By Michael Valenti, Senior Editor

Converting the sun's rays into electricity, or using them to heat water, are proven technologies. Solar energy is abundant in many parts of the world, and does not generate emissions. However, the cost of solar energy systems compared with the amount of power they produce makes them a decidedly uneconomical power source.

Government agencies and laboratories, as well as utilities, are working with private companies to develop technologies that promise to bring down the costs and raise the efficiency of solar power systems.

Last December, the U.S. Department of Energy's National Renewable Energy Laboratory developed a thin film copper indium gallium diselenide photovoltaic cell that converted available sunlight into electricity with 18.8 percent efficiency. This broke the previous world record of 17.7 percent established by the NREL two years earlier and is currently the highest efficiency thin film solar cell.

Thin film photovoltaic cells consist of layers of a semiconductor material such as copper indium diselenide (CIS), deposited on a low-cost substrate, such as glass, plastic, or stainless steel. When sunlight contacts these CIS cells, they generate electric current. The economic benefit of thin film PV cells is that they use much less semiconductor material than the crystalline silicon cells. The film is 4 microns thick as opposed to 300 microns for photovoltaic cells. In addition, manufacturing thin film cells requires fewer processing steps and lends itself to automation and monolithic design.

"We also have greater design freedom in using the multiple elements of CIS as opposed to all-silicon solar cells," said Rommel Noufi, a chemist and team leader of NREL's thin film cell project. "For example, adding gallium lets us manipulate conducting properties of CIS over a wide range."

The NREL designers first sputter a layer of molybdenum onto a glass substrate. A flux of indium, gallium, and selenium is then evaporated onto the molybdenum, followed by a layer of copper and selenium, and a second layer of indium, gallium and selenium. The layers are deposited where the substrate temperature is increased from 300°F to 550°F. The film is immersed in a chemical bath of cadmium sulfate, thiorea, and ammonia, to coat the cell with a layer of cadmium sulfide. A final layer of transparent, conducting zinc oxide is sputtered on top of the cell.

Making a solar module requires three patterning steps, each performed after one layer of material is deposited. This creates an integrated series of connected cells, with each cell about 0.5 cm wide. "This automatic process is much less labor intensive than cutting and stacking silicon cells like tiles," noted Noufi.

APS will examine using the Power Advantage 30 battery charger on grid applications, such as this parking lot solar array, after the utility gains experience using it on off-grid applications at remote locations.

Sunlight shines through the thin film cells' zinc oxide layer, where it is absorbed in the CIS layer. The light energy generates an electric current. The zinc oxide and molybdenum serve as electrodes. External leads are attached to the electrodes to carry the current to the system being powered.

It will be some time before the NREL's 18.8 percent CIS solar cells can be scaled up to several feet square to produce power commercially. However, the NREL partly funded the development of 12.1 percent efficient CIS solar modules that are being marketed by Siemens Solar Industries of Camarillo, Calif. The ST40 CIS modules are 50 percent more efficient than currently available thin film solar modules, according to NREL testing done in March.

Siemens Solar participates in national CIS research and development teams focusing on key thin film photovoltaic technology. Its ST40 CIS modules measure 1 foot x 4 feet, and can generate 40 W of peak power, where voltage is typically measured. This suits the modules for stand-alone applications, including telephone charging and recreational vehicle charging, or grouped into kilowatt-size systems for residential power.

Fire, Wind & Rain Technologies LLC of Flagstaff, Ariz., a developer of electronic systems to convert, control, and measure electric power, introduced its Power Advantage 30 battery charge controller in March, following nine months of testing by Arizona Public Service, based in Phoenix. The computer-based charger lets the system optimize the conversion of sunlight into electrical energy and controls the delivery of current to the deep-cycle lead acid batteries commonly used in solar energy systems. The result is a boost in overall system efficiency of as much as 25 percent.

Fire, Wind & Rain was founded as a strategic business alliance between Arizona Public Service, a major southwestern utility, and Connect Tech International of Flagstaff, an electronic manufacturing services firm. "We had to make the new battery charger efficient, reliable, and affordable," said Bill Schlanger, an electrical engineer and president of Fire, Wind & Rain. Schlanger and his colleagues addressed the efficiency challenge by specifying components that far exceed the minimum design limits of the unit. The design team ensured the charge controller's reliability with protection components, such as an input filter containing inductors, capacitors, and metal oxide varistors to suppress electrical surges. As a result, although the controller is rated at 30 amps, it uses devices specified at 200 amps.

"Lastly, we made the chargers affordable—a $699.95 manufacturer's suggested retail price—by using automated technology in volume production to take advantage of economies of scale, and by including our manufacturing staff early in the design process," Schlanger said.

The Power Advantage is equipped with a processor made by ZiLOG Inc. of Campbell, Calif., to monitor energy production, energy consumption, battery utilization, battery temperature, and illumination of the solar cells. The processor communicates this information with the proprietary Power Advantage Virtual Control Panel software that can be run on a personal computer equipped with Microsoft Windows 95/98 and a serial port. The software shows the voltage, current, and power of the PV panels. In addition, the Virtual Control Panel software shows the status of the charger and battery voltage, current, and power as well as available battery capacity and battery temperature.

Based on these parameters, the Power Advantage 30 will find the maximum power point for the solar panels and constantly adjust their position to convert the optimum amount of solar energy into electricity. "By capturing more power from the PV panels, overall reliability and efficiency of the solar electrical system are greatly increased," said Herb Hayden, the solar program coordinator for APS. Hayden oversees testing of the new battery charger at the utility's Solar Testing and Research Center in a system designed for rural homes and businesses. "With the Power Advantage 30, we've seen up to a 25 percent increase in power in our field application tests," Hayden said.

APS will install the Power Advantage 30 initially on existing solar power systems it builds for customers off the power grid, such as ranches and remote homes. "After gaining some experience in off-grid power applications, we will examine using the Power Advantage 30 on grid applications," Hayden added.

Parabolic trough solar systems do not convert sunlight directly into electricity as do PV systems. Rather, they use the sun's rays to heat a fluid running through the focal line of the trough. This heat can then be used to generate electricity in a thermal power cycle, or be used for thermal loads such as heating water that would otherwise be heated with conventional, fuel-burning energy sources. Water heating is a significant share of a facility's energy costs, accounting for 18 percent of the energy use in residential buildings, and 4 percent of the energy use in commercial buildings, according to the U.S. Department of Energy.

This parabolic trough array designed by Industrial Solar Technology reduces the energy bills of the Phoenix Correctional Institute in Arizona about $6,000 per month by using sunlight to heat 40,000 gallons of water per day for the use of the inmates and staff.

The parabolic systems consist of a curved aluminum trough with a reflective aluminized acrylic film applied to the inner surface. The curve of the parabola reflects light to a linear focus from any point along the curved mirror. At the focal point is a borosilicate glass-encased, black nickel-coated receiver tube through which a water and antifreeze solution is pumped. The fluid picks up heat and takes it to a heat exchanger where it is used either to heat water directly or to heat a thermal storage tank. Another heat exchanger on the load side of the storage tank transfers heat from storage to preheat potable water before it goes to the electric water heaters in each building.

Parabolic troughs are equipped with tracking systems that align the concentrator with the sun throughout the day. This makes the troughs most suitable for clear, sunny regions of the United States, particularly the Southwest.

According to DOE, parabolic trough systems benefit from economies of scale. By dividing balance-of-system components such as the tracking mechanism by a large collector area, costs as low as $27.50 per square foot are achievable. System sizes of 3,600 square feet of solar collector capable of producing about 7,500 gallons of hot water per day might be considered as a minimum economic size. These economies improve for facilities that operate seven days per week with relatively constant populations—places such as prisons, hospitals, and barracks. For these reasons, the Federal Energy Management Program of the DOE has established private-public partnerships called Energy Savings Performance Contracts, or ESPCs, to encourage the use of renewable energy systems such as parabolic trough solar systems.

Under the terms of the ESPC, an energy service company agrees to fund, install, and operate the energy system. The government repays the energy service firm out of the energy savings realized by using the system for the duration of the ESPC. After the contract is completed, the government takes title of the equipment and keeps future energy savings.

The ESPCs spurred the installation of parabolic trough solar collecting systems designed by Industrial Solar Technology of Golden, Colo., the sole U.S. manufacturer of these systems. By the end of 1998, IST had designed and installed its parabolic water heating systems at the Jefferson County Correctional Facility in Golden, the California Correctional Institution in Tehachapi, the Adams County Detention Facility in Brighton, Colo., the Paul Beck Recreation Center in Aurora, Colo., and most recently, the Federal Correctional Institute in Phoenix.

In order to design the parabolic trough system for the Phoenix facility, IST measured the amount of hot water used by the 1,200 inmates and 200 staff, about 40,000 gallons per day, all heated electrically. "We used the solar industrial process heat simulation software originally developed by NREL to determine the most economical amount of water that would be heated by solar energy. The Soliph software also helped us to establish the size of the parabolic trough system and the capacity of the storage tanks needed," explained Ken May, a chemical engineer and president of IST.

Two tanks capable of holding 11,500 gallons each were installed. The tanks are equipped with timers and temperature controls to conserve solar heat and deliver it during the facility's peak demand period. The water is heated by the troughs, and raises the prison's water temperature via heat exchange.

The solar collecting and heat exchange equipment was comparable to other IST installations, but the piping posed a challenge, May recalled. "Unlike some correctional facilities that have a central hot water plant supplying the entire facility, the Phoenix prison has a distributed heating load system that required more complex piping," May said. His company decided to use polyethylene piping in place of standard copper piping because of its low cost, easy workability, light weight, and corrosion resistance.

"However, because polyethylene has much lower temperature resistance than copper, we added controls and redundant temperature sensors to the piping system to limit the heat of the water to 140°F," noted May. The IST system reduced the Phoenix federal prison's electrical use by about $6,000 per month once it began operation.


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