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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a gel in which the liquid has been replaced by air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