High voltage or low voltage – what’s the right solution for household PV-plus-storage systems?

Undoubtedly we all want to see more of this, but what are the best ways to make it happen? Image: SMA
Not long has passed since the Intersolar Europe in Munich closed its doors for another year, and the show confirmed that storage solutions are not the exception any longer but seem to have become an integral part of discussions of solar PV. A complete hall was dedicated to energy storage solutions, but they were also presented in other halls at the booths of many system technology providers. In reporting on the findings, we’ve seen three things:

Solutions become more and more mature, a second or even third generation of products associated with better performance and lower costs are available on the market
New and bigger players including the automotive OEMs are replacing start-ups
There are now different concepts for integrating batteries - as discussed in our previous blog article.

We didn’t describe it in detail last time we blogged for PV Tech Storage, but one of the “hot” topics is the question of the appropriate battery voltage for residential storage systems. This time we want to take a closer look if there is just “one right voltage” to be used.

After checking and clustering the complete offering, we see two general centres of gravity: “low voltage systems” in the range of 48V DC, competing with “high voltage systems” with up to 400V DC, with suppliers of each claiming to provide the more brilliant approach. To evaluate the benefits and drawbacks of each approach, we have to take a closer look at the battery itself and the system technology. Power electronics for coupling the battery to the power generation and distribution system seems especially to make a difference.

System technology – power electronics

To connect a DC battery to an AC distribution grid, power electronics is needed, regardless of the question of AC- and DC-coupling. Turning a blind eye to the battery technology, and anticipating a standard PV inverter with MPP-Trackers (maximum power point), a DC link and a semiconductor bridge on the grid side, the lowest system costs for battery integration will be achieved with the least sophisticated topology engineers can imagine.

Asking experienced power electronics engineers, the results will most likely be that coupling a battery to the DC link of a standard PV inverter with a state-of-the-art buck-boost-converter looked most promising. With a minimum effort on windings and switches (only two are needed), this topology is unbeatable regarding costs.

To allow the utilisation of this technical approach, the voltage-level difference between the DC link and the battery should not exceed a ratio of 4:1 to allow acceptable efficiencies. As a practical example, a battery to be connected to a 400V DC link should not provide a minimum voltage below 100 V. Lower battery voltages would require galvanic separation, and this means a transformer and more switches, and thus higher costs. So in general, high voltage batteries allow for lower system costs – if the system integration is done properly and some mandatory degrees of freedom in the design are available.

But what about the costs of the batteries?

There are many ways to do storage and the answers are rarely 'black and white', SMA argues. Image: SMA.
Battery

Let’s start with the appropriate battery capacity which would perfectly fit the residential system needs. There are several factors to be considered, such as depth of discharge (DOD), energy consumption, PV profile, backup functionality etc. However, let’s assume that the typical usable battery capacity will range between 2kWh and 8kWh. If you want to realise a 2kWh with 48V, the battery cell size will be approximately around 42Ah. If one wants to have it on the 400V-level, the cell has to shrink to 4Ah. Doing the same calculation for the 8kWh, the cell size would be approximately 170Ah for lower and 20Ah for higher voltage.

Now it is time to have a look on the cell availability and the system costs. Generally, the cells with the lower size are the xx650-round type cells, highly commercialized, typically offering lowest costs/kWh. The cells in a range of 40-60Ah are well known and used in the automotive world, so yes, these are also highly commercialised and competitive cells in terms of costs. Now, if the cell costs are not the issue, what else could it be? Most reasonably, it will be the question of the definition of the smallest battery unit, the question of flexibility – fixed pack size or modularity, the retroactive extension of the battery, battery technology and synergies with other markets.

48V-Modules are available and accepted as standard for the telecommunication market – the modules can be found from different manufacturers in different sizes, ranging from somewhere around 1kWh to 4kWh - if Li-Ion Batteries are used. And the 48V are certainly more adequate if you want to use different battery technologies. With a parallelization either on cell or module level, basically all required storage sizes can be covered (but this might be accompanied by higher costs). On the other hand, other technologies can cover a very broad range of storage sizes without any additional system costs. The flexibility of the high voltage system is more limited – the coverage for the smaller storage sizes will result in a very specific design and the voltage level will probably not be at 400V, but lower. High voltage in residential systems somehow seems to be a lithium ion-specific topic, and most other technologies will have difficulties in following that trend.

Conclusion

It seems that in each blog article we have to emphasise that, even though the marketing campaigns of various providers are claiming that they found the one and only right way to do residential storage, it is not a simple “black and white” issue that we are talking about.

From an academic level, lower battery voltages offer better battery costs at higher system integration costs, whereas higher voltages turn that comparison the other way round. In the end, only accurate and diligent system integration can cut costs. To close, allow us one final commonplace observation: the more battery prices decrease, the more system costs move into focus and correspondingly, more accuracy has to be invested in the system design.