When High Temperatures and Low Voltages Collide in DC-Coupled Solar + Storage

By Aaron Shoresman, Data Analyst; Tristan Doherty, Head of Research and Development; Licole Paroly, Marketing Manager, IHI Energy Storage

As DC-coupled solar + storage systems gain popularity, their unique challenges and advantages are coming into focus. Among the challenges is that popular DC-converters often advertise a single nameplate power rating tied to ideal conditions, which brings forward a key question: what happens to the DC-coupled system when converter voltage and temperature ranges are outside of these ideal conditions? IHI Energy Storage has conducted analysis to illustrate how low PV or battery voltage and high temperatures may coalesce and force the system to operate at a lower power than the nameplate capacity (situations also known as “de-rate events”). This analysis evaluates to what degree de-rate events might financially impact DC-coupled systems and what can be done to mitigate the effect.

At low voltages, high currents are required to achieve the nameplate power capacity of a DC-converter. However, the currents required may exceed the maximum current rating of the converter. Therefore, low voltages may wind up lowering the power of the converter (in other words, de-rate the converter). Higher temperatures also reduce the current capacity of the system and may contribute to a de-rate event. Low voltage and high temperature simultaneously raise both the likelihood and impact of a de-rate event.

Battery voltage is determined by the state of charge of the battery and charge or discharge power. As you can see in the following chart, charging or discharging the battery at low SOCs results in low rack-level voltages, making the system susceptible to de-rates. As both battery and PV voltage decline with system age, the frequency and impact of de-rates increase.

For IHI Energy Storage’s analysis, the system is located in New York and uses a battery model with a relatively high voltage. If a battery with a lower voltage were used, the frequency and magnitude of de-rates would increase, raising the financial impact of the de-rate events. In order to present a severe but plausible scenario, the remainder of IHI’s assumptions are conservative.

The impact of de-rate events on the bottom line of a system operator is dependent on the market and on the use-case of the system. For example, systems operating in the New York VDER program rely heavily on the revenue generated by discharging during ICAP and DRV windows and de-rate events happen mostly during the summer when these windows occur. For that reason, the analysis focuses on the impact of de-rates on ICAP/DRV windows, which serve as a close proxy for ESS revenue losses.

The table above summarizes the impact of de-rate events on ICAP and DRV windows during years 1 and 10. In this example case, the system includes a 6.7 MW DC PV array, a 5 MW AC inverter, and a 2.5 MW DC-ESS. The dollar values of missed opportunities are based on typical PV plant and ESS sizes under the VDER program in the regions of New York receiving the lowest and highest combined ICAP and DRV prices, respectively.

Note that these values vary and are dependent on market and use-case of the system.

Note that ICAP and DRV windows overlap during the summer (both occur 2-7pm on non-holiday weekdays from June 24 – Aug 31; DRV extends to Sept 15).

For de-rates to occur and have a financial impact on battery operations, three conditions must coalesce: 1) low PV or battery voltage, 2) high ambient temperatures, and 3) charge or discharge near max ESS power. Unfortunately, these three conditions are highly correlated (for example, an operator often wants to completely discharge the ESS at maximum power on a hot summer day with a peak event).

In DC-coupled solar + storage systems, there are many complex elements that must align properly to work effectively. We have evaluated the impact of DC-converter de-rate events on system revenue, but all other system components may experience similar issues and exacerbate the effect. As DC-coupled solar + storage projects are developed in more locations with a wider variety of characteristics, projects of every size need to be evaluated at the component level to fully understand system performance under all conditions. Evaluating potential de-rate events when designing systems will provide customers with a more nuanced operational view and push DC-coupled solutions to advance even further.

About IHI Energy Storage:
IHI Energy Storage is a Chicago-based subsidiary of IHI, Inc., New York, and provider of energy storage system solutions. As a systems integrator, IHI provides energy storage solutions for both front of the meter and behind the meter applications. IHI’s solutions include adaptive, real-time operation software, technology agnostic system designs, and autonomous operation capabilities, with support from its 160-year-old parent company, IHI Corporation, Tokyo.

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