If we are going to get global consumption of power on to a much more efficient basis, one of the challenges is air conditioning. Could we find a breakthrough that would cut 90% of the electricity required for space cooling?

Air conditioners in use today work on the same principle as refrigerators, but instead of dumping the heat into the room (which is what happens at the back of a refrigerator) it is sent outdoors. Air conditioners use a lot of electricity because they involve repeated cycles of compressing a gas to a liquid, and that consumes power. In Singapore – a small, hot, developed country – about 30% of household power is used for air conditioning, and in commercial buildings it is 40%.



Because much global economic development has happened, and will happen, in hot climates, the demand for air conditioning can be expected to increase markedly over the coming decade. There is a need for a much more energy efficient way to accomplish space cooling. The simplest thing to do is to refrain from excessively air conditioning a work or living space. A few degrees higher setting of the thermostat in those chilly restaurants, hotels, homes and offices would go a long way to saving energy.



But what about the device itself? If the energy cost of compressing the gas back to a liquid is the critical step, is there any way to reduce or eliminate this? There is an old idea that is now being revived as a result of advances in chemical materials technology.

Certain materials have a strong tendency to adsorb (that is bind to the surface) water vapour. When this happens, heat is released. Suppose that cool water vapour picks up heat from a hot room, and is then directed outside to where the adsorbing material is located. The water is trapped and the heat released to the atmosphere.

To get the water back off the material, as cool vapour, only a small amount of heat needs to be applied to the solid. Ideally this would come from solar energy, or from some other source of waste heat. If neither is available, a small electric heater is required. Devices cooling air on this principle are known as adsorption chillers.



The problem is that until recently the materials available at a reasonable cost, such as silica gel (the stuff in those little packets you get with various purchases, to keep them dry) do not trap very much water, so you need a lot of them to cool a space. The resulting adsorption chillers are large and heavy – and weight is a consideration for air conditioners, which are often placed on a roof.

New materials are now being developed that are a step change in the amount of water they can hold. These materials are known as metal-organic framework (MOF) compounds.

Some of the recent ones, developed by Dr Raya al Dadah at the University of Birmingham, at Heinrich Heine University in Dusseldorf, and at MIT in the US, will adsorb more than four times as much water as silica gel. As the name implies, these materials are made of various metals, such as zirconium or chromium, bound together with a loose web of carbon atoms, so that there is a much greater surface area with sites where water molecules can be bound.



But the work of the chemists and engineers is not done. Many materials will work well for a few cycles of adsorption and its reverse, desorption. But a commercial adsorption chiller needs to keep up this performance for thousands of cycles without requiring a change of the MOF.

The first materials are undergoing such tests, and if they succeed we might soon see commercial air conditioning using 10% of the energy required for today’s units.



Bernie Bulkin is a director of Ludgate Investments, HMN Colmworth, and K3Solar. He was chair of the Office of Renewable Energy for the UK government from 2010-2013, and a member of the UK Sustainable Development Commission. He was formerly chief scientist of BP.



More stories like this:

The technology and innovation hub is funded by BT. All content is editorially independent except for pieces labelled advertisement feature. Find out more here.

Join the community of sustainability professionals and experts. Become a GSB member to get more stories like this direct to your inbox