Solving the clean heat conundrum 

A UK developed heat battery aims to provide a fresh solution to the growing need for decarbonisation in how we heat our homes. Melissa Bradshaw reports 

The Caldera Warmstone zero carbon boiler and heat battery stand in the garage of Caldera CEO, James Macnaghten. Image credit: Luke MacGregor

Amongst the many pieces of the puzzle that will make up the UK’s route toward delivering on its net zero ambitions, the way in which we heat our homes is becoming a growing area of concern. Energy used for heating accounts for around 17 per cent of the UK’s carbon emissions, the majority of which comes from domestic buildings heated by natural gas boilers.

Efforts to address this have led to talk of a potential ban on conventional gas boilers. In July 2020, the CBI called on the government to implement the proposed ban on sales of new natural gas boilers from as early as 2025. At the time of writing, the UK was still awaiting the government’s anticipated heat and building strategy which looks set to confirm the suggested ban’s fate.

A range of solutions are being explored, from the installation of heat pumps to a nationwide switch to hydrogen fuelled boilers. Whilst it’s looking likely that a blend of innovations will be key to achieving the ambitious targets, James Macnaghten – CEO of Hampshire heating technology firm Caldera — believes his company’s zero-carbon heat battery could also play a key role.

James Macnaghten, CEO of Caldera. Image credit: Luke MacGregor

Caldera was founded three years ago by Macnaghten and COO Guy Winstanley, both of whom had previously spent around a decade exploring large thermal storage systems. Macnaghten describes a middle of the night lightbulb moment in which he came up with the idea for a ‘sensible, solid’ heat storage material that can be heated to high temperatures — the basis of what now makes up Caldera’s patented ‘Warmstone’ material, from which the heat battery is made.

“The material is dense, it’s solid, it’s non-flammable and it’s got really good conductivity,” Macnaghten said, pointing out that the sustainable material is also made from a mixture of recycled and natural materials which can be reused at end of life.

“I wish we’d known about it ten years ago, but what it allowed us to do was then to look at heat storage in homes — for which we have a view that it’s an important part of the energy mix,” he explained.

The heat battery, intended to be used initially as a replacement for oil or LPG boilers, comprises a block of the Warmstone material weighing 1.7 tonnes, 1.0m in diameter and 1.7m high. Electric elements embedded in the block are heated up to 500°C and the heat is stored ready for use when required via a heat exchanger. Designed to be located externally, or in a property’s garage, the heat battery can hold 100kWh as heat and can be installed without interference alongside a home’s existing radiators or piping system.

We sort of feel we’re killing two birds with one stone… helping with the flexibility needed for renewables, and tackling homes where the government is going to struggle to persuade people to spend the money they need to spend to fit heat pumps

James Macnaghten – Caldera CEO

“To us, it’s absolutely key that we don’t have big thermal losses,” Macnaghten said, explaining that they took a vacuum insulation approach from the beginning. “We’ve done a lot of work around this because making a vacuum that will last 20 years at 500°C is not easy … For it to cool from 500° to 200°, if you just left it, it would take 19 days. Which is a really good number for something that hot and that size.”

Key to the transition toward a greener energy sector will be prioritising flexible demand, Macnaghten stressed, optimising how and when we make use of our energy. This is something he believes the Warmstone batteries will provide significant support with through reduced pressure on the grid, heating up overnight using cheap off-peak electricity then releasing heat during the day.

An illustration of how the Warmstone battery works

“Each of our units has intelligent controls which can sense when prices are low and can also be configured to respond to signals from, say, an energy supplier,” Macnaghten said. “This means that thousands of heat batteries can be set to charge and soak up cheap electricity when required – for example, one million heat batteries could provide 20GW of flexible demand for five hours i.e. 100GWh.

“We sort of feel we’re killing two birds with one stone… helping with the flexibility needed for renewables, and tackling homes where the government is going to struggle to persuade people to spend the money they need to spend to fit heat pumps,” he said, adding that he views Caldera’s technology as complementary to heat pumps rather than an alternative.

He claimed that if a home needs 65°C or above water temperature, the Warmstone heat battery would ‘almost certainly’ be a better solution than a heat pump, however for some homes — for instance, insulated homes with underfloor heating fitted — heat pumps could be a preferred option.

Whilst both routes to carbon-free heating have their benefits, no single solution will be the standalone answer. Macnaghten pointed out that the noise from air source heat pumps can be disruptive, and the process of replacing radiators is intrusive in a home.

“It’s not a simple like-for-like because you potentially have to change all of the piping … People’s engagement with heating is very low, to ask someone to do something that is really quite a big measure, most people I think say ‘no’ and that’s part of why I think how we decarbonise homes is a really tricky subject because every home is different,” he said.

Additional barriers to going down the heat pump route include cost and the necessary training that will need to be implemented for heat pumps to be widely installed, Macnaghten said, due to the complexity of installing a heat pump in comparison with the task of installing a boiler.

In June’s monthly poll carried out by The Engineer, we asked readers for their opinions on the feasibility of the 2025 gas boiler ban. The majority (51 per cent) of respondents said that the idea was necessary but unrealistic.

Opinion: tomorrow’s heat is today’s opportunity

Engineering the hydrogen home

This view seems shared by many experts, including Nottingham University professor and specialist in heat pump technology Saffa Riffat, president of the World Society of Sustainable Energy Technology. Riffat said that a 2025 ban is ‘not possible’, citing poor thermal insulation of homes and cost as some of the biggest challenges in the decarbonisation of heating.

“The UK will require 600,000 heat pumps a year to progressively replace gas boilers,” he told The Engineer. “Assuming £10,000 per unit the total annual cost is £6bn. At present, I think most gas boilers are manufactured in the UK while heat pumps are imported. I suggest the government seriously needs to consider promoting a UK heat pump industry.”

People’s engagement with heating is very low … How we decarbonise homes is a tricky subject because every home is different

Riffat goes on to suggest that UK government should encourage large companies such as Daikin and Mitsubishi to set up manufacturing in the UK, and that such heat pump technologies should also be designed to be reversible to provide cooling due to heat waves caused by global warming.

Additionally, he suggested the government takes a similar approach to that taken with the rollout of Covid-19 vaccines this year, backing more than one design and ensuring that industry and academics have adequate financial resources to bring products to market.

Meanwhile, Macnaghten pointed out that one reason Caldera believes its solution is particularly promising is that it’s already competitive with oil: a report from professional consultancy and data services firm Gemserv showed Caldera’s heat battery to come out ‘only marginally more expensive’ (eight per cent on an annualised cost basis) than oil, he explained, compared to around 40-45 per cent more per annum with a heat pump — with government expected to further tax oil heating, the solution will likely end up cheaper. In comparison with gas however, Caldera recognises that a Warmstone battery will still be a more expensive solution using the lowest off-peak tariffs available and whilst prices of fossil fuels are expected to rise in line with net zero targets, the company is initially focusing on off gas grid homes that use oil or LPG.

Having successfully closed its first crowdfunder, raising just over £1.5m, Caldera will install the units in 12 pilot homes this winter with an intention of rolling out the batteries commercially to customers in Spring 2022.