Zheng, Xiaofeng
(2013)
Exploration and development of domestic thermoelectric cogeneration system.
PhD thesis, University of Nottingham.
Abstract
Due to quiet operation, no moving parts, long lifespan and compact structure, the thermoelectric application has become a potential green technology which has been used in different areas in the efforts of contributing to achieve simplified and compact system structures and environmental friendliness. Its applications cover a wide range from the earliest application on kerosene lamp to aerospace applications, transportation tools, industrial utilities, medical services, electronic devices and temperature detecting & measuring facilities. Its disadvantage lies in the low conversion efficiency which only converts small amount (for Bi2Te3, up to 5%) of harvested energy to electrical power. It makes the use of the TEG system far from being economically feasible due to long cost recovery period. Consequently, its use is limited to specialised area where it is unnecessary to consider the cost of the thermal energy input and system cost recovery.
This research aims to explore a way of widening the application range of thermoelectric generation based on introducing a potential direction of improving energy utilisation efficiency to a higher level by adopting thermoelectric cogeneration concept in residential house. It focuses on investigating the practicality of using thermoelectric applications in domestic sectors where the large amount of heat is exhausted to environment without being used and developing thermoelectric cogeneration system to generate electricity and produce pre-heated water for domestic use by recovering the waste heat from the domestic boiler and utilising the on-site solar energy. With the conversion efficiency given by the current commercially available thermoelectric modules, the optimised heat exchanging regimes and systems for thermoelectric applications have been comprehensively studied from the aspects of system design, integration, experimental study, numerical simulation and modification. The importance and necessity of effective heat exchanging methods have been emphasised by the experimental and numerical proofs for the development of a domestic thermoelectric cogeneration system with higher thermal efficiency. The impacts of this domestic energy solution have been evaluated from the aspects of the improvement for outdoor environment and indoor energy profile, as well as economic benefit.
For the flue gas heat exchanger, the model with sudden expansion and gradual constriction has been identified gives in terms of overall performance. The model with sudden expansion, gradual constriction and staggered pipe layout and the one with sudden expansion, gradual constriction and inline pipe layout show better overall performance than other models. Among these two models, the one with staggered pipe layout shows better performance than the one with inline pipe layout in the velocity range of 3.6m/s-5m/s, whilst the one with inline pipe layout shows better performance between 0-3.6m/s. For the cold side heat exchanger, the one with four ø5mm branch channel angled at 90 against the main channel delivers the best overall performance out of 9 cooling plates built according to three variables. Experimental studies show the one-stage TCS produces more power than the two-stage TCS does when the heat input is supplied at 47W and 60W. As the heat input increases, the power output of two-stage TCS gets closer to that of one-stage TCS.
In the system construction and assembly, uneven assembly can lead to a 20% drop in conversion efficiency. The pressure load at 18lpsi gives the highest power output out of five load values, which are 136psi, 159psi, 181psi, 204psi and 227psi, respectively. In comparison with individual assembly, module thickness difference in whole assembly degrades the system conversion efficiency. The cost recovery period of deploying this system in a residential house installed with a 24kW boiler and a 1 m2 solar collector has been evaluated. Based on the conversion efficiency and thermal efficiency that is 4% and 67% at 130C temperature difference. the house can produce 98W electricity and 1640W useable heat when the boiler is running and the contribution from the solar energy is included. It takes less than 4.2 years to recover the system cost.
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