1“In 2050 U.S. carbon dioxide emissions would be 62% below 2005 levels, putting a major brake on global warming.”What do you think?What do you think about the overall plan? If you were the next President would you implement it?A) Yes, as is.B) Yes, with minor changes.C) Yes, with major changes.D) NoSemi-Active Solar HeatingHow do the numbers work?Imagine you want to heat 100 gallons of water a day from 50 to 120 degrees Fahrenheit for your household needs.We calculated last week that we need 100 million Joules of solar energy every day to accomplish this.We then calculated that at 50% efficiency (for semi-active), we would require about 10 square meters (or 100 square feet) of solar panels.Solar Panels of 10 square meters is about 90 square feet.Thus, one panel 9 feet by 10 feet would do the job nicely.Not at all unreasonable….Panels cost ~ $10/ft2, and many $1000’s to set up. Typical is $5000 for a house. This says you might have a 5-10 year payback time.5x107Joules/day Æ 15 kWatt-hour/day. $1.25 per day saved in power costs!4,000 days Æ 10 years!In Winter solar intensity is down by another factor 2-3. You also need some type of backup for sustained poor weather.You will also need storage for nights (and cloudy days).Æ Again moderate storage of heat energy can be achieved with properly insulated materials (e.g. water, bricks, etc.).Photovoltaics (PV)Direct conversion of solar energy (photons) into electrical energy. Not a heat engine !2λhchfE ==Recall that photons have an energy Thus a photon in yellow light has an energy of approximately E = 4 x 10-19Joules.These photons do not have enough energy to typically remove change molecular bonds between atoms. Good for us!However, if the photon strikes a (cleverly designed) semiconductor (like Silicon), it is just the right amount of energy to lift an electron up and let it “conduct away”, flow as an electrical current !―Si ― Si ― Si ― Si ― Si ― Si ―| | | | | |― Si ― Si ― Si ― Si ― Si ― Si ―| | | | | |― Si ― Si ― Si ― Si ― Si ― Si ―Normal SiliconLattice―Si ― Si ― Si ― Si ― Si ― Si ―| | | | | |― Si ― Si ― X Si ― Si ― Si ―| | | | | |― Si ― Si ― Si ― Si ― Si ― Si ――Si ― Si ― Si ― Si ― Si ― Si ―| | | | | |― Si ― Si ― X ― Si ― Si ― Si ―| | | | | |― Si ― Si ― Si ― Si ― Si ― Si ―P-type, doped withValence 3 atomN-type, doped withValence 5 atomSemiconductor MaterialsAdds extra electrons (N=negative type)Has fewer electrons (P=positive type)N-typeP-type0.5 VoltsLightλ<1.2μm+-Typical efficiencies for PV are 10%, and maybe up to 20% at the technological edge.Efficiency is not the only factor. Cost is also a big factor.1.5m X 0.79m= 1.19m212X6 array=72 cells Input:1000 W/m2x1.19m2=1190WOutput:150W (36V,4.5A)Efficiency=13%35 Watt light12 hours on:Whhw 4201235 =×Recharge in 6 hours:222692538int53813.070706420mwmwareainputenweffoutputinputwhwh=======Cost of electricity saved:yearyearnightsnightkwhnightkwh33.15$36504.0$10.0$42.0=×=×Costs about $5003You install a 4kW (peak) PV solar panel system on your house. You currently have a $50/month electric bill (averaged summer/winter). The solar system with installation runs you $24,000. How long before you've paid it off?A) About 10 yearsB) About 20 yearsC) About 40 yearsD) It will never pay off...Argument #1:You save $50/month*12 months/year = $600/year. So it takes $24,000/ $600/year = 40 years. Clicker QuestionThat's if there's no "net metering!" Your panels are generating:4 kW x 8 hrs/day x 30 days/month = 960 kW-hours per monthYou are only using $50/month at $0.10/kWhr Æ 500 kW-hours per month. You can sell the excess back to the power company. This helps shorten the payback time a little.So if the price of electricity goes up, this looks rapidly moreand more attractive. It's already not unreasonable, and is very close to being economically sensible.With subsidies (tax breaks, cost sharing with the utilities), photovoltaics can be made cheaper to the consumer. One big thing we have forgotten…. We could have made a lot of money by investing our $24k for 20-40 years!At what break-even period would you put PV on your roof (assuming you had one!) A) 2-5 yearsB) 5-10 yearsC) 10-40 years.D) Price doesn't matter, I'd like themE) I would not use them even if they were free. Too much hassle, unreliable, ugly, ...There's no right answer here, it's for you to think about...Clicker QuestionSolar ThermalAnother form of “active solar.”Use collected sunlight to heat water, and then use this hot water to run a heat engine. Need high temperature (1000 Celsius), so not for home use. More like power plants.Need about 10 acres of mirrors for each MegaWatt of capacity.Some companies have gone bankrupt doing this, but as fossil fuels get more expensive maybe these make more sense.Aerial of Solar One, in the Mojave Desert near Barstow, CA have computer controlled mirrors that focus sunlight onto a huge central receiver perched atop a 300 foot tower. Solar energy for individual houses may eventually make sense for a broad section of the population.Tax breaks, incentives, etc. are a good way to maintain a modest market that brings in enough money to support research and development and new ideas!Carter gave tax breaks for passive and semi-active solar use. Cost dropped by a factor of 2 over five years. Reagan removed those breaks. If electricity doubles in price and solar panels go down by a factor of two, the “payback time”might be just a few years even for PV.In California, the power company pays half the cost for you! This saves them from having to build new power plants, spreads production, reduces pollution. California is very strict with regulations so this makes sense for energy companies. What would it take the have Xcel do this in
View Full Document
Unlocking...