Geothermal, Solar, Storage

University Developing Solar Generator that Provides Cooling

A university professor in Florida is developing a device that he claims will generate electricity from solar energy, while providing refrigeration from the same unit.

GAINESVILLE, Florida, US, 2001-03-22 <SolarAccess.com> The solar- or geothermal-powered thermodynamic cycle is being tested at the University of Florida by Yogi Goswami, a professor of mechanical engineering. The system was first described in the Journal of Solar Energy Engineering last year, and will attempt to verify what Goswami describes as a new combination of two classic thermodynamic cycles: the Rankine, or steam cycle, and the absorption-refrigeration cycle. “We’ve seen that it works in theory, and we’ve set up this experimental system to prove that it works in practice,” explains Goswami, who is also director of the university’s Solar Energy & Energy Conversion Laboratory. “This is a development that has a lot of impact on how we use energy in the future.” Both cycles are well known to engineers. The Rankine cycle, typically found in large power plants, uses heat to boil water and create pressurized steam which, in turn, spins a turbine and generates electricity. The absorption-refrigeration cycle, most common in large commercial refrigeration units, chills air through boiling and condensing ammonia. In Goswami’s unit, hot water is used to heat pressurized ammonia past its boiling point, generating ammonia steam. If ammonia is maintained at the pressure required to spin the turbine, it boils at a lower temperatures (212oF) than water in the same condition (up to 500oF). In theory, the hot water would come from deep underground or solar collectors, although for the purposes of the experiment a household hot-water heater is used. With a $175,000 grant from the U.S. Department of Energy, Goswami will see if the the pressurized ammonia vapor can spin a turbine and generate electricity, a process simulated in the experiment through using a heat exchanger and expansion valve. As the ammonia spins the turbine, it falls below room temperature, dropping to 32o or lower, where it can be used for air conditioning or refrigeration. “The unique thing we’re doing is that we can remove so much of the energy from the ammonia in the turbine that it actually becomes very cold,” explains Goswami. “We can then use that cold gas to our advantage for air conditioning or to create ice.” A similar Solar Energy Generating System in California’s Mojave desert has used hot water collectors to produce 354 MW of power, but the collectors are expensive and the power costs more than electricity produced with fossil fuel technology. “The capital cost is about $3,500 per kilowatt of capacity,” says Goswami. “To make it competitive, we really need to bring that cost to less than $2,000.” Using solar PV cells to generate electricity costs about three times as much as coal, says Goswami, which is why solar is not widely used. Using off-the-shelf collectors, he says the system would be ideal for homes that could take advantage of both the electricity and the refrigeration. “What we’re looking at is if we can have a power plant to give you as low as 5 kW, so a power plant is good enough for a household,” he explains. The technology could also extract additional energy from hot waste water that is produced by conventional power plants. Efficient power plants capture less than 40 percent of the energy in fuel, releasing the remainder as hot water with damaging environmental consequences. If installed on the outlet pipes at these plaints, Goswami says his unit could obtain 30 percent more energy from the system while cooling the water.