1. Earth’s Changing Environment Lecture 23 Fred’s Utility Bill

2. Energy Efficiency Utility is promoting energy efficiency in two ways.

3. 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%.

4. Energy Use Energy use shifts with season.

5. Cost of Energy Bill is complicated, but Electricity costs $0.057/kWh Gas costs $0.71/ccf

6. Furnace Efficiency 80% for gas furnace.

7. Air-Conditioner Efficiency - SEER 10% for AC

8. 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.

9. How much heat did the gas heater supply? 100 ccf  100 therm 100 therm x 0.80 = 80 therm = = 8,000,000 Btu

10. What was the cost of the natural gas used? = 100 ccf x $0.71/ccf = $71

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

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

13. 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!

14. 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.

15. 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 = Coeff E / Coeff G x Energy E / Energy G = 1.87 x 2.67 = 5.0 Five times as much CO2 is emitted to supply electric heat as to supply gas heat.