Dynapower is spreading its bets with its recently launched DC-DC Converter series rounding out offerings of utility scale, behind the meter and fully integrated systems. It’s an interesting choice, offering design flexibility while exploiting the final years of the ITC incentive.
First some basics. When solar and storage are paired, they can be connected in four ways:
- Operate independently, with energy storage interconnected directly with the grid
- Have inverters for both solar and storage that are then AC-coupled
- Use a single DC-coupled (hybrid) inverter for both solar and storage
- Connect both solar and storage into a tight DC-coupled system
How the batteries are charged depends on the topology: in the first, they are only charged by the grid, in options two and three, they can be charged by the grid and PV, and in a tight DC-coupled system, they are only charged by PV.
DC side charging
Dynapower’s DC-DC Converters are used in the fourth topology. The 250 kW, 375 kW and 500 kW DPS bi-directional DC-to-DC converters can be scaled with up to 8 units for 2 MW, 3 MW or 4 MW of energy storage respectively. All three have a 98.2% average efficiency, a 550-1500 V input voltage range (both battery and PV), and are compatible with all batteries.
The arguments for the DC-DC connector fall into three buckets: a lower initial system cost, increased design flexibility, and potentially more attractive revenues.
The lower system cost is straightforward: there are fewer components, with the elimination of separate battery inverters, AC collection systems, transformers and switching gear. And streamlined BOS is likely to trim engineering and interconnection study costs as well.
A tight DC-coupled system also offers more design flexibility. Hybrid inverters require a 1:1 DC:AC inverter loading ratio. A tight DC-coupled system can support a more natural 1.4:1 or 1.5:1 DC:AC ratio. It’s the revenue argument that deserves close modeling.
A tight DC-coupled system offers all the standard storage revenue streams—capacity firming, curtailment, ramp rate control—plus the all-DC topology results in lower round-trip efficiency losses, which marginally improves the economics across the board. The topology’s benefits start accruing once you add the value of recapturing energy usually lost to clipping caused by the inverter loading ratio as well as low voltage energy that isn’t harvested before the inverter wakes.
The topology, however, trades off two very significant revenue streams: ITC incentives versus rate arbitrage.
One of the financial arguments for the DC-DC converter is contingent on the ITC: because the battery is never charged from the grid, there’s no chance of ITC clawback. While that makes it easy to claim the full ITC, most battery management systems today already can, if desired, be set to prevent AC-coupled or hybrid systems from charging from the grid. And in the future, those AC-coupled or hybrid systems can be reprogrammed as the ITC is phased out or if it is extended with different rules.
The easy-ITC comes at the cost of the ability to take advantage of rate arbitrage: a tight DC-coupled system is only charged during the day, when rates are relatively high. Other topologies offer the flexibility to store from the grid when rates are at their lowest, enabling the system owner (or the battery management system algorithms) to do the math about whether it’s preferable to keep full ITC benefits or sacrifice some in order to gain arbitrage revenue. In its 2017 paper Evaluating the Technical and Economic Performance of PV Plus Storage Power Plants NREL found that, without the ITC, the loss of arbitrage revenue puts the economics of a tight DC-coupled system behind all others except for fully independent systems. With ITC, its economics outperformed others.
NREL’s study captures a moment in time, looking at 2014 incentives and 2016 estimated prices. The reality is that the market is moving so quickly that any developer considering energy storage needs to model the topologies based on today’s conditions and evolving incentives. What Dynapower’s new offering brings is another option to consider.