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Effective thermal storage systems for competitive Stirling-CSP plants

The project aims to develop, test and verify effective thermal energy storage (TES) systems for Stirling engine based power generation, fueled by concentrated solar irradiation (CSP). With an adequate thermal storage, this type of power plants produces cost-effective solar electricity below $100 / MWh, around the clock and can act as base load and load balancing source to the grid. For Stirling engine-based CSP to be competitive with traditional CSP steam turbine systems with thermal storage, a new storage concept is required that can store heat over 800 °C up to 15 hours to reach a yearly availability of 80%. The project will develop such a storage concept for Stirling engines while meeting the industrial requirements for life time and product cost.

Funded by:

Swedish Energy Agency and Azelio

Time period:


Project partners:



Thermal solar power plants concentrate the solar energy to generate a high temperature heat source that can be used to drive conventional power cycles. Attractive electricity prices and the ability to build industrial facilities in the multi-megawatt class have led to four GW installed thermal solar power worldwide over the past years. In addition, thermal solar power is unique among renewable energy sources, indeed the thermal process allows for integration of thermal storage or hybridization with alternative fuels. This will make the power plant flexible and available around the clock. It is precisely the opportunity to integrate cost effective thermal storage, which today is the main reason for CSP to be installed, even though electricity costs are significantly higher than that of direct PV solar panels. In large-scale steam-driven CSP power plants, thermal storage technology is applied using tank systems where molten salt is led to the solar focal point and is heated to a maximum of 550 ° C and then transferred to a tank for storage. When electricity is generated, the heated salt is led to a steam generator for steam production and the steam then drives a power turbine. Compared to Stirling-based satellite systems, steam-powered CSP power plants have lower efficiency and significantly higher power costs (estimated to be double). However, there are no commercial thermal storage solutions for Stirling-based systems available due to the requirement for operating temperature above 750 ° C.

Aim and objectives

The overall objective of the project is to develop and verify an innovative, efficient and sustainable thermal storage system for competitive Stirling motor-based soles. The focus will be on the development of the necessary key components for such a system. The power plant including thermal storage will reach an electricity cost below $ 100 / MWh, in volume production. The storage system must have a working temperature above 800 ° C and a storage capacity of between four and twelve hours in full scale so that a power plant availability of more than 80% can be reached on a typical solar market. The thermal layer must be integrated and demonstrated in a lab scale scale simulator to validate system dynamics.

In detail, this means that we want to achieve the following concrete goals:

  • Develop and optimize a thermal storage concept based on sensible and / or latent storage materials with a gas, air or carbon dioxide, as heat transfer medium and at a working temperature above 800 ° C and up to fifteen hours of storage capacity.
  • Develop and test a thermal energy storage at lab scale under well-defined conditions in KTH's labeling and identify charge and discharge behavior as well as heat flow in the storage. Simulations of the thermal layer shall be carried out and verified using the measurements.
  • Verify that the pressure loss in a full-scale warehouse does not contribute more than 5% of the total pressure loss.
  • Verify in the lab scale in KTH's solar system an integrated power system including bearings, solar receivers and a realistic presentation of a Stirling engine, with respect to charge and discharge behavior. Especially optimal operating conditions should be studied. Both before and after the test, simulations must be performed to support the test and to translate the results to full scale.
  • Demonstrate by techno-economic simulation that the complete system, in full scale and at the required production volume, can deliver an electricity cost of $ 100 / MWh for two example markets.


Currently a lab scale thermal energy storage is under construction at KTH’s premises. The TES will consist of a 0.2 m3 packed bed of ceramics/natural rocks, equivalent to about 60 kWh of energy capacity.Different techno-economic analyses of the TES unit integration in CSP systems have been performed showing the suitability if the system. Specifically, the packed bed TES unit has been integrated in Stirling based CSP, as well as in hybrid gas turbine CSP plants and supercritical CO2 units. Most of the outcomes have been presented in conference and journal publications.


Initial design of a radial-flow high temperature thermal energy storage concept for air-driven CSP systems. Silvia Trevisan, Rafael Guédez, Hicham Bouzekri, and Björn Laumert. AIP Conference Proceedings 2126, 200031 (2019),

Preliminary assessment of integration of a packed bed thermal energy storage in a Stirling – CSP system. Silvia Trevisan, Rafael Guédez and Björn Laumert. AIP Conference Proceedings 2126, 200032 (2019),

Supercritical CO2 brayton power cycle for CSP with packed bed TES integration and cost benchmark evaluation. Silvia Trevisan, Rafael Guédez and Björn Laumert. Proceedings of the ASME 2019 Power Conference.

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