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Unformatted text preview: UCSD Physics 12 Solar Technologies
Ways to extract useful energy from the sun UCSD Physics 12 Notable quotes I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that. Thomas Edison, 1910 My father rode a camel. I drive a car. My son flies a jet airplane. His son will ride a camel. Saudi proverb Spring 2010 2 UCSD Physics 12 Four Basic Schemes
1. 2. 3. 4. Photovoltaics (Lecture 10) Thermal electric power generation Flat-Plate direct heating (hot water, usually) Passive solar heating Spring 2010 3 UCSD Physics 12 Photovoltaic Reminder Sunlight impinges on silicon crystal Photon liberates electron Electron drifts aimlessly in p-region If it encounters junction, electron is swept across, constituting current Electron collected at grid, flows through circuit (opposite current lines) Spring 2010 4 UCSD Physics 12 Photovoltaic power scheme
Utility grid Sun Light PV-array Battery DC Inverter AC Sunlight is turned into DC voltage/current by PV Can charge battery (optional) Inverted into AC Optionally connect to existing utility grid AC powers household appliances
5 Spring 2010 UCSD Physics 12 Typical Installation 1. 2. 3. 4. PV array Inverter/power-conditioner Indoor distribution panel Energy meter (kWh, connected to grid) 6 Spring 2010 UCSD Physics 12 Putting photovoltaics on your roof The greater the efficiency, the less area needed Must be in full-sun location: no shadows south-facing slopes best, east or west okay
PV Efficiency (%) PV capacity rating (watts) 100 250 500 1K 2K 4K 10K 100K Roof area needed (sq. ft.) 4 8 12 16 30 15 10 8 75 38 25 20 150 75 50 40 300 150 100 80 600 300 200 160 1200 600 400 320 3000 1500 1000 800 30000 15000 10000 8000 Above table uses about 900 W/m as solar flux
2 Spring 2010 7 UCSD Physics 12 When the sun doesn't shine... Can either run from batteries (bank of 12 gives roughly one day's worth) or stay on grid usually design off-grid system for ~3 days no-sun In CA (and 37 other states), they do "net metering," which lets you run your meter backwards when you are producing more than you are consuming this means that the utility effectively buys power from you at the same rate they sell it to you: a sweet deal but very few U.S. utilities cut a check for excess production Backup generator also possible Spring 2010 Q 8 UCSD Physics 12 Photovoltaic Transportation A 10 m2 car using 15% efficiency photovoltaics under 850 W/m2 solar flux would generate at most 1250 W 1.7 horsepower max in full sun when sun is high in the sky Could only take a 5% grade at 20 mph this neglects any and all other inefficiencies Would do better if panels charged batteries no more shady parking spots! Spring 2010 9 UCSD Physics 12 Photovoltaic transportation Quote about solar car pictured above: With sunlight as its only fuel, the U of Toronto solar car, named Faust, consumes no more energy than a hairdryer but can reach speeds of up to 120 kilometers per hour. is this downhill?? Note the mistake in the above quote... The real point is that it can be done but most of the engineering effort is in reducing drag, weight, friction, etc. even without air resistance, it would take two minutes to get up to freeway speed if the car and driver together had a mass of 250 kg (very light) just mv2 divided by 1000 W of power Spring 2010 Q 10 UCSD Physics 12 Future Projections As fossil fuels run out, the price of FF energy will climb relative to PV prices Break-even time will drop from 15 to 10 to 5 years now at 8 years for California home (considering rebates) Meanwhile PV is sure to become a more visible/prevalent part of our lives! In Japan, it is so in to have photovoltaics, they make fake PV panels for rooftops so it'll look like you've gone solar! Spring 2010 11 UCSD Physics 12 But not all is rosy in PV-land... Photovoltaics don't last forever useful life is about 30 years (though maybe more?) manufacturers often guarantee < 20% degradation in 25 years damage from radiation, cosmic rays create crystal imperfections Some toxic chemicals used during production therefore not entirely environmentally friendly Much land area would have to be covered, with corresponding loss of habitat not clear that this is worse than mining/processing and power plant land use (plus thermal pollution of rivers) Spring 2010 2 Q 12 UCSD By concentrating sunlight, one can boil water and make steam From there, a standard turbine/generator arrangement can make electrical power Concentration of the light is the difficult part: the rest is standard power plant stuff Solar Thermal Generation Physics 12 Spring 2010 13 UCSD Physics 12 Concentration Schemes Most common approach is parabolic reflector: A parabola brings parallel rays to a common focus better than a simple spherical surface the image of the sun would be about 120 times smaller than the focal length Concentration 13,000 (D/f)2, where D is the diameter of the device, and f is its focal length Spring 2010 14 UCSD Physics 12 The steering problem A parabolic imager has to be steered to point at the sun requires two axes of actuation: complicated Especially complicated to route the water and steam to and from the focus (which is moving) Simpler to employ a trough: steer only in one axis concentration reduced to 114 (D/f), where D is the distance across the reflector and f is the focal length Spring 2010 15 UCSD Physics 12 Power Towers Spring 2010 Power Tower in Barstow, CA 16 UCSD Physics 12 Who needs a parabola! You can cheat on the parabola somewhat by adopting a steerable-segment approach each flat segment reflects (but does not itself focus) sunlight onto some target makes mirrors cheap (flat, low-quality) Many coordinated reflectors putting light on the same target can yield very high concentrations concentration ratios in the thousands Barstow installation has 1900 20 20-ft2 reflectors, and generates 10 MW of electrical power calculate an efficiency of 17%, though this assumes each panel is perpendicular to sun Spring 2010 17 UCSD Physics 12 Barstow Scheme Spring 2010 18 UCSD Physics 12 Solar thermal economics Becoming cost-competitive with fossil fuel alternatives Cost Evolution: solar thermal plants 1983 13.8 MW plant cost $6 per peak Watt 25% efficient about 25 cents per kWh 1991 plant cost $3 per peak Watt 8 cents per kWh Solar One in Nevada cost $266 million, produces 75 MW in full sun, and produces 134 million kWh/year so about $3.50 per peak Watt, 10 cents/kWh over 20 years Spring 2010 19 UCSD Physics 12 Flat-Plate Collector Systems A common type of solar "panel" is one that is used strictly for heat production, usually for heating water Consists of a black (or dark) surface behind glass that gets super-hot in the sun Upper limit on temperature achieved is set by the power density from the sun dry air may yield 850 W/m2 in direct sun using T4, this equates to a temperature of 350 K for a perfect absorber in radiative equilibrium (boiling is 373 K) Trick is to minimize paths for thermal losses Spring 2010 20 UCSD Physics 12 Flat-Plate Collector Spring 2010 21 UCSD Physics 12 Controlling the heat flow You want to channel as much of the solar energy into the water as you can this means suppressing other channels of heat flow Double-pane glass cuts conduction of heat (from hot air behind) in half provides a buffer against radiative losses (the pane heats up by absorbing IR radiation from the collector) If space between is thin, inhibits convection of air between the panes (making air a good insulator) Insulate behind absorber so heat doesn't escape Heat has few options but to go into circulating fluid
Spring 2010 22 UCSD Physics 12 What does the glass do, exactly? Glass is transparent to visible radiation (aside from 8% reflection loss), but opaque to infrared radiation from 824 microns in wavelength collector at 350 K has peak emission at about 8.3 microns inner glass absorbs collector emission, and heats up glass re-radiates thermal radiation: half inward and half outward: cuts thermal radiation in half actually does more than this, because outer pane also sends back some radiation: so 2/3 ends up being returned to collector Spring 2010 23 UCSD Physics 12 An example water-heater system Spring 2010 24 UCSD Physics 12 Flat plate efficiencies Two-pane design only transmits about 85% of incident light, due to surface reflections Collector is not a perfect absorber, and maybe bags 95% of incident light (guess) Radiative losses total maybe 1/3 of incident power Convective/Conductive losses are another 510% Bottom line is approximately 50% efficiency at converting incident solar energy into stored heat 0.85 0.95 0.67 0.90 = 0.49 Spring 2010 Q 25 UCSD Physics 12 How much would a household need? Typical showers are about 10 minutes at 2 gallons per minute, or 20 gallons. Assume four showers, and increase by 50% for other uses (laundry) and storage inefficiencies: 20 4 1.5 = 120 gallons 450 liters To heat 450 l from 15 C to 50 C requires:
(4184 J/kg/C) (450 kg) (35 C) = 66 MJ of energy Over 24-hour day, this averages to 762 W At average insolation of 200 W/m2 at 50% efficiency, this requires 7.6 m2 of collection area about 9-feet by 9-feet, costing perhaps $68,000 Spring 2010 2 Q 26 UCSD Physics 12 Interesting societal facts In the early 1980's, the fossil fuel scare led the U.S. government to offer tax credits for installation of solar panels, so that they were in essence free Many units were installed until the program was dropped in 1985 Most units were applied to heating swimming pools! In other parts of the world, solar water heaters are far more important 90% of homes in Cyprus use them 65% of homes in Israel use them (required by law for all buildings shorter than 9 stories) Spring 2010 27 UCSD Physics 12 Passive Solar Heating Let the sun do the work of providing space heat already happens, but it is hard to quantify its impact Careful design can boost the importance of sunlight in maintaining temperature Three key design elements: insulation collection storage Spring 2010 28 UCSD Physics 12 South-Facing Window Simple scheme: window collects energy, insulation doesn't let it go, thermal mass stabilizes against large fluctuations overhang defeats mechanism for summer months Spring 2010 29 UCSD Physics 12 The Trombe Wall Absorbing wall collects and stores heat energy Natural convection circulates heat Radiation from wall augments heat transfer 30 Spring 2010 UCSD Physics 12 How much heat is available? Take a 1600 ft2 house (40 40 footprint), with a 40 10 foot = 400 ft2 south-facing wall Using numbers from Table 4.2 in book, a south-facing wall at 40 latitude receives about 1700 Btu per square foot per clear day comes out to about 700,000 Btu for our sample house Account for losses: 70% efficiency at trapping available heat (guess) 50% of days have sun (highly location-dependent) Net result: 250,000 Btu per day available for heat typical home (shoddy insulation) requires 1,000,000 Btu/day can bring into range with proper insulation techniques Spring 2010 31 UCSD Physics 12 Announcements and Assignments Stay in School HW 5 (shorter than usual) due Friday, May 7 Quiz also Friday, by midnight Read Chapter 5 (5.1, 5.2, 5.3, 5.5, 5.7) for next lecture Spring 2010 32 ...
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