The growing adoption of energy storage systems is driven by key objectives such as carbon footprint and energy independence, but it is also supported by technological advances. R&D has led to significant improvements in performance, reliability, safety and cost of energy storage systems. The flexibility of these systems in responding to specific needs represents a strong incentive for those who are still undecided about their adoption. The adaptability of storage systems has allowed them to penetrate sectors previously considered complex and less suitable.
The main challenge is to meet the customer’s expectations and specifications, as this can affect both the price and the actual capabilities of the system. An interesting example, designed by Sparq to custom tailored, is the use of an energy storage system for agricultural use. The goal was to reduce energy costs during peak demand, given that in most cases the time slots in which energy is most expensive are daytime. Sparq has designed a BESS (Battery Energy Storage System) with a power of 400 kW and a capacity of 400 kWh, equipped with an inverter and a fire protection system to ensure active safety. The system uses high-quality LiPo4 batteries managed by a Battery Management System and a cooling system. The particularity of Sparq projects is to deliver an “out-of-the-box” product, thinking not only about the technical aspects but also about those concerning logistics. The solution described has been inserted in a 2.5 x 2.5 x 3.00 meter container.
Energy storage has often been associated with domestic uses. However, these systems can be incredibly flexible, adapting to different contexts.
The BESS is usually designed to work in synergy with the traditional grid, where through energy storage it is possible to balance the energy needs and avoid consumption peaks.
The current economic and political scenario has an impact on many sectors, from agri-food to industrial, due to the continuous fluctuation of energy prices. Therefore, there is a growing need for systems capable of mitigating the consequences of price increases, ensuring as much stability as possible.
New technologies and materials allow energy storage systems to increase their performance, ensuring increasing efficiency and reliability. The resulting advantages are substantial since energy storage will represent an increasingly effective investment and a choice with sustainable costs not only for large companies, but also for all small and micro businesses.
The Battery Management System
The Battery Management System (BMS) is an electronic system that is responsible for supervising and managing the operation of a rechargeable battery, in its entirety, that is that it is always able to operate safely and that its performance is always optimal. In order to function, the BMS requires a whole series of parameters collected from the field that concern the state of the pack as well as of the individual cell. It is in all respects a continuous and real-time monitoring system that provides timely indications on the health of the battery. However, the BMS can also be developed to be able to make, in real time and in complete autonomy, decisions on the management of the available energy. This is one of the peculiar characteristics that allows a prolonged life cycle, not only to the benefit of operating and maintenance costs, but also of the environmental impact, since a battery that lasts longer requires less frequent interventions and, in the case of end-of-life, a delayed disposal process over time.
Architecture
The BMS is divided into a hardware part that includes the parameter acquisition systems and a management software part.
Sensors, processors, communication interfaces and protection circuits are combined to receive and communicate with other systems integrated into the device.
In particular, the basic parameters detected by the sensors are voltage, current and temperature.
The measurement of the voltage value is carried out both at the level of the single cell and of the entire group of cells, each cell has an operating voltage limit that if exceeded can cause damage and in any case unbalance the operation of the battery pack. Therefore, the continuous measurement of this parameter ensures that no cell exceeds the limit. The amount of current measured (charging and discharging) refers to the pack and is also monitored to prevent it from exceeding the pre-established limits.
It should be considered that for these variables the precision of the sensors is fundamental since the smallest measurement drifts affect the quality with which the BMS manages the set of batteries.
In the temperature detection system, NTC (Negative Temperature Coefficient) or PTC (Positive Temperature Coefficient) thermistors are generally used whose resistance values can respectively decrease or increase in accordance with the temperature trend. Thermocouples can also be used, but unlike thermistors, the degree of measurement accuracy is much lower, with tolerances that can exceed one degree centigrade. An approximate precision could generate problems in the event of overheating of the cells; for example, lithium cells are very sensitive to high temperatures, and it is absolutely necessary to avoid the phenomenon of thermal runaway.
Thermal runaway is a particular case of positive feedback in which an increase in temperature creates the conditions that determine a further increase, generating an uncontrolled deviation from the equilibrium conditions of the system.
The hardware includes the part that deals with the protection of the pack. The protection circuitry detects short circuits, overloads and excessive discharge, which in other words means preventing peak values of voltage or current. In cases of drift, the protection circuit is designed to interrupt (for example via a solid-state relay or a MOSFET) the operation of the battery, avoiding any damage.
One of the functions of the BMS is the control and balancing of the cells. Since individual cells can discharge at different rates, the balancing unit re-establishes the balance of the charge level between the various cells.
Balancing can be done mainly in two ways: active and passive. In active balancing, the excess charge of a certain number of cells is redistributed among the less charged cells, obtaining uniformity of charge. In passive balancing, the energy is dissipated through special resistors. As a technique, balancing can be compared to the electric braking systems in vehicles and railway traction, where the inertial kinetic energy is reconverted into electrical energy and then returned to the power source or dissipated through rheostats. It is easy to understand that active balancing, although it is a more complex system, allows batteries to increase efficiency with the result of improving their performance.
Software and virtual models
In BMS, the software controls the numerous control and monitoring functions based on mathematical models referring to the basic parameters described. Logically, precision and accuracy in detecting the state of charge and health of the battery are directly proportional to the precision of the control algorithm.
The electrical model is based on a virtual equivalent circuit with resistances and capacities that ideally represent the battery. The dynamic operation is included in the model, which includes all possible variations of the voltage and current parameters with specific responses.
The working condition is simulated through the virtual circuit, which depends on the load applied to the battery: in the absence of load, there will be a voltage value that corresponds to that of the open circuit, applying the load there are current values corresponding to Ohm’s law (I = V / R). The more complex the real circuitry of the battery, the more complex that of the virtual model will be, to which the various blocks that reflect its composition are added.
The thermal model, considering both convection and conduction, simulates the heat path that concerns the entire battery pack, internal and external temperature of the cells, growth or decrease of the temperature value in relation to the charging and discharging phase and so on. Depending on the performance characteristics of the battery, the thermal model is then prepared, the accuracy of which is as crucial as the electrical model.
In essence, the thermal model is based on the heat exchange capacity that each individual cell can have with the external environment, therefore it is essential to have precise acquisition of the temperature in the various areas of the battery.
(by Roberto Romita)
