DC Coupled Energy Storage

Maximize production and revenue of utility scale solar

Harness the full power of your existing utility scale solar array with our advanced DC Coupled Energy Storage technologies that offer unprecedented control, efficiency, and flexibility for your power needs.

Why DC Coupling?

Value Streams

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Why DC coupling for energy storage?

The addition of energy storage to an existing or new utility scale PV installation gives system owners and operators the ability to capture additional revenue. This topology can be achieved with both AC and DC coupling – but utilizing a DC to DC converter comes with many additional benefits.

A DC Coupled Utility Scale Solar Plus Storage configuration can lead to higher efficiencies and overall cost savings from a variety of sources. With our DPS family of DC converters—available in 500kW building blocks—coupling your energy storage system alongside your utility scale solar on the DC side of the bus lowers the overall cost of installation while allowing for greater harvesting of solar energy. What’s more, this configuration has the ability to open up new revenue streams that are not possible with AC coupled or other storage configurations, including clipping recapture and low voltage harvesting—while also earning you valuable tax incentives.

With their scalability and modularity, our systems are designed to be compatible with any size application and are easy to expand or modify to meet your changing energy needs. With our DPS family of converters, any combination up to 6 units can be joined to create building blocks up to 3MW.

Potential value streams of a DC coupled configuration

Ways to capture additional revenue with DC Coupled Energy Storage

Capacity Firming:
Turn your solar energy into a dispatchable asset with the ability to bid firm capacity into merchant markets.

Adding solar to storage gives the system operator the ability to bid firm capacity into merchant markets. That is, storage makes PV generation a dispatchable revenue generating asset. If your storage system is fully charged entering the next 24-hour period, system owners can confidently bid into the day ahead capacity market at any time of day and with no weather contingencies. Depending on the available local capacity market, this may translate to higher overall system revenue than if operated on a traditional flat $/kWh PPA rate structure.

Energy Time Shifting:
Utilize your generated solar energy when its value is highest to take advantage of peak rates and/or demand.

Energy storage allows bulk energy shifting of solar generation to take advantage of higher PPA rates in peak periods or to allow utilities to address daily peak demand that falls outside periods of solar generation. Similar to capacity firming, energy time shifting is not unique to DC-coupled systems – however the reduced cost and increased efficiency of a DC-coupled system may make revenue opportunities such as time shifting and capacity firming more economically viable.

Clipping Recapture:
Using a DC coupled storage configuration, harness clipped energy by charging the energy storage system’s batteries with excess energy that the PV inverter cannot use.

Given common inverter loading ratios of 1.25:1 up to 1.5:1 on utility-scale PV (PVDC rating : PVAC rating), there is opportunity for the recapture of clipped energy through the addition of energy storage. Using a simplified system for illustrative purposes, consider a 14MWDC PV array behind a total inverter capacity of 10MWAC. Depending on your location and type of racking, the total clipped energy can be over 1,000,000 kWh per year. Without energy storage these kWhs are lost and revenues stunted. With storage attached to the array, the batteries can be charged with excess PV output when the PV inverter hits its peak rating and would otherwise begin clipping. This stored energy can then be fed into the grid at the appropriate time. Note that this ability to capture clipped DC output is only possible using a DC-coupled storage system.

Clipping is a phenomenon where the DC-AC PV inverter has hit its peak AC output and therefore must drive the PV DC array voltage off of the maximum power point in order to effectively curtail the PV array. It is not possible to move or shunt this power to an AC-coupled battery system because doing so would force the PV inverter to exceed its rating to pass any excess PV energy onto the common AC bus.

Using a DC-coupled storage configuration, the DC-DC converter charges the batteries directly from the DC bus with the excess energy that the PV inverter cannot use. In the simple example of Figure 2 where there is a 1MW AC inverter with a 1.4MW DC array, during times when the PV array is producing greater than 1MW DC, excess energy can be used by the DC-DC converter to charge the batteries, then discharged later when the PV output is low or when there is a peak demand on the grid for kWh production.

Curtailment & Outage Recapture:
A DC Converter allows continuous operation, even when the PV inverter is offline for scheduled or unplanned outages or curtailments.

When storage is on the DC bus behind the PV inverter, the energy storage system can operate and
maintain the DC bus voltage when the PV inverter is off-line for scheduled or unplanned outages or
curtailments. Additionally, when the PV inverter is offline or curtailed, the energy from the array can still flow to the batteries via the DC-DC converter ensuring energy can be harvested for later use. This benefit may be particularly valuable when a large utility-scale array is curtailed by the ISO or utility. Curtailment is sometimes seen in areas of high solar penetration — such as California — when there is overall excess production on the grid. As one example, a February 2017 memo from the California ISO warned that with high hydro capacity entering 2017, curtailment of solar and wind at any given time could be up to 6 to 8 GW. With a DC-coupled energy storage system, solar production can continue in that scenario with energy being stored and available for discharge when curtailment ends, mitigating system owner downsides? for both existing and future projects in such resource rich areas of the grid. This capability is only available with DC-DC converter.

Low Voltage Harvesting:
With a DC to DC Converter, avoid having a minimum voltage threshold needed to operate and capture energy in the mornings and evenings and during cloud cover.

PV inverters typically require a minimum threshold DC bus voltage to operate. As a result, available generated energy in the morning and evening when voltage on the array is below the PV inverter ‘wake up’ threshold is not captured. Adding energy storage through a DC-DC converter allows for the capture of this margin-generated energy. This phenomenon also takes place when there is cloud coverage. In both cases this lost energy could be captured by a DC-coupled energy storage system. This capability is only available with a DC-DC converter that has voltage source capability.

Ramp Rate Control:
Monitor your inverter output and self-regulate it to allow for continuous energy injection into the grid.

Ramp rate control is often required by utilities and ISOs for PV and wind systems to mitigate the impact of a sudden injection of power onto the grid or a sudden loss of generation due to the intermittent nature of both generation sources.

A ramp rate of 1MW/minute, for example, has been required by HECO in Hawaii to limit the speed with which a large array can come up to power or trail off in the event of cloud cover.

A storage system coupled with PV can monitor PV inverter output and inject or consume power to ensure the net output remains within the ramp requirements, allowing for continuous energy injection into the grid. Additionally, energy otherwise lost when a PV inverter would self-regulate during a ramp up (by manipulating the I-V curve to curtail power output) can be stored for later use.

That is, not only can the storage system provide a regulatory benefit during ramp up and ramp down (PV inverters alone can only modulate ramp up events), but there is some revenue recapture from the storage of the excess energy during ramp up.

Note that the ramp up phenomenon is much sharper on arrays with higher DC:AC inverter loading ratios. One of the desired effects of a large inverter loading ratio is the “boxier” output curve versus the traditional bell shaped curve associated with traditional PV systems with closer to a 1:1 DC:AC ratio. The potential negative is that for grid areas where the ramp up is regulated, there is greater potential loss of otherwise sellable kWh during the ramp up. Therefore, coupling PV with storage provides one more opportunity to optimize revenue from your utility-scale PV array.

Learn from the experts

Watch the webinar to learn how to maximize the benefit cost ratio of your existing PC array with a DC Coupled configuration.

Contribute to a greener future while maximizing the production of solar energy and unlocking additional revenues with the power of DC Coupled Energy Storage.

More Resources

Learn more about the value of DC Coupled solutions for Solar Plus Storage applications and compare configurations.

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