Four years ago I wrote about a hybrid system that was intended both to make electricity and gather usable heat on residential rooftops. That company, now called Echo Solar, is offering its product around the country.
But the market for such hybrids goes beyond homes, especially if the second product is hot water, which can make steam and then electricity. Now another company, Cogenra, is supplying a hybrid solar electricity and hot-water system for big apartment buildings, dormitories, retirement homes, wineries, food processing plants and, most recently, a dairy, all of which use large amounts of hot water. It has 40 installations worldwide.
The basic problem, according to Gilad Almogy, the chief executive of Cogenra, is that a photovoltaic cell captures only about 15 percent of the sun’s energy. Another 5 percent or so reflects back off the solar cell. “Where’s the rest? It’s heat,’’ he said. On a 100-degree day in a sunny location, the cell, which is black, can reach 160 degrees Fahrenheit, he explained.
So his company mounts the cells on water pipes in the center of trough-shaped mirrors. The mirrors concentrate the sun’s energy by a factor of 10. The result is electricity and water that is warmed to as much as 158 degrees. The temperature can be varied, according to the application; water for sterilizing a food processing tank is hotter than water for a shower.
The trough design is not new; it is similar to the one used at solar thermal power plants, which boil water into steam to spin a turbine, turn a generator and make electricity. But in Cogenra’s installations, the water does not reach the boiling point. If it did, the solar cells would not work very well.
As measured by heat content, the bulk of the production is hot water, but the electricity sells at a higher price. The economics vary by location; in most parts of the United States, the value of the hot water is set by the value of the natural gas that did not have to be burned because the solar system did the work instead.
Since natural gas is currently very cheap, the small electricity production may be more valuable. But in Hawaii or India, where fuel to heat the water has to be shipped in, the value of the hot water would be higher, Mr. Almogy said.
If a quantity of electricity is worth four times as much as the equivalent amount of natural gas, then the hot water and the electricity produced by his system have about equal value, he said. At the moment, though, the electricity is worth far more than four times as much, because the price of natural gas is depressed by a surplus brought on by fracking.
The economics of his system works well in California because that state offers a rebate for the installation of renewable systems for heating hot water, he said. The rebate is equal to $12.82 per therm of gas saved in the first year, which is well above what a therm actually costs most consumers at the moment.
While Cogenra’s system starts with the same concept as Echo Solar’s, they are not quite competitors. They are both backed by the same alternative energy investor, Vinod Khosla.