Experimental Investigations and Simulation of Soil Temperature Distribution at a Depth Above the Neutral Layer
DOI:
https://doi.org/10.31649/1997-9266-2023-168-3-34-46Keywords:
renewable energy sources, geothermal energy, soil heat, neutral layer, geothermal heat pump systemAbstract
In the process of solving the problems of soil accumulation and extraction of heat from the near-surface layers of the Earth, there is a need to obtain information on the depth of annual temperature changes in the soil, which determines the layer of the earth’s surface that actively interacts with the near-Earth atmosphere. In the cold season, the temperature in it drops, and in the warm season it rises. It is known that the efficiency of a heat pump system depends both on the temperature difference at the outlet of the heat pump condenser and the inlet to its evaporator, and on the temperature stability of the heat source. The temperature at the inlet to the evaporator of the heat pump is determined by the temperature of the ground at the location of the heat collector. The greatest efficiency is achieved by heat pump systems with collectors installed below the neutral layer — the temperature of which is constant and equal to the average annual temperature of the soil in the area.
At the geothermal test site of the Institute of Renewable Energy Sources of the National Academy of Sciences of Ukraine, experimental studies of soil temperature changes at the installation site of vertical heat exchangers (collectors) were carried out. The research methodology is described. The characteristics of the measuring equipment installed on the experimental setup and the software used for archiving and visualization of the data obtained during the research are given. The depth of the neutral layer was determined and the obtained dependences of temperature change on depth were substantiated, taking into account the ambient temperature and other factors of exogenous impact.
Mathematical model is presented that makes it possible to determine the soil temperature T(z, t) depending on the depth z ≥ 0 and time t ≥ 0, provided that the change in the temperature of the soil surface or outdoor air over time is given, taking into account the assumption that the soil temperature does not depend on the coordinates (x, y) and the thermophysical properties of the soil do not change with the coordinates (x, y, z) over time. Based on the mathematical model, calculated data were obtained and graphs of the dependence of T(z, t) on depth per day and per year were plotted. The depth of the neutral layer is determined. Experimentally obtained as a result of research work on the thermal regime of the soil at the geothermal test site of the Institute of Renewable Energy of the National Academy of Sciences of Ukraine, correlate with the results obtained during mathematical modelling. The depth of annual temperature changes in the soil h, which determines the layer of the earth’s surface that actively interacts with the Earth’s atmosphere, in both cases is at around 15 m.
In the course of the study, the patterns of seasonal temperature changes in the upper layers of the Earth were confirmed. The analysis of the data obtained made it possible to conclude that it is necessary to take into account changes in soil temperatures during the year when solving the problems of accumulation and extraction of heat by geothermal heat pump systems. The obtained theoretical and practical results make it possible to improve the construction of geothermal systems. There are prospects for further studies of the influence of geological, hydrogeological morphological and anthropogenic conditions on the temperature deviation below the neutral layer and their influence on the efficiency of geothermal heat pump systems.
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