As part of a building’s overall energy usage, elevators consume up to 10 percent of the total energy in a building. From an environmental standpoint, the most significant impact elevators have is the electricity use while the elevator is in service. Therefore, buying or installing elevator equipment that promotes low-energy consumption can help save money and reduce a building’s environmental footprint.
Buildings and Energy
One way to measure overall energy usage is by calculating the power factor (PF) of the building and/or its energy-consuming devices. These are generally motors, transformers, high intensity discharge (HID) lighting, fluorescent devices or other pieces of equipment that require magnetism to operate. The following are three components to the power consumption of an electrical device in alternating current (AC) circuits:
- True power is electricity consumed to do the work of an electrical device. Measured in watts, this is the energy that the meter on a building measures.
- Reactive power is non-working electricity required to generate the magnetic fields needed by some devices in order to operate. This power is not a direct component of a consumer’s electric bill.
- Apparent power is electricity provided by the utility company to meet the demands of a facility. It is measured in volt-amps (VA). Apparent power is what utilities must generate to meet demand.
Power factor is a measurement of electrical system efficiency in the distribution and consumption of electrical energy. It is the percentage of the amount of electric power being provided that is converted into real work and expressed as a number between zero and one. For example, if a device had a .70 PF, then 70 percent of the power that the utilities generate to run the device is actually being converted into real work. The lower the PF number, the poorer the PF efficiency. The higher the PF number, the greater the PF efficiency.
In some areas, utilities use PF in the computation of the demand charge. A low PF for a customer’s facility could result in a demand charge penalty that increases the monthly demand cost. This is where newer, more innovative elevator control systems can contribute to lower energy consumption and improve a buildings’ overall PF.
Because of electrical losses caused during generation, distribution and consumption of electricity, the amount of power needed to be provided by a utility company will be greater than the amount for which they get paid by consumers.
During a recent modernization of two identical traction elevators, before and after energy data was collected. The original, first generation silicon controlled rectifier (SCR), direct current (DC) motor control was measured using a series of fixed run patterns and known loads. After modernization, the new insulated-gate bipolar transistor (IGBT)-based alternating current (AC) motor control for a permanent magnet synchronous motor system was measured using the same run patterns and known loads.
The SCR-DC system used far more energy (watts/hour) to move the exact same load through the exact same distance compared to the IGBT-based permanent magnet AC control (Chart 1). In fact, in these six load tests, the IGBT-based system used less than half the energy. An incredible 383 percent increase in power factor of the IGBT-based system compared to the SCR-DC system (Chart 2). That means more of the energy consumed was being converted into real work with less waste in terms of heat and magnetism.
These kinds of energy usage reductions and PF increases are becoming even greater as newer elevator technology gets incorporated into buildings (Chart 3).
It’s easy to see how reducing energy consumption and increasing power rating can benefit the building’s owners and operators. However, these same improvements benefit the community as well. The electricity not being used in one building can be used by other customers — allowing utilities to meet the community’s electricity demand without increasing electricity generation. That translates into no rolling blackouts or brownouts, no new power plants being built and an overall smaller environmental footprint.
Up to this point, traction elevator technology was discussed where wire ropes pull the elevator from above the car. In contrast, the hydraulic elevator pushes the elevator cab through the hoistway. The way a hydraulic system works is a piston and cylinder are sunk in the ground below the elevator. To go up, a pump forces oil from an oil tank reservoir into the cylinder — causing the piston to rise, making the elevator cab go up. To go down, gravity and the weight of the cab pushes the piston down into the cylinder and forces the hydraulic oil back into the tank reservoir. Historically, hydraulic elevators (or hydros) have been installed where either the building had fewer floors (typically six to eight) or lower material and installation costs were a consideration (when compared to a traction elevator).
So how does a hydraulic elevator measure up in energy consumption?
Charts 4 and 5 show the results of two before and after 2009 studies in which energy data was collected on two hydro-to-traction elevator modernizations. In each case, two identical hydros operating as a duplex were replaced by two identical traction elevators utilizing AC-permanent magnet motors. This is a process where the elevator hoistways and machine rooms were stripped to the walls of their hydraulic elevator components and replaced with traction elevator components. As reflected in the charts, there is tremendous opportunity to lower energy consumption of low-rise elevator applications by replacing aging hydros with modern traction elevators.
Considerations Beyond the Hoistway
Energy reduction of a building’s elevators can also impact heating, ventilation and air conditioning (HVAC) systems. Quite often, elevator machine rooms are air conditioned to support removal of the heat generated by elevator control systems. Motor-generator-based elevator controls create a tremendous amount of heat; the effect is multiplied when several systems are contained in the same machine room.
Additionally, a check should be made of the shut-down timer typically employed with motor-generators (M-G) sets. Is it working? Does the M-G set turn off after a set period of time? Or has the timer failed and no longer shuts down the motor-generator, wasting energy as the M-G set turns but no work is being done by the elevator?
The elevator cab’s lighting can impact both the energy consumption and HVAC systems. A recent survey conducted of a 34-story high rise office building with 18 elevators showed the cab lights were on 24-hours a day. There are 28 incandescent light bulbs per elevator. That worked out to 100-amps of power being consumed continuously. By replacing the incandescent bulbs with compact fluorescents, energy consumption could be cut to 30 percent. And if a 24-hour clock timer is added to shut the lights off at midnight, even more energy could be saved.
Reducing Energy Consumption
Finally, if you’re considering an elevator modernization, call your electric provider or visit their Website to explore the possibility of energy rebates from the local utility provider. It is quite common for utilities to offer dollar incentives for specific building improvements that reduce energy consumption and improve PF.
There are various benefits to building owners and facility managers who lower their power consumption and understand how power factor helps reduce the overall cost of energy, particularly the energy used to run the elevators in their buildings. These benefits go beyond the elevators themselves to include benefits derived from HVAC systems, cab lighting and energy consumed when the elevators are not moving that affect the monthly utility bill.
Images Credit: KONE, Inc.