Choosing a Solar Collector

 Peak oil, climate change and sustainability require developing energy resources that are freely available, affordable and don’t alter the atmosphere or oceans. Why not start providing energy for our homes and public buildings because they don’t move and they consume a third of our fossil fuels? Wind turbines feed power into the grid but many homes and farms do not have reliable wind for direct power, hot water and comfort. Sun shines everywhere and solar collectors can augment many living situations. There are many kinds of solar collectors: flat plate types that simply utilize the sunlight that shines on them, those that intensify sunlight from a few times and up to 50 times, and those that concentrate solar energy a few hundred to over 1,000 times. Which is most useful?

Earth is in the sun’s Goldilocks zone: not too hot, and too cold. Natural sunlight is just right. We wouldn’t be here if the sun didn’t support plants and animals. But to gather enough sunlight to do the useful work we’ve come to expect requires utilizing energy from large areas. Economics favor using inexpensive mirrors to intensify sunlight on a small area that will pay for high-tech approaches (like high temperature materials and insulation) to harvest most of the available energy. Flat plate solar collectors, both photovoltaic and thermal, use materials that would be too expensive without government subsidies and perform poorly (PV delivers <15% and thermal even less when it’s cold). Passive solar designs use windows to contribute heat for buildings but today’s active systems cost too much.

Prototype Dish Solar Collector: Work Begins in 2015 on One That Has About Four Times More Mirror Area

Parabolic trough solar collectors that intensify sunlight from 20 to 50 times are popular for generating power and typically use glass envelopes to surround the metal tube that contains the working fluid inside with the volume between evacuated like a Thermos bottle. To allow the metal and glass to expand at different rates without breaking, they need a flexible metal bellows to join the two materials.  These receiver assemblies are quite expensive and when they lose vacuum they must be drained, cut out and a new one welded in place. Again, without subsidies, they are too expensive.

Concentrating photovoltaic collectors, parabolic dishes and solar power towers intensify sunlight from 300 to a few thousand times. This allows receivers to work well without vacuum enclosures because the high intensity energy can be efficiently captured without a window or vacuum enclosure (typically capturing above 90% even when it's below freezing). Economics favor higher concentrations because inexpensive optical elements direct large amounts of sunlight into tiny receivers. Using 450 mirrors, a square foot each at a dollar, to intensify sunlight 900 times requires a small, well-insulated trash pail size receiver with a 9-inch diameter opening to collect 34 kilowatts in bright sunlight. Each hour this collector would deliver the same amount of steam as a fossil fueled boiler burning a gallon of fuel oil or 1.5 gallons of propane. The same concentrator with a CHP (combined heat and power) receiver would deliver 10 kilowatts of power and 20 of heat.

Taking Apart the Engineering Prototype Solar Dish

The few dish solar collectors available today are complicated and require experts and cranes to install them. This year I hope to demonstrate a new approach to building solar dishes that enables anyone to make the parts in their home shop, put them together, accurately align mirrors and let a simple open-source controller run it for 30 years, custom programming and setup not required. I hope that the availability of inexpensive and fun to make solar equipment will enable others to develop receivers and applications that enable very high performance renewable energy systems to displace major amounts of fossil fuels.