Tech Directions - August 2007 - (Page 26) Student Research Activity Table 1—Power Draw Appliances Power Draw (W) Time of Use Microwave 1,500 30 minutes per day Clock 2 Continuous Home computer 400 60 hours per month 60 W bulb 60 6 hours per day Television 200 6 hours per day Hair dryer 1,200 10 minutes per day ures, students can calculate the annual operating cost for each one, assuming an energy cost of $0.08/kWh. As an additional challenge, students could compute the annual operating cost of a two-door, frost-free, 19 cubic foot refrigerator and freezer that uses 430 W and consumes 188 kWh per month. This exercise should show that determining consumption of electrical energy is not rocket science. With a little bit of math, students can calculate how much it costs to operate appliances in their homes. Note that as power consumption of appliances becomes smaller due to energy efficiency improvements, it will take less electricity to operate those appliances. If you raise the efficiency of an appliance by 10 percent, you reduce its power draw or wattage by 10 percent. If energy cost was to hold steady, this means it would cost 10 percent less annually to operate that appliance. So when buying appliances, pay attention to efficiency ratings and labels. Purchasing more energy efficient appliances will save money every year an appliance is in operation. An interesting exercise to conduct in the classroom is to have students investigate when the various electric appliances that they perhaps most value came into common use. A list might include: • Microwave ovens • Video cassette recorders • Personal computers • Food processors • Video games • Large screen TVs • High-wattage, hand-held hair dryers Students can then try to estimate how often such devices are “on,” and thus drawing energy, along with how much power they consume. More Historical Perspective Another way to view the growth in electric energy use in the home is to realize that before about 1950, most homes had an electrical service from the utility pole capable of delivering 30-60 amperes (A) of current to the home. Today, a 100 A service is common, with newer homes having electrical services of 150–250 A. In addition, most of the earlier homes typically had individual 15 A circuits for lighting and general use and 20 A circuits for kitchen and large appliance use. Today a 1,500 W hair dryer by itself will draw 12.5 A of current—almost the entire capacity of a standard 15 A circuit! A super hair dryer of 2,000 W will exceed the capacity of a 15 A circuit, probably tripping a circuit breaker or blowing a fuse. Circuit sizes of Table 2—Average 20 A or better are Electrical Energy Use becoming common in New Jersey for general home use. Table 2 shows how Year kWh/year the use of electrical 1940 755 energy in the average 1950 1,307 home in my state of New Jersey has 1960 2,489 grown since 1940—a 1970 4,995 span of just 60 years. 1980 5,549 Today, we use 1990 6,035 almost as much energy in our homes in 2000 6,650 a single month as our grandparents used in a whole year! There’s something to think about the next time you flip an electric switch! The Rapid Growth in Electricity Usage Much of the growth in electric energy usage during the 1950s, 1960s, and into the 1970s came from the proliferation of televisions, large appliances, and convenient window-size air conditioners. Whether a large central air conditioner or a window model, air conditioners, on average, run about 1,000 hours per year. They consume vast amounts of energy during peak electric utility load periods, generally experienced during the hot and humid months of July and August. The air conditioner has basically turned most utilities into summer peaking systems, especially in parts of the country where heat and humidity occur together. This makes it difficult for utilities to share power through regional power pools, since they all tend to experience peak or near peak demands at the same time. For ready-to-use lesson plans based on this article, go to www.techdirections.com/ projectsE.html and select Calculating Home Electricity Consumption. Only $6.95! 26 techdirections ◆ AUGUST 2007 http://www.techdirections.com/
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