Purpose

The installation of energy meters in railway vehicles facilitates the exact measurements of energy consumption. This serves mainly two purposes:

1. provide consumption data for an exact energy billing system
2. provide consumption data for the identification and assessment of energy saving measures

Availability of energy consumption data

In diesel traction the energy supply is on-board and therefore the energy consumed can be clearly attributed to one single vehicle (nevertheless an additional diesel flow meter can have an additional benefit). In contrast, in electric systems energy consumption is usually measured at the substations only which does not allow for an exact assignment to one single vehicle, since several vehicles may be in the same feeding section.

As a consequence, energy billing is usually based on theoretical consumption values derived from train data or simulations of train runs. The accuracy of such a system is limited by various factors:

• Simulation algorithms are based on models that make simplified assumptions on real conditions. This creates a small but non-negligible inaccuracy of the outcome.
• Simulations or calculations do not take into account the specific external factors influencing the energy consumption of one particular train run, such as wind, track conditions etc.
• The same is true for internal factors influencing the train run, mainly the driver. The driving style has a significant impact on energy consumption through such factors as use of recuperation brakes, speed pattern and coasting.

The values derived from direct measurements by means of an energy meter do not have these limitations.

Liberalisation of railway markets

As European railway markets are deregulated, the energy consumption of individual trains is no longer just an internal affair of the railway operator. In an effort to ensure the same conditions for all competitors, a clear and transparent billing mechanism has to be introduced. Naturally, an on-board measurement of energy consumption is not a prerequisite for billing but it does put it on a more accurate basis.

Technical details

There are no technical specifications for energy meters on a European or international level, although there do exist standardisation efforts. An on-board measurement of energy consumption and data recording as well as processing requires the following system components:

• The energy meter: The energy measurement requires measurement transformers (voltage and current) as well as the meter itself. The whole measurement device usually has an accuracy of at least 2%. The measurement equipment must be protected against manipulation by unauthorised persons.
• Data collection: Depending on the needs of the operator, the equipment will produce ¼ hour or 1 hour profiles, i.e. the on-board equipment calculates the energy consumed in a period of 15 or 60 minutes by integrating the power intake over this period of time. Double registering allows for a separate collection of energy intake and recovered energy returned to the grid.
• Data recording: A memory is needed to avoid data loss in case the regular read-out is not possible. Usually, 60 days of data storage will in most cases be sufficient.
• Remote reading: A remote reading by GSM or GSM-R radio link improves the efficiency of the process.
• Display: There are controversial opinions on the question whether it is desirable to display the consumed power to the driver. While some argue that this could be an additional incentive to reduce energy consumption, others fear that the additional information could distract the driver from more relevant information.

Energy meters have two positive effects on energy efficiency which presumably are both substantial but difficult to quantify:

Economic incentive

If energy billing is based on calculations, there is no economic incentive for operators to save energy by recuperation or energy efficient driving. Consumption data collected in the TEMA project at DB AG show that the energy consumption on a given line differs by up to 20% from one day to another. This underlines the role of factors such as weather, track conditions, traffic situation or driving style for energy consumption. These conditions are not taken into account by simulations presently used. Often, the calculated energy consumption is not even accurate enough to provide an incentive for using energy efficient stock. These shortcomings of theoretical consumption data obviously don’t apply to on-board energy measurements.

Monitoring of energy saving measures

Measuring or calculating energy consumption in itself does not save energy. However a better knowledge of energy consumption will provide valuable data to identify potential for optimisation as far as regenerative braking, energy efficient driving or stopping patterns are concerned. Experience at DB AG shows that energy meters facilitate the monitoring and thus tend to improve the general acceptance of energy saving measures within the company.

Retrofitting of fleet

Experience from the TEMA project at DB AG shows that retrofitting of old vehicles with energy meters causes costs of about 2.000 € per vehicle.

New vehicles

In new stock the integration of an energy meter will be substantially cheaper if the feature is contained in the specifications and can be integrated into the vehicle design at an early stage.

Energy consumption

Energy saving effects are to be expected but are rather indirect and will be difficult to attribute to the existence of energy meters.