| Abstract |
As of the middle of 2019, more than 15,000 geothermal plants with heat pumps have been built in Lower Saxony, a federal state of North-West Germany. Most of these plants are being operated with borehole heat exchangers as closed systems with the majority of them being used for heating family homes. However, more and more plants supply heating and cooling for commercial, residential, office or industrial buildings. Compared to family homes, these buildings have significantly higher requirements with regards to the the development of the heat source, depending on whether, when, how long and how much heating or cooling is required. To comply with these demands, several adjacent borehole heat exchangers need to be built and combined into a field with an output usually in excess of 30 kW. As of the middle of 2019, about 300 borehole heat exchanger fields have been built in Lower Saxony. Planning these borehole heat exchanger fields requires much more knowledge and input data than planning a borehole heat exchanger for a family home. According to our experience, input data such as monthly heating and cooling demand are usually only based on estimates from an early phase of planning, which may be supplied by the architect or planning company. Dimensioning of the plant, which is part of the application documents in order to obtain the required license issued under German water law, often relies on these estimates. This means that, for example, the length and number of the boreholes are defined by the estimated heating and cooling capacity. First experiences on realized geothermal projects with borehole heat exchanger fields indicate that the actual demand may differ significantly from these estimates. This could be demonstrated by an undersized plant where the heat quantity taken from the field is much larger than the heat flow available to reinstate the required temperature of the ground for sustainable use. As a result, the temperature in the borehole heat exchanger field will drop continuously. This process generally takes place over a period of several years and can only be reversed over a long period of time, typically also in the time frame of years. At a critical point the temperature control of the heat pump would cause the failure of the plant. To detect potentially problematic temperature changes, it is necessary to undertake a proper monitoring of borehole heat exchanger fields. The monitoring, which is a requirement of the license issued under water law, is composed of several aspects, such as: - electric power consumption of the plant, - heat quantity taken from the field, - heat quantity brought to the field, - flow and return temperature of the heat carrier fluid, - subsurface/groundwater temperature at a control point within the field. This data allows the plant operator and the authorities to compare the estimated demand used for planning with the actual demand during operation, as well as the planned versus the actual temperature changes in the field. The operator can react by adjusting the heat demand covered by the geothermal plant or by adding more borehole heat exchangers or alternative heat sources to the system well before a potential failure of the plant. Therefore, the monitoring allows for the optimized running of the plant. Hence, the regulation process in Lower Saxony controls the plants from the application procedure throughout the operation and monitoring phase in order to achieve optimum results for economy and ecology. |