The gas pressure regulating and measuring station (GPRS) Ostheim owned by the company Energienetz Mitte (ENM) has a heat demand of about 1,900 MWh/a for preheating the natural gas, which is throttled in the station from 90 bar to 16 bar in order to feed it into the downstream natural gas network. The heat requirement is due to the fact that the natural gas cools during expansion and must therefore be heated up by around 30 K before the throttle used for the expansion. Without the preheating of the natural gas before the expansion, the freezing point in the throttle would be undercut, which would lead to technical problems due to condensate, ice formation and the deposition of individual gas fractions in the pipelines.
This preheating is usually carried out by gas boilers and consumes approx. 0.2 % of the natural gas passed through the GPRS. In order to reduce this fossil energy demand at the GPRS Ostheim, a solar process heat plant with two large subfields was installed. 198 m² large scale flat plate collectors were installed directly on the roof of the GDRA, another 224 m² are located on a green directly in front of the building. In addition to the solar system, three gas heat pumps with 41 kW each were installed to further reduce the fossil energy demand.
The solar system and heat pumps initially supply the heat to a 21 m³ pressureless buffer storage tank. This heat is then transported via an intermediate circuit to a separate gas heat exchanger, which regeneratively or, in the case of heat pumps, efficiently preheats the natural gas to be expanded. In a separate downstream gas heat exchanger the four fossil gas boilers, each with a thermal power of more than 600 kW, heat the gas to the desired set point temperature if it couldn’t be reached in the first one.
The separate heat exchanger for preheating by the heat pumps and the solar process heat system enables very low return temperatures to the buffer storage tank, which increases the efficiency of the solar system. The expected useful solar heat yield is therefore approx. 600 kWh/m²a, which means that nearly 260 MWh/a of natural gas and 65 t/CO2 can be saved annually by using the solar process heating plant.
This solar process heat plant was originally designed to cover the whole summer heat load. However, due to the lack of available space the plant had to be built a bit smaller, which is why it can only provide just under ¾ of the summer heat demand. Overall, a yearly solar fraction of a good 11 % was thus achieved in the first year of operation. In addition, approx. 30 % of the heat requirement is provided by gas heat pumps.
The plant was realised as part of a heat supply contract. The operator of the GRPS (ENM) obtains the heat from the heat pumps and the solar system from the ESCO Enertracting for a contractually agreed price below the gas price. This means that ENM can achieve savings from day one without having to deal with the planning, financing, installation or operation of the solar process heat and heat pump system itself, as these tasks are performed by ESCO Enertracting.