Performance of a natural circulation solar air heating system with phase change material energy storage

Renewable Energy 27 (2002) 69–86 www.elsevier.com/locate/renene

Performance of a natural circulation solar air heating system with phase change material energy storage S.O. Enibe * Department of Mechanical Engineering, University of Nigeria, Nsukka, Nigeria Received 8 July 2001; accepted 1 October 2001

Abstract The design, construction and performance evaluation of a passive solar powered air heating system is presented. The system, which has potential applications in crop drying and poultry egg incubation, consists of a single-glazed flat plate solar collector integrated with a phase change material (PCM) heat storage system. The PCM is prepared in modules, with the modules equispaced across the absorber plate. The spaces between the module pairs serve as the air heating channels, the channels being connected to common air inlet and discharge headers. The system was tested experimentally under daytime no-load conditions at Nsukka, Nigeria, over the ambient temperature range of 19–41 °C, and a daily global irradiation range of 4.9– 19.9 MJ m⫺2. Peak temperature rise of the heated air was about 15 K, while the maximum airflow rate and peak cumulative useful efficiency were about 0.058 kg s⫺1 and 22%, respectively. These results show that the system can be operated successfully for crop drying applications. With suitable valves to control the working chamber temperature, it can also operate as a poultry egg incubator.  2002 Elsevier Science Ltd. All rights reserved. Keywords: Solar air heating; Passive systems; Crop drying; Solar egg incubation; Phase change materials

1. Introduction The use of solar energy for the heating of fluids, including air, is already well established, and many excellent reviews of the subject are available (see for example * Tel.: +234-42-771-538; fax: +234-42-255-026. E-mail address: [email protected] (S.O. Enibe). 0960-1481/02/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 1 4 8 1 ( 0 1 ) 0 0 1 7 3 - 2

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Nomenclature A c G m ˙a Q ˙ Q T t v h r f

area, m2 specific heat, J kg⫺1 solar irradiance, W m⫺2 air mass flow rate through air heater, kg s⫺1 quantity of heat, J heat flow rate, W temperature, °C time, s flow velocity, m s⫺1 efficiency, % density, kg m⫺3 temperature-irradiance function, defined in Eq. (6)

[1–4]). Most investigators have devoted greater attention to forced circulation air heaters operating under near steady state conditions. In contrast, only a few reports of natural circulation air heaters have appeared in the literature (see for example [5]). Natural circulation air heaters are important in many industrial and agricultural applications, including the drying of crops and medicinal/aromatic plants, timber, natural rubber, tea and coffee products, and fodder for animals [6–9]. They could also be used for poultry egg incubation. In the latter case, as well as in the drying of medicinal/aromatic plants, the heated air temperature is to be maintained within specified ranges. Further, the hot air is to be supplied over a continuous period of several days, including off-sunshine periods. For these special applications, some form of energy storage, possibly combined with an auxiliary heat source, is required. An air-heating system which could be adapted to meet these special applications is considered in this paper. Solar radiation is a periodic energy resource with strong diurnal variations. Its use for poultry egg incubation must therefore incorporate a storage system to take care of the off-sunshine hours. Many thermal energy storage systems have been suggested, and these include sensible heat storage, chemical energy storage and latent heat storage [1]. Several advantages of phase change material (PCM) energy storage strongly suggest its preference in special crop drying and solar egg incubation systems. These advantages include a high energy storage density and the isothermal nature of the heat storage and recovery processes [10,11]. A phase change material is a solid which stores energy by melting upon the application of heat. The melting temperature may be fixed or vary over a small range. The stored energy is recovered upon solidification of the liquid. Many latent heat storage materials have been reviewed recently by Abhat [12]. These are usually hydrated salts (such as Glauber’s salt), paraffins, non-paraffins and fatty acids [13– 16]. Many low-cost paraffins are now available for use as PCMs at different tempera-

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tures up to 110 °C [17]. PCM energy storage devices have been developed for space applications [18], domestic hot water systems, greenhouse heating and solar power plants [10]. Good operating efficiencies have been reported. The use of phase change materials for energy storage is not without its troubles. For salt hydrates, the major drawback is that of recrystallization and segregation of the salt during repeated cycles of heat charge and discharge [13,19]. Another major problem, common to all PCMs, is the low thermal conductivity of the material. The latter problem may be overcome by the use of fins of various configurations [17,20].

2. Description A photograph of the air-heating system considered is shown in Fig. 1, while the schematic diagram is shown in Fig. 2. It consists of a flat plate solar collector integrated with the heat storage system, and uses a paraffin type PCM with known thermophysical properties for energy storage. The PCM is prepared in modules, each module being made of thin rectangular blocks of the phase change material encapsulated by the rectangular walls of a box-like structure which behaves like thin fins. The encapsulating box is made of a material of good thermal conductivity, and may be divided into a number of compartments of identical dimensions. A number of such modules are equispaced across the collector, as shown in Fig. 3. The space between each module pair serves as an air heater, the heaters being connected to common air inlet and discharge header manifolds. By natural convection, ambient air enters the inlet header through control valve A, while heated air leaves the discharge header through valve B, flows into the hot air (working) chamber and is discharged to the environment through valve C (see Fig. 2).

Fig. 1. Photograph of the air heating system. A, collector assembly with energy storage and air-heating subsystems; B, heated space.

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Fig. 2. Schematic view of a natural circulation air heater. The hot air (working chamber) will hold the crops in a cabinet-type crop dryer, and will hold the eggs in an egg incubator.

Fig. 3. Cross-sectional view of the collector assembly. The heated air flows perpendicular to the page, and the little boxes indicate approximate locations of thermocouples.

The absorber plate/PCM module assembly is housed in an insulated box inclined toward the equator at approximately the local latitude angle. In the evenings, the solar collector may be covered with an opaque screen to minimize the night-time heat loss coefficient. The modules containing the phase change material are made of slender rectangular channels whose tops are welded to the absorber plate and the bottoms rest on the bottom insulation of the collector. To minimize unwanted air leakage into the system,

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the absorber plate and module assembly are housed in such a way that air enters only through the inlet manifold and leaves only through the discharge manifold. With other care normally taken during collector construction and installation, air leaks into the system were kept to a minimum. The design is intended to utilise the advantage offered by the large heat transfer area of the PCM walls. To further increase the surface area available for air heating, horizontal metal plates may be welded equispaced along the vertical walls of each PCM module. The single-glazed collector of area 1.503 m2 has a steel absorber plate coated with a non-selective absorber material. A paraffin type phase change material supplied by Repsol Derivados SA of Madrid, Spain, was used. The total mass of the phase change material is about 65 kg. The total volume of the air heating chamber is 0.31 m3, while that of the working (hot air) chamber is 0.26 m3. The Styrofoam insulation at the bottom and sides of the collector are 0.052 and 0.044 m thick, respectively. In contrast, the insulation around the working chamber is 0.022 m. Other specific dimensions of the system are given in Table 1 while the thermophysical properties of the phase change material are given in Table 2.

3. Materials and methods Temperatures were recorded with copper-constantan thermocouples manufactured by Cole-Palmer Inc. The hot junctions were fixed at the points specified in Table 3 using a thermal glue made of araldite impregnated with copper filings. The “coldjunctions” of the thermocouples were joined in a common block directly connected to a self-compensating digital thermocouple display supplied by Cole-Palmer. Using a 10-channel data switch fabricated in our laboratories, the temperatures were obtained directly in °C. Temperatures were recorded at 30 min intervals, generally

Table 1 Physical dimensions of the test air-heating collector Inclination=local latitude angle (=7° for Nsukka, Nigeria) Collector length=1.635 m, collector width=0.945 m, effective glazing area 1.34 m2 Number of PCM modules=6 each of length 1.637 m, spaced 0.1575 m apart Number of compartments per module=1 of height 0.25 m and width 0.033 m Working chamber height, width and breadth (inside dimensions)=0.98, 0.605 and 0.44 m, respectively Height of collector entrance above ground=0.1 m Height of working chamber above top of collector=0.1 m Cross-section of channel connecting air heating and working chambers=0.29×0.18 m Length of channel section connecting air heating and working chambers, 0.510 m Length of entrance channel, 0.05 m with cross-section at air inlet 0.89×0.19 m and removable baffle of 0.79×0.039 m Length of exit channel, 0.05 m of cross-section 0.245×0.05 m Number of air heater plates per compartment, nil Absorber plate absorbance, 0.9 Absorber plate emmittance, 0.9

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Table 2 Thermophysical properties of paraffin wax phase change materiala Melting temperature range Specific heat Thermal conductivity Enthalpy of fusion Solid density Liquid density Viscosityb Thermal expansivityc

58–60 °C 900 J kg⫺1 K⫺1 0.2 W m⫺1 K⫺1 214.4 kJ kg⫺1 850 kg m⫺3 775 kg m⫺3 1.07×10⫺6 kg m⫺1 s⫺1 3.6×10⫺4 K⫺1

a Source: Veraj et al. [18] except as indicated. The melting temperature range and density of the solid phase were confirmed experimentally. The specific heat and thermal conductivity of the solid and liquid phases are assumed equal. b Author’s estimate. c Value for water used.

Table 3 Location of thermocouplesa Thermocouple

Description of position

Distances from origin (mm) xb

T5 T4 Tic Tp0 Tpj Tv

Th

Ts Tg Ta a

Exit of the working chamber Inlet to the working chamber (=exit from the collector channel) Centre of the working chamber Collector plate midway between two modules Collector plate at joint Centre of vertical side of PCM encapsulating wall at third module from the left Centre of bottom side of PCM encapsulating wall at third module from the left Paraffin wax through lower plug near entrance to collector channel Centre of glazing top surface Outside air

yc

470 431 412

1032

122

550

595

zd

1000 900

913

250e

The locations of thermocouples for T4 and T5 are as indicated in Fig. 2, while the thermocouple for Tic is midway between them. The approximate positions of the other thermocouples are indicated in Fig. 3. b Origin of x is located at the left-hand side of collector top. c Distance y is measured vertically downwards from the absorber plate. d Distance z is measured from the air inlet end of the collector channel, except for the thermocouple for Ts which is measured from the collector channel exit. e Measured from the exit of the lower plug at the exit end of the collector channel.

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between 6:30 and 18:30 local time. Night-time performance of the system was not studied. The instantaneous global irradiance on a horizontal surface was measured at intervals of 10 min using an Eppley radiometer, model PSP, serial number 17361F3. The millivolt output of the radiometer was read with a digital millivoltmeter, model number AVD 830B supplied by Alda Corporation of Japan. The voltmeter readings were converted to radiant flux units using a conversion factor of 1 W m⫺2=9.6 µV, supplied by the radiometer manufacturer. Air velocity at the collector inlet, va (point 1 in Fig. 2) was measured with a hot wire anemometer supplied by Airflow Developments Ltd, Wycombe, UK. The air mass flow rate was then calculated from the expression m ˙ a ⫽ ravaAa

(1)

with ra calculated for any given temperature Ta by considering air as a perfect gas at constant pressure. Windspeed at the location of the collector was, however, not measured. The useful heat gain rate of the heated air is given by ˙ us ⫽ m (2) Q ˙ aca(T3⫺Ta) while the useful instantaneous collector efficiency, hus is calculated from the expression hus ⫽

m ˙ aca(T3⫺Ta) AcGT

(3)

where T3 is the temperature at the exit of the collector channel (positions 3 and 4 in Fig. 2, also equal to the temperature of air inlet to the working chamber) and GT is the solar irradiance at the inclination of the collector. GT could be calculated from the measured data for a horizontal surface, G, using the expressions for radiation on tilted surfaces given in Duffie and Beckman [1]. However, since the collector is oriented at the latitude of the location, which is 7° in the present case, GT is essentially equal to G. The cumulative heat gain from start up to any particular time is obtained by integrating the total useful heat gain for the period. Thus,

冕

冕

t

t

˙ us dt ⫽ m Qus ⫽ Q ˙ aca(T3⫺Ta) dt 0

(4)

0

The cumulative efficiency at any time may be obtained by dividing the cumulative useful heat gain from the start by the cumulative irradiation, giving

冕 t

m ˙ aca(T3⫺Ta) dt

huc ⫽

0

冕 t

Ac GT dt 0

(5)

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For solar collectors with storage, the cumulative efficiency, huc is a more useful measure of performance than the instantaneous efficiency, hus. 4. Experimental results Experimental day-time performance data on the system was obtained for 14 different days in May and June at Nsukka, Nigeria (latitude 7° N), usually between the hours of 6:30 and 18:30 local time. As shown in Table 4, the daily global irradiation covered the range 4.91–19.96 MJ m⫺2, while the ambient temperature over the period varied within the range 19.6–41.8 °C. The minimum, average and maximum ambient temperature for each day are also shown in Table 4. The cross-sectional area of the air inlet channel, Aa, was varied as shown between 0.0308 and 0.1691 m2 using a removable baffle plate. Fig. 4 shows the temperature profiles on the absorber plate for three days with the lowest, highest and intermediate daily irradiation for the test period. It may be seen that, as would be expected, the plate temperature midway between the modules, Tp0, is generally higher than the absorber plate temperatures at the joint directly above the phase change material, Tpj, especially when there is some net energy gain. Table 4 Global daily irradiation and ambient temperature for the test datesa Dateb

4 May 5 May 6 May 7 May 8 May 9 May 10 May 11 May 12 May 30 Mayc 31 May 1 June 2 June 3 June

Daily global irradiation (MJ m⫺2)

19.11 15.14 16.91 16.96 16.72 19.87 17.53 16.38 6.48 7.96 15.37 4.91 16.9 16.01

Ambient temperature (°C)

min

mean

max

25.5 24.1 25.5 25.8 25.7 21.2 23.1 24.8 21.7 24.6 22.2 19.7 19.6 22.6

29.8 27.3 29.7 29.9 29.8 27.6 30.7 32.2 25.7 29.6 29.9 23.2 29.7 30.3

34.1 35.4 36.7 33.4 32.6 35.0 38.4 41.0 32.6 33.7 39.1 31.2 38.3 41.8

a The normal cross-sectional area of the air inlet is 0.1691 m2. This was used for tests between 4 and 9 May, but reduced to 0.0308 m2 for the subsequent tests using a removable baffle. b All tests were carried out in the year 2001. c Tests on this day lasted for only 6 h, from 12:30 to 18:30 hours local time.

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Fig. 4. Temperature profiles on the absorber plate: (a)–(c) are plotted for days with global irradiation of 19.87, 15.14 and 4.91 MJ m⫺2, respectively.

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In contrast, when there is net energy loss, such as during the evening hours, Tp0ⱕ Tpj. The plate temperature generally varies in sympathy with the global irradiance. Increasing from a value almost identical with the ambient temperature in the morning hours, it reaches its peak value near solar noon, and then decreases towards the ambient temperature as solar irradiance falls. For days with intermediate or high global irradiance, represented by parts (a) and (b) in Fig. 4, the plate temperatures tend to be around 60 °C, usually about 2–3 h before and after solar noon. This represents the period when some of the PCM change from the solid to the liquid phase, the required latent heat being drawn from the absorbed irradiation. The PCM used has a melting temperature range of 58–60 °C. The temperature at the middle of the vertical and bottom sides of the walls of the PCM holder (Tv and Th, respectively), are plotted in Fig. 5 for the three representative days. Both Tv and Th vary in sympathy with the solar irradiance, but more sluggishly in comparison with the absorber plate temperature Tp. This is due to significant heat exchange between the plates and the phase change material. The peak values of Tv and Th are generally lower than the peak values of Tp0 by about 20–30 K. This shows that there is significant temperature gradient along the vertical side of the PCM module wall, as would be expected. The temperature of the PCM close to the bottom plate of the PCM capsule wall, and near the air exit from the collector channel (i.e. near position 3 in Fig. 2) Ts, generally differs from the latter by no more than 2 K. The value of Ts at the particular position considered was less than the presumed minimum melting temperature value of 58 °C by at least 17 K, showing that some of the PCM used remained in the solid state (or subcooled) for the particular conditions of the test. This is not a surprising result for the position considered, where the lowest PCM temperatures are expected to occur. The temperature of the PCM is expected to be higher at other positions. A plot of the glazing temperature, Tg, with time is shown in Fig. 6. As with Tp0, the glazing temperature rises in sympathy with the global irradiance, but achieves a local peak value of no more than 50 °C around solar noon. The difference between the glazing and ambient temperatures, Tg⫺Ta, also increases from near zero in the morning to a maximum of about 15 K near solar noon, and then falls off gradually in the evenings. The high value of this temperature difference above the ambient value represents significant heat losses (by convection and radiation) through the collector cover system. Fortunately, through the use of multiple glazing possibly in combination with selective absorber surfaces, it is possible to reduce this significantly. The temperature of the heated air at the inlet and exit of the working chamber (T4 and T5, near points 4 and 5 in Fig. 2) is shown in Fig. 7. The ambient air temperature is also shown. The temperature rise of the heated air above the ambient, T4⫺Ta, is shown in Fig. 8 for three sets of four consecutive days of the test. In general, the maximum temperature rise of the air leaving the collector channel

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Fig. 5. Temperature profiles on the vertical and bottom plates of a module of the phase change material: (a)–(c) are plotted for test dates with global irradiation of 19.87, 15.14 and 4.91 MJ m⫺2, respectively.

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Fig. 6. Plot of glazing surface temperature with time: (a)–(c) are plotted for dates with global irradiation of 19.87, 15.14 and 4.91 MJ m⫺2, respectively.

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Fig. 7. Variation of heated air temperatures with time in the working chamber. (a)–(c) are for days with total irradiation of 19.87, 15.14 and 4.91 MJ m⫺2, respectively.

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Fig. 8. Temperature rise of the heated air above the ambient at the exit of the collector channel. The number 1.00 on the horizontal axis corresponds to 0:00 hours of the first day, hence 1.5, 2.5, 3.5 and 4.5 correspond respectively to solar noon of the first, second, third and fourth days: (a)–(c) are for the periods 5–8 May, 9–12 May and 31 May–3 June, respectively.

at point 4 and entering the working chamber is about 15 K, especially during high irradiance periods. The air temperature rise is lower with lower irradiance. The differences between the inlet and outlet temperature of the working chamber are within a narrow range of about 0–2 K. This may be due to the fact that the working chamber was empty. The temperature gradient is expected to be higher when the chamber is loaded, say with crops for drying or eggs for incubation. The airflow velocity at the exit of the working chamber, and the corresponding air mass flow rate, m ˙ a, are plotted in Fig. 9. The airflow rates are seen to increase gradually as the collector heats up from sunrise, reach a peak value for the day around the period of peak irradiance, and fall off as solar irradiance decreases. The daily peak values increase with total global irradiation. Although the airflow rates are small in comparison with forced convection systems, they are quite significant

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Fig. 9.

83

Airflow rates through the working chamber.

Fig. 10. Variation of cumulative useful efficiency with time. The daily global radiation for the test days are 15.37, 4.91, 16.90 and 16.01 MJ m⫺2 for 31 May, 1 June, 2 June and 3 June, respectively.

for a natural convention air heater, and are comparable with the velocities reported by Macedo and Altemani [5] for a system consisting of the collector channel alone without storage. A plot of the variation of the cumulative useful collector efficiency, huc with time is shown in Fig. 10. The peak cumulative efficiency is observed to be about 50%.

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The efficiency curves tend to show higher values for days with high solar irradiation, and lower values otherwise. In the morning hours of most days, the cumulative efficiency curves start from initially high values, drop to a minimum, and then gradually increase. In the same manner, in the evening hours of some days, the cumulative efficiency curves tend to remain flat or continue with a positive slope even towards sunset. These effects are due to a drawdown of energy stored within the collector components, especially the phase change material. From Eq. (5) we define a temperature-irradiance function, f, such that

冕 t

f⫽

(T3⫺Ta) dt

0

冕 t

⫽

T¯ 3⫺T¯ a ¯T G

(6)

GT dt

0

The parameter f is thus comparable to the usual temperature-irradiance function for flat plate collectors operating at steady state as described in Duffie and Beckman [1]. A plot of huc against f is shown in Fig. 11, and this gives the collector characteristic curve for a flat plate collector with storage. In obtaining the figure, we have ¯ T is within a specified range, selected only data for which the average irradiance G as suggested by Macedo and Altemani [5] and Oreszczyn and Jones [21]. In the present case, a minimum irradiance of 150 W m⫺2 was chosen arbitrarily, but no upper limit was imposed. Some scatter is observed in the plots, apparently due to the large time interval between experimental data sets which may tend to reduce the

Fig. 11.

Variation of cumulative useful efficiency with the temperature-irradiance function, f.

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accuracy of the numerical procedures used in approximating the integrals involved in evaluating huc and f. This notwithstanding, a definite trend could be observed in the data points. The line of best fit has a positive slope and an intercept near the origin. A non-linear least squares fit of the data using the Marquardt–Levenberg algorithm of the GNUPLOT software by Williams et al. [22] (also described in [23]) gave the correlation (7) huc ⫽ 0.459 ⫹ 873.7f This expression would be very useful in predicting the system performance for different climatic conditions. 5. Conclusions The design, construction and performance evaluation of a natural convection solar air heater with phase change material energy storage has been successfully undertaken. The day-time performance of the system under no-load conditions was tested under natural environmental conditions involving ambient temperature variations in the range 19–41 °C and daily global irradiation in the range 4.9–19.96 MJ m⫺2. Peak temperature rise of the heated air was about 15 K, while peak cumulative useful efficiency was about 50%. The system is suitable for use as a solar cabinet crop dryer for aromatic herbs, medicinal plants and other crops, which do not require direct exposure to sunlight. Further work is continuing to develop suitable valve control systems to keep the working chamber temperature within a prescribed range, and hence make it possible to utilise the system for poultry egg incubation. Acknowledgements This work was made possible through the Senate Research Grant No. 94/186 of the University of Nigeria, Nsukka, Nigeria. This paper was written while the author was visiting the Abdus Salam International Centre for Theoretical Physics (ICTP) in Trieste, Italy as a Regular Associate using funds provided by the Swedish International Development Cooperation Agency (SIDA). For these, the author is grateful. I thank my former student, Mr A.L. Mabiaku, for his immense contributions during the construction of the experimental model; and current students, Mr G.I. Odomelerun and Mr O.P. Osagie for assisting during the final stages of the construction of the model, and their painstaking efforts in collecting the experimental data. The construction of the physical equipment and thermocouple data switch were done by Mr John Igatha and Mr Chuks Udom of the National Centre for Energy Research and Development (NCERD), University of Nigeria, Nsukka, Nigeria. The solar radiation and airflow measurements were undertaken by Mr Paulinus Ugwoke and Mr E. Nosike of the NCERD, while the extensive tables resulting from the experimental data were prepared on a word processor by Mr Godwin Nwodo and Mr George Ezeh, both of the NCERD. The Director of the Centre, Prof. C.E. Okeke, enthusiastically granted permission for the facilities of the Centre to be used in the construction and testing of the equipment. Special thanks also go to my wife, Ojiugo, for facilitat-

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ing the collection and transmission of the experimental data. Above all, I thank God who made all this possible.

References [1] Duffie JA, Beckman WA. Solar engineering of thermal processes. 2nd ed. New York: John Wiley and Sons, 1991. [2] Loveday DL. Thermal performance of air-heating solar collectors with thick, poorly conducting absorber plates. Solar Energy 1988;41(6):593–602. [3] Choudhury C, Garg HP. Evaluation of a jet plate solar air heater. Solar Energy 1991;46(4):199–209. [4] Parker BF, Lindley MR, Colliver DG, Murphy WE. Thermal performance of three solar air heaters. Solar Energy 1993;51(6):467–79. [5] Macedo IC, Altemani CAC. Experimental evaluation of natural convection solar air heaters. Solar Energy 1978;20:367–9. [6] Ekechukwu OV, Norton B. Review of solar-energy drying systems II: an overview of solar drying technology. Energy Convers Mgmt 1999;40:615–55. [7] Diamante LM, Munro PA. Mathematical modelling of the thin layer drying of sweet potato slices. Solar Energy 1993;51(4):271–6. [8] Fohr JP, Figueiredo AR. Agricultural solar air collectors: design and performances. Solar Energy 1980;38(5):311–21. [9] Palaniappan C, Subramanian SV. Economics of solar air pre-heating in South Indian tea factories: a case study. Solar Energy 1998;63(1):31–7. [10] Conti M, Charach Ch. Thermodynamics of heat storage in a PCM shell-and-tube heat exchanger in parallel or in series with a heat engine. Solar Energy 1996;57:59–68. [11] Kerslake TW, Ibrahim MB. Analysis of thermal energy storage material with change of phase volumetric effects. ASME J Solar Energy Engng 1993;115:22–31. [12] Abhat A. Low temperature latent heat thermal storage: heat storage materials. Solar Energy 1983;30:313–32. [13] Rabin Y, Bar-Niv I, Korin E, Mikie B. Integrated solar collector storage system based on a salthydrate phase change material. Solar Energy 1995;55(6):435–44. [14] Hasan A. Phase change material energy storage system employing palmitic acid. Solar Energy 1994;52:143–54. [15] Aboul-Enein S, Olofa SA. Thermophysical properties of latent heat of fusion storage materials for cooling applications. Renewable Energy 1991;1:791–7. [16] Kimura H, Kai J. Mixtures of calcium chloride hexahydrate with some salt hydrates or anhydrous salts as latent heat storage materials. Energy Convers. Mgmt 1988;28:197–200. [17] Veraj R, Seeniraj RV, Hafner B, Faber C, Schwarzer K. Experimental analysis and numerical modelling of inward solidification on a platened vertical tube for a latent heat storage unit. Solar Energy 1997;60:281–90. [18] Strumpf HJ, Coombs MG. Solar receiver experiment for the space station FREEDOM Brayton engine. J. Solar Energy Engng 1990;112:12–8. [19] Farid MM, Khalaf AN. Performance of a direct contact latent heat storage unit with two hydrated salts. Solar Energy 1994;52:179–89. [20] Lacroix M. Study of the heat transfer behaviour of a latent heat thermal energy unit with a platened tube. Int J Heat Mass Transfer 1993;36:2083–92. [21] Oreszczyn T, Jones BW. A transient test method applied to air heating collectors. Solar Energy 1987;38(6):425–30. [22] Williams T, Kelley C, Lang R, Kotz D, Campbell J, Elber G. GNUPLOT—an interactive plotting program, version 3.7; 1998. Available from: http://www.cs.dartmouth.edu [23] Press WH, Teukolsky SA, Vetterling WT, Flannery BP. Numerical recipes in FORTRAN. 2nd ed. Cambridge: Cambridge University Press, 1992.