Sunamp Heat Batteries Can Help Electrify Everything

Batteries aren't just for electricity, but can store heat or cold.

Heat battery beside washer
The little box beside the washing machine is a heat battery.


One of the biggest problems that we have in getting off fossil fuels is the intermittency of renewables; there is no sun at night and the wind doesn't always blow. You could buy a lot of Tesla Powerwall home batteries, but that gets pretty expensive if you want to use it for heating, cooling, or domestic hot water. According to the Energy Information Administration, that is where most of it goes; 70% of the energy used in an American home goes to space heating (43%); cooling (8%), and water heating (19%).

Another problem is that gas is cheap, while electricity can get really expensive, especially during peak hours in places where there is time-of-use pricing. As an example, where I live in Ontario, Canada, natural gas costs C$0.32 per cubic meter including delivery charges or C$0.031 per kWh. Electricity costs C$0.085 per kWh off-peak (7 p.m. tp 7 a.m.), 2.74 times as much as gas, and C$0.176 during peak hours, 5.67 times as much as gas. It's no wonder that it is so hard to electrify everything.

Sunamp battery in closet
Heat battery vs Electric Water Heater. Sunamp

That's why the Sunamp Heat Battery is such an interesting product. CEO Andrew Bissell tells Treehugger that they have sold thousands of these in the United Kingdom, mostly as hot water heaters where they are replacing dangerous on-demand gas water heaters in housing projects, but also in homes for domestic hot water and for the radiators that almost all British homes have for heating. Their vision:

"Sunamp set out to explore the potential of using thermal energy storage to make buildings more energy efficient, sustainable and self-sufficient, whilst reducing carbon emissions by optimising renewable energy sources on-site, supporting the grid to allow on more renewable electricity sources and harvesting waste heat for re-use."
Sunamp Heat Battery

The Sunamp Heat Battery is basically a well-insulated box filled with a phase-change material. When liquids turn to solids they release the latent heat of fusion; water will absorb 80 calories of energy per gram when it melts, and release the same when it freezes at 32 F. Bissell mixes up different chemicals to change state at different temperatures, absorbing or releasing energy as required. A copper coil runs through the material and picks up enough heat that it's instantaneous, eliminating the need for a space-eating tank or cylinder, and any worries about Legionella bacteria growing.

You can put any kind of heat into the box. It has an electric heating element that can be hooked up to the grid or to rooftop renewables, so if you have solar panels you can use them when the sun shines, and draw down the heat when it doesn't, and use grid power to top it up. For hot water, this makes a lot more sense than feeding rooftop solar power into a heat pump hot water heater, as many people are now doing.

Heat pump or timed power connected to sunamp

You can use the Sunamp battery to take advantage of time-of-use pricing and charge it up when the power is cheap as shown in the drawing above, but where it really gets interesting is when you hook it up to a hot water source like a heat pump, which is really just a different form of phase change device. Units that use CO2 (R744) as a refrigerant pump out hot water and can be connected to the Heat Battery, which is straightforward if you have hydronic (hot water heating) but can also be run through a heat exchanger for hot air systems.

condenser for heat pump
Albert Rooks' Sanden Heat Pump Condenser. Photo Lloyd Alter

Because heat pumps move heat from the air or the ground, rather than actually generating heat, they have a coefficient of performance (COP) that is a multiplier of the output of straight resistance heating. CO2 heat pumps like those made by Sanden can have a COP of as high as 5.2 at 67 F, but even when it is cold out they still pump out the heat, with a COP of 4.5 at 47 F, 3 at 23 F, and 2.25 at 5 F. Where I live, much of this winter was around 23 F, so if I divide that off-peak electricity rate of .085 by a COP of 3, I get $.028, which is cheaper than gas. During the daytime peak hours, it's .0586 per kWh, almost twice as much as gas, but that's why we have a heat battery – to charge the battery at the cheap times and run the heating and the hot water off the battery during the peaks.

So, not including the capital costs of those heat pumps and heat batteries (which is considerable) I am able to get off gas and have pretty much the same or lower operating costs running on clean carbon-free power – because Ontario runs on water and nuclear power with a few gas peaker plants – and I am getting my electricity at night when the peakers are sleeping and the utility has trouble giving away its excess power. The time-shifting that the Sunamp heat battery permits is good for my wallet and it's good for the grid, smoothing out demand, chopping off the peak and filling the valley, dealing with what they call curtailment, when the utility is generating more power than it can sell.

Phase Change points

Curtailment is a problem with solar panels as well in warmer climates like California, where in mid-day there is often way more power than they can use, while demand peaks in the evening when the air conditioners all get turned up, causing the famous duck curve. But Bissell can mix up the chemicals with a phase change temperature that can cool as well as heat, and run a coil through the AC ducts and can kill the duck way more efficiently than Tesla's big batteries, and no lithium mining required.

Of course, the smarter and cheaper solution would be to build a house to a much higher standard so that it actually becomes a thermal battery on its own; as we have noted before, a house built to Passivhaus standard can go for days without heat and could run on a much cheaper heat pump and a tiny Sunamp just for domestic hot water. But there are millions of existing homes like mine that can't do that, and are prohibitively expensive to renovate. This could very well be the solution for them.

In the U.K., Canada, and other countries that burn a lot of natural gas, everything from new nukes and fusion have been proposed as a way to decarbonize our power supply, and they are dreaming about using that electricity to make hydrogen to pipe to our boilers. But as we noted in an earlier post on exergy, this is high grade, expensive energy. As engineer Robert Bean put it, this is like warming your hands with a blowtorch.

To reiterate, in our housing 70% of that high-grade energy is being converted to dumb, low-grade heat. We don't want pipes full of hydrogen or wires full of electrons, we want heat, which is all around us in the air and in the ground, we just have to collect it and concentrate it with heat pumps. With the Sunamp Heat Battery, we have a place to store it. I don't know why we have to make everything so complicated; all the stuff we need to do this is sitting there on the shelves.

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