solair guidelines en

8/20/2019 SOLAIR Guidelines En

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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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Requirements on the design and configuration of small and medium sized solar air‐conditioning applications Guidelines

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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


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


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


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

solair guidelines en

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