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Solar Shootout in the San Joaquin Valley

Side-by-side crystalline and thin-film photovoltaic installations at a water treatment plant in California's Central Valley should provide a clear indication of which provides the best energy production and cost benefit performance over varying climatic conditions within a year.

The 1.6-MW solar array for the South San Joaquin Irrigation District (SSJID) was installed in two phases by Denver-based Conergy Americas in Manteca, CA. Phase One includes 6720 Conergy 175-W crystalline modules mounted on a single-axis solar tracking system that can boost peak-time output by about 15% over fixed systems.

The Phase Two tracking system, which went active in late March, uses cadmium telluride (CdTe) modules from First Solar, chosen because they are expected to perform at a lower cost/watt than crystalline modules, according to David Vincent, Western U.S. project director for Conergy. They add 419 kW to the project, and it is believed to be the first commercial thin-film solar tracking system in the U.S.

Thin-film modules "can outperform monocrystalline in areas prone to hazy, overcast conditions or in industries that generate dust or high degrees of air particulates," according to Vincent. They are also superior when there is frequent fog, such as in coastal areas. The reason, he says, is the sensitivity of the thin-film cells to a broader span of the solar spectrum, including infrared and ultraviolet regions.

Thin-film cells also should perform better when dust covers the surface, he added. Another advantage of thin-film modules is that less interconnect is needed between cells, so that there is less rise in resistivity and heat loss on hot days, he explained.

Early indications, Vincent says, are that the output/DC kW of the thin-film modules is about 10% higher that of the monocrystalline.

The project, known as the Robert O. Schultz Solar Farm, will handle almost all of the power needs for a water treatment plant that provides 40 million gallons/day for 155,000 residents and businesses of four nearby communities, as well as irrigation water for 55,000 farm acres. The main goal of the project is to stabilize electrical costs, which can spike in summer months because of time-of-use metering, according to Don Battles, utility systems director for SSJID. Also, these are times when solar output is at a maximum.

To reduce long-term maintenance requirements for the thin-film tracking system, the number of drive motors had to be minimized. The challenge was to effectively drive more than 30 tons of modules and steel following the sun's trajectory with each 2hp motor. This was done by means of a 30-ton screw jack and engineered counter-balance.

Power generation data for the crystalline and thin-film modules will be fed from equipment that Conergy installed on inverters to Fat Spaniel Technologies, a nearby monitoring and reporting company. The analysis is put online so that it can be tracked by SSJID's Battles and his team from offices located more than 20 miles from the solar arrays.

The data on the Fat Spaniel Web site also allows the group to compare the 1-MW Phase One solar-tracking system with a number of fixed installations, such as a 1-MW fixed-axis rooftop system at a fruit packing firm in Hanford, CA, a system that Conergy also installed. Battles indicates that the output at the water treatment tracking facility is typically 15%-18% ahead, even though he believes the sun is better at the Hanford location.

The irrigation district expects to save nearly $400,000 a year in utility costs due to the solar system, while getting millions of dollars in state incentives.

Conergy's Vincent says that the side-by-side face-off between monocrystalline and thin-film systems is attracting worldwide attention, particularly in Europe where solar has advanced much further than in the U.S.

The performance of thin-film modules under the hazy, often foggy conditions is attracting considerable interest in the California valley region, according to Vincent. For example, a 188-kW thin-film fixed solar array is being installed by Conergy in Hanford, CA, for Verdegaal Brothers, a fertilizer, warehousing and soil and water amendment supplier.

Vincent said that the First Solar CdTe thin-film installation takes about 10%-15% more ground space, but provides more energy and is expected to cost 10%-15% less than a monocrystalline array. The facility is expected to offset Verdegaal's utility bills by 99%, cutting some $60,000 a year, while providing for 82% of the company's energy needs. Over the 25-year life of the system, which is scheduled to start up in July, emissions are expected to be reduced by 6,145 tons of CO2.

This article was originally published by Photovoltaics World and was reprinted with permission.

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