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Active space heating - a collector (generally placed on the roof top) is used to heat the air inside a building. A mechanical circulator pulls cooler air from inside the building into the collector, where it is heated by the sun. The heated air is then pushed from the collector to a thermal storage area, which is generally composed of a large mass of rocks or water. This stored heat can later be pulled back into the house by the circulator when it is needed.

Active water heating - heat trapped in the collector box is transferred to the cooler fluid circulating through the absorber tubing, gradually raising the temperature of the fluid. This fluid is either the household water itself, which goes through the tubing directly into the hot water tank (open loop system), or an antifreeze fluid that goes through a heat exchanger within the tank, heating the surrounding water, but not mixing with it (closed loop system). The tank may be located at a distance from the collector. However, heat loss increases as the distance between the tank and collector increases.
(see also flat plate collectors, concentrating collectors)

Alcohol fuels - see ethanol, methanol

Annual heating degree days - the total number of heating degree days for the year. Higher heating degree days mean more energy is necessary to heat a given space.

Batch heaters - consist of one or more small fluid storage tanks placed in an insulated, glazed collector box that is usually mounted on the roof. A good batch heater can be built for under $1000 and is easy to maintain, providing there is freeze protection.

Batteries - see electrochemical energy storage.

Biomass energy - fuels derived from oganic sources (not fossil fuels such as oil and coal).

BTU - British Thermal Unit, the amount of heat necessary to raise one poudn of water one degree Fahrenheit.

C - Centigrade.

Calorie - the amount of heat necessary to raise one gram of water one degree centigrade.

Concentrating collectors - have convex absorber plates to concentrate the solar radiation and increase its efficiency. These collectors are often used where higher water temperatures are required, as in commercial applications. A technologically advanced type of concentrating collector utilizes evacuated tubes connected in parallel rows. These tubes consist of smaller inner metal tubing, which contains the heat transfer fluid, inside larger outer glass tubes. Air is withdrawn, or evacuated, from the space between the inner and outer tubes to create a vacuum. This vacuum minimizes conductive and convective heat losses and further increases the efficiency of the collector.

Degree day, heating (base 65 degrees F) = the number of degrees Fahrenheit that the average tempreature for the day is below 65 degrees F.

Electrochemical energy storage - includes batteries and fuel cells. New, more efficient battery systems have been developed (lithium, nickel-hydrogen, sodium-sulfur, zinc-air, etc.) and older ones using lead acid batteries have been improved. Batteries are commonly used to store the energy in small photovoltaic, hydroelectric or wind energy systems. However, they remain heavy, expensive and a major source of hazardous waste materials. An exciting and rapidly evolving technology is that of fuel cells - electrochemical devices that convert the energy of fuels such as methane or hydrogen into electrical energy. Fuel cells consist of an electrolyte and two electrodes that produce an electric current by combining hydrogen and oxygen ions. They are efficient, create little air pollution or solid waste, and are silent. Since they are modular they can be used in small or large applications (e.g., automobiles, office buildings, hotels or hospitals) as well as providing peak power for utilities. Fuel cell systems can be designed to supply heat and air conditioning as well as electricity.

Electrolysis - a means of producing hydrogen by the use of an electric current to "split" water into hydrogen and oxygen.

Ethanol - ethyl alcohol (C2H5OH), acohol from corn and other grains

F - Fahrenheit

Flat plate collectors - commonly consist of a glazed and insulated box containing a blackened absorber plate. Mounted on this plate is the metal tubing (also blackened) that carries the fluid to be heated. The glazing, which acts as a cover for the collector, consists of one or more transparent sheets of glass or plastic. This glazing allows the sun's rays to enter the box and also traps the resulting heat inside by means of the greenhouse effect.

Fuel cells - see electrochemical energy storage.

G - gram

Geothermal energy - geothermal means "earth" plus "heat." This heat originates from the molten rock, or magma, deep within the earth; this magma often comes close to the surface where it creates volcanoes or hot springs. Geothermal energy - the second type of renewable energy (along with tidal power) that does not directly result from the sun's radiation - is available from the earth's crust in hot rocks, the magma itself, and particularly in subterranean pools of hot water.

Hydroelectric power, or energy from falling water, is the most commonly utilized form of renewable energy throughout the world. In most cases the water is stored behind a dam and the overflow used for power. As with wind power, the mechanical energy of water was primarily used in the past to directly power various types of mills, but during this century the powering of electrical turbines and generators has been its main application. Including both small and large scale facilities, hydropower generates about 10% of U.S. electricity. A good hydro site must have a constant flow of water and a relatively quick drop in elevation. Therefore, consistent rainfall throughout the year is important. For average precipitation in 168 towns and cities throughout the U.S. see to the Weather Data section.

Hydrogen fuels - see electrolyis, photobiological conversion, photochemical conversion.

Kg - kilogram, about 2.2. pounds

KWh - kilowatt hours

M - meter

Mechanical energy storage - there are three primary methods for storing kinetic energy. These are:
- powering flywheels that can spin for long periods of time at very high RPM's;
- compressing air in places such as underground caves; and
- pumping water up to a higher elevation and then extracting energy as it falls back down to a lower elevation.
Flywheel systems are most often used in transportation - especially in electric vehicles - to store electrical or mechanical energy (by regenerative braking).
Pumped hydro storage systems use falling water to generate electricity. No energy is really saved, but this system can be useful to complement the demand cycles of an electric utility.
Water is pumped up when the utility has excess generating capacity during low demand times; it is released to fall back down and generate excess electricity during high demand (such as in the evenings when people come home from work and turn on their air conditioners, electric fans, etc.)

Methanol - methyl alcohol (CH4OH), also called wood alcohol

Passive space heating - the house or other structure to be heated is designed to absorb the sun's energy during colder months and to block it from entering during warmer periods. Examples of this design include south-facing windows, sunspaces, special window glass that reflects the heat back into the house, thermal mass such as brick or stone that stores heat, and overhangs that block the summer sun. Design with the sun in mind was important in traditional architectures; it can be a cost-effective approach in most climates today.

Passive water heating - see batch heaters, thermosiphon systems

Photobiological conversion - takes advantage of the fact that certain strains of algae and bacteria produce hydrogen as a photosynthetic waste product to convert biomass into hydrogen.

Photochemical conversion - sunlight strikes a container holding a liquid electrolyte and a catalyst. The catalysts absorb the solar energy to create electrical fields that trigger the splitting of water into hydrogen and oxygen.

Photovoltaic cells - when light strikes these cells, it causes electrons to migrate in one direction and positive charges to migrate in the opposite direction, creating an internal electric field within the cell. The current in this field is collected by a grid of contacts on the top and bottom of the cell. PV cells are joined in modules and arrays of modules that can generate anywhere from 5 to several hundred watts of 12 Volt DC (direct current) power. An inverter must be used to convert the DC to AC (alternating current) in most cases. Since the sun shines intermittently, it is important to be able to store the electricity by a means such as batteries (see Energy Storage).

Precipitation - average annual amount of rain and melted snow and ice. More precipitation plus a hilly terrain creates better hydropower potential.

PV - photovoltaic

Renewable energy sources - result directly or indirectly from the heat energy radiated by the sun and can be replenished quickly. Direct solar radiation can be used for heating and cooling, or it can be converted to electricity. Indirectly this same energy drives the heat engine that produces wind and water power. It is also stored as plants grow, plants that later can be used for biomass fuel. Since most forms of renewable energy are available intermittently (i.e., when the sun shines or the wind blows), it is necessary to develop superior methods of energy storage.

Solar collector - see flat plate collectors; concentrating collectors.

Solar radiation - kw hours per meter squared; indicates potential solar electricity or BTU's per square foot; indicates potential solar, water or space heating (see also sun angle)

Solar space heating - see passive space heating and active space heating.

Solar thermal electric power - concentrated sunlight is used to boil water, producing steam that rotates a turbine, which in turn powers an electrical generator

Solar water heating - see active water heating systems; batch heaters; thermosiphon systems;

Sun Angle - at noon facing due south. It is necessary to calculate the approximate average sun angle to determine what angle to slant a stationary collector so that the sun's rays are as close to perpendicular as possible on average.

Thermal energy storage - the storage of heat energy. Various substances can be used. These include:
     - water - from small domestic water tanks to large solar ponds or underground reservoir;
     - pebbles or rock;
     - masonry; or
     - phase-change materials.
   Phase change materials - including eutectic salts, paraffin wax, and polyethylene glycol - generally require a much smaller storage volume than rocks, masonry or water. These materials absorb a lot of heat when they melt (change phase from solid to liquid). The heat to melt them can be supplied by solar energy. After the sun sets and the temperature drops, the phase change material cools; when it gets cool enough to freeze it releases the stored solar heat. Phase change materials are most effective for heating air if they melt/freeze in a temperature range of from 70 deg. F to 90 deg. F.
   Water is actually a good phase change material, but its freezing point is far too low (32 deg. F.) to be useful for space heating. Therefore, it is generally used in greater volume to store heat at temperatures well above 32 deg. F., particularly for liquid heating systems.

Thermosiphon systems - here the storage tank is attached directly to the top of the solar collector. As water in the collector heats up, it becomes lighter and rises naturally into the tank above; at the same time water in the storage tank that has cooled flows down into the collector, replacing the heated water that has risen out of it. Thus a natural convection circulation occurs when as water is heated by the sun. Thermosyphon systems, which may be placed on locations other than rooftops, are reliable and relatively inexpensive, but allowance must be made for the weight of the water and - as usual - for freeze protection.

Tidal power - or energy from the ebb and flow of tides results primarily from the gravitational pull of the moon, and therefore is not strictly a form of solar energy. A large tidal power plant has been generating electricity in LaRance, France for over 30 years. In North America, the greatest potentials exist in Maine, New Brunswick and Nova Scotia.

Wave power - an underutilized type of renewable solar energy that combines the energy of wind and water. Wind blowing over long stretches of ocean transfers its mechanical energy to the seawater as it forms waves. Different energy designs use the rising and falling of the waves on our coastlines to generate electricity. These include buoys, pendulums, oscillating water columns, and air turbines. The highest concentration of wave energy is in latitudes between 40o and 60o in each hemisphere.

Wind power - is extracted from air currents by wind turbines with blades that rotate around a horizontal or vertical axis. Both horizontal- and vertical-axis turbines are now commonly used to generate electricity. To be effective, wind turbines mounted on towers high off the ground and located in areas where the wind blows at a constant speed. Many regions, including the Atlantic seacoast, the Great Lakes, the Great Plains and the Rocky Mountains, have enormous wind power potential.

Wind speed - areas with consistently high average wind speeds are good candidates for wind power.