PROFIenergy - Energy saving potential

01 November 2011

Professor Dr. Frithjof Klasen discusses the results of a PROFIenergy study undertaken at Daimler and Volkswagen which highlighted some potential energy savings for production plants.

It has long been a matter of course in the PC world that products are placed in standby mode, depending on the operating situation. This function is a device feature and only requires parameter assignment. This is exactly the approach taken in the PROFIenergy concept, where standardised control commands are used to place devices and machines into energy saving mode via PROFINET.

The Institute for Automation & Industrial IT, Cologne University of Applied Sciences, has performed a study to provide quantitative data and analysis to support the argument for PROFIenergy. The institute is a member of the PI Working Group that developed the PROFIenergy specification and also serves as a PROFINET Competence Center, specialising in PROFINET diagnostics and in performing energy consumption measurements and analyses for production plants.

The main tasks of the study included performing measurements for recording typical load curves; analysing load curves; determining the relevance of idle times for energy savings; and identifying the potential savings from PROFIenergy use. To achieve representative results, the study included applications and industry sectors in which PROFINET is used and benefits from PROFIenergy are particularly relevant.

Initial analyses and measurements for the PROFIenergy study have been completed on production lines at Daimler's Sindelfingen plant and at Volkswagen Commercial Vehicles in Hanover. The behaviour of the overall plants and their components were analysed with respect to load curve, load distribution, and pauses. In addition, the influence of operating modes on energy consumption was analysed, and pauses were analysed with respect to frequency and duration.

The measurements involved long-term recordings on production equipment to capture planned pauses and idle times as well as unplanned pauses and to determine their relevance. The power measurements were taken at up to 15 different measuring points within a plant, which made it possible to record typical load curves and determine characteristic values at different levels, ranging from the main incoming supply down to individual consumers.

Line-side analysers, capable of simultaneous measurement and recording of values were used to measure the power and all characteristic values of the supply system, including voltage, harmonics, and phase offsets. Up to 15 measuring devices were used in parallel for the long-term recordings. Continuous recordings of voltage, current, and power parameters were made over a 7-day period at 1 second intervals. At the same time, synchronous data on equipment status and operating mode were acquired from PLC log data. The synchronisation of the measuring devices and the PLC ensured that the measured values at the individual measuring points could be attributed explicitly for subsequent analysis.

Based on these measurements it was possible to perform a detailed analysis of operating modes and the related energy consumption of plant units. This analysis covered typical energy consumption of individual plant units; typical reduction of energy consumption during idle times; characteristic duration of idle times; relevance of pauses and relevance of plant concept (effect on energy savings potential).

The load curves in the analysed production plants typically exhibit regularly recurring load profiles that are the direct result of the discrete production steps occurring in production plants. Yet, not all production equipment is active at every point in time. The load curves therefore have typical profiles that are the result of chronological overlapping of individual devices and plant components. However, due to material stores in the infeed or between plant units, there are often no rigid process sequences. The load profiles can, therefore, vary – particularly during the transition to a temporary equipment standstill, in which not all plant components are affected at the same time.

A feature of the load curves in the analysed production plants was the high load peaks obtained over 24 hours in a typical plant segment. While the load level during operation is around 80 kW, the base load is only around 17 kW. This may not seem particularly relevant to the search for potential savings by reducing energy consumption during idle times, as the base load appears to be less than 20% of the upper load level. However, it is important that the high peak load is not allowed to conceal the fact that the actual consumption value is the mean value of the load profile. The base load during a standstill is, therefore, more than 50% of the energy consumption during productive operation and could provide opportunity for savings.

An important aspect of the study was the analysis of the load distribution within the different plants. Due to the structured distribution of the measuring points – extending from the incoming supply to the terminal level – it was possible to analyse the energy consumers separately and to identify their typical characteristics during production and idle times.
A large proportion of the energy consumed in automotive production - typically 30 to 60% - is used by robots, which are also big energy consumers during idle times.

Analysis of idle times
Idle times can occur for different reasons and provide important clues to the operating behaviour of a plant. Brief standstills are often an indicator of opportunities for optimisation with respect to equipment synchronisation and/or the material store; longer standstills occur during planned pauses and planned shutdowns and when problems occur.

Based on the results of the study, both planned and unplanned idle times are relevant for the use of PROFIenergy. Special attention was therefore given to analysing the duration of the idle period. The idle times were classified according to their duration, and the cumulative duration of all the individual events was calculated to produce analysis of idle times.

Idle times of short duration occur relatively frequently, but are typically not candidates for switchover to energy saving mode because of the time required to restart the equipment from standby mode.

Based on previous estimations, it can be assumed that for many plant components, a transition to energy saving mode is appropriate for idle times lasting five minutes or more. If this approach is taken, one can conclude for the plant example in Figure 1 that 64% of the cumulative idle times last more than five minutes and offer significant potential for the use of PROFIenergy.

Savings potential
Most production plants only have a ‘hard switch’ on/off option. Experience dictates that problems will occur when restarting switched-off equipment, so operators often do not switch off production equipment even during extended standstills. These planned idle times can account for a significant portion of the operating hours. Unplanned idle times further contribute to this. Based on the results of the study, it can be assumed that for typical automotive production plants engaged in body construction and assembly a production plant with two-shift operation will consume 47% of its total energy consumption during idle times. Only 53% of the energy consumption is used for productive operation.

The use of PROFIenergy can offer significant potential for savings. It must be noted, however, that all of the energy consumed during idle times cannot be saved. The PROFIenergy concept does not switch-off equipment completely, but places it in an energy-saving mode, which can differ depending on the equipment component. In addition, it only makes use of idle times of sufficient duration.

Based on the previous results, it can be assumed that the use of PROFIenergy can save approximately 70% of the energy during exploitable idle times. The result is a saving of 33% of the total energy consumption of a plant. Based on the energy consumption of a typical production line of 210,000 kWh per year, this yields a potential saving in the order of 7,000 euros per year (based on 0.10 € per kWh).

To fully exploit the savings potential, the use of PROFIenergy-capable control components alone is not enough. Changes to plant concepts are also needed to enable devices or plant units to be placed selectively in energy-saving modes. In so doing, there must not be any impairment of safety-related functions in standby mode for safety-related applications.

PROFIenergy applications
PROFIenergy differentiates the following four main applications:

Application 1: Brief standstills
Examples of brief standstills are breakfast and lunch breaks which range from a few minutes to an hour Here, energy can be saved by placing unneeded consumers in energy saving mode. The energy savings are not as high in this application as in application 2, in order to allow a fast restart.

Application 2: Extended standstills
Nights and weekends are typical examples of these idle times. The duration is significantly longer so that more consumers can be switched to more stringent energy saving modes.

Application 3: Unplanned standstills
Because in this case the duration of the standstill cannot be predicted, it is first classified as application 1, i.e., a limited number of consumers are placed in energy-saving modes, to avoid interfering with a fast switchover to production. If the standstill turns out to last longer, a switch can be made to application 2 to achieve greater energy savings.

Application 4: Measurement of power consumption
PROFIenergy allows acquisition and representation of consumption data of devices during operation which can be visualised on an HMI.

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