In the first of our Mesh Energy Explains series of 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…

A brief history of heat pumps and the basics

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 use in air conditioning and refrigeration.

In the 1940s, Robert C. Webber is credited with creating the first modern day heat pump and 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 low-grade energy from the natural environment 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 domestic hot water system.

The diagram below shows the basic cycle.

Air source heat pumps use an external fan unit to draw air across the evaporator unit, which 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 single fan (or fans) 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 successfully extract energy from the air down to well below -20ºC. Because the refrigerant in the heat pump boils at such a low temperature, even air well below freezing temperatures can give up its heat.

Due to energy in the air affecting outputs from air source units, the efficiencies of these systems typically vary from 450% efficient in the summer and 250% efficient in the winter months. This means that even in the winter one unit of electricity can supply 2.5 units of heat to the home using ‘freezing’ cold air and some clever physics!