High temperature heat storage systems

Metal-based heat accumulators can store process heat at high temperatures and thus make process steam generation in industrial power plants more flexible.

High temperature heat storages for combined heat and power plants and industry

Heat storage units (thermal energy storage units, latent heat storage units), in particular metal-based high-temperature storage units, can make the operation of industrial cogeneration plants more flexible by storing process heat and providing process steam. Operators can thus save costs for fuels in auxiliary boilers.

thermischer energiespeicher
© Fraunhofer UMSICHT, Stadlmayr
Pilot plant for the investigation of high-temperature storages/ heat storages at the Fraunhofer UMSICHT Centre for Energy Storage in Sulzbach-Rosenberg

With the use of metal alloys as phase change materials in heat accumulators / high-temperature accumulators, the process steam supply of industrial combined heat and power plants can become more flexible and efficient.

The costs of conventional process steam protection by steam boilers can be significantly reduced.

Actual situation

In addition to electrical energy, industrial cogeneration plants provide process heat for industrial plants and processes in industrial or chemical parks. During electricity production, steam from the turbine is fed into a process steam network by means of combined heat and power generation.

When shutting down a combined heat and power plant (e.g. for inspections or driven by low prices on the electricity market), mostly fossil-fired auxiliary boilers (safety boilers, reserve boilers) take over the process steam generation until restart of the power plant.

These steam boilers must be kept in permanent minimum load operation in order to guarantee fast steam generation and thus a fail-safe supply of industrial processes. The continuous supply of the auxiliary boilers is associated with high consumption of oil or natural gas and causes significantly higher costs and CO2-emissions compared to the actual operation of the combined heat and power plant.

Metal based high-temperature storage ensures steam supply

By means of a newly developed, highly dynamic heat storage, which uses metal alloys according to the phase change principle, the supply of process steam can be bridged efficiently and cost-optimized during shutdown or partial load operation of the combined heat and power plant.

The auxiliary boilers for steam generation are transferred to base mode, thus saving considerable costs and emissions.

The integration of the heat storage gives the auxiliary boilers sufficient time to move more gently from the more economical base mode to full load operation.

Initial profitability calculations by Fraunhofer UMSICHT show that the investment in a metallic heat storage for securing process steam can pay for itself within a very short time.Metal alloys in the temperature range 250 - 550 °C

Principle of a latent high-temperature heat storage for backing up process steam in an industrial park

Metal alloys in the temperature range 250 - 550 °C

The new heat accumulator is based on the development and investigation of metal-based phase change materials and suitable encapsulation materials in the medium to high temperature range. Due to the high thermal conductivity of metal alloys, high storage dynamics can already be achieved with the simplest heat storage geometries.

The possible areas of application are independent of the heat transfer medium used and extend beyond the provision of process heat / process steam. Metal-based latent heat storages can be a solution wherever heat needs to be quickly stored and retrieved.

The development of phase change materials and suitable encapsulation materials, which is being promoted at Fraunhofer UMSICHT, forms the basis for heat storage in the medium to high temperature range.

The new heat storages are based on the development and investigation of metallic phase change materials and suitable encapsulation materials in the medium to high temperature range. Due to the high thermal conductivity of metal alloys, high storage dynamics can already be achieved with simplest and well-known heat storage geometries.

The possible areas of application are independent of the heat transfer medium used and extend beyond the provision of process heat / process steam. Metal-based latent heat storages can be a solution wherever heat needs to be quickly stored and retrieved.

The development of phase change materials and suitable encapsulation materials, which is being promoted at Fraunhofer UMSICHT, forms the basis for heat storage in the medium to high temperature range.

Project »Thematic«

In the TheMatIK project, metallic phase change materials and metallic encapsulations were investigated for their suitability as heat storages. Besides Fraunhofer UMSICHT, enolcon GmbH, Ostbayerische Technische Hochschule Amberg-Weiden and Uniper Technologies GmbH were involved.

A compromise was found between economic and thermodynamic optima for the PCM and the encapsulation. The heat storage concept developed in the project was developed with the help of flow simulations.

In addition, process simulations were carried out to investigate the storage behaviour in a real process steam network. In a pilot plant, tests were carried out under power plant conditions on a selected combination of phase change and encapsulation material.

Development steps of the joint project at a glance

Definition of the requirement parameters for the materials (PCM and encapsulation)

Development of metallic PCM and the associated encapsulation

CFD simulation of the heat storage concept

Verification of cycle stability by load tests on a laboratory scale

Process simulations for simulating the storage integration in a process steam network

Profitability and potential analysis of the heat storage concept for fossil and renewable cogeneration plants

Preparation of detailed utilisation concepts for storage integration in power plant technology

The project was funded by the Federal Ministry of Education and Research (BMBF).

Comparison of heat storage technologies

Comparison of different storage materials / heat exchanger technologies for heat storage in the high-temperature range The following table gives an overview of the technology concepts in the field of high-performance heat storage tanks. In the project TheMatIK metal alloys are used as storage material.

Comparison of heat storage technologies

Technology   sensitive   latent    thermochemical
Storage material KNO3-NaNO3 volcanic rock ZNAl6    NaNO3 Ca(OH)2
Temperature level °C 290 - 560 600 250 - 550 306 500
Gravimetric storage den -- -- 0 + ++
Volumetric storage den -- -- ++ 0 ++
Power density(kW· m-3) -- -- ++ ++ 0
Investment cost(€ · kWh-1) + +
0 0 -