Article published in the magazine Energética: http://www.energetica21.com/revistas-digitales/revista-energetica-noviembre-2020

What is the relationship between factors such as population growth, global industrial development, more intensive use of electronic devices and new electric mobility plans? These factors, among others, are driving exponential growth in energy demand worldwide.

This trend requires strengthening the infrastructures of the current electricity grid to ensure the quality of electricity supply, in addition to achieving an optimal balance between generated and demanded energy. All this linked to the global commitment to promote clean energy sources to limit the use of fossil fuels. However, the inherent variability of renewable energy sources makes it difficult to integrate them into the electrical system.

In the past, the solution to rising demand was to increase investment in transport and distribution grid infrastructure. The current strategy, however, is based on optimising existing resources because smart power grids enable new business models in which energy storage has become a key pillar, not only as grid backup systems but as a fundamental element in the integration of renewables.

The versatility of energy storage makes it possible to meet current energy needs, among which the following should be highlighted: electrification of isolated populations, grid backup applications, self-consumption facilities and large-scale applications integrated into the distribution grid that contribute to its stability.

If we focus on grid backup applications, energy storage provides another set of additional functionalities, in addition to ensuring continuity of supply to customers, such as:

• Modulation of the demand curve. It allows to dampen demand peaks, injecting or storing energy to avoid working on saturation regimens. At the same time, it prevents the facilities in the area from working on a saturation regime by not having to adapt to demand points.
• Improving the quality of supply. It is done by controlling variables such as voltage, power and frequency of the grid.
• Shifting investments over time. It allows to postpone and even eliminate investments to respond to energy needs in a fast and modular way ensuring a second alternative of feeding the area.
• Ensure energy supply in areas where it is unstable or of low quality.

Energy storage solutions consist mainly of a power conversion system (PCS), which allows energy exchange with the grid, and a physical means of accumulating energy, usually a battery bank.

Power Converter (PCS)

The central element of the storage system is the power converter. ZIGOR’s PCS GRID is a two-way converter capable of charging and discharging batteries in a coordinated manner with the Battery Management System (BMS). All this, without forgetting the main function, to guarantee and improve the quality of the grid at the connection point.

In order to perform these functions, Zigor, for example, has developed a converter that has the following operating modes:

• Power control. The PCS GRID converter responds to direct battery charging and discharge consignments at the request of a customer or a superior monitoring system.
• Voltage and Frequency Control. Thanks to the PCS GRID, it is possible to monitor and correct any variation in frequency and voltage. By injection or intake of active power, the frequency can be modified; for voltage control, both active and reactive power are managed.
• Island mode. In the event of a mains supply failure, the PCS GRID converter has the ability to generate an three-phase isolated grid plus neutral, using the power available in the batteries. This function must be coordinated with the protection and switching elements of the distribution grid.
• Peak shaving Mode. This function is used to optimise existing infrastructures by charging batteries in low demand periods to be able to inject energy in periods of high consumption.

Electrochemical storage

With regard to electrochemical storage, several alternatives can be used depending on the energy needs of the installation (cyclability, energy and power).The most common options are lead acid (Pb acid) batteries and lithium-ion batteries.

Acid lead batteries are undoubtedly the most mature and reliable technology. However, its energy density and cyclability are more limited compared to lithium-ion. A remarkable feature of these batteries is their low maintenance although more space is needed for their installation.

As for lithium-ion batteries, they offer high energy efficiency, high energy and specific power, high cyclability and low self-discharge. However, they require proprietary control (voltage, temperature and protections) and coordination with the battery manufacturer’s BMS.

Example of integration of a storage solution

The ability to calculate, design, engineer, integrate and provide customized solutions to specific customer needs is required to present such integration. The town of San Vicente del Monte (Cantabria) has a storage system fully developed by Zigor for Viesgo. It is integrated into a 20-foot container and is mainly sectored in two areas:

1. Battery area, where the lithium battery (LFP) is housed with a cooling system based on air conditioning with controlled temperature, in order to ensure the maximum availability of the battery.
2. Electronics area, where the rest of the components are installed: the PCS GRID converter, communications system, protection panel, isolation transformer and auxiliary services. As it can withstand a greater range of temperature, this zone is cooled by forced air.

The objective of this deployment is to support a rural population at the end of the distribution grid line that presents the following problems:

• In summertime it has consumption peaks that are compensated with the storage system.
• The energy supply presented voltage variations depending on the load that are compensated with this solution.
• This development, in addition, ensures the grid against possible drops in the medium voltage grid.

All this, integrated and supervised with the control centre in Viesgo.

Energy storage in the future

Cantabria’s successful experience comes after Iberdrola’s deployment of a similar Zigor integration in San Agustin de Guadalix (Madrid). In this case, the objective was to study the improvements introduced by the flexibility provided by storage systems in electricity distribution grids, taking advantage of the potential of smart grids.

In addition, Europe is also making a strong commitment to storage in order to tackle climate change thanks to projects such as Islander, where Zigor is responsible for developing a short- and medium-lasting community electrical storage system (including power converters and ultra-capacity systems), as well as energy conversion systems for buildings. All this in order to continue with the conversion of the German island of Borkum into a fully autonomous and decarbonised island energy system, and subsequently replicate the results in other EU islands.

Another European commitment is the iStormy project. Its aim is to achieve an innovative and interoperable hybrid stationary energy storage system, based on a modular battery pack, a modular power electronics interface (PE) and a universal self-regenerable energy management strategy (Shems). At iStormy, Zigor will design, prototype and test a new modular power electronics architecture based on low-cost DC/DC and DC/AC converters, high reliability and high efficiency for multi-tech hybrid storage for stationary application.

As energy storage confirms its leading role within the power grid, new business models, use cases, and other needs that require customized developments emerge. Evidence of this is the growing interest in the use of green hydrogen as an element of energy storage. This technology implies new challenges in the development of power electronics equipment that will ensure the proper introduction of green hydrogen into the current electrical infrastructure.