Generating Solar Data

Assumptions and Definitions

The solar tables obtained from the Energy Center’s calculator cover two solar technologies: photovoltaic electricity generation and solar thermal for domestic heat and hot water.

Input Data

Calculations are based on one year of experimental solar radiation measurements at five Iowa recording stations. Stations, operated by the Iowa Wind Energy Institute were located in Alta (42.84oN, 95.34oW), Arlington (42.77oN, 91.61oW), Muscatine (41.45oN, 91.05oW), Red Oak (41oN, 95.1oW) and Radcliffe (42.28oN, 93.43oW).

One year of data is generally considered insufficient to fully represent long term local climatological conditions. Also, five experimental stations may be not enough to account for all micro-climatic effects to be found in the State of Iowa. These limitations will be addressed in the future, by developing a cartography of Iowa based upon geostationary satellite observations, providing a ground resolution of about 3 miles. For the time being, however, the current tables are expected to be within ± 5% of probable long-term climatology on a yearly basis, and ± 25% on a monthly basis.

The experimental data measured at the five sites consist of 15-minute global irradiances (in addition to other meteorological data such as wind speed and temperature). Data were processed and controlled to insure calibration consistency between each site. From these measurements, direct and diffuse irradiances were extrapolated using the ASHRAE WYEC2 model that had been previously validated for the central United States [ref]

Other locations in the State of Iowa were derived via geographical interpolation and extrapolation of the five measurement sites.

Photovoltaic Calculations

Photovoltaic (PV) power generation is simulated from, irradiance, temperature and wind speed data. The simulation program is a simplified version of PVFORM [1] developed by Sandia National laboratories.

The tables report PV output as a function of time of day and time of year, normalized to a 1 KW-ac system rated for summer operating conditions. Such a system would deliver 1-KW of ac-power (i.e., including inverter inefficiency) under full sun conditions (1000 W/sq. m plane-of-array irradiance) and 25oC ambient temperature. The reported values are average power output in Watts for the considered time period. The rightmost column of the tables reports energy available from the system in kWh by month, and for the entire year. Results should be prorated from 1-kW nominal size to reflect actual system size.

The considered PV technology is crystalline silicon, which corresponds to over 90% of current installed PV capacity. This technology is characterized by an increase in conversion efficiency inversely proportional to the array operating temperature (hence arrays may deliver more than their rated output in winter). Further, the simulations correspond to PV systems that can make use of all available solar energy, such as grid-connected systems or water pumping systems. Stand-alone residential systems system with batteries and back-up generators would produce less energy because of storage inefficiency and likely overproduction in summer.

The user can select various array geometries, including fixed and tracking arrays. Fixed arrays may be defined in terms of slope and orientation (azimuth). A slope of 0o corresponds to a horizontal array, while a slope of 90o corresponds to a vertical array. An azimuth of 0o corresponds to a south-facing array, 90o to an east-facing array, 180o to a north-facing array and 270o to a west-facing array. Therefore, the azimuth of an array facing 20o east of south will be 20o, while the azimuth of a PV array facing 20o west of south will be 340o. Four tracking arrays may also be selected: (1) 2-axis tracking arrays, designed to face the sun at all times; (2) one-axis tracking arrays with an horizontal north-south axis, and (3) one-axis tracking arrays with an horizontal east-west axis, and (4) one-axis tracking array with an axis tilted to the site’s latitude.

Solar Thermal Calculations

Solar thermal output is also simulated from irradiance and ambient temperature data. The simulation is based on the F-Chart method. [2].

The considered solar thermal systems are ideally operating high efficiency panels effectively making use of all available solar energy. Such systems are small compared to their loads (25% or less). Inefficiency increases (hence nominal output decreases) as the solar portion of the load increases. System efficiency is a function of water inlet as well as ambient temperature and irradiance. The considered water inlet temperature is 12oC (53oF).

The tables report PV output as a function of time of day and time of year, prorated to a one square foot solar panel. The value reported in each cell of the tables is the average heat delivery rate in BTU per hour for the considered time period. The rightmost column reports energy available in BTUs by month, and for the entire year.

1. D.F. Menicucci and J.P. Fernandez/ User’s Manual for PVFORM. Report # SAND85-0376-UC-276, 1988, Sandia Natl. Labs, Albuquerque, NM.
2. W.A. Beckman, S.A. Klein and J.A. Duffie, Solar Heating Design by the F-Chart Method, John Wiley and Sons, (1977-1990)