Recent Advances in Building AC

7/30/2019 Recent Advances in Building AC

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Recent advances in building air conditioning systems

Clito F.A. Afonso *

Faculdade de Engenharia da Universidade do Porto R. Dr. Roberto Frias, s/n 4200-465 Porto, Portugal

Received 10 March 2005; accepted 6 January 2006Available online 9 March 2006

Abstract

Conventional cooling systems are responsible for large amounts of CO2 release to the environment as well as for the use of harmfulrefrigerants regarding the greenhouse effect and the ozone depletion potential. So research has been carried out in order to find out newcooling systems that are free of those problems. This work is a review of cooling systems discussing both classical and more advancedtechnology emerging from recent research, with a respect to their general operating principles and their applications. Special attention ispaid to solar cooling. However focus will not be given to individual systems components as they change very often in design in order toget even better efficiencies. In this paper a classification of cooling systems is presented according to the final energy used to operate them. 2006 Published by Elsevier Ltd.

Keywords: Air conditioning cycles; Review; Solar cooling; Electrical systems; Thermal systems; Hybrid systems

1. Introduction

Conventional systems for cooling and heating of build-ings consume large amounts of energy produced by theburning of fossil fuels. This results in vast quantities ofgreenhouse gases being emitted to the atmosphere andhas serious consequences in terms of global warming, envi-ronmental damage, e.g., acid rain and detrimental effectson human health such as asthma. At Earth Summits inRio de Janeiro, Kyoto and recently Johannesburg, pressurehas gradually been brought to bear on national govern-ments to act in an attempt to control greenhouse gas emis-sions, and the Kyoto Summit secured a commitment from

EU countries to achieve an 8% reduction in CO2 emissionscompared to the 1990 level by 20082012. So developmentof new environmentally-friendly technologies will be vitalto achieve these targets.

On the other hand, chlorofluorocarbons (CFCs) havebeen used as working fluids in conventional air condition-ing systems for over 60 years. However, these refrigerants

are known to deplete the ozone layer and contribute to glo-

bal warming [1,2]. Environment concern about CFCs,HCFCs and some of their replacements [3], has promptedresearch to identify new technologies to provide an alterna-tive to conventional vapor compression systems.

For refrigeration and building air conditioning there areseveral available refrigeration systems. These systems canbe classified in three main categories according to the finalenergy used to operate them: electrical systems, thermalsystems and hybrid systems [4], as is shown in Fig. 1. Whilein the first category the input energy for operation of thesystem is electricity (high grade energy), in the secondone the driving force can be any kind of thermal energy

(low grade energy). The third one is composed of severalenergy forms that are used together in order to provideincreased system efficiency as well as greater balance inenergy supply.

Recently the second group (thermal systems) has beenreceiving increasing interest from both the commercialmarket and research. This is mainly due to the fact thatthermal systems represents smaller ozone depletion poten-tial and smaller contribution to greenhouse effects thanelectrically operated refrigeration plants, using syntheticrefrigerants used in the systems operated electrically

1359-4311/$ - see front matter 2006 Published by Elsevier Ltd.

doi:10.1016/j.applthermaleng.2006.01.016

* Tel.: +351 2250 81746.E-mail address: [email protected]

www.elsevier.com/locate/apthermeng

Applied Thermal Engineering 26 (2006) 19611971
mailto:[email protected]:[email protected]


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(leakage rates vary from 5% to 25% of the total charge).Also as they are thermal operated instead of electricallyoperated, the CO2 emissions are lower. As an example,for the same quantity of final energy, burning natural gasin a boiler releases 0.21 kg CO2/kW h to the environmentwhile electricity releases 0.68 kg CO2/kW h. Electricity pro-duction in a traditional power plant powered by fossil fuelsinvolves several stages of power conversion, first chemicalenergy to thermal then mechanical and finally electricity.There are losses associated to each conversion process aswell as to the grid connection and electricity transportwhich all contribute to the higher emissions of CO2 to

the atmosphere [5]. The International Institute of Refriger-ation has estimated that approximately 15% of all electric-ity produced worldwide is used for refrigeration and air-conditioning processes of various kinds [6]. Due to this dif-ference in CO2 emissions, to regional shortage of electricitythat forces the price of electricity to high levels, there areeconomic incentives in several countries to the replacementof electricity, as final energy, with thermal energy, whichmakes more appealing the thermal operated refrigerationsystems. The third kind of systems is a symbiosis of severalform of energy supply to the refrigeration systems. Thereare also some miscellany refrigeration systems that are stillunder development.

This work presents a review of the classical systems forcooling as well as the new ones that emerged from recentresearch, discussing their general operating principles aswell as their applications. Focus will not be given to indi-vidual components of the systems as they change very oftenin design in order to achieve even better optimal efficiency.

2. Electrical systems

The electrical systems can be subdivided according tothe working fluid used for its operation e.g., vapor, airand CO2 (see Fig. 1). While the first one is based on the

vapor compression cycle, the second one is based on the

inverse of the JouleBrayton cycle and the third one onthe trans-critical cycle. Besides these systems, there is thethermoelectric refrigeration system that, unlike the otherones, accomplishes its objective, the cooling, in a moredirect manner.

The major part of refrigeration systems operated electri-cally are based on the vapor compression refrigerationcycle [7,8], which is composed of four basic components:evaporator, compressor, condenser and expansion valve,as shown in Fig. 2.

Applying the first law of thermodynamics to the wholecycle, and to each of its components [9], neglecting changes

in kinetic and potential energy, and if _m is the refrigerantmass flow rate in the system, it is possible to calculate thedifferent energy fluxes in the cycle by the following set ofequations:

first law of thermodynamics: _Qevap _Qcond _W 0, evaporatorrefrigeration effect: _Qevap. _mh1 h4, compressorcompression power: _W _mh2 h1, condensercondensation heat: _Qcond _mh3 h2, expansion valve: h4 = h3.

Refrigeration Systems

Electrically operated Thermally operated Hybrid

Vapour

Air

CO2

Thermoelectric

Absorption

Adsorption

V. with thermal engines

Desiccants

Ejector

Heat / electricity

Metal Hybrid

Solar / Biomass

Solar / Biomass / Diesel

Solar / Gas

Chemical / Thermal

Fig. 1. Classification of active refrigeration systems according to the final energy used to operate them.

Fig. 2. Basic vapor compression cycle.

1962 C.F.A. Afonso / Applied Thermal Engineering 26 (2006) 19611971


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The coefficient of performance (COP) is then given by

COP _Qevap.

_Wh1 h4h2 h1

where hienthalpy (kJ/kg), _Qevapheat power extractedfrom the evaporator (kW), _Qcondheat power released in

the condenser (kW),_

Wcompressor mechanical power(kW).COP values are always positive and usually greater than

one, due to the fact that the refrigeration effect is greaterthan the compression power. Typical values of COP forthe vapor compression system are in the range of 23. Evenif the evaporation temperature is held constant all over theyear, the COP is not constant due to changes in air or watertemperature feeding the condenser, which causes changesin the condensing temperature and also in the enthalpiesaffecting the COP equation.

Better performances can be achieved with some modifi-cations. There are several possible modifications that can

be implemented, depending on the specific application. Avery common one is the use of multistage compression,i.e., the use of more than one compressor, with intercoolingof the refrigerant between each pair of compressors [10].Intercooling is carried out with the refrigerant at lowertemperature withdrawn from other parts of the systemwhich reduces the system total work input. There are threelevels of pressure, a low in the evaporator, the intermediatebetween the two compressors and the high at the con-denser. The multistage systems usually have higher COPvalues when compared with the conventional compressionsystems. This is due to the fact that there is a decrease in

compression work and an increase in the refrigerant effect.There are several ways to implement this technique, e.g., tocouple the system with several evaporators, each one with atypical operating temperature.

Other modifications can be carried out in the whole sys-tem. For instance, a simple radiation shield placed in therear side of domestic refrigerator-freezers, to avoid the heattransfer by radiation on this surface from the condenserand compressor. It can decrease the inside air temperatureof the refrigerator up to 2 C. Table 1 shows the maximumand minimum inside air temperature in the refrigerator inthree different inside levels with and without radiationshields [11]. This technique can also be applied on standardair conditioning units.

Dry air may serve as a refrigerant in a mechanical com-pression system. In these systems the refrigeration is accom-plished by means of a non-condensing gas. The cycle is basedon the reverse of the Joule-Brayton cycle which have fourbasic components: two heat exchangers, one compressorand one turbine [9]. One of the heat exchangers absorbs heat

from the place to be cooled down, while the other one rejectsheat to the environment. For conventional refrigerationrequirements, the air cycle systems have too low coefficientof performance to compete with the vapor compression sys-tems. These systems are of great interest in applicationswhere the weight of the refrigerant unit must be kept at aminimum, for example, in aircraft cabin cooling.

The trans-critical cycle uses CO2 as working fluid andoperates over the critical point of the refrigerant. Due tothe thermodynamic properties of CO2, the vapor compres-sion cycle and the components of the system should differfrom the ones with low pressure refrigerants. In fact, formoderate ambient air temperatures, the pressure at which

the refrigerant rejects heat must be supercritical, with vari-able fluid temperature. Fig. 3 shows a typical trans-criticalcycle. As pressure and temperature are independent proper-ties on the supercritical region, the system must have a highside pressure adjustment. The COP is pressure dependentand has a maximum value for a given high side pressure [12].

The high pressure (>100 bar), combined with the lowmolar mass of CO2, reduces the volumetric flow and thedimensions of the system components (compressor, valves,piping).

The thermoelectric refrigeration system uses directlyelectrical energy to achieve a refrigeration effect without

any intermediate conversion process [13,14], such as theconversion of electrical energy to mechanical energy todrive the compressor. The electrical energy, rather thanthe refrigerant serves as a carrier. It thereby avoids the costof having a compressor, condenser and evaporator (seeFig. 4). Therefore, the system is compact, quiet, and needslittle maintenance. Like the conventional thermocouple,the thermoelectric refrigeration is based upon the Peltiereffect (1834) in which two dissimilar materials, A and B,are used. There are two junctions between these materials,one located in the refrigerated space and the other in thesurroundings. When an electrical potential difference is

Table 1Minimum and maximum inside air temperature at different heights in therefrigerator (C) for different locations of the radiation shield (rs)

Height(cm)

Withoutrs

Rear wallwith rs

Recesswith rs

Real wall+ recess with rs

83.5 4.5/6.6 4.3/6.9 4.1/5.9 3.9/6.358 3.8/7.0 2.6/6.9 3.7/6.3 2.5/5.253 2.9/7.4 1.8/6.3 2.5/6.1 1.1/5.135 3.1/6.5 2.3/5.7 2.2/5.3 1.8/5.6

Taver. 3.6/6.9 2.8/6.5 3.1/5.9 2.3/5.5r

0.6/0.35 0.93/0.49 0.79/0.37 1.0/0.47

crit

evaporation

h

heat rejection

compressionexpansion

Fig. 3. Trans-critical cycle.

C.F.A. Afonso / Applied Thermal Engineering 26 (2006) 19611971 1963


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applied on these materials, the temperature of the junctionlocated in the refrigerated space decreasesthe refrigera-tion effectwhereas the temperature of the other junctionincreases [1518]. The Peltier effect is one of the three ther-moelectric effects, the other being known as the Seebackeffect and Thompson effect. Whereas the last two effectsact on a single conductor, the Peltier effect is a typical junc-tion phenomenon [19]. The three effects are connected to

each other by a simple relationship [20].Thermoelectric cooling offers many advantages when

compared to other methods. These includes the easy ofinterchanging the cooling and heating functions, no wearand noise from moving parts, no problem in containmentof refrigeration, easy of miniaturization for very smallcapacities systems, easy of controlling capacity varyingapplied voltage and ability to operate under zero gravity.The main applications of thermoelectric systems have beenin portable refrigerators, water coolers, cooling of scientificapparatus used in space exploration, and in aircraft.

3. Thermal systems

There are several kinds of heat driven refrigeration sys-tems that can be generally classified as shown in Fig. 1.

In the next subsections, the principles of each one isshortly described.

3.1. Vapor compression systems driven by thermal engines

These systems are based on the traditional vapor com-pression cycle described in Section 2, but in this case theelectricity supply to the compressor comes from a thermalengine, namely the Stirling engine, instead of coming fromthe main electrical grid [21,22]. For efficient operation theheat source should work between 650 C and 800 C. Thereare a few of those engines running with parabolic solarconcentrators, but these systems are expensive and com-plex, as they must track the sun.

If the electrical energy supply to the vapor compressionsystem comes from a Rankine cycle, it is called DuplexRankine system [23].

3.2. Absorption systems

Absorption cooling, as a method, is as old as the vapor

compression. However it has only started to gain signifi-

cant importance recently. On one hand, because of theundesirable effects of synthetic refrigerants on the environ-ment and on the other hand, because of the increasingenergy prices [24]. An absorption unit differs from thevapor compression system in the way how the compressionof refrigerant is carried out, while having in common the

other three components: the evaporator, the condenserand the expansion valve.Fig. 5 shows the different parts of the cycle, the thermal

compressor. In the absorption cooling the compression isdone using a secondary fluid that has the capacity ofabsorbing the main refrigerant flowing in the other threecomponents. In the absorber outlet, heat is rejected tothe environment in order to carry out the absorption pro-cess. The result is a homogeneous liquid solution that ispumped to the generator. The objective is to separate thetwo fluids using external heat [25]. The work of compres-sion in the absorption system is much lower than in thevapor compression system due to the fact that a liquid

solution is pumped instead of vapor. However a largequantity of heat at higher temperatures (typically over100 C) must be supplied in the generator. These two effectstogether, decrease the COP value of the absorption system,when compared to vapor compression cooling, to valuesbelow one, typically around 0.7. COP can be increasedusing waste heat (found in many industrial processes) orsolar energy in the generator. Because of the need to supplyheat to carry out the compression process, this part of thesystem is also called a thermal compressor in opposition tothe vapor compression system where a mechanical com-pressor is applied. The absorption cooling is nowadays

very common in house and camping refrigerators as wellas in air conditioning equipment.

Absorption systems can be classified according to

Working fluid. The most popular fluids in the absorptionsystem are H2OLiBr (water as refrigerant and lithiumbromide as secondary fluid) and NH3H2O (ammoniaas refrigerant and water as secondary fluid). The firstpair of fluids are suitable for positive temperatures inthe evaporator (water freezes below 0 C at ambientpressure) while the second one can also be used for neg-

Fig. 4. Thermoelectric refrigeration system [28].

Heat

supply

Absor-

ber

Genera-

tor

Refrigerant

From evaporator

Refrigerant forcondenser

Heat

rejection

pump

Liquid solution

(refrig. +absorbent)

Fig. 5. Compression in the absorption system.



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ative temperatures. However the NH3H2O systems arenot very common, due to their low efficiency (averagecooling COP = 0.6), high heat transfer areas and initialcost. Research is being carried out in order do developdifferent pairs.

Number of effects, that describes the number of cycles

that are connected in cascade. A single effect machineis related to a single cycle, while a double effect unit usesthe heat released in the high pressure cycle to the lowpressure generator. Thus, the energy supply to the sys-tem is used twice and the COP of the cycle is averagedoubled (typically 1.4 against the 0.7 of the single effectfor the pair H2OLiBr). The single effect system can usehot water at about 80 C while the double effect systemneeds water or vapor over 120 C that must be producedin a boiler. However, recently developed double effectwater chillers with the pair NH3H2O [26] have achieveda COP of about 1. Results reported for a heat pump [27],indicate that values as high as 1.9 can be reached.

In spite of a fast increase in use, absorption cooling isstill more expensive and huge than the classic vapor com-pression systems.

A number of modifications on the basic absorption cyclehave been recently published [28]. One of them, called Pla-tenMunters system (after its Swedish inventors), the liquidpump is replaced by a third fluid, typically hydrogen. Theadvantage of this solution is that it does not have any mov-ing parts. Another one is the steam ejector recompressionabsorption refrigeration system. It is similar to a singleeffect lithium bromide absorption cycle, with the difference

that there is a steam ejector for enhancing the concentra-tion process [29].

A novel method, the electrochemical absorption refrig-eration system was published in 2000, and consists of fourmain components [30]:

An electrochemical cell is the heat absorber, equivalentto an evaporator in conventional vapor compressionrefrigeration.

A fuel cell rejects heat in a manner similar to a con-denser in vapor compression refrigeration.

Heat exchanger between gas streams and water flowstream.

Current pump for elevating the fuel cells voltage outputto a level sufficient for driving the electrochemical cell.

There are other systems still under development [31].

3.3. Adsorption cooling

Adsorption systems are similar to the absorption onesbut they use a refrigerant/adsorbent solid pair instead ofa refrigerant/absorbent liquid pair. There are several avail-able pairs however the ammonia/activated carbon andwater/zeolite are the most frequent ones. Recent develop-

ments in solar absorption and adsorption cooling systems

showed that they can be implemented with a comparativelyhigh efficiency and low manufacture and operation costs[32].

3.4. Desiccants systems

Desiccant cooling is based on an open cycle where thecooling is done directly in the air by changing its humidity,instead of being cooled trough evaporators as in the othersystems. A desiccant is a synthetic or natural hygroscopicmaterial that is able to absorb or release the humidity ofthe surrounding air. The humidity absorption is followedby an increase of the air temperature because of the latentheat released by the condensing water. In an opposite way,the humidity absorbed by the desiccant can be released tothe air by heating the desiccant to a sufficiently high tem-perature. This results in a decrease of the surrounding airtemperature, because of the water evaporation [33].

In a typical desiccant cooler the air to be insufflated in

the space is dehumidified as it passes through the desiccant.The latent heat is transformed in sensible heat as the air isbecoming drier and warmer. The exhaust air from thespace goes then trough an evaporative cooler becomingmore humid and colder. These two air streams flow intoa heat exchanger where the supply air is cooled to a tem-perature lower then the space air temperature. Energy sup-ply is required to heat the exhaust air after the heatexchanger and for the regeneration of the desiccant.

The most important advantage by using desiccant cool-ing is that both air temperature and humidity can be simul-taneously controlled (World Health Organization

recommends a maximum humidity of 7 g/kg for healthyindoor air). Efficiency and COP can be improved by usingsolar or waste energy for the desiccant regeneration. Indi-rect benefices are associated with low humidity levels,including reduced corrosion and microbial growth.

3.4.1. Solid desiccant systems

Commercial solid desiccant systems are available withtypical COP values about 1 [34]. Several solid desiccantmaterials can be found, such as silica gel, carbon, etc.Fig. 6 shows a typical solid desiccant system as well thepsychometric chart of the air evolution. Typical values ofthe proprieties of the thermodynamic air states shown inFig. 6 are represented in Table 2.

In some cases, the cooler (evaporator) mounted after thethermal wheel, is replaced by an evaporative cooler inorder to avoid the use of refrigerants. In this case there isan increase in humidity and a decrease in temperature afterstate 3.

3.4.2. Liquid desiccant systems

The working principles of both desiccant systems is sim-ilar, however there are some differences in the equipmentdesign. For example, in a cooler using liquid desiccant,the desiccant wheel is replaced by a spray chamber. This

is an important advantage, since desiccant wheels are



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generally large in size in order to enhance heat and masstransfer, but makes them relatively expensive.

A new liquid desiccant system where attention is paidonly to the differences regarding the conventional ones isshown in Fig. 7 [35]. In this new system, the classical ven-tilators have been replaced by rotors with fibers which act

as evaporator or absorber, Fig. 8, enabling an improvedheat and mass transfer by significantly increasing the con-tact surface area [36,37]. As liquid is injected in the centerof the ventilators flowing over the fibers, there is also aneffective removal of air particles promoting, in this way,air cleaning. The thermal wheel was also replaced by amore effective heat exchanger composed of heat pipes.

The absorbent used was lithium bromide. PotassiumformateHCOOKas absorbent is also under investiga-tion since it has a negative crystallization temperature, itis less corrosive and not as expansive as the others.

COP increases with inside temperature (Ti), howeverdecreases with outside temperature (Tamb) as shown in

+

-

Space

Desiccant

wheel

Thermal

Wheel

Cooler

Humyfier

1 2 3 4 5

678910

1

234 5

6

7 8 9

10

Humidity

Temperature

Fig. 6. Solid desiccant system.

Table 2Typical thermodynamic properties of the air in a solid desiccant system

State T (C) w (g/kg) State T (C) w (g/kg)

1 28 11 6 23 6.82 44.4 6.1 7 15.81 9.763 22.9 6.1 8 32.27 9.764 15 6.1 9 70 9.765 16.5 6.1 10 46 16

Absorber

Heat pipes

Building

Evapora-

tor

Water

WaterHeat exchanger

Desiccant solution

Vapor

Generator

Heater

Evapo-

rator

R

1

3 2

S

A

T

23

1

I

S

AR

X

Fig. 7. New liquid desiccant system.

Fig. 8. Fiber rotor.



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Fig. 9. These results were obtained for a relative humidityof 50% and a heat pipe efficiency of 70% [38].

3.5. Ejector

A very attractive feature of the ejector systems is thatthey can provide heating and cooling simultaneously usinga single heat source, such as renewables (biomass, solar,geothermal energy) or waste heat (exhaust gases for airconditioning a vehicle).

For air conditioning applications, the most convenientheat source is the solar energy [39,40]: it enables summercooling and winter heating. It is particularly interestingfor commercial buildings, where it is not necessary to useair conditioning in the evenings, or in remote places wherethere is no electricity or where hot effluents are not avail-able. In this system, the ejector replaces the compressorof the vapor compression system. The ejector has a venturi,a suction section, a converging section and a straight anddiverting section, Fig. 10.

Schematic representation of an ejector cycle is shown inFig. 11. QG represents the primary heat source, which

would be, as mentioned before, a renewable (e.g., solar col-lector) or a kombi system with a conventional boiler.

This system has no moving parts, which makes themvery reliable. There are ejector systems operating for morethan 20 years without any kind of problems.

A new prototype for combined heat and power has beendeveloped that consists of two circuits, a primary circuitusing n-pentane as working fluid and a secondary circuitwith water (boiler and heat exchangers). Using n-pentaneas a working fluid has advantages thermodynamic proper-ties at relatively low temperatures therefore solar energycan be applied as the heat supply. It is also a green refrig-erant in contrast to CFCs. Schematic representation ofthis prototype is shown in Fig. 12 [41], with 50% of solarenergy and 50% heat input from a natural gas boiler.

Global COP was 0.23. The estimated was 0.019/kW hthat is roughly half the price of a conventional system [42].

Other ejector systems have been applied in several build-ings with different kind of refrigerants and good results[4345].

3.6. Metal hybrid system

This is a state of the art refrigeration system developedby a group of Japanese companies in the 90, capable for

26 27 27 28 28 29 29 30 300.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

Tamb

COP

Ti=24C

Ti=26C

Fig. 9. COP as a function of outside air temperature for two differentindoor air temperatures.

Fig. 10. View of an ejector.

Fig. 11. Schematic diagram of the ejector system.

Boiler

1

Heater

2

Condenser

8

9

Evaporator

Ejector3

Turbine4 5

6

71

0

2a 2b

Fig. 12. Representation of the combined ejector/Rankine cycle.



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cold storage below 30 C. The method is based on thehydrogen absorption and desorption capability of metalhybrid alloys in exothermic and endothermic reaction,respectively. This phenomenon can be implemented in arefrigeration cycle using a combination of two alloys, oneworking at high and another at low temperatures. Some

of the advantages of this technique are that it is CFC free,safe, no moving elements except for pumps circulatingwater and brine and low noise [46].

3.7. Solar cooling

Heat and electrical driven refrigeration systems alreadymentioned are suitable to operate with solar energy, whichcan be converted either to heat (solar collectors) and eitherto electricity (photovoltaic panels) [4750]. An importantadvantage of operating a cooler using solar power lieswithin the intrinsic connection between solar radiationand cooling demand. Heating loads, and therefore cooling

demands are generally higher when solar intensity is high,thus more energy is available for running the cooling sys-tem. The appropriate refrigeration cycle depends on thecooling demand, available form of energy input and tem-perature level of the refrigerated objects and environment.From an energy saving point of view, a solar cooling sys-tem can save electrical energy in the range of 2540% whencompared to an equivalent cooling capacity of a conven-tional water cooled refrigeration system [5153].

Solar driven absorption refrigeration was the first solarrefrigeration system using solar thermal panels as heatsource for the generator. The COP of this system was

higher than the COP of other thermal operating systems,typically around 0.60.8 and up to 1.35 for the two stagesprocess [5457].

Solar driven adsorption refrigeration is not as widelyused as the absorption systems. However, it can be inte-grated with a low temperature solar collector, e.g., flatplate solar collectors. Ejector refrigeration can use alsosolar energy to operate the generator, as already mentionedbefore. It can work with a low temperature energy supply,thus a solar collector can be used [58]. The vapor compres-sion system can be also driven by electricity from photovol-taic cells. However this system is quite expensive due to thehigh installation cost and low efficiency of the photovoltaicpanels. It is effective in areas far from the electricity grid,where the cooling capacity is low [59].The duplex Rankinecycle can also use thermal collectors to supply thermalenergy to the boiler. The solar energy is also suitable touse on thermoelectric systems, through solar photovoltaicpanels.

4. Hybrid systems

An interesting solution, shown in Fig. 13, combines in asingle cycle the vapor compression and an absorption sys-tem [60,61], operated on electricity and heat, with the fol-

lowing advantages:

Increase in COP values when compared with the absorp-

tion system. Use of refrigerants of the absorption cycles. Possibility of compression of the vapor flowing out of

the generator and the use of its latent heat of condensa-tion to reduce the necessary amount of thermal energyfor the operation of the generator.

As mentioned, the difference between this system andconventional absorption cooling lies on the generator.The necessary thermal energy for comes from the conden-sation of the refrigerant (process 23 in Fig. 13) previouslyreleased in the generator and compressed to a pressure suchthat the saturation temperature is higher than of the gener-

ator (process 12). COP values of this systems lies between2 and 3.5 [62].

In order to intensify the heat transfer in the system, acentrifugal field was created [63], by placing all heat andmass transfer equipmentevaporator, absorber, generatorand condenserin a rotating ensemble, Fig. 14.

The same kind of centrifugal field was used on a gasfired absorption system, called ROTEX [64]. A solar/bio-

GENE-

RATORHEAT

EXCHANGER

ABSOR-

BEREVAPORATOR

H. SOURCE

H.

4 5

3

21

SINK H.SINK

Fig. 13. Hybrid system.

Fig. 14. Hybrid system with a rotate ensembleevaporator view.



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mass hybrid cooling systems was developed [65] that wasbased on an absorption cycle. Measured COPs are around0.5. With the same energy sources, an other adsorption sys-tem was developed that uses ammonia/carbon pair asworking fluid [66].

Solar/biomass/diesel energy was used on an absorptionrefrigeration system (with cold storage) that uses water

lithium bromide as working fluid [67]. COP values up to1.2 were obtained. A solar/gas driven ejector refrigerationcycle using methanol as working fluid was also proposedfor a hospital in Mexico [44]. Another hybrid co-generationsystem was recently proposed [68], combining a fuel cellwith an ejector. Electricity is produced in a fuel cell gener-ator by reacting hydrogen with oxygen resulting steam thatdrives the ejector [69,70].

Table 3 shows the typical values of COP of severalrefrigeration systems so far analyzed.

5. Miscellany systems

The thermoacoustic refrigeration systems is consideredto be a new technology, attaining cooling without the needfor refrigerants. A loudspeaker creates sound in a hollowtube which is filled with an ordinary gas. The process itselfutilizes standing acoustic waves in an enclosed cavity to gen-erate the mechanical compression and expansion of a work-ing fluid needed for the cooling cycle. The technique has thepotential for high efficiency operation without the need forcooling liquids or mechanical moving parts [7173].

Flash cooling is also important commercially for obtain-ing chilled water and for dry ice production. The flashchamber is maintained under an extremely low pressure

by a compressor, usually a stem jet compressor. The advan-

tage of steam-jet-water-vapor systems is that they haveonly few moving parts and therefore require little mainte-nance. Flash coolers use cheap, non-toxic refrigerant(water vapor) and have minimum power requirements.

6. Conclusions

In this overview, the main trends on air conditioningsystems were presented. They are related with the researchof new refrigerants environment friendly, with the develop-ment of new thermodynamic cycles namely the desiccantsand hybrid and by the development of rotating devices toenhance the heat and mass transfer.

Some of these innovations are still in research, howeverthey are expected to substitute the conventional systems inshort term. The implementation of new technologies willenable to reduce the energy cost associated to operatingair conditioning, the negative environment impact as wellas the initial cost at installation.

Attention was also given to refrigeration systems thatcan use solar energy turning them mote sustainable.

An increase in efficiency, with lower costs, will allow forthe diffusion of air conditioning systems in countries werethey are not widely spread.

The previsible decrease in dimension and costs of newsystems will have benefits in the increase of competitivenessof the air conditioning industry.

Acknowledgements

The author is grateful to Doctor Szabolcs Varga for his

valuable suggestions as well as to review the paper.

References

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Table 3Typical values of COP of several refrigeration systems

Refrigeration cycle COP

Vapor compression Standard 23PV cells 35Duplex Rankine 0.30.5

Reverse JouleBrayton $1Thermoelectric Standard 0.51

PV cells 0.5

Absorption Single effect 0.6Double effect 1.2Thermal collectors 0.61.3

Adsorption Standard 0.20.7Thermal collectors 0.30.8

Desiccants Standard $1Thermal collectors 0.51.5

Ejector Standard 0.7Thermal collectors 0.30.8Combined heat and power 0.250.45

Hybrid Heat/electricity 23.5Solar/biomass $0.5Solar/biomass/diesel 1.2



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