Kinetic energy storage: what to know about a dynamic UPS

Dynamic uninterruptible power supplies (UPS) can be used in place of static UPSs, creating smaller, more efficient power solutions

By Rob Long, PE, RCDD June 3, 2024
Dynamic uninterruptible power supply (UPS) illustrative arrangement. Courtesy: Rolls-Royce Solutions Liege

UPS insights

  • Dynamic UPS systems offer the same functionality as a static UPS system coupled with a separate generator, but provide a solution that requires less space, produces less waste and costs less to run over the life of the facility.
  • Use of a dynamic UPS is a cost-effective alternative to large-scale static UPS systems where on-site generation is used to support extended interruptions.
  • These UPS options can save capital investment by requiring less space and reduces operational costs by requiring less support equipment and reduced utility costs.

The ever-increasing call for electrification has placed a great burden on energy transmission and distribution infrastructure. At the same time, the proliferation of power electronics to reduce power consumption has resulted in substandard electricity, known as dirty power, within the utility.

These two trends have the net result of reducing system reliability while also contaminating power. As demand for power outpaces infrastructure development and we become even more reliant on electronics, the need for clean, uninterrupted power can only be expected to increase.

A dynamic or double-conversion uninterruptible power supply (UPS) solution is one way to address the negative impacts of these energy trends, providing a seamless transition between utility power and customer generation and filtering utility power to maintain the quality within the limitations of the equipment.

Although a static UPS and dynamic UPS serve a similar purpose, the dynamic UPS requires less space, improves power factor on the utility bill and costs less to operate each year.

Static UPS systems

A static UPS typically consists of a double-conversion, online inverter module and direct current (dc) storage using a combination of capacitors and one of several battery chemistries. The amount of storage is a trade-off between the customer’s needs and the cost for batteries. Where longer durations of backup power are required, it is common to pair this with a standby diesel genset rather than expand the dc storage.

Under normal operation, the online converter provides a pulse-width modulated waveform to the load. The inverter front-end may generate harmonics back into the customer power distribution depending on the amount of filtration installed. Power conditioning may be provided upstream of the UPS to provide improved power factor, from the perspective of the utility and cleaner input power to the static UPS.

When an outage occurs or power becomes unstable at the UPS input, the UPS will switch the load to batteries. Separately, the transfer switch will detect the loss of utility, signal the generator to start and transfer to the generator. The transfer switch will transfer back to utility after stability is restored (see Figures 1 and 2).

Figure 1: Static uninterruptible power supply (UPS) illustrative arrangement with standby generator and power filtration. The illustrative arrangement shows an indoor option complete with static UPS, standby generator and power filtration. Courtesy: Rolls-Royce Solutions Liege

Figure 1: Static uninterruptible power supply (UPS) illustrative arrangement with standby generator and power filtration. The illustrative arrangement shows an indoor option complete with static UPS, standby generator and power filtration. Courtesy: Rolls-Royce Solutions Liege

Figure 2: Static uninterruptible power supply (UPS) illustrative diagram. The corresponding schematic to Figure 1 is shown. Courtesy: Rolls-Royce Solutions Liege

Figure 2: Static uninterruptible power supply (UPS) illustrative diagram. The corresponding schematic to Figure 1 is shown. Courtesy: Rolls-Royce Solutions Liege

Dynamic UPS systems

A dynamic UPS provides the same effective solution as static UPS coupled with a diesel genset. However, in a dynamic UPS, the energy is stored in a flywheel, not batteries. Modern solutions may use the traditional, high-speed flywheel or a low-speed, high-mass flywheel. The dynamic UPS produces clean power as it is a true sinusoidal waveform and the combination of the choke with the synchronous machine acts as a power filter.

Under normal operation, the utility power is used to run the synchronous machine as a motor, which works in combination with the choke coil to condition the power. This makes the apparent power factor near unity from the perspective of the utility and prevents harmonics from being passed from the utility into the load. The energy storage device (flywheel) is kept in a charged state.

When the system detects or predicts an outage, the utility feed is interrupted. Loss of utility causes the synchronous machine to become a generator. The energy storage device provides the momentum necessary to support electrical output until the engine can start and couple to the synchronous machine. The result is the system behaving as a diesel genset, with the exception that the energy storage device is recharged to allow a seamless transition back to utility after stability is restored.

In the case of a dynamic UPS system such as the mtu Kinetic PowerPack from Rolls-Royce, the coupling is performed through an electromagnetic clutch, which offers the advantages of removing electrical synchronization and allowing bidirectional power transfer. This means that the energy storage device can also be used to mechanically start the engine should the starting batteries not perform — the most common cause of generator failure is bad-starting batteries.

The illustrative arrangement in Figure 3 shows an indoor option of a dynamic UPS system, using approximately 70% of the space of the static UPS solution. The corresponding schematic is shown in Figure 4, showing the reduction in components needed for system operation.

Figure 3: Dynamic uninterruptible power supply (UPS) illustrative arrangement. Courtesy: Rolls-Royce Solutions Liege

Figure 3: Dynamic uninterruptible power supply (UPS) illustrative arrangement. Courtesy: Rolls-Royce Solutions Liege

Figure 4: Dynamic uninterruptible power supply (UPS) illustrative diagram showing reduction in components. Courtesy: Rolls-Royce Solutions Liege

Figure 4: Dynamic uninterruptible power supply (UPS) illustrative diagram showing reduction in components. Courtesy: Rolls-Royce Solutions Liege

Benefits of kinetic storage

For the purposes of this example, we will compare 2.4 megawatts (MW) of static UPS coupled with a 2.8MW diesel genset against a 2.4MW dynamic UPS.

The dynamic solution provides clean and reliable power to the facility but reduces the storage element to the size of the static UPS inverter cabinet, saving approximately 30% the space needed for the static UPS solution.

The dynamic UPS can also be placed exterior to the building, in the same manner as a standby generator. If we compare a static UPS with the standby generator in an equipment enclosure to a dynamic UPS in an enclosure, the space savings offered by the dynamic UPS is approximately 75%.

Let us equate that to dollars. A sample of new data center construction shows that space can cost from $600 to $1,000 per square foot. For the static UPS solution described, 2.4 MW of UPS will require approximately 250 square feet, resulting in a cost of $150,000 to $250,000.

Using the same size dynamic solution with its 75% lower space requirement, will reduce the construction cost of the building by $112,500 to $187,500. Alternatively, this space savings could be used for increased revenue generation by allowing more space for information technology racks, manufacturing process equipment or additional health care imaging equipment, for example.

Power conditioning, power factor correction

Dynamic UPS systems provide power factor correction, from the perspective of the utility, as an intrinsic feature of construction. In localities that bill for low power factor (typically below 0.8), this added benefit can reduce utility costs in lieu of providing power factor correction. As the system is available in native generation up to 15 kilovolt class, this can have broad application in heavy industry applications, such as water/wastewater treatment.

This is typically not a consideration for static UPS, as high inductance loads, i.e., motors, are historically considered detrimental to the life of the battery and therefore, system reliability. The dynamic UPS provides sufficient reactive power and spinning reserve to manage motor loads.

Dynamic UPS costs

The cost of ownership is often overlooked when comparing a static UPS to a dynamic UPS. Many of the operational costs are expected to be similar — engine maintenance, efficiency losses, annual inspections, etc. — and therefore not crucial to the comparison.

However, there are two major components where significant differences are expected: the conditioning associated with the static UPS power electronics and batteries and the service life of the energy storage devices.

The static UPS solution requires the space for inverter module, system controls and batteries to be climate conditioned space. The dynamic UPS solution only requires the system controls to be climate conditioned, which are assumed to be equal to the static UPS controls. Ignoring the system controls, the static UPS needs climate conditioned space which the dynamic UPS solution does not.

The cost of conditioning is estimated at $36,000/year. The cost of the additional ventilation needed in the generator enclosure is included in the efficiency of the dynamic UPS, which is approximately equal to the static UPS and therefore ventilation cost is ignored.

The added operational costs also represent an additional 116 metric tons of carbon dioxide (CO2) that the static UPS operation produces above that of the dynamic UPS. For comparison, this is about the same as 25 cars. While this seems like a small value compared to typical traffic, this is not the cost of the system, this is just the impact of the heating, ventilation and air conditioning that can be offset by using a dynamic UPS.

With a static UPS solution, the most common cell chemistry is a valve-regulated lead acid (VRLA) battery. A VRLA battery has an expected service life of five years, at which time it can only be replaced. The expected cost of battery replacement for a 2.4 MW static UPS is estimated to be $180,000. The dynamic UPS solution requires a system overhaul in 10-year cycles, which is primarily a bearing replacement, but includes a full recertification of the energy storage device. This is approximately $180,000. A quick review shows that the long-term maintenance of the static UPS solution is expected to be twice the cost of the dynamic UPS solution.

Without sufficient knowledge of disposal options for each technology, comparison of the global warming potential cannot be made, but it is unlikely that that logistical differences will substantially offset the operation. The replaced components are effectively fully recyclable, so no other waste is considered.

The static UPS solution is expected to cost about $1 million more to run over the course of the 20-year service life. It is also expected to produce 2,320 metric tons of CO2 to the environment from operation. The logistics of equipment overhaul are not expected to offset the operational global warming contribution.

While static UPS systems are the industry leading choice for mission critical backup power, dynamic UPS should not be overlooked. For large scale UPS, dynamic UPS can save space, improve power quality and cost less to run, while being better for the environment.


Author Bio: Rob Long, PE, RCDD, is a Senior Applications Engineering Specialist — Electrical, Rolls-Royce Solutions America, in Mankato, Minnesota.