AY Honors/Renewable Energy/Answer Key
Renewables as solar energy
Most renewable energy sources can trace their roots to solar energy, with the exception of geothermal and tidal power -- yet even these can be attributed to the sun's gravity. For example, wind is caused by the sun heating the earth unevenly. Hot air is less dense, so it rises, causing cooler air to move in to replace it. Hydroelectric power can be ultimately traced to the sun too. When the Sun evaporates water in the ocean, the vapor forms clouds which later fall on mountains as rain which is routed through turbines to generate electricity. The transformation goes from solar energy to potential energy to kinetic energy to electric energy.
Modern sources of renewable energy
Solar energy
Main article: Solar power
Since most renewable energy is "Solar Energy" this term is slightly confusing and used in two different ways: firstly as a synonym for "renewable energies" as a whole and secondly for the energy that is directly collected from sunlight. In this section it is used in the latter category.
There are actually two separate approaches to solar power, termed active solar and passive solar.
Solar electrical energy
For electricity generation, ground-based solar power has serious limitations because of its diffuse and intermittent nature. First, ground-based solar input is interrupted by night and by cloud cover, which means that solar electric generation inevitably has a low capacity factor, typically less than 20%. Also, there is a low intensity of incoming radiation, and converting this to high grade electricity is still relatively inefficient (14% - 18%), though increased efficiency or lower production costs have been the subject of much research over several decades.
Two methods of converting the Sun's radiant energy to electricity are the focus of attention. The better-known method uses sunlight acting on photovoltaic (PV) cells to produce electricity. This has many applications in satellites, small devices and lights, grid-free applications, earthbound signaling and communication equipment, such as remote area telecommunications equipment. Sales of solar PV modules are increasing strongly as their efficiency increases and price diminishes. But the high cost per unit of electricity still rules out most uses.
Several experimental PV power plants mostly of 300 - 500 kW capacity are connected to electricity grids in Europe and the USA. Japan has 150 MWe installed. A large solar PV plant was planned for Crete. In 2001 the world total for PV electricity was less than 1000 MWe with Japan as the world's leading producer. Research continues into ways to make the actual solar collecting cells less expensive and more efficient. Other major research is investigating economic ways to store the energy which is collected from the Sun's rays during the day.
Alternatively, many individuals have installed small-scale PV arrays for domestic consumption. Some, particularly in isolated areas, are totally disconnected from the main power grid, and rely on a surplus of generation capacity combined with batteries and/or a fossil fuel generator to cover periods when the cells are not operating. Others in more settled areas remain connected to the grid, using the grid to obtain electricity when solar cells are not providing power, and selling their surplus back to the grid. This works reasonably well in many climates, as the peak time for energy consumption is on hot, sunny days where air conditioners are running and solar cells produce their maximum power output. Many U.S. states have passed "net metering" laws, requiring electrical utilities to buy the locally-generated electricity for price comparable to that sold to the household. Photovoltaic generation is still considerably more expensive for the consumer than grid electricity unless the usage site is sufficiently isolated, in which case photovoltaics become the less expensive.
Centralization and decentralization
Frequently renewable electricity sources are disadvantaged by regulation of the electricity supply industry which favors 'traditional' large-scale generators over smaller-scale and more distributed generating sources.
Solar thermal electric energy
The second method for utilizing solar energy is solar thermal. A solar thermal power plant has a system of mirrors to concentrate the sunlight on to an absorber, the resulting heat then being used to drive turbines. The concentrator is usually a long mirrored parabolic trough oriented north-south, which tilts, tracking the Sun's path through the day. A black absorber tube is located at the focal point and converts the solar radiation to heat (about 400 °C) which is transferred into a fluid such as synthetic oil. The oil can be used to heat buildings or water, or it can be used to drive a conventional turbine and generator. Several such installations in modules of 80 MW are now operating. Each module requires about 0.5 km² of land and needs very precise engineering and control. These plants are supplemented by a gas-fired boiler which ensures full-time energy output. The gas generates about a quarter of the overall power output and keeps the system warm overnight. Over 800 MWe capacity worldwide has supplied about 80% of the total solar electricity to the mid-1990s.
One proposal for a solar electrical plant is the solar tower, in which a large area of land would be covered by a greenhouse made of something as simple as transparent foil, with a tall lightweight tower in the center, which could also be composed largely of foil. The heated air would rush to and up the center tower, spinning a turbine. A system of water pipes placed throughout the greenhouse would allow the capture of excess thermal energy, to be released throughout the night and thus providing 24-hour power production. A 200 MWe tower is proposed near Mildura, Australia.
Solar thermal energy
Solar energy need not be converted to electricity for use. Many of the world's energy needs are simply for heat; space heating, water heating, process water heating, oven heating, and so forth. The main role of solar energy in the future may be that of direct heating. Much of society's energy need is for heat below 60 °C (140 °F) - e.g. in hot water systems. A lot more, particularly in industry, is for heat in the range 60 - 110 °C. Together these may account for a significant proportion of primary energy use in industrialized nations. The first need can readily be supplied by solar power much of the time in some places, and the second application commercially is probably not far off. Such uses will diminish to some extent both the demand for electricity and the consumption of fossil fuels, particularly if coupled with energy conservation measures such as insulation.
Solar water heating
Domestic solar hot water systems were once common in Florida until they were displaced by highly-advertised natural gas. Such systems are today common in the hotter areas of Australia, and simply consist of a network of dark-colored pipes running beneath a window of heat-trapping glass. They typically have a backup electric or gas heating unit for cloudy days. Such systems can actually be justified purely on economic grounds, particularly in some remoter areas of Australia where electricity is expensive.
Solar heat pumps
With adequate insulation, heat pumps utilizing the conventional refrigeration cycle can be used to warm and cool buildings, with very little energy input other than energy needed to run a compressor. Eventually, up to ten percent of the total primary energy need in industrialized countries may be supplied by direct solar thermal techniques, and to some extent this will substitute for base-load electrical energy.
Solar ovens
Large scale solar thermal power plants, as mentioned before, can be used to heat buildings, but on a smaller scale solar ovens can be used on sunny days. Such an oven or solar furnace uses mirrors or a large lens to focus the Sun's rays onto a baking tray or black pot which heats up as it would in a standard oven.
