The Case for Utility Scale DC Coupled Solar + Storage

Why DC Coupling of Large Scale Solar + Storage Makes So Much Sense

No debate, solar plus storage is a hot topic in the alternative energy business these days. But, what’s the best way to go about marrying up solar and storage in a single location?  Essentially, there are two methods for integrating solar and storage. Solar and storage can be integrated on the AC side of the system (known as AC coupling) or on the DC side of the system (DC coupling).

To explain what these strategies mean in terms of system design: In an AC coupled solar plus storage system, the batteries (i.e. storage) are charged after the solar energy has passed through the PV inverter. As a consequence, such systems need two inverters, one for the PV and one for the battery. Additionally, since the batteries technically sit outside of the PV system, essentially on the grid side of the system, not all of their charge will necessary come from the PV. While such an approach could have benefits, it very definitely has drawbacks from a financial perspective in as much as storage, which at this point costs far more than PV, risks not being counted for sake of the solar Investment Tax Credit (ITC), if it is not being charged from the PV.  Click here to read a great article about solar and storage and the ITC.

Here at Alencon, we’ve developed a novel approach to facilitating utility scale DC coupled solar plus storage deployments using our unique, galvanically isolated DC-DC optimizer, the SPOT. In our concept, by coupling solar and storage on the DC side, we can provide the system owner a number of benefits including:

1. Reduced cost: Optimizers cost less than inverters, so you can save money on a DC-coupled system by only needing one inverter.

2. Increased Energy Yield: Coupling solar and storage on the DC side provides a significant opportunity to increase the yield of your PV assets, particularly if you have a high DC to AC ratio, i.e. more PV panel capacity than name plate inverter capacity. When a PV plant has a DC overbuild, PV capacity can be “clipped” when DC energy production exceeds that of the inverter. By coupling PV and storage on the PV side, this excess DC capacity can be diverted directly to the battery. As a result, such a system can better divert PV energy at times of higher supply and lower demand to be available during times of higher energy demand and low energy supply (i.e. early evening).

3. Fewer Roundtrip Losses: Roundtrip losses is the amount of energy lost as power passes from the energy source into the batteries and back out again when that energy gets put onto the grid. In the DC-coupled concept, DC charging of the battery never goes through an inverter, so energy is only lost when the battery discharges, which is a half as much loss as an AC coupled system.

4. More ITC: When charging your storage on the DC side, 100% of the cost of the storage system can be deducted for ITC purposes. As the cost of storage is significant, taking this deduction can appreciably improve the ROI of a solar plus storage deployment.

5. Retrofit Possibility for Existing PV Systems – Alencon’s optimizers, with their roots in use for PV retrofits, makes retrofitting existing PV plants with storage a possibility.  With the specter of the Suniva and SolarWorld casting a long shadow over the industry, considering a storage retrofit to an existing PV plant could be a very effective use of capital. 

Click here to learn more about Alencon’s DC coupled solar plus storage solution.