Tech Directions - December 2007 - (Page 20)

math is involved, but the good news is there are simple solar simulation programs available that allow solar designers to determine how much energy a certain size solar system will produce. A very nice Web site Simple solar simulation programs can calculate how much energy a solar system will produce. for this type of engineering simulation is http://rredc.nrel.gov/solar/ codes_algs/PVWATTS. This is definitely the site students should use to exercise key variables to see how they affect the energyproducing capability of a particular solar system. For a single-size system like the 3 kW one described above, students can run various simulations looking at such basic parameters as collector/panel tilt angles from, say, a vertical to a horizontal position through a variety of intermediate angles to derive a series of charts that show the energy collected per month at each angle and the totals at the end of the year. Students can repeat this for different city locations to learn how dramatically sunlight does vary across the country. In this situation, location really does matter! Generally speaking, a solar system is designed to provide about 40– 60 percent of a home’s annual energy needs, which means it can reduce that home’s energy bill by about half. Students can calculate the actual savings by multiplying the total number of kWh produced in the simulation by the amount of money a local utility charges for each kWh. For instance, if a solar system design simulation says you will produce a total of 3,300 kWhs in a year, and if the local utility charges $0.08 per kWh, then the solar system will save the home owner $264. If a yearly electric bill is $578, then the homeowner will have saved about 46 percent on annual electricity costs. Once students determine how much money the solar system can save, they can divide this into the final installed cost of the system to compute how long it will take for the solar energy investment to pay for itself. To promote more solar installations, many states offer special incentives to homeowners to help them afford this kind of alternative energy system. Some state governments will actually pay for up to half of the cost of the system. Putting It All Together Once the solar system is designed, and the homeowner understands how it will affect the pocketbook, the system must be installed. Great care must be taken in attaching the solar panels to the roof of the house. They must be secure to the roof and not likely to vibrate loose by winter winds. Also, their connection to the roof must be mechanically sealed to prevent water from leaking into the attic. Each panel comes with its own electrical wires that installers can easily connect to the next panel. You can think of each panel as though it were a large battery that is energized by the sun. Since batteries can be connected in series or parallel combinations to produce a variety of voltage and current conditions, the same can be done with solar panels. A device known as an inverter serves as the “brain” of the whole solar system, controlling when the system turns on. The inverter also knows how to take the direct current (dc) electricity from the solar panels and turn it into the alternating current (ac) for use in the home. At the end of the day, when sunlight is low, the inverter shuts the solar system off for the night. As the sun grows strong enough on the next day to produce electricity from the panel, the inverter turns the system on once again. Since the solar system is directly connected to a home’s electric supply, it can be made to feed energy back out into the local utility grid. In the early days of solar energy application, the utility companies did not allow for such feedback because doing so presented a possible safety problem to utility workers working on the pole lines. A solar system feeding energy back would endanger a worker who thought the electricity was no longer on. Today, better and more reliable equipment allows the solar system to not feed back when the utility company shuts the pole line down for maintenance. Those solar homes that do feed energy back to the utility grid are paid a fair price for the energy they send back. The price paid is determined by the state regulatory agency that oversees each utility. Final Thoughts on Solar A well-designed solar system can last for 20 years or more. It is quiet, produces no pollution, and saves the homeowner about half of their electric energy costs. Since the late 1970s, scientists and engineers have worked to develop cheaper and better solar systems. While the sun’s energy is free, the equipment to use it is not. Also, making that equipment survive the harsh outdoors and continue to work well year after year is not easy. Today’s home energy systems are well protected inside the house—usually in the basement. Solar systems are subject to very hot summer conditions and freezing cold temperatures with ice and snow in the winter. As the cost of solar systems drops, more people are trying them in their homes. Perhaps the best way to get more people to use this clean energy source is to build solar systems into new homes at the time of construction. 20 techdirections ◆ DECEMBER 2007 http://rredc.nrel.gov/solar/codes_algs/PVWATTS http://rredc.nrel.gov/solar/codes_algs/PVWATTS

Table of Contents Feed for the Digital Edition of Tech Directions - December 2007

Tech Directions - December 2007 Technically Speaking Contents Direct from Washington The News Report Technology Today Technology's Past Mastering Computers Is This Op-Amp Any Good? Lab-Built Checker Removes All Doubt Here Comes the Sun! Residential Solar Systems Add Up to Savings Pinhole Cameras—For Science, Art, and Fun! Poster Project Maps Out Design Process Gifts for Geeks E-gallery More than Fun

Tech Directions - December 2007

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