Good, Better, Best: The Relative Merits of the PV Centric vs. Battery Centric DC-Coupled Solar + Storage Architecture

Why Galvancially Isolated String Level Optimizers are the way to go in DC-Coupling Solar and Storage

Solar + Storage is hot. Really hot. Like the market is going to double six times in the next 15 years hot, according to Bloomberg New Energy Finance.

While that projected growth is amazing, the amount of collocated Solar and Storage on the grid is still minimal as of time of writing of this article.  At present, much of the installed Solar and Storage capacity is of the AC-coupled variety, a topology whereby the harvested solar energy is sent up to the grid via one set of inverters and transformers and back down to the battery storage via another. More recently, the drumbeat for deploying collocated, utility-scale Solar and Storage via DC coupling has grown louder and louder as the industry embraces several undeniable technical and economic benefits of DC-coupling, where access to solar energy is stored directly at location of harvest. We won’t dive into those benefits in this blog, but will refer you to one we’ve published earlier on that subject by clicking here.

So, while combining the power of solar and storage presents great benefits to energy producers and consumers alike, we here at Alencon would posit, that in many applications, DC coupling is the way to go.  But not at all DC-coupled topologies are the same.

While the DC-coupled approach to combining Solar and Storage on the utility scale is still in its infancy, two predominant topologies for doing so are emerging. One topology, is a battery centric approach which places a larger direct DC-DC converter (typically 250 KW or greater) between the storage and the DC bus to facilitate the charging and discharging of the battery. A second approach, which is PV centric, involves installing smaller, string level DC-DC optimizers throughout the PV array which will in turn map a fixed voltage onto the DC bus to control the state of charge of the battery and charge the batteries directly from the PV.

This second approach, where galvanically isolated, string level DC-DC optimizers are distributed throughout the PV array offers many measurable benefits over the more monolithic, battery centric DC-DC converter strategy to DC-coupled Solar + Storage. The PV-centric approach introduced above is enabled by Alencon’s String Power Optimizer and Transmitter – the SPOT.

The primary benefit of using this PV centric, distributed approach is improved round trip efficiency.  For those not familiar with this term, round trip efficiency represents the net efficiency of the process of collecting energy, using that energy to charge the battery and then later discharging that stored energy. With a battery-centric, monolithic DC-DC converter, every roundtrip involves double the losses.  For example, if the DC-DC converter between the battery and the DC-bus connected to the PV is 98% efficient, the “round trip” efficiency of that charging solution will be 96% (i.e. 98% x 98%). By contrast, using a PV centric DC-DC optimizer to provide the battery its needed level of voltage only involves one conversion, so whatever the efficiency of the optimizer becomes the round-trip efficiency of the energy stored in the battery.  It is important point out however this does not impact the energy production going directly from the solar panels to the grid.

But if you think that’s good, the PV centric solution gets even better when you consider the fact that this approach adds more granular energy harvesting to the system. By using a string level DC-DC optimizer which includes string level maximum power point tracking (MPPT) to couple the Solar and Storage you stand to recover those losses all together, if not perhaps exceed them in certain cases.  This yield improvement can be achieved by mitigating any power production mismatches between strings.  Certainly, over the course of the 20 to 25-year life of a plant, as panels inevitably, unevenly degrade or various parts of the plant see different irradiation conditions, the improvement in power harvest offered by string level MPPT can really add up.

The distributed method for DC-coupling Solar and Storage also comes with several operations and maintenance (O&M) benefits over the life of the deployment of the battery energy storage system (BESS). String level optimizers like the SPOT are similar to string inverters in that their service protocol is “rip and replace” – meaning if there’s a problem with a unit, there’s no servicing it in the field. You just remove it and replace with a working unit while the other unit is returned to the vendor. In contrast, the battery centric approach represents a single point of failure, which should it fail, renders the battery, the most expensive part of the system, useless until that effected piece of power electronics is repaired or even possibly replaced.

These are indeed exciting days in the arena of Solar + Storage, not just because of the amazing growth prospects for the sector but because no one approach for marrying these two technologies has yet to become the standard for doing so.