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Requirements on the design
and configuration of small and medium sized
solar air‐conditioning applications
Guidelines
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Guidelines Requirements on the design and configuration of small and medium sized solar air‐conditioning applications
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Guidelines
This publication has been produced in the framework of the
SOLAIR project which is supported by the Intelligent Energy –
Europe programme of the European Commission.SOLAIR aims mainly at capacity building, promotion and
influencing the process of decision making for the
implementation of small and medium‐sized solar air‐
conditioning (SAC) systems in order to increase the
confidence on the technology and to encourage its
implementation.
April 15, 2009
www.solair‐project.eu
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This report was prepared as deliverable D10 in the SOLAIR project on base of material and
information provided by all partners in the project.
Edited by Edo Wiemken, Fraunhofer ISE. Chapter one – Building cooling and air‐conditioning –
was prepared by Sašo Medved, University of Ljubljana, Slovenia. Chapter seven – Planning tools –
was prepared by Maria João Carvalho, INETI, Portugal.
SOLAIR is co‐ordinated by
• target GmbH, Germany
Partners in the SOLAIR consortium:
• AEE – Institute for Sustainable Technologies, Austria• Fraunhofer Institute for Solar Energy Systems ISE, Germany• Instituto Nacional de Engenharia, Technologia e Innovação INETI, Portugal• Politecnico di Milano, Italy
• University of Ljubljana, Slovenia• AIGUASOL, Spain• TECSOL, France• Federation of European Heating and Air‐conditioning Associations RHEVA, The Netherlands• Centre for Renewable Energy Sources CRES, Greece• Ente Vasco de la Energia EVE, Spain• Provincia di Lecce, Italy• Ambiente Italia, Italy
0
SOLAIR is supported by
The sole responsibility for the content of this publication lies with the authors. It does not necessarily reflect the opinionof the European Communities. The European Commission is not responsible for any use that may be made of the
information contained therein
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Table of content
Introduction.................................................................................................................................7
1 Building cooling and air‐conditioning..............................................................................9
1.1 Indoor thermal comfort ....................................................................................................9
1.2 Cooling demand of buildings ..........................................................................................13
1.3 Energy conservation principles .......................................................................................18
1.4 Fundamentals of solar cooling ........................................................................................22
1.5 Impact of climate changes on thermal indoor comfort
and energy demand for cooling......................................................................................23
2 Technologies applicable for solar thermally driven cooling ...........................................1
2.1 Chilled water systems .....................................................................................................29
2.2 Open cycle processes......................................................................................................37
2.3 Solar thermal collectors ..................................................................................................40
3 General requirements on solar air‐conditioning and cooling systems.........................46
3.1 Primary energy saving.....................................................................................................46
3.2 Requirements on basic system layout ............................................................................50
3.3 Heat rejection system .....................................................................................................52
3.4 Solar collector system.....................................................................................................54
4 Selection of the appropriate technology.......................................................................60
4.1 All air systems .................................................................................................................62
4.2 Full air system + chilled water distribution.....................................................................66
4.3 Supply air system + chilled water distribution................................................................684.4 All water system..............................................................................................................69
5 Small systems: schemes for typical applications ..........................................................71
6 Recommendations on monitoring and quality assurance ............................................77
7 Planning tools .................................................................................................................84
7.1 Design approaches..........................................................................................................84
7.2 Rules of Thumb ...............................................................................................................85
7.3 Simple pre‐design tools...................................................................................................87
7.3.1 SHC‐Softwaretool (NEGST Project) .................................................................................87
7.3.2 SACE Solar cooling evaluation light tool .........................................................................89
7.3.3 SolAC – Solar Assisted Air Conditioning Software...........................................................90
7.3.4 ODIRSOL – Solar Assisted cooling Software....................................................................92
7.3.5 Expected new pre‐design tools.......................................................................................93
7.4 Detailed simulation tools ................................................................................................94
7.4.1 System orientated...........................................................................................................94
7.4.2 Building orientated .........................................................................................................95
7.4.3 Further simulation tool description ................................................................................96
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Introduction
The demand for building cooling and air‐conditioning is still rapidly increasing. To give an
impression: the sales rate in 2008 for small size electrically driven room air conditioners (< 5 kW
chilling capacity) was approx. 82 million units worldwide, of which 8.6 million were sold in Europe.
It is not surprising that in some areas the peak load in the public electricity grid is evoked during
hot summer seasons already by electrically driven air‐conditioning. In Germany, a country with
definitely not the highest demand for cooling and air‐conditioning, the overall electricity demand
for building air‐conditioning in 2006 was estimated to approx. 5% of the total electricity
consumption (14% for the total of air‐conditioning and refrigeration); in other South European
countries this share might be far higher.
Building air‐conditioning is today based mainly on electrically driven mechanic vapour
compression technologies. Although for new developed, predominantly large capacity scale
developments it is reported about high efficiencies in the compression cycle, for the standard of
air‐conditioning in existing buildings it can be assumed that on an average less than 3 kWh ‘cold’
are produced with the electricity input of 1 kWhel. Subsequently this implies that approximately
1 kWh primary energy is used for the provision of 1 kWh useful ‘cold’.
At the same time of peak cooling demand, high amounts of solar radiation are available at many
sites and could be used for thermally driven processes, e.g., cooling and air ‐conditioning. The
processes are in general well known and not new. Thermally driven cooling was applied within the
last decades in niche‐markets preferably in the large capacity range, using waste heat or heat
from combined heat and power production. However, the combination of this technology with
solar heat is new and some more complexity arises with this combination. Solar cooling and air‐
conditioning is demonstrated in a few hundred installations so far.
Solar thermally assisted cooling and air‐conditioning can contribute to an environmentally friendly
building supply system for the following reasons:
• considerable savings in primary energy consumption and reduction of CO2 emissions arepossible
• load relieving of the public electricity grid in terms of both, peak power and energy, thuscontributing to grid stabilisation
• combined use of solar heat for heating, cooling and domestic hot water preparation, thus anall‐season high utilisation of the solar thermal system
• no use of working materials with high global warming potential• less noise emissions and less vibrations than vapour compression technologies.
Thus, support for the market development of this technology is useful; these guidelines, edited in
the frame of the SOLAIR1 project, is one of the supporting activities.
1 SOLAIR ‐ Increasing the market implementation of solar air ‐conditioning systems for small and medium applications inresidential and commercial buildings (SOLAIR). Supported in the Intelligent Energy Europe Programme of the European
Commission. EIE/06/034/S12.446612. Duration: until 12/2009.www.solair ‐ project.eu
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Interaction in the design and layout of a solar thermally driven cooling and air ‐conditioning system, to be considered in
the planning phase.
The proper design of a solar cooling and air‐conditioning system and the choice of the
components interact to a high degree with the site conditions (climatic conditions) and with thedemand for cooling (load conditions). The intention of this guideline is to support the
understanding of the interactions and to provide in parallel a picture on the state of the art of
solar cooling and air‐conditioning.
As one of the most cost‐effective measures in the planning of an air‐conditioning system is the
reduction of cooling loads already in the building planning and design phase, chapter one deals
with general aspects on building cooling and air‐conditioning and prepares the reader for the
subsequent chapters, focusing on the technical aspects of solar thermally driven technologies.
However, some aspects of solar cooling and air‐conditioning may have found not the adequate
attendance in these guidelines, such as e.g. more details on system control or on detailed site
oriented installation information. The reason for this lack is the still ongoing process in the
development and preparation of such information.The thematic structure of the content underlines the target group of technical orientated
planners in the building services and utilities management area, but the guidelines are hopefully
useful to anyone, interested on this subject.
Finally, a more comprehensive description of solar cooling and air‐conditioning can be found in
the handbook for planners ‘Solar Assisted Air‐Conditioning in Buildings’2, elaborated in the Task
25 on Solar Cooling within the Solar Heating and Cooling Programme (SHC) of the International
Energy Agency (IEA). In the current Task 38 ‘Solar Air‐Conditioning and Refrigeration’, a new
edition of this handbook will be launched and available in 2010. In the context with the existing
handbook, these guidelines may be seen in both ways: as a straightforward introduction into solar
cooling and air‐conditioning on the one hand, and as a market and practically oriented
complement to the handbook on the other hand.
2 Hans ‐Martin Henning (Editor): Solar Assisted Air ‐Conditioning of Buildings – A Handbook for Planners. Second revised
edition 2007. ISBN 978‐3‐211‐73095‐9, Springer Wien New York.
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1 Building cooling and air‐conditioning
The main goal of every building planner is to assure the most pleasant and healthy living
environment to people that live in the building. However the challenge here is to attain the
optimal indoor comfort with minimal energy consumption and minimal environmental impact.
From the engineering point of view the quality of indoor environment is defined by four groups of
requirements: thermal comfort, indoor air quality, lighting comfort and noise protection.
Concerning energy consumption the most important issue is fulfilling of thermal comfort
requirements.
Figure 1.1 Indoor environment quality could be assured by fulfilling of four groups of requirements
1.1 Indoor thermal comfort
Human is a warm‐blooded being with constant internal temperature (37 ± 0.8°C), which isindependent of surrounding temperature and muscle activity. The body produces heat in internal
organs with combustion (oxidation) of nutritive substances. This process is called metabolism or
basal metabolism. Metabolism is regulated by our body regarding to momentarily activity. Similar
as with heat machines, the human body has to give off the excess heat to the environment by
means of different heat transfer mechanisms. If such heat transfer from our body to surroundings
does not cause any unpleasant sensation the requirements of thermal comfort are fulfilled.
Thermal confort
Lighting confort
Indoor air qualityIAQ
Noise protection
IEQ
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Figure 1.2 Human body emits sensible and latent heat into environment using different heat transfer processes. If this
process does not cause unpleasant sensation the thermal comfort is provided.The body emits the heat in the form of sensible and latent heat. Sensible heat is emitted with convection and radiation
of the body to the surrounding air and surfaces, conduction of heat on the places where we stand and with exhaling the
warm air. Latent heat is given off to surroundings with diffusion of vapour trough the skin, evaporation of water on the
skin surface and humidifying the exhaled air.
air temperature (oC)
h e a t f l u x ( W )
Figure 1.3 Heat transfer mechanisms and heat flux emitted by human body to surroundings depends on air temperature
and humidity ‐ at low temperatures radiation and convection are the most important mechanisms, meanwhile at air
temperatures above 30°C latent heat transfer is dominant, emitted amounts of water vapour as function of air
temperature are presented as well.
1.1.1 Parameters of indoor thermal comfort
The importance of individual heat transfer mechanisms is varying with regard to the state of
indoor environment which is evaluated with several parameters: air temperature, mean radiant
temperature of surrounding surfaces, air velocity and air humidity. Because the amount of heat
that the body gives off depends on the difficulty of the work and on the clothes we are wearing,
the activity level, which is given in “met” (metabolism) and clothing which is given in “clo” (cloth)
are two very important additional parameters that affect thermal comfort. 1 Met corresponds
with 58 W released by 1 m2 of human surface area or approximately 100 W in total. During heavy
work metabolic rate can reach up to 10 Met and this corresponds to emitted heat flux of 270 W.
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Clo is proportional to thermal resistant of cloths. Characteristic values are 0 clo for a nude body, 1
clo for a business suit and 3 clo for winter clothes.
Indoor air temperature Ti is the most evident indicator of proper thermal comfort. In principle,
the temperature should be higher on lower activity level and lighter clothing. For building cooling
it is important that our body is capable to adapt to the seasonal conditions. Thus the appropriateindoor temperatures are between 20 and 22
oC in the winter and 26 to 27
oC in the summer time
when ambient temperature is above 30°C.
Mean radiant temperature Tr is mean temperature of the surfaces that surround the living space.
It has a strong influence on radiative heat transfer between human body and surroundings. The
difference between the indoor air temperature Ti and mean radiant temperature Tr should not be
greater than 2K. During the summer, the indoor surfaces or internal window blinds exposed to the
solar radiation can warm up to 50 and more °C, which can be disturbing. Bright coloured or
reflective external window blinds are a good solution for decreasing the mean radiant
temperature.
The air velocity in the room affects the convective heat losses and evaporation of water, which
we are excreting trough the skin and sweat glands. During the heating season our body feel as
unpleasant velocities above 0.15 m/s, meanwhile in the summer time we have no comfort
problems with higher velocities up to 0.6 or even 0.8 m/s. For example, we can increase the air
flow around our bodies with a ceiling fan and it results as feeling the environment around us
being cooler.
Air humidity affects the latent heat transfer from the bodies to the surrounding air. Therefore in
case of higher temperatures the humidity level has to be lower. Air humidity in the buildings is
varying because of air conditioning and different sources of water vapour in living spaces (human,
plants, cooking, etc.). The air humidity can be given as moisture content of air x, which is definedwith the ratio of water vapour mass (in g or kg) added to the mass of one kilogram of dry air
(typical values are between 5 to 20 g/kg) or as relative humidity which is defined as ration
between actual water vapour pressure and water vapour pressure in saturated air at the same
temperature. Values are quoted in percents in range between 0% in dry air and 100% in air
saturated with water vapour. At the air temperature Ti between 20 ‐ 26°C air humidity should
be 70 to 35%, or the moisture content x should not exceed 11.5 g/kg. In practice the air humidity
could be reduced by cooling the air beyond its dew point with cooling devices in the rooms or
with central air conditioning units. In both cases dehumidification increase the electricity
consumption, unless thermally driven cooling engines are used instead of compressor driven
cooling systems.
1.1.2 Integrated indicators of thermal comfort
Joint influence of the thermal comfort parameters could be evaluated with the predicted main
vote PMV indicator. PMV is an agreed relative assessment scale of thermal comfort in indoor
environment. The values of PMV are in the range between ‐3 (cold), ‐2 (moderately cold), ‐1
(pleasantly cold), 0 (neutral), +1 (pleasantly warm), +2 (warm) and +3 (hot environment). The
value PMV equal to 0 therefore means neutral environment, positive values mean warmer
environment, negative values mean colder environment. The PMV value is established by a
mathematical expression or based on measurements of thermal comfort parameters and
considering the activity and clothing of the occupancies. The predicted mean vote can be related
to percentage of dissatisfied people (PPD), which tells us the percent of dissatisfied people in
observed room.
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Figure 1.4 Instrument for determination of predicted mean vote of indoor environment (PMV); sensor for temperature,
velocity and humidity measurements, knobs for Met and Clo input.
Figure 1.5 Instrument Correlation between PMV and PPD values. According to the graph at PMV +2 80% of people will
be dissatisfied with their thermal environment. Source: [EN ISO 7730, 2005]
The demands concerning the indoor thermal environment are defined in many international and
national standards and regulations. Thus EN 15251 standard defines three levels of comfort
expectations: class A (high expectations), class B(normal expectations) and class C (moderate
expectations). For class A the PMV must be ± 0.2 (corresponds with PPD < 6%), for class B ± 0.5(PPD < 10%) and for class C ± 0.7 (PPD < 15%). EN ISO 7730 defines thermal comfort as acceptable
if 80% or more inhabitants feel comfortable in such indoor environment.
As cooling of buildings is closely related to indoor air temperatures and humidity some other
comfort indicators could be used as well. Humid operative temperature is the temperature of the
environment with 100% relative humidity in which a human body emits the same total amount of
heat as in real environment. The heat stress index is the ratio of the total evaporative heat losses
of human body required for thermal comfort and maximum evaporative heat losses possible in
the same environment multiplied by factor 100. The decimal value of heat stress index is called
skin wettedness.
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1.2 Cooling demand of buildings
1.2.1 Conventional or mechanical cooling
Most of the buildings today are cooled with mechanical cooling or air conditioning systems. In
both cases a cooling machine is needed. Usually, this is a heat pump which pumps the heat out of
the cooler building to the warmer surrounding of the building. In cases of smaller systems(compact cooling units) the air is directly cooled in the evaporator of the cooling unit placed in the
room. When dealing with larger buildings central air or water cooling systems are commonly
used. In case of air cooling systems the air in the air‐conditioning device is cooled with chilled
water before delivered into the building. In water cooling systems water with temperature
between 5° to 7°C is pumped through chilled water pipe distribution systems to the end heat
exchangers (e.g. fan‐coils) installed in each indoor space.
Figure 1.6 Fan‐coil units with coil heat exchanger and fans are end heat exchangers in central water cooling systems.
During operation the cooling machine consumes electricity. Because it is working as a heat pump,
the amount of heat transferred out of the building is significantly larger than the amount of used
electric energy. The ratio between the heat extract out of the building Qc and the electric energy
demand W is named coefficient of performance (COPel). Modern cooling units have COPel
between 3 and 5 depending on the cooling power and the type of compressor. In spite of highCOPel, these cooling devices still use electricity which is in many countries produced with high
emissions of greenhouse gasses. An increased consumption of electricity is characteristic for all
“modern” societies. In Europe the consumption of electricity has increased by a factor 12 within
the last 50 years. Today the yearly increase of electricity consumption is twice as high as the
increase of fossil fuel consumption. Building cooling systems also have a high factor of
simultaneity, which consequently leads to electricity network overload. In Slovenia for example
the peak electricity demand has changed from 19 PM in the winter time to 15 PM in the summer
time in last three years indicating increased electricity demand for cooling of the buildings.
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Figure 1.7 Air handling unit of central air conditioning system; air is cooled with chilled water provided by cooling
engine.
Figure 1.8 Cooling machines operates as a heat pump, therefore heat transferred from buildings to the environment islarger than consumption of electricity. The ratio is called coefficient of performance or COPel . Modern cooling units have
COPel between 3 and 5.
Building heat
COPelelectricity coefficient of
performance
Q kWhc hCOP el W kWh
e
=
Compressor
Environment
Qc W
Qod
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1.2.2 Cooling loads and energy demand for cooling of the buildings
Cooling loads and energy demand can be calculated using different approaches. In engineering
practice VDI 2078 and ASHRAE calculation procedures are often used. Regardless to the method
the first step in buildings cooling analyses is the determination of heat gains. Heat gains are
divided into sensible and latent heat gains. Sensible heat gains are originated by:
• solar radiation and heat transfer through windows• unsteady heat transfer through opaque building envelopment• internal heat gains (human, lighting, appliances,..)• heat transfer by air exchange between surrounding and building because of infiltration and
ventilation
Heat gains through windows and transparent walls can be characterized by several optical
parameters:
• transmittivity of solar radiation t• total energy transmittivity g• shading factor of shading devices Sf
Transmissivity of solar radiation is the ratio between transmitted and incoming solar radiation.
Since part of solar radiation is absorbed in glazing, radiation and convection heat flux from the
inner glass layer into the interior represent additional heat gains. The sum of heat gains can be
expressed by g‐value as the ratio between sum of solar radiation and heat flux gains and incoming
solar radiation on window surface. The g‐value is the most adequate window characteristic for
cooling load determination. Cooling loads through transparent building envelopment could be
significantly reduced by selection of effective shading devices.
Gi
Gτ =
skGi q qgG
+ +=
f
G'S
G=
Figure 1.9 Transmissivity of glazing is the ratio between transmitted (Gi) and incoming solar radiation (G) (left); total
energy transmissivity g of glazing is the ratio between sum of transmitted solar radiation and heat flux transferred from
inner glass surface by radiation and convection (Gi + qk+qs) and incoming solar radiation G. (middle); shading factor Sf
of shadings is the ratio between transmitted solar radiation G’ and incoming solar radiation G (right)
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Heat gains through opaque building envelopment depends on absorbed solar radiation (wall
orientation and wall surface colour), thermal conductivity of wall materials and heat accumulation
of the wall. Heat gains can be calculated by hour‐to‐hour analyses of steady heat transfer
replacing air temperature differences with reference temperature difference as it is proposed in
VDI 2078. Since contemporary building envelope elements have low a heat transfer coefficient,
heat gains through opaque elements are in most cases small.Internal heat gains are often major reason for overheating. The human body itself emits a heat
flux between 100 W and 250 W in condition of heavy activity. Large number of appliances
characterized for commercial buildings contribute to large internal heat gains as well. Good
daylighting design and use of high efficient compact and LED lamps can significantly reduce the
internal cooling loads.
Contemporary buildings are sufficiently tight to prevent significant infiltration of ambient air into
the building. Nevertheless they must be ventilated to ensure good indoor air quality. Mechanical
ventilation must be regulated according to demand to ensure lower cooling load with supply air.
Latent heat gains are in general generated in buildings because of different water vapour sources,
nevertheless in humid regions supply external air must be dehumidified before supplied to the
buildings. For example, a human body emits up to 50 g of water vapour per hour, plants up to 20g per day.
Cooling load indicates heat flux (removed rate of energy) needed for fulfilling requirements of
thermal comfort especially regarding to indoor air temperature and humidity. Time dependant
heat gains and cooling loads differ by amplitude and time shift because of heat accumulation in
building constructions. Cooling loads are calculated for a climate dependant hot summer design
day and the daily maximum value is taken as design cooling load of the building. More advanced
methods are based on hour‐by‐hour analyses using a computer tool, among others TRNSYS is very
well known. In such tools, a Test Reference Year as meteorological data source is used for specific
locations. The software Meteonorm (CD published by James & James, UK) includes TRY for more
than 5000 location world wide. Such tools are most useful for the calculation of energy demand
for cooling which taks into account hour‐
by‐
hour cooling load, COPel of cooling machine andoverall cooling system efficiency.
Detailed descriptions of planning tools are presented in Chapter 7.
Important note:
The Energy Performance of Buildings Directive (EPBD) requests that the energy demand for
cooling must be included into buildings energy performance indicators. As a consequence, in
some national regulations the rated power of cooling machines is limited. In Slovenia, for
example the permitted power of the cooling machine is 24 W per m3 of building volume.
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1.2.3. Study cases
As an example of computer simulation approaches, annual specific cooling loads and energy
(electricity) demand for four business buildings are presented below. All buildings are built at
locations with continental climate.
Cooling
load
(W/m3)
Cooling
load
(W/m2)
Useful
energy
demand
(heat)
(kWh/m2)
End energy
demand
(electricity)
(kWh/m2)
Office
building 1
7.7 21 8 3.4
Office
building 2
31 84 51 18.7
Office
building 3
14 38 29 10.6
Shopping
centre
21.6 76 58 19.2
Tabel 1.1 Specific cooling loads and energy demand of four business buildings
Remark: useful energy is related to quantity of heat extracted from indoor air, end energy
demand is related to electricity demand of mechanical cooling.
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1.3 Energy conservation principles
The energy demand for cooling of buildings can be reduced by implementation of five principles
presented on Figure 1.10: solar radiation controlling, reduction of heat gains thought opaque
building envelope, intensive night ventilation, reduction of internal gains and implementation offree cooling techniques.
Figure 1.10 Principles of energy conservation for buildings cooling. Source [McQuiston et al., 2005]
Shading devices must be external, high reflective for solar radiation and mounted in such a way
that enables convective cooling as well as daylighting of the interior. Figure 1.11 shows the
temperature profile in an office without shadings and mechanical cooling and in the neighbouring
office with external shadings; shading devices are installed in such a way that convective cooling is
enabled on both sides of shadings and they are movable to improve shading factor Sf all day long
and enable optimal daylighting in offices.
18
23
28
33
38
43
48
3984 4152 4320 4488 4656 4824 4992 5160
Dan v letu
T e m p e r a t u r a p r o s t o r a v t r e t j e m n
a d s t r o p j u ( o C )
Brez senčil in haljenja
Zunanja lamelna senčila, nehlajen prostor
Without shadings
With shadings
Hour starting 1st of January
R o o m
t e m p e r a t u r e ( ° C )
Figure 1.11 Only external, high reflective and movable shading devices controls successfully solar radiation heat gains;temperature in an office without shadings and cooling (gray line), and temperatures in office equipped with external
shadings as presented on photo (orange line).
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Shading devices could be multi purpose. For example PV modules can be used as external shading
device. Following example shows such a case. PV modules are mounted on the part of glass roof
of atrium in office building. The result is the reduction of the peak cooling load from 150 kW to 75
kW, meanwhile the heating load remains practically unchanged. In this particular case PV
shadings have little influence on daylighting as well.
-100
-50
0
50
100
150
200
Jan. Feb. Mar. Apr. Maj Jun. Jul. Aug. Sept. Okt. Nov. Dec.
[ k W ]
heating load
cooling load
-100
-50
0
50
100
150
200
Jan. Feb. Mar. Apr. Maj Jun. Jul. Aug. Sept. Okt. Nov. Dec.
[ k W
]
heating load
cooling load
Figure 1.12 PV modules as external shading devices on the glass roof of an atrium in office buildings reduce peak cooling
load by 50% meanwhile heating demand and daylighting remain practically unchanged (left heating and cooling loadswithout PV modules, right after PV modules were installed)
Heat gains through the opaque envelope could be reduced with light surface colours and quality
thermal insulation in combination with a high building construction thermal mass. As a
consequence, a significant decrease of temperature amplitude swing at the inner side of the
construction and a time lag of several hours can be attained. Modern architecture often requires
dark surface colours of walls and roofs. Selective paints can be used in this case to reduce both,
surface temperature and resulting cooling loads. Such colours have equal reflectivity of light as
ordinary colours, but enlarged near IR reflectivity. This causes a reduction of dark surface
temperature during solar noon by 20°C. Even more effective are green roofs and walls.
Evapotranspiration by grass and plants reduce cooling loads for 5 to 10 times regarding to darkroofs.
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Figure 1.13 Additional selective white paint layer (left) pained below green coating (right) reduces peak wall surface
temperature up to 15 K
Night ventilation can significantly reduce cooling loads but only in case if intensive night
ventilation with at least 4 to 5 exchanges of building volume per hour is provided. On the other
hand ventilation systems can be supplemented by free cooling techniques like evaporative
cooling. Evaporative cooling is most effective in hot and dry areas. It can significantly contribute
to cooling power reduction and therefore to the peak electricity demand for mechanical cooling.
COPel of such systems are 50 or more.
8
10
12
14
16
18
20
22
24
26
28
30
32
34
0 300 600 900 1200 1500 1800 2100
number of hours per year (h)
s u p p
l y a i r t e m e p r a t u r e ( ° C )
T ambient
T afterevaporativecooling
Figure 1.14 Evaporative cooling is most effective at high ambient temperature at solar noon; duration of ambient air
and supply air temperatures after evaporative cooling (left); evaporative cooling can significantly contribute to cooling
power reduction and peak electricity demand additional for mechanical cooling. Source: [Vidrih, Medved, 2006]
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15
17
19
21
23
25
27
29
31
0 12 24 36 48 60 72 84 96Time (h)
T e m p e r a t u r e ( ° C )
Ta (LHTES inlet temperature)
To (measured)
To (numerical model)
Figure 1.15 Latent heat storage integrated into ventilation system are cooled down during the night and provide lower
supply air temperatures during the next summer day; such system can be combined with other free cooling systems to
provide all day free cooling operation. Source: [Arkar, Medved, 2007]
Ground heat exchangers can be coupled to mechanical ventilation systems for pre‐cooling of
ventilation air during the daytime in summer days. They are used in smaller buildings, and they
have to be planned very carefully, to ensure a high COPel. Mechanical ventilation system can be
upgraded with a cold storage as well. Especially effective are the latent storages which are cooled
during the night, and at day time they are used to cool the supply air. These systems are moreexpensive, and are still in a phase of development.
Despite the fact that free cooling techniques are effective and can reduce energy demand for
cooling greatly they alone cannot guarantee that indoor comfort will be fulfilled all the time. In
such cases other energy efficient cooling technology must be implemented – the solar cooling.
night y
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1.4 Fundamentals of solar cooling
1.4.1 Principles of desiccant‐evaporative solar cooling
Air is a mixture of different gasses and water vapour. The change of air state can be a
consequence of sensible heat transfer during the heating or cooling and the transfer of latentheat because of humidification or dehumidification. For that reason the state of the air should be
expressed by the internal energy called enthalpy (h) instead of the air temperature. We can
demonstrate the changes of air states in an T‐x diagram. During the humidification of air,
dispersed drops of water in the air, transforms into molecules of water vapour with assistance of
internal energy of air. Consequently the air cools down. This kind of natural cooling is very
efficient, although it has an side effect of increasing the air’s moisture content and it’s relative
humidity, which can exceed the appropriate levels, defined by thermal comfort.
2
1
0
10
20
30
40
50
60
70
80
90
0 2 4 6 8 10 12 14 16 18 20 22 24
x (g/kg)
T ( ° C )
φ=0,1
φ=0,2
φ=0,3
φ=1
Figure 1.16 The process of evaporative cooling goes on at constant enthalpy. Air temperature drops, but at the same
time moisture content of air (x) and relative humidity ( φ ) increase.
9
10
0
10
20
30
40
50
60
70
80
90
0 2 4 6 8 10 12 14 16 18 20 22 24
x (g/kg)
T ( ° C )
φ=0,1
φ=0,2
φ=0,3
φ=1
Figure 1.17 The process of sorption drying (10 ‐> 9) also goes on at constant enthalpy. Air temperature increases as
moisture content (x) and relative humidity ( φ ) decrease.
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In conventional cooling systems air is dehumidified by cooling below the dew point, resulting in
condensation of water vapour. The second option for drying the air is using special materials
which have the ability of sorption removal of water vapour molecules out of the air. These
materials are for example silica gel or lithium chloride. The first one is a solid, the second one is a
liquid; however, lithium chloride is also applied in impregnated structures, thus appearing as solid
form sorption unit. A side effect of this process is an increase in the air temperature andhumidification of the material, which absorbs the water vapour from the air. When heating the
sorption material above the temperature of 60 to 70°C the water vapour is released from it and
the process can be repeated. In solar driven desiccant‐evaporative solar cooling systems, this
regeneration heat is provided by a solar thermal collector system.
In market available applications, the processes are combined with a heat recovery unit to the
desiccant‐evaporative solar cooling cycle, described in detail in Chapter 2.
1.4.2 Principle of sorption solar cooling
Conventional cooling system use a compressor to compress refrigerant vapour. Sorption cooling
processes run in a similar way. However instead of mechanical compressor which uses electricity,only fluid pumps are applied to pump binary mixture of two substances – the refrigerant and a
substance that absorbs the refrigerant and is called absorbent, in case of an absorption process is
applied. In practice a mixture of water (refrigerant) and lithium bromide (absorbent) on the one
hand, or ammonia (refrigerant) and water (absorbent) on the other hand is used. Circulation
pump electricity consumption is negligible compared to a compressor in a conventional cooling
system. Additional energy needed for the operation of sorption cooling systems must be provided
in form of heat, which can be produced by high efficient solar thermal system.
Alternatively, an adsorption process may be applied, based on the physical process of adsorption
of the refrigerant at a solid state sorption material, such as silica gel or types of zeolithes.
Since the result ab‐ or adsorption processes is coolant water with temperature of 7 to 10°C all
kinds of cooling system can be used. Details of sorption solar cooling can be found in Chapter 2.
1.5 Impact of climate changes on thermal indoor comfort
and energy demand for cooling
Predicted climate changes due to anthropogenic emissions will cause an increase in mean
atmosphere temperatures and atmospheric IR radiation. For that reason the climate changes will
have a strong influence on thermal comfort in the buildings in the summer period and therefore
on the energy demand for cooling as well. Based on simulations of a low‐energy dwelling and an
office building without cooling shown on Figure 1.18 and considering a corrected test reference
years (TRY) one can find out that the number of overheated hours will strongly increase. In case of
mechanical cooling and if the most severe scenario (D, Ta + 3°C, +6 W/m2) is taken into account
the energy demand for cooling will increase by 10 times (depending on the location and
application). It can be expected, that the cooling demand will increase for 3 to 5 kWh/m2 of
buildings living space. The conditions will be similar as in the year 2003. As temperatures will be
also higher in the night time, the free cooling systems will be less efficient.
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Figure 1.18 Low energy and commercial building used in climate change impact simulations.
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
TRY A B C D Year 2003
CTRY
N u m b e r
o f h o u r s [ h / a ]
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
2600
TRY A B C D Year 2003
CTRY
N u m b e r o
f h o u r s [ h / a ]
Figure 1.19 Increased overheating hours (Ti > 26°C) in un‐cooled one family (left) and office (right) building; Scenario A
(+1°C), Scenario B (+1°C, +3 W/m2
), Scenario C (+3°C), Scenario D (+3°C, +6 W/m2
) Source: [Vidrih, Medved, 2006]
0,0
0,51,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
5,5
6,0
TRY A B C D Year 2003
CTRY
C o o l i n g d e m a n d [ k W h / m 2 a ]
0
2
4
6
8
10
12
14
16
18
20
22
24
TRY A B C D Year 2003
CTRY
C o o l i n g d e m a n d [ k W h / m 2 a ]
Figure 1.20 Increased specific cooling demand in cooled one family (left) and office (right) building in kWh per m2 of floor
area per year
Taking all these facts into account, we can expect that more and more buildings will be cooled
in the future, especially every new built building. This gives solar cooling a great possibility to
enforce itself in the market.
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References
[EN ISO 7730, 2005]
Ergonomics of the thermal environment – Analytical determination and interpretation of thermal comfort using
calculation of the PMV and PPD indices and local thermal criteria.
[EN 15251, 2007]
Indoor environmental input parameters for design and assessment of energy performance of buildings addressing
indoor air quality, thermal environment, lighting and acoustics.
[McQuiston et al., 2005]
F. McQuiston, J. Parker, J. Spitler: “Heating, Ventilating, and Air Conditioning, Analysis and Design”; Jonn Wiley&Sons,
Inc, 2005
[Vidrih, Medved, 2006]
B. Vidrih, S. Medved: “The Connection Between the Climate Change Model and a Buildings Thermal Response Model: A
Case of Slovenia”, Journal of Mechanical Engineering, vol. 52, no. 9/06, Ljubljana, 2006
[Arkar, Medved, 2007]
C. Arkar, S. Medved; ”Free cooling of a building using PCM heat storage integrated into the ventilation system”, SolarEnergy, vol. 81, no 9, Elsevier Press, 2007
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2 Technologies applicable for solar thermally driven cooling
The focus in SOLAIR is on solar cooling and air‐conditioning systems in the small and medium size
capacity range. The classification into ‘small’ and ‘medium’ aligns with available chiller products;small applications are in this sense systems with a nominal chilling capacity below 20 kW, and
medium size systems may range up to approx. 100 kW.
Systems in the small capacity range are usually consist of thermally driven chilled water systems,
whereas medium sized systems may be open cycle desiccant evaporative (DEC) cooling systems as
well. While in the first type of system technology the distribution medium is chilled water in a
closed loop to remove the loads from the building, in the latter one supply air is directly handled
in humidity and temperature respectively in an open process. Figure 2.1 visualises the two general
types of applications. Of course, applications using both types of technology at the same time are
possible. In chilled water systems, the central cold water distribution grid may serve decentralised
cooling units such as fan coils (mostly with dehumidification), chilled ceilings, walls or floors; but
the chilled water may be used for supply air cooling in a central air handling unit as well. The re‐quired chilled water temperature depends on this type of usage and is important for the system
design and configuration, but the end‐use devices are not in the focus of SOLAIR and thus are not
presented more in detail.
Cooled /Conditionedarea
Chilled ceiling
Supply air
Fan coil
~18°C
16°C - 18°C(< 12°C)
6°C - 9°C
Chilled watertemperature
Heat> 60°C
ThermallydrivenChiller
Supply air
Heat> 50°C
Return air
Desiccant evaporativecooling (DEC)
Conditionedarea
Figure 2.1 General types of thermally driven cooling and air ‐conditioning technologies. In the figure above, chilled water
is produced in a closed loop for different decentral applications or for supply air cooling. In the figure below, supply air is
directly cooled and dehumidified in an open cycle process. Source: Fraunhofer ISE. The technologies are outlined more indetail below. Heat is required in both technologies, to allow a coninuous system operation. In the applications surveyed
in SOLAIR, the heat is at least to a significant part produced by a solar thermal collector system.
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Figure 2.2 illustrates that any thermally driven cooling process operates at three different tem‐
perature levels: with driving heat Qheat supplied to the process at a temperature level of T H , heat
is removed from the cold side thereby producing the useful ‘cold’ Qcold at temperature T C . Both
amounts of heat are to be rejected (Qreject ) at a medium temperature level T M. The driving heat
Qheat may be provided by an appropriate designed solar thermal collector system, either alone or
in combination with auxiliary heat sources.While in open cycle processes the heat rejection is with the air flow in the system integrated into
the process, closed chilled water processes require for an external heat rejection system, e.g., a
cooling tower. The type of the heat rejection system is currently turning more into the field of
vision, as this component usually is responsible for a considerable fraction of the remaining
energy consumption of solar cooling systems.
A basic number to quantify the thermal process quality in thermally driven chilled water systems
is the coefficient of performance COP, defined as
heat
cold
Q
QCOP = ,
thus indicating the amount of required heat per unit ‘produced cold’ (more accurately: per unit
removed heat). The COP and the chilling capacity depends strongly on the temperature levels of
T H, T C and T M. In open cycle desiccant cooling systems, the performance is more difficult to assess,
since it depends more strongly on the system operation. It is useful, to define here the
performance for the desiccant operation mode only, since in this operation mode heat is required
(section 2.2). The performance is then calculated from the enthalpy difference between ambient
and supply air, related to the required heat input. Experiences from DEC plants have shown that
performance values comparatively to single‐effect chillers may be achieved.
Focussing on chilled water systems, a maximum process performance COP ideal for each
temperature level can be derived from thermodynamic laws:
C M
M H
H
C ideal
T T
T T
T
T COP
−−
⋅= .
This dependency is discussed more in detail in e.g. [Henning, 2006]. As shown in figure 2.3, the
ideal performance of a reversible process is far above the performance, obtained in market
available thermally driven chillers. The COP in realised products ranges from 0.5 to 0.8 in single‐
effect chillers (absorption or adsorption), and may range to 1.4 in double‐effect chillers.
Qcold
Qreject
Qheat
TC
TM
TH
Figure 2.2 Basic scheme of a thermally driven cooling process.
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0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
45 60 75 90 105 120 135 150
Hot water inlet [°C[
C O P
ideal
double-effect
absorption
single-effect
absorption
adsorption
chilled water temperature: 9°C
cooling water temperature: 28°C
Figure 2.3 Exemplary curves of the coefficient of performance COP for different sorption chiller technologies and the
limit curve for an ideal process. The curves are shown as function of the driving temperature and for a constant chilledand cooling water temperature level. Source: [Henning, Wiemken, 2006]
The difference between real and ideal performance of the thermally driven chillers can be
expressed with a process quality number PQ:
PQ = COP real / COP ideal .
Typical vaules of PQ , extracted from market available products, are 0.3. The process quality
number allows to assess the advantages of an improved process quality with respect to the
required driving temperature. This is shown in figure 2.4. The figure presents the driving
temperature as a function of the ‘temperature lift’ ∆T , which is defined as the difference between
heat rejection temperature T M and chilled water temperature T C : ∆T = (T M ‐ T C ). As an example,
the temperature lift is low in case of high chilled water temperature and wet heat rejection (low
cooling water temperatures) and high in case of low required chilled water temperatures and dry
cooling. Driving temperatures for two different COP values are included. For each COP‐curve, the
driving temperature depends furthermore on the process quality; therefore, two different quality
numbers are assumed. The operation areas of different collector technologies are indicated as
well. As an example, a single‐effect chiller with COP of 0.7, working at ∆T = 35 K, may be driven
still with vacuum tube collectors, if the process requires driving temperatures of approx. 100 °C
(process quality number of 0.4). In case of a lower process quality, the required driving
temperature is higher and tracked concentrating collectors are necessary.
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0
50
100
150
200
250
300
350
400
10 15 20 25 30 35 40 45 50 55
useful temperature lift T = TM – TC [K]
r e q u i r e d d r i v i n g t e m
p . T H [ ° C ]
1,1 / 0,4
1,1 / 0,3
0,7 / 0,4
0,7 / 0,3
Flat-plate collector
Vacuum-tube
collector
1-axis tracked
concentrating collector
COP / PQ
Chilledceilings
Fan-coils;wet cooling
Fan-coils;dry cooling
High temperature lift:ice storage, dry cooling
Application examples:
0
50
100
150
200
250
300
350
400
10 15 20 25 30 35 40 45 50 55
useful temperature lift T = TM – TC [K]
r e q u i r e d d r i v i n g t e m
p . T H [ ° C ]
1,1 / 0,4
1,1 / 0,3
0,7 / 0,4
0,7 / 0,3
Flat-plate collector
Vacuum-tube
collector
1-axis tracked
concentrating collector
COP / PQ
Chilledceilings
Fan-coils;wet cooling
Fan-coils;dry cooling
High temperature lift:ice storage, dry cooling
Application examples:
Figure 2.4 Heat source temperature required for different COP/ PQ combinations, plotted as a function of the
temperature lift. Typical operation ranges of solar collector technologies are included as well as different system
application examples (grey marked areas). Source: [Hennng, 2006].
2.1 Chilled water systems
Absorption chillers
The dominating technology of thermally driven chillers is based on absorption. The basic physical
process consists of at least two chemical components, one of them serving as refrigerant and the
other as the sorbent. The main components of an absorption chiller are shown in figure 2.5. The
process is well documented, e.g., in [ASHRAE, 1988]; thus, details will be not presented here.
The majority of absorption chillers use water as refrigerant and liquid lithium‐bromide as sorbent.
Typical chilling capacities are in the range of several hundred kW. Mainly, they are supplied with
waste heat, district heat or heat from co‐generation. The required heat source temperature is
usually above 85°C and typical COP values are between 0.6 and 0.8. Until a few years ago, the
smallest machine available was a Japanese product with a chilling capacity of 35 kW.Recently, the situation has improved due to a number of chiller products in the small and medium
capacity range, which have entered the market. In general, they are designed to be operated with
low driving temperatures and thus applicable for stationary solar thermal collectors. The lowest
chiller capacity available is now 4.5 kW. Some examples of small and medium size absorption
chillers are given in figure 2.6. In addition to the traditional working fluids H2O/LiBr, also H2O/LiCl
and NH3/H2O are applied. The application of the latter working fluid with Ammonia as refrigerant
ist relatively new for building cooling, as this technology was dominantly used for industrial
refrigeration purposes below 0°C in large capacities. An advantage of this chiller type is especially
given in applications, where a high temperature lift (T M – T C ) is necessary. This is for example the
case in areas with water shortage, when dry cooling at high ambient temperatures has to be
applied.
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chilled water cooling water
cooling water hot water
(driving heat)
GENERATOR
ABSORBER
CONDENSER
EVAPORATOR
chilled water cooling water
cooling water hot water
(driving heat)
GENERATOR
ABSORBER
CONDENSER
EVAPORATOR
Figure 2.5 Scheme of a thermally driven single‐effect absorption chiller. Compared to a conventional electrically driven
compression chiller, the mechanical compression unit is replaced by a ‘thermal compression’ unit with absorber andgenerator. The cooling effect is based on the evaporation of the refrigerant (e.g., water) in the evaporator at low
pressure. Due to the properties of the phase change, high amounts of energy can be transferred. The vaporised
refrigerant is absorbed in the absorber, thereby diluting the refrigerant/sorbent solution. Cooling is necessary, to run the
absorption process efficient. The solution is continuousely pumped into the generator, where the regeneration of the
solution is achieved by applying driving heat (e.g., hot water). The refrigerant leaving the generator by this process
condenses through the application of cooling water in the condenser and circulates by means of an expansion valve
again into the evaporator.
Figure 2.6a Examples of small absorption chillers using water as refrigerant and Lithium‐Bromide as sorption fluid. Left:
air ‐cooled chiller with a capacity of 4.5 kW of the Spanish manufacturer Rotartica. Middle: 10 kW Chiller with high part ‐
load efficiency and overall high COP of the German manufacturer Sonnenklima, shown without housing. Right: Chiller
with 15 kW capacity, manufactured by the German company EAW; this machine is also available in capacities of 30 kW,
54 kW, 80 kW and above. Sources: Rotartica, Sonnenklima, EAW.
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Figure 2.6b Further examples of absorption chillers. Left: Absorption chiller with the working fluid H2O/LiBr and a
capacity of 35 kW from Yazaki, Japan. This chiller is often found in solar cooling systems, since it was for several years
the smallest in Europe available absorption chiller, applicable with solar heat. Currently, a smaller version with 17.5 kWchiller capacity from this manufacturer has entered the European market. Source: Gasklima. Right: This chiller uses
water as refrigerant and Lithium‐Chloride as sorption material. The crystallisation phase of the sorption material is also
used, effecting in an internal energy storage. The capacity is approx. 10 kW; the machine is developed by ClimateWell,
Sweden, and can operate as heat pump as well. Source: ClimateWell.
Figure 2.6c Examples of absorption chillers with the working fluid ammonia‐water. In principle, these types of chillers
are foreseen to provide chilled water at temperatures < 0°C for commercial and industrial cooling, but may be applied
for higher chilled water temperature levels under appropriate operating conditions as well. Left: Absorption chiller with
12 kW rated chilling capacity, developed by Pink, Austria; shown without housing. Right: Absorption chiller from Ago,Germany. This chiller is available with 50 kW capacity and with higher capacities. Sources: Pink/SolarNext.
Figure 2.7 displays current available hot water driven aborption chillers, sorted by chilling
capactiy. The presentation makes no claim to be exhaustive.
Double‐effect machines with two generators require for higher driving temperatures > 140°C, but
show higher COP values of > 1.0. The smallest available chiller of this type shows a capacity of
approx. 170 kW. With respect to the high driving temperatures, this technology demands in
combination with solar thermal heat for concentrating collector systems. This is an option for
climates with high fractions of direct irradiation.
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0 20 50 100 150 200 250 300 350 400 450
Chilling capacity range [kW]
York, Carrier, Trane ..
Broad
EAW
ClimateWell
Rotartica
Sonnenklima
Pink*
Ago*
Yazakiwater/LiBr
ammonia/water*
water/LiCl
water/LiBr
ammonia/water*
water/LiCl
Robur*
Thermax
* typical for applicationswith Tcold ≤ 0°C
Figure 2.7 Typical capacity range of hot water driven absorption chillers. The listed products are market available, either
by small series production or fabrication on demand. No claim to be complete.
Adsorption chillers
Beside processes using a liquid sorbent, also machines using solid sorption materials are available.
This material adsorbs the refrigerant, while it releases the refrigerant under heat input. A quasi‐
continuous operation requires for at least two compartments with sorption material. Figure 2.8
shows the components of an adsorption chilller. Market available systems use water as
refrigerant and silica gel as sorbent, but R&D on systems using zeolithes as sorption material is
ongoing.
To date, only few manufacturers from Japan, China and from Germany produce adsorptionchillers; a German company is with a small unit of 5.5 kW capacity on the market since 2007 and
has increased the rated capacity in improved versions to 7.5 kW and 15 kW (models of 2008).
Typical COP values of adsorption chillers are 0.5‐0.6. Advantageouos are the low driving
temperatures, beginning from 60°C, the absence of a solution pump and a comparatively
noiseless operation. Figure 2.9 shows examples of adsorption chillers, whereas figure 2.10
displays current available adorption chillers, sorted by chilling capactiy. The presentation makes
no claim to be exhaustive.
An overview on closed cycle water chillers is also presented in [Mugnier et al., 2008].
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cooling water
cooling water
chilled water
hot water
(driving heat)
CONDENSER
EVAPORATOR
12
Figure 2.8 Scheme of an adsorption chiller. They consist basically of two sorbent compartments 1 and 2, and the
evaporator and condenser. While the sorbent in the first compartment is desorbing (removal of adsorbed water) using
hot water from the external heat source, e.g. the solar collector, the sorbent in the second compartment adsorbs the
refrigerant vapour entering from the evaporator; this compartment has to be cooled in order to increase the process
efficiency. The refrigerant, condensed in the cooled condenser and transferred into the evaporator, is vaporised under
low pressure in the evaporator. Here, the useful cooling is produced. Periodically, the sorbent compartment are switched
over in their functions from adsorption to desorption. This is usually done through a switch control of external located
valves.
Figure 2.9Examples of adsorption chillers. Left: Chiller with 70 kW capactiy of the Japanese manufacturer Nishiyodo,
installed for laboratory cooling at the University Hospital in Freiburg, Germany. Adsorpition chillers of similar medium
capacity are available from the Japanese manufacturer Mayekawa as well. Middle: Small ‐size adsorption chilllers with
7.5 kW and 15 kW chilling capacity from SorTech company, Germany. Source: SorTech. Right: Small ‐size adsorption
chiller in the capacity range 7 to 10 kW of the manufacturer Invensor, Germany. Source: Invensor.
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0 20 50 100 150 200 250 300 350 400 450
Invensor (DE)
SorTech (DE)
Mayekawa (JP)
Nishyodo (JP)
water/silicagel
water/zeolite
{ SJTU (CN) }
Chilling capacity range [kW]
{ } no detailed informationon market status
Figure 2.10 Typical capacity range of adsorption chiller brands. The listed products are market available, either by smallseries production or fabrication on demand. No claim to be complete.
Heat rejection
Figure 2.2 in section 2 indicates that the amount of heat extracted from the building (‘useful cold’)
plus the driving heat of the transformation process has both to be charged to the environment at
(medium) ambient temperature level. This operation is done by means of a heat rejection system.
Figure 2.11 illustrates as an example the difference in the demand of heat rejection between a
conventional compression chiller system and an ab‐ or adsorption chiller system. It is evident that
heat rejection in thermally driven systems plays a central role in the system development.
Compression
Qc = 3 x W
QM = Qc + W
WCompression
1 kWc
0,33 kWe
1,33 kW
Sorption
Qc = 0,7 x QH
QM = Qc + QH
QH
1 kWc
1,4 kWt
2,4 kW
Sorption
Figure 2.11 Example on the demand for heat rejection in a conventional electrically driven compression chiller system
(left) and in a (single‐effect) thermally driven chiller system (right). In the comparison, the chilling capacity is 1 kW in
both systems. Typical efficiency numbers have been used. Source: Tecsol.
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In principle, different possibilities and heat rejection technologies may be applicable:
1. wet cooling, either of open type or of closed type, using the evaporative cooling effect
2. dry cooling without evaporation
3. hybrid cooling, allowing for both options: wet and dry cooling
4. geothermal heat rejection by use of ground tubes
5. heat rejection by use of ground water, sea water, river or spring water6. application of low temperature level cooling water by thus rejecting the medium temperature
level heat
If applicable in any case, the options 5. and 6. should be preferred, as these applications are
connected with the lowest electricity consumption of the different heat rejection possibilities.
Unfortunately, application fields of low temperature level heat (~ 30°C) is rarely identified, and
sea water cooling is for financial reasons limited to applications direct at costal sites and for large
applications. Additionally, the permittance to increase the sea water temperature level by this
means is difficult to obtain.
Heat rejection using ground tubes is a comparatively new approach and may be of interest,
especially when the ground tubes are used for heat pump operation as well during winter, thuscontributing to an annual balanced charging and dischcharging of the ground. However, the
investment cost for ground tubes are currently still high. An example of such an application in
combination with a small adsorption chiller (with heat pump operation) is shown in the SOLAIR
Best practice examples [SOLAIR: Best Practice Catalogue, 2008].
The most applied heat rejection technology in combination with thermally driven chillers today is
still wet cooling by means of open cooling towers. Figure 2.12 illustrates the principle of such a
heat rejection system: the cooling water is sprayed on top of the cooling tower towards the filling
material, which increases the effective exchange area between air and cooling water. The main
cooling effect is obtained through evaporation of a small percentage of the cooling water
(typically < 5%); this loss has to be compensated by fresh water supply. The cooled water then
returns to the cooling circuit of the chiller. A fan removes the saturated air in order to keep theprocess running. The process is very efficient in appropriate climates and in principle, the
limitation temperature of the returned cooling water is not far from the wet‐bulb temperature of
the air (3°C to 5°C above the wet‐bulb temperature). A commercial product is shown in
figure 5.13.
Fan
Drip-catcher
Cooling waterdistribution
Filling material
Air inlet
Sump
Figure 2.12 Typical scheme of an open wet cooling tower. Source: GWA.
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Figure 2.13 Example of a large wet cooling tower installation.
In dry climates, the fan speed of a wet cooling tower can be often decreased in order not to fall
below the minimum cooling water temperature of the chiller (e.g., 25°C often defined for
absorption chillers), whereas in a very humid climate also the wet‐bulb temperature often is high.
Figure 2.14 displays as an example for more extreme climates monthly averages of the wet‐bulb
temperature at Dubai. During summer, the monthly values are approx. 25°C, indicating that
during daytime the obtained return cooling water temperature often may exceed 30°C. Also the
ambient temperature levels are very high and during day, up to 40°C ambient temperature is
detected, which indicates the limit of dry cooling (limitation temperature: a few °C above ambient
temperature).
In the application with adsorption chiller technology, closed wet cooling towers have to be
applied instead of open wet cooling towers. The reason is the connection of the heat rejectioncircuit with the driving circuit for some seconds during the heat recovery phase, which is activated
between the functional interchange of adsorption and desorption partitions of the chiller. The
hydraulic pressure conditions do not usually allow for an open cooling water loop. In the closed
cooling towers, the tower is equipped with a cooling water heat exchanger, which is sprayed by
an external water loop for indirect evaporative cooling. A disadvantage of this technique are
lower efficiencies and higher costs.
In some countries, regulations exist on the application of wet cooling towers with respect to
hygienic aspects. In order to avoid unfavourable growth of bacteria, a water treatment of the
cooling water may be necessary. For this reason and for reasons of improving the optical
acceptance of heat rejection systems especially in small scale applications, dry cooling is still of
interest, although the cooling temperature level as well as the electricity consumption is ingeneral higher (higher power consumption of the fans due to pure sensible cooling). Dry heat
rejection in solar thermally driven cooling systems has been applied in a number of
demonstration systems for testing this opportunity. Furthermore, a supplier of small capacity
adsorption chillers offers a dry cooler with spray function in case of high ambient temperatures,
adapted to the chiller.
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0
10
20
30
40
50
60
70
Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec
A m b i e n t a i r a n d w e t b u l b T e m p e r a t u r e [ ° C ] ,
r e l . h u m i d i t y [ % ]
0
50
100
150
200
250
G l o b a l h o r i z o n t a l r a d i a t i o n s u m
Ta [°C] Twb [°C] RH [%] G_Gh [kWh/m2]
Figure 2.14 Monthly climate data for Dubai site. During summer, very high wet ‐bulb temperautes may be expected
during daytime, thus limiting the efficiency of wet cooling towers. At the same time, also the ambient temperature as
indicator for dry cooling limits is very high as well.
2.2 Open cycle processes
While thermally driven chillers produce chilled water, which can be supplied to any type of air‐
conditioning equipment, open cooling cycles produce directly conditioned air. Any type of
thermally driven open cooling cycle is based on a combination of evaporative cooling with air
dehumidification by a desiccant, i.e., a hygroscopic material. Again, either liquid or solid materials
can be employed for this purpose. The standard cycle which is mostly applied today uses rotating
desiccant wheels, equipped either with silica gel or lithium‐chloride as sorption material. All
required components, such as desiccant wheels, heat recovery units, humidifiers, fans and water‐
air heat exchangers are standard components and have been used in air‐conditioning and air‐
drying applications for buildings or factories since many years. However, the appropriate
combination of the components to form a desiccant evaporative cooling system (DEC), which is
the most common solar driven open cycle system, requires some special experience and
attention.
The standard cycle using a desiccant wheel is shown in figure 2.15. The application of this cycle is
limited to temperate climates, since the possible dehumidification is not high enough to enable
evaporative cooling of the supply air at conditions with far higher values of the humidity of
ambient air. For climates like those in the Mediterranean countries therefore other configurations
of desiccant processes have to be used.
Systems employing liquid sorption materials which have several advantages like higher air
dehumidifiation at the same driving temperature and the possibility of high energy storage by
means of concentrated hygrocopic solutions are note yet market available but they are close to
market introduction; several demonstration projects are carried out in order to test the applica‐
bility of this technology for solar assisted air conditioning. A possible general scheme of a liquid
desiccant cooling system is shown in figure 2.16.
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humidifier cooling
loads
supply air
backupheater
return air
dehumidifier
wheel
heat recovery
wheel
1 2 3 4 56
789101112
Figure 2.15 Scheme of a solar thermally driven solid Desiccant Evaporative Cooling system (DEC), using rotating sorption
and heat recovery wheels (source: Fraunhofer ISE). Below: sketch of the DEC unit (source: Munters). The successive
processes in the air stream are as follows:
12 sorptive dehumidification of supply air; the process is almost adiabatic and the air is
heated by the adsorption heat released in the matrix of the sorption wheel
23 pre‐cooling of the supply air in counter‐flow to the return air from the building
34 evaporative cooling of the supply air to the desired supply air humidity by means of a
humidifier
45 the heating coil is used only in the heating season for pre‐heating of air
56 small temperature increase, caused by the fan
67 supply air temperature and humidity are increased by means of internal loads
78 return air from the building is cooled using evaporative cooling close to the saturation
line
89 the return air is pre‐heated in counter‐flow to the supply air by means of a high
efficient air‐to‐air heat exchanger, e.g. a heat recovery wheel
910 regeneration heat is provided for instance by means of a solar thermal collector
system
1011 the water bound in the pores of the desiccant material of the dehumidifer wheel is
desorbed by means of the hot air
1112 exhaust air is removed to the environment by means of the return air fan.
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diluted solution
concentratedsolution
regeneration air
QH
driving heat
⇒ QM rejected heat
Regenerator
Absorber
supply air
solution storage
LiCl/water
Figure 2.16 General scheme of a liquid desiccant cooling system (top). The supply air is dehumidified in a specialconfigured spray zone of the absorber, where a concentrated salt solution is diluted by the humidity of the supply air.
The process efficiency is increased through heat rejection of the sorption heat, eg., by means of indirect evaporative
cooling of the return air and heat recovery. A subsequent evaporative cooling of the supply air may be applied, if
necessary (heat recovery and evaporative cooling is not shown in the figure). In a regenerator, heat e.g. from a solar
collector is applied, to concentrate the solution again. The concentrated and diluted solution may be stored in high
energy storages, thus allowing a decoupling in time between cooling and regeneration to a certain extent. Bottom: a
liquid desiccant cooling demonstration system is installed at the Solar Info Center in Freibug, Germany, for air ‐
conditioning of 310 m² office area. The air volume flow rate is 1500 m³/h. The system was developed and installed by
the German company Menerga. The ventilation system is at the left side of the figure, the solution storages are located
right hand side in the foreground. The storage in the background is part of the solar thermal driving heat source,
consisting of 17 m² flat ‐ plate collectors. Sources: Fraunhofer ISE.
In general, desiccant evaporative cooling is an interesting option if centralized ventilation systems
are used. At sites with high latent and sensible cooling loads, the air ‐conditioning process can be
splitted into dehumidification by means of a thermally driven open cycle desiccant process, and
an additional chilled water system to maintain the sensible loads by means of e.g. chilled ceilings
with high chilled water temperatures, in order to increase the efficiency of the chilled water
production.
More details on open cycle processes are given in [Henning, 2004/2008] and in [Beccali, 2008].
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2.3 Solar thermal collectors
A broad variety of solar thermal collectors is available and many of them are applicable in solar
cooling and air‐conditioning systems. However, the appropriate type of the collector depends on
the selected cooling technology and on the site conditions, i.e., on the radiation availability.General types of stationary collectors are shown in figure 2.17, and construction principles of
improved flat‐plate collectors and evacuated tube collectors are given in figure 2.17a‐c.
The use of cost‐effictive solar air collectors in flat plate construction is limited to desiccant cooling
systems, since this technology requires the lowest driving temperatures (starting from approx.
50°C) and allows under special conditions the operation without thermal storage. To operate
thermally driven chillers with solar he