physics 140 lecture 15 efficiency of buidlings march 19, 2012
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Physics 140 Lecture 15 Efficiency of Buidlings March 19, 2012. Announcements. 1) Homework 4 is due in class on Wednesday March 28. 2) Prof. Schnetzer will not be holding office hours this week. Efficiency in the Building Sector. Energy Usage in the US. - PowerPoint PPT PresentationTRANSCRIPT
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Physics 140Lecture 15
Efficiency of Buidlings March 19, 2012
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Announcements
1) Homework 4 is due in class on Wednesday March 28
2) Prof. Schnetzer will not be holding office hours this week
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Efficiency in the Building Sector
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Energy Usage in the US
Buildings account for 39% of US primary energy use
This corresponds to 36% of US carbon emissionsWhy not 39% ?
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Growing Demand
Building energy demand expected to grow by 30% by 2030
Goal of no net increase in building energy use requires30% average improvement in building efficiency by 2030
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Where Does the Energy Go?
Residential (55% of building total)
Commercial (45% of building total)
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What Can be Achieved
Use of energy in buildings• heating / cooling• lighting
• electrical appliances
responsible for about 36% of GHG emissions
Improvement of average building efficiency by 30% will reduce carbon emission by 10%
World wide this saves 0.8 Gt of carbon emissions per year
About one wedge
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How to Improve Building Efficiency
Better insulation
Window coatings
Higher efficiency heating and cooling
White roofs
Higher efficiency lighting
Occupancy sensors
Higher efficiency electronics
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Better InsulationAerogel insulation
sometimes called solid air
a gel in which the liquid has been replaced by air
basically a nanofoam• lightweight• strong• extremely high thermal resistance high
R-valuetemperature difference across a materialdivided by incident heat power per unit area
good insulators have large R-values
Silica aerogel has about a ten times largerR-value than standard building wall insulation
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Window TechnologyDouble pane with inert gas (argon) filling to reduce convective heat flow
Low-emissivity coatings on facing pane surfaces to reduce radiative heat flowLow-E coatings are microscopically thin, virtually invisible, metallic oxide layersThey reducing solar radiation into the house (in summer) and radiation of indoor heat to outside (in winter)
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HeatingIf you have a gallon of fuel and a cold house what is the best way to heat it?
Burn the fuel and use the heat produced? No!
Better to use a heat pump. It will amplify the amount of heat produced
Heat Pump
A heat engine running in reverse.
A refrigerator is an example
Using energy it takes heat from a cold temperature and deposits the heat at a higher temperature.
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Heat PumpHeat engine Heat pump
A 33% heat engine operating in reverse would deliver 3 times as much heat as the energy it uses. Gain of factor of 3
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Coefficient of Performance (COP)COP is the amplification
factor.It is the heat delivered divided by the energy used.
COP = QH / W
Recall from thermodynamics the efficiency of a heat engine is given by:
efficiency =
If we run this heat engine in reverse we get a heat pump with COP given by:
COP
maximize efficiency by maximizing temperature difference
maximize COP by minimizing temperature difference
Heat pumps work best when temperature difference is small
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An exampleSupplying 1 unit of home heating energy
70% efficient natural gas furnace
requires 1.4 units of natural gas primary energy
requires 0.9 units of coal primary energy
electric heat pump COP = 3.3 with electricity from coal burning plant
In terms of carbon emissionsthe gas furnace wins !
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Carbon Emissions
1.4 units of natural gas vs. 0.9 units of coal
remember for a given amount of energy coal releases1.8 times as much carbon dioxide as natural gas
But
• COP of 3.3 is typical for today’s heat pumps but in principle COP’s as high as 14 are possible
• in the future a larger fraction of electric power will be produced by natural gas
In the long term heat pumps win!
but remember heat pumps don’t work well(COP is close to one) in very cold climates
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Light Colored RoofsA simple low tech way to save energy
White roofs
In the summer absorbs less solar radiation
In the winter radiates less energy to the sky
seems like a no-brainer
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Lighting
Incandescent Light Bulb
only about 5% of electrical energy converted into light
Compact Fluorescent Light (CFL) bulb about 20% of electrical energy converted into light
Four times less energy for a given amount of light
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Carbon SavingsThe amount of incandescent lighting in the US corresponds to about 3 billion 100 W light bulbsRecall that we calculated that a 100 W bulb on continuously releases about 500 pounds (0.25 tons) of carbon per year(assuming electricity from coal burning plants)
3 billion bulbs on for 2.5 hours per day releases
(3 billion) x (2.5 hours / 24 hours) x (0.25 tons) = 80 Mt
If all were replaced by CFL’s the carbon emission would be reduced by a factor of four
We would save 60 Mt of carbon emissions per year
6% of a wedge
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Economics of CFL’s
Compact Fluorescent Light (CFL) bulbs are expensive
Do it!
but they last 8 to 15 times longer
they cost 3 to 10 times more that incandescent bulbs
But you would save $400 to $1000 over five years
and they use 4 times less electricity
Replacing all of the incandescent bulbs in your house would cost about $90
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Solid State Lighting
Light emitting diode (LED) lamps may be the lighting technology of the future
factor of two better efficiency than CFL’s
Currently niche markettraffic lightsflashlights
Further developments neededwhite LED lightsbrighter bulbs
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Higher Efficiency AppliancesSince 1970 appliance efficiencies have improved dramatically
These trends are expected to continue with expected reductionof about 10% of projected electrical energy useage by 2020.
This corresponds to a carbon emissions savings of 10 Mt per year
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Legislating EfficiencyNote in the plot on the previous slide that efficiency improvements only happened when they were legislated (shown by the location of the arrows on the plot).
The cost savings are real but electricity cost are too small and the future savings too abstract for the consumers themselves to demand action.
California has led the way but we need aggressive national efficiency standards.
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The Standby ProblemMost modern electronic devices:
printers, microwaves, TV’s, DVD players, cable boxes , etc.are in “standby” mode and continue to use power even when turned off.
On average these consume about 2.5 W of continuous power.
It’s estimated that the average household has 40 of these devices. (Not mine!)That’s 100 W of continuous power or 500 pounds of carbon emission per year per household. It represents 8% of the average US household electricity consumption
This is crazy!
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Zero Energy BuildingsA Zero Energy Building (ZEB) is one that is “off grid.” It uses no electricity from the electric power grid.
Goal:• most new residential buildings ZEB by 2020• most new commercial buildings ZEB by 2030
Achievable but will require• significant advancement of building technology
• development and widespread adoption of integrated building design and operation practices
Need national building codes tailored to local conditions
How do we get to ZEB?
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Toward ZEB
Generally accepted approach is to reduce household energy useby 70% using techniques such as those we;ve mentioned
• switch to CFL lighting (30%)• better appliance efficiency (30%)
• elimination of standby mode (10%)
Then get remaining 30% from on-site electricity generation.
In a future lecture on solar energy, we’ll discuss the feasibility of getting 30% of household electricity from on-house solar panels
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Putting it All TogehterExample of Integrated Design for ZEB Commercial
Building