Study On Characteristics Of Buried Pipe-in-Pipe Heat Exchanger In Ground Source Heat Pump System

As a branch of heat pump technique, GSHP (ground source heat pump) has attracted close attention and become a more and more important selection for building energy conservation today, because of rising of energy price and urgent environmental pressure caused by excessive energy consumption, and also because of its significant energy saving effect. Specially, its application will be very significant in Yangtze River valley where whether is warmer in summer and cooler in winter.In order to supply technical guidance for GSHP system design, it is very important to find out heat transfer performance of buried pipe-in-pipe heat exchanger, which is key of GSHP technique. The author gives a completely literature review about buried pipe-in-pipe heat exchanger in GSHP system at first, and then outlines the research subjects to be studied in this Ph.D. thesis, that is, process optimizing of performance of a buried pipe-in-pipe heat exchanger, mathematical modeling of its heat transfer process, and results verification by field tests.Through developing a 2-dimensions heat transfer model of buried pipe-in-pipe heat exchanger and simulating the process by computer, the author figures out inter-relations of factors which influence characteristics of buried pipe-in-pipe heat exchanger, such as pipe diameter, pipe length, ratio of pipe diameters, flow rate, inlet water temperature, patterns of water inlet and outlet, and material features made of pipe, and proposes some optimizing indexes of buried pipe-in-pipe heat exchanger. The author studies the "heat--short circuit" phenomenon as well and gives some important results.By way of analysis of heat transfer process of the field-test model, the author formulated a 3-dimensions heat transfer model which is correspondent to actual performance of buried pipe-in-pipe heat exchanger. Coupled with field testing data, the short and long term operation features has been thoroughly analyzed by Finite Element Solution and Forward Finite-Difference Analysis of the heat transfer differential equations of buried pipe-in-pipe heat exchanger. By interpreting regularity of continuous and intermittent operations of buried pipe-in-pipe heat exchanger, its heat transfer mechanism has been further explained in this thesis. After carrying out a period of testing in which GSHP system operated for three months and then turned off for same period of time, that energy stored in soil and rocks around buried pipe-in-pipe heat exchanger can be used in anti-season manner, that is, winter cool could be used in summer and summer heat could be used in winter, has been proved by calculating temperature distribution in soil and rocks around buried pipe-in-pipe heat exchanger. According to field testing data analysis, the author find out that buried pipe-in-pipe heat exchanger, which act as a important part of GSHP system, can supply sufficient heat flow and its operation performance is better than that of a wind cooled--heat pump. The author also finds out the self-equilibrium mechanism GSHP system operated with buried pipe-in-pipe heat exchanger, which give experiences for design and operation of a GSHP system.A thoroughly study on characteristics of ground pipe-heat exchanger of GSHP has been made in this Ph.D. thesis. The research method used in this thesis is very useful for other ground energy storage system. The research results could be very significant for engineering application of GSHP technique.