Why Ice Might Be the Future of Heating

Heat from ice? It sounds like a joke—or maybe a paradox. But what if the future of warm, cozy homes lies in freezing water? As outlandish as it seems, an English research team is working on a heat pump that runs off actual ice … well, more like slushies. It’s a serious effort to tackle one of our biggest energy challenges: how to affordably convert buildings over to highly efficient electric heat pumps. But how can a heat pump raise temperatures through freezing?

Right now, heating gulps down almost half the energy used in buildings worldwide—and most of that heat still comes from fossil fuels.¹² Conventional heat pumps are a cleaner, more efficient alternative, but there’s a catch: how do we transition large numbers of homes affordably and easily to more efficient electric heat? That’s where the mind-bending idea of harnessing ice as a heat source comes in.

Before we get into what on earth any of that actually means, let’s back up for a moment and review all the key players in this equation. If you’ve been around this channel long enough, you know heat pumps are kind of my thing. They take the concept of electrifying heating to the next level by producing more heat energy than the amount of electricity that’s put in. That’s because heat pumps don’t generate heat on their own — they transfer heat from one place to another, the same way your fridge shuffles heat around to keep your food cold.

Heat pump types are defined by where they source their thermal energy. An air-source heat pump absorbs heat from the air. A water-source heat pump gathers its heat from a nearby body of water. The type of heat pump I have installed at my home, geothermal, relies on the heat available underground. And so on.³ Therefore, an ice-source heat pump (or ISHP) is exactly what it sounds like: It’s a heat pump that makes use of the heat provided by…ice. Wait…hold on a minute.

If you’re feeling some brain freeze coming on as you’re trying to wrap your head around how that works, don’t worry. I felt the same way, at first. To understand how ice can be used as a heat source, we’ll first have to go over a bit of physics, specifically in the world of thermodynamics. Here’s how the team of University of Nottingham researchers describe their slushy supply of energy in a 2024 paper:

“This process takes advantage of the high enthalpy of phase change, allowing significant energy absorption with minimal mass or volume. The dimensionless energy ratio for freezing water versus a 1-degree temperature change is about 335. This heat pump concept is effective with water as the heat source due to water’s high latent heat of fusion, enabling substantial energy absorption with low mass.”⁴

For those of you that aren’t physicists, that paragraph might have just sent a chill down your spine. So let’s pick it apart. First: high enthalpy. In this context, enthalpy is the amount of energy it takes to cause a phase change: for our ice-source heat pump, that would be the conversion of liquid water into solid water.⁵ Funny thing about ice…as it freezes, it releases heat, making that enthalpy value in our physics calculation negative.⁶ That’s a good thing, though, because that heat is now available for use. This energy is also known as latent heat, or, more specifically, the heat of fusion. (Fun fact: it’s called “heat of fusion” for both freezing and melting. Just in case you wanted these terms to be more confusing.)⁷⁶

What about that energy ratio? Well, an energy ratio compares how much energy you put in with how much you’re getting out. It’s a measure of energy efficiency. So, when the Nottingham researchers discuss the freezing ratio, they’re referring to the heat of fusion of water at 0 C, which is about 335 kilojoules per kilogram.⁷ But let me put this information in a different way: The same amount of heat energy that’s released by freezing one kilogram of water is the same amount of heat energy that it would take to raise the temperature of a kilogram of water to 80 C.⁸⁷ In other words…if you can access ice, you can access heat.

And that’s the magic of this theoretical system. It draws on two plentiful resources in the UK: natural gas pipes and precipitation. As part of the clean energy transition, the natural gas network would take on a new role. Instead of delivering gas, the pipes would deliver non-potable water. That water would feed into heat pumps inside the home that are augmented with ice generators. As the heat pump captures the latent heat from the water it starts the freezing process. The system is also benefiting from the heat that’s released when the ice forms. To keep the system from freezing up, an auger-like device would keep the ice that’s forming in a slushy, slurry state. This resulting ice slurry would return through another pipe where it would begin to melt.⁹

To understand how this would work in a residential setting, all you have to do is picture the same kind of appliance that makes your local equivalent of snow you can sip on. For me, that would be the mesmerizing machines that dispense Slurpees stateside. When I recently interviewed Nottingham researchers Ramin Mehdipour, Seamus Garvey, and Zahra Baniamerian, Garvey put it like this:

“Effectively, it’s a Slush Puppie maker.”
“Most of the work done so far has been practical work making an ice generator that is first of all, small enough to fit in a house, and secondly, cheap enough that you could imagine putting it into every house. And then has the right properties of being able to make a slurry that you could put down the drain.”

The whole point of having the Slurpee maker as part of this is to prevent the ice that’s forming from attaching to the sides of the pipes as it freezes. Keeping it in that slushy state allows it to continue flowing through the system. Superhydrophobic coatings have been developed for anemometers to keep ice crystals from nucleating on the surface, and perhaps these could be used to prevent ice accumulation.¹⁰

Now that I’ve introduced what this icy idea is, I need to clarify what it’s not.

It’s not simply thermal energy storage using frozen water. That’s already a separate practice long used in HVAC systems.¹¹ However, with Nottingham’s open-loop strategy, water comes in, slurry goes out.

And to be crystal clear, ice source heat pumps themselves are not new inventions.⁸ Garvey’s interest in latent heat pumping was sparked by a 2019 conference. While there, he attended a presentation on the topic given by fellow researcher Ziping Feng from the Chinese Academy of Sciences in Guangzhou, China.¹²¹³ As Garvey explained,

“The only piece that I added to his idea was to say that you could deliver the water for the latent heat pumping through what is our existing gas network.”

So, what makes this plan unique isn’t the heat pump, or even the ice generator. It’s the fact that it gets mileage out of the UK’s…gas pipe mileage. Meterage?

“Where what you’re talking about is, okay, we already have infrastructure in there. There’s already these pipes in the ground. Can we reuse these in an intelligent way? So I’m assuming that is to you the key here of what is the most practical, easiest way to get better heat pumps into people’s homes.”
“That’s absolutely spot on. So Ramin and I, we’re engineers, we’re not scientists, so we’re not interested in discovering new laws of physics. We’re just interested in finding the lowest cost solutions, and we have an asset in the ground…And if it’s not carrying natural gas, you know, it’s, we should try to sweat that asset for something good.”

With that in mind, this is also not a district heating system, even though it is similar on the basis of using shared pipeline infrastructure. District heating involves sending heat outward from a central location.¹⁴ What the Nottingham team is proposing involves individual heat pumps for each home.

Garvey says that little to no alterations would need to be made to the natural gas network to adapt it for water transmission. After all, gas pipes are usually made from materials like plastic and steel, the same stuff you’d see in water ones.⁴ There is some potential to use underground polyethylene (PE) gas pipes as water pipes, though part of the installation cost will be upgrading the materials at the entrance of each home. But why choose ice over any other heat source?

For one thing, ice-source heat pumps avoid some of the pitfalls associated with its alternatives. In general, heat pumps have a bit of a bad reputation for poor performance in the cold. While this has improved over time, it’s true that they have to work harder as temperatures drop. At extreme ends, this means bringing in reinforcements in the form of a backup heater, leading to a spike in electricity consumption and an overall reduction in efficiency.¹⁵

The research team’s argument goes like this: air-source heat pumps work well, but need expensive control systems for maintenance, including defrosting mechanisms to keep humidity at bay. Potential adopters also scrutinize them for the noise that their fans generate. More than anything, though, they’re vulnerable to the cold.¹⁵

Geothermal heat pumps sport high efficiencies that are guarded against the weather. That comes at a high cost, though — I can speak personally to that. It’s a lot of work to drill deep enough to reach those nice and consistent soil temperatures. The further those ground loops go, the better the protection against the elements…and the more expensive the project. Plus, you need the land to dig into at the start. That’s a lot more of a headache in city centers.¹⁵

What about water? Water source heat pumps can better handle the cold, but one of the tradeoffs is the source itself. Without a stable water supply, you’re out of luck. For people living in metropolitan areas or in arid climates like the desert southwest, there’s less opportunity to benefit from the likes of rivers and lakes.¹⁵

In a November 2024 presentation, the study authors summed up the pros and cons using this table:¹⁶

So, Nottingham’s response is basically “If you can’t beat ‘em, join ‘em.” Ice and snow are problematic for heat pumps, so why not take the Kirby approach and integrate ice into the system itself — and make use of what infrastructure we already have while we’re at it?

The researchers explain that ice-source heat pumps don’t perform as well as their water-based counterparts. They also incur a small cost increase by switching what would normally be the heat pump’s evaporator with components to separate and send out the ice.⁴ What they do have to offer, though, makes them stand out. These heat pumps can not only weather the cold but use significantly less water in the process. In a 2024 paper, the team estimates that ice source heat pumps require “approximately 36.7 times less water volume than traditional heating systems for residential use.”¹⁵

They also noted a bunch of other perks. Without air components, the heat pump is quieter, compact, and appropriately sized to be placed indoors, which also eliminates environment-based concerns like snow buildup. In regions where both water and space are limited, ice-source heat pumps can provide high performance without an abundance of water or the heavy financial investment that geothermal systems require.¹⁶ You can do more with less. This goes beyond the heat source itself, too, as our frigid friends also don’t consume as much electricity.¹⁵⁴ The researchers contend that by using latent heat and gas infrastructure, ice source heat pumps can take pressure off the grid. Otherwise, using electricity-based heating as opposed to gas would increase the overall electricity consumption load.⁴

Of course, every new idea comes with its limitations, and every approach comes with its challenges. And this absolutely has its share of challenges. First of all, this research is very much specific to its place of origin. This theoretical switch solidified with the UK’s extensive existing network in mind. Out of roughly 30 million homes, about 85% of them use natural gas for heating.¹⁷ Talk about frozen assets. While Great Britain isn’t the only region in the world to use this form of transmission system, it does stand out in comparison with the EU, where about 40% of households are connected to a gas network.¹⁸

There’s also questions around corrosion if these are steel pipes being reused. Will they be able to withstand the extra weight of the water and strain? Slush doesn’t flow through a pipe all that well, so how well will the system be able to evacuate the slushy water back out of the system and home?

That brings us to environmental constraints. Icing out natural gas naturally necessitates having these pipelines in the first place. So, as much as ISHPs might be convenient for population-dense areas, they might not be applicable to more rural ones.

Another major hurdle to this vision is scale. You’d have to have multiple households in a given region adopt the system all at once. These changeovers aren’t unprecedented, but the effort involved could cause a bit of a…chilling effect.⁹

So, that wraps up the pitch, but where does it stand today? It’s important to note that the gas-to-slurry swap is still in the research phase. As Garvey explained to me:

“We need to convince people that it’s not noisy, it’s not unsafe, and it’s not unattractive. And then there’s a process of just introducing this gently into the policy space. …It’s going to take a little bit of time for this to get into people’s minds. We have to bring the water companies with us. We have to bring the owners of the gas network with us. So the engineering research will be finished in September. And after that, there’s a translation, which is probably beyond the skillset of a humble engineer.”

But the Nottingham team are definitely not the only people interested in solving this challenge. Here in my home state of Massachusetts, two utilities, Eversource and National Grid, are experimenting with geothermal systems. They, too, are applying their natural gas and utility know-how toward efforts to transition existing customer bases to thermal energy networks.¹⁹

It was the Massachusetts-based nonprofit Home Energy Efficiency Team (or HEET), alongside the organization Mothers Out Front, that took the initiative to approach state gas utilities with the intent to collaborate back in 2016. They’ve since worked together to create the first geothermal system ever built by a gas utility.²⁰ As of June 2024, residents of the Massachusetts town of Framingham have been enjoying the benefits of geothermal through Eversource’s pilot program, and National Grid plans to establish its own network of geothermal heat pumps in Boston.¹⁹²¹

As for the Nottingham team’s idea. It’s obviously just an idea at this point, but that’s what drove me to make this video. Whether or not the UK adopts their concept, I get inspired by the clever approaches and ideas that engineers, like the those at the University of Nottingham, come up with to solve the challenges we have in our daily lives.