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31% Efficient Stirling Engines Used to Convert 1.5MW of Arizona Solar Power


Stirling engines are known to mankind for about 200 years but they are not so widely used today, except maybe for the pacemakers and long-distance robotic spacecraft. This will soon change as 60 Stirling engines will be used in Phoenix, Arizona, to harvest solar power which will be converted into electricity.

A conventional stirling engine operates by cyclic compression and expansion of a gas at different temperature levels, converting the heat into mechanical energy (a piston is pushed into another chamber while the gas is heated, expanded, and flows back when the gas cools down, contracting itself). The movement of the piston is able to generate mechanical work, and thus electricity via an alternator.

The demonstration site, called Maricopa Solar, will begin its activity this month and will be able to produce 1.5 MW of energy. The stirling engine units will be provided by the Arizona-based company called Stirling Energy Systems (SES). Since 1996, SES refined the technology together with their partner Sandia National Laboratories.

After almost 14 years of developments, they came up with an ingenious system: a circle of curved mirrors, resembling to a satellite dish, tracks the sun on two axes and reflects the sun’s heat onto a single focus point, the power conversion unit (PCU). The PCU makes use of 4 pistons that are able to move into their own cylinder. Inside the cylinder, the gas that expands and contracts is hydrogen.

Stirling engines are one of the best options on the market to harvest solar power as they can reach a 31% efficiency compared to just 16% for parabolic trough technology or 14-18% achieved by photovoltaic panels. Because of the high price and high maintenance costs, stirling engines do not stand a chance compared to the photovoltaic systems. Even though the future of the stirling engine may sound crazy today, we never know what tomorrow may bring.

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  1. A few years ago, I interviewed for a job for this company in Albuquerque.  I blew the interview because I chuckled when they said the engine assembly was located in front of the mirror’s focal point.  I told them that, having built telescopes for a living, modern telescopes place a secondary mirror at that position and reflect the light to a hole in the primary mirror, so all equipment could be mounted there.  Not only is the equipment more accessible at the back end of the primary mirror, but it’s much easier to balance and rotate the entire assembly with a lower center of gravity.  (PS – I didn’t get the job…)  Actually, in my opinion, they could have just built a collector plate to heat a transfer fluid at this position, then moved the fluid to a larger, more central generator system.  It’s far less maintenance to have one engine/generator for say, 100 collectors, than 100 engine assemblies.

  2. “Spacecraft” is an unchanging irregular verb and never becomes “spacecrafts” unless you are referencing wicker basket making and other fun activities in which astronauts perhaps engage in their off time in orbit. This is why you might hear the term “small craft advisory” on NOAA radio but never “small crafts advisory.”  Other than that, good article and thank you for writing it.  Please do not mistake my criticism as a lack of appreciation for the information you successfully gave me.

  3. I find the sentence in the article: \Because of the high price and high maintenance costs, stirling engines do not stand a chance compared to the photovoltaic systems.\ to be misleading. The quoted price of this system is $3000. per KW installed. This is about half the installed cost of PV if you include the inverters. The engines drive alternators that put out 3-phase 60 Hz power and do not need inverters. And one primary reason for the 1.5 MW pilot plant is to measure maintenance costs. The 6 units that have been operating for several years at Sandia Labs show low maintenance.


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