ARE Member CMI Energy, well known as a global player in steam generation, has diversified into renewables, in particular CSP, and is now taking steps into the battery storage area with completion of the MiRIS facility at CMI Group’s international headquarters in Seraing (Belgium). The aim of the MiRIS (Micro Réseau Intégré Seraing) pilot plant, which incorporates a microgrid and PV, is to look at a range of battery options and their integration with renewables.
Renewable energy has many environmental and economic benefits, but intermittency of solar and wind resources is by far the most formidable technical barrier to widespread system integration. Wind and solar generation are not dispatchable thus grid operators must wrestle with transmission and distribution system upgrades in addition to grid capacity constraints, spinning reserve margins, and frequency regulation to offset the effects of intermittency. Duplicate fossil-fired resources are often constructed to backstop renewables when the wind doesn’t blow or the sun doesn’t shine. The elegant solution to each of these problems is energy storage.
Small-scale energy storage projects are typically used in either high power, short duration applications such as frequency regulation or in low power, long duration applications such as time shifting of energy (kWh) and demand (kW) within a facility or local grid. Time shifting is the process of storing energy produced by an intermittent generating resource and releasing it at a later time when the energy and/or demand reduction is needed, typically during costly on-peak hours. Unfortunately, most energy storage technologies are unable to perform well in both roles.
There are many other economic advantages available to facilities with excess capacity to adopt energy arbitrage practices by storing low cost, off-peak energy for resale to the grid during on-peak hours at a premium price. A properly designed energy storage system may supply valuable grid ancillary services, such as grid voltage support, frequency regulation, and spinning reserve to the grid in some regions. This option is especially attractive for facilities with self-generated renewable energy that have surplus capacity at certain times during the day.
As an alternative, energy storage may allow a facility with essential services to avoid the cost of expensive conventional backup power equipment.
CMI Energy, part of CMI Group, a well established player in steam generation (notably HRSGs, as well as industrial and LNG boilers), is also active in renewable energy system design and integration, particularly concentrated solar power with thermal storage. The natural extension of that experience is battery energy storage, particularly when integrated with intermittent renewable energy resources, on either side of the meter. CMI Energy also acts as an EPC integrator within the energy storage sector, providing optimised solutions depending on each customer’s specific needs and the best available technical-economic features.
CMI Energy is currently constructing the MiRIS (Micro Réseau Intégré Seraing) facility, Europe’s largest industrial energy storage pilot, located adjacent to the CMI Group’s international headquarters in Seraing (Belgium) (Figure 1). The purpose of the full-scale pilot plant is to demonstrate advanced integration of intermittent renewable energy resources with battery- based energy storage thereby producing a fully dispatchable renewable energy resource.
MiRIS, scheduled for completion in late 2018, consists of renewable power generation and energy storage systems. The renewable portion of the project includes a 2 MWp 1.75 GWh/y photovoltaic system consisting of 6500 roof top panels (Figure 2) and carports, and 36 inverters. The energy storage portion of the demonstration project includes two types of flow battery systems plus a lithium-ion battery system for a combined total of 4.2 MWh of storage capacity. The technology showcase will interconnect with the building’s electrical network and its DSO 15 kV distribution service connection. The CMI complex consumes approximately 1.3 GWh/y.
CMI Energy plans to use the MiRIS project to investigate the interoperability of renewables and different energy storage technologies for a variety of energy user energy profiles, particularly with respect to renewable energy time shifting and energy resale to the grid. Another important goal is to demonstrate off-grid or “islanding” operation of the MiRIS microgrid. Potential ancillary services that may be provided to the local grid will also be evaluated as well as the impact of user demand response.
Jean-Michel Gheeraerdts, president of CMI Energy spoke to the importance of MiRIS when announcing the project: “We now have ways of use green energy sources that eradicate their major flaw: intermittent production. Energy storage and management can be applied in a number of fields as an alternative todieselgeneratorsforunconnectedregions, as a way of deferring investment in parts of the network, as a means of optimising existing photovoltaic or wind systems, and as an enabler of participation in the primary or secondary reserve markets.”
A single energy management system (EMS) ensures optimal energy flows within the MiRIS microgrid thus maximising the profitability of the overall system while contributing to its safe and reliable operation (Figure 3).
The EMS performs data management, modelling of alternative operating scenarios, as well as facility energy and demand forecasting. In addition, the EMS can evaluate the most economic combination of operating variables, electricity market signals, and weather projections in real-time to optimise the operation of the entire facility and its grid interconnection. CMI Energy developed the EMS in close collaboration with the University of Liège in Belgium.
The EMS is uniquely capable of adapting to a variety of different applications by employing a suite of sophisticated and innovative algorithms. System optimisation considers various inputs, such as forecast PV panel electricity production, expected loads, current and expected electricity tariffs, and grid constraints to derive optimal control decisions for each component, with both grid-tied and off-grid operation of the micro grid. Among the grid-tied applications are behind-the-meter segments, such as energy price arbitrage for consumer bill optimisation, self- consumption and peak shaving, including participation in reserve and capacity markets. The EMS will be enriched with added capabilities as experience is gained with operation of MiRIS.
Most energy storage projects today rely on packaging very large numbers, often tens of thousands, of individual lithium- ion (Li-ion) cells to meet a project’s energy storage requirement. Li-ion batteries, commercialised in the early 1990s, have found many commercial and residential uses. Lithium-ion batteries have been the preferred energy storage technology for much of the past decade, particularly due to scale and manufacturing efficiencies from electric car production. Li-ion batteries are currently used in applications ranging from small-scale residential systems to grid- connected containerised battery systems that supply ancillary services.
The operation of a Li-ion battery, is in principle, the same as a conventional battery. However, instead of metallic electrodes and an acid-based electrolyte, lithium ions are injected into the structure of the electrode materials and lithium ions flowing between the electrodes produce current. The typical Li-ion battery used for energy storage applications uses a lithiated metal oxide positive electrode and a carbon negative electrode (Figure 4).
Modern Li-ion cells are generally available in different formats, such as prismatic and cylindrical. Depending on a project’s energy density requirements, individual batteries are grouped into multi-cell modules in series/parallel arrays to form a battery string that will produce the desired voltage and capacity. Each string is usually controlled by a battery management system. Battery strings are then combined to provide the required amount of energy storage (kWh). For the MiRIS project, 1260 kW/1340 kWh of Li-ion batteries packaged within a single shipping container will be used.
As an alternative to Li-ion, CMI Energy determined that flow batteries exhibit the best combination of life cycle cost, expected near-term economies of scale, and reasonable technology risk for bulk energy storage.
Navigant Research recently released a Leaderboard Report that examined the “strategy and execution” of 13 companies
offering non-lithium-ion battery technologies for grid energy storage. The two companies judged to be market leaders
were Sumitomo Electric Industries (Sumitomo vanadium redox flow battery) and ViZn Energy Systems (ViZn zinc-iron redox flow battery).
For the MiRIS project, CMI Energy concluded strategic partnership agreements in late 2017 with Sumitomo and with ViZn for the supply of flow batteries for the MiRIS and future projects.
Sumitomo Electric’s redox flow battery (600 kW/1.75 kWh) uses vanadium dissolved in sulphuric acid as its electrolyte, with inert graphite electrodes.
The ViZn flow battery (400 kW/1200 kWh) uses a zinc-iron (hybrid) solution as its electrolyte.
Flow battery technology is distinctly different from conventional batteries. A flow battery stores energy in the electrolyte, unlike conventional lead-acid or Li-ion batteries, which store energy in the electrodes.
Modern flow batteries use two dissolved chemical components to form liquid electrolytes, positively or negatively charged, as energy carriers. The electrolytes are simultaneously pumped through two half cells separated by an ion-selective exchange membrane. The thin exchange membrane between the cells prevents the electrolytes from mixing but allows specific ions to pass through to complete the redox (reduction-oxidation) reaction and thus produce a flow of electric current.
For example, in Sumitomo’s vanadium flow battery, the battery reactions change the valence of the vanadium in both the positive and negative electrodes (Figure 5). The valence change moves protons through the membrane, charging or discharging the battery. In a similar fashion, ViZn uses a zinc-iron electrolytic solution for its flow battery design.
The power rating (kW) of a flow battery is determined by the size, number, and configuration of electrodes in the cell stacks. A bipolar design describes cells stacked in a sandwich configuration so that the negative plate of one cell becomes the positive plate in the next cell. The voltage of a single cell is approximately 1.4 V. To obtain the design voltage, multiple layers of cells are connected in series to form a cell stack. The energy storage rating (kWh) is determined by the amount of electrolyte in the two electrolyte tanks (see Figure 5).
Unlike Li-ion batteries, redox batteries can be fully discharged with no impact on the life of the battery. Redox batteries are characterised by very high power output, capable of deep discharge and fast recharging of spent electrolyte, can undergo complex charge/discharge cycles (particularly attractive for remotely controlled grid ancillary services), very quick ramp rates, and have a long life because the electrolyte can be replaced. The electrolytes are also incombustible. ViZn’s electrolyte has the added advantage of being globally abundant, non-toxic, and easily recycled. The output and capacity of a redox flow battery is expected to remain close to 100% of rated capacity for the first 20 years of operation. Li-ion batteries typically lose storage capacity with age and the charge/discharge cycles must be carefully managed in order to maximum battery life. Conversely, flow batteries are much more complex that conventional Li-ion batteries (except for the battery management system) and have a much lower energy density.
The capacity of a redox flow battery is also easily expandable as the power output (kW) and the energy storage capacity (kWh) may be independently specified because the number of cell stacks determine power output and the energy storage capacity is a function of available electrolyte (the size of the storage tanks). Flow batteries are also uniquely capable of providing both rapid, high-power discharges as well as long-duration low-power releases, ideal for grid-connected applications. In merchant applications, for example, flow batteries can provide two daily charge/discharge cycles and millisecond switching for wholesale grid regulation services, which are substantial economic advantages over conventional batteries.
In the immediate future, CMI Energy will complete MiRIS and begin developing a deep understanding of how to economically optimise renewable energy sources coupled with energy storage for a range of users and demand load profiles. An added complexity in assessing the economics is the impact of grid interoperability (purchases, sales, and supply of ancillary services), which is site specific. Once these design and engineering hardware and EMS issues are well in hand, CMI Energy plans to expand its MiRIS project in the future by incorporating further energy storage and management innovations.