Stirling Is Gold

“We are excited to be part of a technology that is the first of its kind in the nation,” said Lori Singleton, Salt River Project’s (SRP) project manager for sustainability initiatives and technology. “The test project allows us to have a hands-on look at the technology to see how it is performing and how it integrates with the grid.”

Located about 30 minutes from the heart of Phoenix, 60 SunCatchers are installed at the 12-acre site, which SRP is leasing to SES’ sister company, Tessera Solar, for 10 years. The installation is adjacent to one of SRP’s steam generation plants, so transmission lines are already in place.

SES has been working for more than two decades to perfect the SunCatcher’s technology to make it utility-scale-friendly. In 2009, the right design breakthroughs were made and projects ranging in size from 54 MW to 850 MW in California and Texas have been announced. Singleton said Tessera was also awarded a project with the city of Phoenix to site a 250 MW SunCatcher plant on a landfill just outside the metropolitan area.

The news that dish/Stirling joins other concentrating solar power (CSP) technologies in commercial-scale applications is good. But what makes it even more interesting is that dish/Stirling is also a viable candidate for distributed generation.

Dish/Stirling could give photovoltaics (PV) a run for their money on small-scale, distributed installations like rooftops. Why? Because manufacturing costs for dish/Stirling units are not based on underlying raw materials, as is the case for PV, and standard manufacturing methods such as robotics and assembly lines can be used. As production increases, costs will likely come down.

SRP is interested in distributed technologies, said Singleton. With the utility’s EarthWise solar rebate program, small solar projects can sell their power back to the grid.

“We are hearing more and more from other solar developers that are interested in installing some kind of technology within our system,” she said.

In a dish/Stirling engine system, parabolic mirrors placed on a dish concentrate and focus the sun’s rays onto a receiver, which absorbs the energy and transfers it to the Stirling engine. The engine converts the heat energy to mechanical power and an electrical generator or alternator converts the mechanical power into electrical power. SES’ SunCatcher, for instance, comprises a 38-foot mirrored parabolic dish, an automatic sun tracking system and a Stirling engine.

Invented in 1816 by a Scottish minister named Robert Stirling, the engine promised efficiency, quiet and safety. In Stirling engines, a gas–air, helium or hydrogen–is sealed inside the engine. The Stirling cycle needs an external heat source, which can be provided by solar energy or even heat from combusting biomass or propane. The gas moves from a hot side to a cold side, expanding and contracting in the process and pushing pistons. There are no exhaust valves and no combustion takes place inside the engine’s cylinders.

Theoretically, Stirling engines are the most efficient engines, but they were eclipsed by a later invention, the internal combustion engine. That’s because while Stirling engines can stop instantly, they can’t start up quickly. For CSP applications, though, rapid startup is not an essential characteristic and Stirling engines use no fossil fuels and emit no carbon dioxide.

Stirling engines provide other benefits. For example, maintenance on the Stirling engine is low and they are easy to work on–that’s why the systems can be located in remote locations, operating unattended and automatically.

Dish/Stirling sets like the SunCatcher are modular, scalable, require minimal grading and trenching and no foundation excavation. Dish/Stirling technology does not use as much water as some other CSP technologies, one of SRP’s critical concerns, said Singleton.

I first wrote about dish/Stirling systems in 2003 after a visit to Sandia National Laboratories’ solar facilities, part of Sandia’s Distributed Energy Technologies Laboratories. Several dish/Stirling systems were operating there. In fact, that’s where SES perfected its SunCatcher model, achieving one of the world’s highest solar-to-electricity conversion efficiency records and getting the technology ready for manufacturing and commercialization.

Today, solar incentive programs like SRP’s EarthWise have proliferated at utilities all across the country. Utility-scale solar installations of all CSP types are being developed. In this kind of electric utility environment, I can imagine small grid-connected dish/Stirling sets on rooftops, in shopping mall parking lots or installed by the thousands for multi-megawatt projects. The Maricopa Solar project may be just the start.