Physics 140
Lecture 15
Efficiency of Buidlings
March 19, 2012
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
Buildings account for 39% of US primary energy use
This corresponds to 36% of US carbon emissions
Why not 39% ?
Growing Demand
Building energy demand expected to grow by 30% by 2030
Goal of no net increase in building energy use requires
30% average improvement in building efficiency by 2030
Where Does the Energy Go?
Residential (55% of building total)
Commercial (45% of building total)
What Can be Achieved
Use of energy in buildings
heating / cooling
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
How to Improve Building Efficiency
Better insulation
Window coatings
Higher efficiency heating and cooling
White roofs
Higher efficiency lighting
Occupancy sensors
Higher efficiency electronics
Better Insulation
Aerogel insulation
a gel in which the liquid has been replaced by air
sometimes called solid air
basically a nanofoam
• lightweight
• strong
• extremely high thermal resistance high
temperature difference across a material
divided by incident heat power per unit area
good insulators have large R-values
Silica aerogel has about a ten times larger
R-value than standard building wall insulation
Window Technology
Double pane with inert gas (argon)
filling to reduce convective heat flow
Low-emissivity coatings on facing pane
surfaces to reduce radiative heat flow
Low-E coatings are microscopically thin,
virtually invisible, metallic oxide layers
They reducing solar radiation into the house (in summer)
and radiation of indoor heat to outside (in winter)
If 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?
Better to use a heat pump. It will
amplify the amount of heat produced
Heat Pump
A heat engine running in reverse.
Using energy it takes heat from a cold temperature
and deposits the heat at a higher temperature.
A refrigerator is an example
Heat Pump
Heat 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
Coefficient of Performance (COP)
COP is the amplification factor.
COP = QH / W
It is the heat delivered divided by the energy used.
Recall from thermodynamics the
efficiency of a heat engine is given by:
efficiency =
maximize efficiency by maximizing temperature difference
If we run this heat engine in reverse
we get a heat pump with COP given by:
Heat pumps work best
when temperature
difference is small
maximize COP by minimizing temperature
An example
Supplying 1 unit of home heating energy
70% efficient natural gas furnace
requires 1.4 units of natural
gas primary energy
electric heat pump COP = 3.3 with
electricity from coal burning plant
requires 0.9 units of
coal primary energy
In terms of carbon emissions
the gas furnace wins !
Carbon Emissions
1.4 units of natural gas
0.9 units of coal
remember for a given amount
of energy coal releases
1.8 times as much carbon
dioxide as natural gas
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 very15cold climates
Light Colored Roofs
A 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
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
Carbon Savings
The amount of incandescent lighting in the US
corresponds to about 3 billion 100 W light bulbs
Recall 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
Economics of CFL’s
Compact Fluorescent Light (CFL) bulbs are expensive
they cost 3 to 10 times more that incandescent bulbs
but they last 8 to 15 times longer
and they use 4 times less electricity
Replacing all of the incandescent bulbs in
your house would cost about $90
But you would save $400 to $1000 over five years
Do it!
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 market
traffic lights
Further developments needed
white LED lights
brighter bulbs
Higher Efficiency Appliances
Since 1970 appliance efficiencies have improved dramatically
These trends are expected to continue with expected reduction
of about 10% of projected electrical energy useage by 2020.
This corresponds to a carbon emissions savings of 10 Mt per year
Legislating Efficiency
Note 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.
The Standby Problem
Most 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!
Zero Energy Buildings
A Zero Energy Building (ZEB) is one that is “off grid.”
It uses no electricity from the electric power grid.
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?
Toward ZEB
Generally accepted approach is to reduce household energy use
by 70% using techniques such as those we;ve mentioned
switch to CFL lighting
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
Putting it All Togehter
Example of Integrated Design for ZEB Commercial Building
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