Power-to-heat systems can be used both in the low-temperature range as well as in the high-temperature range (steam).
Immersion heaters or heating elements are used predominantly in the decentralized low-temperature range, while electrode steam boilers are used in the high-temperature range. The generation of process steam of up to 30 bar is technically possible by means of an electrode steam boiler. The saturated steam generated in this fashion can be superheated to higher temperatures by means of a downstream electrode flow heater and hence also fulfil higher requirements with regard to the steam generation.
Steam generation is based on the electrical resistance of the boiler water, whose electric conductivity should be analogously low (adjusted to 60 μS/cm). Efficient water conditioning is mandatory to achieve this. The efficiency of the electrode steam boiler is dependent in large part on the available water quality and is generally considerably higher than the efficiency of a steam boiler, because no thermal losses caused by hot waste gas occur.
Heat accumulators are not part of the standard outfit of a power-to-heat system, but they can enable a more flexible use. For instance, in the event of demand for balancing power, power-to-heat systems with heat accumulators can participate in the secondary and minute reserve market, without also being required to feed the heat into the district heating network (e.g. in the summer).
Investment costs for electric boilers used predominantly in the district heating network amount to approximately 75-100 euros per kilowatt. Costs vary depending on the area of application and the existing infrastructure. In the high-temperature range, the system must meet stricter requirements, and investment costs may therefore be twice as high. They amount to approximately 100-200 euros per kilowatt. Large-scale systems are particularly lucrative; at current prices in the balancing market, they can be amortized as early as after three to five years. Smaller, decentralized systems are less profitable, because they are associated with higher specific costs, while the specific revenue is the same.
Annual maintenance costs amount to nearly 3% of total investment and are comparable to those of a natural gas boiler system. There are no traditional wear parts, but a power-to-heat system requires still two days of maintenance each year. The pumps, valves and water/steam/condensation systems etc. are subject to the same wear as a conventional steam boiler system.
With a central power-to-heat system, the heat converted by the power is fed into a supply grid. The district heating system of a municipal utility is a classic example of such a heat supply grid. With it, the heat generated is transferred to the consumer through an isolated pipe system.
The advantage of CHP plants, such as a biogas CHP unit combined with a power-to-heat system is that they work very efficiently together, and there is often a connection with the subsequent utilization of the heat. The control of the power-to-heat system achieves a continuous operation of the CHP system, which can thus provide full capacity.