Energy Management in Connected Lifts
Global energy consumption has to be significantly reduced. The ecological footprint of Europeans is too big. We live beyond our means and at the cost of others and future generations. A large share of the ecological footprint consists of burning fossil fuels and the emission of the greenhouse gas carbon dioxide for energy consumption. To achieve the goals of reduction in carbon dioxide emission defined in the Kyoto protocol international legal guidelines to reduce energy consumption were adopted. Some of them already affect the lift industry.
In Germany the VDI 4707 describes the energy certificate for lifts. At the international level the ISO25745-2 is work in progress. It describes a similar classification of lifts and escalators.
The constructor of the lift hands it over to the lift operator after completion. This way the guideline is a basis for energetic comparability of lifts.
To issue the certificate the following is required:
- the energy demand for a defined reference drive
- the standby-demand 5 min after the last drive
- and the usage category, as a result of the number of drives and duration per day.
After the classification according to usage category the efficiency class of the lift can be defined with the measured values and the certificate can be issued. A detailed analysis of VDI 4707 can be found in several articles of the trade press in the last 3 years (compare the sources at the end of this article).
This procedure possesses two disadvantages. On the one hand, the demand can be determined only after the lift is completed and not during the proposal phase. On the other hand, measuring the energy demand for the certificate according to VDI 4707 is complex and therefore expensive. As explained in the article on the measurement of energy . It would be better were it possible to calculate the energy demand right at the beginning. More about that later.
With the release of the guideline the first developers of components already had solutions to offer in the market place, for example BÖHNKE + PARTNER with their solution Blue Modus® or Kollmorgen Steuerungstechnik GmbH with the system MPK Green, to reduce the energy demand of a lift. With this solutions it is primarily the standby-demand that is being reduced. This is done by switching off modules that are not necessary at the time. For example, the floor indicator and the light of the cabin do not have to be switched on while the lift is not in use with nobody in the car.
By switching off components there must not be an impairment of security, of comfort (longer waiting periods) nor of a decrease in longevity of components. If that is the case, the ‘energy-saving function’ will be deactivated by the operator. Therefore, there have to be solutions that save energy without impairing users.
Considering the energy demand of a lift in relation to its usage one can speak of three categories. The basic demand occurs through electronic modules that have to be active at all times, independent of the state of the lift. Emergency call devices, for example, have to be active all the time. A standby-demand occurs through modules that are active if the lift is in standby-mode and waiting for calls. Travel demand occurs by a person issuing a call for the lift, including signals, door movements, lighting etc.
The basic demand is occurring 24 hours a day. Every saved Watt saves 1.60 Euro per year (at a price of energy of 18 cents/kWh). Here is the greatest potential for saving energy. By using highly effective switching power supplies with an efficiency factor of above 85% instead of linear regulated power supply with transformers, which have a low efficiency factor, the basic demand can be significantly reduced.
The standby-demand occurs if the lift is not in use and ‘awaits’ new calls. The time the lift spends in standby-mode depends on the usage, for example, whether it is a highly used lift in a hospital or a lift in an apartment building that is rarely used. The standby-demand can be reduced by switching off single modules that do not have to be active, or by switching them into a mode where they use less energy.
Energy saving modes of the modules
For the modules to activate their inner energy saving modes they need a signal from the lift controller. However, input terminals for such signals are not available at most modules. Therefore only the complete switch off of the modules helps to save energy. This is not always possible. Partly because modules are not allowed to be switched off, like emergency telephones, or modules need too much time to function again, or they are not allowed to be switched off that often (reduction of longevity), or parameters change, or there has to be learning drive / reference drive after switching them on again.
Only if the modules are linked by a bus extensive energy savings are possible. In the open standard CANopen-Lift (CiA 417) messages were defined to capture the inner states of the modules, read energy measurements, and to switch single modules into energy saving modes.
Once a drive is executed in a lift whose components are linked via CANopen-Lift, the lift controller can send a command to all modules after closing the doors to activate their energy saving function. If the modules support the soft-standby-mode they can then decrease their energy demand by switching off inner functions. This happens until the lift controller sends the command to be ready-to-operate again. Displays in the car are switched off, the lights will be dimmed or switched off, drive inverters switch to savings mode and fans off, door controllers switch off. Because all these devices still need energy they are ready-to-operate after fractions of a second once the “wake up command” occurs. Because this wake up happens very quickly the mode can be used after every drive and not only at night or during the weekends.
In those times, when the lift is not used for a longer period time, additional energy saving modes will be used. Here different modules will be switched off completely to further reduce the energy demand. This brings the disadvantage of additional seconds needed to wake up the modules until the lift is ready-to-operate again.
If one wants to shift the lift into energy saving mode some things have to be considered. The lighting and the door controller must not be switched off if there is still a person in the car. Here the network of the electronic modules in the car helps as well. The load measuring unit connected via bus submits not only zero load, full load, and overload, but directly the load in kilograms. This way, it is easy to determine whether the car is empty before switching off the light. The signal ‘person present’, which is sent by entrance monitoring sensors or by acceleration sensors, indicate whether a person is inside the car as well.
With lift groups there are further possibilities for saving energy. Here, we have the opportunity to switch off single lifts completely and this way reduce energy demand significantly. A group of four lifts in a hotel or an office building is used to capacity during peak hours only. In times of less demand the energy manager can switch off a single lift so that a group of three lifts remains active. This way the standby-energy demand is reduced by 25%. If the remaining group of three lifts recognizes increasing demand it can ‘wake up’ the dormant lift and it will process its share of the incoming calls after a few seconds.
This principle can be applied in other group constellations as well and in case of very little demand the group participants can be switched off one after another, at night possibly with just one lift remaining available. For the realization of this function it is very important that the lift controller knows about the situation of the whole group and the internal state of many modules as well, before simply switching them off. For example, drive inverters cannot be switched on/off any number of times. At the construction time of these modules this was not a requirement, as a converter used to be always active except for commissioning and maintenance. In the technical specification the maximum switch-on frequency per hour is declared, for example, 10 times per hour. If the components are linked the lift controller is able to consider the internal state of the modules and therefore increase their longevity.
Measuring Energy Demand
Every once in a while lifts are to be connected to the building automation and are to submit, besides the state of the system, the energy demand. This measuring procedure is extensive because all three phases have to be measured and the difference between the biggest voltage during full load and the small voltage during standby amounts to several decimal powers. If errors in measurement are expected to be low several or very expensive instruments have to be used. By linking the modules a further advantage materializes.
Modern frequency converters internally determine the demand for energy needed by the power section. With the definition of the measurement values as CANopen-Lift-messages these values are available for all participants and do not have to be determined with an expensive three-phase instrument. The remaining energy demand can be determined with a simple one-phase instrument and the overall energy demand is determined by adding both values.
Calculation to determine the energy demand of lifts
As was mentioned at the beginning of this article, a method of calculation to determine the energy demand is needed. Within the framework of the VDI there is the guideline work group for VDI 4707-2. This group developed calculation methods to determine the energy demand of a lift. The guideline is currently available in greenprint (June 2012) and is to be published this year.
To determine the energy demand it is necessary to know among others:
- Construction type
- Usage category
- Type of drive
- Efficiency factors
To calculate the standby-demand it is necessary to have the values of the energy demand of every single electronic module in their specific states of use, as well as the length they stay in specific states of use.
For electronic modules the VDI 4707 paper 2 defines four modes of operation:
- P0: the module is active,
- S0: the module rests, but is immediately ready-to-use (< 250 ms),
- S1: the module is in a soft-off-mode and after 3 seconds ready-to-use again (< 3 s),
- S2: the module is off / power down and in a maximum of 60 seconds ready-to-use again (< 60 s).
If the energy demand of all modules in these four modes of operation is known and the length the lift stays in each of them, the standby-demand of the lift can be calculated.
In the application profile CANopen-Lift 5 energetic modes of operation were defined, which are also meant to cover the modes of operation of VDI 4707-2. An energy manager, who is usually embedded into control, sends a soft-off commando to the components after each run. This way they can activate their internal energy savings options. These functions are already implemented by the first manufacturers and have been tested successfully.
After guideline VDI 4707 has been published many manufacturers of components have become engaged with the subject of energy savings. In the beginning phase the saving of energy was realized by switching off entire modules, which is limited in time to when the lift is not used for a longer period of time, like at night or at the weekend.
Only by linking the components via the open standard CANopen-Lift the possibility of an all-encompassing energy management arises. Many modules can be switched into internal energy saving modes right after each run. By doing this energy savings of almost 50% of the standby-mode are possible, without the necessity to install further hardware.
Further options for saving energy are possible because of optimization of the control software according to energetic aspects, for example, the reduction of frequency of adjustment in parking floors, direct runs with high load in the cabin or optimizing group algorithms according to energetic aspects. This process is not finished yet and we will continue to find new solutions to further reduce a lift’s energy demand in the future.
- Article Liftreport 3/2010 'VDI 4707 „Energy efficiency of lifts“' by Conradin Jost
- Article Liftreport 5/2012 'Studie über den Energieverbrauch von Aufzugsanlagen' von Lazaros Asvestopoulos und Nickos Spyropoulos (German)
- Article LiftReport 2/2009 'Energy efficiency in elevators' by Jörg Hellmich
- Article LiftReport 1/2008 'Energieeinsparung bei Aufzügen' von Werner A. Boehm (German)
- Richtlinie VDI 4707 Blatt 1 beim VDI (German, PDF)