High-temperature heat storage systems for flexible CCGT power plants

A high-temperature heat storage system for combined cycle gas turbine (CCGT) plants has been under development since 2009. The integration of such a storage system in the power plant process enables the electricity and heat provision to be decoupled for specific periods. The process therefore enables, for example, the surplus heat produced in addition to customer requirements to be “parked” in the storage system during the day and then made available again at night when the gas turbines are switched off. This form of separation in CCGT plants, which are operated using cogeneration (CHP) and therefore have a high fuel energy utilisation and low CO₂ emissions, was previously not possible.

The demands for flexible power plants are also increasing in view of the increasing non-needs based capacities in the generation pool, for example from the increasingly larger offshore wind farms. CCGT plants operated using cogeneration currently often have the disadvantage that the operation is no longer based on electricity requirements but on heat requirements. For this reason, power plants that also generate heat must also continue to operate when there are low electricity requirements in the mains grid, for example at night. The use of the storage technology is therefore intended to increase the economic efficiency of the CCGT plants. For this reason, many industrial enterprises and municipal utility companies are also very interested in developing the heat storage system.

Around two and half years have been earmarked for the project development. Once the first development phases have been successfully concluded, the cooperation partners are looking to construct a pilot plant with an output of ten megawatts. A possible location from 2011 could be the CHP power plant in Dortmund that belongs to RWE Power. A preliminary study has already confirmed the underlying technical and economic viability of the project. The feasibility of a CCGT plant with an integrated thermal storage system depends on the availability of a cost-efficient, reliable and durable high-temperature heat storage system in a large-scale commercial design.

The project encompasses the development of storage concepts in accordance with operator requirements, their testing at a laboratory scale and the basic engineering for a pilot plant to be constructed following the project. Laboratory studies to determine suitable materials for storage fixtures and insulation will consolidate the storage concept. In addition, issues relating to the integration of storage systems in the power plant process and operational aspects are being investigated, whereby the target application of a full-scale CCGT plant with a heat storage system shall be kept in mind and developed in the project in the form of front-end engineering and design (FEED).

In addition, a globally competitive power plant process will be available which – following the planned demonstration of its reliability on a large scale – will have excellent prospects for commercial success. This will particularly apply when the expansion of electricity generation not based on electricity consumption is realised on a large scale (CHP plants and wind turbines).

The core of a new test rig for investigating high-temperature storage systems at the DLR Institute for Technical Thermodynamics in Stuttgart is provided by a five-metre high storage container (HOTREG). It enables the scientists to test out different storage concepts, operating methods and materials.

In the centre of the new system is a solid storage tank for operation with hot air with adjustable air pressures. The storage tank contains a replaceable internal container. This enables scientists to quickly substitute various test structures consisting of different storage fixtures and high-temperature insulation for different test purposes. These include, for example, ceramic or stone storage materials and differently shaped storage materials. In addition, all determining operating parameters such as the temperature, pressure, air flow and air humidity can be varied across a wide range.

A characteristic feature of heat storage tanks are their highly specific requirement profiles in accordance with the respective applications. This working area therefore requires the use of a wide range of storage technologies, materials and methods, whereby the work focuses on medium- and high-temperature applications.

In particular, the following activities are being focussed on:

• The development and experimental qualification of components and subsystems
• The selection and qualification of solid storage tank materials for sensitive heat storage up to 1,000 °C
• The selection and qualification of phase change materials for latent heat storage up to 350 °C
• The optimisation of operating strategies