Strategies for solar heating systems

Strategies for solar heating systems

Solar & Wind Technology Vol. 5, No. l, pp. 83-91,1988 Printed in Great Britain. 0741-983X/88 $3.00+.00 Pergamon Journals Ltd. STRATEGIES FOR SOLAR H...

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Solar & Wind Technology Vol. 5, No. l, pp. 83-91,1988 Printed in Great Britain.

0741-983X/88 $3.00+.00 Pergamon Journals Ltd.

STRATEGIES FOR SOLAR HEATING SYSTEMS S. M. HABALI, M. A. S. HAMDAN, B. A. JUBRAN a n d ADNAN I. O. ZAID Department of Mechanical Engineering, Faculty of Engineering and Technology, University of Jordan, Amman, Jordan (Received 20 November 1986; accepted 19 June 1987)

Almtract--A computer program simulation based on the F-chart was carried out to study five active solar heating systems and two passive heating systems in Jordan. Cost analysis for all systems were carried out over 20 years. The general heating system tends to have the best life cycle saving. The analysis indicated that heating a swimming pool using solar energy is very promising, on average 30% of the yearly heating load for a swimming pool is met by solar energy. The passive heating systems analysis indicated that 60% of the heating load may be met by solar energy if passive solar houses were designed. 1. INTRODUCTION

relations by Ward [8], who has used only January results, by Barley and Winn [9] who used a two points curve fit to obtain dependent annual results, and by Lameiro and Bendt [10] who also obtained dependent annual results with three points curve fits. The other approach which comes under the second group of designing methods is by Balcomb and Hedstrom [11], which correlates the output simulations for specific systems and two collector types. The third group of designing methods are based on short cut simulations. Representative days of meteorological data were used to relate long term performance. This paper reports the performance results for seven heating systems based on the two year meteorological data of Amman (1983, 1984), using a computer simulation based on the F-chart method. The meteorological data were obtained from the Royal Scientific Society. The analysis was carried out for a fiat with a UA value of 275 W °C- 1. The area of the collectors for all active heating system is 52 m 2.

Heating in buildings may be provided from solar energy by systems that are similar in many respects to the conventional heating system. F o r the last 20 years there has been an increasing interest in solar heating. Telkes and Raymond [1] reported the solar house that was constructed at Dover, Massachusetts. The system was designed to carry the design heating load for 5 days. George Lof [2] constructed the first solar house to use active air collectors, in which air instead of water, was circulated through a solar collector and the hot air was used to heat the house directly or was directed to a bed of pebbles that served as a thermal storage medium. Close et al. [3] described a heating system used for a partial heating of a laboratory building that has been in operation for many years. Design methods for solar thermal process may be classified into three major groups depending on the assumption used in the calculation. From collectors with known operating temperature, comes the utilizability method, which is based on the analysis of hourly weather data to obtain the fraction of the total month's radiation that is above a critical level [4]. Proctor [5] developed the heat table method which is the tabulation of integrated collector performance as a function of collector characteristic, location and orientation, assuming fixed fluid inlet temperature. The second group for designing heating system includes correlations of the results of a large number of detailed simulations. A widely used method based on this technique is the F-chart method of Klein et al. [6, 7]. The results of many simulations are correlated in terms of easily calculated dimensionless variables. The F-chart method has been used for further cor-

2. DESCRIPTION OF SYSTEMS 2.1 Active heating systems (1) A common arranyement o f a solar heating system usiny a pebble bed is shown in Fig. 1. Air is heated in the flat plate or evacuated tubular collectors and circulated to either the house or the pebble bed. Energy is stored in the pebble bed when hot air circulates over the pebbles. In the absence of the sun, the solar energy is not enough to meet the heating load directly, however the energy in the pebble bed storage system is used together with the auxiliary energy from a boiler or immersion heater. The specifications of the system used are shown in Table 1. The performance 83

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Fig. I. Solar heating system using a pebble bed storage.

of the above system was predicted using the F-chart method by Beckman et al. [12]. (2) Water storage space and domestic water heating system. Energy is stored in the form of sensible heat in the water tank, Fig. 2. A water to air load heat exchanger is used to transfer heat from the storage tank to the building. A liquid to liquid heat exchanger is used to transfer energy from the solar heated water to a conventional water heater. The specification for this system is the same as in Table 1 except that the

Table 1 Parameters

Specifications

Volume of pebble bed storage Building UA Fuel Efficiency of fuel usage Daily hot water usage Water set temperature Environment temperature DHW storage tank size UA of Aux. storage tank Inlet duct UA Outlet duct UA Collector panel areas Collector slope

12.5 m ~ 275 W °C Electricity/diesel 90/70% 300 1 60 °C 20 °C 300 1 10 W ~'C10 W °C 10 W °C52 m 2 45 ~

water storage tank volume is 3750 1 and the heat exchanger effectiveness is five. (3) Active collection with building storage. This type of heating system is very simple, and does not have a separate storage component, Fig. 3. The useful energy gain of the fluid is transferred either directly when air is used, or via a heat exchanger when liquids are used, to the building space. In this type of heating system when the solar contribution is less than the instantaneous load, all of the energy entering the space at this time is useful in offsetting auxiliary energy use. During periods when there is a high solar gain the temperature of the building will increase, which will activate the collector pump to turn off. The building storage capacity is 23.5 MJ ° C ~and the daily internal generation is 10.7 k W h d a y - ~. (4) General solar heating system. This type of system represents the general classes of closed and openloop solar energy systems which can be used for a variety of applications including space heating absorption air conditioning, water heating and process heating. Solar energy is usually collected by flat plate collectors and stored as sensible heat in a liquid storage tank, Fig. 4. (5) Swimming pool heating system. The system used here for collecting the solar energy is the same as that used for the general heating system. In this simulation the temperature of the swimming pool is kept fixed at

85

Strategies for solar heating systems

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Fig. 4. General solar heating system.

S. M. HABAL1 et al.

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Table 2 Water storage system flat plate collector

Pebble bed storage system flat plate collector

Jan. Feb, Mar. Apr. May Jun. Jul. Aug. Sep Oct. Nov. Dec. Year

Solar GJ 20.7 18,3 28.2 29.0 32.0 33.4 34.8 35.9 35.0 43.6 24.2 24.5 359.5

Heat GJ 11.0 8,8 7.5 4.3 1.6 0.9 0.2 0.3 0.5 2.3 4.6 6.5 48.6

DHW GJ 2.2 2,0 2.2 2.2 2.2 2.1 2.2 2.2 2.1 2.2 2.2 2.2 26.2

Aux GJ 4.7 3,5 0.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.4 9.1

F 0.~ 0,67 0.97 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.98 0.96 0.88

Solar GJ 20.7 18.3 28.2 29.0 32.0 33.4 34.8 35.9 35.0 43.6 24.2 24.5 359.5

Jan. Feb, Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. Year

Load GJ 10.6 8.5 7.1 4.1 1.6 0.9 0.2 0.3 0.5 2.2 4.4 6.2 46.6

Aux. GJ 5.3 3.9 1.9 9.0 0.0 0.0 0.0 0.0 0.0 0.0 0.4 1.7 13.2

F 0.50 0.54 0.73 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0,91 0.73 0.72

Solar GJ 20.7 18.3 28.2 29.0 32.0 33.4 34.8 35.9 35.0 43.6 24.2 24.5 359.5

Jan. Feb. Mar. Apr. Oct. Nov. Dec. Year

Qpool GJ 12.4 12.8 22.0 26.1 27.3 14.4 13.4 128.4

Load GJ 78.7 64.9 57.l 38.8 28.9 54.1 62.8 385.2

Aux. GJ 69.3 56.6 42.9 23.4 2.3 40.9 49.8 285.1

Aux GJ 5.5 4.3 0.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.2 1.1 11.9

F 0.58 0.61 0.91 1.00 1.00 1.00 l.O0 1.00 1.00 1.00 0.97 0.88 0.84

Load GJ 15.5 14.0 15.5 15.0 15.5 15.0 15.5 15.5 15.0 15.5 15.0 15.5 182,5

Qtank GJ 0.67 0.60 0.70 0.68 0.71 0.70 0.71 0.74 0.72 0.77 0.67 0.69 8.38

Aux. GJ 11.2 10.3 8.2 7.1 6.3 5.2 4.4 3.0 2.7 0.0 8,2 8.8 75.6

F 0.28 0.26 0.47 0,53 0.59 0.65 0.72 0.81 0.82 1.00 0,45 0.43 0.59

Pool heating system evacuated tubular collector

Pool heating system fiat plate collector Qcoll. GJ 9.4 8.3 14.2 15.3 26.6 13.2 13.0 100.0

DHW GJ 2.2 2.0 2.2 2,2 2.2 2.1 2.2 2.2 2.1 2.2 2.2 2.2 26.2

General solar heating system fiat plate collector

Active-building storage system flat plate collector Solar GJ 20.7 18.3 28.2 29.0 32.0 33.4 34.8 35.9 35.0 43.6 24.2 24.5 359.5

Heat GJ 11.0 8.8 7.5 4.3 1.6 0.9 0.2 0.3 0.5 2.3 4.6 6.5 48.6

F 0.12 0.13 0.25 0.40 0.92 0.24 0.21 0.26

28°C, The heating o f the pool starts in O c t o b e r and ends in April. 2.2 Passive heating system (1) Passive direct-gain system. In a direct-gain structure, solar energy is transmitted t h r o u g h a southfacing glazing. The solar energy collected by such a system is used to provide the daytime heat require-

Qcoll. GJ 10.1 8.8 14.2 14.8 24.0 12.7 12.8 97.4

Qpool GJ 12.4 12.8 22,0 26.1 27.3 14.4 13,4 128.4

Load GJ 78.7 64.9 57.1 38.8 28.9 54.1 62.8 385.2

Aux. GJ 68.6 56.1 42.9 24.0 4.9 41.4 50.0 287.8

F 0.13 0.14 0.25 0.38 0.83 0.24 0.20 0.25

ments o f the building, and the excess is stored in the form o f sensible heat in massive elements o f the building. In this simulation a double glazing w i n d o w at 90 ° is used, the total area o f the w i n d o w is 23 m 2. The m e t h o d used for predicting the p e r f o r m a n c e o f this system is that o f M o n s o n et al. [13]. (2) Passive storage wall system. In this type o f system the material which serves as thermal storage

Strategies for solar heating systems

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Table 3 ~ivedir~tg~nsystem Solar GJ 8.1 6.4 8.6 7.3 6.4 5.7 6.3 7.9 9.8 15.2 9.4 9.9 100.9

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. Year

Lo~ GJ 10.2 8.2 6.9 4.0 1.5 0.8 0.2 0.2 0.4 2.1 4.3 6.0 ~.8

~ssivestor~ewaU

A~. GJ 5.7 4.5 2.8 0.9 0.0 0.0 0.0 0.0 0.0 0.0 0.9 2.1 16.8

F

Solar GJ 8.9 7.1 9.5 8.0 7.1 6.3 6.9 8.7 10.8 16.8 10.3 11.0 111.4

0.~ 0.45 0.60 0.77 1.00 1.00 1.~ 1.~ 1.~ 1.00 0.79 0.65 0.62

in the building is placed immediately behind the south facing glazing. The wall area is 26 m 2 with thickness 0.46 m and the thermal conductivity is 1.73 W m - l oC. The performance of this system was predicted using the Balcomb and McFarland [14] method for evaluating the performance of the passive storage wall system.

Load GJ 7.3 5.9 4.9 2.7 1.0 0.5 0.1 0.1 0.3 1.4 2.9 4.2 31.3

Aux. GJ 4.7 3.8 2.4 0.9 0.0 0.0 0.0 0.0 0.0 0.0 0.4 1.3 13.3

F 0.37 0.35 0.51 0.68 1.00 1.~ 1.~ 1.00 1.~ 1.00 0.88 0.70 0.57

and evacuated tubular collectors over a year. The results of the yearly fraction solar energy for different heating systems using fiat plate and evacuated types is shown in Fig. 8. It can be seen from Fig. 5, that for all heating systems, except the general one, the fraction of solar energy from April through to October is one. This means all the heating is met by solar energy. During the months of January to March, the pebble bed storage heating system covers 64% of the heating load. During this period 36% of the heating load is to be supplied by an auxiliary heating system. The water storage heating system is the second best with the fraction solar energy in the range 58-91% and the fraction of the energy from the auxiliary heating sys-

3. RESULTS AND DISCUSSION 3.1 Solar energy results Seven types of solar heating systems are evaluated using the F-chart method. Figures 5, 6 and 7 show the monthly fraction (F) of the space and water heating load which is supplied by solar energy using flat plate

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tem is in the range 42-9%. The least efficient heating system with solar energy fraction of 28-47%, is the general heating system. Another useful way of utilizing solar heating energy is to use it for heating swimming pools. The results in Fig. 6 indicate that a swimming pool of 50 m 2 can utilize 12-90% of the solar energy over the period October to April. Figure 6 also indicates that the fraction of solar heating is independent of the type of collector used in this analysis. The results for heating systems using evacuated tubular type collectors are shown in Fig. 7. Generally the evacuated tubular collectors give a better solar energy fraction especially when it is used with the general solar heating system. There is an increase in

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the solar fraction of 15-20%. The effect is not significant for the other systems. The results for two types of passive heating system are shown in Fig. 9. It can be seen that the passive direct gain heating system gives a higher solar energy fraction than the passive storage wall system. During the period from January through to April the values of the solar fraction are in the range of 44-77%. For the months of November and December the fractions of solar energy are 89% and 70% respectively. The results obtained show that on the average during the months of winter the passive direct gain tends to give a solar fraction which is 5% more than the passive storage wall system. The yearly fraction of solar energy together with

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the yearly fraction of auxiliary energy for the seven systems are shown in Fig. 8. The results indicate that the best system for solar heating from the solar fraction point of view is the pebble bed storage heating system with evacuated tubular collectors which gives a 90% solar energy fraction. Although the swimming pool heating system only gave 26%, it is still a good system since this is the fraction of solar energy which is obtained during the winter months of October until April during which heat is needed. 3.2 Cost analysis The cost analysis carried out in this investigation is based on the life cost method as applied to solar energy systems as reported by Duffle and Beckman [4]. The first fuel cost is calculated by multiplying the cost per unit of auxiliary energy by the annual

auxiliary energy requirements as determined in the solar analysis and dividing by the fuel delivery efficiency. The first year fuel cost and fuel saving when electricity is used are shown in Fig. 10 for different types of systems using flat plate collectors and evacuated type collectors. The figure indicates that there is no significant difference for the first fuel cost and fuel saving when different types of collectors are used. The best of the seven systems considered is the general heating system which has the highest first year fuel saving. As expected the passive heating systems have the lowest first year fuel saving. The life cycle gain type is used. However this analysis indicates that combined passive and active solar heating is promising. Such a combined system is being investigated by the authors.

REFERENCES

Table 4 Abbreviations

Solar heating system

PBSS WSS ABSS GSHS PHSF

Pebble bed storage system Water storage system Active building storage system General solar heating system Pooling heating system (fiat plate collectors) Pooling heating system (evacuated tubular collectors) Passive direct gain system Passive storage wall

PHSE PDGS PSW

1. M. Telkes and E. Raymond, Storing solar heat in chemicals--A report on the Dover house. Heatin9 and ventilating 80 (1949). 2. George O. G. Lof, M. M. EI-Wakil and J. P. Choice, Design and performance of domestic heating systems employing solar heated air--The Colorado solar house. Proc. U.N. Conf New Sources of Energy, Rome 5, 185 196. August (196l). 3. D. J. Close, R. V. Dunkle and K. A. Robeson, Design and performance of a thermal storage air conditioning system. Mech. Chem. En.qng Trans Inst, Enyrs, Australia MC4, 45 (1968). 4. J. A. Duffle, W. A. Beckman, Solar Engineering of Thermal Processes, p. 485. Wiley (1980). 5. D. Proctor, Methods of predicting the heat production of solar collectors for system design. Paper at Int. Sol. Energy Soc. Meeting, Los Angeles (1975).

Strategies for solar heating systems 6. S. A. Klein, W. A. Beckman and J. A. Duffle, A design procedure for solar heating systems. SoL Energy 18, 113 (1976). 7. S. A. Klein, W. A. Beckman and J. A. Duffle, A design procedure for a solar air heating system. Sol. Energy 19, 509 (1977). 8. J. C. Ward, Minimum-cost sizing of solar heating systems. Proc. Int. Sol. Energy Soc. Conf., Winnipeg 4, 336 (1976). 9. C. D. Barley and C. B. Winn, Optimal sizing of solar collectors by the method of relative areas. Sol. Energy 21,279 (1978). 10. G. Lameiro and P. Bendt, The GFL method for designing solar energy space heating and domestic hot water systems. Proc. 1978 Meet. Am. Int. Sol. Energy, Denver (2)1, 113 (1978).

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I 1. J. D. Balcomb and J. C. Hedstrom, A simplified method for sizing a solar collector array for space heating. Proc. Int. Sol. Energy Soe. Conf., Winnipeg 4, 281 (1976). 12. W. A. Beckman, S. A. Klein and J. A. Duffle, Solar Heating Design by the F-chart Method, p. 448. Wiley, New York (1977). 13. W. A. Manson, S. A. Klein and W. A. Beckman, Prediction of direct gain solar heating system performance. Sol. Energy 27, 143-147 (1981). 14. J.D. Balcomb and R. D. McFarland, A simple empirical method for estimating the performance of a passive solar heated building of the thermal storage wall type. Proc. 2nd Nat. Passive Sol. Conf., Passive Sol. of the Arts, 2, 377-389 (1978).