Battery System Classification
Grid-scale battery systems can be broadly classified into two types: AC-side and DC-side battery systems. Addressing project needs for a particular battery system design, its performance and risk, and the associated techno-economic models which can be built comes down to examining their differences. Let’s settle on some basic concepts first by using this partial list of characteristics:
| AC-side Battery Systems | DC-side Battery Systems |
| Requires own power electronics, such as an inverter | Relies on pre-existing power electronics, such as a conventional solar PV inverter |
| Charges from the grid | Charges from renewable or conventional generation (solar, wind, mini-hydro, diesel) |
| Power flows AC to DC then back to AC | Power flows DC to AC |
| Requires active cooling | Uses passive or active cooling |
| Requires separate interconnection | Relies on existing generation’s interconnection |
This is an early in development classification scheme. In conversations, some have suggested that a better split would be along functional rather than integration characteristics such as:
Unidirectional Battery Systems in which power flows from an AC source to the battery system and into a DC load OR from a DC source to the battery system to an AC load
Bidirectional Battery Systems in which power flows are more circular, charging a battery system from an AC source and delivering power back to an AC load
To be honest, a functional approach allows you to ask questions about the appropriateness of a battery system to a particular use case. But, unsurprisingly, I prefer the integration view as it facilitates uncovering areas of risk. Both schema require further work to be broadly useful as a tool in industry.
On the risk side, I like to look at:
Project planning (e.g., determining charge-discharge efficiency to determine deliverable kWh; do any system elements raise any issues regarding decommissioning)
Project timeline impacts (e.g., are the battery systems commercially available; lead times for complete units and any required components; construction and integration timeline)
System integration issues (e.g., is a battery system delivered as complete units or is onsite battery system integration required; if other equipment is required, how many vendors need to be coordinated; what’s the availability of workers at the appropriate skill levels)
Operations & Maintenance (e.g., how much do I have to pay for station power; what performance guarantees (insurance, bonds) are available and how are they priced; what are the requirements for and costs associated with scheduled maintenance under the warranty and what systems/subsystems does the warranty cover; what are lead times on replacement battery system parts and for any required hardware such as HVAC, inverters, communications equipment)
Given available revenues and taking all the costs into consideration, can the project meet its investment and cover ratio targets?
How do you evaluate battery systems? Email me at <my first name> at electroferocious.com or leave a comment below.
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