Chapter 14.1 Putting Solar Energy to Work solar energy background originates with thermonuclear fusion in the Sun radiant energy reaches Earth, with wavelengths throughout the electromagnetic spectrum about ½ of energy reaches the Earth’s surface; 30% is reflected and 20% is absorbed by the atmosphere the energy in 40 minutes of sunlight is equal to a year’s worth of fossil fuels using some of this solar energy will not change the energy balance of the biosphere despite its abundance, solar energy is diffuse (widely scattered) and varies with season, latitude, atmospheric conditions, and time of day

solar heating of water solar hot-water heating is already popular in warm, sunny climates solar collector consists of thin, broad box with a glass or clear plastic top and a black bottom with embedded water tubes (known as flat-plate collectors) active system: heated water is moved by a pump passive system: relies on natural convection currents; system constructed so that collector is lower than tank in temperate climates where the water might freeze, a heat-exchange coil circulates antifreeze

solar space heating uses same concept as water heating greatest efficiency is gained if building acts as its own collector have windows face the sun (usually south-facing) in winter, light can come in to heat interior at night, shades can be drawn to insulate windows well-insulated buildings act as heat-storage unit heat load for windows with sun exposure can be minimized by using owning or overhangs

other ways of increasing efficiency landscaping deciduous trees and vines on the sunny side of buildings can block excessive summer heat while allowing winter sunlight to pass through evergreens opposite sunny side can insulate from wind and cold earth-sheltered housing combines insulation and solar heating build up soil against the building walls, with south-facing windows left exposed soil has large capacity for storing heat; at night, building will radiate heat stored during the day in summer, building is kept cool by its contact with soil

other ways of increasing efficiency Energy Star in 2001, EPA extended Energy Star program to include buildings buildings awarded the label use at least 40% less energy than other buildings in the same class, along with passing a number of other criteria in 5 years, more than 2500 buildings have earned the Energy Star label, saving $350 million in lower energy bills good insulation helps minimize the need for a backup heating system for most well-insulated solar homes, a small wood or gas heater is usually sufficient

photovoltaic cells each cell consists of two very thin layers of semiconductor material lower layer has atoms that easily lose electrons; upper layer has atoms that easily gain electrons energy from light photons knock electrons from lower layer, creating an electrical potential between the two layers electrons flow from lower side, through motor or other device, to upper side light energy is converted to electrical energy with efficiency of about 20% lack of moving parts means they do not wear out silicon is main material for layers

photovoltaic cells uses: toys, calculators, watches, irrigation pumps, traffic signals, offshore oil-drilling platforms many installations involve “net metering,” where rooftop electrical output is subtracted from customer’s use of power from the grid cost is calculated by dividing the cost manufacturing the PV cell by the total amount of power they’re expected to produce over their lifetime (usually about cents / kWh) [cost of traditional residential electricity is 6-12 cents / kWh]

photovoltaic cells inverters most complicated, yet necessary component of PV systems acts as interface between solar PV cells and electric grid or batteries changes incoming direct current (DC) to alternating current (AC) utilities companies are considering solar power plants with multiple enormous arrays of PV cells also providing incentives for customers to install PV panels on their roofs

photovoltaic cells Million Solar Roofs Initiative Solar America Initiative new technologies to compete with traditional sources, cost of solar cells needs to come down (1) thin-film PV cells in which inexpensive amorphous silicon is used instead of crystals, allowing a PV coating to be spread over roof tiles or glass (2) electrically conductive plastics fashioned into inexpensive solar sheets that can be spread out over roofs

concentrated solar power (CSP) several technologies have been developed that convert solar energy into electricity by using reflectors such as mirrors to focus concentrated light onto a receiver that transfers the heat to a generator solar trough: light hitting collector is reflected onto pipe in center which contains oil or other heat- absorbing fluid (9 facilities in California desert) power tower: array of sun-tracking mirrors that focus sunlight falling on several acres of land onto a receiver mounted to a tower in the middle of the field; receiver transfers heat to a molten-salt liquid dish-engine system: set of parabolic concentrator dishes focus sunlight onto a receiver

future of solar power solar energy is growing fast PV panel business is $7.5 billion industry and is growing about 40% / year avoids hidden costs associated with traditional sources—pollution, strip mining, greenhouse gas emissions still disadvantages to solar power technology is more expensive than conventional energy sources works only during the day (requires backup or storage battery) certain climates are not suitable (especially in winter)

matching demand although solar can only provide energy during the day, 70% of demand is during daytime hours as a result, there is still a great potential for savings also is useful for supplying electricity in developing countries costs are decreasing by about 5% per year