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• A constant-temperature thermal response test was proposed and demonstrated. • The new concept enables simultaneous heat injection and extraction tests. • A test data analysis method was developed based on the finite line source model. • Ground thermal conductivity was evaluated for both heat injection and extraction. • Convective effect enhanced ground heat transfer when heat injection was involved. Getting an accurate estimate of site-specific ground thermal conductivity from thermal response tests (TRTs) is crucial to the efficient and sustainable use of ground source heat pump systems. In a conventional TRT, the ground thermal conductivity is estimated by perturbing the ground with a positive heat injection and the response is measured in time. Despite the simplicity, the conventional TRT does not take into account the ground thermal response to cooling pulses. This may lead to mode-biased estimation due to the presence of natural convective effect. To address the problem, a constant-temperature TRT that comprises simultaneous heat injection and extraction was proposed. A test data analysis method was developed based on the finite line-source model. On this basis, a very first field test was performed in Taiyuan, China, demonstrating the use of the proposed TRT concept with full-scale measurements. Results show that, in the case of this field test, the ground thermal conductivities derived separately from heat injection and heat extraction are 1.83 W/(m K) and 1.65 W/(m K), respectively. The significant difference (10.9%) indicates that when heat injection is involved, the natural convective effect has enhanced ground heat transfer to increase the conductivity estimate. Thus, as compared to the conventional approach, the use of the proposed TRT can improve the characterization of the ground thermal response and conductivity. Relevant results and the test protocol reported in this study will be useful in providing the background information for this technology to be adopted in the ground source heat pump industry. [ABSTRACT FROM AUTHOR]

The "Thirteenth Five-year Plan for Geothermal Energy Development and Utilization" sets forth the guidelines and targets for the development and utilization of geothermal energy, and that becomes the basis of Chinese geothermal energy development and utilization during the "13th Five-Year" period. Due to the impacts of geological conditions, the use of geothermal energy is limited. Light and heat are abundant in the desert, however, because of its harsh conditions, shallow geothermal energy has not been effectively utilized. At present, China has actively explored the development of modern and sci-tech agriculture in the desert region. The application of shallow geothermal energy provides a better opportunity for the development of desert agriculture, especially the greenhouse. Ground source heat pump technology as an important way to apply shallow geothermal energy in China has been widely used. Geothermal conductivity has a great influence on the determination of the number and depth of boreholes, thus it affects the initial investment of the system. At the same time, inaccuracy of geothermal thermal and physical parameters lead to mismatches between the designed system and the actual load, so that they cannot give full play to their energy-saving advantages. Therefore, determining the geothermal thermal properties of accurately is the basis for the design of ground source heat pump system. In this paper, a desert source heat pump project in Ningxia desert area was used to carry out geothermal thermal response test. There were large sandstorms and no stable water source in the test area. The testing process in this project was designed according to the unique geomorphology and climatic conditions of the desert area. The heat transfer coefficient of ground heat exchanger in this desert area was tested by analyzing the average geothermal coefficient. The testing data were as follow: The soil temperature was 16.52 ℃; The average thermal conductivity of the soil was 1.12 W/(m·℃) by using the theory of linear heat source. In the cooling mode, the ground heat transfer was 40.94 W/m; and the heat transfer of the ground heat exchanger in the heating mode was 25.28 W/m. By comparing with the data of the related tests in the eastern part of China and combining with the operation scheme of the climatic conditions analysis system in the desert area, the operation scheme considers adding heat source in winter to assist heating, and a hybrid energy storage desert source heat pump system suitable for summer operation in the desert area was proposed, which could provide a solution for the limited use of shallow geothermal energy in desert areas. The results can provide a reference for the thermal response experiment in the desert area and the application of desert source heat pump system. [ABSTRACT FROM AUTHOR]