Impact of the natural temperature regime of the upper layers of Earth on efficiency of a hydrothermal heat pump system

Abstract

This article focuses on current issues of alternative geothermal heating development, by using low-potential thermal energy of upper aquifers by heat pumping technology. This type of heat supply is sustainable, does not require fossil fuel consumption and does not pollute the environment. In recent years, heat pumping technologies have become common. A lot of heating systems are implemented, using low potential energy in the air, open bodies of water or soil. Although, now, the heat of groundwater in upper aquifers is a more efficient and reliable source of energy for heat pumps. It was tested in this study. The purpose of this study is to impact assessment of the heat regime of the ground upper layers on efficiency of operation of heat pumping units, using groundwater of near-surface horizons. The factors founded, which form the natural heat mode within the studied area, also the correlation established between temperature changes in the horizon and the operation of the heat pump unit, based on the experimental heat pump unit, established by the Institute of Renewable Energy of the National Academy the Sciences of Ukraine. The article presents an experimental hydrothermal heat pump system developed and constructed at the Institute of Renewable Energy of the National Academy of Sciences of Ukraine, which consists of a heat pump and two wells through which water circulation from the underground horizon to the heat pump. The study describes the characteristics of measuring equipment installed on a hydrothermal heat pump system and described developed an interactive scheduling system based on the software product ESM (Engineering Systems Manager) using the programming language FBD (Function Block Diagram | Continuous Function Chart). This software product was used to create the visual system and archival data system that were obtained in the course of this work. The benefits of this study are that the experimental installation uses the thermal energy of groundwater of the Poltava-Kharkov aquifer as the primary energy source. The interval of the productive horizon is 32-57 m. The groundwater level in the horizon is set at about 40 m. In addition, the air temperature between the pipe space of the observation well and the groundwater temperature in the aquifer are monitored. The duration of regime observations was six months, the frequency of measurements – 5-15 seconds. The monitoring results indicate that despite the fact that the productive horizon is at a depth that significantly exceeds the usual depth of the neutral layer for the territory of Ukraine, the for- mation temperature is not stable and its amplitude is 2 oC. According to the authors, the increase in the depth of the surface of constant annual temperatures may be due to local features of the studied area, namely: increasing the absorption surface of solar radiation due to terrain, the presence of lateral heat inflow, the presence of water-saturated layers in the upper part. Consequently, based on the analysis of geological and hydrogeological conditions of the site, as well as technological processes occurring in the installation, the main factors that form the natural temperature of the upper layers of the earth are identified. As a result, the percentage of energy efficiency drop of the installation is calculated depending on the decrease in the temperature of the natural heat carrier in the aquifer. It is established that it is necessary to conduct additional research to assess the environmental impact of the use of aquifers for energy purposes and to ensure the optimal mode of operation of aquifers, which would be as close as possible to their natural regime.