Preferences between the gas and the electric stove for cooking are mostly a matter of personal taste. (However, gas certainly has an advantage when electrical power has failed.) Here, we will look only at how to determine the economics of each. The efficiency with which heat is conducted (or convected) to the food varies with every model of stove and generalizations cannot be made concerning the difference between gas and electric. Therefore, we will compare only the energy consumption of each and estimate the cost per unit energy consumed by the stove.
The heat of combustion for propane specified by most commercials suppliers is 50 MJ/kg. Since most propane companies sell propane by the pound, we will convert this value to kWhr/lb. A standard 100 lb tank of liquid propane has a volume of 23.1 gallons at a cost of $54 here on St. Thomas. (Thanks to Antilles Gas for the information.) Using this information, you should be able to show that the heat of combustion of propane is 6.3 kWhr/lb. Using $.54/lb for propane here in the VI, the energy cost is about $.09 /kWhr. That is indeed cheaper than the approximately $.14/kWhr for electricity here on St. Thomas. However, the cost of both electricity and propane changes significantly within the Caribbean and the World. You will have to determine the economics of each based upon the costs in your area.
Here in the Caribbean, gasoline-powered electric generators are quite common. The frequency of large storms and the subsequent loss of electricity from the public utilities has encouraged many homeowners (and renters as well) to invest in these types of generators. Perhaps it would be more economical to use a private generator at all times? Let's look at the cost.
The heat of combustion for gasoline is approximately 43 MJ/kg, not significantly differenct from that of propane. However, the efficiency of the gasoline engine is critical. The greatest possible "ideal" efficiency for any engine using the Otto cycle, which is similar to that for a gasoline engine, is about 50%. In practice, the efficiency runs from about 15% for small engines to 30% for larger ones. That means that, at best, we can expect to extract about .3 x 43 = 13 MJ/kg from gasoline and convert it into electrical energy. The density of gasoline is 800 kg/m3 = 3.02 kg/gallon. Using these numbers, you should be able to show that the effective heat of combustion for gasoline is 5.03 kWhr/gallon. To calculate the cost, we need the cost of a gallon of gasoline. Now that's a serious variable. Gas prices vary by a factor of two even within the Caribbean. If the cost is $2.00 per gallon, then electric generator electricity would cost about $.40 per kWhr! Even at half that price, it is clearly more expensive than commercial power. We have not factored into account the initial cost, maintenance and lifetime expectancy for these generators. That subject is left for the special projects.
A Detailed Look at Hydrocarbons
Hydrocarbons are an important class of compounds formed with hydrogen atoms attached to a carbon backbone. The general chemical formula for hydrocarbons is CnH2n+2. The simplist hydrocarbon is methane or natural gas. For methane, n =1 and the chemical formula is CH4. The next three hydrocarbons are ethane (C2H6), propane (C3H8), and then butane (C4H10). The structure diagrams are shown below. Note that each carbon forms four single bonds and each hydrogen just one single bond. You should review the location of carbon and hydrogen on the periodic chart and convince yourself why this works.
The pattern seems to suggest that the backbone could be extended indefinitely. This is esssentially correct. Gasoline is a mixture of hydrocarbons from n = 5 to n = 12. Kerosene, diesel, and lubricating oils follow next. The first four hydrocarbons are gaseous at room temperature, while gasoline through oil are liquid. Paraffin (n = 20) and asphalt (n = 40 and up) are solid.
Essentially all hydrocarbon products are produced from petroleum or crude oil extracted from the earth. The origin of petroleum is believed to be from the anaerobic decomposition of marine plankton and algae. The crude oil is a variable mixture of many hydrocarbons and other chemicals. Separation is achieved with a cracking plant.
Methane gas is not readily available in the Caribbean. Propane gas, on the other hand, can be liquified under pressure and stored in steel tanks and bottles. This makes it economical to manufacture propane and compress it into a liquid for shipping. Butane can be maintained as a liquid at even lower pressures and can be safely held in plastic butane lighters.
Heat energy is released when any of these hydrocarbons "burn" or oxidize. The reaction for methane is
The reaction for butane is similar to that of methane, but more complicated because of the many variations. Essentially, the backbone breaks apart into many different molecular combinations, mostly of lower hydrocarbons. If the propane stove is properly adjusted, all the constituents will be oxidized and the products will be only carbon dioxide and water. The flame will "burn clean" and will be blue or colorless. The "net" reaction would be
If the stove is not properly adjusted, the correct amount of oxygen will not be entrained into the gas as it burns. As a result, there will be un-oxidized products. The flame will look yellowish and black soot (the unoxidized products) will form on cooking pans.
The burning of gasoline is far more complicated. First, there are many different hydrocarbons. Second, those hydrocarbons burn too rapidly to work efficiently in an engine. For many years, lead additives were introduced to slow the burn rate of the hydrocarbons. But concern over pollution has led to another method. The long straight chain molecules are converted into "branched" molecules by a high temperature catalytic process. The branched hydrocarbons burn at a slower rate.
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