DC COUPLED ENERGY STORAGE SYSTEMS

Combining energy storage with solar generated power has long been the “holy grail” for the renewable
energy industry. By adding energy storage, the constantly varying nature of solar energy can be turned
into a dispatchable energy source.

The earlier system solutions were actually parallel combinations of PV power systems and energy
storage system (BESS). These paralleled systems were combined at the AC outputs of the respective
inverters (Power Conditioning Systems or PCS) and were designated as AC coupled power systems.
Where this type of integrated PV and energy storage system is still used is in cases where there is an
existing PV system or roof mounted PV system that uses small inverters.

A more efficient and more cost effective way of combining solar generated energy and energy storage is
to use the PV energy to charge the batteries on the DC side, and use a common PCS to deliver the AC
power to the grid. There are two ways to accomplish this DC coupled system architecture. One is to use
a PV inverter that is connected on the DC side to both the PV array and a DC to DC converter that
charges/discharges a battery. In this way, surplus solar energy is stored in the battery during daylight
hours so that it can be used effectively to extend the “solar day”. This stored energy will supply the PV
inverter after there is no longer sufficient energy from the PV array to fully power the inverter.

In the block diagram below, the DC to DC converter follows the constantly varying voltage at the PV
array input to the PV inverter at the PV side of the converter and manages the battery voltage at the
battery side. This capability is inherent to the buck/boost converter design in both the charge and
discharge modes.

The graph shows the surplus energy available during the middle of the day that can be diverted to the battery through the DC to DC converter. This is a relatively inexpensive way to add energy storage to a conventional PV power system. The functions that are normally available with this type of system include the following:

• Capacity firming
• Energy time shifting
• Ramp rate control

Factors to consider for this type of DC coupled system:

• In order to have enough surplus energy available for battery charging from month to month, the
PV array is usually sized for a higher than normal capacity relative to the PV inverter capacity.
• The PV system architecture must include central PV inverters with capacities of 500kW or more
in order to use the 500kW DC to DC converters.
• String inverter systems are usually incompatible with this type of DC coupled systems, so string
inverter and microinverter systems are normally used in AC coupled systems only.
• The batteries can only be charged from the PV system so charging from the grid to ensure full
battery charging is not an option.
• The short winter days as well as extended cloudy days throughout the year will limit the
system’s ability to maintain a high battery state of charge (SOC) which can impact the
performance and life expectancy of the battery cell blocks.
• Since these systems use PV inverters that can’t operate without the presence of a grid, they
can’t be used for resiliency, microgrids, black start, etc.
The DC coupled system that provides the most application options uses a bi-directional, battery inverter
instead of a PV inverter and uses 100% of the PV array energy to charge the batteries

With this system architecture, the DC to DC converter acts as a solar charge controller that manages the
charge voltage and current from the PV array to the battery. The PCS (Storage Inverter) can be supplied
with energy from the battery, the PV array, or both depending on the availability of solar energy, the
state of charge of the battery, and the
load requirements. Since there is no PV
inverter in this DC coupled system, it can
operate on or off the grid and the
batteries can be charged both from the
PV array and from the grid. This allows
for better battery state of charge
management than with PV charging
only.

The functions that are normally available with this type of system include the following:

• Capacity firming
• Energy time shifting
• Ramp rate control
• Peak demand reduction
• Frequency management
• Resiliency to grid failures
• Microgrids for extended off-grid operation
• Black start capabilities

A typical Commercial/Industrial, DC coupled, PV and energy storage system would look like the pictorial
diagram shown below. In this example, the PV array is connected to the FlexGen FlexPod battery
system via a DC to DC converter, while the AC output of the PCS is connected to the building loads. The
standby generator is connected in parallel with the FlexPod to support the building loads during
extended grid outages.

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FlexGen designs and integrates storage solutions and the software platform that is enabling today’s energy transition. Leveraging its best-in-class energy management software and power electronics, FlexGen delivers utility-scale storage projects integrated with traditional and renewable power generation globally. Our clients and partners include the most technically and commercially demanding developers, utilities, government agencies and industrial companies in the world.

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