Solar-powered Rankine system for domestic applications

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Pergamon SOLAR-POWERED

Applied Thermal Engineering Vol. 16, No. 4, pp. 281-289, 1996 Copyright © 1996 Elsevier ScienceLtd Printed in Great Britain. All rights reserved 1359-4311/96 $15,00 + 0.00

1359-4311(95)00032-1

RANKINE

SYSTEM

FOR

DOMESTIC

APPLICATIONS J. L. Wolpert and S. B. Riffat Institute of Building Technology, Department of Architecture and Building Technology, University of Nottingham, University Park, Nottingham NG7 2RD, UK (Receit~ed 8 August 1995)

Abstraet--A new environmentally friendly device for solar-powered electricity generation and storage is described in this paper. Emphasis is given to a computer model which calculates the optimum features of the electricity-generating phase of the system; these features include working fluid efficiencies, solar collector size and efficiency and power output. Weather conditions for the UK and Mexico City are analysed and computer simulations are made using reliable updated meteorological data for both latitudes. Results show that, for an annual power output of 4000 kW.h, in the UK, the collector area required by the system proposed would be 92 m:; whereas in Mexico, the area of collector required for the same output would be 48 m 2 under similar simulation conditions. A small increase in the efficiency of the turbine, feed pump and generator would bring about a significant reduction of the collector area required. Keywords--Rankiue cycle; HFC refrigerants; solar energy; electrolysis; absorber.

NOMENCLATURE A asi aec Fr G L0 1D~ /do L PO Q5-2 /"1 T4 T~ U W

area of collector, m 2 apparent solar irradiation, W/m 2 atmospheric extinction coefficient, dimensionless empirically determined correction factor, dimensionless incident of solar radiation, Wh/m 2 total solar irradiation on surface, W/m-" direct radiation from sun, W/m 2 diffuse radiation component from sky, W/m 2 short-wave radiation reflected from other surfaces, W/m: power output, kWh/m-' heat supplied, kJ/kg temperature of inlet fluid to absorber, K source temperature, K ambient temperature, K overall heat-transfer coefficient, W/m2K actual work performed, kJ/kg

Greek letters absorptivity of absorber plate, dimensionless B solar altitude angle, degrees ? solar-surface azimuth angle, degrees 0 incident angle, degrees efficiency of collector, % rlo Z tilt angle, degrees T¢ transmittance of collector's cover plate, dimensionless surface azimuth angle, degrees O~a

INTRODUCTION

This paper investigates the feasibility of a new device [1] capable of generating power by use of solar energy to satisfy the electricity demand of a dwelling. The new device could use solar heat to drive a Rankine system and the electricity surplus could be stored in the form of hydrogen using the electrolysis of water. When a shortfall of electricity occurs or when little or no solar energy is available (i.e. at night or winter daytime), hydrogen could be converted back into electricity via a fuel cell. A gas burner could be used as a backup source of heat. This would allow the system 281

282

J.L. Wolpert and S. B. Riffat

to generate electricity at all times without necessitating a storage system and also allow a smaller solar collector to be used, reducing the cost of manufacture of the system. Exigting large-scale plants for power generation consume vast quantities of fossil fuels and dissipate pollutants into the atmosphere. Solar-powered Rankine and Stirling cycle engines were first studied in 1874 and offer an alternative means of electricity generation. Although these were initially uncompetitive on economic grounds, diminishing reserves of fossil fuels and environmental concern over pollutants released by their combustion have encouraged renewed interest in solar energy. In addition to solar thermal energy conversion, solar cells may also employ solar photo-voltaic conversion. Although this technology developed slowly due to its low efficiency, it is currently the most actively investigated method of solar energy conversion [2]. The CHP system proposed in this paper uses renewable energy and an ozone-friendly working fluid to generate electrical power; its widespread deployment would reduce the load on existing powerplants and so reduce pollutant emission. It also provides some by-products, such as hot water and pure drinking water, which would allow low-grade water to be piped to the building for washing, etc. In certain circumstances it may be even possible to eliminate water supplies altogether by capturing rain water. Hot water produced by the system would also enable the building to forgo a supply of fossil fuels for heating purposes. Storage of electricity in the form of hydrogen could also help reduce energy losses involved in electricity transportation via the national grid. DESCRIPTION OF THE SYSTEM Figure 1 shows the solar-powered Rankine system as described in this paper. Heat obtained from an evacuated heat-pipe solar collector is used to increase the temperature of the liquid contained in heating chamber 1 so that the pressure increases in reservoir 2. In countries where little solar energy is available, such as the UK, the system could be assisted with a gas burner backup to generate electricity as required. The pressure build-up in reservoir 2 causes the vaporised working fluid 3 to drive a turbine where the Rankine cycle 4 begins. The turbine is connected to electrical generator 5. Lower-pressure vapour is then released from the system into the condenser 6 where it is condensed into liquid 7 and pumped by the feed pump 8 back into the heating chamber I. Only environmentally safe refrigerants will be used to run the Rankine system. A storage system would be implemented in places where solar energy produces more energy than required during the hours of sunshine. When the voltage output of the electricity generator is above a certain predetermined or calculated threshold established by controller 9 (i.e. electricity load requirements of the dwelling are below the power currently being generated), the surplus power is channelled via a relay to the electrolysis plant 10, where water is electrolysed. The hydrogen resulting from this process is stored in a hydrogen tank. When the output voltage generator falls below a predetermined threshold, and at night when solar energy is unavailable, then controller 9 opens a valve allowing hydrogen into fuel cell 11. Fuel cell 11 enables a voltage regulator to compensate for the shortfall in the available solar heat and so maintains the power output of generator 5. A fuel cell is an electrochemical device that produces electricity silently and without combustion. Hydrogen fuel and oxygen from the air are combined in a fuel cell to produce electricity, heat and water. This is the reverse of the process of electrolysis. As there is no combustion, fuel cells produce minimal pollutants. Hot water may be obtained from the Rankine system and pure drinking water from the electrolysis plant. W O R K I N G FLUIDS AND ENVIRONMENTAL CONSIDERATIONS Environmentally friendly refrigerants are to be used as working fluids in the Rankine cycle and ozone-friendly refrigerants such as R134a, R152a, Klea32 and Care30 have been analysed. Their thermophysical saturation properties were fed into a computer program designed specifically to obtain their Rankine efficiencies and use them to calculate the power output, the solar collector's efficiency and the area of collector required to generate a predetermined electrical demand along with updated meteorological data. This enables the best refrigerant for the Rankine cycle to be

Domesticsolar-poweredRankinesystem Solar

Energy

///,1,

Heat Pipe Solar Collector

283

~ R e sRoe s e r v o i r

I~

( 2):

] I

High Pressure Vapour (3)

Feed

Pump~ ( 8 )(

Outlet

~ ~ j j Water Inlet

Vapour Low Pressure

Engine

mmlm

) Condenser (6)

Load (12)

Electrical

Generator (5)

02 S,or.

(Fuol) c'" /

H2

Store

02

Fig. 1. Solar-poweredRankinecyclefor domesticapplications. determined. Figure 2 shows the ozone depletion potential versus global warming potential for various refrigerants. New refrigerants must not contribute to direct or indirect global warming and candidates such as R134a, and blends using Klea32 and R134a with a possible addition of R125, may be the first members of a range of environmentally acceptable HFC refrigerants which are non- flammable and have a high refrigeration capacity [3]. R152a is completely ozone safe and has a low global warming potential compared with CO2. Although it is flammable (flammability limits are 4.2-20.4% by volume in air), some basic safety precautions would enable its safe use. A non-flammable mixture of R134a and R152a has optimum performance when blended in the ratio: 86.2%/13.8%, respectively. It shows an approximate increase in the coefficient of performance of 2.7% over R134a. The addition of R152a to R134a provides a slight improvement in the overall evaporator heat-transfer coefficient [4]. The refrigerant Care 30, is a product containing an optimised blend of hydrocarbons, essentially propane (R290) and iso-butane (R600a). Its flammability limits are a lower explosive limit in air

J. L. Wolpert and S. B. Riffat

284

R12

.j z~O°

o

R 123

|

R 124

U

Ill

UJE

Cam 30 O

1

R 1S2a

Klea 32 R 134a

R 125

ra ~J

0

N

[]

10

100 1000 1000o GLOBAL WARMING POTENTIAL RELATIVE TO CO 2 ON A 500 YEAR BASIS

Fig. 2. Ozone depletion potential versus global warming potential for various refrigerants.

of 2.0 vol% and an upper explosive limit of 9.1 vol%. Most gases are flammable under certain conditions and the behaviour of hydrocarbons is well known and understood [5]. All the refrigerants analysed in this paper meet current environmental standards. As Fig. 2 shows, their ozone impact and global warming potential are negligible compared with those of the known fluorocarbons in current use. Some of them are still being tested and their saturated properties are not yet available, COMPUTER MODELLING There are many variables that affect directly or indirectly the system described. To calculate its optimum performance, a computer model was designed where all these variables were taken into consideration. Figure 3 shows the flowchart followed in the modelling. The program was primed with the thermophysical properties of the refrigerants chosen. The computer model includes data for the refrigerants from - 8 . 1 5 ° C up to a temperature close to their critical point using 5°C increments. For the purpose of this paper, different source temperatures were assumed to run the program. Higher temperature ranges for calculations were considered as follows: T~ for T~ for T, for T, for

R134a was measured from 41.85°C up to 81.85°C. R152a was measured from 41.85°C up to 91.85°C. Klea32 was measured from 41.85°C up to 76.85°C. Care30 was measured from 46.85°C up to 87.85°C.

Thermophysical properties for R134a and R152a used in this paper were calculated by means of developing a simple correlation representing reliable experimental data [6]. Data for Klea32 were obtained from ICI published charts [7] and the properties used for Care 30 were taken from recently published data sheets. The model uses these properties at the temperatures indicated to calculate enthalpy at different states of the Rankine cycle. Figure 4 shows a pressure--enthalpy diagram where the cycle is simulated. The real Rankine efficiency is then calculated with equation (1): ~oyc~o = W / Q s -

2.

(1)

Domestic solar-powered Rankine system

285

Thermophysical properties for Refrigerants : R134a, R152a, Klea32 and Care30

I

J Input working fluid for simulation,,,J

I

.,,

I InputT1 and T4

ICycle Efficiency

Input: date, hour, month, latitude, weather data and assumed conditions.

T1 + 5K

I

i co,,c,o,sI efficiency and

power output at

T1

I

Optimisedarea of collector, 2

Ill

)

Results

I

Fig. 3. Simulation flowchart.

Solar geometry calculations are then made given a particular date and orientation of the collector. Geographical and meteorological information such as latitude, aec, asi, hour angle, angle, Ido and L, were input to the program for calculation of the total solar irradiation at the surface of the collector from the following equations: sin fl = cos L cos H A cos 6 + sin L sin 6

(2)

d = 23.47 sin (360*(284 + N))/365,

(3)

where N = day of the year numbered from 1 January. cos 0 = cos fl cos 7 sin _r + sin fl cos S,

(4)

where the value of Z, which is the tilt angle of the collector, is automatically determined as S = 90 -

ft.

(5)

The model assumes that the inclination angle of the collector is perpendicular to the sun's rays at the date and hour specified. The direct solar radiation is calculated with equation (4): IDN

=

asi/exp.(aec/sin fl).

(6)

286

J . L . W o l p e r t a n d S. B. Riffat

temperature critic alp oint

saturated vspou~ me

satmated liquid line

specific entropy Fig. 4. R a n k i n e cycle s i m u l a t e d in a t e m p e r a t u r e - e n t r o p y d i a g r a m .

Taking the results from equations (2)-(6) and using input data, the total irradiation can be calculated as /i0 = IDN COS 0 + Ido + L.

(7)

Values for transmittance, ambient temperature and overall heat-transfer coefficient must be input. The program uses them along with the value obtained from equation (7) to calculate the efficiency of the collector accordingly to the following equation: r/o = {zeta - [(TI - -

r~)U]/IioFr},

(8)

Fr is assumed to be equal to 0.90; ~a is equal to 0.90 as the absorber plate is assumed to be made of aluminium. With the values obtained from equations (1) and (8) the power output is then calculated as follows:

PO = {r/o - U/G([(T, + r,)/2] - ro~)} x G x r/cy¢l,.

(9)

The area of collector is determined using the value obtained from equation (9): A = electrical annual demand/(PO x generator efficiency).

(10)

Electrical demand and all efficiencies have to be input to the program when requested. When the routine is completed, the program takes T~ to be 5°C higher than the previous input and follows the procedures as indicated above. It continues to loop as long as the area of collector Table 1. Mexico's weather and assumed data for simulation Latitude Hour angle Date Surface azimuth Ambient temperature Apparent solar irradiation Atmospheric extinction coefficient Global irradiance, G value Overall heat-transfer coefficient, U value Diffuse radiation component from sky Short-wave radiation reflected from other surfaces Turbine efficiency Feed pump efficiency Generator efficiency

19.4° north 0 ° (noon) 15 June 0 ° (collector facing south) 16.3°C 1093,2 W/m 2 0.20 405 Wh/m 2 1.6 W/m2K 273 Wh/m 2 30 W/m 2 80% 80% 70%

Domestic solar-powered Rankine system

287

Table 2. UK weatherand assumeddata for simulation 53.48° north 0° (noon) 15 June 0° (collectorfacingsouth) 9.6°C 1093.2 W/m2 0.20 (average from 1 January 1994at 00:00hr, to 31 December1994at 24:00hr) 219.05 Wh/m~ 1.6 W/m2K (average from 1 January 1994at 00:00hr, to 31 December1994at 24:00hr) 142.83 Wh/m2 30 W/m2 80% 80% 70%

Latitude Hour angle Date Surface azimuth Ambient temperature Apparent solar irradiation Atmospheric extinction coefficient Global irradiance, G value Overall beat-transfer coefficient, U value Diffuse radiation component from sky Short-wave radiation reflected from other surfaces Turbine efficiency Feed pump efficiency Generator efficiency

gets smaller and stops when the higher temperature is equal to the highest for which thermophysical properties are recorded. In this way, the optimum meteorological conditions, highest achievable performance and smallest collector for the system described can be obtained so that decisions can be made concerning the supplementing of solar energy with other heat sources. SIMULATION CONDITIONS In this paper both calculations and computer simulations were carried out using data for the refrigerants analysed and meteorological information for Mexico City, Mexico, and Finningley, England. Weather data for Finningley, including the hourly global and diffuse radiation and the mean air temperature were obtained from meteorological station 4238. The global and diffuse radiation figures used in the simulations represent an average of the actual data measured at the meteorological station between January 1990 and December 1994. Mexico's weather data were obtained from several sources. The global solar radiation was obtained from a publication by the National A u t o n o m o u s University of Mexico (U.N.A.M.) [8] and the diffuse radiation was calculated from cloudy and clear day data for a similar latitude issued by the CIBSE guide [9] in the UK; temperature means were obtained from the Institute of Engineering of the U.N.A.M. Tables 1 and 2 show geographical and weather conditions and assumed data used for simulations in Mexico and the UK. Figure 5 shows the variation of Rankine efficiencies obtained for various refrigerants tested, assuming a turbine and feed pump efficiency of 80°/'0, against different sink temperatures T4; in all cases, the higher temperature, T~, is the highest recorded for each working fluid. For all simulations, the date assumed to calculate the solar altitude is 15 June. The solar time was assumed to be 12:00 hr for the calculation of the hour angle. The electrical 16 >~ 14

u ¢:

.2 o ul

---

0 J¢ ¢: ¢B tr

10 8

•

R134a (T1 = 81.85 °C) 0 R152a (T1--91.85 C)

-•

K]ea32 (T1 =76.85 °C) o Care30 (T1=87.85 C)

o

6

i

0

1I0 210 30 Sink Temperature T , ° C 4

410

50

Fig. 5. Variation of Rankine efficiencywith sink temperature.

288

J. L. Wolpert and S. B. Riffat

8O

%

70 8o ~

S

5o 40 10

20

30

Sink Temperature

----

care30

•

r152a 40

50

T 4 , eC

Fig. 6. Variation of area of collector against sink temperature with Mexico's weather data.

demand input for simulations was 4000 k/W.h in a 1-year period. Simulation conditions were assumed equal for both latitudes, except for meteorological information, which was obtained from reliable sources. Simulations were also carried out, increasing the efficiency of the turbine, feed pump and generator by up to 5%. RESULTS AND DISCUSSION The efficiency of the collector obtained from simulations was, in all cases, between 63 and 64%. Figure 6 shows the variation of the area of collector obtained for different sink temperatures with Mexico's weather data. Figure 7 shows the variation of the area of collector against sink temperature for UK weather data. In all cases, the optimum performance of the working fluid was achieved at the highest T~ for which data was recorded in the program and at the smallest T4 input. Refrigerants R152a and Care30 produced the best performance and simulations were carried out for these working fluids. Using R152a, the system located in the UK would be able to generate 4000 kW.h per year with a collector area of 92 m 2 when the sink temperature is 16°C; and 134 m E if the sink temperature is 41°C. Use of Care 30 as the refrigerant would require a 12% increase of the area of collector. In Mexico, the system proved to be substantially more efficient due to the 160 150 ~,

140 130 120

=

11o 100

care30

9O 10

20

30

Sink Temperature

40

5O

T 4 , @C

Fig. 7. Variation of area of collector against sink temperature with U K weather data.

Domestic solar-powered Rankine system

289

greater solar energy available and better geographical and weather conditions. The area required to meet the same electrical demand varied from 47 m s when the sink temperature was 16°C to 79 m s when the sink temperature was 41°C. In every case the higher temperature assumed was the highest for which data are recorded. It may be possible for the refrigerants studied to work at higher temperatures than the ones for which thermodynamic data are available. If hot water is to be obtained directly from the system, higher sink temperatures would be expected. This would necessitate larger collector areas, which would be comparable with those required by photo-voltaic energy conversion systems to generate electricity alone. Results show that if the efficiencies of the turbine, feed pump and generator are increased by 5%, the collector areas required could be reduced by 13%. Tables 1 and 2 show that the turbine and feed pump efficiencies assumed for the simulations are 80%; and the generator efficiency assumed is 70%. CONCLUSIONS Theoretical analysis of the C H P system for electricity generation described in this paper has shown that the system is much more efficient when used at lower latitudes. In Mexico humidity is lower than in the UK, which might reduce the need for evacuated heat-pipe solar collectors for the system. A simple fiat plate could perform as efficiently if the glass covering has an overall heat transfer coefficient of 1.6 W/m2K. This would reduce the cost of the system significantly. In the U K there are options to optimise the system's performance. Alternative heat sources need to be studied. Refrigerant Care30 is the recommended working fluid for the Rankine system and its environmental impact is negligible compared with the other refrigerants analysed in this paper. R152a required a smaller area of collector to satisfy the electrical demand, The flammability issue needs careful consideration as R152a may prove to be useful for the system if certain safety measures are taken. It is important to build a small prototype to continue the development of this new device and test it using alternative energy sources. The storage system would be economically justified and of major importance at lower latitudes. Acknowledgements--The authors wish to thank Mr F. Best for his technical advice and the Institute of Engineering of the

U.N.A.M. for providing data for simulations. REFERENCES 1. Energy Supply System, UK Patent International Publication No. W092/21861, December 1992. 2. C. Haddock and J. S. C. Mckee, Solar energy collection, concentration, and thermal conversion--a review. Energy Sources 13, 461-482 3. S. Corr, F. T. Murphy, B. E. Gilbert and R. W. Yost, Characteristics of refrigerant-lubricant mixtures containing R32 and R32 blends. A S H R A E Trans. Syrup. DE-93-20-1, pp. 1123-1128. 4. J. W. Linton, W. K. Snelson, P. F. Hearty and A. R. Triebe, Systemperformance of near azeotropic mixtures of R134a and R152a. A S H R A E Trans. Research DE-93-9-1(3732), pp. 400-405. 5. Calor Gas Refrigeration. The Care range. Calor TJR(01)-06/94. 6. A. Kamei, C. C. Piao, H. Sato and K. Watanabe, Thermodynamiccharts and cycle performance of FC134a and FCI52a. ASHR,4E Trans. 96(1), 141-149 (1990). 7. ICI KLEA. Thermodynamic Property Data for Klea32. November 1994. 8. R. Almanza, V. Estrada-Cajigal and J. Barrientos, Actualizaci6n de los mapas de irradiaci6n solar en la Rep6blica Mexicana. Series del Instituto de Ingenieria, No. 543 (1992). 9. The Chartered Institution of Building Services Engineers, The Chartered Institution of Building Services Engineers, 5th Edition. Staples Printers St Albans Ltd, A2.84. (1988).