Heat pumps are an increasingly popular option for hot water heating in many homes. They work by extracting heat from the air and using it to heat water, which can result in significant energy savings and a reduced environmental impact. But how do heat pumps actually extract heat from the air, and what makes them so energy efficient?
Heat pumps can be thought of as fridges that operate in reverse. They work by drawing in a large volume of low-temperature air and using a compressor to compress and heat it, before transferring the heat to the water in the tank. This process is known as “heat harvesting” and it’s the key to the high efficiency of heat pumps.
To understand how this works, let’s imagine that the outside air is at 10°C. A heat pump can extract 8°C of heat from this air as it passes through. So, the air that enters the heat pump at 10°C comes out the other side at 2°C. But the trick here is the volume of air that the heat pump is extracting heat from. A heat pump can churn through over 150 litres per second of air, which equates to over 550,000 litres of air in an hour.
Now, let’s assume that there is 300 litres of water inside the heat pump tank. Over an hour of operation, the heat pump would pass 550,000 litres of air (extracting the 8°C of heat from it), and the 300 litres of water in the heat pump will also heat up by approximately 6°C. This might not seem like a lot, but it’s important to consider the difference in volume between the air and the water. Low-temperature heat is extracted from 550,000 litres of air in an hour, while high-temperature heat is added to 300 litres of water, which is an 1833x difference in volume. This is how the heat pump “pumps” the heat from a large environment (the air) to a small environment (the tank water) and concentrates it.
The process of harvesting and concentrating heat is also the secret to the high efficiency of heat pumps. When you consider an electric hot water unit, the energy from the electricity is simply converted into heat energy. This happens at around a 99% efficiency level, meaning that for every 1kW of electricity, 0.99kW of heat is emitted from the heating element into the water. But with a heat pump, you don’t have a direct relationship between the amount of electricity you input and the amount of heat that is output. Instead, the heat pump gains 3/4 of the heat from harvesting and concentrating the heat from the air, and only 1/4 of the heat is gained from the conversion of electrical input into heat from the compressor. This means that you get 4 units of heat for the cost of 1, resulting in an efficiency of 400%.
It’s important to note that the efficiency of a heat pump, also known as its Coefficient of Production (COP), can vary depending on factors such as the temperature of the air and the water, as well as the brand and model of the heat pump. However, by understanding the basic principles of how heat pumps extract heat from the air and why they’re so energy efficient, homeowners can make an informed decision when choosing a heat pump for their home.