Store smart: 4 steps to successfully design DC-coupled solar+storage

By Ray Saka, senior director of sales at IHI Energy Storage

The advent of California’s 100% carbon-free energy mandate has positioned solar project development for substantial growth in the next decade. With the rise in solar spurred by the new mandate, the industry will need to address various challenges, including the long anticipated issue of the “duck-curve,” wherein over-generation of solar energy during the day and ramp-rate concerns in the evening necessitate tying solar systems together with energy storage systems (ESSs). One way to efficiently address the duck curve issue is by using DC coupling instead of AC coupling.

Figure 1: AC coupling (red lines) vs. DC coupling (blue lines)

There are two major ways in which solar can be coupled with an energy storage system: either by coupling on the DC side (DC-coupled system) or on the AC side (AC-coupled system), as shown in Figure 1. While AC-coupled ESSs are more conventional and have been the standard in the industry for many years, the systems have higher losses compared to DC-coupled systems (see Figure 2).

Figure 2: DC-coupled system vs. AC-coupled system efficiency

DC-coupled ESSs are more advanced–they can capture clipped energy and be designed with higher efficiency. However, PV integration in utility-scale DC-coupled systems requires a deeper understanding of system integration and controls.

In this article, we will visit four key items to pay attention to while planning for a DC-coupled solar+storage system.

1. Complete in-depth performance modeling

As with any solar and energy storage project, studying performance is crucial. Fortunately, a DC-coupled system is simple to model, as backfeed charging from the grid is not possible unless the PV inverter is configured for bidirectional power flow and conversion.

While modeling the recapture of DC clipped energy is simple, it is very important that developers investigate project-specific LCOE sensitivity analysis of the PV system and energy storage system combined. Loading high DC/AC ratio is possible with the latest PV inverter technology; however, the optimal ratio depends on land availability, module pricing and the value of shifted clipped energy. Additionally, it is important to take an account of performance related losses against the theoretical maximum based on solar uncertainty, system component losses and controls related losses.

2. Design thoughtfully

When coupling an ESS on the DC side, careful planning and implementation of multi-connections between PV, ESS and the DC/AC inverter is needed. While normal daytime operation is straightforward, a sudden drop in irradiance or end-of-day battery discharging into the DC system could reverse the current, backfeeding power into the array. For a smooth and safe transition and operation, proper preventative coordination is essential, such as including the use of a reverse current blocking diode or circuit breaker.

3. Beware of topology pitfalls

A DC/DC converter is the key to controlling and stabilizing the voltage condition between PV and ESS and optimizing the design.

The solar industry has historically experienced inverter failures where voltage withstands the capability of the IGBT (insulated gate bi-polar transistor) device. It is important to evaluate the system voltage during various conditions, such as forced power plant curtailment (when the ESS is more likely to be charging). Substantial curtailment boosts the PV voltage to a maximum operational voltage closer to Voc.

Additionally, even during nominal operation, PV power plants are subject to the cloud edge effect where irradiance levels can reach as high as 1,300 W/m2 and voltage spikes can occur.

There are various DC/DC IGBT-based converter technologies in the market that can mitigate these issues, including 2-level vs. 3-level NPC (neutral point clamped) technology. While 2-level conversion technology rated with 1,200 Vdc IGBT is simplistic, maximum operational voltage is limited and the cloud edge effect could cause the operational voltage to exceed the allowed voltage. The more advanced IGBT architecture, 3-level NPC technology parallels additional IGBT and distributes the voltage stress evenly, enabling the converter to safely operate up to 1, 500 Vdc.

(from left to right) 2-level topology, T-type NPC topology, NPC topology

4. Implement a centralized intelligent control system

Operating a PV plant at optimal conditions requires a centralized intelligent control system to manage both the PV array side (DC/AC inverter) and the ESS side (DC/DC converter).

The centralized control format will enable the system to operate smoothly in “clipped energy” mode while the DC/AC inverters track the maximum power point (MPP) of the PV array. The DC/DC converter must be well coordinated such that it “follows” the voltage condition of the PV array set by the MPP algorithm or array voltage during power limit, while the system controller dynamically operates both the DC/AC inverter and the DC/DC converter to keep plant generation in clipped energy mode. This entire sequence of strategic control must be closed-loop to prevent time delay related losses.

When the DC/AC inverter is at maximum power, the inverter moves off the MPP algorithm to power limit mode. During this period, the DC power from the PV system is wasted; the PV inverter capacity is limited as it has reached its maximum allowable current, which is the best time for the DC-coupled ESS to charge.

A central control is important not just for physical control, but also for dispatch intelligence. The data provided guides the forecasting engine, which leverages several algorithms based on historical data availability and the project case, as well as the optimal dispatch engine, which captures the greatest customer value out of foreseeable future scenarios.

The future is bright for solar

To bring both solar and storage to their greatest potential, the two industries must work together. While DC-coupled storage solutions are not without challenges, if we keep in mind these four aspects of DC coupling, DC solutions may affect the industry shift and enable utility-scale solar+storage systems to reach true efficiency.

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