Research On The Performance Of Underground Heat Exchanger Of Heating (Cooling) System With Solar Soil Storage

Solar with ground source heat pump (SGSHP) is taken as an air conditioning technique which has the greatest future, energy saving and environmental protection in the 21st century, since it utilizes solar radiant and renewable underground heat. Solar energy is regarded as the main heat source of the heat pump. The development of SGSHP is restricted by a lot of factors, and storage of solar energy, heat transfer of underground heat exchangers and heat balance of soil are three key points of research.Based on the researches done on the SGSHP and energy storage technology, a novel system combined the two systems was proposed and named as heating (cooling) system of solar seasonal soil storage. It consists of a solar energy collecting system, soil storage system, a heat pump unit, floor radiant terminal device. The main study object is underground heat exchanger. The research is based on the following aspects:First, three-dimensional numerical model of underground heat exchanger was established using finite element method. Simulation was done for observing soil temperature field variation and resumption and soil temperature was enhanced 1℃after two years running. The system is suitable in cold weather areas. Solar energy is the main heat source and ground source heat pump is the auxiliary heat source in winter; cooling can be extracted by underground heat exchangers directly in summer. Soil storage is a long period energy storage process that goes along in three seasons (spring, summer and autumn). It can not only maintain the balance of soil, but also supply the guarantee for heating of next winter.Second, based on the numerical model, operation characteristics of single U-type heat exchanger were simulated. Variation of temperature of different depths (5m, 30m, 50m), influence of different operation conditions on soil temperature, influence of soil properties and pipe materials on soil temperature were studied for two years'operation with period of year.Third, operation characteristics of muti U-type heat exchangers were simulated, too. Heat transfer effect between soil and heat exchangers was analyzed in four aspects: thermal physical properties of soil, material of pipes, and space of pipes and arrangement mode of pipes. Material of pipes has little influence on long period running solar storage soil storage system. It can be considered economically when choosing. Common PE plastic pipe has been chosen as the material in the system because of its low price.Forth, the most influential factor is arrangement mode of pipes. The best heat transfer ability is arrangement of circuit rotundity; the better ones are polygon and square arrangement; the worst one is rectangle one. Arrangement of strong heat transfer ability has large heat extraction per day of heat exchangers and high outlet water temperature in winter; meanwhile, it has low outlet water temperature in summer. It goes against floor radiant cooling. Rectangle arrangement has been used in the system. It can not get to the requirement of heating in winter though its heat transfer effect is the worst in four arrangement, but also accord with the demand of low temperature radiant floor cooling in summer.Last, demonstration engineering started to operate from Nov. 2007 which was established in Songbei district Harbin in order to research the operation characteristics of the solar seasonal soil storage heating (cooling) system which combined with solar heat pump, soil storage and floor radiant system together, and meanwhile, to validate the simulation results. Experimental test data: mean heating COP is 5.01 in middle heating period and 13.01 in end heating period. The results between the experimental measurements and simulations under the same (similar) conditions tally well, so that the numerical model was validated within acceptable accuracy and reliability.The novel heating (cooling) system of solar seasonal soil storage cannot be quite popular at present due to certain financial constrain. However, with energy sources decreasing, energy prices increasing and an awareness of the sustainable development increasing, it promises a bright future as an alternative means for heating (cooling) system. It is believed that the system could be popular with the development of the solar energy heating and air conditioning technologies. The study done in this thesis provides a fundamental and theoretical support for the further application of the system.