Concentrated Solar Thermal Heat and Power for Homes

December 22, 2011 – Vol.16 No.40

CONCENTRATED SOLAR THERMAL HEAT AND POWER FOR HOMES.
by Bruce Mulliken, Green Energy News

First, a warmer upper, so to speak.

For small companies it’s very difficult to launch new products. Where big companies can have huge bank accounts behind product launches, small companies are often operating on a shoestring.

But it’s often the small companies that have the best “thinking outside the box” ideas. In the world of alternative energy “outside the box” is really needed. Still, going too far “outside” may not lead to product success either. (People – potential customers – may simply not believe radical claims.) So a blend of conventional mixed with clever thinking may prove the best formula for success in the marketplace.

Some thoughts along this line:

1) The chance of success of a new product is probably much greater if proven scientific or engineering principles are employed in clever ways to create that product. A short bumper sticker phase might be “Product success equals using proven principals in clever ways.”

2) The chance of success of a new product is probably much greater if off-the-shelf materials are used in clever ways. A bumper sticker here might be “Product success equals using off-the-shelf stuff cleverly."

3) The chance of success of a new product is probably much greater if research and development costs are small. Try “ Low R&D costs equal high returns.” (l like that one.)

4) The chance of success of a new product is probably much greater if the very first product off the assembly line is reasonably priced. That is, don’t wait for a economies of scale or a learning curve to bring production costs down. “If the first one’s cheap the rest will be cheaper.” ( I like that one too.)

Now on the meat of the story.

Concentrating solar thermal energy using parabolic troughs has been used successfully since the 1980’s to generate grid-scale power. The giant SEGS (Solar Energy Generating Systems) operating in California’s Mojave Desert, with 354 megawatts installed capacity of clean power, are the proof.

SEGS uses long rows of parabolic troughs to reflect and concentrate sunlight onto a receiver tube filled with a working fluid. The receiver tube, or absorber tube, is placed at the focal point of the parabola for maximum solar concentration. That working fluid, a synthetic oil, is heated to over 750 degrees F (400 degrees C) and piped to heat exchangers where high pressure steam is produced to spin a turbine generator to send electricity to the grid.

There’s nothing particularly exotic about SEGS or other similar systems around the world using the same proven technology. SEGS, and others, are built with common with off-the-shelf materials and use scientific and engineering principles that were proven long before SEGS was built.

While SEGS is meant to feed many megawatts of clean power to the grid, smaller, similar installations are possible too. Sopogy of Honolulu, Hawaii makes its MicroCHP systems that can be used to generate power from 200 kilowatts to as much as a grid-scale 50 megawatts, as well as provide process heat for commercial and industrial applications.

Another variation on solar trough technology is from Cogenra which combines solar thermal energy with photoelectricity to provide heat and power for commercial applications.

But what about individual homes? Can solar trough technology be scaled down enough for domestic use? Could you put a solar trough on your roof or in your back yard to generate both heat and power?

Yes, says inventor Dean Rowland of High Latitude Solar of Bath, Maine. In an email Dean said, “I’ve been looking for this (solar combined heat and power (CHP)) for years but nobody seems to have it. So I had to go and invent it all.”

In the High Latitude system a working fluid, either water or glycol, is heated by concentrated sunlight from parabola-shaped, polished stainless steel mirrors. The computer controlled troughs follow the Sun throughout the day for maximum solar energy absorption. To control heat build up, the troughs will also turn away slightly from the Sun. The troughs can also flip upside down at night, during vacations or before snowstorms to keep the mirrors clean. The whole collector with four troughs is only 5 by 8 feet and 9 inches deep.

To add to the efficiency of the trough, the receiver tubes, that sunlight is focused on, are made of polycarbonate which transmits the same amount of sunlight as glass, but because of the insulating qualities of the plastic material, solar generated heat inside is trapped. Polycarbonate is the same material used in some eyeglasses.

From the receiving tubes, the heated working fluid goes to a heat exchanger to make steam to run a small steam engine, which High Latitude calls its Super Expansion Engine. The steam can be vaporized water or vaporized methanol. Methanol is preferred since it boils, and thus creates methanol “steam” at as low as 148 degrees F, perfect for a solar thermal system that operates at fairly low temperatures, under 300 degrees F.

highlatcollector

High Latitude's back yard parabolic solar collector.

The methanol driven Super Expansion Engine and generator is kept outside the home in case of leaks. To make the little engine more efficient, its cylinder is made of plastic which like the plastic collector tubes, traps heat instead of losing it to the surrounding air. High Latitude claims that some on the efficiency of conventional steam engines is lost because common metal cylinders conduct heat away from the engine into the atmosphere. It’s better to trap the heat with insulating plastic, the company says. Operational temperatures aren’t high enough to melt the plastic, if you’re thinking that.

When there’s no sunlight, alternative sources of heat can be used such as a wood-fired boiler to keep the Super Expansion engine running.

To make the system cogenerating, waste heat from the engine is piped to another heat exchanger to either heat the home or make domestic hot water, or both.

Cleverness runs throughout the design. Polycarbonate, a common replacement for glass, helps the troughs collect and retain more heat. Building the steam engine (itself a tried and true piece of engineering) partially out of plastic instead of metal helps the engine run more efficiently and thus generate more power. And the use of methanol instead of water to make steam allows the system to operate at low temperature.

The company offers full specifications for the system on its website. At maximum operation under optimum conditions the steam engine generator will produce 2760 Watts of electricity and the solar collector can provide 9000 Btu/hr of thermal energy.

Overall, too, the system has a relatively small cost. Totaled, all components on the company price sheet add up to a little over $8000, not including the cost of a boiler. Mr. Rowland says, “Sales of at least 50 units of either product would well justify my funding a manufacturing ramp up.”

He continues, “This is a new slant on green energy because it makes heat, the simplest energy form to realize, the total energy solution. It could potentially make power grids unnecessary, at least in Northern residential neighborhoods.”

Maybe. Maybe not. But this is something new and incorporates the clever use of proven engineering principles and materials at a relatively low price, right from the start.

Links:

High Latitude Solar
http://www.highlatitudesolar.com

Sopogy
http://sopogy.com

Cogenra
http://www.cogenra.com