Tech Directions - December 2007 - (Page 19)

quite complex. Any good textbook on solar engineering can run you through the trigonometry. Fortunately, most of this laborious mathematical work has already been solved and codified into simple design software that people can use to determine how much energy can be captured from a solar system of a specific size and orientation. I recommend using the PVWATTS program, which is freely available on the Internet (Its version 1 simulation program is http://rredc.nrel.gov/ solar/codes_algs/PVWATTS/ version1/). This is a very nice educational tool, showing how collected energy depends on a variety of conditions—from which students can graph results and explore the practical limits of their designs. Without bright sunlight, solar panels will not collect enough energy to work properly. If a house’s roof is shaded, the panels could be located nearby on the ground, but they must get lots of sunshine. Not all roofs face the sun and that will certainly limit the usage of solar energy—again, unless a nearby ground area can be used. In my state, about 27 percent of the homes face the proper direction, or have the best orientation to the yearly transit of the sun. An additional factor is that my state also has many trees. After a few years’ growth, nearby trees might shade the solar panels. For that reason, solar engineers pay attention to trees and other structures that could cast a shadow on the panels, especially when the sun is at a low angle in the sky during winter. How Big a Solar System? Selecting the right-size solar system ensures production of enough electricity to supply the power needs of the home. This is where the math comes in, and it starts with understanding how much electricity the house will need during the 24hour period of its operation. For periods of the day when need for electricity matches the available sunshine, there is no need to store energy for use later. The sun can do the work when it is out and shining. If no one is home during the day, then the energy must be stored or sold back to the local utility. (More about this in just a few paragraphs.) How much total power the house requires determines the size of the solar system—in other words, how many solar panels will be needed. Each solar panel can produce only so much power in The PVWATTS program helps students understand sunlight. Typihow different factors affect collected energy. cally, a home the floor times its width. A typical could require about 3,000 watts (W) room in a home might be 12' × 12', or of peak power to operate. If each 144 sq. ft. A 600 sq. ft. solar system panel can produce 40 W of power in is almost five room floor areas in full sunlight, then we need to install size! That equals the entire first floor 75 panels (3,000 ÷ 40 = 75). area of most homes. Solar panels come in a variety of Solar systems are big so they can sizes, energy conversion efficiencollect the weak rays of the sun. In cies, and power capabilities. So the fact, solar panels are often called specific type of panel used will desolar collectors. termine the size of the system Think of it this way. A tree is very needed on the roof. much like a solar system. The leaves The 75 panels we just calculated of the tree are tiny solar collectors might require anywhere from 250 to that together power water and nour600 square feet of roof area. The ishment throughout the entire volmore efficiently a panel converts ume of the tree. When the leaves fall sunlight to electricity, the smaller it in autumn, the homeowner (who will be, and hence the smaller the must rake them) comes to realize rooftop solar array will be. Generhow much area all those leaves ally, the efficiency of a solar panel really take up! falls in the range of 5–15 percent, and cost increases with higher efficiency. Solar system design quickly Energy Production becomes a cost/efficiency tradeoff, Capability since roughly half the cost of an Once we have determined the installed solar system is in the panpower rating of the solar system, we els. can calculate the amount of enUsing a little more math, we can ergy—the kilowatt-hours (kWh)— produce a mental picture to underthat the system will produce. A stand what panel area means. The home’s monthly utility bill is based area of a room in your house is meaon the amount of energy, or kWh, sured by multiplying the length of the home consumes. Again, more www.techdirections.com POWER & ENERGY 19 http://rredc.nrel.gov/solar/codes_algs/PVWATTS/version1/ http://rredc.nrel.gov/solar/codes_algs/PVWATTS/version1/ http://rredc.nrel.gov/solar/codes_algs/PVWATTS/version1/ http://www.techdirections.com

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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