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Successful Strategies for Ecologically Sustainable Development

Assoc. Prof. M Sivakumar Dr Judy Messer

Futureworld: National Centre for Appropriate Technology ( NCAT) Inc

PROCEEDINGS OF THE NATIONAL CONFERENCE ON SUCCESSFUL STRATEGIES FOR ECO LOGICALLY SUSTAINABLE

DEVELOPMENT

WOLLONGONG, NEW SOUTH WALES, AUSTRALIA S - 7 DECEMBER 1994

PROTECTING THE FUTURE:

ESD IN ACTION

Edited by: Assoc Prof M Sivakumar Dr Judy Messer

FUTUREWORLD: NATIONAL CENTRE FOR APPROPRIATE TECHNOLOGY (NCAT) INC

MAY 1995

Solar Powered Air Conditioning - Is it a Viable Option ?

Paul Cooper and Antonio Chan Department of Mechanical Engineering,

University of Wollongong, Northfields Ave, Wollongong, NSW 2522, Australia

ABSTRACT: Air conditioning of offices and homes in Australia consumes a significant proportion of our national primary energy and is a major source of ozone depleting gases. Currently conventional air conditioning systems use vapour-compression refrigeration plant which requires high grade electrical energy for power to cool the air in a building. However, there are a number of methods by which air conditioning can be powered using renewable energy sources and solar radiation in particular. This paper examines the viability of air conditioning systems powered by solar thermal energy with emphasis placed on the lithium­ bromide absorption refrigeration system which uses environmentally benign working fluids (i.e. water and lithium-bromide salt).

Here the principles of operation of a solar powered absorption system are described and details are provided of a system currently in place at the University of Wollongong. The current and future viability of the technology is briefly discussed in terms of both economics and environmental impact.

1 INTRODUCTION

There is a world wide recognition that global warming and ozone depletion are critical environmental problems. CFCs (chlorofluorocarbons) are destroying the ozone layer which protects life on Earth from damage by UV radiation. Both CFCs and carbon dioxide are held as contributing to global warming and climate change. One major source of emissions of both these pollutants is air conditioning systems. Conventional air conditioning systems use vapour compression refrigeration with CFC or HCFC working fluids to cool the air in a building. The mechanical energy required to drive the compressor is usually gained from electric motors and this perhaps is not the most appropriate use of our non-renewable primary energy resources. ·

One potential means of greatly reducing emissions of CFC's and HCFC's is the replacement of vapour compression air conditioning systems with systems based on the absorption refrigeration cycle. In the absorption cycle the conventional mechanical compressor is replaced by a "thermal compressor", thus, only a very small amount of mechanical energy is required to drive the system, the majority of the driving energy being provided by heat. The second advantage then of the absorption air conditioning system is that it can be driven by a solar thermal collection system eliminating the need for production of carbon dioxide in the course of the air conditioning process.

Solar powered air conditioning has been considered by many researchers and manufacturers in the past. Close (1978) and Basu and Cogger (1985) have reviewed the options available In Australia research projects have been carried out various groups

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including those at the Universities of Queensland and of Western Australia (Langridge and McCorrnick, 1981 ). There are a number of possible ways in which solar energy may be harnessed to provide air conditioning. Perhaps the simplest method in concept is to driv t:. the electric motor of a conventional air conditioning system with photovoltaic cells While perfectly feasible m principle this system would be enormously expensive due to the larse area of solar cells required to dnve a relatively modest air conditionrng plant. e

A quite different approach is that where the refngeration system is driven by heat energy rather than electrical energy as in the case of the absorption refrigeration system. There are many different types of absorption refrigeration systems available. Many have been around for a long time such as the "kerosene fridge" or the "gas fridge". In air conditioning systems today the most widely used absorption system is the Lithium Brornide . Water system. This uses the most benign working media possible i.e. a salt and water!

We will briefly consider firstly how significant the production of CFC's, HCFC's and electricity consumption from air conditioning systems is at present. Secondly, a demonstration solar air conditioning plant at the University of Wollongong is described. Finally, a case study of the economics of solar powered air conditioning of an office building is considered.

Considerable attention has been given to our use of CFC's since the Montreal Protocol and efforts have been made to clearly quantify the amount of CFC's and other halocarbons that are used in various applications. In Australia in 1992 air conditioning and refrigeration accounted for the use of approximately 5900 tonnes of CFCs; 63% of the total national usage (IEAust, 1992). Since that time the refrigeration and air conditioning industry has been moving toward replacement of CFC's with HCFC's and HFC's with much less ozone damaging potential. However, HCFC's do still have a significant ozone depleting potential and they will also have to be phased out in the not too distant future. Indeed, some refrigeration applications such as water chillers for the cooling systems in large commercial buildings cannot as yet use HFC's. It is estimated that there are approximately 4000 of these units in high-rise buildings in Australia alone (IEAust, 1992). Much of the CFC emissions comes from maintenance operations on these systems when refrigerant may escape, accidentally or otherwise. Commercial absorption refrigeration machines which have been available for many years have the potential to replace the great majority of these conventional chillers resulting in reduced ozone depletion.

From the National Energy Survey for 1986-87 (Australian Bureau of Statistics, 1987) and the report on Australia's Environment (Australian Bureau of Statistics, 1992) it can be deduced that approximately I 0% of electricity consumption by both industry and domestic users is employed for air conditioning. This suggests that the total electricity usage for air conditioning throughout Australia is of the order of 40,000 TeraJoules. Thus. a much wider use of absorption air conditioning systems powered by heat from solar radiation, or other industrial sources which might otherwise be simply wasted, has the potential to greatly reduce the amount of carbon dioxide we release to the atmosphere in Australia.

2 ABSORPTION REFRIGERATION

Absorption refrigeration technology has been around for many years and has found application in the past in many industries, notably for food preservation. However, the introduction of CFC refrigerants some forty years ago resulted in a great improvement in the efficiency of vapour compression refrigeration systems. Absorption refrigeration systems were then displaced by this technology for most applications.

The absorption refngeration system comprises two media, the refngerant and the absorbent. There are many possible refrigerant-absorbent pairs. The most common refrigerant-absorbent pair for air conditioning applications is Lithium Bromide (a hygroscopic salt) and water A schematic of the basic absorption cycle is shown in figure 1. The cycle is

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essentially driven by heat energy which is added at high temperature to the generator. In the solar powered system hot water from the solar collectors is supplied to the generator. Cooling of the air from the room is effected by passing "chilled water" through the evaporator coil of the refrigerator to remove heat and then through a fan coil unit. As with any refrigeration system the energy from the cooled air and from the driving device (in this case the heat energy from the collectors) must be rejected to the environment through the condenser.

Cooling Water

Condenser

Separator

Solar Heated Hot Water

Evaporator

1 Chilled Water

Cooling Water

Generator Heat Exchanger Absorber

Figure 1. Schematic of the absorption refrigeration system.

The great environmental benefits of the solar powered absorption system are that there is a negligible energy use above the energy already available from the immediate environment and that a minimum of environmentally deleterious materials are used.

3 DEMONSTRATION SOLAR POWERED AIR CONDITIONING SYSTEM

A demonstration solar powered absorption air conditioning system has been constructed at the University of Wollongong. The purpose of this plant is firstly to publicise the potential benefits of this type of technology and, secondly, to provide a research facility for improvement in the design and operation of these systems. A schematic of the system is shown in Figure 2.

Twelve solar flat-plate collectors; each having an absorber area of 2m2, have been mounted on a laboratory roof. The heart of the system is a commercially manufacture absorption chiller designed for servicing the requirements of a domestic dwelling. The unit is a Yazaki WFCC-400S Lithium Bromide (LiBr-H20) hot water powered absorption chiller with a nominal cooling capacity of 4.7 kW. [This unit was previously used at the Solar Energy Research Centre at the University of Queensland and was kindly donated to the present research project in 1990]. A 350 litre hot water storage tank is used to maintain the temperature of the supply hot water to the chiller.

The system has only recently been installed and final commissioning is currently in progress. System performance will be monitored using a computer-based data acquisition system. Performance of the chiller itself has been checked and found to be close to the original manufacturer's specification. Since the unit had not been operational for some years this was a great relief as absorption refrigeration systems can be sensitive to inappropriate

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handling and shipment. The most important factor m maintainmg the efficienr-, of the absorption chiller is maintenance of the vacuum in the unit through removal of non­ condensable gases; that can quickly render an absorption refrigeration machine inoperable

The chiller can operate with generator hot water supply temperature . .., of between 75°c to 95c.c. As the unit is principally for demonstration and research purposes the collector array is somewhat undersized and auxiliary electrical heating is used to boost the ener£v collected by the solar array. The performance of absorption system is dorrunated by the operating temperature of generator. The higher the hot water operating temperature the higher the COP (coefficient of performance) One of the important research activities in the future will be to opnrruse the control S) stern for cooling water and generator wate­ temperatures and flows.

FEED&. EXPANSION TANK

AIR VENTS

SOLAR COLLECTORS

FAN COIL UNIT

AUXILIARY IJATER HEATER

12 kw'

350L HOT IJATER

STORAGE TANK

YA ZAKI ABSORPTION

CHILLER

CODLING TDIJER

Figure 2. Schematic of demonstration solar powered absorption air conditioning system at the University of Wollongong

4 ECONOMICS OF SOLAR POWERED ABSORPTION AIR CONDITIONING OF A COMMERCIAL BUILDING

Deterrrurung whether a new technology will be economically viable is always difficult task particularly as unit costs are often unrealistically high in the early development of the technology. From the outset it should be noted that the solar powered absorption air conditioning system is not presently a viable alternative to the conventional systems when viewed purely agamst economic criteria. Here we make a very simple estimate of the economics of installing a solar powered absorption system in a commercial office building with a water-cooled air conditioning system. It should be noted that this analysis does not attempt to optimise the configuration of the solar cooling system vis a vis economic viability but serves to give the reader an indication of the issues involved. The floor area of the office is ta.ken as 2700m2 Many of the cost estimates of construction, operation and fuel costs for such a building are taken from the very useful set of data in the Building Energy Manual (NSW Public Works, 1993)

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The data relating to design conditions, equipment performance, etc are listed in Table I. The base case of a conventional air conditioning system is taken as a typical water-cooled air conditioning system with a reciprocating chiller and cooling tower. The solar case represents a solar powered absorption air conditioning system with flat plate collectors, absorption chiller, hot water storage tank and auxiliary heater. The additional cost implications of the absorption system are the cost of the collectors, solar hot water circuit, the cost of the absorption chiller over and above that of a conventional chiller and the additional heat rejection capacity of cooling water tower. Currently very few (if any) absorption chillers are used in commercial buildings in Australia and are only economically viable if a source of suitable waste heat is available on site from an industrial process. These units must be imported from overseas and one of the smallest units available (400kW cooling capacity) costs of the order of $130,000 compared with just $60,000 for a conventional reciprocating chiller of the same capacity.

Table 1. Estimate of the present economic viability of solar powered absorption air conditioning based on current fuel and plant costs

Description Conventional Solar system Remarks system

Office area (m2) 2700 2700 Max. cooling load (kW) 400 400 Chiller Vapour LiBr-water

compression Absorption Average COP 2.2 0.58 Fuel cost p.a. $ $12,000 Assuming electricity 16¢/kWh Fuel cost p.a. $ $6,760 Assuming solar fraction of 0.34

(as Bong et al, 1987) gas as auxiliarv suonlv 1 ¢/MJ

Estimate of total cost of 350,000 Cost estimate provided by conventional air con. svstem Southern Air Conditioning Add. cost-solar collectors --- $800 Assume 5m2/kw (similar to per kW cooling Bong et al, 1987) costs by

Solarhart™ Add. cost of cooling tower --- $56 Double the capacity of cooling lper kW cooling tower reauired for solar Total additional cost per kW --- $856 cooling Add. cost for absorption --- $70,000 Prices by Trane™ Australia Pty chiller Tot. Additional cost for solar --- (856*400 + 70,000) air conditioning $412,400 Simple payback period 412,400/(12,000-6,760)

78 years!

The data in Table 1 clearly indicates that even a crude estimate shows the economic feasibility of solar powered absorption refrigeration to be very remote given the current pricing of equipment and fuels. However, this is not to say that this will always be the case in the future. It may only be a decade or so before the greenhouse effects on climate preclude the use of fossil fuels in many applications that are accepted now. Unit costs of both solar components and absorption refrigeration machines could dramatically fall under high volume manufacturing regimes.

Furthermore, conventional refrigeration machines face an uncertain future as our understanding of the importance of ozone depletion mechanisms increases and it is possible that there may have to be a complete shift away from the use of any type of halocarbon working fluids for refrigeration purposes.

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5 CONCLUSIONS

So 1s solar air conditioning a viable option? Of course the answer depends on the critena by which one measures viability.

Technically, solar thermal powered absorption air conditioning is a viable technolooy that has been proven in the past by several researchers using the technology on\ demonstration scale.

From the environmental point of view the lithium-bromide absorption solar air conditiorung system is not only viable but is greatly superior to conventional air conditioning systems. It not only uses renewable energy as a source of power but employs environmentally benign working fluids. As with any solar thermal system non-renewable auxiliary backup energy is used to maximise the economic viability of the system. However, use of the technology has the potential to substantially reduce greenhouse gas enussions and to eliminate the release of ozone depleting refrigeration working fluids.

Economically, however, this type of system is currently not viable when compared to the installation and operating costs of conventional systems. Nonetheless, it is essential that non-conventional energy systems continue to be developed and demonstrated to ensure that the community is aware of the non-polluting alternatives that exist and may be yet further developed in the future.

6 ACKNOWLEDGMENT

The authors wish to thank both the Environmental Research Institute and the Department of Mechanical Engineering of the University of Wollongong for financial and technical support of the solar air conditioning project. We would also like to thank the Department of Mechanical Engineering, University of Queensland for donation of the Yasaki chiller.

7 REFERENCES

Australian Bureau of Statistics (1987) National energy survey (energy consumption in industry, Australia) 1986-87.

Basu, R. N. and Cogger, L. L. (1985) Cooling by Solar Energy, Australian Refrigeration Air Conditioning and Heating, Feb. 1985, pp26-33.

Bong, T. Y., Ng, K. C. and Tay, A. 0. (1987) Performance of a solar-powered air conditioning system, Solar energy, Vol. 39, pp 173-182.

Close, D. J. (1978) Building Heating and Cooling Systems, Search, V 9, No. 4, pp138-143.

IEAust ( 1992) Mixed response to CFC phaseout, Engineers Australia, Journal of the Institution of Engineers Australia, Nov. 1992.

Langridge, D. and McCormick, P. G. (1981) Solar air conditioning using concentrating collectors, Proc. !SES meeting, Brighton, UK, pp 554-561.

NSW Pubhc Works (1993) Building Energy Manual.

Yazak.i water fired absorption chiller: Technical manual, Yazaki Corporation, 1975.

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Author index

Adorni-Braccessi, G. 449 Izmir, G. 417 Seidlich, B. 357 Alla, P. 249 Snashall, D. 437 Anselme, C. 261 James, K. 437 Sperling, K. 475 Audie, J.M. 261 Jeans, P.E. 23 Swarbrick, G.W. 311

Johnson, M.K. 339 Sweatman, A. 429 Beder, S. 119 Johnston, K. 385 Birkeland, J. 397 Taylor, J. 273 Bishop, R.J. 311 Kanako, M.K. 373 Thomas, C. 175

Katsof, E. 33 Thwaites, R. 221 Caines, G. 383 Keating, P. 3 Trainer, T. 87 Cairnes, L. 165 Turner, A. 93 Caswell,T. 435 Laird, P.G. 449 Turpin, T. 141 Chan, A. 325 Lambropoulos, N. 311 Clarke, B. 311 Lawson, B. 313 Usback, R.G. 339 Clarke, G. 363 Lefevre, F. 261 Cooper, P. 325 Lofthouse, A. 385 van der Broek, B. 311 Craik, W. 127 Loken, S. 43 Verhey, R. 199 Cullen, P. 7 Lovins, A. 45

Wadhwa, L.C. 103 Denlay, J. 303 McCotter, B. 437 Watkinson, P. 221 Deville, A. 141 McDonald, R. 193 Wescott, W. 201 Dunstan, C. 331 McKelvey, M. 243 Whitaker, 0. 381

McLaren,N. 423 White, S. 279 Evans, R. 383 Winterbottom, D. 207

Mandra, V. 261 Woods, P. 113 Falk, J. 13 Mills, T. 391 Ferry,B. 423 Mowbray, P. 461 Figgis, P.J. 231 Munro, D.A. 49 Fritz, S. 251 Fry, T. 403 Newman, P. 59

Galloway, D. 213 Okraglik, H. 77 Gayler, D. 411 Gertsakis, J. 77 Partridge, H. 313 Goldie, J. 153 Greene, D. 159 Radovic, D. 467

Rathur, A.Q. 287 Haggen, K. 311 Reeve, D. 81 Hales, R. 319 Reynolds, C.W. 339 Hall, C. 165 Rismiller, P. 243 Hart, K. 311 Rolls, E. 485 Hindmarsh, M. 257 Rosier, P. 349 Hunt, C. 183 Rudolph, V. 311

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