HEAT PUMP BASICS
This course assumes that you are already
proficient in the operation of comfort cooling. The cooling function of
a heat pump is identical to that of a straight air conditioner with the
exception that the refrigerant flows through the reversing valve and
accumulator (if one is present). All the power circuitry is identical
and all the components are the same. Some of the earliest experiments
with refrigeration were done by Lord Kelvin in 1852 See article in Wikipedia on Lord Kelvin a.k.a William Thomson. Note: there is an article on Wikipedia
on heat pumps which has some serious errors because there is no mention
of Latent Heat, only gas laws which relate the pressure of a gas with
temperature not the change in state of the refrigerant from liquid to
gas and back..
A heat pump is an air conditioner
that can be reversed to move heat from outside to inside. In some
countries a heat pump is also known as a "reverse cycle air
conditioner". In most cases heat pumps are air to air but there are
systems that use air and fluid or fluid and fluid. A Ground source or
Geothermal heat pump uses fluid that passes through the earth to
transfer the heat energy instead of the air but will be almost
identical in function.
Without getting into a major physics discussion at this time, the fact is that a comfort
cooling system will move more heat from the evaporator to the condenser
than the amount of electrical energy supplied to the compressor. The
heat that is discharged from the condenser is the latent heat that is
picked up from the evaporator plus the motor heat from the compressor.
For comparison it is practical to get a 3 to 1 return on the energy
input to the compressor. Even today as a rule of thumb for comfort
cooling 1 electrical horsepower equals 12,000 BTUs/hr (1 Ton ) of energy
moved. So a compressor motor which is rated at 1HP (746 Watts) which is
close to 3,000 Btus/hr (2536.4 to be exact) Btus/hr will move 12,000 Btus/hr or a 4 to 1
return.
If the cost of electricity is 10 cents a kWh, to produce heat with straight resistance heat will be 100,000 (1 Therm)
divided by 3,400 Btus or $2.94 a Therm. Using $3.00 a therm divided by
a COP. of 3 a heat pump will cost $1 a Therm to operate all things
being equal, which makes it very competitive will all forms of energy
including coal and wood. To get the base cost per therm of electricity
take the base at 10 cents a kWh and multiply it times your rate in
cents per kWh divided by 10 (8 cents a kWh= $3.00 a Therm times .8 ).
For space heating purposes it is possible to to move enough heat to
satisfy the load at typical Winter conditions such as 30F outdoor and
70F indoor conditions if the system is properly sized for Summer
cooling load.
The closer the temperature between the evaporator and condenser and the
larger either coil the more heat is transfered for a given time and compressor capacity. If either coil is
reduced in size and the temperature difference is increased then the
amount of heat transfered is reduced for a given time and compressor capacity.
The major advantage of a ground source heat pump over an air to air is
that the temperatures of the ground fluid are closer to the indoor
temperature or in the case of cooling may be below the indoor
temperature resulting in extreme efficiencies.
Some buildings have loop systems where heat pumps are placed throughout
the building and in cooling mode the loop water is fed to a cooling
tower or ground loop and in heating mode sourced from a boiler.
Copyright © 2007 Scott Meenen
All rights reserved