Back to Basics: Heat Pumps
An introductory guide to how heat pumps work by Alan Pears AM (RMIT University) and Geoff Andrews (Genesis Now)
Basically, a heat pump (or chiller) works by concentrating heat to a higher temperature at the condenser and a lower temperature at the evaporator, and shifting (pumping) it. The technology takes advantage of the large amount of energy stored or released as the refrigerant changes phase between gas and liquid. For example, when changing between liquid and gas, the common refrigerant R134a absorbs or releases 150 to 250 times as much energy as it would when its temperature is changed by one degree Celsius.
The only difference between a heat pump and a chiller is that one is designed to remove heat from a space or process stream, making it cooler and rejecting heat to the environment, while the other is designed to extract heat from the environment and use it to provide useful heat.
Consider a simple example: the evaporator in a refrigerator removes heat from the space around it (within the refrigerator) by evaporating a refrigerant, absorbing heat as the liquid changes into a gas. It ‘concentrates’ this heat to a higher temperature using a compressor (just like a bike pump gets hot as you pump up the tyres).
Through design of the system and selection of refrigerant, this temperature is higher than the ambient temperature around the condenser, so the condenser can lose heat to its surroundings and the refrigerant condenses back to a liquid. So the coils on the back of the fridge (or attached to the sides and back) get hot, and lose heat to the room the fridge is in. The refrigerant is then allowed to flow into the evaporator via a pressure reduction valve. As its pressure falls, it evaporates and becomes colder, absorbing heat from the evaporator, just as spray from a spray can full of liquid cools as it is released into the air.
For different circumstances, different refrigerants (sometimes called working fluids) with varying boiling points and operating pressures are used. Indeed, even water can be used as a very effective refrigerant if the right operating pressures can be established so that it boils and condenses at suitable temperatures (its designation is R718 – see here for more)!
Heat pumps can seemingly defy the laws of thermodynamics, because they can deliver much more than one unit of heat (or cooling) per unit of electrical energy consumed. This is because they are extracting heat from around the evaporator and dumping heat to the environment around the condenser. Electricity is being used to concentrate and shift heat, not to produce heat directly as in a resistive electric radiator or fan heater. And, as motor, compressor and heat exchanger efficiencies improve, they can do more useful work per unit of electricity consumed. Indeed, the exciting thing about heat pumps is that (in theory) they can deliver up to 10-15 times as much energy as they consume driving the compressor! So a leading edge (at present) residential heat pump can deliver heat at 600% efficiency, compared with a gas heater at 50% to 95% efficiency. When the electricity is produced using renewable electricity, this delivers astounding reductions in greenhouse gas emissions.
The overall efficiency with which heat is transferred by a heat pump or chiller is called its Coefficient of Performance (COP) or Energy Efficiency Ratio (EER) – these terms really mean the same thing. This is simply the ratio of useful heating or cooling provided divided by the input power. An air conditioner with a COP of 5 delivers five times as much heating or cooling as the amount of electricity it uses.
How can you get heat from ‘cold’?
Many people struggle to understand how a heat pump can extract heat energy from ‘cold’ air (or water). This reflects our limited understanding of the amount of energy in our environment. Lord Kelvin provided the key to this in the 19th century. Matter only has its theoretical minimum heat energy at ‘Absolute Zero’ or zero degrees Kelvin, which is minus 273 degrees Celsius. The temperature in outer space is close to this. So air or water at, say, 7C actually contains a lot of energy, because it is 280 degrees above Absolute Zero (or 280 degrees Kelvin). Indeed, it has 90% as much energy as air or water at 37C (310 Kelvin).
So, even in very cold climates, there is a lot of heat energy available in our ambient environment. Heat pumps can change the temperature by concentrating this energy and shifting it around.
We also need to understand the difference between ‘heat’ and ‘temperature’. Heat is a form of energy that is directly related to how much the sub-atomic particles, atoms and molecules move. Temperature is the ‘pressure’ driving the heat flow. Heat energy flows from a higher temperature to a lower temperature. So temperature is like water pressure: water flows from a place of higher energy state to one of lower energy – so it flows downhill to a location of lower gravitational potential energy. So heat energy flows ‘downhill’ from hotter to colder places. But the amount of heat flowing depends on many factors, including insulation, conductivity and heat capacity of materials and heat transfer across surfaces.
For a given heat pump, the bigger the temperature difference between its evaporator and condenser, the less efficient it can be, due to the fundamentals of thermodynamics. Conversely, by minimising the temperature differential, efficiency can be improved. By improving the efficiency of heat transfer across the condenser and evaporator heat exchangers, overall efficiency can also be improved.
This article was written by Alan Pears and Geoff Andrews for the Energy Efficiency Council's Efficiency Action e-zine, February 2016.
Alan Pears AM is a Senior Industry Fellow at RMIT University and Senior Advisor to the Energy Efficiency Council. Alan has worked across all aspects of sustainable energy in policy, program development, public education and specific projects and is a well-known commentator on sustainable energy issues.
Geoff Andrews is the founder of energy efficiency engineering consultancy Genesis Now, co-founder of six of sustainability focused businesses and inventor of Organic Response. Geoff was named ‘Energy Efficiency Leader 2013’ at the National Energy Efficiency Awards in recognition of his long-standing contribution to the sector.
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