USGS Taps the Florida Sun for Energy in St. Petersburg
In line with the U.S. Department of the Interior (DOI) strategy to reduce its environmental footprint, the U.S. Geological Survey (USGS) St. Petersburg Coastal and Marine Science Center in St. Petersburg, Florida, is installing a new solar-heating system. The new system will supplement the current boiler with evacuated-tube solar collectors to provide heat in the winter and reheat conditioned air in the summer.
The solar collectors are being installed on the newest USGS building in the C.W. Bill Young Marine Science Complex, which houses the USGS and several other ocean-science institutions in St. Petersburg (see related Sound Waves article "USGS St. Petersburg Office Dedicates New Building to Congressman C.W. Bill Young"). Designed by USGS operations manager Terry Kelley and constructed in 2005, the building is cooled with water received from the nearby University of South Florida, St. Petersburg. The chilled water, at around 40°F, passes through radiators in the ventilation system just as compressed refrigerant would in a standard air-conditioning system. Because the resulting air is very cold, a reheat system employs a heated-water radiator to warm the air and control the final temperature. The result is continuously circulating air of optimum temperature, important because the building contains several laboratories.
"We recycle some of the cool air before it exits the building and must use 100-percent recirculated air for the lab environment," said Kelley. The necessary recirculation and Florida's high humidity require that the air be cooled even in the winter.
The existing natural-gas boiler used for heat and reheat not only contributes to the center's carbon footprint but also costs about $800 a month to run. The new system will significantly reduce natural-gas consumption and integrate almost seamlessly into the existing mechanics of the building. The evacuated-tube system uses no fossil fuels and is expected to make up the cost of the parts and installation in 4 to 6 years.
St. Pete Solar Energy is installing the new solar collectors and built the structure supporting the array of tubes. Positioned above the boiler to allow for easy connection to the existing storage tank, the evacuated tubes function considerably differently than the photovoltaic solar systems that are usually installed, said St. Pete Solar Energy President Barry Flaherty.
"Photovoltaic actually works on an atomic level. A photon, a little packet of energy from the sun, actually strikes an atom and excites certain electrons that want to go somewhere else," Flaherty explained. "There's a positive and a negative side to the solar cell, so when the photon strikes that electron, it wants to go to the other side, and creates current."
In contrast, evacuated-tube technology takes advantage of the wave properties of light rather than its particle properties. The evacuated tubes consist of a copper "heat pipe" in the center with two flat aluminum absorber plates attached. The absorber plates are sprayed with an aluminum nitride coating to maximize the absorption of radiation energy of the sun. This heat is then circulated within the heat pipe via a salt and synthetic-oil substance up to a header pipe where the heat is transferred to the water. The heat pipe and absorber plates are surrounded by a vacuum tube made of borosilicate glass, similar to laboratory glassware, with the air removed. Removal of the air inside the tube reduces the effect of outside weather conditions and is the "evacuated" part of evacuated tubes.
Flaherty will be working with Frank Derkovitz, facility maintenance manager, to fine-tune the system and meet the needs of the center. "It's all about temperature rise, and we really don't know how much we're going to get," Flaherty said. "There are so many variables in this, so I like running it in real time."
Flaherty will be installing several temperature probes to measure temperature change at various points and valves to allow for different configurations in the flow of the system. Once the outside configuration is finalized, the primary method of regulating the heat will be controlling the flow of the water. Increasing the rate at which the water flows across the heat exchanger, for example, will decrease the amount of heat that is absorbed.
"Once it's up and running, we'll find an optimum value for it with the water pumps," said Kelley, who organized bringing the solar tubes to the facility and recently retired this year after 43 years with the USGS.
The solar installation should be up and running by the end of summer. This project reflects the DOI's recently released strategic plan, Building a 21st Century Department of the Interior:
"… to reduce Interior's environmental footprint over the coming years with a goal to incorporate sustainable practices throughout and more effectively utilize resources and protect the environment."
in this issue:
Solar-Heating System Reduces Environmental Footprint