2 TYPES OF ENERGYKinetic - energy in motionPotential - stored energyChemical - energy stored in the bonds of molecules and atomsNuclear energy- energy stored in the nucleus of atomElectrical energy- electrons moving through a wireMechanical, sound, motion, thermal and radiant are other examples
3 Energy Efficiencypercentage of energy input that does useful work in an energy conversion system
4 When we’re energy efficient we use less energy to do something as good as before or better. We have to remember there is a limited amount of non-renewable fuel sources such as coal and gas. Even if we don’t run out of fuel, we can damage our environment by using too much and wasting energy.
5 Energy transformations are great, but very inefficient Energy transformations are great, but very inefficient! Our bodies are about 20% efficient, and burning the gasoline in your car is only 10% efficient! Most of the energy is lost in the form of heat!
6 Efficiency of Some Common Devices Device Efficiency (%)Dry-cell flashlight battery 90Home gas furnace 85Storage batteryHome oil furnaceSmall electric motor 62Steam power plant 38Diesel engineHigh-intensity lamp 32Automobile engine 25Fluorescent lampIncandescent lamp
7 UNITS OF ENERGY JOULE (J) or kilojoule (kJ) Calorie (cal) amount of heat required to raise the temp of 1 g of water 1⁰CBTU (British thermal unit) amount of heat required to raise the temperature of 1 pound of water 1⁰ F Used in US to rate water heaters, furnaces and air conditioners. (coal)Therm (thermal units) often used by gas companies (natural gas)
8 CONVERSIONS 1 cal = 4.186 joules 1 BTU = 1.05 kJ = 252 cal 1 therm = 100,000 BTU1 kWh= 3413 BTU1 cubic foot of gas contains 1031 BTUs1 ton of coal contains 2.5 x 107 BTUs
9 POWER- THE RATE AT WHICH ENERGY IS USED P = E/t E= energy t= timeTHE UNIT FOR POWER IS THE WATT1 W = 1 J/sA 100 watt light uses 100 J/s of electrical energyPower (watts)= current (amps) x voltage(volts)
10 FIRST AND SECOND LAWS OF THERMODYNAMICS If a bulb is 20% efficient, then the bulb converts 20% of the electrical energy into light and 80% is lost as waste heat.First Law of ThermodynamicsEnergy can be converted from one form to another ( electrical → heat)Second Law of ThermodynamicsIn any energy conversion, some energy is converted into lower quality energy
11 ANSWER: E = Pxt E = 75 J/s x 60s/min x 25 min = 110,000 J or 110 kJ Knowing the relationship between energy and power allows you to find the energy used when an appliance of known power (watts) operates for a known amount of time.EXAMPLE: how much energy (kJ) does a 75 watt light bulb use when it is turned on for 25 minutes?ANSWER: E = Pxt E = 75 J/s x 60s/min x 25 min = 110,000 J or 110 kJ
12 The kWh (kilowatt hour) Kilowatt hour is a unit of energy. This is the term used by the electric company.E= P x t1 kWh is equal to 1 kW (1000 W) delivered continuously for 1 hour (3600 seconds)1 kWh = 1000 J/s x 3600s = 3,600,000 J or 3.6 e6 J or 3600 kJ
13 1 kWh is the energy required to power 10 100-watt light bulbs for 1 hour. The average US home uses about 10,000 kWh electric energy/year.
14 Remember your metric conversions! GMKHD – DCMMNP5000 w = ____ kW4800 J = ____ MJ88 kc = ____ cal
15 Energy Calculations Worksheet Take a look and highlight terms.See if you can do the first problem tonight.
19 Energy Calculation Answers 1. A.) 100 kg x 1kcal x 20 `C = 2,000kcalkg/`CB.) 2,000 kcal x 4.186j x 1,000 cal= 8.37 MJcal kcalC.) Btu x 8.37 x 106J = 7934Btus1,055JD.) 7,934Btu x h x 60m = 11.9 min40,000Btu hr
20 2. A.) 1KWH x 3.6 x 10J x 1BTU = 3,412Btu KWH 1,055J B.) The second law of thermodynamics prevents 100% conversion of heat to mechanical or electrical energy. A typical coal fired plant operates at 33% efficiency, meaning that only 1/3 of the energy in coal is converted to electricity.
21 B. 1165ccf = 1,942ccf x $0.9 = $1,748 to heat house@ 60% eff .6 ccf 120MBtu x 1,000,000 = 1.2x10 Btu x 1 ft3 x 1/ccf = 1,165ccf (gas)1 MBtu Btu 100ft3B. 1165ccf = 1,942ccf x $0.9 = $1,748 to heat 60% effccf C = ccf x $0.9 = $1,310.6 to heat 80% effccfMoney saved per year in heating costs = $1748-$1,310.6 = $437.40Years to payback cost of furnace = $4000/$ = 9.1 years
22 4.A.)8760hrs/yr x 160 W/m2 x 10m2 x .20 =2,803.2 KWH2,803.2 KWH x 3.6x106 J x 1BTU = 9,565,421 BTU = 9.57 MBT KWH JB.) 2,803.2 KWH = =28%10,000 KWH9,565,421 Btu = 1,200 BATHS7,934 Btu/BATH
23 5.A.) W/m2 x 10m2 x .10 x 1KW x 8,760HR = 2,190 KWH1,000WB.) 2,190 KWH = .219 x 100 = 21.9%10,000KWHC.) ,000KWH x 10m2 = m22,190KWH
24 6. 8,760 x 200W/m x 106m x x Btu = 498 Tons12 X Btu/ ton W
25 7A.) 1,000MW x 1,000KWMW = 4,000 Wind Turbines250 KWB.) Solar energy can help to decrease the total amount of energy provided by the power company by 21.9 %. Solar energy used to heat water could be feasible for small families. It would not be economical or practical to use solar energy as your only energy source at this time. Wood is very inefficient as a heat source. The forest in example 7 could provide enough energy for almost 50 homes. The trees would require continuous replenishment; the soil would require fertilization to maintain……
27 $1.69 x 1 gal x 1x 106 BTU = $18.37/MBTU Gal 92000 BTU MBTU 9.Natural gas cost is $1.14 x 1cf x ccf = $11.07/MBTUccf BTU cf120/.8 x $11.07 = $Propane gas cost:$ x 1 gal x 1x 106 BTU = $18.37/MBTU Gal BTU MBTU120/.8 x $18.37= $2775.
28 Gal 133,000 BTU MBTU 120/.8 x $14.51= $2177 Electricity: Fuel Oil cost:$ x Gal x 1x106 BTU = $14.51/MBTUGal ,000 BTU MBTU120/.8 x $14.51= $2177Electricity:x 1KWH x x 106 BTU = $29.51/MBTUKWH BTU MBTU120/.8 x $ = $3,517
30 SAMPLE PROBLEMS!1. Dr. Jones’ Nov-Dec power bill shows that his home used 1355 kwh over a 30 day period.How much energy was used (kJ) for the 30 day period?Find the energy used in J/day.At the rate of $ .0749/kwh, what is the doctor’s power bill (w/o tax)?
31 A. 1355 kWh x 3600kJ/1 kWh = 4878000kJ ( 4. 9 e 6 kJ) B A kWh x 3600kJ/1 kWh = kJ ( 4.9 e 6 kJ) B kWh/30 days x J/1kWh = 1.6e10 J/day C kWh x $0.0749/kWh = $101.49
32 2. The current through a toaster (110 V) is 8 amps A. What is the power (in watts) of the toaster?B. How much energy (J) willthe toaster use in 5 minutes ofoperation?
33 A. W = 110 V x 8 A = 880 W = 880 J/secB. E = P x t so,880 J/sec x 60 sec/1 min x 5 min = 264,000 J
34 3. A 100 watt light bulb is 20% efficient. A. how much energy does it use in 12 hours of operation?B. How much energy does thebulb convert into light over the12- hour period?C. How much energy does thebulb convert into heat over the12-hour period?D. Convert the total energy use into kwh.
35 A. 20 J/sec x 60sec/1min x 60 min/1 hr x 12 hrs = 864,000 J B A. 20 J/sec x 60sec/1min x 60 min/1 hr x 12 hrs = 864,000 J B. 864,000 J C. 80 J/sec x 60 sec /1min x 60min/1hr x 12 hrs= 3,456,000 J D. 4,320,000 x 1kwh/3.6e6J = 1.2 kwh
36 4. An electric clothes dryer has a power rating of 4000W 4. An electric clothes dryer has a power rating of 4000W. Assume that a family does 5 loads of laundry each week for 4 weeks. Also assume that each load takes 1 hour.Find the energy used in both Jand kJB. If the cost of electricity is$0.758/kwh, find the cost ofoperating the dryer for 3 months.
37 A. E = P x T = 4000J/sec x 60 sec/1min x 60 min/1hr x 20 hrs x 1kw/3 A. E = P x T = 4000J/sec x 60 sec/1min x 60 min/1hr x 20 hrs x 1kw/3.6e6B. 80 kwh x $0.0758/kwh = $6.06
38 5. Dr. Nick’s natural gas bill states that his household used 110 therms of energy for a 30-day period.Convert 110 therms to kwhHis charge for the energywas $ Find the costof this natural gas in $/kwh.
39 110 therms x 100,000 BTUs/1 therm x 1. 05kJ/1BTU x 1 kw/3500kJ= 3208 110 therms x 100,000 BTUs/1 therm x 1.05kJ/1BTU x 1 kw/3500kJ= kwh$88.78/ kwh = $0.03 kwh
Your consent to our cookies if you continue to use this website.