The Role of Natural Refrigerants in Future Refrigeration and Heat Pump Systems

ALEXANDRU SERBAN, FLOREA CHIRIAC, IOAN BOIAN, LUCIA BOIERIU

Building Services Department Transylvania University of Brasov

Turnului Street, No. 5, Brasov ROMANIA

alexandru.serban@unitbv.ro http://www.unitbv.ro

Abstract: - Global warming potential of refrigerants as a huge challenge for the future, and for all countries, leads to natural refrigerants as an efficient, cost-effective and safe solution for many applications in refrigeration, air conditioning and heat pumps. For technical and safety reasons they often require different equipment than « chemical » refrigerants (CFCs, HCFCs, HFCs). Even if natural refrigerants still require improvements, research and development in various applications they should be better promoted for new equipment but for the replacement of old equipment too being a cost efficient alternative. Present research on refrigerating system and heat pumps is mainly focused on energy optimization, use of environmental protection refrigerants, and compact equipment all aimed for first cost reduction and low size refrigerant charge. Key-Words: - refrigeration, optimization, environment protection 1 Introduction Energy optimization is focused in principal on renewable energy use as source (as the solar, geothermal, wind energy). Using solar energy for air conditioning-SOLAR COOLING is one of the most important topic for research, Heat Engineering Chair from the Technical University of Civil Engineering Bucharest UTCB being involved in such a research. Solar driven refrigeration systems are absorption type or mechanical compression type. As refrigerants/absorbent pairs the absorption systems use LiBr/Water or Ammonia/Water. A 17 kW, YAZAKI, LiBr/Water system was installed inside the laboratory of the Faculty as shown in Figure 1. The operation of the experimental setup: the 10% ethylene-glycol/water mixture circulating through the solar collectors, 1, transfer heat to the water from the 4000 l tank 3, by means of the plate heat exchanger 2. The hydraulic unit 9, is controlled by the temperature sensors TC: only positive temperature differential between solar panels and storage tank keep in operation the circulator. The vapor generator inside the absorption chiller is drove by the hot water prepared in the tank 3. The system was designed for bivalent operation: if the solar radiation is not sufficient to heat the water at the temperature requested by the vapor generator then a back-up boiler 4, supplements the necessary thermal energy. The distributor 5, prepares the water at the

requested temperature for the vapor generator. To improve the efficiency of the solar collectors the temperature of the hot water leaving the generator has to be lowered at approx. 30…35 °C, which is realized by the plate heat exchanger 2.

Figure 1. Solar Cooling installation diagram Legend: 1 – flat solar collectors; 2- plate heat

exchanger; 3 – warm water tank; 4 – boiler; 5 – distributor; 6 – absorption chiller Li Br/water; 7 –

cooling tower; 8 – circulators; 9 – hydraulic unit;10 – hydraulic unit control; 11 –chilled water consumer for air conditioning; 12 – hot water consumer; TC –

temperature sensors. The chilled water, approx. 6 °C, is stored in a collector before being used in the air conditioning system.

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Professors, Florea Chiriac, Alexandru Serban and Ioan Boian proposed an air-conditioning/ventilation system to be used in the University Auditorium and also for the New Campus, the Research Center PRO-DD of the Transilvania University in Brasov. Figure

2 shows the air-conditioning diagram for the Auditorium located in the central Library, which consists of a 90 kW absorption LiBr/Water system powered by solar energy together with a 60 kW mechanical compression system PV electrical driven.

Figure 2. The air-conditioning diagram of the Auditorium and the central Library The system proposed for the Research Center is presented in Figure 3: for air conditioning during the summer an absorption LiBr/Water SOLAR COOLING was proposed, and for the winter a mechanical compression heat pump having the soil as source and a water coil is working bivalent. Another issue was analyzed for the SOLAR COOLING, i.e. Ammonia/Water and Figure 4 presents the proposal offered to ARIZONA STATE UNIVERSITY in USA. Considering the high temperature and low humidity climate we have analyzed the operation limits of the one stage/two stages systems.

2 Environment Protection The environmental protection by using natural refrigerants is a worldwide concern reflected by the scientific research. After the MONTREAL Protocol the HCFC refrigerants have been replaced by HFC ones. The Global Warming Potential GWP is presently the index taken into consideration when analyzing refrigerants. The greenhouse effect produced by gases, is compared with the CO2, considered as having a GWP=1. Refrigerants escaping into the atmosphere have a very strong effect in terms of global warming. For example, 500 kg of refrigerant R134 escaped into the atmosphere during one year has the same result as the exhausted gases from a car after 2.5 million miles.

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Figure. 3. Summer process Legend: G – generator; C – condenser; E –

evaporator; A – absorber; 1 – solar collectors: 25-30 kW; 2 – hot water boiler Q=25-30 kW, when

collectors do not work; 3 – heat exchangers 30 kW; 4 – absorption cooling machine BrLi/H2O Yazaki Qo=17 kW; 5 – dry cooling tower; 6 – cold water

storage tank, T=12…15 oC, V=2m3; 7 – vapor compression cooling machine Qo=17 kW.

Coefficient of performance for absorption C.M. COP=0,6

Coefficient of performance for vapor compression C.M. COP=2,5-3 (powered with electricity)

Figure 4. Ammonia-water absorption Solar Cooling

Legend: PS – flat solar collectors; Pc- circulating pumps; R – warm water tank; G – generator; A – air

cooled absorber; V – evaporator; C – air cooled condenser; E1,E2 – economizers; VR1,VR2 –

expansion valve 1,2; Co – consumer; Natural refrigerants resulted through biochemical processes in the environment as R717 (ammonia), R744 (CO2), hydrocarbons can be used for refrigeration systems and heat pumps. Water is also a natural refrigerant to be used in the future. Hydrocarbons R600a (isobutene), R290 (propane), R1150 (ethylene), R1270 (propylene) and mixtures of these substances became popular with time as a

result of their harmless effect on the ozone layer, but also for their zero or negligible GWP. Some fields where natural refrigerants are used: At least 75% of the new domestic refrigerators

and freezers are predicted to use isobutene within the next ten years.

R-22 which is largely used for commercial refrigeration either in SPLIT or in centralized systems is replaced step-by-step with R-134a and R-404a, but hydrocarbons, ammonia, and carbon dioxide are prepared for the future replacement. Centralized systems are becoming more and more indirect ones.

For transport refrigeration hydrocarbons and carbon dioxide are the issues developed for the future.

Ammonia and carbon dioxide (as an alternative) have been adopted as the refrigerants for large size refrigeration systems.

For Air-conditioning systems HFC-32 will replace R410a, and R407C in unitary systems and hydrocarbons are substitutes for low charge applications. CHILLERS based on centrifugal compressors still use R-134a and R-123. Small and medium size systems use ammonia, hydrocarbons being only used in limited applications. Carbon dioxide will increase to be used, being a good issue for preparing of the domestic hot water

R-134a employed in bus and train air conditioning will be replaced in the future by carbon dioxide, R-152a and R-1234yf.

Some examples of refrigerating systems and heat pumps operating with natural refrigerants will follow.

Figure 5. CO2 refrigeration system operating in the

transcritical regime: pv ≈ 30 … 40 bar ; tv ≈ -5 ºC ;

pi ≈ 60 … 70 bar ; ti ≈ 40 … 50 ºC ; pf ≈ 120 bar ; tf ≈ 90 ºC.

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Figure 6. Two-stage refrigeration system CO2 / NH3

CO2 pv ≈ 15 bar ; tv ≈ -30 ºC ; pi ≈ 45 bar ; te1 ≈ 10 ºC ;

NH3 pv2 ≈ 15 bar ; tv2 ≈ 5 ºC ; pc ≈ 145 bar ; tc = 36 ºC.

Figure 7. Ammonia heat pumps system used for district heating

pv ≈ 4,3 bar ; tv ≈ 0 ºC ; pi ≈ 7,7 bar ;

pc ≈ 141,4 bar ; tc ≈ 80 ºC.

Figure 5 presents the diagram of a CO2 based two stages system operating in the transcritical regime producing cold water at temperatures between -5 oC ...+5 oC and hot water at +90 oC. The unit also provides warm water at +40 oC...+50 oC. The multistage (cascade) refrigeration unit operating with CO2 in the low pressure stage and with ammonia in the high pressure one, presented in Figure 6, was designed to realize temperatures in the range of -300oC … -400oC. The ammonia heat pump shown in Figure 7 is used for district heating (hot water 90 oC) and also for chilled water (+4oC), realizing a coefficient of performance of approx. 3. 3 Conclusions

Limiting the charge size in case of the natural refrigerants leads to compact units. As a result one rotor screw compressors are almost exclusively used but for reduced power scroll compressors are usual. These compressors realize the high pressure ratios required by the natural refrigerants. Plate heat exchangers are used as evaporators and as condensers, and some companies already use micro channel heat exchangers (less than 2 mm diameter).

The switch to natural refrigerants is a matter of concern for governments, and for international institutions as UNEP, IIR, and Council of Europe: new standards and procedures for the design, construction and operation of the refrigeration systems and heat pumps have to be endorsed. Replacement of the transition refrigerants by the natural refrigerants are already in use by companies from developed countries. Romania through professional association has to contribute to the protection of the environment introducing new standards.

References: [1] X1. BROUWERS, C., The Ultimate CO2

Refrigeration System for every Food Retail Store Format, Atmosphere 2010, Brussels 27.09.2010;

[2] X2. ColBOURNE, D., Guidelines for the safe use of hydrocarbon refrigerants – A handbook for engineers, technicians, trainers, and policymakers. Atmosphere 2010, Brussels 20.09.2010;

[3] X3. GIROTTO, S., Carbon Dioxide Domestic Hot water Heat Pump. Atmosphere 2010, Brussels 20.09.2010;

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