An Introduction to Heat Pumps (for non-specialist citizens)

Executive Summary:
  • The colloquial phrase "air conditioner" is the consumer name used to describe an air-cooling appliance.
    • Technically speaking, most air conditioners fall under the label of "air-to-air" heat pump.
  • The colloquial phrase "heat pump" is the consumer name used to describe a dual purpose appliance which can produce heat during winter then cool during the summer.
    • 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 installation quote
    • Heat extracted from a too-short loop will eventually freeze the loop
  • 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.

Prologue (2018.12.xx)

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.

Caveats:

  1. I have gone out of my way to not mention anything about fossil fuels here.
  2. I will attempt to introduce the non-scientific reader to some scientific terms (kept to the secondary school level)
  3. For simplicity, all explanations here will ignore conversion losses.

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

    K C F R Notes
    Absolute zero      0 -273 -459      0 no kinetic motion whatsoever
    Water Freezing 273      0    32 491 contains some heat energy even though it is cold to the touch
    Water Boiling 373 100 212 671  
    Notes:
    • The difference between freezing and boiling is 100 degrees in the first two columns (Kelvin and Celsius)
    • 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 two 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.
  • The 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
      comment: 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
      comment: 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.
    For example:
      from to
    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 water 1 gram of 100°C steam
  • notes:
    • 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:

Cooling

refrigeration cycle
  • You would never use water as a refrigerant but let's assume you did just for the following discussion:
    1. 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.
    2. 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 appliance)
  • Since water requires too much effort to force a changes-of-state either way, other substances are used.
    • More than 100 years ago, the cooling industry was using Ammonia which has a heat-of-vaporization of 5,613 calories (ref)
    • Before 1995, Freon-12 (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 the Montreal Protocol
    • 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 / 50F)
    than an air-to-air heat pump can move inside heat (21C / 71F) into outside air (30C / 86F)

Heating

Electric Heat

  • 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 inside air.
  • 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 loops
  • 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 Loops

  • 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

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Neil Rieck
Waterloo, Ontario, Canada.