ENV SCI 15 Lecture 16

• ENERGY UPDATE!?!• Homework Exercises• Solar Energy: electric and thermal

1,364

The round Earth intersects the same amount of solar energy as would a flat disk with an area equal to Earth’s cross- sectional area (πRE

2).The quantity S measures the power per unit area incident on an area oriented perpendicular to the incoming sunlight.

Stanford University’s Solstice, a solar-powered car that won the stock class competition in the 2005 North American Solar Challenge.

Insolation measured on a mid- October day in Middlebury, Vermont. The dashed curve represents an ideal cloudless day. Can you tell from the numbers whether the insolation was measured on a horizontal or a tilted surface?

Dashed curves are trends in the insolation data, indicating the summer insolation peak at around 30°north latitude, above which the actual insolation drops because of increasing cloudiness. Cities, in order of latitude, are Singapore, Jakarta, Bangalore, Dakar, Manila, Honolulu, Abu Dhabi, Riyadh, Cairo, Atlanta, Ankara, Madrid, Detroit, Bucharest, Paris, London, Edmonton, Oslo, Anchorage.

Theoretical maximum horizontal-surface insolation (solid curves) versus latitude for January and June. Also shown are actual data points for selected world cities in January (gray circles) and June (black circles).

Annual average insolation on a horizontal surface for selected world cities.

Annual average insolation in the continental United States, for surfaces oriented perpendicular to the incident sunlight. Values range from about 150 W/m2 for the lightest shading to over 500 W/m2 for the darkest.

Distribution of solar energy among infrared, visible, and ultraviolet portions of the spectrum. Wavelengths between 380 nm and 760 nm make up the visible region.

A passive solar house using direct gain and a concrete slab for thermal mass.

Indirect-gain solar house using a Trombe wall.

Cross section of a flat-plate solar collector using a liquid heat-transfer fluid.

An evacuated-tube solar collector. Inside the evacuated glass tubes are smaller tubes containing fluid that boils and transfers heat to circulating liquid at the top of the collector. The vacuum in the tubes eliminates conductive and convective heat loss.

A solar domestic hot-water system.

The solar domestic hot-water system in your author’s home, with collectors built into the roof structure. Inset photos show the basement storage tank (right) and control unit (left). Insulated pipes at the bottom of the tank carry heat-transfer antifreeze to and from the heat exchanger at the bottom of the tank. The control unit displays the collector temperature in degrees Fahrenheit, in this case on a hot, sunny summer day..

Solar concentrators. (a) A parabola reflects parallel rays of sunlight to a common focus. (b) A parabolic dish focuses sunlight to a point and requires two-axis tracking. (c) A trough concentrator focuses sunlight to a line and requires only one-axis tracking.

A compound parabolic concentrator funnels incident light from a range of angles, providing concentration without imaging.

Stirling-engine solar concentrators at the 1.5-MW solar power plant in Maricopa, Arizona. Each unit produces 25 kW of electric power.

Spain’s 19-MW Gemasolar power-tower system uses a molten-salt heat-transfer fluid, and can generate power for 15 hours in the absence of sunlight.

Parabolic trough concentrators at the 64-MW Nevada Solar One plant near Boulder City, Nevada. The plant uses molten salt for energy storage, so it continues to produce electricity when the Sun isn’t shining.

A PN junction. When the junction is first formed, electrons (minus signs) migrate from the N-type side to the P-type side, and holes (plus signs) migrate from P to N. This sets up a strong electric field (arrows) pointing from N to P.

A photovoltaic cell. The energy of sunlight photons creates electron-hole pairs at the PN junction, and the electric field at the junction sends electrons into the N-type semiconductor and holes into the P-type semiconductor. Metal contacts at the top and bottom become negatively and positively charged, respectively, and as a result a current flows through an external circuit.

A polycrystalline solar cell. The mottled appearance results from silicon crystals with different orientations. Horizontal structures and the wider vertical bands are the metallic contacts that channel current to an external circuit. This particular cell measures 12.5 cm by 12.5 cm and produces just over 2 W of electric power, with a 14% conversion efficiency.

Solar World’s SW 240 solar panel measures approximately 1m×1.7m and produces 240W of power in direct sunlight.

Maine hardly qualifies as a “sunshine state,” but the rooftop PV (arrow) panels on this Maine home generate more electric power than the home uses; the excess is sold to the local power company.

This 97-MW photovoltaic power plant in Sarnia, Ontario, Canada was the world’s largest when it went on line in 2010.

Global PV capacity grew at some 40% annually during the first decade of the twenty- first century. Grid-connected facilities comprised about two-thirds of the total in 2000, and by the end of the decade more than 99% of new installations were grid connected.

Energy payback time for a typical PV system is about 2years. If the system is manufactured using fossil energy, then a system with a 30-year lifetime produces 2 years’ worth of “dirty” energy followed by 28 years of “clean” energy. Note that the cumulative energy production after 2 years is zero, since at that point the system has produced as much energy as it took to manufacture.

Retail prices for photovoltaic modules fell substantially in the first decade of the twenty- first century. Prices are in dollars (U.S.) or euros (Europe) per peak watt of power.

The president’s budget request for U.S. Department of Energy research programs for fiscal year 2011.This budget represents substantial increases for renewable energy, but fossil and nuclear subsidies still dominate. Not shown is funding for research on energy-efficient buildings and vehicles.