Regenerative drives are controllers that can also operate motors in reverse so that they act as a generator when braking. They are commonly used to increase efficiency and extend range in electric vehicles. The same principle can be applied to commercial and industrial drives, in applications such as lifts, cranes and conveyors. These are energy-intensive devices and significant energy savings can be achieved by using regenerative drives.
The benefits are reduced energy costs. For example, Schindler claims energy consumption reductions of up to 30% than comparable lifts with its Power Factor One (PF1) regenerative drive technology, which was recently supplied to 34 lifts for two 34-storey residential towers in Singapore’s Marina One development (see also video: www.is.gd/apilar). A rival technology is Otis’s ReGen drive, fitted to its Gen2 lift.
How do they work? First, let’s review braking in non-regenerative systems, where there are essentially two methods. The first is to use a mechanical brake that applies pressure to a rotating body, which slows the rotation through friction. The same types of drum and disc brakes used on vehicles can be used as mechanical brakes. The second method of braking a drive is through the motor. Many drives are actually braked through the motor, generating an electrical current, but then, perhaps surprisingly, dissipate this energy with a resistor rather than using it to supply power for a device. This is known as resistor braking, and has the advantage of simplicity. Resistor braking means that mechanical braking components are not required. This can reduce both installation and maintenance costs, as well as reducing the size of the installation. It also means that there is no need to deal with the complexities of feeding energy back into the network.
With both mechanical and resistor braking, the energy is dissipated as heat. If the braking loads are high, this may require dedicated cooling or place additional demand on the building’s air conditioning system. In either case, additional energy is consumed and there may also be additional capital costs.
Regenerative drive technology diverts this lost power back to the grid. “When power flows into the regenerative drive motor, it creates a lifting torque on the shaft and sheave that lifts the cab. As the cab descends, the motor transforms the mechanical power to electrical power and pumps it back into the building’s power grid,” says Phillip Collins of Sheridan Lifts.
FEEDING BACK
If electrical current is fed back into the power network, it must maintain the power quality. To maintain a clean power supply, the frequency and voltage must be consistent across the entire national grid. An increase in the electrical demand causes the electromagnetic load on the generator to increase, slowing it down and reducing the AC frequency. Large generators have significant inertia to damp out this effect, giving time for the mechanical power delivery to respond to the demand without serious fluctuations in the frequency. Wind and solar power lack this physical inertia. Some generators, as well as battery storage, can also be rapidly switched on and off to provide frequency response. Fluctuations in voltage, known as transients, must also be controlled within acceptable limits, typically by absorbing reactive power. The grid is transitioning from a small number of centrally-controlled, large generators with significant inertia, to a distributed system with many operators providing small generators. This creates huge challenges for frequency and voltage stability.
Regenerative drives are a part of this trend towards an increasingly distributed and complex power system. They can improve efficiency while also presenting challenges for power quality. A particular issue with drives is that they can cause high-frequency voltage disturbances such as noise and harmonics. Regenerative drives should therefore be specified that produce low harmonic content, conforming with standards such as IEEE519, IEC61000-3-2, IEC61000-3-4 and IEC61000-3-12. High power factors are also important to prevent excessive reactive power being generated. To achieve this, an active supply unit and line filter may be required. These electrical devices may be built into a regenerative drive, simplifying installation.
POTENTIAL SAVINGS
Under the right circumstances, the energy saved by installing a regenerative drive can result in a payback within two years and good return on investment (ROI). However, for a drive that is only used occasionally, resulting in a minimal energy consumption, it might not make sense to retrofit a regenerative drive. US lift installer KEB offers an online tool (www.is.gd/enowis) to calculate the ROI for a particular application. The most important parameters determining the feasibility of a regenerative drive are the efficiency of the drive, its utilisation and the energy cost.
More efficient direct drive permanent magnet motors will be able to recover more energy than less efficient induction motors with gearboxes. The least efficient drives use worm gearing, which can see efficiencies drop below 60%. The efficiency of the drive results in losses both when power is delivered and when regenerative braking takes place.
Utilisation includes both the frequency and the duration of use. For a lift this means how often people use it, the height of the building and the number of floors. Taller buildings generally have higher acceleration and deceleration rates, resulting in both a higher energy demand and greater potential for recovery using a regenerative drive.
Energy costs can vary widely between regions and between different industry sectors. This often has less to do with generation costs and more to do with the differing costs of transmission, distribution and mark-ups. For example, in the USA, industry pays $70/MWh, commercial users pay $109/MWh and residential users pay $133/MWh, while in Europe prices are often higher, with residential users in the UK paying $170/MWh.
OTHER FINANCIAL INCENTIVES
Rebates must also be taken into account; there may be grants or tax incentives for installing energy-saving technology. Another consideration regarding cost might be whether a feed-in tariff is available so that the energy consumed can be offset by energy generated if it gets fed back into the grid. However, for most applications, other energy demands in the building will simply consume the regenerative energy, reducing the overall energy bill.
As well as energy savings, regenerative braking also reduces installation size and complexity. There are no brake pads to change, mechanical parts to service, or resistor wiring. There is also no need to cool the dissipated energy. This all adds up to give the simplest and smallest installation footprint possible.
In essence, regenerative drives should be installed when the energy they will save justifies their cost. This can be viewed in purely financial terms, considering whether the reduced energy bill will result in a good ROI for the investment in the drive. The investment may also be justified in terms of emissions reductions, considering the carbon payback period and the contribution to a corporate objective to cut carbon.
BOX: Hong Kong case study
Passenger lifts at the Hong Kong Tamar central government offices were fitted with regenerative drives and studied. In particular, the regenerative module was connected in parallel with the rectifier of the motor drive, between the DC bus and AC supply, it said. The results indicated that energy savings depended on design capacity (studied capacity was 1,600kg), travelling speed (lifts there ranged from 2.5-6m/s), loading profile and travel distance. It found savings ranged from 17-27%. For more information, see ww.is.gd/pibezo