November 2, 2021
DC-Coupled: On the Benefits of Smart String Management
As energy storage costs continue to fall, Solar energy power plants plus storage are generating a higher benefit to cost ratio. There are multiple types of photovoltaic (PV) plus storage plants, namely AC coupled (AC/AC), DC coupled (DC/DC) and DC coupled bi-directional with grid mode charging. A DC coupled PV plus storage system is particularly beneficial because the balance of system costs can be further reduced. Currently, PV and storage share the same inverter. The loss of revenue due to inverter power clipping is marginal — typically about 1%. When using bi-directional inverters, storage can also be charged using grid electricity which can further increase the arbitrage revenue.
As the PV market continues to steadily grow, the capacity credit of PV stand-alone systems will see a significant decline, making PV plus storage even more attractive. Per the DOE energy storage grand challenge 2020 report, annual global deployments of stationary storage (excluding PSH) is projected to exceed 300 Gwh by 2030. The grid-related storage represents a 27% compound annual growth rate, within which a significant expansive growth comes from 4-hour-duration PV plus storage.
The benefits of DC-Coupling are extensive and further generate efficiency as a result. First, DC-Coupling fully utilizes battery string capacity. As a result of impedance variations and cell state of health (SOH), battery strings have slightly different ideal charging and discharging rates. The battery cell variations are inevitable and begin to manifest as early as during the manufacturing process. As this aging process is inevitable, current will circulate from one string to another; thus, causing unnecessary energy loss. As a result, not all the battery strings will have their capacity utilized fully. DC/DC converters eliminate this impedance and SOH as the charging rate is independently controlled per string. When each string can be fully charged and discharged, this yields 1.5% more capacity utilization than standard AC/DC coupling.
Secondly, DC/DC allows for a higher bus voltage (over 1350V) for the full range of the battery, reducing DC current of the PCS. Thus, a higher DC bus voltage results in much more power. Since the PCS power is not limited when battery voltage drops to less than 950V at the end of discharge, less PCS power blocks are needed to fulfill the same power output; which in turn reduces the PCS costs for specific installations.
In addition, DC Coupling, battery modules are more flexible in configuration resulting in ease of repair and augmentation. Battery units, therefore, don’t need to match each unit per string in DC/DC converters as they are able to be adjusted. When each string operates independently from each other; it is easier to repair faulted battery modules without impacting the remaining string operations. String level management allows for new battery string augmentation and allows mixed usage of both new and old batteries.
Moreover, DC/DC creates a lower short circuit current resulting in higher safety measures. For conventional configuration, all battery strings feed into the fault together causing the short circuit current to exceed 250kA for a 2.5MWh unit. When using a DC/DC converter, the short circuit current is reduced up to five times the standard. This results in fewer fuses and interconnection busses requiring installation and adding points of failure which substantially reduces the arc-flash incident energy.
Finally, a DC/DC converter can be agnostic to AC Coupled or DC/DC configurations. DC Coupled solar plus storage can use the same battery unit, meaning battery projects are interchangeable between AC/AC and DC/DC without changing the battery unit.
In conclusion, DC Coupling is the paradigm of smart string management as it completely utilizes the full battery string capacity, allows for a higher DC bus voltage, is easily configured, safer, and results in agnostic configurations without needing to change the battery unit. As the industry moves away from PV stand alone systems, smart string management is imperative for future solar plus storage systems.