Geosolar District Heating in ”Feldlager” in Kassel, Germany
The planning area „Zum Feldlager“ in Kassel is surrounded by existing buildings of the district and is located in an urban ventilation path. For that reason, combustion of oil or wood (fine dust emissions) should be avoided. Due to the location of the area a connection to the existing district heating network of Kassel is not feasible because of logistical and economic reasons. Instead, a local district scheme is implemented. The concept involves principally the use of renewable energy sources (RES) such as geothermal and solar energy for LTDH supply. The new housing estate will be characterized by a very compact and south oriented construction; 1-2 storey detached and semi-detached houses in the north, two-storey terraced houses in the centre and large three-storey apartment buildings in the south. All buildings have a specific heat demand of 45 kWh/ m²·a and a specific domestic hot water (DHW) demand of 730 kWh/person·a. Thus, the demand is below the maximum energy demand for new buildings (<50 kWh/ m²a) according to the valid German energy saving ordinance EnEV 2014.
Unlike typical new (smaller) district heating systems in Germany the district heating in the geosolar concept shall have only 40°C supply temperature. This temperature level is, however, enough to provide the space heating of buildings via surface heating systems. The LTDH network is fed by the central heat pump and the electric boiler during the heating period of 7 months. The heat pump uses the ground via borehole heat exchangers (92 boreholes, 120m depth) as a heat source. The geosolar heat supply concept combines central and decentralized heat sources (see Figure 84 below).
Domestic hot water preparation requires higher temperatures of minimum 45°C for single-family houses and 60°C for multi-family houses. It is mainly provided by distributed solar thermal systems in every building with an electrical back-up. Due to the rather high electricity prices in Germany, it is advantageous to preheat the hot water storage during winter using the district heating supply.
Simulation studies showed the collector area and preheating by district heating in winter will reduce the auxiliary energy demand to 17% of the total energy demand for domestic hot water. Decentralized solar domestic hot water systems enable low supply temperature and seasonal operation of the district heating. Both factors lead to very low heat transport losses of 2.5 % of annual heat generation. Such low heat losses are crucial for the economic feasibility of the district heating in areas with very low linear heat density (e.g. 500–1000 kWh/a·m). The results show that the geosolar heat supply concept leads to 61% lower primary energy demand and to 64% lower CO2 emissions comparing to the reference system (gas condensing boiler with solar DHW).
Furthermore, the total annual costs of both systems (include capital expenditures, operation, and maintenance costs) were calculated using current prices and interest rates. The total costs for the geosolar heat supply concept were estimated to be about the same or even lower than the reference system (Schmidt et al., 2017 / Schmidt and Kallert, 2017).
Finally, it was decided by the city council and the local utility company not to realise the suggested project concept. The main reason was that it would take too long time to get a drilling permission for the ground heat exchanger. Instead, a standard district heating system (supply temperature of about 80–90°C) operated with a gas-fired CHP unit will be realised and no innovative and future concept will be built. The chance to realise a good and carbon emission saving concept has not been taken, which is a tragedy for the implementation of the necessary energy transition.