Decarbonising heat

A key challenge facing the UK is decarbonising the energy system. The UK is legally committed to reduce its carbon emissions by 80% by 2050 and eventually to net-zero under the Paris Agreement.  Most of the focus in the public mind has been on low carbon electricity, but heat is equally critical, and the Climate Change Committee says it needs to be almost completely decarbonised by 2050. 44% of the UK energy consumption is for heat, a little less for transport, and the rest for all other uses.

We use heat everywhere. We are immediately aware of its use in buildings, for space and water heating, but it is equally important in industry. Manufacturing industries use significant quantities of heat; from the glowing heat of metals and ceramics production, to the much more modest temperatures of food production and processing. Just over half our heat demand is domestic, and about a quarter for all industrial applications.

Consider this diagram. It shows the UK domestic energy consumption as electricity and heat over a year. The electricity demand varies significantly over a day, but is remarkable constant over the seasons. Heat demand in contrast is low in summer, when it is close to electricity demand, to nearly 6 times the electricity demand in the depths of winter. Overall, decarbonising heat is a much bigger challenge than decarbonising electricity.

Many people assume that we need to dramatically increase our production of low carbon electricity so that we can use it to generate heat. But that is a tall order, both in terms of generation and the strains it will place on the grid. Add in the growth in electric vehicles and it is even worse.

Ideally, we should decarbonise heat without using electricity.

Fortunately, there are some interesting approaches to doing exactly this, and I was able to see one of them in action last week at an Open Day in Derbyshire run by Rotaheat.

Heat from magnetic braking

Rotaheat take advantage of the properties of powerful magnets to turn mechanical energy from a rotating shaft directly into heat. We are used to seeing on television the brake disks of a Formula 1 car glowing bright red as they heat up to 1200 C from the friction of slowing to safely corner after a fast straight. You also get a braking effect if you spin a conducting disk between powerful magnets. The interaction between the conductor and the magnetic field sets up a counter force that slows the disk down. There is no contact with the spinning disk and no friction, but the braking force converts mechanical energy into heat in exactly the same way as those Formula 1 brakes.

These magnetic brakes are widely used to slow high-speed trains and roller coasters, and to stop powered tools quickly.

The Rotaheat unit I saw looks rather like a large flat centrifugal pump. A drive shaft spins a disk inside a housing. The inner part of the disk is a pump impeller and the outer part is aluminium. This section is rotating between high power permanent magnets in the housing that are continuously trying to slow the disk down, converting the rotation of the shaft into heat. Water flows into the unit at the axis and is forced by the impeller out across the disk cooling the hot disk and heating the water.

In this demonstration the drive was a waterwheel, and the unit was easily able to heat two radiators to a temperature too hot to touch comfortably. Pretty impressive.

This size of unit is designed to deliver 10kW continuous heat output. It can be easily scaled by increasing the number or strength of the magnets, increasing the shaft speed (the faster the rotation the bigger the braking force and the greater the heat output), or building bigger units.

Where can you use it?

This could be used wherever you have a low-carbon source of rotational energy, e.g. a water or wind turbine, and a need for heat nearby. Although domestic heating is the dominant use of heat, it is probably impractical to use this system on individual houses, but for farms, factories, public and commercial buildings where there is access to water or wind power it is ideal.

What are the benefits?

Generating heat this way brings a number of advantages. It is extremely efficient compared to generating electricity and then turning that into heat.

The braking force increases with the speed of rotation, so the system is self-governing. You could attach a unit to a wind turbine without the problem of high wind speeds damaging gearbox or generator.

Where heat is being generated by a boiler you typically have an intermediate heat storage system, such as an insulated tank, and a separate pumped heat distribution system. So this is a direct and simple replacement for the boiler. You don’t have to reconfigure the entire system as you might with a heat pump. Alternatively, instead of a hot water tank you could improve the ability of the system to smooth any intermittency of heat generation by using a phase-change heat battery for storage, such as those developed by Sunamp.

Whilst there are clearly applications in the UK and other countries with relatively centralised energy grids, perhaps the biggest opportunity is in other countries where there is a bigger demand for low-carbon off-grid heat.

I am looking forward to see how Rotaheat and their innovative heating system develop over the coming years. I think it could be an important contribution to a very difficult problem.


Renewable Low-Carbon Heat
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