Wang, Zhangyuan
(2012)
Investigation of a novel façade-based solar loop heat pipe water heating system.
PhD thesis, University of Nottingham.
Abstract
Solar thermal is one of the most cost-effective renewable energy technologies, and solar water heating is one of the most popular solar thermal systems. Based on the considerations on the existing barriers of the solar water heating, this research will propose a novel façade-based solar water heating system employing a unique loop heat pipe (LHP) structure with top-level liquid feeder, which will lead to a façade-integrated, low cost, aesthetically appealing and highly efficient solar system and has considerable potential to provide energy savings and reduce carbon emissions to the environment.
The research initially involved the conceptual design of the proposed system. The prefabricated external module could convert the solar energy to heat in the form of low-temperature vapour. The vapour will be transported to indoors through the transport line and condensed within the heat exchanger by releasing the heat to the service water. The heated water will then be stored in the tank for use.
An analytical model was developed to investigate six limits to the loop heat pipe’s operation, i.e., capillary, entrainment, viscous, boiling, sonic and filled liquid mass. It was found that mesh-screen wick was able to obtain a higher capillary (governing) limit than sintered-powder. Higher fluid temperature, larger pipe diameter and larger exchanger-to-pipes height difference would lead to a higher capillary limit. Adequate system configuration and operating conditions were suggested as: pipe inner diameter of 16 mm, mesh-screen wick, heat transfer fluid temperature of 60oC and height difference of 1.5 m.
This research further developed a computer model to investigate the dynamic performance of the system, taking into account heat balances occurring in different parts of the system, e.g., solar absorber, heat pipes loop, heat exchanger, and tank. Data extracted from two previously published papers were used to compare with the established model of the same setups, and an agreement could be achieved under a reasonable error limit.
This research further constructed a prototype system and its associated testing rig at the SRB (Sustainable Research Building) Laboratory, University of Nottingham and conducted testing through measurement of various operational parameters, i.e., heat transfer fluid temperature, tank water temperature, solar efficiency and system COP (Coefficient of Performance). Two types of glass covers, i.e., evacuated tubes and single glazing, were applied to the prototype, and each type was tested on two different days of 8 hours from 09:00:00 to 17:00:00. By comparison of the measurement data with the modelling results, reasonable model accuracy could be achieved in predicting the LHP system performance. The water temperature remained a steady growth trend throughout the day with an increase of 13.5oC for the evacuated tube system and 10.0oC for the single glazing system. The average testing efficiencies of the evacuated tube system were 48.8% and 46.7% for the two cases with the testing COPs of 14.0 and 13.4, respectively. For the single glazing system, the average testing efficiencies were 36.0% and 30.9% for the two cases with the COPs of 10.5 and 8.9, respectively. Experimental results also indicated that the evacuated tube based system was the preferred system compared to the single glazing system.
This research finally analysed the annual operational performance, economic and environmental impacts of the optimised evacuated tube system under real weather conditions in Beijing, China by running an approved computer model. It was concluded that the novel system had the potential to be highly-efficient, cost-effective and environmentally-friendly through comparison with a conventional flat-plate solar water heating system.
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