Activity 8 Renewable Energy Sources– Solar and Wind EarthComm Energy Resources Goals In this...

872EarthComm

Energy Resources

GoalsIn this activity you will:

• Construct a solar water heaterand determine its maximumenergy output.

• Construct a simple anemometerto measure wind speeds in yourcommunity.

• Evaluate the use of the Sunand the wind to reduce the use of nonrenewable energyresources.

• Understand how systems basedupon renewable energyresources reduce consumptionof nonrenewable resources.

Think about It

The solar energy received by the Earth in one day would take care of the world’s energy requirements for more than twodecades, at the present level of energy consumption.

• How can your community take advantage of solar energy to meet its energy needs?

• Is your community windy enough to make wind power feasible for electricity generation?

What do you think? Record your ideas in your EarthCommnotebook. Be prepared to discuss your responses with your small group and the class.

Activity 8 Renewable Energy Sources–Solar and Wind

CS_Ch13_EnergyResources 3/1/2005 5:35 PM Page 872

873Coordinated Science for the 21st Century

Activity 8 Renewable Energy Sources – Solar and Wind

Investigate

Part A: Solar Water Heating1. Set up a solar water heater system as

shown in the diagram. Put the sourcecontainer above the level of theheater so that water will flowthrough the tubing by gravity. Do theexperiment on a sunny day and facethe surface of the solar water heaterdirectly toward the Sun. This willenable you to calculate maximumoutput.

2. In its simplest form, a solar waterheater uses the energy of the Sun toincrease the temperature of water asthe water flows through a coil oftubing.

A Btu (British thermal unit) is theamount of heat energy required toraise the temperature of one pound ofwater one degree Fahrenheit. Youneed to determine the number of Btusof heat generated. If you divide thisvalue by the surface area of the heaterand by the time it takes to heat thewater, you will have the Btu/ft2/min

output of your solar water heater. It will help to know that 1000 mL of water weighs about 1 kg, or 2.205 pounds.

a) Decide how you will do theinvestigation and record the data.

b) Prepare a data table to recordrelevant data.

c) Think about the conversions thatyou will need to make in order toend up with Btu/ft2/min.

3. Run about one liter of cold waterthrough the heater. Refill the sourcecontainer with two liters of cold tapwater and run the experiment.

4. Answer the following questions:

a) What is the output of your solarwater heater in Btu/ft2/min?

b) Convert the output of your solarwater heater from Btu/ft2/min toBtu/ft2/hour.

c) Assume that you could track theSun (automatically rotate thedevice so that it faces the Sun allday long). Assuming an average of 12 hours of sunlight per day,what would its output be per day?

d) Convert this value to kWh/m2/dayby dividing by 317.2.

5. Examine the maps on the followingpage of average daily solar radiationfor the United States. The first mapshows the values for January, and thesecond one shows the values for July.The maps represent 30-year averagesand assume 12 hours of sunlight perday and 30 days in a month.


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a) How does the average daily solarradiation per month for Januarycompare with that of July? Howmight you explain the difference?

b) Find your community on the map.Record the values for average dailysolar radiation for January and July.

c) How does the daily output of yoursolar water heater compare to theaverage daily solar radiation forthe two months shown?

d) Percent energy efficiency is theenergy output divided by theenergy input, multiplied by 100.Calculate the estimated efficiencyof your solar water heater. If youhave Internet access, you canobtain the average values for themonth in which you do thisactivity. Otherwise, calculate theefficiency for whatever month isclosest—January or July.

6. In your group, brainstorm aboutways to increase the efficiency ofyour solar water heater. Variablesthat you might explore include therole of insulation, diameter of thetubing, type of material in the tubing,and flow rate of the water throughthe system. If time and materials areavailable, conduct another test.

Part B: Harnessing Wind Energy1. Set up an anemometer as shown in

the diagram below.

Average Daily Solar Radiation per Month

January

kWh/m2/day

flat plate tilted south at an angle 15° greater

than the latitude

Alaska

Hawaii

4.96

4.875.50

5.29

5 to 64 to 53 to 42 to 30 to 2

5.91

5.156.51

5.59

Average Daily Solar Radiation per MonthJuly

kWh/m2/day

flat plate tilted south at an angle 15° lessthan the latitude

Alaska

Hawaii

7 to 86 to 75 to 64 to 5




2. To find the wind speed, count thenumber of revolutions per minute.Next, calculate the circumference ofthe circle (in feet) made by therotating paper cups. Multiply therevolutions per minute by thecircumference of the circle (in feet per revolution), and you will have the velocity of the wind in feet per minute. (This will be anunderestimate, because the cups donot move quite as fast as the wind.)

3. Take your anemometer home duringa weekend. This will give you achance to make measurements atdifferent times of the day and overtwo days.

4. Make a map of your house and theyard around the house. Plan to placethe anemometer at different locationsand make velocity measurements atdifferent times of the day.

5. Using a watch, count the number oftimes the colored cups spins aroundin one minute. Use your calibrationto convert this to wind velocity.

a) Keep a record of the wind speedsyou are measuring for the nextfew days.

6. Measure the wind speed at differenttimes of the day.

a) Is the wind speed the same in themorning; the afternoon; theevening?

b) Move your anemometer toanother location. Is it windier inother places?

c) Do trees or buildings block thewind?

7. Back in class, determine the bestlocation for a wind turbine at yourhome.

a) On the basis of the data that youhave taken, make a plot of windvelocity throughout the day. Youwill assume that the wind willblow the same way every day. (Is this a good assumption?)

8. Following the example given below,calculate the electricity that can beproduced at your home for a windturbine with a 2-m blade (the cost of this wind turbine is about $3000).

a) What percentage of yourhousehold electricity needs doesthis represent?

b) How much wind power could beproduced with a 3-m blade? (Costabout $5000.)

Example calculation of wind power:Wind power per square meter ofturbine area is equal to 0.65 times the cube of the wind velocity. Forinstance, if a wind turbine has a blade 2 m long, then the area that theturbine sweeps is π(1 m)2 = 3.14 m2.

If a constant wind of 10 m/s (22 mph) is blowing, the power is(103)(0.65)(3.14) = 2000 W. Theefficiency of wind turbines is about40%, so the actual power that can be produced is about 800 W. If thewind blew a constant 10 m/s everyday of the year, this wind turbinecould produce 7000 kWh ofelectricity.


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9. Get together as a class and discussthis activity.

a) How do your calculationscompare with those of otherstudents?

b) What are the characteristics of thelocations of homes that have thehighest wind velocities?

c) What are the characteristics ofthose with the lowest windvelocities?

d) Overall, how well suited is yourcommunity for wind power?

e) Which locations in yourcommunity are optimally suitedfor wind turbines?

f) How realistic is the assumptionthat two days’ worth ofmeasurements can be extrapolatedto yearly averages?

g) For how long shouldmeasurements of wind velocity betaken to get an accurate measureof wind velocities in yourcommunity?

Reflecting on the Activity and the Challenge

Water heating can consume as much as40% of total energy consumption in aresidence. In this investigation, youdetermined the energy output of a flat-plate collector, a device used to heatwater using solar energy. You also builta simple anemometer in the classroomand used it to measure wind speedaround your home at different times of

the day. Finally, you calculated whetherit would be feasible and cost-effectivefor you to generate part of your family’selectricity needs using wind power. Youmay wish to consider the use ofdifferent forms of solar energy as analternative energy source for yourcommunity.




Geo Wordsinsolation: the rate atwhich a given area ofland receives solar energy.

Digging Deeper

SOLAR ENERGY

Forms of Solar Energy

Forms of solar energy include direct and indirect solar radiation, wind,photovoltaic, biomass, and others.Tidal energy is produced mainly at theexpense of the Earth–Moon system, but the Sun contributes to the tides aswell.The three forms of solar energy with the most potential are solar-thermal (direct solar radiation to generate electricity), wind power, andphotovoltaic cells.

Solar energy is produced in the extremely hot core of the Sun. In a processcalled nuclear fusion, hydrogen atoms are fused together to form helium atoms.A very small quantity of mass is converted to energy in this process.Theongoing process of nuclear fusion in the Sun’s interior is what keeps the Sun hot.The surface of the Sun, although much cooler than the interior, is still very hot.Like all matter in the universe, the surface radiates electromagnetic energy in alldirections.This electromagnetic radiation is mostly light, in the visible range ofthe spectrum, but much of the energy is also in the ultraviolet range (shorterwavelengths) and in the infrared range (longer wavelengths).The radiation travelsat the speed of light and reaches the Earth in about eight minutes. Only anextremely small fraction of this energy reaches the Earth, because the Earth issmall and very far away from the Sun. It is far more than enough, however, toprovide all of the Earth’s energy needs, if it could be harnessed in a practical way.The Sun has been producing energy in this way for almost five billion years, andit will continue to do so for a few billion years more.

Capturing the Sun’s energy is not easy, because solar energy is spread outover such a large area. In any small area it is not very intense.The rate atwhich a given area of land receives solar energy is called insolation. (That’snot the same as insulation, which is material used to keep things hot orcold.) Insolation depends on several factors: latitude, season of the year, timeof day, cloudiness of the sky, clearness of the air, and slope of the landsurface. If the Sun is directly overhead and the sky is clear, the rate of solarradiation on a horizontal surface at sea level is about 1000 W/m2 (watts persquare meter).This is the highest value insolation can have on the Earth’ssurface except by concentrating sunlight with devices like mirrors or lenses.If the Sun is not directly overhead, the solar radiation received on thesurface is less because there is more atmosphere between the Sun and thesurface to absorb some of the radiation. Note also that insolation decreaseswhen a surface is not oriented perpendicular to the Sun’s rays.This isbecause the surface presents a smaller cross-sectional area to the Sun.


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The Earth rotates on its axis once a day, and it revolves in an almostcircular but slightly elliptical orbitaround the Sun once a year.TheEarth’s rotation axis is tilted about23º from the plane of the Earth’sorbit around the Sun, as shown inFigure 1. It points toward the Sun atone end of its orbit (in summer)and away from the Sun at the otherend (in winter). In summer, whenthe Earth’s axis points toward theSun, the Sun traces a path high inthe sky, and the days are long. Inwinter, the Sun’s path across the skyis much closer to the horizon, andthe days are short.This hasimportant effects on seasonalinsolation and on building designs.

Solar Heating

Home heating is one of the main uses of solar energy.There are two basickinds of solar heating systems: active and passive. In an active system, special

equipment, in the form of a solarcollector, is used to collect anddistribute the solar energy. In a passivesystem, the home is designed to let inlarge amounts of sunlight.The heatproduced from the light is trappedinside. Passive systems do not rely onspecial mechanical equipment, but theyare not as effective as active systems.

Water heating is another major use ofsolar energy. Solar water heating systemsfor buildings have two main parts: a solarcollector and a storage tank.The solarenergy is collected with a thin, flat,rectangular box with a transparent cover.As shown in Figure 2, the bottom of thecollector box is a plate that is coatedblack on the upper surface and insulated

flat plate collectorat an angle 15° greater

than the latitude

flat plate collectorat an angle 15° lessthan the latitude

SummerNorthern Hemisphere

WinterNorthern Hemisphere

Winter - Sun lowerin the sky

Summer - Sun higherin the sky

Figure 1 The tilt of the Earth on its axis duringwinter and during summer.

inlet connectionenclosure

absorber plate

flow tubes

insulation

glazing frame

glazingoutlet connection

Figure 2 Flat plate collector.




on the lower surface.The solar energy that strikes the black surface isconverted to heat. Cool water is circulated through pipes from the hotcollector box to a storage tank.The water is warmed as it passes through thecollector box.These collectors are not very expensive, and in sunny regionsthey can provide most or all of the need for hot water in homes orswimming pools.

Many large commercial buildings such as the one shown in Figure 3, can usesolar collectors to provide more than just hot water.A solar ventilationsystem can be used in cold climates to preheat air as it enters a building.Theheat from a solar collector can even be used to provide energy for cooling abuilding, although the efficiency is less than for heating.A solar collector isnot always needed when using sunlight to heat a building. Buildings can bedesigned for passive solar heating.These buildings usually have large, south-facing windows with overhangs. In winter, the Sun shines directly through thelarge windows, heating the interior of the building. In summer, the high Sun isblocked by the overhang from shining directly into the building. Materialsthat absorb and store the Sun’s heat can be built into the sunlit floors andwalls.The floors and walls then heat up during the day and release heatslowly at night. Many designs for passive solar heating also provide daylighting. Day lighting is simply the use of natural sunlight to brighten up theinterior of a building.

By tapping available renewable energy,solar water heating reducesconsumption of conventional energythat would otherwise be used. Eachunit of energy delivered to heat waterwith a solar heating system yields aneven greater reduction in use of fossilfuels.Water heating by natural gas,propane, or fuel oil is only about 60%efficient, and although electric waterheating is about 90% efficient, theproduction of electricity from fossilfuels is generally only 30% to 40%efficient. Reducing use of fossil fuel forwater heating not only saves stocks ofthe fossil fuels but also eliminates theair pollution and the emission ofgreenhouse gases associated withburning those fuels.

Figure 3 Solar collectors on the roofs of thesebuildings can be used to provide energy forheating water and regulating indoor climates.

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Photovoltaics

Generation of electricity is another major use of solar energy.The mostfamiliar way is to use photovoltaic (PV) cells, which are used to power toys,calculators, and roadside telephone call boxes. PV systems convert lightenergy directly into electricity. Commonly known as solar cells, thesesystems are already an important part of your lives.The simplest systemspower many of the small calculators and wristwatches. More complicatedsystems provide electricity for pumping water, powering communicationsequipment, and even lighting homes and running appliances. In a surprisingnumber of cases, PV power is the cheapest form of electricity for performingthese tasks.The efficiency of PV systems is not high, but it is increasing asresearch develops new materials for conversion of sunlight to electricity.The use of PV systems is growing rapidly.

Wind Power

People have been using wind power for hundreds of years to pump waterfrom wells. Only in the past 25 years, however, have communities started touse wind power to generate electricity.The photograph in Figure 4 is takenfrom an electricity-generating wind farm near Palm Springs, California.Thetriple-blade propeller is one of the most popular designs used in windturbines today.Wind power can be used on a large scale to produceelectricity for communities (wind farms), or it can be used on a smaller scale to meet part or all of the electricity needs of a household.

Figure 4 Wind turbines on a wind farm in California.




1. How does the Sunproduce solar energy?

2. What factors determinethe rate at which solarenergy is received by agiven area of theEarth’s land surface?

3. What are thedifferences betweenpassive solar heatingsystems and activesolar heating systems?

4. Why must solar energysystems occupy a muchlarger area, per unit ofenergy produced, thanconventional systemsthat burn fossil fuels?

Check YourUnderstanding

Figure 5 shows typical wind velocities in various parts of the country.California leads the country in the generation of electricity from windturbines (in 2000, California had a wind-power capacity of 1700 MW, with20,000 wind turbines), Many other areas in the country have a high potentialfor wind power as well.These areas include the Rocky Mountains, the flatMidwest states,Alaska, and many other areas. Commercial wind turbines canhave blades with a diameter as large as 60 m.

The cost of producing electricity from wind has dropped significantly sincethe early 1990s.This is due mainly to design innovations. In 2000, the cost of wind power was less than six cents per kilowatt-hour, which makes itcompetitive with electricity produced by coal-fired plants.The price isexpected to decrease below five cents per kilowatt-hour by 2005.

Figure 5 Location of electricity-generating wind-power plants relative to wind-power classes in the United States.


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Preparing for the Chapter Challenge

Understanding and Applying What You Have Learned

1. Suppose that you live in a regionthat requires 1 million Btus (or293 kWh) per day to heat a home.An average solar water-heatingsystem with an output of 720Btu/ft2/day costs $50 per squarefoot to install.

a) What size of system would berequired to meet these heatingrequirements?

b) What would the system cost?c) Find out how much it costs to

heat a home for five months inthe winter where you live(November through March).Calculate how many years itwould take to recover the costof the solar water-heatingsystem.

2. Why is metal used for tubing(rather than the plastic tubing thatyou used in the activity) in flatplate solar collectors? Use whatyou have learned about heatconduction to explain.

3. Why is wind power more efficientthan generating electricity from allthe methods that involve heatingwater to make steam?

4. Why is wind power less efficientthan hydropower?

5. A wind turbine is placed where itis exposed to a steady wind ofeight mph for the entire day exceptfor two hours when the windspeed is 20 mph. Calculate thewind energy generated in the twowindy hours compared with the 22 less windy hours.

6. Looking at the United States mapof wind velocities, where does yourcommunity fall in terms of windpotential?

7. Assume that you live in acommunity in eastern Colorado.

a) Use the wind map above todetermine the number of windturbines that would be neededto produce the electricityequivalent to a large electricpower plant (1000 MW).Assume that each of the windturbines has a blade diameter of 50 feet.

b) If these wind turbines arereplacing a coal-fired powerplant that burns coal with aratio of pounds of SO2 permillion Btu of 2.5, calculate thereduction in the amount of SO2 emitted per year.

c) What are some problems thatmight be encountered with thewind farm?

Your challenge is to find a way tomeet the growing energy needs ofyour community. You have also beenasked to think about how to reducethe environmental impacts of energy

resource use. Using what you havelearned, describe how some of theenergy needs of your communitycould be accommodated by wind orsolar power.




Inquiring Further

1. Solar-thermal electricity generation

Research how a solar thermalpower plant, like the LUZ plant inthe Mojave Desert in California,produces electricity. Diagram howsuch a system works. What are thekiloWatt-hour costs of producingelectricity using this method? Whatdoes the future hold for powerplants of this kind?

2. Photovoltaic electricity

Investigate photovoltaic electricitygeneration and discuss the resultsof your investigation with the class.

3. History of wind energy or solar energy

Investigate how people in earliertimes and in different cultures have

harnessed the Sun or wind energy.What developments have takenplace in the past hundred years?How is wind energy being usedtoday? Include diagrams andpictures in your report.

4. Wind farms

Prepare a report on how electricityis generated on wind farms.Describe types of wind generators,types and sizes of wind farms, theeconomics of electricity productionon wind farms, and the locationsof currently operating wind farmsin the United States. Includediagrams.


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