Optimising battery energy storage systems operation

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Optimising battery energy storage systems operation

02/26/2015 - 6.17 pm

Grid-tied battery energy storage systems (BESS) are promising smart grid solutions that are now commercially available. However, utilities and investors face the challenge of evaluating their economic viability. An energy storage management system can minimise the losses in the BESS, prevent premature ageing and maximise the return on investment.

Governments around the world are promoting the use of renewable energy sources. However, these tend by their nature to be variable. Hence the increasing need for energy storage. There are several types of grid-level storage solutions, among them pumped hydroelectric (PHS), compressed air, flywheels and batteries. According to the IEC 2011 white paper on energy storage, PHS represented almost 99% of worldwide electricity storage capacity. “But”, says Dr Maialen Boyra, Alstom Energy Storage R&D Engineer, “battery energy storage systems are a relatively affordable – and growing – alternative.” Affordable, yes, but a challenge nonetheless.
 
 

Summary chart of storage technologies
 

The smallest unit of a battery energy storage system is the battery cell. A group of cells constitutes a battery module and a group of battery modules makes up a battery pack. Finally, a group of packs forms a battery container. The container also houses a battery management system (BMS) to balance the current between battery cells and guarantee safe operation of the battery as well as auxiliary systems such as lighting, air conditioning, pumps (in redox-flow batteries, for example), heating and fire-extinguishing equipment. Then, the battery container is connected to a power conversion system (such as MaxSineTM e-Storage) that interconverts DC power and
AC power, and to the balance of plant (BoP) equipment such as transformers, switchgear and protection.

1_Three main scenarios for BESS
BESS, together with other storage and Smart Grid technologies, enable large-scale integration of renewable energy sources. Their modularity and their capability to integrate different types of energy storage technologies also allow them to be used in a wide range of applications.
 

Simplified single-line diagram of e-Storage BESS substation associated to external sources and loads

 
Connected to a power plant:
  • defer peak power plant investment;
  • provide ancillary services enabling the power plant to focus on scheduled power;
  • deliver black-start and uninterruptible power supply;
  • smooth, shift and ramp the power output of renewable power sources.
 
Connected “alone” to the T&D grid:
  • delay investments in new transformers, lines and cables;
  • regulate voltage at a particular point;
  • participate in electricity markets (also known as “arbitrage”).
 
In microgrids:
  • guarantee the stability of the microgrid and optimise the power dispatch among sources in microgrids with renewable energy sources and loads;
  • increase the energy autonomy of the microgrid (islanding) – which mostly relies on its own distributed resources – with regard to the wider electricity system.
 
“There is also a fourth scenario,” adds Boyra, “where the BESS is connected to a load – a household or production facility – with the objective of reducing the electricity bill or increasing power quality by mitigating the impact of power supply interruptions and voltage sags.”
2_Breaking down the barriers to BESS deployment
For these scenarios to become a dynamic market reality, a number of barriers have to be overcome. The main barriers are economic. Therefore, any means of reducing capital expenditure (capex) and operating expenditure (opex) will contribute to greater BESS deployment. Then there is the issue of uncertain ageing of the batteries, as well as the tendency of battery manufacturers to oversize their products to ensure they comply with contractual specifications. “The energy storage management system (eSMS) proposed by Alstom (one of the few such software solutions on the market), together with its complementary SISTOS sizing tool co-developed with the CEA, is designed to mitigate these constraints,” Boyra explains.
 

“The eSMS is a collection of algorithms – that rely on embedded battery models – integrated into the substation control system of a multi-MW BESS. Its three main optimisation criteria are efficiency, availability and ageing.”

The ratio between energy retrievable from and energy charged to the BESS defines its energy efficiency. Boyra points out that “by modelling influences on efficiency and keeping track of state of charge and temperature, charge and discharge efficiency maps can be determined. Using such maps, the eSMS can find optimal power setpoints that minimise system energy loss and ultimately increase system efficiency.”

Battery availability depends on the state of charge and the temperature of the battery. Often (depending on the battery technology), the battery management system limits the maximum discharge current when the state of charge of the battery reaches low levels (near zero) and the maximum charge current when the state of charge reaches high levels (near unity). The eSMS can distribute the power through the battery fleet so that charge/discharge current limiting is avoided.

A battery’s performance is not constant over its service life but decreases with time and usage. This battery ageing has two origins – cycle ageing (loss of capacity due to the effects of charge/discharge cycles) and calendar ageing – and two effects: a reduction of available capacity, and an increase in conversion losses. “The eSMS can optimise the BESS ageing by considering the ageing factors when distributing the overall power setpoint among battery units.” 

3_Maximum flexibility due to the eSMS’s modular design
The eSMS calculates the power setpoints (target values) of the battery units and optimises the operation of the BESS to maximise the internal rate of return (IRR). “Specifically, it calculates the optimal production plan of the BESS subject to a given grid code or set of requirements and dispatches the global active and reactive power setpoints within the power units. It also monitors and diagnoses the state of the battery units within the BESS.”
 
The modular design of eSMS allows it to adapt to the different aforementioned scenarios and different specifications (smoothing, ramping, etc.). When the BESS is “alone”, the eSMS can choose:
 
  • No optimisation;
  • Loss optimisation (the eSMS seeks to minimise energy losses);
  • Ageing optimisation (the eSMS considers ageing factors when distributing power setpoints among battery units);
  • State of available power optimisation (the eSMS seeks to increase the limitation on maximum active power). 
 
When the BESS is connected to an AC source or load, the objective of the eSMS is to calculate the production/consumption profile that maximises the profitability of the system. And when the BESS is connected to a microgrid (islanded or grid-connected), the aim of the eSMS is to assure frequency and voltage stability as well as to optimise a specified economic indicator (for example, to promote the use of renewable sources rather than diesel generators).
 
Thanks to its modular design, Alstom’s eSMS solution adapts to a wide range of configurations and applications simply by modifying the interfaces and the modules. It is also battery technology “agnostic”, meaning that it can manage different battery technologies by integrating battery models: efficiency, available power and ageing models. “And although the eSMS described here deals with electrochemical storage, it could also be used to manage and control any other type of storage device with minor changes,” Boyra concludes.
4_Getting battery sizing right
Nicolas Martin, head of the CEA-tech smart grid laboratory, explains the purpose of the SISTOS tool.
 

What is SISTOS and how does it work?
Battery manufacturers often oversize their products to avoid surprises and especially penalties. But an oversized storage system is costly and an undersized one undergoes undue stress and shortens its useful life. SISTOS is a modelling tool to accurately size a grid-connected storage system depending on the storage system’s application and how it is to be used and managed. So there is a strong link between the sizing and the way the storage system is operated.

 

How is SISTOS complementary with eSMS?
SISTOS is used at the beginning of a project. SISTOS contains control algorithms in a simulated environment. Then these are integrated into eSMS to implement its control strategy. The controller is not just for basic control, but also look-ahead control, which helps to plan and optimise the system.

 

What are the results achieved?
Lower investment cost. Plus the assurance that you have the best-sized storage system adapted to your application and the control mechanism.

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