Solar energy is not one, but three types of energy. They are heating and cooling applications, generation of electricity, and fuels from biomass. Heating and cooling using the sun has occured since the Mesa Verde cliff dwellings were built to let in the low winter sun, but keep out the high summer sun. Many modern buildings now use such passive solar techniques. Generation of electricity is achieved in a simple way using an array of solar collectors that heat water to produce steam that turns a turbine. Photovoltaic cells convert sunlight directly into electricity. These cells are grouped on panels used to power remote outposts like irrigation pumps, space stations, and unattended railroad crossings. Fuels from biomass are alcohols, wood and dung. Wood and dung are used a lot as fuel in developing countries by being burned.
The most potential lies in the generation of electricity because we can influence how much energy is drawn from the sun. As an energy source that will last five billion more years, it'll last longer much longer than our oil deposits. In 1993 scientists had made solar cells with 30% efficiency, which has almost certainly increased by now. With large arrays of cells, the cost of using them was about twice as much per kilowatt-hour as major cities payed for electricity in 1989. The major con is cost, but the pros, which are being able to store electricity in batteries, having access to in anywhere, and never running out of it, easily outweighs the cost.
Wind power has been in use since the first sail was used on a boat. We have many machines that use the wind like sailboats, airplanes, windsurfing boards, windsocks, and weathervanes, but the main machine we use is the windmill. It has been in use since as early as the 7th century A.D. in Persia. The machine was on a vertical shaft, as today, but the sails were horizontal, allowing only one-half of the machine to be in use. It was used for irrigation and milling grain. The vertical sails were devised in the 12th century and began appearing all over Europe. In the 18th century, improvements like shutters, which allow the rotation to continue at a constant speed in varying wind speeds, air brakes, which can stop rotation, and propellerlike airfoils in place of sails, which increase usefulness in light winds. Windmills were widely used in Denmark to pump water in the late 19th century and early 20th century. In the U.S. in the early 20th century, windmills called turbines were used to generate electricity.
Modern wind turbines use either lift (as on an airplane wing) or drag (pushing) to operate. The ones using lift turn faster and are more efficient. They are used to pump water or generate electricity. High-torque, low speed machines used for power pump water. They are 2-5 m in diameter with a number of oblique blades radiating from a hub with a rudder that points them into the wind. When the wind is too high a safety devices points it away from the wind. Electricity generators consist of a rotor that converts wind power to rotating power, a gearbox that increases speed, and a generator that converts shaft power to electrical power. Most start operating at 12 mph, achieve their rated power at about 45 mph, and shut down at 60 mph. The best spots have an annual average of 13 mph. The power available is the cube of the wind speed, and wind turbines can theoretically be 59.3% efficient, but most are 30%-40% efficient. A wind farm in California has an array of turbines 50-100 feet in diameter that has a 1400 MW rating. A typical nuclear plant has a 1000MW rating.
Currently Denmark gets 2% of its power from wind. Many remote areas and islands use wind power to augment their power supply. It is believed 10% of the world's power will be wind power by 2050. The pros of wind power are lack of pollution, renewable energy, and few greenhouse gas emissions. The cons are that huge tracts of land would have to be covered in turbines for its percentage to be above 50% of the world's energy supply. Also, it can't be directly converted to anything without a plug, like cars.
Geothermal energy is energy derived from the earth's natural heat. One object is to find concentrations of hot water and use them for direct applications like space heating and industrial drying processes. Another is to find a volcanic region and pour water onto the hot rocks and use the steam as other power plants do. Geothermal energy has been used since early Norse settlers in Iceland used water from nearby hot springs for heating purposes. Geothermal energy was developed for electrical power in 1904 in Italy. The world's largest geothermal power complex is at The Geysers of northern California. They generate approximately 1400 MW, enough to light up metropolitan San Francisco. In 1990 geothermal plants provided 5800 MW worldwide. The potential in the US is 23,000 MW per year for 30 years. The pros are that it's environmentally safe, and it's renewable. The cons are that we only get as much steam as water we put in, it's only available near "hot spots", and that it's quite unsafe working in a plant on top of a potential volcano or rift. In fact, a plant already got swallowed.
Tidal energy is harnessing the tides of the oceans. The plants are built where tides are extreme between high and low. In France a plant built in the Rance estuary has an 18 foot tide and produces 240 MW. It uses the water that flows in through the dam to turn turbines. When the tide goes out, water is trapped behind the dam and when it's released it turns the turbines again. In the Bay of Fundy in Canada the difference between high and low tides 60 feet. As of the early 90's only plans had been made to build a tidal power plant. Pros are that it's clean, and the source is renewable. Cons are that the tides don't move that fast, and that it's an eyesore.
Wind energy at my house would be interesting because we'd have the only wind turbine in Eagle-Vail. We could put it on our roof and put the shaft through the middle of our house and put an electricity converter in the garage. It would probably supply power for our house and a few neighbors. We could charge dirt cheap energy and make a few bucks. It gets windy when fronts move through, since we're in a valley, and it gets windy when the storm gets there. For an added bonus we could put solar panels on the roof and sails. I'd have to sacrifice nothing, but the rest of my family would have to sacrifice sleep because they sleep upstairs. Although the rhythmic swoosh-swoosh-swoosh could be soothing.
Sources - "Funk and Wagnalls New Encyclopedia", copyright 1993, vols. 9, 11, 24, 25, 27.
