Learn how to assess your property’s weather conditions to determine the right source for your self-sufficient energy home: solar power vs wind power setups.
Solar Power vs Wind Power
Wind and solar are two of the most accessible sources of renewable energy. Large farms of wind turbines and solar arrays can provide power for multiple sources, but there are also smaller home units available that generate power for a single property. Unless your goal is purely environmental, however, the economics of your situation will determine whether it makes sense for you to invest in a renewable energy system. Will the money you save by generating your own power cover the initial costs? This will depend on a number of factors, but perhaps most importantly on the energy source itself: wind or sunlight. Both of these are elements of the weather, and they’re impacted by other weather factors and situations. Homeowners wishing to install solar or wind systems need to know as much as possible about these weather effects before they try to utilize nature’s energy.
Harnessing the Wind
If you’re interested in wind power, first check into and follow any local zoning laws and permits for installing a wind turbine on your property. Once you’ve established legality, the best way to determine if wind power is a viable option is to have a professional conduct a wind resource assessment of your property. Such professionals are typically associated with a provider of wind power equipment. In addition, see “Wind and Solar Data” below for a number of resources where you can access basic wind information for your location. You can also check with local airports, which have extensive wind records, or any local National Weather Service office. (Keep in mind that these wind measurements are typically taken at 30 feet above the ground.) Overall, a large, open area is necessary; 1 acre is usually considered the minimum. This typically rules out urban and even many suburban sites. Rural locations with flat terrain are ideal.
Wind speed and direction. To be effective, wind turbines have to reach a minimum wind speed, and winds of this speed must be reasonably consistent over time. The U.S. Department of Energy recommends an average annual wind speed of at least 9 mph for stand-alone units and 10 mph for systems connected to the grid. Much of the Great Plains is prime for wind power, as are some coastal areas. Besides wind speeds, you’ll need to ascertain the prevailing wind direction of your location for proper siting of the equipment. To illustrate prevailing winds, meteorologists develop “wind roses,” which are graphical displays of wind direction occurrence.
Wind turbine height. How high should your wind turbine be? Keep in mind that wind speed increases with height, as you move away from the frictional drag of the Earth’s surface. Standard height for a home wind turbine is 100 feet. Wind speeds are typically 15 to 25 percent faster at this height than at 30 feet. But even 100 feet is considered the minimum tower height to have an economically viable unit; an increase in height of 20 to 40 feet can produce significantly more power.
Wind obstacles. Evaluate your property for any obstacles that may block wind flow to a turbine. These can be buildings, trees, or topographic features, such as hills. The effects of such obstacles can extend well downwind. Besides current conditions, you must also allow for future changes, considering your wind turbine will be operational for decades. Are you planning to build any new structures? Consider all future plans for your property when making the decision to install a turbine. Don’t worry about trees; allow them to grow normally.
Other than physically blocking the wind, obstacles can produce turbulence far downwind of the actual blockage. Wind turbines function best with a uniform wind flow, and disturbances in the flow can significantly decrease efficiency for home units. It’s been estimated that for small home turbine systems, turbulence can reduce the maximum power produced by 15 to 25 percent compared with a standard estimated nonturbulent wind flow. A general rule is that your wind turbine should be 30 feet above any obstacle within 500 feet. This is especially true of obstructions typically upwind of the prevailing wind direction, which is a major reason you must know your prevailing wind direction.
Air density. Although not nearly as influential as wind speed, air density can also affect the power output of a wind turbine. Less dense, lighter air isn’t as effective at turning turbine blades. When it’s extremely hot, air density decreases, reducing power output. This is especially true at higher elevations, because air density decreases exponentially with height. Most projected power outputs are based on the air density at sea level.
Weather impact. To lessen the danger of damage due to strong wind gusts, many turbines have an automatic braking system. An ultraviolet (UV) coating will help protect the system from prolonged exposure to solar radiation. If ice is a problem in your location, systems can be equipped with de-icing apparatuses.
Sourcing from the Sun
The amount of solar energy varies by time of day, time of year, and latitude. The strength of sunlight (the amount of energy it has) is primarily a function of the sun’s height in the sky. The higher the sun is in the sky, the less atmosphere the sun’s rays have to penetrate, and, consequently, the stronger the sun’s rays.
For a day, solar radiation starts at zero at sunrise, rises to a peak value at “solar noon” (halfway between sunrise and sunset), and goes back to zero at sunset. For the year, the sun is highest in the sky on the first day of summer. Summer, as a whole, receives the most solar energy during the year. On the other extreme, the sun is lowest in the sky on the first day of winter, and winter months receive less solar energy. For example, Denver, Chicago, and New York City receive three times the amount of solar energy in June that they do in December. Day length, which is greater in summer and less in winter, contributes some to the energy budget as well. This is especially important in higher latitudes, where winter days are very short. Latitude also affects the sun’s height in the sky. In lower latitudes (southern locations in the Northern Hemisphere), the sun is higher in the sky and the sun’s rays are stronger. All of these factors can affect solar power. In the U.S., the often cloud-free desert Southwest gets the most solar energy overall for the year, and the northern states get the least. Even in Alaska, though, solar power can be economically viable.
How much solar energy is available in your location? Unlike wind, solar radiation is pretty consistent across rather large regions, allowing for shading effects or local climate anomalies. See “Wind and Solar Data” below for a list of websites you can use to determine your solar power potential.
Cloud cover. Direct solar radiation — where the direct rays of the sun hit — is the source of maximum energy, and is where solar panels are most efficient. However, the sky itself emits sunlight diffused by water molecules, dust, and other particles in the air. This diffused solar radiation, or sky radiation, will still power solar panels, although at a lower efficiency. How much lower depends on the amount of sky covered by clouds and the density of the clouds. Low, thick stratus clouds can cut production in half or more. But even Seattle, with its famously cloudy winters, has functioning solar units.
Panel location. If you’ve decided to go with solar power, where should you put your panels? Most people install home units on the roof, but ground racks are also an option. Overall, you’ll need a location that receives direct, unobstructed sunlight for much of the day and throughout the year, if possible.
Panel direction and angle. To get the most out of the available solar energy at your location, consider two major concerns when mounting the panels: the direction they face and their vertical angle.
The direction the panels face is critical. In the Northern Hemisphere, the sun is primarily to the south at noon. Therefore, for maximum energy production, solar panels should face south. If the peak demand period is a major concern, however, this typically coincides with the warmest temperatures of the day, which occur in the afternoon. Since the sun is in the western part of the sky in the afternoon, the panels could be installed facing west. Avoid shading by trees or other structures; anything to the south or west of your panel location can be problematic.
Although not as important as the direction they face, the angle of solar panels also impacts their performance. This is due to the height of the sun above the horizon, which is a function of latitude. Something between 25 and 40 degrees is usually best, but more precise information for your location is available online. And don’t forget to allow for the slope of your roof or the angle of the installation rack. You can adjust the angle of your panels throughout the seasons to get peak performance as the sun’s height in the sky changes.
Weather impact. High-quality solar panels are built to withstand most severe weather conditions, including heavy rain, small-to-moderate-sized hail, snow cover, and normal cycles of hot and cold temperatures. Even strong winds are typically not a concern with high-quality, well-installed panels, which are usually able to withstand wind speeds over 100 mph. Proper grounding reduces the risk of damage due to a lightning strike, and other protective features can be added.
Anything that covers the tops of the solar panels will reduce their effectiveness. Snow typically slides off sloping panels, but if it doesn’t, the panels should be cleaned off; there are even solar panel snow rakes specially designed for this. If you live in an area with significant dust or sand in the air, hose off panels following any accumulation.
Typical of most electronic apparatuses, solar panels are more efficient in colder temperatures. High temperatures can reduce power generation, which can be significant in hot climates where temperatures can reach or exceed 100 degrees Fahrenheit.
For more than 30 years, Dr. Ed Brotak taught thousands of college students about weather, and he’s helped many of them pursue careers in meteorology. He lives in Asheville, North Carolina, with his wife and daughters.
