Wind and Solar Resource Assessment for Iowa

Grant# 93-04-02
Principal Investigator: Tom Factor
Organization: Iowa Wind Energy Institute
Technical Area: Renewable Energy

Background and Significance
Iowa has abundant wind resources. An Iowan’s impression standing on exposed land is that it is usually windy. However, measured average annual wind speeds in different parts of the state at the tower height of a large wind turbine will range from about 4.9 to 7.8 meters per second (11 to 17.5 miles per hour). The same wind turbine at a high wind site will produce over three times the amount of electricity as a low wind site. Even differences of a fraction of a mile per hour in wind speed can make a significant difference in the economic viability of a turbine. The purpose of the Iowa Wind Energy Institute’s (IWEI) wind resource assessment has been to accurately measure and map wind speeds in Iowa. This map will allow any individual, company, utility or independent power producer to perform an initial assessment of the potential feasibility of a chosen wind site without the usual cost and delay of erecting a tower to measure the wind speeds for a long period of time. This report details the progress of IWEI’s mapping effort. This report also updates IWEI’s effort to create a similar mapping of Iowa’s solar resources for predicting output from photovoltaics and solar thermal panels as well as solar gain from double-pane glass.

Wind Resource Assessment Objectives and Work Summary
By June, 1994 IWEI had erected twelve 50-meter wind towers equipped with state-of-the-art wind monitoring equipment geographically distributed throughout the state on sites with good potential for wind development. The distribution put more towers in the windier Northwest region of the state but some towers in each area. Sites were chosen for their high exposure, access to transmission lines, favorable terrain, and size of developable area. Each site was instrumented with anemometers at 10, 33 and 50 meters, vanes at 33 and 50 meters, and a temperature sensor. During the study, some towers were added and moved around to enhance the assessment. By December, 1999, IWEI had completed data acquisition from twelve sites for five and a half years, plus data from four additional sites with varying durations. Hourly averaged data is available for the entire period. 10-minute averaged data is available for half of the period.

After the first two years of data was collected IWEI together with Brower & Company created a wind resource mapping of the state. The map incorporated USGS terrain elevation and roughness data together with IWEI wind speeds and long-term national weather service data to provide a map with a 1-kilometer by 1-kilometer resolution in 1-mph gradients. This map was accompanied by a tabular form of the data that predicted average wind speeds at 2,400 towns and cities in Iowa by month and for an average year.

This year we are in the process of redoing the map using the entire 5.5-year data set. The map in being created with WindMap software which incorporates wind speed by direction as well as terrain elevation and roughness data to give further refinement to the results. The map will then be tiled and imported into IDRISI format to allow it to be refined to the 100-meter resolution. Thereafter it will be imported into ArcView software which together with ArcView Internet Map Server will allow an on-line user to zoom from a state map into a detailed section which shows graphically the 100-meter resolution and .1 mph gradients. The user will be able to display local roads and waterways to aid in the identification of the site. IWEI has already created several maps of specific areas in Iowa for wind energy developers using this technique who have found them invaluable in siting larger wind projects. The availability of these maps on-line will make it possible for any person or company to have access to this information without cost.

Figure 1: Web Interactive Iowa Annual Wind Resource Map

Figure 2: Detail of a county zoomed in from state map showing rivers, roads and highways.

The mapping process is dependent upon the quality of the data used to create it. IWEI has achieved an average data recovery rate of 95 percent from the stations. The data is sampled by the data loggers every second and interval averages are stored and then transmitted weekly by cellular link to IWEI’s base station computer. Data is then translated from hex format and compiled using NRG Microsite software. It is then exported into a database created by IWEI for quality assurance to the Utility Resource Assessment Program standards of the National Renewable Energy Laboratories. Care is taken to clean data of icing events and sensor failures and to compensate for the tower’s wind shadow on the instruments. The clean data is used to create hourly, daily, weekly and monthly averages of wind speed and direction at the measured heights. Wind shear is also calculated as well as wind power density, a wind speed and energy distribution by direction rose, and frequency distribution of wind speeds in 1-mph bins. All of these calculations are useful in applying to the power curves of available wind turbines to predict energy output.

January 1, 2000, nine wind monitoring stations were decommissioned. It was determined that sufficient long-term data had been stored from these stations to narrow the margin of error in long-term assessments. The Iowa Energy Center will continue to fund data acquisition from the Radcliffe site roughly in the center of the state. Having established a correlation between this site and each of the other sites, this will aid in understanding the long-term weather patterns effect on wind energy. It will also provide a basis for correlation studies to specific sites. With co-funding, IWEI will also continue to acquire data from four stations in the northwest region of the state near Sibley, Algona, Forest City, and Estherville. The Algona wind measurement station provides data to the EPRI Turbine Verification Program. This data has unique value in that it is adjacent to three Zond 750 kilowatt turbines assisting in verifying their power curves.

Processing of the complete data set began in January, 2000 after updating all loggers and software to Y2K compliance. Good progress is being made toward the completion of the project. The IWEI quality assured database and new zoomable wind maps should be available on the web site of the Iowa Energy Center by May.

Solar Energy Assessment of Iowa Objectives and Work Summary
IWEI has worked solar researcher Richard Perez to create a solar mapping of Iowa also available on the web site of the Iowa Energy Center at . These maps are based on hourly averaged solar insolation data from li-cor pyranometers mounted at five IWEI wind-monitoring sites. The five measurement sites were well distributed, one each in the northwest, southwest, southeast, northeast and center of the state.

As of November, 1999 a satellite data set of solar data became available. The satellite data measures solar energy on a 1-km grid of the whole state and has a far greater accuracy and reliability than the li-cor pyranometer ground measurements. The solar maps of Iowa are now being remade to incorporate this newly available solar data.

The map interface is being revised as well to match the interface used by the wind resource assessment maps. The solar calculator currently allows calculation for any location in Iowa of photovoltaics, solar thermal, and solar gain through double-pane glass. The user can select the orientation and type of tracking device used for the solar panel. In the new version, the user will be able to enter an efficiency coefficient to accommodate different photovoltaic materials.

The new wind resource and solar maps and web-based calculators should be on-line by the end of May. Through the publicly available wind and solar mapping and calculation tools, anyone should be able to calculate the benefits of a wind turbine or solar panel. IWEI has also completed an analysis with the National Renewable Energy Laboratories of the impact of a ten percent renewable energy portfolio standard consisting primarily of new wind energy in Iowa. This report, which is being linked to the Iowa Energy Center web site, has been possible only because of the wind resources assessment work completed in this grant. This extends the understanding of the feasibility and benefits of wind energy from the individual level to the level of consideration of the utilities and public policy makers. Under this grant, IWEI has also provided direct assistance to numerous schools and individuals interested in implementing wind energy at their location. We will continue this assistance and sufficient wind measurements assist in correlation studies and to quantify the long-term changing weather patterns effect on wind and solar energy.

Iowa has the potential to serve five percent of the national renewable energy needs. This assessment should give Iowa a competitive advantage in delivering this resource by reducing the time required to plan and finance projects by several years.