Types of Renewable and Alternative Energy

There are several renewable energy sources that are in use today. Listed below are brief descriptions of these
resources; later we will discuss how some of these can be used in residential applications.

Hydropower:

Hydropower represents one of the oldest and largest renewable power sources and accounts for close to 10%
of our nation’s electricity. Existing hydropower capacity is about 80,000 megawatts (MW – one million watts or one
thousand kilowatts). Hydropower plants convert the energy of flowing water into electricity. This is primarily
done by damming rivers to create large reservoirs and then releasing water through turbines to produce electricity.
Hydropower results in no emissions into the atmosphere but the process of damming a river can create significant
ecological problems for water quality and for fish and wildlife habitat.

Biomass

Biomass is second to hydropower as a leader in renewable energy production. Biomass has an existing capacity of over 7,000 MW. Biomass as a fuel consists of  organic matter such as industrial waste, agricultural waste, wood, and bark. Biomass can be burned directly in specially designed power plants, or used to replace up to15% of coal as a fuel in ordinary power plants. Biomass burns cleaner than coal because it has less sulfur, which means less sulfur dioxide will be emitted into the atmosphere.
Biomass can also be used indirectly, since it produces methane gas as it decays or through a modern process called gasification. Methane can produce power by burning in a boiler to create steam to drive steam turbines or through internal combustion in gas turbines and reciprocating engines.
The largest use of biomass energy in Virginia is the forest products industry. Furniture plants, sawmills, and paper mills usually burn their wood waste to produce heat and electricity. Many homeowners use firewood or pellets
for winter heat.

Geothermal

Geothermal electric capacity in the United States is over 3,000 MW. Geothermal power plants use high temperatures deep underground to produce steam, which then powers turbines that produce electricity. Geothermal power plants can draw from underground reservoirs of hot water or can heat water by pumping it into hot, dry rock.High underground high temperatures are accessed by drilling wells, sometimes more than a mile deep. In one
sense, this geothermal energy is not renewable, since sometime in the future the core of the earth will cool.
That time is so far off (hundreds of millions of years) that that we think of it as renewable. Most geothermal power
plants are located in the western United States, but some costal regions of Virginia (near Wallops Island) have
geothermal power potential.
Geothermal heat pumps use compressors to pump heat out of the earth (for winter heating) or into the earth (when running as air conditioners in summer). The energy they pump into and out of the earth is renewable, since it is
replaced by the cycle of the seasons. The energy that runs the compressor can either be renewable or conventional.

Solar Energy

Solar energy comes directly from the power of the sun and is used to produce electricity, to produce heat, and
for light. Solar represents a small share of the electric market in the United States – about ½ of one percent of
electrical capacity. Solar’s contribution to heating and lighting is much larger.
Solar-electric power can be produced either by power plants using the sun’s heat or by photovoltaic (PV) technology, which converts sunlight directly to electricity using solar cells. PV technology is more practical for residential use.
Systems to use the heat of the sun directly can be either active or passive. In active systems, air or liquid
circulate through solar collectors and bring heat to where it is used. In passive systems, buildings are built with
windows and heat-absorbing surfaces set up to maximize solar heating in winter. Either technology is suitable for
residential use.
Systems to directly use the light of the sun are most common. The most usual device for using sunlight is the
window, but skylights and skylight tubes are also used.

Wind Power

Wind energy represents 4,700 megawatts (MW) of installed electric capacity in the United States. Wind has been the fastest growing energy source in the U.S. over the last decade mainly due to very significant improvements
in wind energy technology. The American Wind Energy Association predicts that 6,000 MW of windpower
will be installed by the end of 2004. This is enough to power 1.5 million homes. Wind power is produced by the
energy of the wind turning aerodynamic blades mounted to a hub. The hub is connected to a shaft that turns a generator.
Large utility-scale wind turbines range in size from 50 kilowatts to over four megawatts. Smaller wind towers
(under 50 kW) are suitable for residential and agricultural use.

Fuel Cells

A fuel cell is an alternative energy device, but it is not necessarily a renewable energy device. It is only
renewable if the source of the fuel used is renewable. A fuel cell is an electrochemical device, like a battery in that it converts the energy from a chemical reaction directly into electricity and heat. But unlike a battery, which is
limited to the stored chemicals within, a fuel cell has the capability of generating energy as long as fuel is supplied.
Currently produced fuel cells combine hydrogen and oxygen without combustion to produce electricity. The
oxygen comes from the air, while the hydrogen can either be produced from water (using electricity) or extracted
from fossil fuels. New fuel cells are being developed that can use fossil fuels directly. Fuel cell technology has been
around for over 150 years and it shows great promise in powering vehicles and in providing energy for residential
applications.