Solar Powered Air Conditioning System for a Hospital in Manzanillo

World Renewable Energy Congress VI (WREC2000) © 2000 Elsevier Science Ltd. All rights reserved. Editor: A.A.M. Sayigh

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SOLAR POWERED AIR CONDITIONING SYSTEM FOR A HOSPITAL IN MANZANILLO Jorqe L. Wolpert, Minh V. Nguyen and Saffa B. Riffat Institute of Building Technology School of the Built Environment University of Nottingham University Park, Nottingham, NG7 2RD KEY WORDS:

ejector cycle, air conditioning, solar cooling, motive fluid

Abstract: An ejector air conditioning system using solar thermal energy for operation is described in this paper. The solar cooling unit consists of a corr,mercial solar parabolic concentrator that drives an ejector cycle cooling system. Water is used as working fluid making it an environmentally sound device. There is a growing need for new technologies that are safe for the environment and which use alternative energy sources to the buming of fossil fuels. A 13kW cooling capacity prototype was designed to operate in a hospital in Manzanillo, Mexico. The solar collector for this unit will be a parabolic concentrator fitted with and gas burner to compensate for periods with low solar irradiance. Vaporised water from the boiler enters the ejector and is expanded to a lowpressure though a converging-diverging nozzle creating a pressure- differential with the refrigerant in the evaporator and causing it to flow into the ejector and mix with motive fluid. Vaporisation of the fluid in the evaporator causes a cooling effect and hence refrigeration is produced. A COP of up to 0.62 can be achieved by the system under typical operating conditions encountered in Manzanillo.

1 .-INTRODUCTION Production use of CFS's has decreased in a significant manner; but their use in conventional vapor compression air-conditioning and refrigeration systems persists and has continued to contibute to ozone depletion and global warming. In addition, these systems consume vast quantities of electricity that is generated primarily with the burning of fossil fuels. The new solar cooling device described in this paper uses solar thermal energy for operation. Water is used as the refrigerant making it an environmentally sound system. The system consists of a solar heating loop and an ejector cycle. Ejector systems have been proposed in the past as alternative cooling devices (AI-Khalidy, 97; Da-Wen, S and Eames, I, 95). As ejectors have no moving parts, their construction is simple and their maintenance requirements are low (Petrenko, Vet. al, 94).

2. DESCRIPTION OF THE SYSTEM The major components of the solar cooling system are the solar collector, gas burner, boiler, ejector, evaporator, condenser and feed pump. The solar collector is a concentrating parabolic trough type. The integration of solar collectors with a natural gas-fired boiler will ensure that the system can operate during periods of low solar irradiance. Figure 1 shows a diagram of the ejector cooling cycle. The boiler is a compact brazed plate (CBE) type heat exchanger that transfers heat from the solar heating loop to the refrigerant. The refrigerant boils at a high pressure and temperature iq the boiler. The resulting vapour is exhausted through the ejector compressor.

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ejector live steam dr'opletseparator

|

I

I /

/

~

CBEevaporator water/glycol refrigerant : secondary 13kW~;[OC

I

I Cilhebat~12r 21.02Kw@160C

I 1

/

CBEcondenser groundwatercooled 34.02kW(d~30C

~ metering valve

~l) ed pump 0.48Kg/mindP 6.18bar 4.9Wminimumduty Figure 1 Ejector cycle's layout

A typical ejector cross section is shown in figure 2. Inside the ejector this primary stream is accelerated to a high velocity (typically mach no. >2). This jet passes across the inlet from the evaporator and entrains fluid. This causes a low pressure inside the evaporator resulting in the refrigerant boiling at a low temperature. The evaporator is a direct expansion unit in which heat for the boiling process is extracted directly from the air, thereby cooling the air down. The refrigerant vapour flows into the ejector and is compressed. The mixed stream from the boiler and evaporator is exhausted from the ejector to the condenser. The condenser is a CBE type heat exchanger which cools the refrigerant stream using ground water. The refrigerant condenses on cooling. Condensate is passed to the boiler using a liquid feed pump and to the evaporator using a metering valve. A unit of this design, with a 13kW cooling capacity, is currently being fabricated for installation at an operating hospital in Mexico.

primary nozzle

constantpressure constantarea mixingchamber mixingchamber :~

~1~

-~i~

diffuser

5~ 6 :

I

secondaoj inlet

Figure 2 Ejector cross section

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3. SOLAR EJECTOR UNIT FOR MANZANILLO HOSPITAL Manzanillo is a city located on the Pacific coast of Mexico. Table 1 shows geographical data for Manzanillo. This site maintains a high solar irradiance throughout the year (1845 kWh / m 2 per annum). Table 2 shows a solar radiation and temperature chart for Manzanillo. The hospital is currently under construction and it forms part of a new hospital construction program under taken by the Instituto Mexicano del Seguro Social (IMSS) for 1999-2001. The area within the hospital where the system will be installed is physiotherapy. This has a total area of 275 m 2. Calculations for the cooling load were made with reference to the architectural design of this particular space, including its building materials, so that an accurate assessment could be made. The solar collect')r considered for the Manzanillo project is a concentrating non-tracking parabolic trough solar collector. Use of a concentrating collector allows a high boiler temperature to be used. This results in increased performance (Sokolov M and Hersgal, D, 1993). Solar concentrators have been used in the past to run solar thermal power plants (Haddock, C and McKee, J S C, 1991). Due to the high condenser-boiler pressure differential a feed pump must be included to return condensate to the boiler. The refrigerant used is water and the heat exchange medium between the solar collector/gas burner and the boiler is oil.

Latitude

19.35 °

Climatic Zone

503

Situation

City

Time Zone

-6G

Azimuth angle (solar collector)

0°

Inclination angle (solar collector)

0°

Solar concentrator facing

South

Table 1 Geographical Data for Hospital Site

Jan

Feb

Ma

Apr

r Global Irradiation (kWh/m2) Temperatu re (°C)

141

151

184

Ma

Jun

Jul

Aug

Sep

Oct

Nov

Dec

y 182

173

Yea r

163

159

153

132

141

130

134

184 5

~ l P~WJl P ~ l II P ~ J

P~

! P # ~ T P~:~J P~:~Jl P~:gl • P~:g • P ~ ' S l PJb~ ! P ~ I

Table 2 Solar Radiation and Temperature for Manzanillo

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4. ECONOMICAL CONSIDERATIONS A payback time of 5 years for the ejector cooling system was calculated. After this period, the vapour compression system will continue to consume electrical power and consequently continue to contribute to the burning of fossil fuels with its associated environmental effects. The comparative

analysis

was

made

using the net present value method. This method is a

comparison between the investment made at present, using the present value of money considering interest rates and inflation over a period of time. In this case two lifetime periods were considered for analysis. The lifetime considered for the ejector unit was of 30 years and that assumed for the vapour compression system was of 15 years. The lifetime of a vapour compression system is relatively short due to high rates of wear in the reciprocating compressor.

5. CONCLUSIONS The solar powered ejector system described is an environmentally safe alternative to conventional vapour compression air conditioning systems. Solar powered cooling has the advantage that performance increases with solar radiation intensity, the period when cooling is generally required. Solar radiation intensity at the site chosen for installation of the proposed solar ejector cooling unit is high. A COP of 0.62 has been predicted for the air-conditioning system to be installed in Manzanillo.

REFERENCES AI-Khalidy, N. A. Performance of a solar refrigerant ejector refrigerating machine. ASHRAE Transactions: Research 4016 Sun, Da-Wen and Eames, I. Recent developments in the design theories and appfications of ejectors- a review. Journal of the Institute of Energy, June 1995, no. 68, pp. 65-79. Petrenko, V., Bulavin, I. V. and Samofatov, Y. A. Investigation of the methods increasing the efficiency of solar ejector cooling and refrigeration systems. (1994), Technical report for the Odessa State Academy of Refrigeration,Odessa, 270100, Ukraine. Sokolcv, M and Hershgal, D. (1993) Solar powered compression enhanced ejector air conditioner. Solar Energy, vol. 51, no. 3, pp. 183-194. Haddok, C. and McKee (1991), Solar energy collection, concentration, and thermal conversion-a review. Journal of Energy Sorces, UK, Volume 13, Pp. 461-482.

AKNOWLEDGEMENTS The authors wish to acknowledge the IMSS authorities for considering the system for the Regional Manzanillo Hospital.