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The greatest challenge to
any thinker is stating
the problem in a way that
will allow a solution.
– Bertrand Russell
A Renewable Energy Primer
by Sascha Deri
This article is a primer to help you decide upon the best form of renewable energy to use for your home or business. You can assess the potential at your site for solar-, wind-, and microhydro-electric power before you seek professional assistance from the solar suppliers who advertise in Green Living. They can help you more easily if you know what resources are available at your site. -LP
Without taking into account financial incentives available from state and national governments, renewable energy systems for solar air heating, solar water heating and microhydro-electric systems have the best financial payback periods. However, in part due to rebates and people's familiarity with solar-electric and wind-power systems, these two types of systems are the most popular with home owners and businesses.
Selecting the type of renewable energy to use to power your home or business is not always about what you prefer, but what is available and economically most feasible. Here, we will discuss reviewing your site for its potential in generating electricity from solar, wind, and hydro (water) power. We won't cover solar air heating and solar water heating systems in this article, even though they are exceptionally cost effective for most home owners.
Generally, the most cost effective systems for generating electricity are (going from least to most expensive): microhydro-electric systems, wind-electric systems, and solar-electric systems. Ironically, the types of systems that are most frequently used go in exactly the opposite order, with a great many solar-electric systems, some wind-electric systems, and just a few microhydro-electric systems being installed. The main reasons for this reversal are the actual availability of these energy resources, government and utility rebates and incentives offered, and zoning laws in effect at particular locations.
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Assessing Microhydro Potential
Only 5% of the population in North America is fortunate enough to have a location suitable for generating microhydro-electric power. To take advantage of this form of renewable energy, you need a river or stream that provides sufficient water flow rate and head. Head is the vertical distance between where you would divert the water to your turbine and where it would re-emerge to be joined back with the original water source. You can have heads as low as 6 feet (2 meters), but you will need to have relatively high flow rates to generate any significant power.
To estimate the amount of instantaneous power (watts) you could generate from your location, multiply the head (in feet) by the flow rate (in US gallons per minute) and then divide by ten:
Equation for estimating power for microhydro: Head (ft.) x Flow (US gpm) ÷10 = Output (watts)
For example, let's say you had a site with a head of 60 feet and a flow rate of 100 gallons per minute. Your instantaneous power available would be 60 x 100 ÷ 10 = 600 watts. This may not sound like a lot of power, however, remember that in most cases the river is flowing all day and all night long. As a result, to estimate the total daily energy being produced, you would multiply the 600 watts times 24 hours, which equals 14,400 watt-hours. Over the period of a month (~30 days) that would be 432,000 watt-hours (or 432 kilowatt-hours). This particular scenario would provide enough electricity to power most energy efficient homes.
Determining the flow rate of a stream or a river is usually more challenging than determining the head. One method for determining the flow rate is to see how quickly the stream fills up a bucket of a known volume (e.g. a 5-gallon bucket). For instance, if a five-gallon bucket can be filled up in 10 seconds, then you know that over a period of a minute (60 seconds) there would be 35 gallons (5 gallons x 6 refills of the bucket in a minute), or 35 gallons per minute (gpm). This method is somewhat challenging since you sometimes need to dam up and divert the entire stream through a single tube that outputs into your bucket of known quantity.
Another method for estimating the flow rate of a river or stream is to figure out how fast the water is traveling through a particular cross-section. To do this, you need to find a length of the stream that is relatively consistent in its width and depth. Measure what those cross-sectional dimensions are (width and depth). Then at the beginning of that known stretch of the stream, drop in a floating object, such as a ball or stick, and time how long it takes to float to the other end of your marked stretch. With this information, you can estimate the total flow rate of the stream.
Assessing Wind Potential
The basic rules for determining whether your site is suitable for wind power are less complicated than for microhydro electricity. Generally speaking, you need at least a 1/2 acre (0.2 hectares) of open land where you can mount the turbine on a tower, and your average wind speed should average at least 10 mph (16km/h), either annually or during the months you intend to use the turbine.
You can use several resources for estimating the average wind speeds for your location, some less accurate than others.
Here is a list of possible methods:
When reviewing your location to see if you have enough land available to mount the turbine, keep in mind that the tower also needs to extend the turbine at least 30 feet (9 m) above nearby obstructions such as trees, buildings, or hills. Any obstructions within 300 feet (91 m) create turbulence in the air, causing the wind turbine to work very inefficiently. The turbine needs to get up above the turbulence to smoother airflows.
There are exceptions to these rules, such as for people who would like to use small wind turbines for their boats or RVs. In these cases, the location and weather conditions can vary widely from day to day. Usually, in these applications the turbine is used in conjunction with other power sources such as solar-electric panels or an engine-generator to charge up a battery bank.
Sizing a Wind Power System
Once you've determined the average winds for your location and determined if you have a suitable location for a wind turbine, you can then select the specific wind turbine to meet your needs by consulting manufacturers' specifications -- charts or tables that tell you how many kilowatt-hours of energy you can expect to produce given the average wind speed for your location. If the estimated energy isn't enough for your needs, go to a larger turbine.
If you find that the turbine you prefer doesn't supply enough energy at the average wind speeds for your location, you have a few options besides going with a larger wind turbine. One option is to consider using a hybrid system. That's a system that combines two different renewable energy technologies together, such as a wind turbine and a solar-electric system. A benefit to a hybrid system is that the two different technologies frequently complement each other. For instance, when sunshine is low due to a rainstorm, there tends to be more wind -- providing more power to the wind turbine.
Another option is to look again at how you can reduce energy needs in your home. Are there more creative ways you can come up with to reduce how much electricity is used in your home? Review Energy Star's Web site for tips on how to further make your home more energy efficient. Consider buying a book that provides even more tips on how to maximize the energy efficiency of your home. The general rule is that for every $1 you spend on making your home more efficient, you reduce the purchasing cost of a renewable energy system by $3 to $5.
Assessing Solar Power Potential
Most locations can make use of solar energy as long as you install the solar panels where they have a clear view of the sun for most of the day (e.g. 9 AM through 3 PM). Most solar-electric panels rapidly decrease in performance with just a little bit of shading. Literally, the shadow of a twig on a solar panel could cause it to decrease its output by 30% or more.
The amount of energy you can get from solar electricity at your site depends on your location in the world and the time of the year. Sites closer to the equator will tend to receive more solar energy throughout the year than those far north or south of the equator. Locations north of the equator receive more sunlight between the months of April through September, and those south of the equator receive more during the opposite time of the year (October through March). Sunlight is also affected by the weather. Sites that frequently have long-lasting fog or are overcast during large parts of the year will have less available solar energy potential.
The measurement for the strength of the sunlight striking the earth at your location is defined as solar insolation. Using this value, you can determine how much energy will be generated throughout the year for your site. Solar insolation data for all over the world and at different times of the year has been researched and recorded. Charts of the peak, average, and lowest annual insolation values for several different US cities can be found at NREL (rredc.nrel.gov/solar).
To get an idea about how much energy you could produce each day at your location, multiply the average annual solar insolation value times the total wattage of your planned solar panel array. As an example, let's say that you had a single, 100-watt solar panel and you lived in Columbus, Ohio, which has an average annual solar insolation value of 4.15 sun-hours. To determine your average daily energy output from that 100-watt panel, multiply 100 watts times 4.15 sun-hours (100 x 4.15), which equals 415 watt-hours (or 0.415 kilowatt-hours). This amount of energy would be about enough to power a color TV for a couple of hours every day.
Sizing a Solar Electric System
The process for sizing your solar-electric system is different from what we just went through in the previous example. First, determine how much energy you need on a daily basis in kilowatt-hours (kWh). This data can be found on your electric bill on a monthly basis if you have one for your location. To estimate the energy usage on a daily basis, take your monthly usage and divide by 30. If you don't have an electric utility bill, you will need to go through the process of estimating your energy consumption. Convenient "load" calculators and worksheets can be found on the Internet.
Second, you need to determine the solar insolation value for your site. If you're going to be totally disconnected from the electric grid/utility company, then you will need to use the minimum (worst-case) solar insolation value for your location. If you're going to be feeding your electricity into the electric grid, then use the average solar insolation value. Last, you need to divide your estimated daily energy usage (kilowatt-hours) by the solar insolation value and multiply by a system inefficiency factor:
Total Watts of Solar Electric Panels Needed = [(Daily Kilowatt-Hours) ÷ (Solar Insolation)] x Inefficiency Factor
The inefficiency factor for systems that are disconnected from the electric grid (off-grid) is 1.3. For systems that are connected to the grid (also called on-grid, grid-tied, grid-intertied or grid-connected), the assumed inefficiency factor is usually 1.2. The reason that the inefficiency factor is higher for off-grid sites is because these systems have to store the energy in battery banks, which are not perfectly efficient. Most on-grid systems either don't use batteries at all or use them in such a way that their inefficiencies are minimized
Choosing the Right Solar-Electric Panels
Now that you know the total wattage of the solar-electric array you need, you can select the specific solar panels you'll use to reach your total wattage. For example, if your home needs a total of 2,000 watts of solar panels, and you decided you liked a certain company's 102-watt solar-electric panels, then you would need 20 of them (2,000 watts divided by 102 watts = approximately 20 panels).
People make selections of solar panels based upon many different factors. Some people prefer only larger solar panels (150 watts or above) because it means they have to do less wiring for interconnecting the panels. Others choose moderate-sized panels (80 to 120 watts) because they're easier to lift and maneuver. Other people will choose panels based upon color, electric properties, whether or not the manufacturer is a petroleum company, which country the panels are manufactured in, whether the panels are immediately available, and of course, price per watt, which plays a big role for most people. Keep in mind that qualified technical assistance is only a phone call or mouse click away.
Author Sascha Deri is president and cofounder of the Alternative Energy Store. More articles and renewable energy resources may be found at their Alt-E University Web site: http://howto.altenergystore.com
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