Earth’s Changing EnvironmentLecture 23

Fred’s Utility Bill

Energy Efficiency

Utility is promoting energy efficiency in two ways.

Space Heating with Natural Gas

Advisory claims that heating with an electric resistance furnace cost $1,400 and heating with natural gas cost $555. This is about 40%.

Energy Use

Energy use shifts with season.

Cost of Energy

Bill is complicated, but

Electricity costs $0.057/kWh

Gas costs $0.71/ccf

Furnace Efficiency

80% for gas furnace.

Air-Conditioner Efficiency - SEER

10% for AC

Problem and Solution

During February, a San Antonio homeowner used 100 ccf of natural gas for space heating.

Complete combustion of 1 ccf of natural gas releases 1 therm of heat

1 therm = 100,000 Btu of heat. AFUE of the Fred’s furnace = 80. Electric resistance heating releases 3413 Btu

per kWh Natural cost $0.71 per ccf. Electricity costs $0.057 per kWh.

How much heat did the gas heater supply?

100 ccf 100 therm 100 therm x 0.80 = 80 therm = = 8,000,000 Btu

What was the cost of the natural gas used?

= 100 ccf x $0.71/ccf = $71

How much heat would have to be supplied if an electric heater were used? 80 therms = 8,000,000 Btu

How much electricity would the electric heater use? 8,000,000 Btu / 3413 Btu/kWh = 2,343 kWh

How much would this electricity cost?

2,343 kWh x $.057 = $134

So the ratio of the cost of the electricity to the cost of natural gas would be $134 / $71 = 1.9

It would cost almost twice as much to heat with electricity!

How much energy must be supplied at the utility to supply the heater’s electric energy?

2,343 / 0.30 = 7813 kWh 7813 kWh x 3413 Btu/kWh =

26,700,00 Btu = 267 therm So the ratio of the energy

needed to supply electric heating to the energy to supply the gas heat = 267 therm / 100 therm = 2.67

Almost three times as much primary energy is needed to supply electric heat as to supply gas heat.

Compare the CO2 emission for electric and gas heating

Coal releases 26 tons / QBtu Gas releases 14 tons / QBtu Ratio of CO2 emission coefficients =

26/14 = 1.87 Ratio of CO2 emission =

(Coeff. x Energy)E / (Coeff. x Energy)G = CoeffE / CoeffG x EnergyE / EnergyG = 1.87 x 2.67 = 5.0

Five times as much CO2 is emitted to supply electric heat as to supply gas heat.