For the last few years, much of the innovation around solar technology has been aimed at generating more electricity from sunlight. But a new crop of entrepreneurs is working to use the sun more efficiently by using its heat, as well as its light, for individual business, factories and, perhaps one day, even homes. These companies say that providing both electricity and heat reduces the amount of time it takes customers to recoup their investments. "It's a single system that provides the greatest impact on a homeowner's energy bill," says Sam Weaver, CEO of heat and power solar startup Cool Energy.

These technologies stand to benefit if natural-gas prices rise, considering that natural gas is used both for heating and for electricity. Those prices rose after the Deepwater Horizon oil rig sank and began leaking oil in the Gulf of Mexico, amid concerns that the spill could reduce natural-gas production, and prices could rise again if hurricanes hit the Gulf Coast and reduce production. A gas pipeline explosion in Texas probably doesn’t help either.

In general, the new combined-solar-heat-and-power systems are based on the same concentrating solar-thermal technology used by the big solar projects — both already built and in the works — in the Mojave Desert. (The 392-megawatt system BrightSource is building with its $1.37 billion federal loan guarantee is one of these.) The large desert systems concentrate sunlight and use it to heat fluid running in tubes. Then, that steam can be used to run a turbine and make electricity.

With the systems for homes and businesses, the heat would be used directly for warm air or hot water, or — although it may seem counterintuitive — could be used to run an air conditioner. (Heat paired with an absorption chiller, for example, can power air conditioning.) Meanwhile, some of the systems siphon a portion of the steam to make electricity, such as via a generator, while others use photovoltaic cells, and in some cases concentrators that direct sunlight to those cells, to produce electricity.

A Symbiotic Relationship

Pairing solar heat and electricity is an attractive idea because, companies say, a combined system would cost less than two separate systems, and could be far more competitive compared to the price of both a heating and an electric system added together. On top of that, the technologies fit well together, according to Jenny Chase, lead solar analyst for Bloomberg New Energy Finance. “Generally speaking, you have something that wants heat and something that wants to get rid of heat,” she says. All photovoltaic cells lose their efficiencies at higher temperatures, and that challenge is exacerbated with concentrating-photovoltaic technologies, which get very hot, she explains. Solar-thermal technologies could potentially use that heat, keeping the photovoltaic systems cool.

And as solar prices continue to fall and conventional electricity prices remain low, companies are looking for ways to stand out from the competition and to do more with less, such as by boosting the value of solar energy. Technologies have improved, reducing the expected prices of combined heat-and-power solar systems and raising their efficiencies, and state and federal policies pushing more renewable energy. “People are starting to think more broadly,” Weaver says. “People are realizing that in a lot of places, electricity is not the largest part of the energy bill and are taking a more holistic [view] of energy in people’s homes and commercial facilities.” While nearly everybody understands electricity, making solar electricity an easier sell, heat is much more regional, he adds. “In some places, such as the Southwest, electricity is dominant; in other places, heating is dominant.”

Some companies and researchers are approaching combined heat and power from two directions: concentrating photovoltaic (PV) systems and concentrating solar-thermal systems. They’re also targeting a variety of markets, including industrial, commercial and residential uses. 

Concentrating PV vs. Concentrating Solar-Thermal

The Center for Architecture Science and Ecology (CASE) in New York is developing a concentrating PV technology integrated into a glass façade with a 'gem-like' architectural design (see lead image for a close-up look at the product). CASE claims the system brings more daylight into buildings, while removing much of the heat that comes with the light, and delivers electricity and usable heat with 60 percent efficiency.  The Fresnel lens concentrators direct light into gallium-arsenide solar cells, while a heat sink behind those cells collects heat. If architects and developers like the look and the lighting features, this technology could gain traction, Chase says. “It sounds like it could be attractive and useful,” she says. “It depends on how good they are at marketing this stuff.”

In California, Entech Solar is also taking a concentrating-photovoltaic approach. It’s developing a system for commercial and industrial customers that concentrates sunlight 20 times to make electricity, while also capturing the heat for hot water, according to a CNET article published back in 2008.

Meanwhile, Chromasun, also based in California, is working on a concentrating solar-thermal system that uses shiny metal strips to concentrate and reflect light onto oil-filled pipes, creating heat to run commercial air conditioners. CEO Peter Le Lievre, formerly the chief executive of utility-scale solar-thermal company Ausra, said last year that Chromasun is also developing a concentrating PV panel, which would reflect light onto solar cells and produce more heat for the pipes, to add to the system. In 2008, the company partnered with researchers from the Australia National University and China’s Tianjin University to explore the technology.

And Colorado-based Cool Energy wants to provide space heating, water heating and electricity with a system that uses weather predictions and other data to predict the most cost-effective combination for customers on any given day. Founded in 2006, the company is developing a system that uses off-the-shelf evacuated solar collectors, usually used for solar hot-water systems, to heat a mineral-oil-based fluid that retains heat well. The fluid, which can be stored in an insulated tank until it’s needed, will be able to heat both water and air, while some of the heat will be used to run a Stirling engine (pictured, left) the company calls SolarHeart. Stirling engines use pressure created by hot air on one side and cooler air on the other side to move pistons and generate electricity.

The system also includes an Internet-based control system that will use publicly available information, such as predictions about the weather, temperature and sunlight, to decide how much heat and electricity to produce. The idea is that in the winter, or whenever it’s cold and cloudy, the system will store heat, while in sunny temperatures, it will deliver electricity. With a heat exchanger, the system also could potentially make use of waste heat from appliances like dishwashers.

Cool Energy, which is targeting residential customers that use fuel oil and propane for heat, believes it will be able to address a potential market of roughly $1 billion annually in North America alone. The system would make the most sense for customers in places with long winters, who need space heating for winter, electricity for air conditioning in the summer and hot water all year round, Weaver says. In some parts of the United States and Canada, in places with long winters, air and water heating accounts for more than half of customers’ annual energy bills, he adds.

The technology won’t be available for a few years, however. The eight-person company has developed a 1.8-kilowatt prototype running at 15.5 percent efficiency and expects to bring that to around 20 percent efficiency by the time it hits the market in 2011. Cool Energy is looking for around 10 pilot projects now, then plans to ramp to a few hundred in 2011, thousands in 2012 in 2013 and between 10,000 and 20,000 per year by 2014, Weaver says. The company’s already raised a few million in angel funding and about $900,000 of government funding from the National Science Foundation, and is now seeking $5 million for its first round of venture-capital funding.

Canada-based Conserval Engineering also makes a PV/Thermal hybrid solution that pairs with its SolarWall, which RenewableEnergyWorld.com recognized with an Excellence in Renewable Energy award last year.

Finally, Swedish startup Absolicon has developed a solar collector that tracks the sun, uses a parabolic reflector to concentrate sunlight onto solar cells to produce power, then collects heat for hot water and cooling. (See image of the collector, right.) For cooling, the heat is used to dry a salt that then cools water, the company says. Founded in 2007, Absolicon already has completed a handful of installations on large buildings with flat roofs, including an eight-collector array installed at a hospital in Härnösand in April, and has raised 1 million euros from angel investors.

Challenges

Of course, these systems face plenty of challenges. Combining two systems, no matter how symbiotic, is rarely as easy as it sounds, and complexity can lead to unexpected costs in production or installation. “The big problem is new solar products are all a bit risky,” Chase says. With that risk, customers and financiers may not be willing to lay out the upfront cash to buy these heating technologies right away. In the meantime, prices for regular photovoltaic solar panels keep falling. By the time these technologies hit the market, “it might just be cheaper to shove a PV system on the roof and run your conventional AC from it,” Chase adds.

Traditional heating and cooling technologies aren’t standing still, either, and those improvements could make these new solar heat and power systems less competitive. Hallowell International, for example, sells an energy-efficient electric heating and cooling system that uses boosted compression to absorb more heat, which it claims can deliver 30 to 70 percent cost savings, and this year also began offering a hybrid option to allow customers to switch between electricity and natural gas depending on prices. CEO Duane Hallowell says the company conservatively forecasts it will sell 3,000 systems this year.

Jennifer Kho is a freelance reporter and editor based in Oakland, Calif. Aside from RenewableEnergyWorld.com, her stories have appeared in The New York Times' Green Inc. blog, The Wall Street Journal, Los Angeles Times, AOL's DailyFinance, MIT's Technology Review, The Christian Science Monitor, Reuters.com, Earth2Tech and other publications. She has more than a decade of journalism experience and has been covering green technology since 2004.