Where do Heat Pumps get their Heat?

A common question we get asked is, where do heat pumps get their heat from? How do Heat Pumps still manage supply heat when the temperature outside has dropped below freezing?

Heat is Energy

First and foremost, Heat is energy. More specifically, heat is a measure of how fast the individual molecules that make up a substance are moving. Even in solid ice that appears still – as it’s heated the molecules of water are vibrating in place. As it melts to liquid water they gain energy enough to be able to move freely against each other. Heat them up further still and they gain enough energy to separate from each other entirely and boil, to form steam.

Zero Degrees isn’t Absolute Zero.

The Celsius scale which we are most familiar with was defined by Andre Celsius based upon the melting point of ice and boiling point of water – effectively meaning the Zero-point of the scale is arbitrarily placed. A useful analogy is the A.D. and B.C. dating system commonly used. An arbitrary year was chosen as Year 1 and everything after that year was considered positive, while everything before that year is, effectively, negative.

Year 100 B.C. is more recent than Year 200 B.C., for example. A person born in 4 B.C and who died in 33 A.D. would’ve been 39 years old when they died.

What this means is that just because something is at a temperature below 0°C, doesn’t necessarily mean it’s at ‘negative energy’. Something that’s sitting at -10°C, will still contain more heat energy than something sitting at -20°C, but both will have a positive amount of energy. One simply has less than the other.

In fact, the Celsius scale goes all the way down to -273°C – to what’s known as Absolute Zero. No temperature lower than Absolute Zero can exist – it’s a physical impossibility in this universe. At Absolute Zero there is no energy whatsoever left to be extracted. Above -273°C, there’s always some positive energy available.

Just because something is at 0°C does not mean it has no energy left to give. Even air at -20°C still has a lot of heat energy available. Because it’s still 250 degrees above Absolute Zero and so still has 250 ‘degrees’ of heat energy available to use in some way.

Just because it feels cold out doesn’t mean there isn’t heat energy available. It just means the air is cooler than you are.

The hidden heat.

All matter has three phases; Solid, Liquid and Gas. For water, this is Ice, Liquid Water and Steam. It’s well known that water, under normal atmospheric pressure, will begin to boil at 100°C. To actually turn water into steam however, requires far more energy to be given introduced – energy which goes towards actually creating steam. This additional energy is called the Latent Heat of Vapourisation.

To turn a litre of Water to Steam requires 2260kJ of energy. All this energy goes towards turning that water into steam powerful enough to move a train.

However, when Steam is condensed back down to Water, all of this Latent Heat will be released and has to be removed in order to fully condense the Steam. The Steam is said to Give Up its Latent Heat of Vapourisation. There’re two ways to condense Steam – either by cooling it to remove the Latent Heat, or by compressing it somehow to a point where the pressure is so high that it begins to condense on its own.

If 1 kilogram of Steam at 100°C and at atmospheric pressure were to be compressed up to 3 Bar (3 Atmospheres), it would begin to condense back to Water. In the process of being compressed, it will still give up its Latent Heat of vapourisation – that 0.63kWh will be forced come out again.

Some energy input however, is required to compress the Steam again.

This difference between the energy required to compress the Steam to the point where it condenses and the Latent Heat of Vapourisation given up is equivalent to the Coefficient of Performance for a Heat Pump. If it’s above 1.0, more Latent Heat energy is given up than energy supplied to compress the Steam.

Futhermore, as the condenser side gets hotter and hotter it gets harder and harder to condense the Steam. It has to be compressed more and more to a higher pressure before it starts condensing, meaning more energy is required by the compressor to reach those pressures. More and more steam needs to be compressed to get more energy and reach higher temperatures.

This is one of the reasons why Heat Pumps get less efficient as the temperature difference between the source temperature and the output temperature increases.

Cool Gases.

While the principal can be demonstrated with Water and Steam, in practice Water and Steam would be poor choices for use in a Heat Pump. Water doesn’t boil until it reaches a temperature of 100°C. This is far beyond any useful temperature for a domestic Heat Pump. For a Heat Pump, what’s needed is a gas which vapourises at common atmospheric temperatures.

This is called a Refrigerant Gas.

Under normal atmospheric conditions most refrigerant gasses will boil at temperatures well below 0°C. R134a is a common refrigerant gas which has a boiling point under normal atmospheric pressure of -26°C. Below this point, it’s a clear liquid that looks a lot like water.

This means that air at a temperature of -20°C still has enough heat energy within it to boil R134a under atmospheric pressure.

When R134a boils and turns to gas, it absorbs its Latent Heat of Vapourisation. When it’s compressed again and Condenses into a liquid, this heat is then given up. In a Heat Pump, the Heat of Vapourisation comes from an external source, such as the atmosphere, or a geothermal ground loop. This heat, when given up at the compressor, can then be used within a heating circuit or similar.

How refrigerant affects performance

Different refrigerant gases have different performance characteristics, which place different limits on the capabilities of various Heat Pump models.

A refrigerant gas like R134a that boils at -26ºc will struggle to deliver heat in temperatures below -20c – it just doesn’t boil fast enough to extract any useful energy. On the other hand, it can be relatively easy to compress and reach higher domestic temperatures.

Conversely Heat Pump filled with a gas that has a lower boiling point – say, R13 which has a boiling point of -89°C – will reliably work in even the coldest of temperatures but will require more and more effort from the compressor to condense the refrigerant and give a high enough output temperature for domestic use.

The lower the boiling point of the refrigerant gas the better the Heat Pump can operate under cold conditions, but the less efficient it gets as the output temperature increases. A Heat Pump with a Higher Boiling point can offer higher output temperature but will struggle to deliver heat during the coldest of winters.

Each particular refrigerant gas has a range of temperatures that it offers its best efficiencies at. Below that temperature, it just doesn’t vapourise fast enough to extract a useful amount of heat. While above that temperature band the compressor is doing too much work to extract the heat from the gas, such that the efficiency of the Heat Pump starts to suffer. Different manufacturers choose different refrigerants based upon the specific performance characteristics they’re looking to achieve, the production cost of the gas and the environmental conditions it’s being designed for. A Heat Pump designed to be efficient in Finland may use a different refrigerant than one designed for France.

Greentherm have taken a compromise solution, to give the best of both options. The Yutaki heat pumps offered by Greentherm are rated by Hitachi to operate at minimum temperatures of -20ºC -well below the record low Winter temperature in Ireland. They also have an upper temperature limit of 55ºC which is more than hot enough for domestic hot water and most heating systems. Even at these extremes of temperature – delivering 55ºC output with an atmospheric temperature of -20ºC – the COP is still approximately 2.

The Bottom Line.

Just because it feels cold outside, doesn’t mean there’s no energy available for your Heat Pump. Refrigerant gasses which boil at low temperatures enable the Heat Pump to extract heat from ‘cold’ sources such as a ground loop or ambient air. Allowing the refrigerant to boil absorbs heat from the source to turn it to a gas. Compressing the refrigerant back down to a liquid allows this atmospheric heat to be extracted. Compressing, however, requires energy input. The ratio between input energy to the compressor and output from the Heat Pump is the Coefficient of Performance, and is a measure of how efficient the Heat Pump is. The higher the output temperature the more work the Compressor has to do to achieve it, reducing the efficiency of the Heat Pump.

Exactly how efficient your Heat Pump will be is determined by the refrigerant gas chosen by the manufacturer and the temperature it’s operated at. Different gases have different temperature limits and performance characteristics, suitable for different installations or environments.

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