Increasing the flexibility of CHP plants by means of high-temperature thermal storage facilities

In combined cycle gas turbine plants (CCGT) including steam extraction for production processes in industry or for district heating of residential homes (CCGT/CHP plant) power generation, as a rule, is heat controlled, which is why such power stations – being must-run capacities – are unable to react flexibly to the current demand for electricity. Thermal storage facilities permit electricity and heat production to be decoupled timewise and have been used in combined heat and power (CHP) plants for a long time. However, their use has so far been restricted to the provision of hot water. The "FleGs" research project (Increasing the flexibility of combined cycle power plants by using high-temperature thermal storage systems), funded by the German Federal Ministry of Economics and Technology, developed a plant scheme with a novel solids-based thermal storage facility. The aim was to increase the deployment flexibility of CCGT/CHP plants to compensate for the fluctuating input of renewables-based electricity to the grid, to stabilise the network and to make balancing energy available.

Adapting CHP operation to electricity demand

The integration of a high-temperature thermal storage facility into the power plant process enables the supply of electricity and heat to be decoupled timewise. The operation of the gas turbine is restricted to times of high demand for electricity, and heat produced beyond the customer's demand is "parked" in the storage system. In times of low demand for electricity – for instance at night or when wind or solar power output is high – the gas turbine can simply be taken out of operation since heat is still supplied from the thermal storage facility.

In this way, "FleGs" will widen the applications of efficient and resource-conserving cogeneration by making the deployment of these power plants more flexible. Especially increasing, non-demand-driven capacities in the generation pool, for instance in growing offshore wind farms, impose greater flexibility requirements on other power stations.

Key component: High-temperature thermal storage facility

Zoom FleGs investigates ceramic shaped bricks and natural stone fillings as internals for thermal storage facilities. .The centrepiece of the "FleGs" project, which was coordinated by RWE, was the development of a novel solids-based thermal storage system for CCGT/CHP plants suitable for temperatures above 500°C. The project was carried out in collaboration with the companies Hitachi Power Europe Service, Paul Wurth Refractory & Engineering and the Institut für Technische Thermodynamik des Deutschen Zentrums für Luft- und Raumfahrt (DLR) between end of 2009 and end of 2012.

In April 2012, RWE commissioned a new trial plant at the innovation centre at Niederaussem. Here we tested if and which natural stones are suitable for the process. Above all, different volcanic rocks were being explored.

The use of this innovative technology improves the security of supply of heat and electricity produced in CCGT/CHP plants. At the same time, demand-driven, market-oriented power generation improves the revenue situation and renders the economic efficiency of cogeneration more attractive. The target group for marketing the newly developed thermal storage facility, besides power stations, is industrial plants and municipal utilities.

The market environment for storage technologies has become more difficult since the completion of the "CHP Heat Storage (FleGs)" project. This is due, above all, to the decreasing value of storage facilities on the electricity market. Nevertheless, we will continue to explore this issue, for instance in a joint investigation with a company from the food industry, which was launched in the autumn of 2015. The aim of this investigation is to ascertain to what extent high-temperature thermal storage facilities, in the current market environment, are an attractive option for more flexibility in existing CHP plants, taking the Company's demand for energy into account..

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