The Latest In Solar Tech: A Generator That Doubles As A Refrigerator

February 21, 2001

GAINESVILLE, Fla. — A network of steel pipes and tanks tucked behind a small building at the University of Florida could lead to a new method of creating two seemingly unrelated products — electricity and refrigeration — by tapping into the power of the sun.

The pipes and tanks are the guts of a just-launched experiment to test what Yogi Goswami, a UF professor of mechanical engineering, describes as a novel solar- or geothermal-powered thermodynamic cycle.

“We’ve seen that it works in theory, and we’ve set up this experimental system to prove that it works in practice,” said Goswami, a specialist in solar energy who also is director of UF’s Solar Energy & Energy Conversion Laboratory.

The system, first described in the Journal of Solar Energy Engineering last year, 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.

Both cycles are standard fare in engineering textbooks. The Rankine cycle, typically found in large power plants, uses heat to boil water and create pressurized steam, spinning a turbine and producing electricity. The absorption-refrigeration cycle, seen in large commercial refrigeration units, chills air through boiling and condensing ammonia.

In Goswami’s experimental set-up, hot water is used to heat pressurized ammonia past its boiling point, generating ammonia steam. This is possible because ammonia maintained at the pressure required to spin the turbine boils at a far lower temperature, around 212 degrees Farenheit, than water in the same circumstances, which requires temperatures of at 400 to 500 degrees. 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.

The next step is for the pressurized ammonia vapor to spin a turbine and produce electricity (a process simulated in the experiment through using a heat exchanger and expansion valve). The unique part is what happens next. As the ammonia spins the turbine, it actually falls below room temperature, reaching lows of 32 degrees or lower — cold enough to make ice. The result can be used for refrigeration or air conditioning.

“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,” Goswami said. “We can then use that cold gas to our advantage for air conditioning or to create ice.”

The system is not the first to attempt to use solar or geothermal power to drive a turbine, Goswami said. The Solar Energy Generating System, a mammoth facility in the Mojave desert in Southern California, has used specially designed hot water collectors to produce as much as 354 megawatts of power — enough for 70,000 homes. But the collectors are extremely expensive, making the power more costly than electricity produced with fossil fuel technology.

“The problem has been that the capital cost is about $3,500 per kilowatt of capacity,” he said. “To make it competitive, we really need to bring that cost to less than $2,000.”

Goswami said his system is more economical because it can use off-the-shelf collectors. Although it could be used on a large scale, the system would be ideal for homes that could easily take advantage of both the electricity and the refrigeration, he said.

“What we’re looking at is we can have a power plant to give you as low as five kilowatts, so a power plant is good enough for a household,” he said.

Another application for the technology is to milk additional energy from the hot waste water produced by conventional power plants. Even the most efficient power plants today capture only 30 to 40 percent of the energy in the fuel, releasing the bulk of the remainder in the form of heat — much of it as hot water with sometimes damaging environmental consequences. Goswami said his system, installed on the outlet pipes for the hot water, could leach 20 to 30 percent more energy from the system while also cooling down the water. As a result, the plant could generate extra electricity while gaining cooling capability for on-site refrigeration or air conditioning needs, he said.

The research is funded with a $175,000 grant from the U.S. Department of Energy, which is interested in developing the geothermal application of the technology.