Greentech Media

Thermal energy is the energy content of a system related to raising or lowering an object's temperature. Heat is the flow of thermal energy between two objects caused by a difference in temperature. Grab a hot coffee cup on a cold day and you experience thermal energy in action.

The British Thermal Unit (BTU or Btu) is commonly used to describe the energy content of fuels and the power of heating and cooling systems. One BTU is the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit. There are several different definitions of the BTU based on the initial water temperature, but in general, one BTU equals about 1,055 J, about 780 ft-lb and about 0.3 watt-hours.

Combustion transforms the chemical energy of the fuel into thermal energy or heat. Burning No. 2 heating oil yields nearly 138,000 BTU per gallon. Burning a pound of coal produces about 15,000 BTU; burning a cubic foot of natural gas, about 1,000 BTUs. It takes roughly 95,000 BTU/h to heat a 2,000-square-foot house in New England.

One of the problems faced by biofuel proponents is ethanol's lower energy content compared to gasoline. A gallon of gasoline contains about 115,000 BTU, while a gallon of ethanol contains around 80,000 BTU. Thus, burning ethanol produces less mechanical energy than burning gasoline, and cars get fewer miles per gallon. With E10 fuel (10 percent ethanol, 90 percent gasoline), the mileage reduction is negligible. With E85 (85 percent ethanol, 15 percent gasoline), drivers see mileage reductions of at least 15 percent. Some auto manufacturers are installing larger fuel tanks, so that the range of their flexible-fuel vehicles is similar to gasoline vehicles.

Other thermal-energy units include the calorie, the therm and the quad. The small or gram calorie (cal) is the amount of energy needed to increase the temperature of one gram of water by one degree Celsius. The large or kilogram calorie (kcal) is the energy needed to raise the temperature of one kilogram of water by 1 °C. Like the BTU, the calorie has different values depending on initial water temperature. On average, one cal equals about 4.18 J, and one kcal equals about 4.18 kJ or nearly 4 BTU. Nutritional calories are based on kilogram calories.

The therm (thm) equals 100,000 BTU and is approximately equal to the amount of energy released by burning 100 cubic feet of natural gas.

The quad equals a quadrillion (1015) BTU and is used when discussing the energy budget of whole countries. In 1950, the U.S. consumed 34.6 quad of energy. By 1970, total consumption grew to 67.8 quad; by 1990, 84.7 quad; and, by 2006, 99.9 quad. The amount attributable to renewable energy sources - hydro and biomass - in 1950 was 8.6 percent. By 2006, renewable-energy consumption - hydro, biomass, geothermal, solar and wind - dropped to 6.9 percent of the total.

Thermal power is measured in BTU per hour (BTU/h), often abbreviated to just BTU. Most heating and cooling ratings in BTU are really BTU/h. One watt equals about 3.41 BTU/h. One horsepower equals more than 2,500 BTU/h.

Cooling power is often rated in tons. One ton of cooling is the amount of power needed to melt one ton of ice in 24 hours and equals 12,000 BTU/h. A typical home's central air-conditioning system is rated at 4 to 5 tons (48,000 to 60,000 BTU/h). Room air conditioners run anywhere from 5,000 to 15,000 BTU/h.

The U.S. Department of Energy currently enforces a seasonal energy-efficiency rating (SEER) standard of 13 for new residential central air conditioners. SEER is defined as total cooling output in BTUs divided by total energy input in watt-hours (SEER = BTU / w·h). By raising the SEER standard from 10 to 13, the DOE expects the U.S. to save 4.2 quads of energy between 2006 and 2030, with a parallel reduction in greenhouse-gas emissions.