Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

Chapter 2 | 81

the food to the energy consumed by the appliance (Fig. 2–56). Electric
ranges are more efficient than gas ranges, but it is much cheaper to cook
with natural gas than with electricity because of the lower unit cost of nat-
ural gas (Table 2–2).
The cooking efficiency depends on user habits as well as the individual
appliances. Convection and microwave ovens are inherently more efficient
than conventional ovens. On average, convection ovens save about one-third
and microwave ovens save about two-thirdsof the energy used by conven-
tional ovens. The cooking efficiency can be increased by using the smallest
oven for baking, using a pressure cooker, using an electric slow cooker for
stews and soups, using the smallest pan that will do the job, using the
smaller heating element for small pans on electric ranges, using flat-bot-
tomed pans on electric burners to assure good contact, keeping burner drip
pans clean and shiny, defrosting frozen foods in the refrigerator before
cooking, avoiding preheating unless it is necessary, keeping the pans cov-
ered during cooking, using timers and thermometers to avoid overcooking,
using the self-cleaning feature of ovens right after cooking, and keeping
inside surfaces of microwave ovens clean.
Using energy-efficient appliances and practicing energy conservation
measures help our pocketbooks by reducing our utility bills. It also helps
the environmentby reducing the amount of pollutants emitted to the atmo-
sphere during the combustion of fuel at home or at the power plants where
electricity is generated. The combustion of each therm of natural gaspro-
duces 6.4 kg of carbon dioxide, which causes global climate change; 4.7 g
of nitrogen oxides and 0.54 g of hydrocarbons, which cause smog; 2.0 g of
carbon monoxide, which is toxic; and 0.030 g of sulfur dioxide, which
causes acid rain. Each therm of natural gas saved eliminates the emission of
these pollutants while saving $0.60 for the average consumer in the United
States. Each kWh of electricity conserved saves 0.4 kg of coal and 1.0 kg of
CO 2 and 15 g of SO 2 from a coal power plant.

TABLE 2–2

Energy 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 of

Cooking appliance temperature time used energy

Electric oven 350 F (177C) 1 h 2.0 kWh $0.16

Convection oven (elect.) 325 F (163C) 45 min 1.39 kWh $0.11

Gas oven 350 F (177C) 1 h 0.112 therm $0.07

Frying pan 420 F (216C) 1 h 0.9 kWh $0.07

Toaster oven 425 F (218C) 50 min 0.95 kWh $0.08

Electric slow cooker 200 F (93C) 7 h 0.7 kWh $0.06

Microwave 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.

5 kW

3 kW

2 kW

Efficiency =

Energy utilized

Energy supplied to appliance

3 kWh

=5 kWh= 0.60

FIGURE 2–56

The efficiency of a cooking appliance

represents the fraction of the energy

supplied to the appliance that is

transferred to the food.