Submitted to: National Solar Conference
Publication Type: Proceedings
Publication Acceptance Date: 4/1/2005
Publication Date: 7/15/2005
Citation: Vick, B.D., Clark, R.N. 2005. Effect of panel temperature on a Solar-PV AC water pumping system. In: Proceedings of the International Solar Energy Society (ISES) 2005 Solar Water Congress: Bringing water to the World, August 8-12, 2005, Orlando, Florida. p. 159-164. 2005 CDROM.
Interpretive Summary: Determining the exact number of solar-PV (photo-voltaic) panels needed to pump water from a water well at a specific pumping depth for watering livestock is very important to ranchers. If too few solar-PV panels are purchased, this could result in the rancher having to haul water to the livestock. However, if too many solar-PV panels are purchased, then that will result in having more power available than the pump motor can utilize resulting in the rancher having wasted money on unused available solar panel power. Data were collected at the USDA-ARS laboratory at Bushland, TX on a solar-PV AC water pumping system over a 6 year period which included: intensity of solar radiation, water flow rate, power (DC and AC, DC electricity from solar-PV panels converted to AC electricity by a controller), solar-PV panel temperature, wind speed, and air temperature. Since solar-PV panels generate DC electricity then they perform better at lower temperatures -- opposite trend with solar thermal hot water systems which collect heat from the sun. So for Bushland, there can be a decrease in solar-PV panel performance of as much as 15% from a cold windy day in winter to a hot calm day in summer (wind has the effect of cooling the solar-PV panels). For Bushland, TX (assuming the solar-PV panels are tilted to the correct angle in the winter and summer), the solar-PV panels will perform at rated or slightly above rated in the winter, but will perform 6.5% worse than rated in the summer. In order to estimate the expected solar-PV panel performance at other locations besides Bushland, an equation was derived which estimates the solar-PV panel temperature given the solar radiation, air temperature, and wind speed. By using data already measured at most sites in U.S. – namely solar radiation, air temperature, and wind speed – an installer using this equation can estimate the correct number of solar-PV panels required for a solar-PV water pumping system.
Technical Abstract: Panel temperature was monitored during a solar-AC water pumping experiment by the USDA-Agricultural Research Service, Bushland, TX. Using 6 years of average monthly panel temperature data, the solar panel performance during winter was estimated to be at rated power, but the solar panel performance during the summer was estimated to be only 93.5% of rated power. In addition, the water pumped on a hot day (36oC) was shown to be 7 to 10% less than on a warm day (20oC) – the entire performance loss was attributed to higher panel temperatures. Solar panel temperature was also shown to be dependent on wind speed which means solar-PV water pumping performance can also be affected by wind speed. In order that other locations besides Bushland can estimate the temperature effect on solar panel performance, an equation is derived in this paper for estimating the solar panel temperature.