Energy derived ultimately from the sun. It can be divided into “direct” and “indirect” categories. Most energy sources on Earth are forms of indirect solar energy, although we usually don’t think of them in that way. Coal, oil and natural gas derive from ancient biological material which took its energy from the sun (via plant photosynthesis) millions of years ago. All the energy in wood and foodstuffs also comes from the sun. Movement of the wind (which causes waves at sea), and the evaporation of water to form rainfall which accumulates in rivers and lakes, are also powered by the sun. Therefore, hydroelectric power and wind and wave power are forms of indirect solar energy. Direct solar energy is what we usually mean when we speak of solar power it is the use of sunlight for heating or generating electricity.
The basic building block of a solar electric system. Solar cells are made of a semiconductor material and convert sunlight directly into electricity.
A group of solar cells, modules are the smallest solar electric components sold commercially, ranging in power output from 10 watts to 300 watts.
A device that converts Direct Current (DC) into Alternating Current (AC).
1,000 watts of electric power.
A unit of electric power consumption indicating the total energy developed by the power of one kilowatt acting for one hour.
A service option available from your electric utility that allows your electric meter to spin backwards when excess energy is generated by your solar electric system. The excess energy is effectively stored by the utility until it is required for future use by the customer.
SREC stands for Solar Renewable Energy Certificate and is a tradable certificate that represents all the clean energy benefits of electricity generated from your solar electric system. A SREC can be sold or traded separately from the power. It is issued once a solar facility has generated 1,000 kWh, through either estimated or actual metered production, and can be listed on the bulletin board on this Website.
Alternative Energy Sources
Energy sources different from those in widespread use at the moment (which is referred to as conventional). Alternative energy usually includes solar, wind, wave, and tidal, hydroelectric and geothermal energy. Although they each have their own drawbacks, none of these energy sources produces significant air pollution, unlike conventional sources.
Carbon or hydrocarbon fuels, derived from what was living material, and found underground or beneath the sea. The most common forms are coal, oil and natural gas. They take millions of years to form. Their energy is only released upon burning, when the carbon and hydrogen within them combine with the oxygen in air to form carbon dioxide (CO2), or carbon monoxide (CO) and water (H2O). Other elements within the fuels (such as sulfur or nitrogen) are also released into the air after combining with oxygen, causing further pollution with sulfur dioxide (SO2) and nitrogen oxide gases. In the case of coal, ash particles are also a problem.
Used to describe energy sources that exist in limited amounts on Earth. Thus all available material could eventually be completely used up. Coal, oil and gas (see fossil fuels) are considered as non-renewable energy sources because the rate of their formation is so slow on human timescales that they we are using them without them being replaced. Uranium (used in nuclear power) is also non-renewable, although its reserves are very large compared to its rate of use.
Passive Solar Heating
The use of the sun to heat buildings. Careful design and positioning of buildings can ensure that sunlight in the winter months will warm them by day, with much of the warmth remaining during the night. Summer sunlight is usually kept out. This does not involve the conversion or harnessing of solar energy.
Photovoltaic (PV) Cells (also known as solar cells)
A photovoltaic cell is made of thin wafers of two slightly different types of silicon. One, doped with tiny quantities of boron, is called P-type (P for positive) and contains positively charged ‘holes’, which are missing electrons. (Electrons are negatively charged particles that orbit the nuclei of atoms.) The other type of silicon is doped with small amounts of phosphorus and is called N-type (N for negative). It contains extra electrons. Putting these two thin P and N materials together produces a junction which, when exposed to light, will produce a movement of electrons and that constitutes an electric current. Photovoltaic cells thus convert light energy into electrical energy.
Used to describe energy sources that are replenished by natural processes on a sufficiently rapid time-scale so that they can be used by humans more or less indefinitely, provided the quantity taken per unit of time is not too great. Examples are animal dung, ethanol (derived from plant sugars), wood, wind, falling water and sunlight.
Devices for capturing the sun’s energy over a large area and focusing it on a small area, thereby concentrating it. In this way it can be made to provide extremely high temperatures, used to generate steam that will expand, or to carry out a chemical reaction to produce a portable fuel such as hydrogen. Solar collectors may be curved dishes – like satellite receiving dishes coated with reflective material, or can consist of an array of reflectors, arranged like flower petals, focusing onto a central point. Usually the dish or the individual reflectors can be steered to follow the sun across the sky.