Gas and heating networks store electricity
Germany’s electricity provision needs to be made more flexible in future in order to integrate the increasing proportion of fluctuating renewable energy. Heating and gas networks can already balance out some of these fluctuations and help to stabilise the future energy system. In the Multi-Grid Storage project, scientists at Fraunhofer IFAM have investigated how the energy from the surplus electricity can be stored as heat or gas, and then made available again as electrical energy when required.
In the Multi-Grid Storage research project, researchers at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) have analysed how electricity, gas and heating supplies can be interlinked in order to balance out inflexible electricity generation. Energy from surplus electricity can be fed via electric boilers or heat pumps into heating systems or fed into the gas network by means of electrolysis. “If an electricity generation plant – such as a CHP plant – is connected to the same network, electricity is in effect discharged from the system,” explains Max Fette, project manager of the Multi-Grid Storage project.
A conversion chain consisting of an electrolyser, gas network and CHP plant – optionally in combination with a methanation system, can be understood as a so-called storage chain. The same applies – if only in balancing terms – for a chain consisting of an electric boiler or heat pump together with a heating network and CHP plant. In this case, the heat itself is not converted back into electricity but rather the gas saved through the heat being produced from electricity rather from gas. Storage chains therefore fulfil the same function as conventional electricity storage systems. In a first step, researchers from the Multi-Grid Storage project – MuGriSto for short – compared the efficiency and costs of the different storage chains. “Heating chains can already be relatively efficient and cost-effective,” says Max Fette and adds: “This initial analysis showed that the costs of the ‘gas chains’ currently lie far above those of the other storage chains investigated.”
Model analyses the electricity market
The scientists have also developed a model called MuGriFlex. It calculates the interactions between different energy converters within heat- or gas-based storage chains. The model analyses energy conversion chains in terms of their economic and technical aspects, which are dependent on variable system characteristics and the energy industry-related economic framework conditions.
Based on current electricity prices, MuGriFlex optimises on an hourly basis the electricity procured from electric boilers or electrolysers as well as the electricity produced from CHP plants. Surplus heat is stored and the remaining heating requirement is covered by the storage system or from a gas boiler. The course of the electricity prices and the heating requirement are inputted into MuGriFlex as default values. System parameters such as the plant capacity, efficiency and fuel costs, as well as energy industry-related economic framework conditions such as the duties paid on the electricity procured or CHP subsidies, can be varied as desired. The parameters investigated include, for example, the dependence of the full-load hours and the payback time on:
- the size of the heat storage system in the Power-to-Heat system,
- the size of the duties for electricity for Power-to-Heat and Power-to-Gas systems,
- the size of the CHP surcharges,
- the gas prices or efficiencies of Power-to-Gas systems.
Further developed market mechanisms
The scientists discovered that certain framework conditions such as grid charges are detrimental for the flexible use of heating and gas networks. The researchers have therefore developed proposals for changing the existing framework conditions. “Our analyses show, however, that care is required when intervening in the framework conditions. For example, a slight change in the duties paid on electricity for Power-to-Heat systems can make it attractive to operate heat pumps during hours when the heat pumps do not benefit the system. This is because they would further increase the residual load during this time,” explains the project manager.
Based on underlying assumptions such as the investment costs and the development of future spot market prices, efficiencies and plant costs, the MuGriSto researchers additionally discovered that it may also not be possible to operate the Power-to-Gas systems economically in future. In order to increase the number of full-load hours and thus help to balance out the fluctuating renewable electricity provision, subsidies or different sales channels are required for the green gas generated with the systems.
The researchers at Fraunhofer IFAM are also planning to use and develop the model in future in order to answer further questions concerned with the interaction between the electricity, heating and gas sectors.