How on earth can an ASHP be above 100% efficient?

How on earth can an ASHP be above 100% efficient?

This is where the magic (and often confusion) of heat pumps comes in! It seems counter-intuitive, but it's completely real and explained by the laws of thermodynamics.

The key to understanding how an Air Source Heat Pump can be over 100% efficient is to realise that it's not generating heat from the electricity it consumes. Instead, it's moving existing heat from one place to another.

Let's break down the concepts:

1. What is "efficiency" in this context?

When we talk about a boiler or an electric heater being 90% or 100% efficient, we're talking about energy conversion.

• Boiler: Burns fuel (gas, oil). 100% of the energy in the fuel could theoretically become heat, but some is lost through the flue gases or radiation. So, a 90% efficient boiler means 90% of the energy in the fuel is converted into useful heat for your home.

• Electric Heater: Converts electrical energy directly into heat. Nearly 100% of the electrical energy is converted into heat (COP of 1).

2. The heat pump difference: heat transfer, not generation

A heat pump operates on a completely different principle.

It uses a small amount of high-grade energy (electricity) to move a larger amount of low-grade thermal energy (heat) from one location (the cold outside air) to another (your warmer home).

Think of it like this:

• Elevator Example: Imagine you need to move 10 people from the ground floor to the 10th floor.

  • "Generating" Approach (like a boiler): You could pay a team of bodybuilders to carry each person up, one by one. This would take a huge amount of effort (energy) for each person moved.

  • "Transferring" Approach (like a heat pump): You could use an elevator. The elevator itself consumes some electricity to run its motor, but that small electrical input allows it to move a large number of people much more efficiently than carrying them. The people (heat) already exist; you're just moving them against their natural tendency (from cold to hot).

3. The laws of thermodynamics and COP

First Law of Thermodynamics (Conservation of Energy): This law states that energy cannot be created or destroyed, only transferred or changed from one form to another. A heat pump does not violate this law. The total energy always balances. The electrical energy input + the heat extracted from the source = the heat output.

• Electrical Input (Work) + Heat from Outside = Total Heat Delivered Indoors

Second Law of Thermodynamics (Entropy): This law explains why heat naturally flows from hot to cold. To move heat from a colder place to a warmer place (which is what a heat pump does), you need to do work on the system. This "work" is the electricity that powers the compressor and fans.

• The Coefficient of Performance (COP) is the measure of a heat pump's efficiency. It's calculated as: COP=Electrical Energy Input (kWh)Useful Heat Output (kWh)

• Since the heat pump is transferring heat from the environment in addition to the heat generated by the electrical input (the compressor creates some heat as a byproduct of its work), the useful heat output can be greater than the electrical energy input.

Example:

Let's say a heat pump uses 1 kWh of electricity to run its compressor and fan. During that time, it extracts 2 kWh of heat from the outside air.

The total heat delivered to your home would be:

1 kWh (electrical input) + 2 kWh (heat from air) = 3 kWh of heat output

Now, let's calculate the COP:

COP = 3 kWh (output) / 1 kWh (input) = 3

Expressed as a percentage, this is 300% efficiency.

Why it's not a "Perpetual Motion Machine":

It's crucial to understand that the heat pump isn't creating energy out of nothing. It's simply taking advantage of the vast amount of thermal energy that already exists in the environment (even cold air has a lot of heat content relative to absolute zero, -273.15°C). The electricity is the "work" needed to make this natural heat flow against its usual direction.

So, while it sounds unbelievable at first, the "over 100% efficiency" of a heat pump is a direct consequence of its fundamental operation as a heat transfer device, rather than a heat generation device.