystalline PV modules are considered together with a highly efficient
compression chiller system with integrated direct dry het rejection. Dynamic system
simulations with INSEL are used to analyse the performance of the different cooling
systems. To compare the overall performance of the analysed solar cooling systems,
the primary energy consumption required to cover the whole cooling load of the
building and the resulting primary energy ratio are calculated for each system.
1. Introduction
The overall efficiency of solar driven absorption cooling machines (ACM) is mainly
influenced by the thermal COP of the absorption chiller and the electricity
consumption caused by the heat rejection system, the chiller and all connected
system pumps. Single effect absorption chillers reach only quite low thermal COPs
which are typically in the region between 0.55 and 0.75. In consequence, large solar
collector areas and large heat rejection systems are required to reach high solar
fractions and to remove the waste heat. This often cause high electricity
consumptions which reduces the primary energy efficiency of the systems [1].
However, the main advantage of single effect absorption and adsorption chillers is
the relatively low driving temperature which varies between 65°C and 95°C. Such
temperatures can be provided by efficient flat plate or vacuum tube collectors.
Double effect absorption chillers reach much higher thermal COPs of 1.3 and above
but typically require much higher driving temperatures of around 180°C. To provide
such high temperatures higher concentrating solar systems like parabolic trough or
Published in:
Proceedings of 4th Solar Air Conditioning Conference, Larnaka, Cyprus,
12.-14.10.2011
linear Fresnel collectors are required [2]. Due to decreasing PV module prices PV
driven highly efficient compression chillers become more and more attractive as a
competitive solar cooling technology. In the present paper a detailed simulation
based case study of a solar cooling application for an office building project in Cairo
Egypt is presented. Here, four different systems are regarded: Single effect, double
effect and triple effect absorption chillers and a PV driven compression chiller. The
single effect absorption chiller is considered to be purely solar driven. For the double
and triple effect absorption chiller backup heating with a gas boiler / integrated gas
burner is considered. In case of the PV driven compression chiller additional
electricity is imported from the local grid if not sufficiently provided by the PV system.
2. Description of the Analysed Building and its Location
The analysed office building is located in Cairo in Egypt and has a total useful floor
area of 15 100 m² with a conditioned volume of 55 116 m³. It consists of a central
core and three starlike adjacent ‘fingers’ with four office floors each. Double glazed
windows with sun protecting coating are considered for the fully glazed facades with
an U-Value of 1.16 W/m²K and g-value of 0.265 with 3.8% framing fraction and an U-
Value of the frames of 2.04 W/m²K. Additional shading is provided by a roof
overhang of 2.5 m in the upper floors of the south, southeast and southwest facing
facades. For all opaque building elements like external walls, roof and floors a
insulation of 20 cm is considered resulting in U-values around 0.18 W/m². According
to dynamic building simulations performed with TRNSYS the maximum cooling load
of the building is about 800 kW (52 W/m²) and the annual cooling energy demand is
1970 MWh/a (130 kWh/m²a). Due to the necessity of dehumidification in summer the
temperature level of the cold water distribution system is 7°C/14°C. The local
weather conditions used in the dynamic simulations are taken form METEONORM
weather data base.
3. Analysed solar cooling systems
The limiting factor for the size of the solar cooling systems is the available and usable
roof area for the solar thermal or PV system, which is 2 000 m² only. For the system
design several simulations were performed for single effect, double effect and triple
effect absorption chillers. The single effect absorption chiller was combined with
efficient vacuum tube collectors (Jiangsu Sunrain TZ47/1500-10U) with an optical
efficiency of 0.65, a linear heat transfer coefficient of 1.585 W/m²K and a temperature
dependent quadratic heat transfer coefficient of 0.002 W/m²K². The maximum
possible collector size at horizontal orientation is 2050 m² brut collector area which is
equal to a collector aperture area of 1350 m² (1.500 collectors). For the double effect
absorption chiller linear concentrating Fresnel collectors of Industrial Solar are
considered. The optical efficiency of the Fresnel collectors is 62% with a linear heat
transfer coefficient of 0.1 W/m²K and a temperature dependent quadratic heat
transfer coefficient of 0.00043 W/m²K². For the linear Fresnel collectors the maximum
collector aperture area is 1320 m² (60 collectors; length 4 m; width 8 m).
To evaluate the optimum system configuration the size of the hot water storage and
the capacity of the absorption chillers were varied. For the comparison of single,
double and triple effect absorption chillers the optimum system design found for each
of the solar thermal cooling systems was selected. For the PV driven compression
chiller the available and useful roof are of 2000 m² allows the installation of 1200 m²
mono crystalline PV modules with an optimum inclination of 25° towards south. The
following four system configurations are analysed:
Case 1: Single effect ACM (LiBr) 422 kW (THERMAX, ProChill LT12C), 7°C/12.2°C cold water, wet
cooling tower (29.4 °C/36.6°C), vacuum tube collector field for hot water supply.
2025 m² brut collector area, 1350 m² collector aperture area, 3.3 kW electricity consumption
solar pump, 20 m³ hot water storage and 10 m³ cold water storage
Case 2: Double effect ACM (LiBr) 500 kW (Shuangliang 500 KW) 7°C/12°C cold water, wet
cooling tower (37°C/42°C), linear concentrating Fresnel collectors (Industrial Solar) for
hot water supply. 2 050 m² brut collector area including spaces between the rows, 1 320 m²
collector aperture area, 3.2 kW electricity consumption solar pump, 20 m³ pressurised hot
water storage (max. 200°C) and 10 m³ cold water storage.
Case 3: Triple effect ACM (LiBr) 563 kW vapour driven (250°C) (Kawasaki Sigma Ace CF01-10-
0001), 7°C/15°C cold water, wet cooling tower (32°C/36.6°C), linear concentrating
Fresnel collectors (Industrial Solar) for water steam supply (max. 250°C at 3.9 MPa)
1280 m² brut collector area including spaces between the rows, 880 m² Collector aperture
area, 1.8 kW electricity consumption solar pump, no hot water storage and 10 m³ cold water
storage.
Case 4: Compression Chiller 795 kW (Quantum A090 3C12 with R-134a as refrigerant), 7°C/12°C
cold water, with integrated direct dry heat rejection for refrigerant condensation (power given
at 35°C ambient air temperature), the electrical COP is 2.9 at 100%, 3.9 at 75%, 4.9 at 50%
and 6.5 at 25% cooling capacity. A 10 m³ cold water storage is considered
PV modules
875 Aleo S17 modules of ALEO Solar GmbH with 180 W per module at maximum power
point (I_SC = 8.42 A, U_OC =30.4 V). 25° Inclination towards south, 1206 m² total module
area, 156 kWpeak total installed power at maximum power point.
Inverter:
Sunny Central SC 150 of SMA, 150 kW nominal power, Umax,DC = 900 V; Imax,DC = 354 A
(Grid connected without battery)
For the thermal cooling systems additional cooling is considered to be provided by an
efficient compression chiller with an average electrical COP of 2.8 including the
electricity consumption of the compression chiller, of the dry heat rejection system
and of all connected pumps. For heat rejection of the thermally driven LiBr chillers
wet cooling towers are considered with frequency inverters for fan speed control at
part load conditions. The single effect and triple effect absorption chillers are
combined with an AXIMA EWK 680/9 and the double effect chiller with an AXIMA
EWK 450/9 cooling tower. Compared to the single effect absorption chiller, the
required heat rejection energy is much lower for the triple effect chiller but due to the
high mass flow rate in the absorber/condenser circuit the bigger cooling tower is
required.
4. Results and Discussion
The main simulation results found for the analysed solar cooling systems are shown
in Figure 1 to Figure 3. The fraction of the thermally driven absorption chillers on the
overall cooling energy demand of the building is shown in Figure 1 together with the
solar system efficiency. The lowest absorption chiller fraction of 37% is reached for
the single effect absorption chiller, since no backup heating is used in this case. This
system reaches compared to the higher concentrating systems the highest overall
solar thermal system efficiency of 40%. The much lower solar system efficiency of
the systems with higher concentrating collector results mainly from the fact, that
these collectors can only use the direct solar radiation and not the diffuse part of it. In
the annual average the direct beam radiation part is in Cairo only 60% of the total
solar radiation. The system with the double effect absorption chiller and higher
concentrating collector reach 91% ACM fraction on the cooling load, since only the
peak loads above 500 kW need to be covered by the compression chiller. The triple
effect absorption chiller reaches a higher maximum cooling power of 563 kW and is
therefore able to cover 93% of the annual cooling load of the building.
The solar heating energy and the additional heating energy provided to the
absorption chillers is shown in Figure 2 together with the average thermal COP of the
chillers which are 0.7 for the single effect, 1.31 for the double effect and 1.83 for the
triple effect chiller. Due to the higher thermal COP the double and triple effect chillers
require much lower heating energy than the single effect system.
Figure 3 shows the primary energy consumption of the four analysed solar cooling
systems compared to the primary energy consumption of the reference system with
efficient compression chiller. The resulting average primary energy ratio of all
analysed systems is also shown in this graph. From this graph it becomes clearly
obvious, that the primary energy ratio of the single effect absorption chiller with
vacuum tube collectors and additional cooling is with 1.43 only slightly lower than that
the double effect absorption chiller with Fresnel collectors, additional heating and
additional cooling which reaches a value of 1.50. The overall best energetic
performance is reached for the triple effect absorption chiller which reaches a primary
energy ratio of 1.6 which is 12 % higher than in case of the single effect system.
37%
91% 93%
40%
31%
27%
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Single effect ACM + vacuum
tube collectors Double effect ACM + Fresnel
collectors Triple effect ACM + Fresnel
collectors
Solar system efficiency / %
ACM fraction / %
ACM fraction Solar system efficiency
Figure 1: Fraction of the ACM on the cooling load and solar system efficiency
1050
792
469
577
534
0,70
1,31
1,83
0
0,2
0,4
0,6
0,8
1
1,2
1,4
1,6
1,8
2
0
200
400
600
800
1000
1200
1400
Single effect ACM + vacuum
tube collectors Double effect ACM + Fresnel
collectors Triple effect ACM + Fresnel
collectors
COP
th
/ -
Heating energy consumption / MWh a
-1
Qh_solar Qh_additional COPth
Figure 2: Solar heating, additional heating and average thermal COP
If the local electricity grid is considered as ideal storage the PER of the PV driven
compression chiller is with 1.59 only slightly lower than the best thermal cooling
system (33% PV contribution). If only the produced electricity is considered, which
can be directly used by the chiller (22% PV contribution), the PER decreases to 1.37
which is even worse than the single effect absorption cooling system. Compared to
an efficient standard compression cooling system only fed by the local grid, all
analysed solar cooling systems reach significantly higher primary energy ratios of
+38% in case of the single effect absorption chiller up to +54% in case of the triple
effect chiller with Fresnel collectors. This highlights the main advantage of efficient
designed and controlled solar cooling systems.
183
443 462
1240 1443
692 640
1194
176 128
1900
1,43 1,50 1,60 1,59
1,37
1,04
0
0,2
0,4
0,6
0,8
1
1,2
1,4
1,6
1,8
2
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Single effect
ACM +
vacuum tube
collectors
Double effect
ACM +
Fresnel
collectors
Triple ef fect
ACM +
Fresnel
collectors
CCM with PV
collectors,
grid as idea l
storage
CCM with PV
collectors,
electricity
directly used
Reference
system with
compression
chil ler only
Primary energy ratio PER / -
Primary energy consumption / MWh a
-1
Electricity Additional heating Additional cooling PER
Figure 3: Primary energy consumption and average primary energy ratio (PER)
5. Conclusions
The results found in this paper clearly demonstrated that double effect absorption
chillers with backup heating (1st choice) and backup cooling (2nd choice) are from
the primary energy point of few not necessarily better than single effect absorption
chillers with backup cooling only. The overall best performance with a primary energy
ratio of 1.6 was reached for a triple effect chiller with backup heating (1st choice) and
backup cooling (2nd choice). The analysed highly efficient PV driven compression
chiller reaches a comparable primary energy ratio, if the local electricity grid is
considered as ideal storage. Otherwise, the primary energy ratio of this system is
lower than that of the analysed thermal cooling systems. However, it could be shown
that all analysed solar cooling systems reach significantly higher primary energy
efficiencies than standard systems with compression chillers only. Further analyses
will focus on the comparison of the economic performance of all analysed systems.
References:
[1] Albers, J., Kemmer, H., Ziegler, F. “Solar driven absorption chiller controlled by hot and cooling
water temperature“,3rd International Conference Solar Air-Conditioning, 30th September – 2nd
October, Palermo, Sicily, Italy, proceedings pp 338-343, 2009
[2] Rubio, H., ‘Solar cooling systems and natural gas; experiences and results’, in proceedings of the
7th European Forum Gas, Madrid, Spain, 2009
