Putting heat only where it’s needed
Tom Shelley reports on a simple, low cost technology that heats water by direct application of electric current
Energy efficiency can be vastly improved by heating things directly by passing electric current through them.
The idea was originally invented and patented back in about 1900, and has for some time been used in the food industry – mainly in niche applications. Now, because of the reduced cost of power electronics and the widespread use and reliability of RCDs, it has become feasible to apply such techniques to applications that range from domestic kettles and shower heads to large scale sewage treatment.
In the food industry, ‘ohmic’ heating avoids the very real possibility that food will be burned onto the inside of an externally heated vessel, thereby improving quality. Applied to water, it greatly reduces or in some cases totally eliminates the formation of scale in water heating units even in very hard water areas.
C-Tech Innovation in Capenhurst in the North West of England is a private management owned company, whose roots are in the old Electricity Council Research Centre. While it still devotes much of its energies to the innovative and efficient application of electricity to industrial production and manufacturing, it is applying some of its expertise to smaller scale production and even the domestic market.
A recent development is an ohmic heated electric kettle in which electricity is passed directly through the water rather than through a heating element. This speeds up heating and is completely silent, because there are no steam bubbles being nucleated on hot element surfaces. It is also slightly more energy efficient, as there are no elements to oxidise and eventually fail, and greatly reduced generation of lime scale even from very hard water.
Researcher John Collins explains: “In a conventional electric kettle, the surfaces of the elements are heating the water far in excess of 100ºC which promotes the precipitation of lime from the water, turning it into scale on the element surfaces. In the new kettle, there is still some formation of calcium carbonate but it appears in the water in finely divided form.”
However, it is not just a matter of connecting up the electrodes to the mains and turning the power on.
“Water from different places has different resistivities, and the electrical properties change significantly as it heats up,” Collins says.
This is the area in which the company’s patents lie. Indeed, the unit could be seen to be run by an electronic controller that maintained current flow at or very close to 13A, regardless of the resistivity of its contents.
“We also have an instantaneous electric shower, and under-sink water heater, whose designs are more like our industrial applications,” Collins continues. “We started developing ohmic heating in the 1980s, licensing it to the food industry, moving onto heating acids and treating waste. To apply it, we are always looking for products that are hard to heat by normal methods – such as things that degrade or leave deposits on a hot surface. It’s a very low maintenance system. We sell it as a robust heating technique over which users have a very high level of control. Its main competitor is steam heating.”
Steam heating may be popular in chemical, food and process industries but it is not energy efficient. All steam-raising systems inevitably suffer from limits to efficiency because of the need to transfer heat across a solid interface – and there is always the need to provide latent heat of vaporisation. Steam-raising power stations suffer from the same problems but tend to be more efficient than most industrial steam heating systems because of the very large scale of their operations and high operating pressures and temperatures.
The alternative and slightly more efficient way of heating by steam injection adds water to that which is being heated.
Enquiring further about the technology behind the kettle, we were told that the latest designs had smaller electrodes than the one shown to us, and it was intended to package the electronics into the kettle base to minimise size. On the subject of the ohmic heated shower, operations director Michael Harrison told us: “There are no ohmic-heated showers that we are aware of.” As it happens, the Shelley household has personal experience of a spa bath, made in Canada, that uses an ohmic heater.
Many will not like the idea of bathing or showering in water that is heated by passing electricity through it. Safety in ohmic heaters is ensured by having relatively long, insulated pipes between the ohmic heating section and the place where a person makes contact with the water. As electric current will always prefer to pass between the electrodes by the shortest route, it will never make an excursion down a pipe. Additional safety mechanisms can be added if required, such as making use of an isolating transformer. However, experience with conventional electric showers is that provided that the shower is directly connected to a modern consumer unit with an RCD, any fault – however unlikely – will immediately lead to an immediate cut out when the mains current goes to earth through earthed metal pipes forming part of the system.
Tests on the prototype shower have equated to running the unit for several years under normal and safe operating conditions. Even on water deliberately made as hard as might be encountered, results have shown that there is only a slight formation of film. Harrison says: “The drive is for showers to become smaller and more beautiful. There is also the opportunity that the use of ohmic heating will also make them cheaper.”
Back on the industrial scale of things, C-Tech is doing a lot of work on applying the technology to a variety of processes, including the treatment of sewage. Harrison says: “It’s quite a good way of heating suspensions of particulates. Lumps and bumps don’t cause problems. Ohmic heating can do as much as microwaves and is much cheaper.”
C-Tech has not particular axe to grind on this one, since is also an expert in the application of energy by microwaves and radio frequency heating – but provided there is plenty a flow of liquid water or other conducting solution, ohmic heating can be much simpler and cheaper.
* Ohmic heating is the passage of electric current through whatever it is that has to be heated. It is essentially 100% efficient, with the only possible heat loss by thermal conduction through the electrodes.
* Applied to liquids, it avoids the need to transfer heat into the medium. Applied to water, it causes negligible build up of lime scale
* Electronic control is essential because water and water-based mixtures and solutions decrease in resistance by a factor of around three between room temperature and 100ºC
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