Hot tub Technology

Dr Barrett says the heaters could be switched on and off rapidly to compensate for the erratic output of wind turbines and solar panels, each heater controlled by a gadget that responds to signals sent through the electricity grid – a system used since the second world war. “Everybody is always looking for a shiny new silver-bullet solution” says Dr Barrett, “but this idea is cheap, safe, and based on technology that’s been around for decades”.

Tea-time troubles

Renewables are a problem for the grid, as currently configured, because supply has to match unfettered demand minute-by-minute. In Britain power consumption ranges between about 20GW and 60GW (gigawatts) depending on season and time of day. But unlike coal- and gas-fired power stations, wind turbines and solar panels are “non-dispatchable”, meaning they cannot be cranked up at a moment’s notice during half-time in the cup final if the nation is gasping for tea. This limits the proportion of renewables that can be absorbed into the grid – although the level of that ceiling is hotly debated.

But renewables are only a problem when demand is taken as the given. If demand could be actively managed as well, a far greater proportion of renewables could be absorbed, slashing carbon emissions and raising energy security. And that’s where the immersion heaters come in.

Dr Barrett explains that 19m domestic tanks, each fitted with a standard 3kW (kilowatt) immersion heater, would provide over 55GW of potentially flexible demand, which could be adjusted to suit the output of renewable generators. The immersion controller would ensure the water temperature stays above a set minimum – so the house would never be without a hot shower – but within a range of 45C-65C the grid would be in control. Along with hot-water storage in commercial buildings, this would provide balancing capacity greater than peak consumption today, and is a key feature of the computer model Dr Barrett has devised to investigate how Britain could best achieve a high proportion of renewable power.

The model assumes a massive increase in wind and solar capacity; smaller amounts of wave, tidal and hydro; expanded interconnectors to France; and increased electricity storage such as the Dinorwig pumped storage facility in Wales. Existing fossil fuel stations are “mothballed” for use only as a last resort. Using a range of hourly demand forecasts and weather data, the model has shown Britain could on average generate 95% of its electricity consumption from renewables.

In this system, hot-water storage is crucial for balancing supply and demand: when renewable generation exceeds demand, the surplus is exported to the continent, and used to recharge electrical storage and hot water tanks; when demand exceeds renewable generation, the shortfall is made up by turning off water heaters, drawing on electricity storage and imports, and firing up old fossil fuel stations.

Dr Barrett claims the immersion heaters could be controlled using a system called ripple control, where high-frequency pulses are sent through the mains and received by a device on each water heater that turns power off and on as required. The system has been used for decades in New Zealand, where the grid company can now reduce peak demand by about 13%, and so defer expensive investments in new power stations. In Florida, where the local power company has struggled to cope with demand caused by a 50-year housing boom, 700,000 customers receive a monthly rebate for handing over control of their hot water heaters, and the utility has avoided building a 1GW power station as a result. In South Africa, ripple control is being introduced to prevent a repeat of the rolling blackouts that crippled the country last year.

Peak practice

Experts warn balancing the entire grid in real time is massively more complicated than occasionally reducing peak demand, and question whether ripple control could do the job. Dr Graeme Bathurst, technical director of the Manchester-based grid consultancy TNEI, says that different numbers of water heaters would need to be turned on and off every minute of the day, yet a traditional ripple-control system – which only transmits instructions, and cannot receive information from water tanks – would not know how much flexible capacity was available at any moment, nor how many heaters to control. “There is massive potential in heat storage, and this concept is eminently achievable,” says Bathurst, “but I think it will need a more intelligent system to make it work”.

Dr Barrett argues that the aggregate heat demand of 20m households would be fairly predictable, but concedes that a modern interactive system would be better. He says it is vital the new “smart meters” the government plans to install in every home by 2020 should be capable of controlling hot-water storage. “But this isn’t rocket science,” says Dr Barrett. “It is quite clear we can go hell for leather installing renewables because we can deal with intermittency using heat storage.”

• David Strahan is the author of The Last Oil Shock: A Survival Guide to the Imminent Extinction of Petroleum Man

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