Ripasso Energy, based in Sweden, isn’t in the business of reinventing the wheel – rather, it’s reinventing the engine. The Stirling engine, to be precise, first patented by Robert Stirling, a Scottish minister, in 1816. Stirling engines operate by compression and expansion of air or other gas, and convert heat energy to mechanical work. They were originally designed to be a competitor to steam engines and are rather like external combustion engines, as all heat transfers to and from the working fluid and takes place through a solid boundary, the heat exchanger.
The design of Ripasso’s Stirling engine is based on a licence from Kockums, a shipyard in Malmö, Sweden, which in turn is owned by the Swedish defence company Saab Group. The director of Ripasso is Gunnar Larsson, formerly with Kockums. I chatted to Larsson to learn more about what Ripasso does. Essentially Ripasso’s hybrid Stirling engine makes it possible to generate electricity 24/7. “In 2012 we demonstrated an efficiency of this engine that is more than twice as great as any other technology for converting solar energy,” said Larsson. “The key to our technology is the hybridisation technique we have developed” says Larsson. “This enables us to provide power on a demand basis of 24/7 – as opposed to photovoltaic solar energy which doesn’t provide much power when the sun isn’t shining.”
The rapid advance of Ripasso can be seen from the fact that the company was formed in 2008 and this year is commercialising its first major venture in Sicily, with sales of slightly more than 150 Stirling engines which together will supply just over 5 MW of electricity. In South Africa a research plant has been at work for almost five years, testing the Ripasso engine’s capabilities more or less continuously. The company reckons it can – together with its partner Sibbhultsverken, in southern Sweden – scale-up production to some 100,000 engines a year, although in 2018 it expects a more modest 150.
How does it actually work? Mirrors attached to a parabolic dish focus the sun’s rays onto a receiver, which transfers this heat to a Stirling engine. The engine is a sealed system filled with hydrogen. As the hydrogen heats up and cools down, its pressure rises and falls. This change in pressure drives the piston inside the engine, producing mechanical power, which in turn drives a generator and makes electricity.
Using the sun’s rays to generate electricity is nothing new of course and it’s becoming a pretty crowded marketplace. Indeed, there have been some rather high-profile examples of Stirling engine-based solar power electricity generators going bust. Stirling Energy Systems (SES) went into bankruptcy in late 2011 along with a couple of smaller Stirling engine companies. According to Eric Wesoff, an analyst with Greentech Media, “Stirling engines are another utility-scale solar technology done in by crystalline silicon and cadmium-telluride solar panels.”
But Ripasso is convinced that its particular technology, of using a dish collector to capture the rays to drive the Stirling engine, has the right ingredients for long-term success. In the end it will all be down to a matter of costs and efficiencies and it would take a far greater expert that I to have a clear and objective view on how that will pan out. According to Larsson, the Ripasso energy proposition has a lower ‘Levelized Cost of Energy’ (LCOE and a lower environmental impact compared with competing technologies. It is certainly true that solar thermal dish Stirling technology has the highest efficiency of any form of concentrated solar power (CSP), converting up to 32% of incoming solar power to electricity, compared to around 15% for other designs. It’s also the case that CSP is a very modular design, with engines that can be run singly or in a collection – you don’t have to install and pay for an entire solar field before a return on the investment might be seen. This can be a vast advantage to people and communities in developing countries who do not want to be dependent on an unreliable national transmission grid.
So for remote areas; for areas of remote and inaccessible population; for arid areas or areas of intense sunshine that need electricity 24/7, then Ripasso Energy is perhaps ideal. According to Ripasso for 2016: “The final product is an engine that is able to deliver electricity around the clock, and that has the highest possible fraction of renewables and such a high fuel conversion rate that its operating costs rate well compared to the alternatives, both in major installations that tie into a national electricity grid, and in small installations for private customers, for instance in the mining industry. The company’s hybrid solution is expected to be in demand on the large and growing market for renewable energy because, unlike existing technology, it is able to deliver electricity at a competitive price even when the sun is not shining or when the wind is not blowing.”
Ripasso Energy’s mission to bring round-the-clock electricity to remote areas that perhaps are ill-served by national grids clearly has a potential powerful social impact. To conclude, I asked Sven Sahle, Chairman of the company, why they had decided to join the Social Stock Exchange: “The reason we joined the Social Stock Exchange was to set ourselves to a higher standard that, through the impact report, was approved by an external body and will be measured and improved on, on a regular basis.”
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