The 19th-century engine that could give solar power a boost
First developed in the early 19th century, the Stirling engine is regaining favor as a way to squeeze even more power out of the sun's energy.
A new attempt to generate electricity from solar power using a Stirling engine created a ripple of interest recently when an article in the Guardian hailed it as the most efficient method of converting the sun’s energy into electricity.
But renewable energy experts were quick to point out that previous attempts to use the Stirling engine technology together with concentrated solar power (CSP) – solar energy that focuses the heat of the sun to drive turbines – failed because the technology could not compete with the rapidly declining price of photovoltaic (PV) panels – which convert the sun’s energy directly into electricity. (Related: The Great Divide Between EIA And OPEC Data)
Virtually all of the CSP operating plants, located mostly in the U.S. and Spain, utilize parabolic troughs capture and store heat, which is used to produce steam to drive turbines.
The South African pilot project mounted by Ripasso Energy of Sweden claims to have achieved the most efficient conversion of solar energy into power – at 34%, nearly double the efficiency of PV technology – and is, according to the Guardian report, ready for commercial deployment.
The 100-square-meter dishes track the sun across the sky and focus its heat into a tiny point, which then allows the alternate heating and cooling of enclosed gas in a Stirling engine to drive pistons that turn a flywheel to generate electricity.
This engine was developed in the early 19th century by Robert Stirling, a Scottish clergyman, as an alternative to the steam engine, but found little practical application until the Swedish defense firm Kockums adapted it for use in submarines in the 1980s.
Kockums’ CEO Gunnar Larsson then left the defense contractor and, in 2008, founded Ripasso Energy to use the Stirling engine in conjunction with solar power.
Ripasso’s solar dish technology produces energy much more efficiently because the large dish, much like a satellite dish, captures a good deal of sunlight. With greater conversion efficiency, the solar dish could use a lot less land than PV to produce a given amount of energy. (Related: Oil Majors Falling Out Favor With This Hedgefund Boss)
However, the dish technology misses out on the main advantage of CSP over PV – namely, the ability of the parabolic trough system to store the sun’s energy as heat in the molten salt that is piped through the trough of the parabolic mirrors. The Ripasso system wouldn’t use molten salt, and therefore, wouldn’t have a built-in way to store energy. This limitation has held back the concept for quite some time.
A U.S. company called Stirling Energy Systems developed a similar technology to the one now being touted by Ripasso, but the company collapsed into bankruptcy in 2011 as PV prices plummeted. Its solar power projects were largely converted to PV.
On the other hand, the rapid improvements in battery technology could change that. If Tesla and other manufacturers can bring the price down on lithium ion batteries to store the electricity produced by solar panels of all types, the Ripasso system could be a big improvement over traditional CSP.
But there are two other drawbacks. Ripasso has yet proven that it can compete on cost with PV. And with its big dishes, it isn’t as well suited to the residential solar sector.
A breakthrough in residential energy storage indicated by the late April unveiling of Tesla’s Powerwall could be the key to a boom in distributed energy as homeowners, who have been embracing PV panels in increasing numbers anyway, will be able to store electricity for consumption when the sun is not shining. (Related: This African Country Sees A Nuclear Future)
Utilities, prodded by regulators, are adapting their business models to a more customer-focused approach that incorporates distributed resource platforms, smart metering and micro-grids as well as the traditional base load grid power.
Ripasso has its work cut out for it as PV is growing rapidly. But with a much greater conversion efficiency rating, it could have enormous advantages over other forms of solar power.
By Darrell Delamaide of Oilprice.com
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