Overview
The future electricity grid will be confronted with new challenges. One challenge are additional loads by electrical consumers such as electric vehicles. But on the other side electric vehicles offer higher energy efficiency and have the potential to reduce greenhouse gas emissions. Without electrification of the whole power train the ambitious climate goals cannot be achieved and the Austrian oil import dependence in the transportation sector cannot be reduced.
In the project ZENEM, the impact of a high penetration of electric vehicles on the distribution grid including charging station infrastructure is analysed under the assumption that a whole Viennese taxi fleet is operated with electric cars.
Since 2006, the observed taxi fleet is equipped with GPS devices for purposes of operation and routing. From this GPS data the relevant transport characteristics are extracted and the feasibility of taxi ranks as locations for charging infrastructure will be examined. By the means of three selectable parameters "battery capacity", "charging power" and "charging stations expansion", various scenarios are defined, which are examined in subsequent analyses using load flow and mathematics software.
The basic requirement for the further considerations is the fulfilment of the mobility needs of taxis. Based on these necessities and frameworks the additional charging profiles at the taxi ranks with charging infrastructure are calculated and the impact on the distribution grids related to voltage stability and overloads is investigated. In order to make a qualitative statement, the current household and commercial electricity loads are identified through long-term measurements.
By using simplified thermal models a temporary overloading of electrical grid components is considered. These temporary overloads allow the supply of a strong electric taxi penetration requiring minimal grid expansion and may be acceptable if the effects on the individual components are negligible.
If the overloads for the electric distribution grid cannot be tolerated, grid oriented charging strategies are developed in order to avoid grid extension. As a consequence, it must be shown that the adapted charging profiles do not negatively impact the fulfilment of the mobility needs.
The results and knowledge of the project ZENEM allow the distribution grid operator to avoid unnecessary expansion of grid infrastructure. In the last phase of the project, a workshop was held on the ecological and economic aspects and frameworks with experts from the energy, transportation and political sector.
Funding
Results
In the project ZENEM, the impact of a high penetration of electric vehicles on the distribution grid including charging station infrastructure is analysed under the assumption that a whole Viennese taxi fleet is operated with electric cars.
Peak demand for taxi services varies strongly across week days. While demand on working days concentrates on morning hours, weekend demand shifts to shortly after midnight. In the study it was assumed that taxis are loaded at taxi stations only and no hybrid vehicles are used.
The study found that current taxi demand with the prevailing short waiting times at taxi stands cannot be satisfied with normal 220V / 16A power facilities. Feasible, however, would be power ranges of 22 kW (AC) and 50 kW (DC). Battery capacities of 24 kWh and fast loading infrastructures at 20 taxi stands could satisfy 25% of demand. Doubling battery at supplying 55% of taxi stands, a satisfaction rate of 80% could be achieved.
None of the power grid components in Vienna was pushed toward their thermal capacity at any time of the simulation. The core problem, however, is the direct power line from the grid to the taxi stand, which may be overloaded by 200%. Decentralized local charging control is found to be essential for a sustainable network operation.
The greenhouse gas reduction potential is up to 80% with full equipment of the Vienna taxi fleet with electric vehicles. Grid investment costs are around 90000 euros per loading point with a strong regional divergence. Investment and operation of charging stations adds another 294 000 euros per year. Due to the high driving performance of taxis, the lower operating costs of electric vehicles is found to outweigh the higher investment costs completely.
Policy implications
Core recommendations of the study are:
- A complete replacement of conventional taxis by electric vehicles in Vienna is possible.
- A demand-sensitive local charging management is required to prevent for overload of local grid facilities.
- The establishment of an adequate charging infrastructure requires annual investment costs in the order of magnitude of one million euros.
- Electric taxis in combination with renewable power sources provide a considerable contribution to the reduction of greenhouse gases.