To develop a system for heating water and air through solar and wind power (Research Project No. 1), it is necessary to have an idea of what one’s local resources are. Luckily, we have baseline data from the National Renewable Energy Laboratory. This laboratory, under the Department of Energy, has national resource maps for solar, wind, geothermal, marine, and other renewable resources. Here we’ll look at our local resources and site-specific questions that will have to be addressed as we obtain our permanent location.
Most places in the lower forty-eight states experience winds over 4 m/s (9 mph) at 30 m (100 ft) in the air. Maps for winds at 100 m are also available but that elevation is out of reach for most home-built systems. In southern Wisconsin we experience winds between 5.0 and 5.5 m/s (11 and 12 mph). Obviously this is a long-term average.
Day-to-day wind speeds can vary in all times and seasons. For example, since 1948, Madison has seen winds ranging from 24.5 to 0 m/s (55 to 0 mph) with no marked seasonality.
Buildings, vegetation, and the landscape itself can dictate local wind resources. Tall trees and forests impede low-level winds and make them more turbulent, while cropland and fields are more likely to give steady wind currents. Many buildings create shapes that winds must navigate, slowing them down. Flat land is more conducive to a wind farm, but a single wind turbine at the top of a hill gains height, just as one sited in a steep valley may only experience strong winds when they run up or down the valley floor.
We’re not going to get into too much detail on the wind turbine itself, because the focus of this build is domestic water and space heating. We’re going to run three pumps to move solar fluid into the collectors, move hydronic heating fluid through the short loop, and an additional pump to move the heating fluid through the whole house in winter. Each motor draws about 0.1 kWh, that means 100 Watts per hour. In addition, when the sun doesn’t provide enough energy to keep our hot-water reservoir up to temperature, an electric heating element will kick in. This thing is a heavy drain on our electricity system, coming in at 4.5 kWh (the equivalent of 45 100-Watt light bulbs), but I estimate it will only run for an average of 4 hours per day when it runs. If the pumps and element are running on a cold, cloudy winter day, it would draw 23.4 kWh. So we’ll aim for this as our average daily output for the whole year as we can use the extra electricity elsewhere.
The rough amount of power put out by a wind turbine is given by the following equation:
AEO = 0.01328D²V³
AEO = Annual Energy Output in kWh
D = Rotor Diameter in ft
V = Annual Average Wind Speed in mph
If we need about 24 kWh per day in the winter, we would plug in the known wind average (low average = 11 mph) and solve for rotor diameter:
D = √(AEO/(0.01328 ×V³))
22.26 ft = √((24 × 365)/(0.01328 × 11³))
This is about a 5-kWh turbine. We’ll discuss wind turbine design and energy storage in future posts.