The electric mobility fair, Expoelectric, is a leading initiative that brings zero emissions electric vehicles and renewable energies to citizens through a series of fun dissemination activities to promote electric mobility and the energy transition. In 2019, the ninth edition of the fair was held in Barcelona, in which for the first time a commitment was made to a new concept, considering the ideal of sustainability of zero emissions and the generation of 100% efficient, green energy.
In previous editions, Expoelectric was in contradiction with this ideal, as it had been obliged to use engine generators to supply the electricity needed for the exhibitor’s tents. In this way, a green fair generated polluting gases and noise by consuming fossil fuels in its diesel electricity generators. In contrast, the electric vehicles on display had energy in their batteries that, along with the aforementioned renewable energy resources that were also exhibited, could be sufficient to power the exhibition tents. Faced with this innovative possibility, the organisers approached the centre for Technological Innovation in Static Converters and Drives (CITCEA) at the Universitat Politècnica de Catalunya – BarcelonaTech (UPC) to study the feasibility of this self-generation proposal, to achieve energy autonomy of an event dedicated to electric vehicles and renewable energies.
CITCEA-UPC accepted the challenge of developing electronic power equipment that could meet all the necessary requirements: achieve an isolated system for electricity generation that could at each moment adapt to the electricity demand for consumption, based on the batteries of various electric vehicles that are available and the renewable electricity generators that participate in the fair.
In response, the concept of Energy Ring Generator (ERG) was developed. A prototype was constructed that was tested in the laboratory and a pilot was implemented at Expoelectric’18, which validated the concept and powered half of the tents at the fair. At that time, the concept of energy ring was created.
Consequently, in 2019 Expoelectric launched the “self-consumption space”. This made it the first fair in the south of Europe to generate its own electrical energy through photovoltaic self-consumption and from vehicles interconnected to the grid (Vehicle to Grid, V2G). The energy ring enables two-way interaction of electric vehicles with the electricity network generated by the ring. In this way, the electricity for the event comes from renewable energy sources. All of it is generated by photovoltaic panels, storage batteries and energy contributions from electric vehicles.
The energy ring
The energy ring is a proposal derived from the concept of a micronetwork that involves the interconnection of many sources of distributed generation and consumption. As this is a system that is completely isolated from the grid, it needs a central convertor and energy storage to establish the network and act as a voltage source and to keep the value of the supply voltage and its frequency within the range of normal operation. This guarantees the power balance and the safety of the system at all times. The central convertor is the Energy Ring Generator (ERG) that was developed by the CITCEA-UPC and manufactured by its spin-off TEKNOCEA. As backup energy, the ERG uses the battery of an electric van designed for urban cleaning services that was provided by the company URBASER and complemented with second-life batteries to reach a useful capacity of 45 kWh with a range of voltage between 240 and 295 Vdc. That is, when the generation of the distributed nodes that power the energy ring is greater than the energy consumed by the loads connected to the ring, the central ERG convertor stores the surplus by charging the battery of the electric van. In contrast, when the load is greater than the generation, the ERG adjusts the balance of power by extracting energy from the battery.
For safety reasons, the ERG generates the three-phase energy ring through a star transformer/star with a neutral of 30 kVA that enables a regime of neutral TT to be established in the distribution ring. The three-phase voltage system that the ERG generates is obtained through an inverter of three-phase topology of four threads that enables the voltage in each phase to be controlled independently. In this way, adaptability to situations of high imbalance in the loads is managed individually, affects the other phases minimally, and provides greater flexibility than any conventional network. As most of the distributed generation nodes throughout the ring are single-phase, whether they are photovoltaic or V2G, situations can occur in which one phase is fully charged, another is empty, and the third phase injects rated current in the opposite direction. The four-thread structure enables the quality of the wave in the three phases to be maintained by balancing them internally so that they are not affected by the reactive power consumed by the loads or by the harmonics produced by non-linear loads.
Protection and control of the energy ring
Due to the nature of a fair such as Expoelectric, a profile of erratic consumption is expected due to the continuous connections and disconnections of devices. A short-circuit situation could occur. In a conventional network, the short-circuit current that is offered by the network is dozens of times higher than the current of the magnetothermal protection. This enables protection to be implemented easily in the installation, with selectivity and fast action times. In contrast, in an energy ring with the characteristics of Expoelectric the maximum short circuit current is limited to the current that the ERG can supply. As this is power electronic equipment, it does not enable overload levels comparable to a conventional network, which makes it difficult for conventional protection to work adequately.
For this reason, the sizing of the energy ring protection should take into account this special circumstance of low short-circuit current to prevent the energy of the short-circuit being insufficient for the protection to activate.
Results of the energy ring
In the 2019 edition, the fair had two active devices that could inject energy into the energy ring. The company Audit Energía connected a photovoltaic installation of 5.67 kWp and the company Wallbox connected two bidirectional V2G chargers that could each provide 3 kW based on two Nissan Leaf vehicles with 34 kWh of usable battery capacity. The battery of the Urbaser electric van provided a usable battery of 45 kWh. With these resources, an electric service was provided to all the exhibition tents that had contracted a total supply of 18 kW power, with total energy consumed during the two days of 150 kWh.
During the fair, the correct functioning of the energy ring was checked. The tents were supplied with a peak power of 15 kVA. The ring was implemented by integrating equipment of different technologies and managing their functioning through control of the central node, integrated by an ERG. This experience showed that with sufficient distributed energy resources, engine generators are not needed to guarantee the power supply to this type of events. In addition, a considerable task of dissemination was carried out, in which the concept was explained and demonstrated to people who were interested.
As recognition of the innovation that this concept of “energy ring” (ERG) represents, the “EmErgEnt” award was given by the Catalan Efficiency Energy Cluster (CEEC) during the “Seventh Night of Efficiency”, for the best energy management project developed by a start-up from the Catalan energy efficiency sector in the “Product” category.