An Introduction to Heat Pumps (for non-specialist citizens)
- The colloquial phrase "air conditioner" is the consumer name used to describe an air-cooling
- Technically speaking, most air conditioners fall under the label of "air-to-air"
- The colloquial phrase "heat pump" is the consumer name used to describe a dual
purpose appliance which can produce heat during winter then
- Technically speaking, anything that moves heat from one place to
another can be called a "heat pump"
- Heat pumps can be air sourced or ground sourced (includes soil as
well as rivers and/or lakes)
- Heat pumps with their outside loops connected to soil (rather than air) go by the name of ground-source heat pumps.
- when in heating mode during the winter:
the soil 2m (6f) below the
surface provides much more heat energy (typical
temperature: 10C/50F) than outside air
(0C/32F) ever could
- when air-source heat pumps are unable to keep up on cold
days, most usually energize one-or-more banks of electric heaters
- when in cooling mode during the summer:
a ground-source heat
pump can more easily move indoor heat (typical temperature: 21C/71F) into a ground loop (10C/50F) than
an air-to-air heat pump can move indoor heat into outdoor air (30C or 86F)
- technically speaking, ground-source is a misnomer when in cooling
mode; ground-sink would be more accurate
- When real-estate is plentiful (think farm) most ground-source installations will
employ a field of horizontal loops
- the length of the loop is dependent upon local conditions. Moist
soil will always transfer heat to/from the loop than drier soil
associated with sand or gravel. Therefore, moist soil requires shorter
loops while sandy and gravel soils require medium and long loops.
- The installer must do a soil survey before writing an
- Heat extracted from a too-short loop will eventually freeze the
- When real-estate is scare (think city) most installations will
employ one-or-more vertical loops. This involves drilling (typically 18m/60ft to
36m/120ft) which can be more expensive than horizontal loops.
I just read an
article in New Scientist magazine where the author states that the only way to for
the EU and UK to
meet their CO2 Reduction Targets is to introduce heat pump technology
to heating-cooling refurbishments, both residential and industrial. Since I know
a thing or two about heat pumps, I decided to create this web-page in order to
pass on some of my knowledge to the less informed. What follows is a very simple
description targeted at secondary school students -and- adults with minimal science education.
- I have gone out of my way to not mention anything about fossil fuels
- I will attempt to introduce the non-scientific reader to some scientific terms (kept to the
secondary school level)
- For simplicity, all explanations here will ignore
Heat Energy, Temperature, and
Changes of State
jump: click here if you already know this stuff
Heat Energy and Temperature
- Heat is best described as the
motion and collisions of molecules.
- Numerous fast collisions are called hot while
less numerous and slower collisions are called cold.
These are relative descriptions of temperature.
- Temperature is a measure of the average collision rate (of a volume of
molecules) and thermal science is usually done on the Kelvin scale because
it starts at zero.
- technically speaking, you cannot have a temperature colder than 0 K
which is also called absolute zero.
- Other popular temperature scales include Celsius, Fahrenheit and Rankin
||no kinetic motion whatsoever
||contains some heat energy even though it is cold to the touch
- The difference between
freezing and boiling is 100 degrees in the first two columns (Kelvin and
- The difference between freezing and boiling is 180
degrees in the second two columns (Fahrenheit and Rankin)
- Absolute zero is the absence
of all heat energy. This means that heat can be extracted from the
environment even when the temperature is below the freezing point of
water (more on this in
the next section)
- The Rankin scale was more popular in Britain when the laws of
thermodynamics were being developed. That is one reason why many
countries still use Rankin and BTU (British Thermal Unit). More on this
in the next section.
- Energy is defined as the ability to do work. So when talking about heat
energy, we need to know more than just the temperature of one molecule since
molecules at a given temperature could do twice as much work as one molecule. Steam engines were
once popular so let me point out that bigger pistons (representing more
steam) can do more work
(moving the wheels) than smaller pistons. Also, hotter steam (higher
temperature) can do more
work than cooler steam.
First Law of Thermodynamics states: Energy can never be created or destroyed but can be converted
from one form to another.
- In the case of heat energy, decreasing the volume of a gas
will increase the temperature (Boyle's
Law) but not increase the energy.
- For example, a compressor can be used to
reduce/compress the volume of a gas which will make it hotter. Why? The temperature of the latent heat in the gas (which is always
present when the temperature is above 0°K) will increase because we have
forced the molecules closer together (diagram).
Now let me be very clear: while temperature has gone up, the total amount of
heat energy remains the same.
- comment: an increase increase temperature,
while keeping the volume fixed, is interpreted as an increase in energy.
This happens when a spark ignites fuel and oxygen in the cylinder of
your gasoline-powered automobile
- Just as electricity flows from high voltage to low voltage, heat energy
flows from high temperature to low temperature
- comment: mechanisms exist for increasing or
changing voltage in an electrical circuit. Take alternating current as
an example, a step-up transformer might double the voltage but the
resultant current (in amperes) is half since "power (in watts) =
EMF (in volts) x current (in amperes)"
Calories and other units
- non-SI units
- One BTW
(British Thermal Unit) is defined as the energy required to raise the
temperature of 1 pound of water by 1°F
this always sounds dumb whenever I hear it. To make matters worse,
commercial HVAC system engineers often reference system capacity as
"tons" of heating or cooling.
- One calorie is
defined as the energy required to raise the
temperature of 1 gram of water by 1°C
Calories are usually associated with the metric system so why were they not incorporated into the SI
system? One explanation involves the confusion introduced when the food
industry began to use an uppercase "C" in calorie to represent a
kilocalorie. This means you could read documents containing "calories"
or "Calories" but could could never have a conversation containing
any precision. Yikes!
- SI units
- One Joule is
defined as the heat energy required to raise the temperature of
1 gram of water by 0.24 °C
- Therefore, one calorie = 4.16 Joules (you
will need this conversion when looking at data found at Wikipedia)
To keep things as simple as possible, the remainder
of this article will only use calories
- Not only can additional energy change increase temperature, it can cause
also induce changes-of-state.
|80 calories is the energy required to convert
||1 gram of 0°C ice
||1 gram of 0°C water
|540 calories is the energy required to convert
||1 gram of 100°C
||1 gram of 100°C steam
- removing 80 calories will convert water back into ice
- removing 540 calories will convert steam back to water
- the values just presented are only for the water molecule.
Other molecules have different values as shown in these two charts:
- You would never use water as a
let's assume you did just for the following discussion:
- A refrigerant is forced to change state from a liquid to a gas. This begins inside an expansion valve
(where the pressure drops) but continues expansion inside
another device known as an evaporator (a fan is moving air over this point).
Approximately 540 calories is pulled out
of the air stream for every gram that changes state.
- A compressor is used to pump the refrigerant into a condenser
where the refrigerant is forced change state again but this time back into a liquid.
At this point 540
calories is transferred from the refrigerant to the condenser which may
look like an external heat
exchanger. (A device with copper-colored fins behind or under your
- Since water requires too much effort to force a changes-of-state either
substances are used.
- More than 100 years ago, the cooling industry was using
Ammonia which has a heat-of-vaporization of 5,613
- Before 1995,
(also called R-12 or CFC-12) was very popular with a
heat-of-vaporization of 40,132 calories (ref).
By the way, Freon-12 and other CFC-based refrigerants were outlawed by
- Lots of
other refrigerants are available today with one of the most popular
gases being R-410A (an HCFC) which does not deplete ozone but contributes to global
warming (oops!). So now there is a movement to replace HCFC with HFC
- The diagram just to the right could be a kitchen refrigerator, an automobile
air conditioner (cooler), an apartment air conditioner (sometimes known as a
"window rattler"), an air-to-air heat pump, or an air-to-ground heat pump.
- As stated previously in the executive summary,
a ground-source heat pump can
more easily move inside heat (21C / 71F) into a ground loop (10C /
than an air-to-air heat pump can move inside heat (21C / 71F)
into outside air (30C / 86F)
- I mention this first because almost all heat pumps (air-to-air or
ground-source) employ electric heat whenever the heat source is too low to
efficiently heat the inside air
- although lots of heat energy is available below the freezing point
on the Kelvin scale, your hardware might
expend considerable energy attempting to extract it. At some point it
makes more sense to stop using the heat pump then switch over to "electric
heaters" (technology is not much different than what you might find in
your kitchen oven).
- This switch-over is usually controlled by an out-door thermostat.
Air-Source Heat Pumps
- No magic here. This process is similar to your summer air-conditioner
except everything is turned around to move heat energy from outside air to
- Think of this as trying to cool the outside air.
Ground-Source Heat Pumps
- These are similar to Air-source heat pumps with one exception: while
outside air can get very cold (even below the freezing point of water) the
ground 2m (6ft) down never freezes.
- So while there is an added expense of adding "3-4-5 vertical loops in
residential locations" or "a field of horizontal loops in rural locations",
there will always be more heat available.
- Now there is one draw back which is this: you will need to electrically
power one-or-more loop pumps to move a water-antifreeze solution through the
- note: some heat pumps will (optionally)
allow you to divert heat to a domestic hot-water tank
COP (coefficient of performance)
- COP is a marketing number used to gauge how efficient your heat-pump
might be under ideal circumstances
- For example:
- scroll compressors are more efficient than reciprocating compressors
but scroll compressors cost more.
- longer loops offer more heat energy but then loop pumps will consume
more energy pushing a greater volume of liquid through the longer loops.
- So lets go back to electric heat for a moment where we can theoretically
get one watt of heat energy for every watt of electrical
energy consumed. This is known as a COP of one.
- Ground-source heat pumps will reliably provide 5-6 watts
of heat energy for every watt of electrical energy consumed. This is
billed as a COP of 5 or a COP of 6
- caveat: this sounds like a scam but it is not. You
are consuming electricity in order to harness heat energy naturally
found in your local environment.
- Other schemes exist for providing both open and closed loops into rivers
and lakes. We all know that these can freeze so I will leave these
technologies out of the current discussion
Waterloo, Ontario, Canada.