Thermal batteries - could they replace our boilers?

It's time for big heat

For most of us batteries are those small cylindrical objects you pop into torches or TV remotes – either that or heavy black boxes, located in inaccessible regions of your car, which mysteriously lose their charge on cold winter mornings when you urgently need to be somewhere.

At least, that was how it used to be before renewable energy became widespread in the UK, either through the national grid or on thousands of our rooftops. Electricity created by wind turbines or solar panels may be essentially ‘free’, but it depends on sunny skies and steady winds, neither of which can be guaranteed. Consequently, solar- and wind-generated energy turns up randomly – and not always when we need it most in our homes.

So the obvious answer is to store that power until we need to use it. Which brings us back to batteries.

Some years ago, I spent a weekend in a holiday cottage in a wind-swept glen on Speyside in the north-west of Scotland. It was actually a renovated former croft, miles from any mains services. It ran on electricity produced by a large wind turbine on the hillside above it. The electricity was stored in several bulky batteries – allegedly from a submarine – housed in a shed beside the cottage.

According to the owner, these would provide more than enough power for a weekend stay, though if they didn’t there was a back-up diesel generator which would fire up automatically if needed.

Few of us have the space for our own wind turbine, or much likelihood of getting planning permission for one outside fairly remote locations. But, if you have – or plan to have – a south- or near-south-facing roof, installing photovoltaic (PV) panels can provide a similar source of renewable energy.

According to a report by Solar Energy UK (formerly the Solar Trade Association), a typical 2,512kW array could reduce your annual electricity bill by between £329 and £963.

But that report dates from last February. Given the explosion in fuel prices since, today’s savings will be considerably higher. They would be even higher if all the electricity your panels produce is used. And for that you need a battery.

Solar batteries not only store unused PV electricity, they can also be programmed to store cheaper off-peak mains electricity.

They are, however, expensive, costing an average of around £4,500, even with zero VAT if installed with PV. And they will only last for around half the 20- to 30-year lifetime of solar panels, their efficiency declining as they age.

But they aren’t the only form of domestic energy storage. So-called thermal batteries can offer similar advantages – and, manufacturers claim, at a lower cost over much longer lifetimes and with no loss of effi ciency.

Storing energy

Storing energy in the form of heat isn’t new. Most householders already have a form of thermal battery – in their hotwater cylinder. Water is a good medium for heat storage, but today’s thermal batteries use newly developed materials which can retain heat at much higher temperatures and in much smaller volumes.

The best established – in fact, the first commercially viable thermal battery system – is the Sunamp heat battery, designed and manufactured in Scotland. Its storage medium is a patented saltbased phase change material, able to absorb large amounts of heat as it changes from solid to liquid and release the heat when it changes back. The high temperatures achieved are maintained by highly efficient vacuum panel insulation.

The Sunamp can be heated electrically, using solar and off-peak electricity, or by means of a heat pump or conventional boiler. Connected to hot and cold feeds, it will provide domestic hot water, central heating or both, but in much less space than a conventional hot-water cylinder. The smallest model – suitable for a fl at or small house – is only 440mm high, 365mm wide and 555mm deep, compact enough to fi t under a kitchen worktop.

More recent is Wokingham-based Tepeo’s ZEB for ‘zero emission boiler’. The size of a fridge, it has a solid, highly insulated core which can be heated up to 800°C. This makes it heavy (375kg) so best suited for an easily accessible ground fl oor.

Like the Sunamp, conventional hotwater and central-heating pipework can be easily connected, though the ZEB can only be heated via mains electricity and PV panels; an app will automatically choose the cheapest rate for your domestic energy needs.

Promised soon is the Warmstone from Hampshire-based Caldera. It’s the bulkiest offer yet – a weatherproof cylinder one metre in diameter, almost 1.8m high and weighing 1,850kg. Designed for installation in garages or gardens, it aims to store up to 100kW of energy for up to 23 days. Caldera is clearly targeting current oil and LPG boiler users.

So, should you opt for a thermal battery in your self build? If you plan to have solar panels it’s well worth considering. Manufacturers also claim a thermal battery can match the performance of a heat pump, but at a lower purchase price, up to twice the lifetime and with minimal maintenance.