Thermodynamics

the food to the energy consumed by the appliance (Fig. 2–56). Electricranges are more efficient than gas ranges, but it is much cheaper to cookwith natural gas than with electricity because of the lower unit cost of naturalgas (Table 2–2).The cooking efficiency depends on user habits as well as the individualappliances. Convection and microwave ovens are inherently more efficientthan conventional ovens. On average, convection ovens save about one-thirdand microwave ovens save about two-thirds of the energy used by conventionalovens. The cooking efficiency can be increased by using the smallestoven for baking, using a pressure cooker, using an electric slow cooker forstews and soups, using the smallest pan that will do the job, using thesmaller heating element for small pans on electric ranges, using flat-bottomedpans on electric burners to assure good contact, keeping burner drippans clean and shiny, defrosting frozen foods in the refrigerator beforecooking, avoiding preheating unless it is necessary, keeping the pans coveredduring cooking, using timers and thermometers to avoid overcooking,using the self-cleaning feature of ovens right after cooking, and keepinginside surfaces of microwave ovens clean.Using energy-efficient appliances and practicing energy conservationmeasures help our pocketbooks by reducing our utility bills. It also helpsthe environment by reducing the amount of pollutants emitted to the atmosphereduring the combustion of fuel at home or at the power plants whereelectricity is generated. The combustion of each therm of natural gas produces6.4 kg of carbon dioxide, which causes global climate change; 4.7 gof nitrogen oxides and 0.54 g of hydrocarbons, which cause smog; 2.0 g ofcarbon monoxide, which is toxic; and 0.030 g of sulfur dioxide, whichcauses acid rain. Each therm of natural gas saved eliminates the emission ofthese pollutants while saving $0.60 for the average consumer in the UnitedStates. Each kWh of electricity conserved saves 0.4 kg of coal and 1.0 kg ofCO 2 and 15 g of SO 2 from a coal power plant.Efficiency =Chapter 2 | 815 kW3 kW2 kWEnergy utilizedEnergy supplied to appliance3 kWh= = 0.605 kWhFIGURE 2–56The efficiency of a cooking appliancerepresents the fraction of the energysupplied to the appliance that istransferred to the food.TABLE 2–2Energy costs of cooking a casserole with different appliances*[From A. Wilson and J. Morril, Consumer Guide to Home Energy Savings, Washington, DC:American Council for an Energy-Efficient Economy, 1996, p. 192.]Cooking Cooking Energy Cost ofCooking appliance temperature time used energyElectric oven 350F (177C) 1 h 2.0 kWh $0.16Convection oven (elect.) 325F (163C) 45 min 1.39 kWh $0.11Gas oven 350F (177C) 1 h 0.112 therm $0.07Frying pan 420F (216C) 1 h 0.9 kWh $0.07Toaster oven 425F (218C) 50 min 0.95 kWh $0.08Electric slow cooker 200F (93C) 7 h 0.7 kWh $0.06Microwave oven “High” 15 min 0.36 kWh $0.03*Assumes a unit cost of $0.08/kWh for electricity and $0.60/therm for gas.