Re: Air-Conditioning Efficiency
- From: Williamknowsbest <William.Mook@xxxxxxxxx>
- Date: Thu, 28 Aug 2008 22:35:21 -0700 (PDT)
In the summer months lets say we're reducing the temp from 92 to 72
and in the winter months we're raising the temperature from 22 to
72. The airconditioner reduces the temperature by 20 degrees and the
furnace raises the temperature by 50 degrees.
Converting these to engineering units -
20 degrees fahrenheit is 11.1 degrees Celsius
50 degrees fahrenheit is 27.8 degrees Celsius
Each degree of temperature change in air requires a change of 1
kilojoule per kilogram. So, to heat air 27.8 degrees requires 27.8
kilojoules of thermal energy added to the air. To cool the air by
11.1 degrees requires that 11.1 kilojoules be removed from the air.
Now, if the energy is added to a tonne of air by direct combustion of
fuels we have
To heat 1,000 kg of air 27.8 degrees celsius
Burn Coal 23 MJ/kg 1.20 kg 4.42 kg CO2
Burn Oil 45 MJ/kg 0.62 kg 1.92 kg CO2
Burn NG 55 MJ/kg 0.51 kg 1.39 kg CO2
Using electricity to power a resistance heater to provide heat we have
the efficiency of the generator and then the losses in the lines to
contend with, which reduces the EFFECTIVE heat capacity increasing CO2
emissions
To heat 1,000 kg of air 27.8 degrees celsius with electricity
Coal fired generator 23 MJ/kg 38% 93% 8.12 MJ/kg 3.42 kg
12.55 kg CO2
Oil fired generator 45 MJ/kg 32% 93% 13.39 MJ/kg 2.08 kg
6.47 kg CO2
NG fired generator 55 MJ/kg 28% 93% 14.32 MJ/kg 1.94 kg 5.33
kg CO2
Most refrigeration systems used for room airconditioning are vapor
compression systems. They consist of four major components;
1) a compressor
2) a condenser
3) an expansion valve
4) an evaporator
Energy is consumed by the compressor. It compresses a vapor in a
closed cycle system. That compressed vapor is passed to a condenser
which rejects heat from the vapor into the environment. When cooled,
the vapor passes through an expansion valve, and into the evaporator.
The evaporator absorbs heat from the surrounding environment and
passes again to the compressor.
The efficiency is measured by the heat flow passing through the vapor
cycle - heat absorbed in the evaporator and later rejected by the
condenser - divided by the energy used by the compressor.
Most refrigeration units have an overall efficiency of about 68% -
that is for each Joule moved by the system, 0.68 joules of energy is
used by the compressor,
So, to cool 1,000 kg of air by 11.1 degrees Celsius
Coal fired generator 23 MJ/kg 38% 93% 68% 5.53 MJ/kg 2.00 kg 7.36
kg CO2
Oil fired generator 45 MJ/kg 32% 93% 68% 9.10 MJ/kg 1.22 kg
3.80 kg CO2
NG fired generator 55 MJ/kg 28% 93% 68% 9.74 MJ/kg 1.14 kg 3.13
kg CO2
Now all these numbers ranked by CO2 emissions are
Coal fired generated electric heater 12.55 kg CO2
Coal fired generated electric AC 7.36 kg CO2*
Oil fired generated electric heater 6.47 kg CO2
NG fired generated electric heater 5.33 kg CO2
Oil fired generated electric AC 3.80 kg CO2
NG fired generated electric AC 3.13 kg CO2
Oil fired heater 1.92 kg CO2
Gas fired heater 1.39 kg CO2**
*NOTE - most common source of electricity in the USA
**NOTE- most common source of home heat in the USA
Using electricity to heat your home in all cases produces more carbon
dioxide than using electricity to cool your home. However, the most
common situation (in Ohio) is a house is heated by Natural Gas
furnace, and cooled by a coal fired generated electric air conditioner
- which is just the opposite of the conclusion of the article cited in
another post.
That is, if you use natural gas or oil fired heaters in your home in
the winter, you are producing less carbon dioxide than you running
your AC in the summer.
ICE
The heat of fusion of water is 333 kJ/liter - and another 92 kJ/liter
to raise it from freezing to room temperature - a total of 425 kJ/
liter. To cool 1,000 kg of air with ice requires that 11.1 MJ be
removed from the air. This requires 26.11 liters of ice be stored for
each tonne of air cooled in this way. A cubic meter - cubic yard
approx - of ice can cool 38.2 tonnes of air. A superinsulated ice
tank of sufficient size that freezes ice in the winter and melts it
with heat from an intake vent can eliminate conventional air
conditioning and the carbon load represented by it.
This allows the cooling of air without any production of CO2.
HEAT PIPES
Putting a heat exchanger between the exhaust vent in a house and an
intake vent in a house can recapture a good portion of the heat energy
in the air.
UNDERGROUND CONSTRUCTION
Below the frost line the Earth is a constant 52 degrees fahrenheit.
With appropriate control of air exchange it is possible to cool air in
the summer and heat air in the winter by passing it underground.
Alternatively heat pipes may be installed to pass heat to and from the
ground as is appropriate, or the house itself may be constructed
underground, and air exchange modulated in the house itself to
maintain the desired temperature.
http://www.mokindustries.com/solar-housing.html
.
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