Solutions To Key Geological Problems of Super Large-Scale Ground-Source Heat Pump Underground Heat Exchange System

Solutions to key geological problems of super large-scale ground-source heat pump underground heat exchange system

WEI Wanshun1, 3, LI xiang2, 3, ZHENG Jia2, 3, LIU Shaomin2, 3, ZHANG Jinping2, 3 (1. Beijing Academy of Science and Technology, Beijing 100083; 2. Beijing Institute of Geology, Beijing 100195; 3.Key Laboratory of Shallow Geothermal Energy, Ministry of Natural Resources , Beijing 102218)

Abstract: In the past 15 years, the global ground-source heat pump industry has developed rapidly, and the large-scale trend of ground-source heat pump system has become obvious. The super large-scale ground-source heat pump systems with more than one million m2 appears constantly. In terms of the development situation of ground-source heat pump industry, it can be described as “mingled hope and fear”. "Hope" is that the shallow geothermal energy is rapidly utilized on a large scale using ground source heat pump, due to the government's attention, social recognition and technological advances. “Fear” is that the rapid and rough development has also brought a series of problems, especially the geological problems of the super large-scale ground-source heat pump underground heat exchange system, which has seriously restricted the large-scale, sustainable and efficient development and utilization of shallow geothermal energy by using ground-source heat pump technology, which needs high attention. The key geological problems of super large-scale ground-source heat pump underground heat exchange system are the distribution law of shallow geothermal energy in subsurface rock-soil body, thermal response properties and the characteristics of heat exchange contribution rate. Through the research on the monitoring data of a ground-source heat pump underground heat exchange system for more than ten years, it is found that: (1)The subsurface rock-soil body in the ground heat exchanger zone is not a "homogeneous body". There are obvious differences in the original temperature of rock-soil body at different depths, and the stratification of variable temperature zone, constant temperature zone and increasing temperature zone is not obvious. The geothermal field is a relatively open system controlled by geological conditions, and its shallow geothermal energy distribution law is obviously restricted by geological conditions and is irregular. (2)During the operation of ground-source heat pump, different parts of subsurface rock-soil body have different thermal (cold) responses to the water temperature change at the ground-source side of the system, both positive and negative response, and there are obvious differences in the response degree of the same response properties, which can be characterized by the positive and negative temperature difference of thermal (cold) response. (3)During the operation of ground-source heat pump, the characteristics of heat (cold) exchange contribution rate of different parts of subsurface rock-soil body and different heat exchange holes are obviously different. In the heating season, about 1/7 heat exchange holes have a negative contribution rate, which means 1/7 holes "do not supply heat" in the heating season. In the cooling season, about 2/7 heat exchange holes have a negative contribution rate, which means 2/7 holes "do not provide cooling" in the cooling season. It appears that he laying of ground heat exchanger in the form of equal spacing grid was obviously unscientific in the past and the larger the laying area of buried pipes, the more unscientific it is. Therefore, we propose a "four methods" solutions to the key geological problems of the super large-scale ground-source heat pump underground heat exchange system in order to scientifically support the efficient, sustainable and large-scale development of ground-source heat pump industry: the “quantitative study “on the genetic mechanism of shallow geothermal energy; the "refinement" of the exploration and evaluation of shallow geothermal energy in subsurface rock-soil body; the "intellectualization" of the operation and control of ground-source heat pump system; the "scientization" of multi energy coupling.