Parameters of capacitor devices used for storing energy from recuperative braking and the profitability of their application in tram traction
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Simulation results have been used for producing estimates of economic profitability of replacing braking systems with recuperation of energy directly into the traction network by systems which store braking energy in devices installed in vehicles. The estimates have indicated that, with the current price relations of energy and supercapacitors, using energy storage devices is not justified from the economic point of view. However, it must be noted that these estimates were based on the assumption that the storage device capacity is 0.9 kWh, which makes it nearly always possible to store all of the braking energy. Similar calculations performed for the storage device with a capacity reduced to 0.6 kWh, which results in a decrease in energy savings by approximately 12%, but reduces the cost of a storage device by one-third, indicate that such a solution, is on the verge of profitability. It could be expected that in the future, with the decreasing prices of supercapacitors and improvements in their parameters, and, on the other hand, with the increasing energy prices, using energy storage devices in the rolling stock will be fully justified economically. In addition, using braking systems equipped with energy storage devices is justified by certain benefits of this solution, e.g. off-induction of relatively high voltage surges in traction network (in systems with recuperation of energy directly into the network, voltage could surge as high as ca. 800 V). In addition, a tram equipped with a storage device becomes, to some extent, an autonomous vehicle, e.g. it can leave a crossroads on its own in the event of voltage loss in the traction network. The cost of installing storage devices at substations is recovered relatively soon, but a fundamental question arises about the longevity of such devices installed at substations. The frequency of charge cycles of storage devices installed at substations is several times higher than that of storage devices installed in vehicles. On the other hand, as it could be seen from the histograms in Fig. 6.8, 6.10 and 6.11, full charge cycles of storage devices occur rather rarely. Most often, storage devices are charged until 1/3 of their capacitance. One can expect that, as a result, their longevity, expressed in a number of charge cycles, will increase substantially. Unfortunately, a valid answer to the question how often the storage devices will need to be replaced can be obtained only from operational tests.