Thermal storage – Thermal battery

Thanks to the thermal battery, huge possibilities are now being opened for united Sun Systems. The electricity produced from the sun during the day can now be delivered in accordance with demand.

High temperature thermal storage

The principle of a thermal battery is to store energy as heat. This storage technology is brand new and patented by the Savannah River National Laboratory. USS has an exclusive agreement and collaboration with Savannah River National Laboratory regarding IP rights for this new thermal battery, which is based on metal hydride technology. This agreement gives USS the right to use the technology within the temperature range 600 – 950 degrees Celsius until year 2041. The temperature is within the range that allows the Stirling solar engine to work as efficiently as possible.

Metal Hydride thermochemical storage

The thermal battery consists of a hot and a cold side. When the battery is heated, hydrogen will move from one to the other side, and there is a chemical reaction between a metal alloy and the hydrogen so that ”metal hydride” is formed. The technology is ten to twenty times more efficient than previous heat storage techniques, according to reports from the US Department of Energy. Thanks to this new technology, we can now save energy for up to 100 years with minimal energy losses.

The battery capacity is the same as the system capacity, which is calculated at more than 40 years. After this, the battery can be 100% recycled. Until now, the thermal battery has been tested in a demonstration plant in smaller scale and will now be scaled up to fit the new USS system (Oasis), which needs a capacity of 14 hours.

Cheap storage with unique characteristics

The development of the metal hydride heat energy storage technology (TES) was supported by contributions from U.S. Department of Energy, Energy Efficiency & Renewable Energy Solar Energy Technologies Office under the so-called Concentrating Solar Power Program. The program is aimed at promoting solar energy technologies that can meet the goal to reach a cost of USD 6 cents per kWh, set by the US Department of Energy. USS estimates that the new system, within a few years, will be commercially available to produce and distribute electricity 24 hours a day, 365 days a year. Prices are expected to be up to 95 % cheaper than current solar technologies in combination with lithium-ion batteries and also be fully competitive against fossil fuels.

Electricity generating technology – CSP Dish-Stirling

The USS solar energy technology is called “Concentrated Solar Power Thermal Dish Stirling – CSPt Dish-Stirling,” and is based on a fourteen (14) meter in diameter parabolic dish covered with glass mirrors, that reflects and focuses the solar rays to create temperatures of 750 degrees Celsius. This concentrated energy will be stored in the company’s new thermal battery. The battery powers a heat engine (Stirling Engine) that creates electricity through a generator. The system is provided with a “gas hybrid” to guarantee electricity production during longer periods of solar interruptions and the possibility to create independent grids.

Scalable and the most efficient of all solar electricity technologies

The technology is scalable from one system, that can generate power up to eleven American homes, to hundreds or thousands of systems that could generate power for larger cities, states or countries – cheaper than fossil fuels.

United Sun Systems solar electricity generating technology, CSP Dish-Stirling is the most efficient of all solar electricity generating technologies. US Department of Energy confirms it as the most efficient at over 30%, compared to other technologies such as PV, CSP Solar Trough and Solar Tower, at approx. 15%.

New designed Solar Concentrator

In order to incorporate thermal storage into the traditional CSP Dish-Stirling technology, the solar concentrator had to be completely redesigned. The thermal battery has a lot of weight and can therefore not be placed in the focal point of the solar concentrator.

Working with our German concentrator design partner and US heat transfer specialist, United Sun Systems has come up with a design where heat is transferred from the focal point to the back of the parabolic shaped dish where both the thermal storage solution and Stirling PCU is situated.

This design also has other benefits. Because of the change in balance, the parabolic dish will not have the traditional “cut out” for the pillar, giving it extra strength and more solar energy is collected. United Sun Systems has a patent for this solar concentrator design.

V4-90 Stirling Engine

The V4-90 is the only mature solar Stirling engine that has been developed in several generations for performance as well as for large scale manufacturing. These engines have up to today done approx. 221,000 hours of testing without any major problems or breakdowns. This work was done by the company United Stirling AB (Kockums) together with, Ford Motor Company and US space agency NASA. Swedish Intersol AB then acquired the rights for the X36 which became the V4-90 Stirling engine. USS has since acquired the right to the V4-90 from Intersol AB.

The V4-90 Stirling engine has the highest energy output to weight ratio of all Stirling engines. The maximum power output has been measured at 46kW.

There are several reasons why USS V4-90 is ranked among Stirling engineers as the world’s best Stirling engine.

1. Design: The V4-90 is a V-shaped engine with the highest output to weight ratio of all Stirling Engines.

A V-shaped engine has one crank shaft. A U-shaped, two crank shaft engine, has a need for a gearing system that brings the two crank shafts into one axis to power the generator. The V4-90 does not need this. This means:

  • The V4-90 contains fewer components
  • It is much lighter
  • Cheaper to install and service
  • Cheaper to produce
  • The V-shaped and its lack of a gearing system also means:
  • Much reduced vibrations and noise
  • Virtually silent engine.
  • Less vibration also reduces – meaning longer Service Intervals (SI – time between services)
  • The V4-90 has been designed with high scale production in mind.

2. Weight:

The weight of the V4-90 engine is 110 kg. It is less than a competing 11 kW engine and nearly 50% less than a comparable size system. In high level production, the production cost is almost linear to weight, making is cheap to produce


The heat transfer system or the heat pipe is one of the key components in the Generation II system and made it possible to move the heat engine and the generator (PCU) from the focal point in front of the system to the back of the dish. Heat Pipes are used in among others military jets and space rockets. Or partner AAVID Thermacore in Pennsylvania, has developed and tested the heat pipe for many years in co-operation with NASA. As the demand for long lasting components in new space shuttles is key for long journeys to other planets, NASA and AVVID Thermacore has been concluding a 14 year lifespan test on the Heta Pipe with very good results. The Heat Pipe is an expensive component in the GII system but are expected to last for minimum twice the depreciation time on the system (+100 years)

Gas Hybrid

No renewable energy source is always there. The sun does not always shine, and the wind does not always blow. However, demand from the market will still be there.

The new patented design of the CSP-Dish system where a thermal battery is heated by the sun in order to power the Stirling engine opens up for the possibility of creating a hybrid system. The Stirling PCU only needs heat, so the thermal battery is heated by gas during none solar hours or longer solar interruptions, creating a 24/7/365 system.

Electricity generating technology – CHP-Combined Heat and Power

The V4-90 Stirling engine can also be used as a CHP system, delivering energy from hydrogen gas contained in: Landfill gas, Bio gas and Natural gas. USSI has created a company called Stirling Gas AB. This company that has an enormous amount of synergies with the mother company. USSI will utilise the V4-90 engine for this market. Stirling Gas AB is 100% owned by United Sun Systems International Ltd.