|Title:||A multi-functional solar assisted ground coupled heat pump system for space heating and hot water supply|
|Subject:||Ground source heat pump systems.|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Building Services Engineering|
|Pages:||xxi, 120 leaves : ill. (some col.) ; 30 cm.|
|Abstract:||Solar energy and geothermal energy are considered promising renewable sources for energy saving applications in buildings. Solar heat can be collected for space heating and domestic hot water (DHW) supply, while geothermal energy could be ideal heat source or sink for heat pump systems. Geothermal heat pump systems with vertical/horizontal heat exchangers, also called ground coupled heat pump (GCHP) systems, are considered relatively efficient for heating, cooling or hot water supply in cold areas. Because underground soil temperature is rather constant compared with ambient air temperature, the GCHP could achieve higher energy efficiency as well as more stable performance compared with traditional air source heat pumps. However, geothermal potential as heat source or sink could be impaired under short-time continuous operation or long-term imbalanced-load conditions, which has been observed and discussed by many researchers. As a result, the solar assisted ground coupled heat pump (SAGCHP), a technology to couple solar energy collecting system with the GCHP, could provide a possible solution to the abovementioned problems of a GCHP system and improve the stability and efficiency of the heat pump system for space heating. Furthermore, additional solar energy could be utilized for DHW supply and soil recharging during non-heating seasons. This thesis intends to carry out optimization, numerical simulation and experimental studies on the energy performances of this combined system for space heating and hot water supply in cold areas of northern China. First of all, a SAGCHP system with an energy storage tank is proposed and its mathematical models are constructed in the TRNSYS environment. The new system mainly consists of a ground heat exchanger (GHE) subsystem, a solar collecting subsystem with heat storage, a water to water heat pump and a DHW supply subsystem, which could achieve multi-mode space heating, water heating, heat storage as well as soil recharging based on heating requirements, weather conditions and control strategies. The model optimization is then performed under the weather condition of Beijing. The typical meteorological year (TMY) weather data of Beijing has been generated from the Meteonorm. Five models in different system configurations are compared by simulation to explore the optimal coupling method of solar collectors and GHEs. The optimal mass flow rate in the solar collectors and the storage factors are determined by evaluating the heating performance and the solar fraction. The ratio of the collector size and GHE loop length is further investigated by parametric studies including the heat pump efficiency, energy savings and economic factors. In addition, effects of alternative control strategies are discussed by comparing the system performances to realize flexible change between diverse working modes and maximum utilization of solar heat.|
After optimization procedures are finished, numerical simulations are performed for operation of 20 years under the meteorological conditions of Beijing. The simulating results show that the long term yearly average space heating efficiency is improved by 26.3% compared to a traditional GCHP system because the solar thermal collecting system is used to elevate the thermal energy in the soil and to provide direct space heating with heat storage. At the same time, the underground heat load imbalance problem for a heating load dominated GCHP is solved by soil recharging during non-heating periods, while extra solar energy is utilized to supply DHW. The operational characteristics of the system in one simulation year are also investigated with specific analysis on each working mode. Furthermore, the energy balance of the optimized design is confirmed with a minor difference of 0.75%, and the system is proved more efficient and economical for its application in Beijing area than in other cold weather regions like Harbin. Finally, experiments are carried out under the weather conditions similar to Beijing to study the practical operation features and confirm the applicability of the SAGCHP system. The test rig was installed at the Hebei Academy of Sciences in Shijiazhuang (lat. N38° 03', long. E114° 26'), China. Solar collectors are in series connection with the borehole array through plate heat exchangers. Four operation modes of the system were investigated throughout the coldest period in winter (Dec 5th to Dec 27th). The heat pump performance, borehole temperature distributions and solar colleting characteristics of the SAGCHP system are analyzed and compared when the system worked in continuous or intermittent modes with or without solar assisted heating. The SAGCHP system is proved to perform space heating with high energy efficiency and satisfactory solar fraction, which is a promising substitute for conventional heating systems. It is also recommended to use the collected solar thermal energy as an alternative source for the heat pump instead of recharging boreholes for heat storage because of the enormous heat capacity of the earth.
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