[pagebreak:Energy and Power Units: The Basics]
If you're learning about greentech, especially renewable energy, you can't help running into statements like these:
- A 26-watt CFL produces light equivalent to a 100-watt incandescent bulb.
- The energy value of a gallon of ethanol varies from 75,700 BTU to 84,000 BTU.
- The Toyota Prius Hybrid Synergy Drive includes a 67-horsepower electric motor.
But what are watts, BTUs and horsepower? What do they measure and how do they relate to renewable energy? How many watts, for example, does a wind turbine generate and how many homes will that power? How many BTUs does it take to heat an average house and how much natural gas does that require?
Before you can answer such questions, you have to master some basic concepts and vocabulary:
- What are energy and power and how do they relate to each other?
- What are the standard units of energy and power used by scientists?
- What are the traditional units used in industry, and how do they relate to the standard units?
- How do the various units apply to applications like lighting, heating and transportation?
This report is a quick review of energy, power and the units used to measure them. But don't worry; it's not high school physics all over again. It's more like a Berlitz course on energy-speak - just enough to let you read the menu and maybe eavesdrop on the natives.
Here's a list of the contents:
- Page 2 - SI: The International System of Units
- Page 3 - Energy 101: Joules, Watts & Newtons
- Page 4 - Mechanical Energy: Foot-Pounds & Horsepower
- Page 5 - Electrical Energy: Volts, Amps & Kilowatts
- Page 6 - Thermal Energy: BTUs, Calories & Tons
- Page 7 - Comparing Units & Conversion Factors
- Bob Bellman is a freelance technology writer and marketing consultant.
[pagebreak:SI: The International System of Units]
Over the centuries, scientists followed diverse paths as they investigated energy and power. So each type of energy - electrical, mechanical, chemical, thermal and nuclear - acquired its own system of measurement, and each energy-related industry developed its own terminology. Car dealers talk about horsepower. HVAC contractors install tons and BTUs. Electric utilities deliver kilowatt-hours. Scientists refer to newtons and joules.
In 1960, the International System of Units (SI) was derived from the metric system to provide a standard vocabulary for all things physical. The SI is built on seven base units (see Table 1) from which all other physical quantities can be derived. Table 2 lists some standard derived units. For example, a newton (derived unit of force) is defined as one kilogram (base unit of mass) accelerated at the rate of one meter (base unit of length) per second (base unit of time) squared. Table 3 lists some standard prefixes used to denote multiples and fractions of units. For example, a megawatt (MW) is equal to a million (106) watts; a milliwatt (mW) is one-thousandth (10-3) of a watt.
Energy-related industries are beginning to use SI terminology, but traditional terms still dominate. Many car companies now list engine power in kilowatts, but in parentheses after the horsepower rating: 187 hp (140 kW). Starting with a brief tutorial on energy, power and force, the next few sections review the units used most commonly in renewable energy applications.
[pagebreak:Energy 101: Joules, Watts & Newtons]
Simply put, energy is the capacity to do work (W) - everything from propelling an automobile to heating a home to lighting a room. Many forms of work involve an energy transformation. A light bulb transforms electrical energy into heat and light energy. An internal-combustion engine converts chemical energy into heat and mechanical energy. A dynamo changes mechanical energy into heat and electrical energy.
Renewable-energy solutions exploit energy sources that will not be exhausted by these transformations and reduce energy consumption by making the transformations more efficient. Photovoltaic (PV) panels create electricity from sunlight instead of burning nonrenewable fossil fuels. Compact fluorescent light (CFL) bulbs use less energy than incandescent bulbs because they convert more electricity into light and less into heat.
Since energy and work are two sides of the same coin, they are measured with the same units. The SI unit of energy/work is the joule (J), named for English physicist James Prescott Joule (1818 - 1889). Joule discovered the relationship between heat and mechanical work, which led to the development of the laws of thermodynamics.
One joule equals the work done by a force of one newton moving an object one meter (J = N · m). That's about the amount of energy required to lift a small apple one meter against Earth's gravity. One joule also equals the energy required to move an electric charge of one coulomb through an electric potential difference of one volt (J = C · V).
Power (P) is the rate at which energy is transferred or converted. Thus, power equals work divided by time (P = W / t). The SI unit of power is the watt (W), in honor of Scottish inventor James Watt (1736 - 1819). Watt's improvements to the steam engine helped to launch the Industrial Revolution. Ironically, Watt himself coined the term "horsepower" to characterize the benefits of his steam engine.
One watt equals one joule per second (W = J / s). A person climbing stairs does work at a rate of about 200 W. In electrical applications, one watt equals one volt times one ampere (W = V · A). Incandescent light bulbs use electrical energy at a rate of 40 to 150 W.
Force is rarely mentioned in renewable-energy conversations, except in a nonscientific sense: "The high price of gasoline is forcing me to walk to work." Still, force is an important concept. Physicists have identified four fundamental forces or interactions: The electromagnetic force acts between electric charges, the gravitational force acts between masses and the strong and weak forces hold together atomic nuclei. The push and pull of these forces manifest as energy. For example, the electromagnetic force pulls electrons through a conductor, creating electric current. Gravity pulls water through the turbines at a hydroelectric plant.
The SI unit of force is the newton (N), in honor of English physicist Sir Isaac Newton (1643 - 1727). Newton, the person, is felt by many to have had the greatest impact on the history of science, beating out even Albert Einstein. Newton, the unit, is the force that accelerates a one kilogram mass at the rate of one meter per second squared (N = kg · m / s2). The force of Earth's gravity on a 70 kg (154 lb) human is about 686 newtons.
[pagebreak:Mechanical Energy: Foot-Pounds & Horsepower]
Of all the forms of energy, mechanical energy is probably the easiest to understand-just try lifting a heavy suitcase. Thus the traditional unit of mechanical energy is the foot-pound (ft-lb), the amount of work required to move a one pound object through a distance of one foot. One foot-pound equals about 1.36 J. The metric analogy to the foot-pound is the newton meter (N·m). One newton meter equals one joule.
Probably the most familiar unit of mechanical power is the horsepower (hp), conceived by James Watt in 1782 to position his steam engine against the competition. Watt determined that an "ideal" mine pony could lift a 33,000 lb bucket of coal one foot in one minute and defined mechanical horsepower accordingly.
Although 33,000 ft-lb/min sounds like a lot, the horsepower is a relatively small unit, equal to about 746 W. A toaster oven draws about 1,000 W (1.3 hp), and it takes at least 5 hp just to spin the blade on a power lawnmower. The four-cylinder engine in a 2007 Honda Accord sedan generates 166 hp; the 12-cylinder engine in a new Rolls-Royce Phantom produces 453 hp.
Greentech companies are addressing mechanical energy issues on multiple fronts. Biofuels, hybrid gasoline/electric engines, plug-in hybrids and other technologies reduce the amount of greenhouse gases produced in the creation of mechanical energy. They also help to wean automobiles and other machinery from fossil fuels. The hybrid engine in a Toyota Prius uses less gasoline than a conventional engine because its internal combustion side generates only 76 hp.
Materials research is further reducing mechanical energy costs. Remember, work equals weight times distance. As much as 50 percent of the Boeing 787 Dreamliner is made of lightweight composite materials. This, along with increased engine efficiency, allows the 787 to use 20 percent less fuel than other airplanes of similar size.
[pagebreak:Electrical Energy: Volts, Amps & Kilowatts]
Electrical energy is less intuitive than mechanical energy because it acts invisibly. The closest analog to lifting a heavy suitcase is the force you feel when you play with magnets.
Electrical energy is based on the attraction and repulsion of charged particles, i.e., the electromagnetic force. The strength of the charges and the distance between the particles combine to create an electric potential difference or voltage. In electrical applications, voltage pulls electrons through a conductor to create a current, not unlike gravity pulling water molecules through a pipe.
The standard unit of electric charge is the coulomb (C). Charles-Augustin de Coulomb (1736 - 1806) was a French physicist who discovered the relationship between electric charges, distance and force. A coulomb is the amount of charge transported by a current of one ampere in one second (C = A · s) and is a surprisingly large unit. The repulsive force between two +1 coulomb charges held one meter apart is 9 x 10 9 N, or more than a million tons! Thus, charge is most often measured in micro- or nanocoulombs.
The standard unit of electric potential is the volt (V), in honor of Count Alessandro Volta (1745 - 1827), known for his development of the electric battery. A volt is equivalent to one joule of energy per coulomb of charge (V = J / C). Household electric service in the U.S. is typically 110 V, although 220 V may be used for heavy appliances. A common flashlight battery delivers 1.5 V, while lightning can be around 100 MV. Long-distance transmission lines operate at 110 to 1,200 kV.
The standard unit of electric current is the ampere (A), or amp. French physicist André-Marie Ampère (1775 - 1836) was one of the main discoverers of electromagnetism. One ampere equals the displacement of one coulomb of charge per second (A = C / s). Most household circuits pull less than 15 A.
Most electric power is produced by burning fossil fuels. PV, wind turbines and other technologies offer clean, renewable alternatives, but they have a long way to go to replace existing generating plants. In 2006, fossil fuel-burning plants in the U.S. generated 2,874 billion kWh, and nuclear plants generated 787 billion kWh. All renewable-energy sources put together generated 385 billion kWh, less than 10 percent of total U.S. production.
Part of the challenge is a matter of scale. A large oil-, gas- or coal-burning plant cranks out 2 to 3 GW at full capacity. Most concentrating solar installations generate tens of megawatts, while an up-to-date wind turbine generates around 3 MW. The proposed Cape Wind project needs 130 turbines to provide just three-quarters of Cape Cod's electricity. A typical grid-tied home photovoltaic system produces less than 6 kW.
On the other hand, plenty of renewable energy is available if we just can figure out how to put it to use. The amount of energy in the sunlight that falls on one square meter of the Earth's surface is roughly one kW per second, or 3,600 kW per hour. Refrigerators and toaster ovens pull 1.0 to 1.5 kW each. Incandescent light bulbs draw from 40 to 150 W, while CFLs deliver the same amount of light at 10 to 40 W. Altogether, an average U.S. home uses about 1,000 kWh per month, a tiny fraction of the sun power that hits its roof.
[pagebreak:Thermal Energy: BTUs, Calories & Tons]
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.
[pagebreak:Comparing Units & Conversion Factors]
Because of their diverse heritage, energy and power units vary widely in size. Figure 1 graphs energy units, while Figure 2 graphs power units. Note that the vertical scale in both graphs is logarithmic; each horizontal line represents a tenfold increase over the line below.
Tables 4 and 5 list conversion factors between selected units of energy and power.