By Dr David Oliver, Materials Development Manager, Sunamp | February 2023
If you’ve found this blog, you’re probably interested in installing a heat battery in your home. Maybe you want to decarbonise your hot water, make use of an off-peak electricity tariff or replace an aging gas boiler.
You’ve probably done some research and found that several companies make thermal batteries, and you’re wondering what the differences are.
This blog explores where Sunamp sits in the battery (!) of options available to you.
So, first things first – what are we comparing in this blog?
We’re focusing on systems that deliver heat in the same way as Sunamp thermal batteries. Sunamp heat batteries use circulating water to deliver heat, and can therefore be described as hydronic heat batteries.
A hydronic heat battery is a device that stores and releases heat on demand, and transfers the heat through circulating water (hence the word ‘hydronic’) in a piped system. It can be used to heat water for radiators as well as for sanitary hot water in bathrooms and kitchens.
We’re not covering night storage heaters (such as those made by Dimplex) in this blog, as they are designed to heat the room directly without the energy passing through a water circuit or pipework.
Other than Sunamp, who else produces hydronic heat batteries for homes?
I’m aware of a few other companies that produce hydronic heat batteries to serve the residential market, including UK-based start-ups Tepeo and Caldera, and Steffes Hydronic Furnace in Canada.
All these batteries use water to circulate the heat, but do they all store the heat in the same way?
No, they don’t. What they do have in common is that they all have high energy densities and store a lot of energy per unit volume of material. However, this is achieved in very different ways.
At Sunamp, we use our Plentigrade phase change materials (PCMs) to store thermal energy – operating at temperatures similar to the desired water temperature and using the latent heat of melting and crystallising to achieve high energy density. (If you’re interested in the science behind Plentigrade, I explore this topic in another of my blogs here).
The companies I mentioned above all use materials with high heat capacities (a measure of how much energy it takes to raise a kilogram of material by 1°C) and use direct resistance heating to heat them to very high temperatures. Above 500°C in some cases. When a material has a high heat capacity, it needs a lot of energy to heat it up. It cools down to release that energy. The hotter you heat it, the more energy it stores. Materials such as concrete, graphite or ceramics are good examples of materials with high heat capacities, and they’ve been used in night storage heaters for many years.
A hydronic battery for domestic applications that uses a third (and much rarer) approach is from Cellcius’ who use thermochemical materials to store energy. Here, heat is stored and released in a reversible chemical reaction. I believe they put potassium carbonate through a series of dehydration and rehydrations to store thermal energy, which is complex and requires many more component parts to make it work (due to the use of gases) than the Sunamp phase-change method. Cellcius’ products also must store the gas created by their processes so there’s a requirement for space for that needs to be built in, which is not necessary with the other methods of heat storage. It could, however, potentially store heat for weeks at a time with no losses if left unused. I’ll say no more about this approach since it is currently at a prototype stage and not yet available commercially.
What sort of energy densities can high-temperature ceramic batteries achieve and how does this compare to Sunamp’s Plentigrade?
There’s no doubt that high temperature batteries store a great deal of energy.
Materials such as the metals, aggregates, refractory oxides or ceramics used can have very high heat capacities.
It is entirely possible that a high-temperature heat battery could demonstrate, across a specific temperature range, that its material could store a higher number of watt hours (energy) per litre than Sunamp’s PCM.
However, material level energy density isn’t the full story.
The difficulty for high-temperature heat batteries is extracting all that energy from the material and mitigating the high levels of heat loss. Imagine the heat loss (reduction in efficiency) of a heat battery is 500°C – that’s over double the temperature of a standard kitchen oven. Sunamp’s residential thermal batteries operate at less than 80ºC, which greatly reduces the temperature differential, and so as you’d expect, our heat losses are much lower. So, when it comes to delivering the heat at a system level, the materials’ energy density numbers are less relevant, it’s all about the device performance.
Tell me more about the heat loss. How much of a consideration is it for each type of battery?
Since the temperature involved in storing energy in an energy-dense non-phase change energy storage medium (for example, ceramic) is very high, the difference between the temperature of the energy storage material and the surroundings is also very high when the battery is charged. This means that the rate at which energy is lost to the surroundings (unwanted discharge of the heat battery) is more rapid. A lot of energy – and therefore money and carbon dioxide – would be wasted in heating the surroundings while the battery is kept in a charged state.
Insulation is usually used to mitigate this. However, the high temperatures limit the type of insulating material that can be used. And of course, the more insulation needed, the bigger the product gets.
Because the temperatures used by Sunamp heat batteries are relatively low and the heat battery volume-to-surface-area ratio is also low, heat loss is much lower and less space-hungry insulation is required. Whilst non-phase change high-temperature batteries may boast comprehensive insulation, Sunamp’s lower heat thermal batteries are always going to win on heat loss. We also use extremely high-performance vacuum insulation panels, which not only minimise heat loss but also the amount of material used, the floorspace taken up and therefore the overall product cost.
What about working with a heat pump? Is operating at a lower temperature is an advantage?
It’s true that you can store vast amounts of heat in high-temperature batteries, but hotter isn’t always better!
For example, a major advantage of using a heat battery sustained by Plentigrade is the ability to charge the heat battery via a heat pump. Heat pumps are orders of magnitude more efficient than direct resistance heating but can’t reach high temperatures (>80ºC). This has the benefit over high-temperature heat storage systems which are unable to be integrated with heat pumps.
The lower temperatures at which Sunamp heat batteries operate, plus their highly effective insulation, ensures that the external case remains cool even when the battery is fully charged and holding at its maximum temperature. This also means that no clearance space is needed between the unit and the wall, and the casing will always be safe to touch.
Sunamp heat batteries can also transfer heat out of the PCM very effectively as they’re optimised to work around the melting point of the PCM. Depending on the design of the system, some high temperature batteries would struggle to deliver heating via hot water effectively if the storage material temperature is below 100ºC. That’s because you need a method of transferring heat from the material to the water, and this needs to be optimised for a specific ‘differential’ temperature. Because these devices operate over a very wide temperature range (this is their core function) this will always be challenging.
Is there anything else that sets a PCM heat battery apart from other heat batteries?
Sunamp’s PCM heat batteries don’t require a hot water tank to be installed. This could save you a lot of space! And because very little water is left standing in the battery’s pipework, there is no legionella risk.
Just how big are these batteries? Do they take up a lot of space? And where can they be installed?
The size of the battery is dependent on the amount of energy someone wants to store.
It’s difficult to compare like for like across the industry, but if it’s simply a question of looking at the space a battery might take up in a cupboard, for instance, then it’s easy enough to check the range of other suppliers’ battery dimensions online.
Our Thermino heat batteries for hot water all have the footprint of a microwave and come in four different heights, roughly from the size of a suitcase to a large fridge.
In terms of installation, the Sunamp Thermino range is particularly straightforward. It has plumbing ports on three sides, making the orientation very flexible meaning they can be installed under a kitchen counter, in an airing cupboard or under the stairs.
