How It Works: How Do Heat Pumps Work?

In the first of many “How it works” blog posts we’re looking at just how heat pumps work; specifically how energy is harvested and transported into the home using little more than an electrical supply. Those of us working in the renewable technology sector often forget that sometimes we need to return to basics and explain some of the fundamentals to those just entering into the field, or researching for their personal provision. So here goes…

In the 19th century William Thompson, otherwise known as Lord Kelvin (of temperature scale fame), pioneered the concept of the heat pump using a refrigerant cycle to prove that heat energy could be used to cool. In fact, he foresaw its use in air conditioning and refrigeration.

In the 1940s Robert C. Webber is credited with creating the first modern day heat pump, we haven’t looked back since. Today’s heat pumps work in exactly the same way; with developments in efficient compressor technology and refrigerant gases incrementally improving operational efficiency of the units.

Heat pumps are often described as “fridges working in reverse”. Energy in fridges is extracted from the enclosed interior of the fridge, keeping your food cool and using a tiny amount of electrical energy to power a small compressor. This extracted heat is “processed” and ultimately expelled at the back of the fridge via an element.

A heat pump uses an identical principle and strips energy from the atmosphere using the process of gas evaporation. Once compressed by the compressor cycle it is during its condensation phase that high temperature energy is exchanged into your under floor, radiator or hot water system. The diagram below shows the basic cycle.

As a general rule heat pumps, as we know them today, use three distinct mediums to harvest energy; earth/ground, air and water. Being pedantic it is the water moisture in all of these mediums that helps with energy transfer, but for clarity we will stick with these three.

So it is the changing heat source that helps the evaporation phase of the refrigerant cycle that gives heat pumps their names.

Ground source heat pumps

Ground source heat pumps (sometimes called geothermal heat pumps) use two practical methods for harvesting energy. The first is horizontally buried pipework in the ground which has pumped around it a glycol/water mixture to stop it freezing. The pipes are buried at approximately 1.5m below the ground level and the ambient heat of the earth provides energy to the liquid flowing through the buried pipes. The fluid which warms up just a few degrees in its journey in the ground heads back to the heat pump to be extracted and to ultimately heat the home.

Alternatively vertical boreholes (circa 100m) can be drilled which require significantly less ground area. These still rely on the grounds low level thermal energy to warm the fluid but because of the depth much more stable ground temperatures are found, and efficiency of the system improves.

Air source heat pumps

Air source heat pumps use an external fan unit to draw air across the evaporator unit, it exits at the back of the unit cooler than it entered. Because of varying seasonal air temperatures these heat pumps are less efficient than their ground source cousins. As a rule the colder the outdoor air gets the harder the system has to work to strip energy from the air and elevate it to heat the home.

Air is sucked into air source fan units using a large fan and refrigerant pipes strip heat from the air. The air exits the fan units at a cooler temperature. You may be surprised to hear that air source heat pumps can successful extract energy from the air down to well below -20C. Because the refrigerant in the heat pump boils at such a low temperature, even air a below freezing temperatures can give up its heat.

Water source heat pumps

Water source heat pumps are so called as they are used in lakes, ponds and streams to harvest energy. “Open loop” and “closed loop” systems are used. Open loop systems can be likened to a hoover sucking up water and using this direct energy to transfer to the evaporator via a plate heat exchanger. The closed loop system is basically a ground source system with loops or panels buried or pinned under water to harvest energy from the body of water. There are many advantages regarding the amount of energy that can be harvested from pure bodies of water especially stream and rivers as flow rates are high and ensure an almost endless supply of energy to the heat pump.

So there you go. A whirlwind introduction to the magical, yet practical basics behind the increasingly more common heat pump.

Doug Johnson

Doug is a chartered mechanical engineer and has had the pleasure of being involved in the UK renewable sector since 2008. With wide-ranging operational and design experience in both the domestic and commercial sectors across a wide range of recognised technologies he hopes to be able to share his experience to help enlighten and save you hassle along the way.