Rooftop Solar Isn’t Just for Photovoltaics Anymore

DSC05878Think of rooftop solar and you likely envision photovoltaic panels. But a group of solar startups are working to put concentrating solar-thermal systems – more commonly seen in large solar projects in the desert – on roofs too. One such startup, San Jose, Calif.-based Chromasun, unveiled its first collector at the Solar Power International conference in Anaheim, Calif., this week.

The 4-by-10-foot collector, called the Chromasun Micro-Concentrator, is intended for commercial roofs. It includes strips of shiny aluminum, made by Alanod Solar, that look like window blinds and use sensors to automatically track the sun. These strips concentrate light 25 times and reflect it upon two pipes to generate temperatures of up to 428 degrees Fahrenheit.

Companies like Chromasun say that concentrating rooftop solar power systems will cut costs compared to conventional photovoltaic solar panels and also take up less room, so that they can generate higher electricity savings in confined spaces. Chromasun’s panels will be 20-50 percent cheaper than PV systems’ current prices, claims CEO Peter Le Lievre.

In Chromasun’s case, the heat created will be used to run an air conditioner, as it takes heat – in this case, hot oil – to chill air. Combined with absorption chillers, the collector can deliver twice as much air conditioning per foot as a PV system, Lievre said. (See a diagram of how this works here.)

It’s an appealing idea because air conditioners play a large part in creating peak electricity, the times when electricity is in greatest demand, in many areas. In the United States, air conditioners consume more electricity than any other household appliance, according to the Energy Information Administration. In California, air conditioning can cause electricity demand to nearly double on hot summer days, the California Energy Commission said last year.

Large concentrating solar-thermal projects in the deserts generate the most electricity on hot days, when the grid already suffers the worst congestion, and that electricity has to be transported into the cities, further straining the grid, said Le Lievre, who was previously the chief executive of concentrating solar-thermal startup Ausra. Shrinking these systems to rooftop size solves that transmission problem and reduces the demand for electricity during hot days, he added.

Chromasun is targeting buildings with big air-conditioning-related electricity bills, such as data centers, hospitals, universities and office centers, Le Lievre said. The idea is to replace the most expensive electricity, the electricity used at times of peak demand, with solar. In a six-month pilot test, a data center in Sunnyvale, Calif., consistently reduced 1.5 MW of its 6-MW electricity demand, he said. The systems also keep air conditioners running during blackouts.

Still, price could be an issue in the recession, as a Chromasun-powered air conditioner costs about twice the amount of conventional systems. The energy savings pay off the difference in 5-20 years — then provide free air conditioning after that, Le Lievre said.

Chromasun has also tried to make its product more building-integrated and user-friendly. The concentrator is enclosed in aluminum and glass, with no moving parts outside of the panel, making it easier to install and clean, and the panel lets natural light through, so that it can double as a skylight, said Andrew Tanner, a senior product development engineer. “This is the first truly building-integrated rooftop concentrator solution able to efficiently generate high temperatures,” he said.

Founded last year, Chromasun plans to start taking orders and installing pilot projects next year, including projects with Santa Clara University and Sempra Energy, Tanner said. The company expects to start pilot production for those projects in the first quarter and also is raising its first round of venture funding, he added. In addition, it’s developing a concentrating PV panel, which will reflect light onto solar cells and collect heat in pipes behind those cells, generating both electricity and air conditioning at the same time, Tanner said.

Other startups also are working to bring concentrating solar thermal to commercial roofs, including HelioDynamics and Sopogy, which announced a new micro concentrating solar power system, called SopoFlare, for rooftops this month. The system produces steam, which can be converted into electricity with a steam turbine, as well as solar thermal air conditioning, drying, dehumidification, desalination and hot water.