Investigation of novel thermoelectric refrigeration systems
Concern over global warming and depletion of the ozone layer has stimulated research to develop cooling methods that do not employ environmentally damaging working fluids such as CFCs and HCFCs. Two methods that have been considered are absorption and thermoelectric `Peltier' cooling systems. Absorption systems, using H20/LiBr have the advantage of being able to use low-grade waste heat. However, the large volume, high capital cost and low performance of these systems have inhibited their widespread application. Thermoelectric systems were developed in the 1950s and use of this technology for air-conditioning applications was investigated as early as the 1960s. However, the continued development of thermoelectric systems was slow owing to technical difficulties and the superior performance of vapour-compression systems in terms of coefficient of performance (COP). It is known however that most working fluids employed in vapour-compression systems are damaging to the environment, and as vapour-compression systems contain moving parts, they have the further disadvantage of being noisy and requiring regular maintenance. In recent years therefore, there has been stimulated interest in using thermoelectric "Peltier" cooling systems for domestic refrigerators and air conditioning. Investigation of novel thermoelectric refrigeration systems was carried out in this research. The systems use thermoelectric "peltier" coolers (thermoelectric modules) to produce cooling or heating. Thermoelectric modules are solid state heat pump, which have the advantage of being environmentally friendly, being quiet, have no moving parts and can operate using direct current supplied from photovoltaic solar cells (PVs). This work mainly investigated a passive technology based on integration of a thermal diode and thermoelectric modules for building integrated heat pump. The heat pump uses thermoelectric modules to produce cooling or heating, and the thermal diode to transfer heat in or out of the building, and prevent reverse heat flow in the event of power failure. The heat pump was designed to have the following features: • Very compact and suitable for incorporation within the building structure; • Does not require a plant room and simple to construct; • Easily switched between cooling and heating modes; • Can prevent reverse heat flow in the event of power failure; • Low manufacturing cost; • Environmentally friendly; • Can be driven by solar photovoltaic panels. This work also investigated the potential application of phase change materials (PCMs) in the thermoelectric refrigeration system. The system employs phase change material to improve the performance of a thermoelectric refrigerator as well as the cooling storage capability. The refrigerator employing phase change material was designed to have following features: • Be able to overcome the peak loads and losses during door openings and power-off periods. • Prevent reverse heat flow via thermoelectric modules in the event of the power being turned off by integrating the thermosyphon with the phase change material. • Low manufacturing cost. • Environmentally friendly. • Can be driven by solar photovoltaic panels The research initially involved the investigation of the performance of the components of the thermoelectric refrigeration systems, including thermoelectric modules, heat pipes and heat sinks. The analytical models were developed to evaluate the heat transfer and optimise the design of these components. Correlations between heat transfer and fluid flow inside the heat pipes were explored by computer modelling. The research work further involved the design, modelling, construction and tests of a thermoelectric heat pump prototype. A computer model was developed to evaluate the performance of the heat pump system for two different modes, i. e., cooling and heating, under various operating and ambient temperatures. Laboratory tests were carried out to validate the modelling predictions and experimentally examine the thermal performance of the heat pump. Comparison was made between the modelling and testing results, and the reasons for error formation were analysed and correction was given. Further experimental investigation showed that reducing the temperature of the condenser of the thermal diode could provide a significant improvement of the efficiency of the coefficient of performance (COP) of the system in cooling mode. This can be achieved by using the evaporation of water on the heat sink attached to the condenser. The research work also involved the design, construction and tests of a thermoelectric refrigerator employing phase change material. The work intended to investigate the potential application of phase change materials (PCMs) in the thermoelectric refrigeration system. The system was first fabricated and tested using a conventional heat sink system as the cold heat sink. In order to improve the performance and the storage capability, the system was reconstructed and tested using an encapsulated PCM as a cold heat sink. Results of tests of the latter system showed an improved performance compared with the former system. However, to improve the storage capability, in particular during off-power periods, it was found necessary to integrate the PCM with a thermosyphons, which would allow heat flow in one direction only. Results of tests carried out on the system employing phase change material integrated with thermosyphons showed considerable improvement in the storage capability of the thermoelectric refrigeration system compared with the previous ones. On the basis of the above investigation the further work for improving the performance of the thermoelectric refrigeration system was suggested, which is illustrated in Chapter 7, and its key technical issues are discussed.