“Our business is based on mastering the challenges and to take advantage of the Stirling engine to produce electricity,” says Magnus Lindvall, at Cleanergy’s offices in Beijing, China. “The plan is to set up solar power plants where the Stirling engine generates electricity, but our product portfolio also includes Stirling engines that run on natural gas. The latter is particularly interesting in houses that are heated with natural gas and here the Stirling engine can simultaneously generate electricity,” says Lindvall. “With our Stirling motors, the natural gas pipelines installed in our service buildings can generate electricity.”

The dish concentrates solar radiation and follows the sun across the sky so that electricity is generated throughout the day.

A Stirling engine can actually be considered as a clean tech solution several hundred years before the concept was born. Swedish company, Cleanergy utilizes the engine’s unique features on a small and large scale to generate electricity in an environmentally friendly way.

Magnus Lindvall is the company’s representative in China with the task of establishing contacts with Chinese investors who want to invest in full-scale solar parks.

An engine with interesting properties

It was a Scottish clergyman Robert Stirling in the early 1800’s that invented the Stirling engine. It was an ingenious invention but still after almost 200 years had only limited use. One of the earliest benefits was that the engine was virtually harmless compared to contemporary steam engines that were unsafe for people and equipment and could explode or leak hot steam.

Stirling engines were fueled with wood and the hot air that formed warmed the top of the container where the bolt was.

The air in the container was warm and pressed the plunger and the movement could, for example, run a flywheel that in turn drove a threshing machine or water pump. When the container’s top cooled, it formed a vacuum that required that the plunger be pulled up to the original position. The engine worked slowly and safely, but the efficiency was unfortunately only a few percent.

When diesel engines appeared, Stirling engines could not compete with diesel’s efficiency and compactness. Stirling supporters did not give up and there were many projects to bring up the pressure in the engine and to increase efficiency. Hydrogen and helium were used to increase the pressure and ingenious improvements arose eventually for the modern vibration and pollution-free Stirling engine. Still tricky and difficult to quickly get up to operating temperature, and control the speed, the interest from the automotive industry waned.

Swedish Kockums did not give up and the Stirling engine was revitalized as an engine for submarines. The engine was used to recharge submarine batteries, and by burning diesel fuel and pure oxygen the by-products were carbon dioxide and water.

Exhaust fumes could be released directly into the water where they dissolved and it is therefore not possible to detect the submarine on the power of exhaust bubbles. The Stirling engine worked 450 meters under the ocean, but how does it work suspended from a solar power plant in the interior of Mongolia?

Satellite tracks the sun

The solar power plant converts the Stirling engine solar energy into electricity. The dish that concentrates solar radiation follows the sun across the sky once, which means that the equipment will generate electricity as long as the sun is up. The Stirling engine is started by the heat of the sun (2) and drives a generator that provides electric power.

The basic principle is that the engine has a “warm” and a “cold” side and the left piston (1) is actuated by the heated gas, in this case hydrogen. In the right part of the motor (3-5), the gas is cooled down and it is the interaction between the heated and cooled gas that make the pistons move. Compared with Stirling engines from the 1,800’s, the engine is quick and efficient. At 1,500 revolutions/minute the gas is heated up to 700 °C and cooled to 80 °C. This happens 25 times per second and the solar power plant can deliver 11 kW.

The power module with engine and generator sits suspended above the center of a solar parabol. The module is about one cubic meter high and weighs 250 kg. The 55 m2 dish is made up of 16 pieces of three-meter long panels. The panels, engine and generator fit in a 20-foot container and can be shipped anywhere in the world.

Compared to other technologies, utilizing solar power with Stirling engines has higher yields. This applies to technologies where water is heated up to generate steam which in turn drives a turbine. Another advantage is that the system does not require access to cooling water, which increases the usability of places with water scarcity. Stirling motors are quiet and vibration-free.

Compared to traditional fixed photovoltaic systems, the benefits include higher yield of electrical energy both during the day and in the longer term, and no use of rare earth metals, as well as less need for cleaning.

“Our Stirling engine was developed 20 years ago and we now have more than two million operating hours. We buy mirrors and “tracking systems” in China, the engines in Europe and we assemble it all in Åmal, Sweden. We have sold a demonstration plant with 10 engines and dishes to a company in Inner Mongolia that delivers 110 kW to the local network and the experience so far is very good,” says Magnus Lindvall.

“The next goal for us is to establish a full-scale plant with 4,500 engines in China. Such a solar park would supply 50 MW and such a scale would offer very competitive electricity prices,” concludes Lindvall.

This article was published in December 2013