Energy-Storage-Control-Options

Energy-Storage Control Options: Optimizing Batteries for Maximum Value

By Adam Todorski

Introduction

In this blog, we discuss energy-storage control options to manage battery storage units. As the price of battery storage falls and its usage continues to increase, it is important to look at different programs and systems that can optimize its usage. It is expected that 1,290 GW of new batteries will be commissioned worldwide by 2050, and energy providers will need to find a way to extract maximum value from this unique resource. This need for optimization—especially for aggregating, integrating and orchestrating batteries in concert with other grid assets—will involve digging deeper into which energy-storage control options work best in different applications, and the pros and cons of each. Here, we will introduce several key terms and consider different use cases and communication scenarios for the variety of storage control options.

Use Cases and Communication Scenarios for Storage Control

Controlling Battery Charging and Discharging

Electrochemical battery energy-storage systems (BESS) bank and release electrical power based on digital-control inputs through a solid-state power-conversion system (PCS). The PCS, or inverter, converts the direct current (DC) of the battery cells to alternating current (AC). It may do so at a variable-phase angle to inject or absorb reactive power. And it may also be able to vary other aspects of its output to manage different AC power system–quality metrics.

These digital-control inputs may be generated in one of two ways: locally, by a control device integrated with the BESS, or remotely, by an upstream system that can communicate with the PCS.

In the case of a local BESS controller, the device itself will act on external inputs such as energy-price data, solar-production forecasts and power-quality data. It will decide whether to charge or discharge the BESS based on these inputs and an algorithm that runs locally on the controller. These external inputs may be set statically or updated periodically or continuously (such as by sensing the local system frequency).

In the case of a remote BESS controller, it may act on instantaneous real-power set-points from an upstream, external system to control the amount of power drawn from the grid into the battery (charging) or injected from the battery into the grid (discharging).

Open vs. Closed Standards

The remote BESS controller interface may accept control inputs in either an open or closed standard.

With open standards, the BESS can be used with any third-party control platform that is capable of generating control signals in the required format, such as a SCADA protocol or a published web-service API. Or it may only communicate with a specific upstream system that is controlled by the system vendor.

With closed standards, the BESS can only be used with the system provided by the vendor. The vendor may or may not provide the capability to take control of the BESS through this proprietary platform, but this capability will most likely be in a proprietary format and may require payment of an ongoing usage fee to utilize.

Fleets of BESS hardware may be managed by merchant operators using either in-house or proprietary solutions. In either case, the solutions can accept signals from upstream systems as well as provide availability and interval meter data.

These systems capture multiple value streams from the BESS on behalf of the operator. They also permit the upstream system, such as that of the distribution utility, to dispatch capacity at certain times in discrete quantities for specific use cases. These interactions are usually communicated through an API using either a proprietary structure or an open standard such as IEEE 2030.5 or OpenADR.

Direct vs. Indirect Control of BESSs

Directly controlling a BESS means either giving it a stream of real-time power set-points or giving it a schedule to follow.

Indirectly controlling a BESS calls for providing tariff details or a forecast and expecting the local controller to steer the battery to accomplish some objective function. Indirect control is appropriate in some situations, but indirect control does not allow for portfolio-level optimization of the BESS with other assets, especially if they are from a different vendor.

In all cases, several data streams represent the status of the BESS and are relevant to monitoring and controlling it. These include the state of charge, alarm status and potentially other system-health parameters such as DC circuit current and voltage, cell temperature, HVAC status and more. Some of these parameters may not change often, while others change frequently. Some are time-critical (such as alarm status), while others are simply informational.

Communication Scenarios for Optimizing Value Streams

Use cases range widely. They include shaping the load profile of the host facility, time shifting load or local generation, or rapidly charging and/or discharging to provide ancillary services such as frequency regulation, spinning or non-spinning reserves, and voltage support. Use cases may benefit the host facility, a merchant demand-response aggregator (or independent power producer) or the distribution utility.

Customer-sited BESSs will either be owned and managed by the host facility, by an aggregator (demand response or otherwise) or by the utility. The entity having direct control over the BESS regulates any optimization and value that the BESS creates. An aggregator- or facility-owned BESS may derive some of this value by offering it into distribution- or transmission-level markets or opportunities. In so doing, the controller may be required to convey control signals from that entity to the BESS, but the system that sits directly upstream of the BESS will always be able to leverage the most value.

Some value streams, such as ancillary services, require high-speed telemetry and control of the BESS. They may even prescribe that there be no more than one system between the dispatching entity and the resource (the BESS, in this case).

In these scenarios, it may be cumbersome or even impossible to route the control signals and telemetry through an aggregator’s platform in a way that provides the speed and precision required to provide these functions. Redundant connections to the BESS can resolve this issue when both an aggregator system and separate control system interact with the same BESS, but this has cost and maintainability implications.

Co-optimization of BESS value streams requires visibility into facility load, distributed generation output and forecast data, as well as frequent control of the BESS (or even continuous real-power setpoint-based steering). Co-optimization of value streams across multiple BESS installations or a BESS and other DERs requires this type of visibility and control into all of these applicable resources.

Performing this type of co-optimization through an aggregator interface is either impossible or difficult. Impossible because the relevant data is not available or the level of control is not achievable. Or difficult (or costly) because the interface to the aggregator system will be a custom API, and some back and forth may be required to incorporate all of the required data streams and control inputs.

Direct SCADA Control for C&I Use Cases

Commercial and industrial behind-the-meter BESSs, as well as grid-connected BESSs, can and do use common SCADA protocols for direct control such as DNP3 or Modbus over a secure transport such as mutual TLS or a VPN. In this case the systems that control them define the available optimizations and value streams. Implementing non-direct SCADA telemetry to a BESS can represent a significant cost as well as proprietary vendor lock-in for that integration.

AutoGrid Flex universally controls all DERs for all applicable value streams. AutoGrid is vendor neutral and performs custom API integrations when it makes sense, such as for residential smart thermostats. Direct SCADA control is often considered optimal for C&I-scale battery storage use cases.

That being said, each of these different control systems lends itself to particular use cases and applications. To co-optimize the BESS as part of a portfolio of energy assets, the energy provider should always choose what makes sense for their specific situation.