Energy Conversion Chuck Carpenter, W5USJ Conversion of energy forms is often needed when you install off-the-grid power systems. You may want to make cost justifications using alternate systems. Or, you may want to know what's going on in an existing system. Regardless of the reason, you will want to make your conversions using known standards. Energy, Force, Work and Power These terms establish the basis for their electrical equivalents. Energy is the capacity to do work. Force is a factor of work and, to do work, has to be applied over a distance. Work is done when energy is used and is calculated as the product of the magnitude of a force and the distance traveled in overcoming a resistance. Power is the rate of doing work. Our applications of these physical basics are primarily to generate electricity. Electrical Work Electrical force (often called electromotive force or EMF) exists between the terminals of a battery or generator. When there is no load connected to the terminals, the resistance of the air is too great; no current flows and no work is done. By connecting a load (e.g., lamps, motors, electrical appliances), across the terminals, the electrical force is able to overcome the resistance of the wires connected to the load. Current will flow, and electrical work is done. In the case of the light, the work is in the form of heat and light. Work by the motor is done by turning its shaft and the load connected to the shaft. Electrical Power Power is the rate that work is done, and is independent of the total amount of work accomplished. Using the " / " to indicate division, mechanical power is expressed as: Power = work / time or P=W / T For electrical power, then, the expression would be: Electrical power = electrical work / time The unit of electrical power is the watt. Using " x " to indicate multiplication the expression is: Watts = volts x amperes Using conventional symbols, electrical power (P) equals the electrical pressure in volts (E) times current in amperes (I), and is expressed as follows: P = E x I For example, with an EMF of 110 volts (E), and a current flow of 10 amperes (I), then: P = 110 x 10 = 1,100 watts Electrical Horsepower Motors are generally rated in horsepower or fractional horsepower. The relationship of mechanical horsepower to electrical horsepower shows that one foot-pound of force is equal to 1.356 watts. Also, it is known that 550 foot-pounds per second are equivalent to 1 mechanical horsepower. The equivalent rate of electrical power would be: 1 electrical horsepower = 550 x 1.356 = 746 Watts If you have a motor that has a rating of 220 volts and 20 amps, then you can determine its horsepower (H.P.) by the following relationships. H.P. = watts / 746 = volts x amperes / 746 = (220 x 20) / 746 = 5.898 Kilowatt Hours A value of 1,000 is represented by the letter "K" (for kilo). If you maintain a power level of 1,000 watts for one hour, your load has used 1 kilowatt hour (KWH). It is also equal to 500 watts used for 2 hours, or 2,000 watts used for one-half hour. The value of 1,100 watts from the previous example is equal to 1.1 KWH. The KWH is the unit of measure found on the electric bill of (us) grid customers. British Thermal Units Heating appliances often use ratings that include British Thermal Units or BTUs. A BTU is the quantity of heat needed to raise the temperature of 1 pound of water one degree Fahrenheit. Don't confuse quantity with intensity. A cup of water at 150 deg. F will contain less heat that a pail- full at 70 deg. F. The BTU is also equal to 778 foot-pounds, 1055 watt seconds, and 0.000293 kilowatt hours. Fuel Conversion Having gotten through all the electrical descriptions and conversions, how would you convert from one form of energy to another? More specifically, what do you do to convert from the various types of fuel to other energy uses. Examples would be (1) gasoline, natural gas and propane powered generators to KWHs, (2) electricity to BTUs, and (3) natural gas or propane to BTUs. The various conversions you might use depend on your application. Gasoline is not normally used as a fuel in the sense that natural gas and propane are. Therefore, there does not seem to be a direct multiplier for conversion. However, most gasoline driven generators indicate average fuel consumption related to load and time. You can check the specifications of a particular generator for these values. For example, a 5 KW generator may use 1.5 gallons for one hour at full load. That's 5 KWH at the cost of one and a half gallons of gas. Using previous information you can convert 5 KWH to horsepower, or to BTUs from the following information. Natural gas is generally converted to BTUs at 1,000 BTUs per cubic foot. If a generator uses 100 cubic feet per hour to generate 5 KWH, the cost of 1 KWH is equal to the cost of 20 cubic feet of gas. Note that some sources use the value of 750 BTUs per cubic foot. Propane converts to BTUs at 2,500 BTUs per cubic foot. The calculations would be the same as natural gas. However, the fuel use rate will be less because of the higher conversion factor. It does not happen that you get an improvement of 2.5 times, however. The efficiencies are not the same, and the cost of a cubic foot of propane is higher than a cubic foot of natural gas. Electricity converts to BTUs at 3413 per kilowatt hour. In the example above, 5 KWH times 3413 is equal to 17,065 BTUs. If the cost of electricity is 0.08 per KWH, then the cost of 5 KWH is $0.40 and you can relate this to the cost of energy in BTUs. Usually, these costs are figured in systems that use millions of BTUs (like my gas heating and electrical cooling system). Fuel Oil conversion to BTUs is another useful quantity. The standard unit is 100,000 BTUs per gallon. Cost Estimating Don't forget to include all of the factors involved in your cost estimates. Fuel used in a combustion engine is more efficient than in a furnace where up to 30% of the heat goes up the chimney. Electricity to heat conversion is considered to be 100%. And, you have all the costs of facilities, installation, maintenance, supplies, and replacement to consider too. ACCESS Chuck Carpenter, 3714 Bishop Hill, Carrollton, TX 75007 ¥ 214-306-8140.