Energy storage vs heat batteries: what's the difference and which is better?

hi I have a Lithium-ion solax 5,8kW battery as part of my solar array. I chose this type of battery because it is proven and its light weight. I fits in the loft space on heavy duty loft boards close to the solar panels. I am very happy with the performance. In the summer months it is charged by my solar panels. In winter months it is charged off peak electricity from the grid. fits really well with the heat pump. I did avoid the energy dealing solar model. The so called stockbroker in the attic that buys  electricity cheap then sells it when demand is high. Having no control over the cycles the battery would be put through was a concern.

Today my specialist subject is.... Heat batteries.

We have four Sunamp batteries, two providing our hot water and two connected to our central heating.

How do they work? They contain a material that changes from solid to liquid when you heat it up. It remains a liquid until it is triggered to change back to a solid, releasing the stored heat. This means that once it is charged it loses very little heat (far less than a hot water cylinder). There are other types of heat battery that use different materials to store the heat but the Sunamp batteries all work the same way and don't get hot to the touch.

Sunamp batteries can be charged in different ways. One of ours has an electric heater and the others are charged by a heat pump. You can also hook them up to a gas boiler. They use different phase change materials for different applications so a battery with an electric heater will charge at a higher temperature than a battery linked to a heat pump.

What's good about a heat battery?

For hot water they work just like a hot water tank but they take up less space.

The phase change materials are non-toxic and don't have the environmental issues associated with the metals used in electricity batteries.

They last a long time. Sunamp have tested them over more than 10,000 cycles with no degredation.

Because they have minimal heat loss you can charge them at any time during the day, to take advantage of excess solar, cheap rate electricity, low carbon electricity etc.

What's not so good about a heat battery?

It isn't flexible when compared to storing electricity. You can use electricity for all sorts of things. You can only use heat for.... well, heating!

They're expensive compared to using a hot water tank, although they are cheaper per kWh of storage than an electricity battery.

The control system on Sunamp batteries isn't user friendly. There's no user interface so I can't tell if they're charged and how much heat they're holding.

Would I recommend a heat battery?

That isn't an easy question to answer. I would say... Maybe!

If you don't have space for a hot water tank then they're a great option for a hot water supply.

They are a cheap and long lasting way to store energy but within 10 years most of us will have cars with a large electricity battery and that's a much better storage option (using vehicle to grid chargers).

If Sunamp improve the control system to include a user interface and easy integration with an electricity management platform, like Kaluza, then I would be more likely to recommend them.

We have a 7.3 kWp of Solar, Tesla Powerwall 2 with Backup Gateway 2… if I break those two down;

First the Battery - a 13.8kWh all in one battery with inverter, liquid cooling all in one unit that can be floor mounted, wall mounted, even outside as fully waterproof. Install cost was circa £7.5k. Did I consider other batteries? Yes the Solid State Pylontech modular batteries is an ideal way to cheaply enter battery storage and then expand BUT the app and support looked patchy. 

The App - is almost perfect. It's the same Tesla app to manage a Tesla car which for me worked well. There are some incredible data and usage charts to really dig into the detail of what energy you have used from which source. It works with great with Time of Use tariffs where you can set cheap off peak and it will charge the battery using cheap off peak electric. 

What’s Backup Gateway 2? So strictly speaking this is NOT a UPS but it works just as a UPS does. You can run totally off-grid either by clicking button in the app, or it will just kick in when there is a power cut. It is insanely quick to cut in. It even has a storm mode so if there is a major storm approaching it fully charge the battery to provide continuity of power, though that doesn't happen to often here on the South Coast! 

Is it perfect? Nearly. Tesla use their own Algorithm to choose how much the battery needs to charge using off-peak until solar power kicks in the next day (we have 7.3kWp of Solar). It routinely gets this wrong. It has no idea of you have a house full at the weekend and needs loads of cheap stored power, so there is a couple of manual solutions; 1) I have a Raspberry Pi running a custom app that allows me to set charge level, times, it even integrates with Octopus Agile (back in the day when that use to be cheap). Before switching to OVO I used to fully charge the PowerWall on the Octopus Go and routinely paid 6p per kWh over the course of a month - brilliant !  2) Home Assistant can also control Powerwall and perform the same Tesla open API. 

Heat Storage - Mixergy - those who have read my other posts know I am a huge fan of Mixergy. It’s a hot water cylinder born out of a University Project, made in the UK and works  like no other. I use this as a secondary heat store in the height of the summer. On a very sunny day our Solar PV system can produce ~50kWh of free electric, once the Powerwall is full the MyEnergi Eddi then diverts the free electric into the Mixergy storing that energy as free hot water. WAY cheaper than battery storage to install. A brilliant smart phone app giving complete control and loads of data. As part of the OVO heat pump trial they disabled my ability to use use this Solar Diverter feature and cut the cable from the Eddi to the Mixergy. Meanies.  As soon as the trial is over the Mixergy will once again be the primary heat source and the ‘dumb’ energy wasting Sunamp Heat Batteries will be ditched.  Mixergy Customer Service and technical team are UK based and utterly brilliant.  

Heat Storage - Sunamp Heat Batteries - I have the same configuration as Mister W above with 4 batteries acting as heat stores for heating and hot water instead of the buffer tank and hot water cylinder you normally get with a heat pump install. The marketing looks great, clever phase change materials storing energy that can create instant hot water and heating. In reality ‘dumb’ batteries with no real controls or data just absorbing huge amounts of energy keeping them hot just in case you need the heat. You can’t even see how full they are, or how much energy they have used..  And for the hot water side it has an immersion that brings the hot water up to a useable temperature but that immersion has used almost 40% of the electric that the whole heat pump system used to heat the batteries. Can’t wait to ditch Sunamp and install a Mixergy and Buffer tank. Sunamp Customer service non existent having not replied to any emails, phone calls. And that’s before you look at the price point with just one battery costing TWICE the cost of a Mixergy Cylinder.  Avoid. 

What upgrades am I considering? A second Powerwall, already have the DNO approval.

I don't think the fact that we have no control over our heat batteries should be held against Sunamp. In a conventional installation the user would, I hope, be able to switch them off so they don't charge in the summer.

For those who aren't aware... @hambrook and I have heat batteries as part of a trial. Our systems are managed remotely so we don't control when the batteries are charged.

Is this because of the trial or are Sunamp just missing some of the basic control/monitoring software?
I’ve an interest in these in the long term but having fine control over knowing how “full” they are and when they are heated would be crucial to me.

Making all these things play nicely together is going to be a long term challenge.

In 5 years I expect to add to my existing PV and home battery (OVO trial) with a ASHP, heat battery, V2G from an EV and more solar/battery. Goal is net zero but also to move to an off peak tariff for almost all my consumption. Getting all these to cooperative will be...interesting.

I know they’re relatively new. Perhaps are light on the software development function at the moment (we ain’t cheap). I know they got some investment recently so they really should be spending on that area pretty soon or, as you imply, they’re making a significant strategic error.

We have recently installed Li-ion battery storage to support our PV solar system. Although there is a future consideration about disposal of the cells, this is the same technology in use in modern EV builds as well as powering virtually all smartphones and other household electronics. 
A 7.8kW system allows us to take advantage of the generation more effectively and is small and light enough for domestic use. 
The system installed also allows a modified consumption pattern, charging at off-peak times and flattening the load level locally (if only OVO would let me use off-peak!)

With a 10 year warranty on the technology in use, this seems to me to be the next level (after solar PV) in domestic power systems

Some really interesting points raised here, @jenthomson.

Particularly in stacking up the sustainability aspects of each battery model with the price/efficiency. 

Was this a major consideration for others when choosing your energy storage solutions? @BPLightlog , @malc_smith , @JoeC, @Heatherd  

While we look forward to EV in the future, we currently use a hybrid but certainly there is a consideration of off peak supply to be available either directly or via storage systems. 
As the current grid will struggle to supply a full EV overnight demand, it would seem practical for suppliers to promote off peak use.