Michael Lippert, of Saft’s Energy Storage System business, draws on practical experience in Europe and the US to explain how the correct sizing and selection of operational mode are crucial to the successful deployment of large scale Li-ion based energy storage systems
There are two main families of large scale energy storage system (ESS) projects. The first is to aid renewable energy integration by smoothing, shaping and time-shifting the intermittent and unpredictable output of photovoltaic (PV) plant. The second is grid storage in which containers or storage systems are deployed to support grid stabilization by providing auxiliary functions, such as frequency regulation and voltage regulation.
In many cases, energy storage systems can offer mutual benefits across a range of applications. For example, sustainable development can be boosted by increasing the share of renewables in the overall energy mix, making regions (especially islands) independent of fossil fuel resources and reducing CO2 emissions. In the smart grid realm, it is possible to reduce the number of grid interruptions/blackouts and to optimize the cost of electricity infrastructure, particularly in deferring the need for costly upgrades.
Today, islands are important markets for ESS projects for renewables integration. Lithium-ion (Li-ion) battery technology has been shown to be ideally suited to these applications due to its versatility in energy to power ratio, high energy density, proven performance and the ability to be scaled to meet a wide range of power and energy needs.
Setting operational priorities
When considering a Li-ion ESS for a PV plant, the first step is to select a battery and management philosophy that satisfies both the technical and economic objectives of the customer, whether they are the plant operator or the distribution network to which it is connected.
It’s important to understand the impact of the chosen operational mode on the battery’s performance as it can influence both system sizing and the economic performance.
Grid operators and renewable plant operators tend to have differing priorities. On one hand, grid stability is a top priority for network operators, who want power from renewables to become a predictable component in their dispatching strategy as they seek to balance supply with demand, at the same time maintaining the constant voltage and frequency required to supply commercial and industrial customers.
On the other hand, plant operators want to maximise their return on investment by injecting the maximum possible energy to generate revenue while avoiding penalties such as curtailments, such as when there is oversupply of PV power into the grid.
Power smoothing versus power shaping
There are two basic modes in which an ESS can support PV production: power smoothing & reserve and power shaping.
Firstly, power smoothing uses energy storage to smooth out short-term variations in generation to control fluctuations in the system. Smoothing keeps production within a given forecast window, compensating for power sags, controls ramp rates and also provides reserve power.
Power shaping, on the other hand, provides a stable power out over several hours. It is defined a percentage of the nominal PV power output and also involves controlled ramping up and down of power.
Sizing a Li-ion energy storage system
A number of factors come into play when sizing a large-scale ESS. The primary aspects to consider are power (in charge and discharge, measured in kW) - related to the application requirements and operational strategy; and the energy (the capacity, measured in kWh) – related to the variations in state of charge (SOC).
It is also important to consider the battery lifetime in terms of both calendar age, generally determined by ambient temperatures and the SOC, as well as the cycle life, generally determined by a combination of number of cycles, depth of discharge (DOD) and charge rate.
There are also economic factors relating to system costs, both capital expenditure (CAPEX) for initial purchase and operational expenditure (OPEX) for ongoing servicing and maintenance.
When sizing a battery system for a PV plant, the normal practice is to use the technical specification to obtain a first sizing. In the example of a shaping application, this typically assumes that:
• Constant power injected to the grid = 40% of maximum PV generation power
• Production above the 40% maximum peak is either stored or lost
• The energy stored can be injected at a later stage
This ‘first sizing’ must be done with a simulation of the PV power profile over several days or months, at a location close to the actual plant location and with the highest possible time resolution (less than five seconds).
A strategy to create a specific injection profile acceptable to the grid operator can then be defined and the battery power profile is obtained accordingly.
The next step is to obtain the energy and power ratings to meet the requirements for the desired operational lifetime.
An economic evaluation during the ESS lifetime is then performed, comparing investment costs and maintenance costs versus revenues and penalties. If this does not provide a satisfactory outcome, then the process is reiterated based on altered variables such as more penalties, increased PV curtailments and reduced lifetime.
Intensium Max containerized Li-ion battery system
To meet the specific needs of renewable ESS projects, Saft has developed the Intensium Max 20 as a ready-to-install, megawatt scale, fully integrated containerized Li-ion solution. With a typical capability to provide 560 kWh of energy storage and 1 MW power, Intensium Max is delivered in a standardized 20-foot container for ease of transportation and installation and integrates the communications interface, battery management and cooling and fire prevention systems. Saft’s well proven Li-ion technology ensures long calendar and cycle life for the system, with an expected operating life of at least 15 years.
Megawatt scale energy storage for grid integration in Spain
Global clean energy operator Acciona Energia has installed an Intensium Max system at its 1.2 MW PV plant at Tudela in Navarra, northern Spain, which is the first European application of a megawatt scale PV plant and battery connected to the mainland grid.
This ILIS (Innovative Lithium-Ion Systems) project is being carried out within a three-year Eurogia+ European framework program of support for clean energy technologies, aimed at improving the viability of photovoltaic and other renewable energy power plants.
The ESS has four modes of operation, which allow it to adapt to suit different priorities, which are all based around power smoothing: charge and discharge of the battery at defined set points; control of ramp rates; frequency regulation; and voltage regulation. The ESS meets the conditions of different Grid Codes and offers ancillary service based on hourly predictions of PV production, meteorological data and energy prices.
Preparing Hawaii’s grid to accept more renewable energy
Two Intensium Max units supplied by Saft in 2012 are now in operation on Hawaii’s Big Island, where PV plant operator HELCO (Hawaii Electric Light Company) is using them to smooth PV production, reduce the impact of renewable energy curtailments and provide ancillary services such as spinning reserves.
Each Intensium Max 20E system has the capacity to store 248 kWh of energy and is coupled to a 100 kW power conversion system.
The first installation is in operation time-shifting PV power production from the morning to support the high afternoon loads created by the use of air-conditioning systems.
Conversely, the second installation is in place at a bottling plant, where it has improved the quality of power supplying the facility’s drives and equipment, boosting production with high quality and reliable power.
Supporting grid stability in Gran Canaria
The Spanish island of Gran Canaria is home to an Intensium Max 20 system rated at 1 MW power and with an energy storage capacity of 3 MWh, supplied as part of the STORE (Storage Technologies of Reliable Energy) project to demonstrate how energy storage can maximize the integration of renewable energy within utility networks and optimize the grid infrastructure
A high priority for Endesa when specifying the ESS was to address the problems inherent in the isolated network and reduce the need to upgrade the grid infrastructure, which is both expensive and difficult to carry out in remote locations.
The Intensium Max is being used for substation peak shaving to minimize the use of diesel generators as well as for wind and PV ramp control and grid frequency and voltage control.
Li-ion ESS technology is now a practical commercial proposition
Li-ion energy storage systems are making the transition from trial projects to fully commercialized installations. Whether the aim is increased grid stability, the desire to sell the maximum energy to the grid or wishing to delay infrastructure investment, the experience and technology is now available to tailor the ideal solution to match PV supply and demand.