Taking the heat out of climate change

The ‘invention’ of fire is often seen as the first step along the path to modern civilisation. Whenever people needed heat, they burned something, whether it was to cook food; for warmth; or to make pots and pans, tools and weapons. Controlling fire and heat took us from the Stone Age through the Bronze and Iron Ages. 

As we have come to realise the effects on the climate of burning fuels, particularly fossil fuels, we’ve been searching for alternative ways of generating heat that do not release carbon dioxide. But combustion to produce heat is so entrenched in civilisation’s infrastructure that heat generation is seen as the hardest use of energy to decarbonise.

In 2022, the International Energy Agency (IEA), which tracks and forecasts the world’s energy needs, estimated that heat accounted for almost half of all energy consumed. This amounted to 38% of the world’s 36.8 gigatonnes of energy-related CO2 emissions that year. Of that 38% (nearly 14 gigatonnes), the IEA estimated that industry emitted 9 gigatonnes. To reach net zero targets by 2050, this needs to fall to 7 gigatonnes by 2030. So far, only a quarter of the energy produced to generate heat currently comes from non-carbon emitting sources. 

An important factor in this is the range of temperatures needed from heat. Keeping homes and other spaces comfortable for human habitation, using space heating, needs fairly low temperatures. Proven technologies such as air- and ground-source heat pumps can deliver this. It takes much higher temperatures to transform raw materials into the products that make up modern society. 

However, according to the IEA, industry needs more than half of its heat at temperatures over 300°C. Producing ammonia, an essential component of synthetic fertiliser, requires temperatures of 300°C. Many raw materials, such as the lime used for steelmaking, must first be purified by heating them to up to 1,000°C without oxygen. Sand is melted at over 1,300°C to eventually become glass, while the furnaces and kilns that make steel and cement burn at up to 2,000°C.

Standard heat pumps cannot provide heat at these temperatures, but renewable sources can. For example, arrays of mirrors can concentrate solar energy, and electricity from wind turbines or photovoltaic solar cells can generate heat. These sources are intermittent, and can’t supply the round-the-clock needs of industry. Therefore, attention has turned to ways of storing heat in a way that can be drawn upon at any time. 

Canned heat

This thermal energy storage (TES) is analogous to the development of batteries to store electricity, according to Professor Yulong Ding FREng, founder of the Birmingham Centre for Energy Storage at the University of Birmingham. But there has been much more research and development on batteries. 

Professor Ding explains that there are three main methods of storing heat. The first, sensible heat, involves simply heating up a material, then keeping it insulated. The second approach, with phase change materials (PCMs), most often involves melting a solid material, which absorbs heat as it transforms into a liquid, and then recovering the heat as this liquid solidifies. The third approach is thermochemical storage. An example is heating a material to drive off water, before adding water again to release and recover that energy.

Of these three approaches to TES, sensible heat has received the most development. “It already has a long history in industry,” Professor Ding says. “Steelmaking has used a system called hot stove for more than 200 years.” Here, hot exhaust gases from the furnace heat up a ceramic material that preheats the air being blown into the furnace. Preheating the air reduces the furnace’s fuel consumption and improves its efficiency.

Using sensible heat for other industrial applications follows similar low-tech lines, for example, with air or an inert gas heated by low-carbon electricity or concentrated solar heat circulating around a solid material in an insulated tank. To recover the heat, fans blow air or an inert gas over the hot material. The cache can store heat for hours or days and can provide heat at temperatures over 1,000°C. For lower temperatures, water reservoirs or underground caches of meltable salts can store the heat. 

University of Edinburgh spinout Exergy3 is commercialising ultra-high-temperature sensible TES technology, invented by technical lead Adam Robinson. Its storage medium is ceramic bricks, which are enclosed in an insulation material. This is in turn surrounded by a heat exchanger to recover radiated heat. The core is at a temperature of over 1,200°C, and the whole system is housed in a module the size of a standard shipping container.

Exergy3’s technology is being trialled in a £3.6 million UK Department of Energy and Net Zero-funded project to produce zero-carbon whisky at Annandale Distillery in the Scottish Borders. In June 2024, the company installed a 36 megawatt-hour TES unit to power a 4-megawatt boiler to run the distillery’s stills. The system charges using excess renewable electricity from the National Grid, at a rate that allows it to quickly store energy from wind turbines that would otherwise have to be switched off.

Also using this approach is German startup Kraftblock, which has its sights set on industries such as steel, glass, food, and paper. The company fills shipping containers with spheres of its heat media, developed by founder Martin Schichtel while researching nanoparticle composites and smart coatings at Saarland University. The exact recipe is undisclosed, but Kraftblock says 85% is based on upcycled materials, including steel slag. 

Kraftblock’s first collaborations are underway. One replaces a 25-megawatt gas-fired boiler for heating oil at a PepsiCo crisp factory in the Netherlands. Its storage temperature is 800°C. Another will store heat from a local steel company at temperatures up to 1,300°C.

In France, meanwhile, Eco-Tech Ceram has installed two TES units at a brick and tile manufacturer, Wienerberger, near Paris, storing energy recovered from ovens so that it can be reused in dryers. The system uses ceramics heated to 600°C as a storage medium. Previously, the company lost half the heat from its ovens in chimney-stack emissions.

Not just a phase

Professor Ding’s current research focuses on PCMs. These have two advantages over sensible heat materials. First, they can store heat per unit volume or mass, several times more efficiently than typical sensible heat storage materials, for a given temperature range – meaning less storage material is needed. Second, because their temperature doesn’t change, thermal management is easier. 

The disadvantage is that the heat has to be transferred from a solidifying liquid, which poses an engineering challenge. Chemical plants and power stations normally use systems called heat exchangers to transfer heat from one material to another. (A radiator is a simple example of a heat exchanger.) But these only work with flowing liquids or gases. 

To cope with this, the PCM is incorporated into bricks of a solid composite (also known as a skeleton material). “We use a porous material as a matrix to contain the PCM, which is generally an inorganic salt,” explains Professor Ding. Typically, the chosen salt is a magnesium nitrate, but his group has also used magnesium carbonate, sulfate and chloride salts.Each of these store heat across different temperature ranges: nitrates will cover temperatures of between about 100°C to 550°C, carbonates between about 400°C to 800°C; and sulfates up to 900°C. Other materials can be mixed with the PCM, such as graphite flakes, to enhance heat transfer into the PCM.

Much of Professor Ding’s team’s research goes into ensuring that the components of the composite are compatible with each other; salts can be corrosive and could potentially degrade the skeleton material. Because the heat transfer enhancing material heats up, the composites are a hybrid between a PCM and a sensible heat material, known as a composite phase change material (CPCM). Capillary action, the same mechanism that pulls water into a sponge, draws the liquefied PCM into the pores of the matrix. “This avoids leakage of PCM and problems of change of shape as the material heats up; if the material were to soften and slump, it would not be effective. The PCM remains as a stable block. It also accommodates a small volume increase that accompanies the phase change,” Professor Ding explains. 

Thanks to the composites, the actual heat storage installations are very similar to those for sensible heat storage. Arrays of blocks, easy to handle and stack, are arranged so that hot gases can circulate around them. This charges the system up by melting the PCM, before cold gases are warmed up by the material resolidifying.

Following a successful 24-kilowatt (100 kilowatt-hour) lab-scale test in Leeds and a 200-kilowatt (2 megawatt-hours) pilot plant in Changsu, China, a first industrial-scale unit was installed in northwest China in 2016. Built by Chinese firm Jinhe Energy, this 6-megawatt (36 megawatt-hours) unit stores excess energy from wind turbines and uses it for space heating. In its first three years of operation, it stored almost 30,000 kilowatt-hours of energy, which would otherwise have been wasted. It reduced CO₂ emissions by some 10,000 tonnes. 

In a collaboration with another Chinese company, CRRC Shijiazhuang, Professor Ding’s team has developed CPCM technology to chill shipping containers. Aimed at replacing diesel-powered refrigeration units, the containers are ‘passively’ cooled, working more like a cold box containing an ice pack than a fridge. It takes about two hours to chill down the PCM with electrical refrigeration – potentially green energy – and the containers can maintain an internal temperature of between 0°C and 14°C for over a week.

Last year, Professor Ding’s group licensed its CPCM Intellectual Property to Vital Energi, a Blackburn-based energy specialist, which plans to commercially develop it for space and water heating. Vital’s development director, Chris Taylor, told Ingenia that its clients, including hospitals and universities, had previously depended on gas-fired heating systems. 

“Lots of companies were setting net zero goals and pretty much everything involving gas was going,” he says. “About five years ago, we started to look ahead and realised that electrification of heat was probably the way it was going to go.” However, he added, electrified heating systems cannot be operated the same way as gas-fired, in part because the price of electricity is much more volatile than that of gas. This means that energy storage is an essential part of any system, with two options. “One is to store electricity and then convert it to heat. The other is to store the heat, and we realised that was cheaper.”

Elsewhere, a European project called SEHRENE – Store Energy and Heat foR climatE Neutral Europe – is exploring another approach to storing waste industrial heat and renewable electricity with PCMs. Funded by the EU, the project includes two UK partners participating via the Horizon scheme: the University of Leicester’s Materials Innovation Centre (MatIC) and Manchester-based digital technologies firm Technovative Solutions (TVS).

SEHRENE, a three-year project coordinated by the French Alternative Energies and Atomic Energy Commission (CEA), is a multinational initiative that is developing an electrothermal energy storage (ETES) technology. Like Ding’s, it is based on a composite PCM, but also adds in a high-temperature heat pump as the source of the energy to melt the PCM. The team reckons the PCM’s metallic foam matrix, selected to ensure good heat transfer, increases energy density so that it is 30% better than the current state of the art. The goal is to store energy eight to 12 times longer than lithium-ion batteries, at a storage cost that beats pumped hydroelectric energy, currently the cheapest commercial energy storage method. By the end of the project, which has a budget of over €3 million, the consortium aims to build a proof-of-concept prototype system at CEA in Grenoble.

MatIC will test the compatibility of the PCM and the metallic foam. “Our role is to understand how construction and phase change materials interact, as relying solely on one without considering the other is not effective,” explains MatIC Director Dr Shiladitya Paul. “You can have a great phase change material but if it is not compatible with any material of construction that’s used, then you have a problem. Likewise, excellent construction materials are rendered ineffective if they are not compatible with available phase change materials.” The centre is studying how corrosion, ageing, stress, and thermal cycles affect the materials.

In its work on this, TVS is developing digital models of the component systems to assess their environmental impacts and compare these to the impact of lead-acid and lithium-ion batteries.

The next step

The IEA’s analysis suggests that renewable sources will meet just 70% of the projected global increase in total heat demand between 2023 and 2028, with the use of fossil fuels increasing to fill the gap. Electricity from renewable sources isn’t likely to keep up with the rising demand for heat. Any attempts to reach net zero clearly need new approaches to delivering the heat that underpins much of manufacturing. TES can be a part of the answer, but only if we begin to match the research effort into this aspect of energy technology as has gone into developing new types of batteries for storing electricity.

The SEHRENE project has received funding from the European Union’s Horizon Europe Research and Innovation Action under the Grant Agreement nº 101135763 (SEHRENE). Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union nor the European Climate Infrastructure and Environment Agency (CINEA). Neither the European Union nor the CINEA can be held responsible for them.