Introduction

HYBRIS system, based on the hybridization of Lithium Titanate Oxide battery and Aqueous Organic Flow Battery, constitutes an advanced hybrid energy storage for microgrid applications able to show high-performance, be cost-effective and environmentally friendly, compared to common storage solutions.

One of the main activities performed during the project was to clearly define how the system can be tested and validated, when installed in real demo sites. In particular, 3 demo sites (and contexts) were chosen for this scope, to highlight the flexibility offered by a hybrid storage system for a better electric energy management and more efficient use. Indeed, Hybrid Energy Storage Systems (HESSs) enable a higher integration of local renewable energy generation with loads’ requests and multiple grid service both energy and power intensive. It is well known that unpredictability of renewable energy sources (RES) and general mismatch between this kind of generation and energy users’ request constitutes a problem.

The issue regards not only the effective energy release but also the performance of the whole energy grid (i.e., in terms of voltage level and frequency value, which must respect operating thresholds). Evaluating of the positive impact to the grid brought by HESSs is crucial to underline the importance of the development and deployment of this kind of technology.  

More in detail, HESSs enable the participation to the energy market services, regulated through the intervention of several factors, such as national TSO (Transmission System Operators), local DSO (Distribution System Operators) and aggregators, to help the main grid in maintaining good performance standards. Although access to the services market and their implementation are still being defined in various countries, it is possible to classify the most important services both for the main grid and customers as in the figure below.

Application Contexts and Services To Prove the HYBRIS System Performance

In HYBRIS project, to demonstrate potentialities of the proposed HESS, some specific services were selected for the various use cases, taking into account the peculiarities of the sites and how they are integrated into the whole energy system.

The three pilot sites represent different contexts of application:

• Italian pilot (located in Messina, IT), will demonstrate the realization of community services for a residential microgrid with islanded operation.
• Belgian pilot (located in Brasschaat, BE), will show the development of new flexibility services under Energy as a Service (EaaS) model for a commercial and industrial business park with high solar penetration.
• Finally, Dutch pilot in Tholen (NL, Delta Glass factory) will constitute a virtual demonstration of customer-owned system able to enhance green consumption in a SME (Small and Medium sized Enterprise) and enable slow and fast grid energy services.

After a preliminary evaluation of the pilot sites features, the following services were chosen to prove the HESS performances:

• Community demand response/ Load Shedding: In this service the HESS will supply directly local loads until allowed SoC level. The integration of an AORFB with LTO batteries enable a combined intelligent use of the storage system to fully serve the scope while taking care of the system overall lifetime. LTO batteries guarantee a high level of DoD but the energy content is less than the AORFB. Therefore, the combined contribution of both will maintain no energy exchange with the grid. From the grid point of view, it represents a load shedding service because load request is directly served from the HESS and no power is needed from it.
• SAIFI reduction: System Average Interruption Frequency Index (SAIFI) is a metric used to describe the quality of the grid stability. The number of power interruptions per year is an important parameter, especially in remote or less developed areas. The HYBRIS system can avoid power interruptions on the installed side in an automatic way, thus ensuring the absence of this kind of disservice for the users. In particular, the LTO inverter will be able to read the interruption and start the local grid forming. AORFB intervention can be programmed in case of longer interruptions.
• Island Operation: Island mode operation consists in locally balancing production and consumption, while the main grid is disconnected. Differently from the power outage case, in this case the microgrid goes intentionally in this operation mode, when favourable conditions occur. The HESS can provide power and energy to the local households in case of total detachment from the grid, ensuring load following with true and stable 50Hz frequency.
• Grid forming/ following switch: During the previous operation mode two transient events occurs, when disconnection and reconnection to the main grid must be done. These two moments are particularly critical and need to be analysed with specific testing activity. In particular, the regular power supply of the local loads and the correct hooking to the main grid (absence of voltage drops and/or dips, frequency deviation, etc.) must be checked.
• Peak shaving (load side, also known as “load shedding”): The HESS can be used to reduce costs to the end user and stress on the DSO’s network by reducing loads’ peaks. In particular it will be used to reduce capacity costs and prevent problems for the DSO, by the curtailment of the peak power consumption on site. The practical implementation of this service will use the cloud-based Energy Management System (EMS): it will calculate a peak target based on what has happened in the past and what is expected in the future. Then, with a one-minute control loop, it will instruct the battery to discharge if the energy in a previous 15-minute time slot is likely to exceed the target peak. In addition, peak shaving for local RES production is possible. In this case storage is used to cut production peaks not balanced by local loads.
• Increase of Self-consumed energy: To reduce energy costs and grid losses and congestions, the HESS will be used to store excess energy and deliver it later (when it is requested), with main aim to exploit locally high-value energy production from the solar source (not injecting it into the grid). For the end user can be financially advantageous to avoid injecting energy into the grid during times of excess solar production and, rather, store it in a battery for later use. The HESS will thus improve self-consumption managing solar production excess and releasing it later.
• Arbitrage: Another very useful service from the end-user point of view (in reducing energy costs) is to strategically buy and sell energy considering prices fluctuations during the day through battery deployment. This service can be activated by the user. The developed HESS will use EMS to perform the logic of energy/price management. It will try to maximize profit for the end user by buying low and selling high daily.
• FCR: This service consists of participating in the Frequency Containment Reserve FCR market. Due to mismatch between generation and consumption, one direct effect on electric power signal is the deviation from the nominal frequency value. Fast responsive elements are present in the generation system to avoid this deviation within certain margins, by modulating injection of active power into the grid. HESS could represent one of these elements due to its performance in terms of responsiveness. Note that due to the need of pooling significant energy capacity to participate in ancillary service markets (e.g. at least 1 MW), this service will be implemented only at a demonstrative level.
• Energy Community: This innovative kind of association aims to share RES among their participants, to independently generate and manage green energy at advantageous costs, significantly reducing CO2 emissions and energy waste. HESSs can represent essential elements of the energy infrastructure of these communities, due to the flexibility offered in energy management. In particular, the proposed HESS will perform the algorithm for overall energy management to enhance green energy consumption.

Written by Davide Aloisio, Francesco Sergi and Giovanni Brunaccini from CNR-ITAE

[1] Fitzgerald, Garrett, James Mandel, J.M.; Touati,  and H. The Economics of Battery Energy Storage: How Multi-Use, Customer-Sited Batteries Deliver the Most Services and Value to Customers and the Grid 2015.