Energy and the Environment-Lecture 09 Energy Footprint spring 2010-Physics, Lecture notes for Energy and Environment. University of California (CA) - UCLA
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Energy and the Environment-Lecture 09 Energy Footprint spring 2010-Physics, Lecture notes for Energy and Environment. University of California (CA) - UCLA

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All houses/apartments have energy meters to monitor electricity usage.Energy Footprint, Electricity meter, Measuring electricity, wheel rate, Natural Gas Meter, Water meter, gasoline, Energy Profile, The Fix, Big Re...
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09_home_energy.ppt

Energy Footprint Spring 2010

Lecture 9 1

UCSD Physics 12

Energy Footprint A Case Study

Spring 2010 2

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 3

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 Watt-

hours 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 4

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

Energy Footprint Spring 2010

Lecture 9 2

Spring 2010 5

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 6

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 7

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 8

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 1°F 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

Q

Energy Footprint Spring 2010

Lecture 9 3

Spring 2010 9

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 10

UCSD Physics 12

Energy Profile

• Looking at my bills April 2006–March 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 11

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?!

Q Spring 2010 12

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?

Energy Footprint Spring 2010

Lecture 9 4

Spring 2010 13

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 14

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 15

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 16

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

Energy Footprint Spring 2010

Lecture 9 5

Spring 2010 17

UCSD Physics 12

dashed line: started seriously cutting back

pilot light

Tracking home usage of electricity and natural gas since 2006

Spring 2010 18

UCSD Physics 12 trend-line for previous year total: keeps trucking down!

Spring 2010 19

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 20

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

Energy Footprint Spring 2010

Lecture 9 6

Spring 2010 21

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 22

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

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