Executive Summary of the report:
The present work describes the activities performed to develop the prototype of the electric energy storage system of the HYBUILD project. The final purpose of the work done was to choose, test, and assemble the electric energy storage system.
The first action performed was to identify the behaviour of the use cases in realistic conditions, considering a wide range of operation. To this aim, the two Mediterranean and Continental systems were considered. In particular, the optimized behaviour for summer conditions (Mediterranean HYBUILD solution) and winter conditions (Continental HYBUILD solution) were taken into account. In the Mediterranean system, the electric storage mainly serves the vapour compression heat pump for cooling production, whereas in the Continental system the electric storage serves the vapour compression heat pump for heating production. Furthermore, in the Mediterranean system, DHW is mainly produced directly from solar or through a back-up, whereas in the Continental system DHW production is obtained through the RPW-HEX accumulating condensation heat of the heat pump during its operation. In both cases, therefore, no extra operation of the heat pump for DHW was considered. A 4.5 kWp system was considered for the Mediterranean solution and a 6 kWp one for the Continental case. This is due to different size of case studies, since the Mediterranean HYBUILD system is intended for a single-family house, while the Continental solution is intended for multi-family houses with 2-3 apartments with shared renewable energy production. Cyprus solar irradiation profile was used for the Mediterranean case and Bordeaux for the Continental one.
The worst-case scenario was considered for testing: in the Mediterranean case, a day in July (high cooling demand), determines an electric consumption of the heat pump of 1.5 kW, corresponding to a heat pump with 5 kW cooling capacity and EER=3.3 (thanks to operation in combination with the sorption module). In the Continental case, a typical winter day was selected, corresponding to very low irradiation and therefore a lower production from the PV field but a higher demand from the user.
After definition of the applications, a selection process among most performing electric storage technologies was performed. In particular, attention was immediately focused on Lithium-ion batteries due to guaranteed performances. Lithium ion batteries offer countless advantages over other types of electrochemical storage such as:
- very high specific energy (Wh/kg) achieving considerable weight and space savings;
- low internal resistance, allowing them to achieve higher currents, therefore charges and discharges at high c-rates, and making them suitable for high power applications;
- limited self-discharge rates, making them the best solution for long-term energy storage;
- no memory effect;
- high lifetime, especially for some specific chemistries;
- high open-circuit voltage (typical values of 3 – 4 V except for lithium titanate where the cell voltage is in the 1.5 V – 2.7V range);
- relatively flat discharge curves (Voltage – SoC) in a wide range of SoCs.
The performance comparison of the technologies already present in the market brought to the choice of LTO (Lithium Titanate Oxide) as possible solution for the considered application in terms of long life and greater safety, no maintenance during lifetime, excellent power density, and good cyclability at high C-rate in both charge and discharge. In particular, the model chosen is a high-energy type, carrying out also a good compromise in terms of energy density.
Preliminary tests at cell and module level were carried out using CNR equipment for battery testing to evaluate performance and achieve a comparative characterization. Tests at different C-rates and operative temperature were performed and results are shown in the report. In particular, a lifetime evaluation was performed with a defined profile to evaluate the degradation. After a 210 days test campaign no degradation was recorded, confirming its appropriateness.
Great effort was spent on the BMS adaption in terms of both hardware and software developments. The main goal was the adaptation of the existing BMU (Battery Management Unit) / CMUs (Cell Management Units) signals exchange (process variables, alarm and failure states, hardware commands) and data structure to the PCS interface (to communicate with the new main controller). This goal implied to develop a system to bidirectionally convert data (i.e. implement the transcoding functions) among a CAN network (within the battery pack/unit) and the Modbus network mastered by the PCS. An Anybus converter by HMS (the “gateway”) was selected as the main device of the transcoding system. The gateway exchanges CANbus frames with the BMU, and then it parses, splits, and re-arrange the process variables values into data structures compliant with the Modbus RTU. On the other hand, by acting as a Modbus slave, it receives data from the PCS Modbus line and assemble the data into the CANbus frames compliant with the BMU communication protocol by adding control/header information of the CANbus stream. The software development activity was addressed to implement the transcoding algorithm and the communication between the PCS (from CSEM) and the storage system (from CNR) by synthesizing a common data structure description including the frame timing, signals priorities, system operation limitations to achieve a common FSM (Finite State Machine) implementation. To develop the FSM, a definition of the procedures to implement in case of single status/error/alert/fault coil commutations was realized and described in detail.
After this phase, the whole storage system (3x45Ah LTO modules mounted in series, BMU, protections systems, current sensor, and power supply and communication infrastructure) was cabled for testing and debugging of the control software. The system was arranged to be connected to a DC link in which a dedicated DC/DC converter will impose the current set point (for charge and discharge) following selected algorithm decisions. The electrical layout, including BMS interfaces, power suppliers, wiring, switches and connections was developed in order to achieve a final design for the installation into a cabinet. An external switch for reset, able to restart the storage system when fault conditions are reached, was directly linked to the overall system supervisor (in agreement between CNR and CSEM) to give the total control to it in any operating case.
When the system was ready to be tested, the whole pack was subjected to cycles representing the chosen test days. This test campaign was launched to evaluate the behaviour of the whole system in real operating conditions. The results of the performed tests are shown in the report.
A small electrical cabinet was chosen to ensure right protections (fuses, contactors) to the storage system for testing the DC-Link with CSEM and for achieving a final device to install into the demos. A further external protection system, which bypass the same BMU, was inserted to control the operation inside the rated maximum and minimum voltage limits. The intervention of this protection system occurs in any case, independently from the BMU state, ensuring not over or under voltage operations of the storage system. A necessary pre-charge system needs to be foreseen between the converter and the storage to avoid dangerous voltage unbalance between them.
The objective to obtain a working electric energy storage prototype was achieved by solving all the related issues (hardware and software).
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