Glossary of Common Solar Energy Terms
Active Solar Thermal - technologies, which use fans or pumps
to convert solar energy into usable heat or cooling (thermal
energy). Active solar technologies use electrical or mechanical
equipment, such as pumps and fans, to move the heat transfer
fluid, air, water, anti-freeze, etc. from the surface where
it is collected to the location where it is used to heat,
cool, or dry something else.
Balance of System (BOS) – All the
non solar collector components needed to harness the solar
energy that falls on the collector surface. For a solar air
system this usually includes a fan, ducts, and a simple thermostat.
For water based heating systems this usually includes a pump,
piping, antifreeze, a storage tank, thermostat and heat exchangers
to transfer the heat to domestic water or space heat. For
a solar electric system it usually includes wiring, an inverter
to convert direct current to alternating current, electrical
switch gear to connect to the existing electric grid or building
power panels or both, and sometimes batteries and battery
charging and monitoring equipment to store electricity.
Btu – British thermal unit,
a measure of heat. One Btu is the amount of heat energy needed
to raise the temperature of one pound of water by one degree
F. This is the standard measurement used to state the amount
of energy that a fuel gives off when burned as well as the
amount of output of any heat generating device.
You might be able to imagine it this way. Take one gallon
(~8 pounds) of water and put it on your stove. Assume that
the water starts at 60 degrees F and you want to bring it
to a boil (212 degrees F.). To reach the boiling point, you
will need about 1,200 Btu's.
Here’s the math: You need to raise each pound of water
152 degrees F. (212-60 = 152) and you have to raise temperature
for 8 pounds, requiring 1,216 Btu's (152 x 8 = 1216).
To give some examples,
- It takes about 50 BTU to take a cup of cold tap water to hot
- It takes about 600 BTU to warm a gallon of cold water
to hot water for sinks and showers.
- A typical home furnace in the central United States
produces about 100,000 BTU in one hour.
- One square foot of well oriented solar roof (or wall,
or collector) can produce about 100,000 BTU in one winter
and 200,000 BTU in one year.
Concentrated Solar Thermal Power Systems – Technologies
using the sun’s heat to produce electricity. These systems
typically concentrate the sun’s heat to boil water or
another liquid at high temperatures. The steam from the boiling
water rotates a large turbine, which spins a generator that
Heat Engine - A device that converts thermal energy
to mechanical output. The mechanical output is called work
and the thermal energy input is called heat. Heat engines
typically run on a specific thermodynamic cycle (Rankine,
Stirling, etc.). Heat engines are often named after the thermodynamic
cycle by which they are modeled. They often pick up alternate
names, such as gasoline/petrol, turbine, or steam engines.
Passive Solar - Technologies capture sunlight for heat energy
without use of active mechanical systems such as fans or pumps
(as contrasted to active solar). Such technologies convert
sunlight into usable heat (water, air) or store
heat for future use, with little or no use of other energy
sources. A common example is a solarium on the sunny side
of a building. Passive cooling is the use of the same design
principles (no fans or pumps) to block solar heating to reduce
summer cooling requirements.
R-Value - A measure of thermal resistance used in the building
and construction industry. A bigger number means the building
insulation is more effective. The units of R-value are (hr
* degree F*ft squared)/Btu. You can interpret R-Value units
as the number of hours it takes for heat to flow across one
square foot of assembly, where there is a 1 degree F temperature
differential between the inside and outside temperatures.
If an assembly of several components has a higher R-value
it means that it poses more resistance to the flow of heat.
A solar re-roof creates a warm solar air space above the old
roof, which helps to increase the R value of the overall roof
Solar Cell or Photovoltaic Cell - An electronic device that
converts solar energy into direct current electricity by the
photovoltaic effect. Photovoltaics is the field of technology
related to the application of solar cells for solar energy.
Assemblies of cells are used to make solar modules, or photovoltaic
arrays, which generate electricity.
Solar Energy - The radiant energy from the Sun that influences
Earth's climate and weather and sustains life. It includes
visible light and several non-visible frequencies such as
ultraviolet light. All of these frequencies can be used to
produce heat using solar thermal techniques and many can be
used to produce electricity using photovoltaics.
Solar Energy Technologies - Convert solar energy to useful purposes.
They can provide: electrical generation by heat engine or
photovoltaic means; space heating and cooling in active and
passive solar buildings; potable water via distillation and
disinfection; daylighting; hot water; thermal energy for cooking;
and high temperature process heat for industrial purposes.
Solar Heat Gain - Measurement of the amount of solar heat that is
captured for use by a solar collector surface including a
solar roof, solar siding or ground-mounted solar system. A
typical measure is Btu per hour per square foot.
Solar Heated-Air - Air heated by the use of solar energy.
Solar Hot Water - Water heated by the use of solar energy.
Solar Power - Sometimes used as a synonym for solar energy
or more specifically to refer to electricity generated from
Solar Thermal Power Systems – Technologies using the
sun’s heat to produce electricity or mechanical power.
There are two kinds of systems. The most common is Concentrating
Solar Thermal Power. A new system uses low temperature solar
heat (without concentrating sunlight) to boil a special liquid
that passes through a heat engine driving a generator to produce
Transpired Solar Collector - A specialized solar collector
that passes air through holes or openings in metal panels.
Sunlight heats the metal panels. The outside air is heated
as it moves through the openings in the panels. Other systems
such as solar roofs of rustic shakes, shingles or tiles provide
a similar effect but are not usually referred to as transpired
Solar Ventilation Preheating - Solar preheating
outdoor air before it enters the ventilation system of a building.
Most large buildings require a large volume (thousands of
cubic feet per minute) of outdoor air to be continuously pulled
into the building to maintain acceptable indoor air quality.
During the long heating season in most of North America, the
air needs to be heated to comfort levels before being distributed
through the building. Many buildings do this with expensive
gas, oil, or electric heating systems. Even efficient modern
buildings often use expensive electric re-heat systems at
the ventilation terminals.
Large buildings often have large roofing or siding areas.
These solar roofing or siding surfaces can provide 1-2 cubic
feet per minute of solar preheated air for every square foot
of roof or siding. This approach to solar ventilation preheating
uses existing building components to produce low cost solar
heat and reduces the need for expensive electric or fossil
fueled heating, saving energy and money.
U-Value - Measurement of the heat flow that occurs during
one hour through one square foot of material , when there
is a 1 degree F temperature difference between the inside
and outside air temperatures. The units are Btu/(hr *degreesF*ft
squared ). If an assembly of several components has a higher
U-value, it means that it loses more heat than one with a