Without the hype
How much do ACs increase the urban heat island effect?
There has been a surprising amount of chitter-chatter recently about whether air conditioning heats urban environments and makes the urban heat island effect worse. What is especially amusing about many of these arguments is that they often fail to understand how air conditioning actually works, and they also tend to ignore some fairly basic thermodynamics. Fortunately, there is a relatively easy way to think about it.
An air conditioner does not magically produce huge amounts of extra heat. It moves heat. During cooling, the outdoor unit rejects two things: the heat removed from inside the building, and the electrical energy used to run the compressor and fans. The important point is that only the second one is really new energy being added to the urban system. The heat removed from inside the building was already in the city. It was already in the air, the walls, the roof, the floors, the furniture, or on its way into those things. The air conditioner moves it from one place to another. It does not summon it from another dimension.
For a properly installed split air conditioner, during cooling in the UK, a rough figure might be that the compressor uses about one unit of electricity to move around four units of heat. So if the system removes 4 kW of heat from inside, it might use about 1 kW of electricity to do so. The outdoor unit would then reject about 5 kW of heat, but at the city scale, the key addition is not 5 kW. It is the 1 kW of electrical energy, which eventually becomes heat.
This is because the 4 kW of heat was already in the system: it was going to make its way outside whatever happened.
So let us try to put some rough numbers on this. A decent split air conditioner in a maisonette or flat might use something like 800 W of electricity while running. Obviously this varies enormously depending on the size of the property, the insulation, the temperature outside, how cold people set it, whether the unit is correctly sized, and so on. But 800 W is good enough for a rough estimate. London has around 4 million dwellings. If every one of those dwellings were running an air conditioner using 800 W, the total electrical power going into air conditioning would be 4 million × 800 W, which is 3.2 GW. To be generous, round that up to 4 GW.
That 4 GW is the extra heat being added to the city by the air conditioning. Not 20 GW, not 50 GW, not some magical enormous number. The additional heat is the electricity used to run the machines. So is 4 GW a lot? Well, it depends what you compare it to. The land area of London is about 1.5 billion square metres. Around midday in summer, solar irradiance in the UK might be somewhere around 800 to 1,000 W per square metre. Not every square metre is flat, unshaded, or receiving maximum sunlight, but for a back-of-the-envelope calculation we can use 1,000 W/m² and then account for reflection using albedo.
Urban albedo is complicated, because London is not just tarmac and roof tiles. It has parks, trees, water, light surfaces, dark surfaces, shaded streets, glass, brick, and all sorts of awkward geometry. But suppose we use an albedo of 0.25. That means roughly 75% of incoming solar energy is absorbed. So 1.5 billion m² × 1,000 W/m² × 0.75 gives about 1.125 TW, or 1,125 GW. Compared with 4 GW from the electricity used by air conditioning, the absorbed sunlight is hundreds of times larger. This does not mean air conditioning has no effect at all. It means that if we are talking about the energy budget of a city, we should keep some sense of scale.
Obviously this is a simplification, but it is a simplification based on the physics of how these units actually work. If you walk outside in a city at night and put your hand on a dark road, you can feel the heat coming off it. That is not an air conditioner. That is solar energy which was absorbed during the day and then stored in the urban fabric. Realistically, the urban heat island effect is largely a consequence of low-albedo surfaces and thermal mass: dark roofs, dark roads, brick, concrete, asphalt, and all the other materials which absorb a lot of sunlight and then release that heat slowly later on.
You can go and sit on dry, crusty grass in a park in summer and it might not be especially pleasant, but it usually does not feel like sitting on a hotplate. You are not going to casually sit down on a black road surface that has spent all afternoon absorbing and retaining an enormous amount of heat. So if you wanted to reduce urban heat island effects, the most obvious place to start is not shouting about air conditioners. It is the surfaces.
For example, you could look at adding reflective pigments or lighter aggregates to road surfaces. Obviously you have to be careful with this. You do not want to blind drivers, create glare problems, make road markings harder to see, or produce surfaces that are unsafe in wet conditions. But fundamentally, the more incoming solar radiation you can reflect rather than absorb, the better your outcomes are likely to be.
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