Solar Arrays Do Double Duty

A startup called Cogenra Solar recently installed a bank of solar arrays with a difference at a Northern California winery. The arrays combine conventional photovoltaic solar cells with a system for collecting waste heat. This produces electricity for lighting and bottling equipment, and it heats water that can be used for washing tanks and barrels.

Cogenra plans to install these “hybrid” solar arrays at businesses that use large quantities of electricity and water, and then charge them for supplying both. The company has not released an estimate for the cost per watt of its electricity, but it says that the cost of heated water will be considerably less than the norm.

At the winery, owned by Sonoma Wine Company, several parabolic dishes, each 10 meters long and three meters wide and lined with mirrors, concentrate sunlight onto two strips of monocrystalline-silicon solar cells suspended above. The parabolic dishes sit on top of mechanical arms that move them to follow the sun. Heat is collected with a mixture of glycol and water that flows through an aluminum pipe behind the solar cells. The glycol solution is fed into a heat exchanger, where it heats up water. The water is then pumped to a storage tank, and the cooled glycol solution is fed back to the solar arrays.

Similar hybrid solar systems have failed in the past because the solar cells have overheated. Cogenra uses sensors to monitor the temperature of its solar cells and an automated control system to draw fluid away more quickly if they need cooling down.

Overheating impairs the performance of a solar cell and is a big problem for hybrid solar systems, says Tim Merrigan, a senior program manager at the National Renewable Energy Laboratory in Colorado. Merrigan notes that more sophisticated equipment for monitoring the buildup of heat and adjusting the flow of liquid away from the cells can help prevent this, but “it is certainly not an easy thing to do correctly.” With Cogenra’s technology, there is also a trade-off between the amount of heat that can be produced and the efficiency of the solar cells.

The winery installation will serve as an important test bed for Cogenra’s technology and for hybrid solar technology in general. The system will generate data showing how efficiently it can produce electricity and heated water under different weather conditions and how well it can meet the fluctuating needs of the winery’s operation.

The solar arrays will be able to produce 50 kilowatts of electricity, and the equivalent of 222 kilowatts of thermal energy. Gilad Almogy, the CEO of Cogenra, says this will cut the winery’s use of natural gas for water heating by 45 to 50 percent and meet about 10 percent of its electricity needs.

Making the technology cost effective will be another challenge for Cogenra. But a growing number of government programs that dole out rebates for installing solar water heaters could help. One such program was launched in California in October. Lasting through 2017, it will provide $350.8 million in rebates for installing solar water heaters. Most water heaters in the state currently run on natural gas.

Vinod Khosla, whose venture capital company Khosla Ventures has invested $10.5 million in the project, says the technology is remarkably cost-effective. “Other solar companies used hundreds of millions of dollars to go to market,” he says.