Document Text (Pages 201-210) Back to Document

CHARACTERISATION OF A THERMAL ENERGY STORAGE SYSTEM DEVELOPED FOR INDIRECT SOLAR COOKING

by MAWIRE, ASHMORE, PhD


Page 201

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-

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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

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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.

169


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