CHARACTERISATION OF A THERMAL ENERGY STORAGE SYSTEM DEVELOPED FOR INDIRECT SOLAR COOKING
Conclusions and recommendations Overall Summary
The thermal performance of the storage was evaluated in terms of the axial temperature
distribution, the thermal stratification, the total energy stored and the total exergy stored
under different charging temperatures and flow rates. The energy and exergy delivery
rates in relation to thermal performance of the storage system were evaluated during
charging. Charging results of the storage under different temperatures indicated that
there was an optimal charging temperature for optimal thermal performance. Exceeding
this temperature resulted in the reduced thermal performance due an increase in the heat
losses. Charging at nearly constant temperature conditions under different flow regimes
suggested that there was an optimal charging flow rate. The optimal flow rate was a
compromise between obtaining a greater heat transfer rate in the energy delivery device
(EDD) and achieving a greater degree of thermal stratification in the storage. Due to
the small size of the storage, the TES system can be used for rapid thermal performance
testing of different oils and pebbles to be used in TES systems for domestic applications
that require low to medium temperatures like solar cooking.
Volumetric heat transfer characteristics of a small glass tube containing oil and glass
pebbles were determined experimentally during charging. The small size of the glass tube
allowed for rapid heat transfer experiments under different average charging flow rates.
The average fluid and bed temperatures were measured at different average charging flow
rates. An increase in the flow rate resulted in an increase in the volumetric heat transfer
coefficient. The coefficient was also found to be linearly dependent on the average charging
flow rate. An expression for the correlation between the superficial mass flow velocity,
the average particle diameter and the volumetric heat transfer coefficient was formulated
using the experimental results. It was suggested that the expression should also include
the average charging temperature of the bed since the volumetric heat transfer coefficient
was seen to increase with an increase in the temperature of the hot plate.
From the results of the simulations and experiments, it is concluded the oil/pebble–bed
TES system is a potentially viable method to enhance the performance of a solar cooker.
The oil/pebble–bed TES system is particularly useful for small scale solar domestic ap-
Conclusions and recommendations Recommendations for Future Work
plications which include solar cooking and solar drying. The simulation and experimental
results obtained in the present thesis can be used for the design and operation of indirect
solar cookers with thermal energy storage
6.2 Recommendations for Future Work
The experimental work was carried out with a relatively small TES system for fast heat
transfer experiments during the charging cycle only. A larger practical TES system is to be
constructed to evaluate the charging and discharging thermal performance characteristics
with the methods simulated earlier. The performance of the larger TES system is also to
be evaluated with an actual parabolic dish concentrator focussing direct solar radiation
onto a receiving absorber circulating the oil. Discharging experiments using the larger
TES system should also be carried out with different load devices to evaluate the thermal
performance of the different devices.
To evaluate the heat transfer effects like thermal mixing and loss of stratification in
the oil/pebble–bed TES system, computational fluid dynamics (CFD) models will be
developed. These models should be compared with experimental results as well as with
the predictions of the simplified models described earlier. CFD models would help in
the detailed understanding of the complex heat transfer mechanisms in oil/pebble–bed
TES systems which were not studied here due to time and financial constraints. Other
parameters such as the pressure drop and friction factor of the oil/pebble–bed TES system
need to be evaluated thoroughly. The detailed heat transfer correlations for oil/pebble–
bed TES systems need to be developed since there seems to very little literature related
to these type of systems.
The possibility of integrating the solar TES system and for electric cooking using low
wattage electrical energy during low solar radiation conditions is to be investigated. Longterm
performance simulation of the TES and cooking system is to be performed using
Conclusions and recommendations Recommendations for Future Work
models available in the TRNSYS package. The experimental performance of different
types of pebbles is to be investigated further. To enhance the storage capacity of the TES
system, phase change materials (PCMs) based TES will be studied in detail.
Abdel-Rehim, Z. (2007). Heat transfer analysis of different storing media using oil as working
fluid. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects,
Abhat, A. (1983). Low temperature latent heat thermal energy storage: heat storage
materials. Solar Energy, 30:313–331.
Abu-Hamed, T., Karni, J., and Epstein, M. (2007). The use of boron for thermochemical
storage and distribution of solar energy. Solar Energy, 81:93–101.
Adams, W. (1878). Cooking by solar heat. Scientific American, 38:376–377.
Ahmad, A., Saini, J., and Varma, H. (1996). Thermohydraulic performance of packed–bed
solar air heaters. Energy Conversion and Management, 37:205–214.
Al-Nimr, M., Abu-Qudais, M., and Mashaqai, M. (1996). Dynamic behaviour of a packed
bed energy storage system. Energy Conversion and Management, 37:23–30.
Alizadeh, S. (1998). An experimental and numerical study of thermal stratification in a
horizontal cylindrical solar storage tank. Solar Energy, 66:409–421.
Amer, E. (2003). Theoretical and experimental assessment of a double exposure solar
cooker. Energy Conversion and Management, 44:2651–2663.
Ataer, O. (2006). Storage of Thermal Energy. Encyclopedia of Life Support Systems
(EOLSS), www.eolss.net, accessed, January 2009.
Balzar, A., Stumpf, P., Eckhoff, S., Ackermann, H., and Grupp, M. (1996). A solar cooker
using vacuum-tube collectors with integrated heat pipes. Solar Energy, 58:63–68.
Beasley, D. and Clark, J. (1984). Transient response of a packed bed for thermal energy
storage. International Journal of Heat and Mass Transfer, 27:1659–1669.
Bhavsar, V. and Balakrishnan, A. (1990). Pebble bed–oil thermal energy storage for
thermo-electric power systems. International Journal of Energy Research, 14:233–240.
Binark, A. and Turkmen, N. (1996). Modelling of a hot box solar cooker. Energy Conversion
and Management, 37:303–310.
Bouhdjar, A. and Harhad, A. (2002). Numerical analysis of transient mixed convection
flow in storage tank: Influence of fluid properties and aspect ratios on stratification.
Renewable Energy, 25:555–567.
Bowman, T. (1979). Solar cookers: Test results and new designs. In Second International
Symposium of Engineering., pages 378–404, San Salvador, El Salvador.
Bowman, T. and Blatt, J. (1978). Solar Cookers: History, design, fabrication, testing and
evaluation. Technical report, Florida Institute of Technology, Florida, USA.
Buddhi, D., Sharma, S., and Sharma, A. (2003). Thermal performance evaluation of a
latent heat storage unit for late evening cooking in a solar cooker having three reflectors.
Energy Conversion and Management, 44:809–807.
Cabeza, L., Ibanez, M., Sole, C., Roca, J., and Nogues, M. (2006). Experimentation
with a water tank including a PCM module. Solar Energy Materials and Solar Cells,
Calflo-Datasheet (2005). www.petro − canada.de, website accessed December 2005.
Camacho, E., Berenguel, M., and Rubio, R. (1997). Advanced control of solar plants.
Canbazoglu, S., Sahinaslan, A., Ekmekyapar, A., Aksoy, Y., and Akarsu, F. (2005). Enhancement
of solar thermal energy storage performance using sodium thiosulfate pentahydrate
of a conventional solar water-heating system. Energy and Buildings, 37:235–
Chandak, A., Dubey, D., and Kulkarni, R. (2006). Innovative balcony model of concentrating
solar cooker. In International conference on solar cooking and food processing.,
pages 12–16, Barcelona, Spain.
Cheema, L. (1984). Two step asymmetrical reflector solar cooker of box type. In Proceedings
of National Solar Energy Coneference., pages 15–18, Vadodara, India.
Chen, S., Chen, C., Tin, C., Lee, T., and Ke, M. (2000). An experimental investigation
of cold storage in an encapsulated thermal storage tank. Experimental Thermal and
Fluid Science, 23:133–144.
Chikukwa, A. (2007). Modelling of a Solar Stove: Small Scale Concentrating System
With Heat Storage (Potential For Cooking In Rural Areas, Zimbabwe). PhD thesis,
Norwegian University of Science and Technology, Trondheim, Norway.
Coutier, J. and Farber, E. (1982). Two applications of a numerical approach of heat
transfer process within rock beds. Solar Energy, 29:451–462.
Crandall, D. and Thacher, E. (2004). Segmented thermal storage. Solar Energy, 77:435–
da Silva, M., Schwarzer, K., and Medeiros, M. (2002). Experimental results of a solar
cooker with heat storage. In RIO 02–World Climate and Energy Event., pages 89–93,
Rio DeJanairo, Brazil.
Dhavraj, M. (1999). The Setting Up and Calibration of Radiometers for the Measurement
of Direct and Diffuse Solar Radiation at the University of Durban–Westville. Master’s
thesis, University of Durban–Westville, Durban, South Africa.
Dincer, I. and Rosen, M. (2002). Thermal Energy Storage: Systems and Applications.
West Sussex, Wiley.
Diver, R., Miller, J., Allendorf, M., Siegel, N., and Hogan, R. (2008). Solar thermochemical
water–splitting ferrite–cycle heat engines. Journal of Solar Energy Engineering,
Domanski, R., El-Sebaii, A., and Jaworski, M. (1995). Cooking during off–sunshine hours
using as storage media. Energy, 20:607–616.
du Toit, C., Rousseau, P., Greyvenstein, G., and Landman, W. (2006). A systems CFD
model of a packed bed high temperature gas-cooled nuclear reactor. International
Journal of Thermal Sciences, 45:70–85.
E-C-S-C-R (1995). Second international solar cooker test: Summary of results. Technical
report, European Committee of Solar Energy Research, Germany.
Ekechukwu, O. and Ugwuoke, N. (2003). Design and measured performance of a plane
reflector augmented box-type solar-energy cooker. Renewable Energy, 28:1935–1952.
El-Sebaii, A. (1997). Thermal performance of a box type solar cooker with outer–inner
reflectors. Energy, 22:969–978.
El-Sebaii, A. and Ibrahim, A. (2005). Experimental testing of a box-type solar cooker
using the standard procedure of cooking power. Renewable Energy, 30:1861–1871.
Fang, P. and Susan, S. (1979). Chinese solar cookers. Appropriate Technology, 5:4–5.
Farid, M., Khudhair, A., Razack, S., and Al-Hallaj, S. (2004). A review on phase change
energy storage: materials and applications. Energy Conversion and Management,
Fernández-Seara, J., Uhía, F., and Sieres, J. (2007). Experimental analysis of a domestic
electric hot water storage tank. Part 2: Dynamic mode of operation. Applied Thermal
Flores-Irigollen, A., Fernández, J., Rubio-Cerda, E., and Poujol, F. (2004). Heat transfer
dynamics in an inflatable–tunnel solar air heater. Renewable Energy, 29:1367–1382.
Franco, F., Saravia, L., Javi, V., Caso, R., and Fernandez, C. (2008). Pasteurization of
goat milk using a low cost solar concentrator. Solar Energy, 82:1088–1094.
Funk, P. (2000). Evaluating the international standard procedure for testing solar cookers
and reporting performance. Solar Energy, 68:1–7.
Funk, P. and Larson, D. (1998). Parameteric model of solar cooker performance. Solar
Furnas, C. (1930). Heat transfer from a gas stream to a bed of broken solids. Industrial
Engineering Chemistry, 22:26–31.
G-A-T-E (1979). Comparision between different systems of solar cookers considering both
technical and economic sspects. Technical report, GATE, Eschbroom, Germany.
Garg, H., Mann, H., and Thanvi, K. (1978). Performance evaluation of five solar cookers.
Sun Mankind’s Future Sources of Energy, 3:1491–1496.
Gosh, M. (1976). Domestic multipurpose type of solar energy unit. In Proceedings of
National Solar Energy Conference., pages 237–239, Calcutta, India.
Grupp, M., Pierre, M., and Mathis, W. (1991). A novel advanced box type solar cooker.
Solar Energy, 47:107–113.
Habeebullah, M., Khalifa, A., and Olwi, I. (1995). The oven receiver: An approach toward
the revival of concentrating solar cookers. Solar Energy, 54:227–237.
Hafner, B. (1999). Modelling and optimisation of solar process heating systems. PhD
thesis, RWTH–Aachen, Aachen, Germany.
Hahne, E. and Chen, Y. (1998). Numerical study of flow and heat transfer characteristics
in hot water stores. Solar Energy, 64:9–18.
Halacy, D. (1974). Fun with sun. The Mother Earth News, 25:26–28.
Harmim, A., Boukar, M., and Amar, M. (2008). Experimental study of a double exposure
solar cooker with finned cooking vessel. Solar Energy, 82:287–289.
Herrmann, U., Kelly, K., and Price, H. (2004). Two-tank molten salt storage for parabolic
trough solar power plants. Energy, 29:883–893.
Hussein, H., El-Ghetany, H., and Nada, S. (2008). Experimental investigation of novel
indirect solar cooker with indoor PCM thermal storage and cooking unit. Energy Conversion
and Management, 49:2237–2246.
Ismail, K. and Henriquez, J. (2002). Numerical and experimental study of spherical
capsules packed bed latent heat storage system. Applied Thermal Engineering, 22:1705–
Ismail, K. and Stuginsky, R. (1999). A parametric study on possible fixed bed models for
pcm and sensible heat storage. Applied Thermal Engineering, 19:757–788.
Jain, D. (2005). Modeling the performance of greenhouse with packed bed thermal storage
on crop drying application. Journal of Food Engineering, 71:170–178.
Kaushik, S. and Gupta, M. (2008). Energy and exergy efficiency comparison of
community-size and domestic-size paraboloidal solar cooker performance. Energy for
Sustainable Development, 12:60–64.
Kenjo, L., Inard, C., and Caccavelli, D. (2007). Experimental and numerical study of
thermal stratification in a mantle tank of a solar domestic hot water system. Applied
Thermal Engineering, 27:1986–1995.
Khalifa, A., Taha, M., and Akyurt, M. (1987). Design and testing of a new concentrating
type solar cooker. Solar Energy, 38:79–88.
Khartchenko, N. (1998). Advanced Energy Systems. Taylor and Francis, Washington.
Kürklü, A. and Bilgin, S. (2004). Cooling of a polyethylene tunnel type greenhouse by
means of a rock bed. Renewable Energy, 29:2077–2086.
Kreysig, E. (1999). Advanced Engineering Mathematics. Wiley, NewYork.
Kulkarni, G., Keedare, S., and Bandyopadhyay, S. (2008). Design of solar thermal systems
utilizing pressurized hot water storage for industrial applications. Solar Energy, 82:686–
Kumar, R., Adhikari, R., Garg, H., and Kumar, A. (2001). Thermal performance of
a solar pressure cooker based on an evacuated tube solar collector. Applied Thermal
Kundapur, A. (2009). Compendium of solar cookers. www.solarcooking.com, website
accessed June 2009.
Kunke, F. (1987). Solar cookers for developing countries. In ISES Conference on Advances
in Solar Energy Technology., pages 2676–2682, Hamburg, Germany.
Kurt, H., Atik, K., Özkaymak, M., and Recebli, Z. (2008). Thermal performance parameters
estimation of hot box type solar cooker by using artificial neural network.
International Journal of Thermal Sciences, 47:192–200.
Lane, G. (1983). Solar Heat Storage: Latent Heat Materials, Vol. I. CRC Press, Florida.
Lane, G. (1989). Phase Change Thermal Storage Materials. In: Hand Book of Thermal
Design. McGraw Hill, New York.
Löf, G. and Hawley, R. (1948). Unsteady-state heat transfer between air and loose solids.
Industrial and Engineering Chemistry, 40:1061–1070.
Lovegrove, K., Luzzi, A., and Kreetz, H. (1999). A solar–driven ammonia–based thermochemical
energy storage system. Solar Energy, 67:309–316.
Lovegrove, K., Luzzi, A., Soldiani, I., and Kreetz, H. (2004). Developing ammonia based
thermochemical energy storage for dish power plants. Solar Energy, 76:331–337.
Marlin, T. (1995). PROCESS CONTROL: Designing Processes and Control Systems for
Dynamic Performance. MacGraw–Hill, Inc, Singapore.
Mawire, A. (2005). A data acquisition and control system for a solar thermal energy
storage (TES) and cooking system. Master’s thesis, Department of Physics, Faculty
of Science and Agriculture at the University of KwaZulu–Natal (Westville campus),
Durban, South Africa.
Mawire, A. and McPherson, M. (2007). Design of a control system used to simulate the
daily variation of solar radiation using electrical power. In ISES, Solar World Congress
2007,Solar Energy and Human Settlement., pages 881–885, Beijing, China.
Mawire, A. and McPherson, M. (2008a). Experimental characterisation of a thermal
energy storage system using temperature and power controlled charging. Renewable
Mawire, A. and McPherson, M. (2008b). A feedforward IMC structure for controlling
the charging temperature of a TES system of a solar cooker. Energy Conversion and
Mawire, A. and McPherson, M. (2009). Experimental and simulated temperature distribution
of an oil–pebble bed thermal energy storage system with a variable heat source.
Applied Thermal Engineering, 29:1086–1095.
Mawire, A., McPherson, M., and van den Heetkamp, R. (2008). Simulated energy and
exergy analyses of the charging of an oil–pebble bed thermal energy storage system for
a solar cooker. Solar Energy Materials and Solar Cells, 92:1668–1676.
Medved, S., Meglic, B., and Novak, P. (1996). SOLAR–BALL: Extremely light, efficient
and low cost solar cooker. Renewable Energy, 9:741–744.
Mills, A. (1999). Heat Transfer. Prentice Hall, New Jersey.
Moffat, R. (1988). Describing the uncertainties in experimental results. Experimental
Thermal and Fluid Sciences, 1:3–17.
Mohammed Ali, B. (2000). Design and testing of Sudanese solar box cooker. Renewable
Morrison, G., Di, J., and Mills, D. (1993). Development of a solar thermal cooking system.
Technical report, School of Physics, University of Sydney, Sydney, Australia.
Mullick, S., Kumar, S., and Kandpal, T. (1987). Thermal test procedure for box–type
solar cookers. Solar Energy, 39:353–360.
Mumma, S. and Marvin, W. (1976). A method of simulating the performance of a pebble
bed thermal energy storage and recovery system. In ASME Paper No. 76–HT–73,
ASME–AICHE Heat Transfer conference., pages 73–78, St Louis, Missouri, USA.
Murty, V., Gupta, A., and Shukla, A. (2006). Design, development and thermal performance
evaluation of an inclined heat exchanger unit for SK–14 parabolic solar
cooker for off–place cooking. In International green energy conference., pages 8–15,
Nahar, M. (2003). Performance and testing of a hot box storage solar cooker. Energy
Conversion and Management, 44:1323–1331.
Nahar, N., Gupta, J., and Sharma, P. (1996). Performance and testing of two models of
solar cooker for animal feed. Renewable Energy, 7:47–50.
Nallusamy, N., Sampatha, S., and Velraj, R. (2007). Experimental investigation on a
combined sensible and latent heat storage system integrated with constant/varying
(solar) heat sources. Renewable Energy, 32:1206–1227.
Nasr, K., Ramadhyani, R., and Viskanta, R. (1994). An experimental investigation on
forced convection heat transfer from a cylinder embedded in a packed bed. Journal of
Heat Transfer, 116:73–80.
Navarrete-Gonzalez, J. and Cervantes-de Gortari, J. (2008). Exergy analysis of a rock
bed thermal storage system. International Journal of Exergy, 5:18–30.
Nsofor, E. and Adebiyi, G. (2001). Measurements of the gas–particle convective heat
transfer coefficient in a packed bed for high-temperature energy storage. Experimental
Thermal and Fluid Science, 24:1–9.
Nyahoro, P., Johnson, R., and Edwards, J. (1997). Simulated performance of thermal
storage in a solar cooker. Solar Energy, 59:11–17.
Oliveski, R., Krenzinger, A., and Vielmo, H. (2003). Comparison between models for the
simulation of hot water storage tanks. Solar Energy, 75:121–134.
Oliveski, R., Macagnan, M., Copetti, J., and Petroll, A. (2005). Natural convection in
a tank of oil: Experimental validation of a numerical code with prescribed boundary
condition. Experimental Thermal and Fluid Science, 29:671–680.
Oztürk, H. (2004a). Energy and exergy efficiencies of a solar box cooker. International
Journal of Exergy, 2:202–214.
Oztürk, H. (2004b). Experimental determination of energy and exergy efficiency of the
solar parabolic–cooker. Solar Energy, 77:67–71.
Oztürk, H. (2005). Experimental evaluation of energy and exergy efficiency of a seasonal
latent heat storage system for greenhouse heating. Energy Conversion and Management,
Oztürk, H. and Bascetincelik, A. (2003). Energy and exergy efficiency of a packed-bed
heat storage unit for greenhouse heating. Biosystems Engineering, 86:231–245.
Pacheco, J., Showalter, S., and Kolb, W. (2001). Development of a molten salt thermocline
thermal storage system for parabolic trough plants. In Proceedings of Solar Forum 2001,
Solar Energy: The Power to Choose., pages 1–9, Washington, USA.
Palavras, I. and Bakos, G. (2006). Development of a low–cost dish solar concentrator and
its application in zeolite desorption. Renewable Energy, 31:2422–2431.
Patankar, S. (1980). Numerical Heat Transfer and Fluid Flow. Hemisphere Publishing
Corporation/ McGraw–Hill Company, NewYork.
Petela, R. (2005). Exergy analysis of the solar cylindrical–parabolic cooker. Solar Energy,
Pomeroy, B. (1979). Thermal energy storage in a packed bed of iron spheres with liquid
sodium coolant. Solar Energy, 23:513–515.
Rabl, A. (1985). Active Solar Collectors and Their Applications. Oxford University Press,
Ramadan, M., El-Sebaii, A., Aboul-Enein, S., and El-Bialy, E. (2007). Thermal performance
of a packed bed double–pass solar air heater. Energy, 32:1524–1535.
Reddy, A. and Rao, A. (2007). Prediction and experimental verification of performance
of box type solar cooker–part i. cooking vessel with central cylindrical cavity. Energy
Conversion and Management, 48:2034–2043.