Unformatted text preview: UCSD Physics 12 Energy Footprint
A Case Study UCSD Physics 12 Electricity meter Electricity meters read in kWh (kilowatt-hour) this is a unit of energy: power times time 1 kWh is 1,000 W over 1 hr = 3,600 seconds or 1 W over 1000 hours, or 100 W over 10 hours thus 1 kWh = 3,600,000 J (= 860 Cal) My electricity bill indicates a cost of $0.13 per kWh try getting 860 Cal of food for $0.13 lesson: eat your electricity--it's cheap! tastes bad, though: burnt tongue smell/taste Spring 2010 2 UCSD Physics 12 Measuring your electricity consumption All houses/apartments have energy meters to monitor electricity usage this is what the bill is based on Dials accumulate KWh of usage Disk turns at rate proportional to power consumption Kh value is the number of Watthours per turn (1 Wh = 3600 J) Example: one turn in 10 sec
(7.2 Wh) (3600 J/Wh)/(10 sec) = 2592 J/s 2.6 kW Takes 138.9 turns for 1 kWh Spring 2010 3 UCSD Physics 12 Reading those tricky dials Let's say you want to read a utility electricity meter... Be careful to note the direction of the numbers (usually flips back and forth) Round down is the safe bet Note the third dial below looks like 5, but it's really 4.9 (next digit is a nine) so looking at next dial helps you figure out rounding note second dial halfway between 0 and 1: next digit ~5 This meter reads 5049.9 the 9.9 reads between the lines in the last dial Spring 2010 4 UCSD Physics 12 Measuring the wheel rate Recall that the Kh constant is Watt-hours per turn of the disk so power is Kh 3600 disk rate units are: (Watt-hour) (sec/hour) (turns/sec) On top of the rotating disk are tick marks with labels every 10 units. 100 units around disk If disk is moving slowly, can measure half a rotation example: from 30 to 80 or 70 to 20 If disk is moving fast, can measure time for 5 or 10 rotations The the turns/sec could be, for example: 0.5 turns / 132.0 sec 98 W for Kh = 7.2 10 turns / 44.0 sec 5890 W for Kh = 7.2 0.2 turns / 35.0 sec 148 W for Kh = 7.2 Spring 2010 5 UCSD Physics 12 Example day electricity profile Run microwave (1000 W) for 12 minutes total (0.2 hr) 0.2 kWh Clothes washer (300 W) for 1 hour 0.3 kWh Clothes dryer (5000 W) for 1 hour 5 kWh Movie on TV/DVD (200 W) for 2 hours 0.4 kWh Desktop computer (100 W) on all day 2.4 kWh Refrigerator (average 75 W) on all day 1.8 kWh Lights (total 400 W) for 5 hours 2 kWh Total comes to 12.1 kWh: not too different from average usage costs about $1.50 at $0.13 per kWh Spring 2010 6 UCSD Physics 12 Natural Gas Meter Dials work just like electricity meter same round-down method Lowest dial usually indicates 1000 cf per revolution cf means cubic foot, or ft3 Thus each tick is one hundred cf (hcf) therefore numerical reading in hcf 100 ft3 delivers 1.02 Therms of energy 1 Therm is 100,000 Btu = 105,500,000 J = 29.3 kWh my gas bill indicates $1.30 per therm equivalent to $0.044 per kWh: cheaper than electricity My meter also has a 0.5 cf dial and a 2 cf dial which I have used to monitor slow usage Spring 2010 7 UCSD Physics 12 Water meter Though not a measure of energy, this can be important because one thing we do with water is heat it Meters typically measure in cubic feet 1 ft3 = 7.48 gallons 1 gallon is 8.33 lb, so 1 ft3 = 62.3 lb recall that heating 1 lb H2O 1F takes 1 Btu = 1055 J The meter at right reads 82.114 ft3 the ones digit usually snaps into place quickly so it's not halfway between numbers for very long the little triangle spins if water is flowing Spring 2010 Q 8 UCSD Physics 12 And finally, gasoline Gasoline energy content is: 34.8 MJ/liter 47 MJ/kg 125,000 Btu/gallon = 132 MJ/gallon = 36.6 kWh/gallon At $2.50 per gallon, this is $0.068 per kWh cheaper than electricity, more expensive than natural gas Spring 2010 9 UCSD Physics 12 Energy Profile Looking at my bills April 2006March 2007, I saw that my household (2 people) used: 3730 kWh of electricity in a year 10.3 kWh/day 330 Therms of natural gas in a year 0.9 Therms/day = 26 kWh/day 10 gallons of gasoline every 2 weeks 26 kWh/day Total is 62 kWh/day = 2580 W or 1300 W per person 13% of 10,000 W American average says most activity in commercial sector, not at home Spring 2010 10 UCSD Physics 12 Something doesn't add up... Something wasn't making sense 0.5 Therms/day = 50,000 Btu/day during summer months when the only natural gas we use is for hot water A typical 10-minute shower at 2 gallons per minute means 20 gallons or 166 lbs of water To heat 166 lbs water from 60 F to 120 F (60 F change) requires 166 60 = 10,000 Btu Averaging 1 shower/day, we should be using 5 times less natural gas, or about 0.1 Therms/day Where is the 0.5 Therms coming from?! Spring 2010 Q 11 UCSD Physics 12 Watching the dials I started watching the 2 cf/turn dial on my gas meter no gas was being used (no furnace, no hot water) it was making about 0.72 turns per hour, so 1.44 cf/hr steady rate, hour after hour that's 34.6 cf/day, or 0.346 hcf/day = 0.35 Therms/day this is close to the missing amount! Where was that gas going? Spring 2010 12 UCSD Physics 12 The Fix Shutting off gas to the furnace resulted in a much slower dial progression rate was about 0.11 Therms/day this part must be the water heater pilot the rest (0.24 Therms/day) was the furnace pilot this means the (useless) furnace pilot matched the (useful) hot water heater gas consumption! also, half the hot water heater gas (0.11 Therms/day) is the pilot The resultant cost for both pilots was (0.35 Therms/day) (30.6 days/month) ($1.30/Therm) $14 per month save almost $10/month by turning off furnace pilot Spring 2010 13 UCSD Physics 12 But I'm not done measuring yet! How much does a shower take? 10 minute shower: measured 2.75 ft3 = 20.57 gallons via meter gas kicked on and used 15.3 ft3 = 0.156 Therms before it stopped at rate of 0.5 cf/minute 0.005 Therms/minute = 500 Btu/minute = 30,000 Btu/hr = 8800 W water heater says 34,000 BTUH on side Used 15,600 Btu for shower 20.57 gallons = 171 lbs heating by 60 F requires 10,280 Btu at 100% efficiency so must be about 10280/15600 = 65% efficient actually less since shower used 20.57 gallons, but not all hot Spring 2010 14 UCSD Physics 12 Average Americans 830 kWh electricity per month per household about 300 kWh per person per month (10 kWh/day) 6 1012 ft3 of natural gas use in residences per day 480 kWh gas equivalent per month per person (16 kWh/day) 0.5 gallons gasoline per day per person 560 kWh per month equivalent (18 kWh/day) Total power is 1340 kWh/month (44 kWh/day) = 1820 W this is 18% of the average American's total of 10,000 W so again, most is outside the home (out of sight, out of mind) Spring 2010 15 UCSD Physics 12 How much better can we do? Starting in 2007, my wife and I challenged ourselves to reduce our energy footprint never turned furnace/pilot back on low power electric blanket helps! shorter showers, with cutoff for soaping up line-dry clothes all bulbs compact fluorescent, some LED diligent about turning off unused lights bike/walk around neighborhood (and bus to work) install experimental (small) solar photovoltaic system (off-grid; battery-based) to run TV & living room since expanded to 1kW peak system; fridge, TV, modem/wireless Spring 2010 16 UCSD Tracking home usage of electricity and natural gas since 2006 Physics 12 dashed line: started seriously cutting back pilot light Spring 2010 17 UCSD trend-line for previous year total: keeps trucking down! Physics 12 Spring 2010 18 UCSD Physics 12 Big Reductions Most substantial savings was gas (no furnace) Immediately went from 0.84 Therms/day to 0.28 Therms/day equivalent to 25 kWh/day, now down to 8 kWh/day now at ~5 kWh/day now using a fifth of what we used to! Line-drying clothes had largest electricity impact some space-heater activity to compensate for no heat Immediately went from 10.5 kWh/day to 5.5 kWh/day now at ~3 kWh/day now using a third of what we used to but this requires about three times the energy in natural gas due to the inefficiency of generation, plus some transmission loss, so the real post-reduction usage is about twice that of natural gas Spring 2010 19 UCSD Physics 12 Carbon Footprints Each gallon of gasoline contributes 20 lb CO2 Each kWh of electricity from natural gas plant (at 33% net efficiency) contributes 1.2 lbs CO2 Each Therm of natural gas contributes 11.7 lbs CO2 So my annual household CO2 footprint (2 people): 4600 lbs + 3600 lbs from elec. plus N.G. before April 2007 2400 lbs + 1200 lbs from elec. plus N.G. just after April 2007 7130 lbs per year from gasoline (@ 10,000 miles per year) 15,000 lbs from air travel (at 0.48 lbs/passenger-mile) See: http://www.earthlab.com/carbon-calculator.html also Google: carbon footprint calculator Spring 2010 20 UCSD Physics 12 Lessons It is illuminating to assess your energy footprint how much do you get from which sources? how much would you have to replace without fossil fuels? how can you cut down your own usage? Again we see that the bulk of energy expenditures are not at home or in our cars but in the industry, agriculture, transportation, commercial sectors. Spring 2010 21 UCSD Physics 12 Announcements and Assignments Read Chapter 4 for next lecture HW #3 due Friday 4/23: primarily Chapter 2-related problems: (show work or justify answers!); plus Additional problems (on website) HW drop box outside my office (SERF 336) for early turn-in Remember that Quizzes happen every week available from Thurs. afternoon until Friday midnight three attempts (numbers change) the better to learn you with Spring 2010 22 ...
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