The Sun's lifetime as star will be about 10 billion years. Half of that time has passed……..
Brief History….. • 1st written account of solar energy use: 4th century BC when a scarcity of wood forced fuel wood imports from Middle East • Socrates laid out principles of passive solar design: • 1. main rooms should face south • 2. north side of buildings should be shielded from the cold winds • 3. eaves should be added to provide shade for south windows in summer • 212 BC Archimedes allegedly used solar energy to "reduce the Roman navy (which was attacking Syracuse) to ashes" by having soldiers reflect sunlight off their shields toward Roman sails • Central heating and large central baths quickly consumed forests around Rome • Romans expanded use of solar energy: • 1st century AD introduced glass (recognized greenhouse effect) • used dark colors and pottery to store thermal energy • 1st to legislate domestic solar rights (2nd century AD) • 37 AD: 1st greenhouse used to grow cucumbers for Tiberius Caesar • 1700: Antoine LaVoisier built a solar furnace that could melt platinum (3236°F, 1780°C)
XX century • circa 1900: solar water heater industry emerged in S. California • circa 1920: natural gas discovered in S. California, ending solar industry there • 1923: solar water heater industry moved to Florida • 1939: MIT built 1st active solar house • 1941: government froze civilian use of copper due to WWII, halting production of solar water heaters • circa 1950: US lifestyle changes made solar water heaters too small • 1950's: cheap electricity and fossil fuels made solar products too expensive • 1973: OPEC Energy Crisis causes US to reexamine use of renewable energy sources; federal and state tax credits result in rapid growth for a new solar industry • 1986: tax credits for residential solar systems ended causing 30,000 US workers to lose jobs • 1996: Solar energy used to heat water for swimming events at Atlanta Olympic Games
SUN “structure” One of the most puzzling features of the Sun is what has been dubbed "the solar corona problem." There is a region around the Sun, extending more than one million kilometers from its surface, where the temperature can reach two million degrees. This region, called the Solar Corona, is where the solar wind originates. The corona has been found to emit X-rayradiation (the corona is a plasma; at temperatures greater than a million degrees a plasma will radiate a lot of X-rays). The corona can be seen during solar eclipses, when the main radiation from the Sun's surface is blocked by the passage of the Moon.
Solar flux At top of atmosphere, 1400 watts/square meter. But: – The atmosphere absorbs energy: At sea level, 1000 W/m2 on sun-facing surface when sun is high in the sky – When the Sun is close to horizon, then there is more absorption. – Day/night cycle: power only available 1/2 time. – Clouds reduce availability further 200 W/m2 on average
Ways to capture solar power Many forms of energy are essentially powered by the sun, including: - hydro (sun powers the water cycle) - wind (sun powers air motion in atmosphere) - direct thermal capture - biomass - photovoltaic (Even fossil fuels are a kind of solar energy, but being depleted at much greater rate than they were produced -- not renewable.)
Efficiency of energy capture Plants aren’t that efficient at capturing solar energy. - Averaged over a whole year, a cornfield captures 1.5% of solar energy as part of corn plants (including leaves and stems.) - Some forests a bit better, deserts much worse (only 0.01% of solar energy captured in desert plants.)
Direct thermal capture, a.k.a. “passive solar” power Sunlight absorbed on dark surface is converted directly to thermal energy. Black surface absorbs best, can reach 90 deg C. Used for water heaters, or to heat home.
Stored solar energy provides a firm capacity of 31MW until midnight at which time fossil fuel backup us used
Solar generation of electricity When light strikes properly constructed piece of silicon, electrical voltage and current are generated. photovoltaic cell Simplest are silicon crystals, but those are very expensive. “Polycrystalline” cells work almost as well, and are cheaper. “Amorphous” silicon cheaper still.
Specs of photovoltaic cells today Efficiency is electrical power output divided by solar power falling on cell. Real Goods catalog offers Photowatt PW 1000-100 polycrystalline cell array. Area = 0.9 m2 intercepts 0.9 m2 * 1000 W/m2 = 900 W. Rated power = 100 W So, its efficiency = 100 W/ 900 W = 0.11 = 11%.
Area required to power our lives At 150 W/m2, how much area do we need? Depends also on efficiency; assume 10%. So we can collect in useful form 15 W/m2. Direct household power: 1000 W Area needed: 1000 W / 15 W/m2 = 67 sq meters, or a square a little over 8 meters (26 feet) on a side. Roughly the size of the roof on a house. Total needs including energy used to produce things we buy: 10 kW. Need 10 times a house’s footprint per household.
Cost of “free” power • Sunlight arrives for free, so why does solar power cost money? • Collectors (converters into useful form) aren’t free. • Costs to manufacture and install, i.e. capital costs.
Interest and the cost of • photovoltaic power • From a table of interest rates: • $1000 borrowed at 7% requires payment of $142/year for 10 years. • If $1000 buys you a 10% efficient collector of 2 sq. meter area, you bought 30 W averaged. • In a year, cell generates 30 J/sec * 3.15 107 sec = 4.7 108 J, or 260 kWh per year. • $142 / 260 kWh, a bit more than $0.50/kWh. • Compare typical utility charge of $0.10 / kWh.
Photovoltaic technology in the home To use in home, we need an inverter to make 60 Hz AC power from the DC generated by photovoltaic cells. What about night-time? Off-grid: charge batteries in daytime, draw them down at night On-grid: hook to grid through an intertie. Sell excess power to utility in daytime, buy back from utility at night. Process is called “net metering.”
Net Metering Rules www.dsireusa.org September 2006 25 kW 100 kW 50 kW NH: 25 MA: 60 RI: 25 * CT: 100 100 VT: 15/150 100 * * 50 100 * 40 * 25 30 * 10/400 20 * 25/100 * 25 * * 500 no limit * PA: 50/1,000/2,000 NJ: 2,000 DE: 25 MD: 500 DC: 100 VA: 10/500 * 40 * * 30 * * 2,000 10 25 * 1,000 15 * 20/100 * * 100 25/100 10 10/100 10 * 50 25/100 10 50 Net metering is available in 40 states + D.C. State-wide net metering for all utility types * State-wide net metering for certain utility types (e.g., IOUs only) Net metering offered by one or more individual utilities #s indicate system size limit (kW); in some cases limits are different for residential and commercial as shown
Trough systems convert the heat from the sun into electricity. Because of their parabolic shape, troughs can focus the sun at 30 to 60 times its normal intensity on a receiver pipe located along the focal line of the trough. Synthetic oil captures this heat as the oil circulates through the pipe, reaching temperatures as high as 390°C (735ºF). The hot oil is pumped to a generating station and routed through a heat exchanger to produce steam. Finally, electricity is produced in a conventional steam turbine.
Solar Power Towers Schematic of electricity generation using molten-salt storage: 1. sun heats salt in receiver; 2. salt stored in hot storage tank; 3. hot salt pumped through steam generator; 4. steam drives turbine/generator to produce electricity; 5. salt returns to cold storage tank
Solar power, fuel cells, and the hydrogen economy For transportation, solar power is hard to harness directly. Car doesn’t have enough surface area. Night-time and cloudy-day driving would be problems, too. One proposal is to use photovoltaic cells hooked to fuel cells to generate hydrogen gas from water. H2 then distributed through network of pipes (now used for natural gas), to be pumped into fuel-cell cars at filling stations. Not competitive yet, but perhaps in 10 years?