Energy Storage Europe 2019

Part 3


7 April 2019

Following on from the previous conference reports that described the overall potential for VRFBs in grid-scale Energy Storage and the expected future development of a $50 Billion dollar worldwide grid-scale Energy Storage sector I now turn to discussions with some of the VRFB manufacturers present at the conference and cover one of the most interesting recent technical developments in the sector.

First up we turn to one of the more established VRFB manufacturers - Schmid Energy Systems and their EverFlow range of small (<50kWh) to large scale (>500kWh) VRFB units. A model (the real thing is 2m high) of their small (5kW/30KWh) unit, optimised for powering telecommunications towers, is shown on the front counter below.

Discussions with Vice President Henrik Buschmann and Sales Manager Marcel Schonleber revealed that as well as all the normal advantages of VRFBs - no degradation of performance, 100% to 0% operation and intrinsic fire safety, etc there is also an unexpected advantage of VRFBs in the Telecomms tower application - it turns out that batteries at remote towers suffer rather a lot from unwanted attention by those that might wish to steal them.

Unlike a Lithium-ion based battery system which contains large numbers of individual Li-ion cells and interconnecting copper busbars, which are easily stolen and reused for other purposes, a VRFB based battery system appears to contain only plastic piping, a pump or two and a larger container of highly acidic liquid. In short it is much less knickable. Someone opening the back cover of such a system might not even realise that it is a battery - certainly there is nothing that is obviously resalable. 

Next up is a more recent entrant to the market - VisBlue, a spinout from an earlier collaboration between the Universities of Aarhus in Denmark and Porto in Portugal. Their systems are pitched at the small (<50kWh) to medium sized (50-500kWh) energy storage markets - one of the VRFB modules that they build their systems up from is shown on the right of the picture below. 

Interestingly Visblue seem to have opted for a slightly longer than average 5 hour ratio of battery energy to power - I believe that the module shown is their smallest unit - 5kW/25kWh. This would be useful for domestic or small industrial storage, as well as the Telecomms application described above. These modules of course can be duplicated to obtain higher power and storage ratings, as has been done for their recently installed battery on the Danish island of Livø. In this installation the VRFB was coupled to a small wind turbine, and this is perhaps no accident as Denmark of course has the highest proportion of wind-power generated energy in the world. 

Next up we come to Volterion who were also showing a small VRFB that could be useful for commercial, industrial and telecomms applications. Their director of Business Development, Kees van de kerk also described how the deep discharge and negligible degradation of VRFB systems made them particularly useful in heavily-cycled electric vehicle (eV) charging systems.

Electric vehicle charging hubs such as the recently announced £1 Billion Gridserve network despite being fitted with PV panels would have to manage significant variation of supply and demand over the course of a single day - morning rush hour, noon PV charging, evening rush-hour, overnight grid charging would yield at least two major charge-discharge cycles over the day - this is twice as much as the single daily cycle assumed by Lazard in their Levelized Cost of Storage (LCOS v 4.0 (2018)) techno-economic analyses. More cycles per day favours VRFB's over Lithium-ion.

If eV hubs like these use batteries with a non-degrading technology such as VRFBs then they can also offer energy storage/frequency regulation services back to the grid and earn revenue from the market when not otherwise utilised.

If when using one of these charging hubs the user will be expected to book a charging slot in advance, rather than simply turning up and expecting to plug straight in (they could offer both at different prices of course), then the charging hub would have even greater capability to plan its charge-discharge schedule and offer energy stabilisation services back to the wider grid. Not an idea I have heard mooted yet, but fine, you're very welcome.

The most interesting thing is perhaps not the large item on the left of the Volterion picture, but instead the small one sitting on the plastic box to its right - this is a 2.5kW flow battery stack and to understand why it is so interesting we need to go back and learn a little more about stack construction.

As described in this introduction to VRFBs each cell produces about 1.25 Volts. A single cell is shown in an exploded view below (diagram from the very helpful article by Noack, Roznyatovskaya, Herr and Fischer

If 1.25V is all you require then this is sufficient. However if we want a higher operating voltage then just as when we stack AA batteries end to end we need to combine multiple cells together to build up the voltage. In this configuration the graphite plate c) acting as the positive side of one electrochemical cell also gets pressed into service to act as the negative side of the next cell - the overall structure then becomes a,b - c,d,e,f,e,d  - (c,d,e,f,e,d  repeated) - c,a . The series of cells is called a cell stack and the plates c in between each flow cell are now referred to as bipolar plates - the positive terminal of one cell is the negative terminal of the next. A typical stack might contain 40-60 cells in total producing 50-75V.

A good example of a commercial cell stack from Schmalz is shown below. 

This is a 6kW stack just over 1/2m long, and weighs in at a fairly hefty 60kg+ - it takes two people to lift it. The image below shows these units installed in banks at the 2MW/20MWh RedoxWind project at the Fraunhofer ICT in Pfinztal near Karlsruhe:

Much of the weight of these stacks derives from the massive metal end plates and the significant clamping structure that provides the external force to provide a long term compressive seal on each and every one of the stack layers.

The notable thing about the Volterion stack (albeit 2.5kW instead of 6kW) is that it weighs only 6kg - so quite remarkably you can pick it up with one hand: 

Notice how the large clamping structure has been almost entirely eliminated - this is because the stack layers have all been welded together - the only remaining clamping structure is provided the 4 steel straps that just act to hold the endplates in position. The resultant reduction is weight is about 80%.

Not only are Volterion using these fully welded stacks in their own VRFB systems, but they are also supplying these as OEM components for other manufacturers' VRFB systems. It is technical improvements just such as this that yield the observed 'experience rates'  reductions in cost referred to by Oliver Schmidt in the first of these conference reports.

The long term reliability of these welded stacks will no doubt be established in time but if they do prove successful then this development may open the way to not just cheaper stationary VRFB energy storage systems but also potentially other, more mass-sensitive, applications as well - watch this space !

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