VDC FAQs

Frequently Asked Questions About VYCON Direct Connect
Products & Technology

The Voltage Direct Connect or VDC is a new DC energy storage solution from VYCON Corporation, a Southern California based company that is a leader in the design, manufacturing and integration of flywheel-based energy storage systems. These systems are used in power quality (UPS) and energy cycling applications such as electric rail systems. The VDC is designed so that it can integrate easily with most leading brands of UPS systems. The VDC is a more reliable and “Green” alternative to the traditional use of lead-acid battery energy storage for a UPS.

A flywheel energy storage system is a “mechanical battery” that stores energy kinetically in the form of a rotating mass. When required during a utility outage, the energy stored by the rotating mass is converted to electrical energy through the flywheel’s integrated electric generator. The system provides the DC energy to the UPS until a standby emergency diesel generator can be started. Once either the utility is restored or the genset provides power to the input of the UPS, the flywheel system will be re-charged by taking some current from the DC bus of the UPS until it is back up to full charge speed.

How is the energy stored in a flywheel? It is stored by means of a rotating mass; hence the energy stored in a flywheel is derived by the following formula: E = kMw2 k – Depends on the shape of the rotating mass M – Mass of the flywheel ω – Angular velocity A first consideration can be made by observing that the energy is proportional to the square of the angular velocity. This is the reason why the technology trend line is to have a flywheel where energy is stored by having higher speed (RPM) values. This allows for lower weight, smaller footprint and allows for using full magnetic levitation of the flywheel mass.

When compared to the traditional lead-acid battery in UPS application the VDC has the following advantages:

Reliability - Up to a 20x higher reliability than a single string of VRLA (Valve Regulated Lead Acid) batteries that are typically used in UPS applications.

Predictable availability – The flywheel energy storage availability and status are always known with its built-in monitoring and reporting system, unlike a battery system which requires a very expensive add-on monitoring system. Even then the state of availability is only really known once the battery is called upon to be used.

Longer Life – The VDC has a 20 year expected life and runtime performance never degrades during that period. VRLA batteries are typically replaced every 4 years and runtime performance degrades with age and usage. The VDC can perform thousands of discharges and the runtime on the last discharge will be the same as the runtime on the first.

Reduction of Required Maintenance – The VDC requires little maintenance as it utilizes a magnetic levitation system rather than mechanical bearings of previous generations of flywheels. Batteries, on the other hand, must be checked on a quarterly basis for leakage, cracks, capacity, and corrosion.

Footprint Savings – The high power density of the VDC means it can free up 50 to 75% of space that would be taken up by an equivalent power rated battery bank. This lowers construction costs or allows space to be freed up to be used more productively.

Does Not Require a Temperature Controlled Environment - The VDC can operate in temperatures from 0° to 40°C, so not only does it take up less space than batteries, it can be installed in less expensive space such as electrical equipment rooms instead of a computer room. Batteries require a regulated environment for optimum performance and life.

Environmentally Friendly – Unlike batteries which contain toxic lead and acid which require safety spill containment, the VDC does not contain any toxic materials.

Lower Life-Cycle Costs – The VDC provides all the benefits noted above and at the same time provides a significantly lower cost vs. batteries over the life of the installation. The ROI is typically within 2-3 years and over a 15-20 year operation the VDC total life-cycle cost is a fraction in comparison to batteries.

The VDC can be used as a battery replacement or as a supplement in parallel with a battery bank. Generally, there are three configurations that our customers are utilizing the VDC which are described below:

Ride-through “Bridge” to generator

Most of our clients for the VDC are using it as a direct replacement for batteries on the UPS. The VDC provides the back-up energy required by the load when the utility fails and until the emergency standby diesel generator is online. This “Bridge” time required is typically 6-10 seconds. The VDC can provide the bridge time more reliably and predictably than lead-acid batteries.

Battery “Hardening” and life extension

Because the VDC provides a programmable DC voltage it can be paralleled very easily with a bank of batteries. This provides the client with redundant energy storage on the UPS DC bus and the VDC can programmed to be the first line of defense against outages so the batteries are never cycled unless the VDC has exhausted its energy. This shields battery banks from virtually all cycling events, preserving battery capacity for longer disturbances. Consequently, this “battery hardening” effect extends battery life expectation and improves UPS reliability.

Short-duration ride-through

Some of our clients, particularly in the industrial sector don’t have emergency standby generators and utilize the VDC with a UPS to provide protection against short-duration “glitches” or outages using only the available energy from the VDC. The Electric Power Research Institute–EPRI—reports that only 2% of all outages are longer than 10 seconds in duration. So many industrial customers may only experience a longer-term outage every 3 years, but they may experience 12 or more short-term outages or “glitches” per year that have the same effect as a long-term outages causing computer controlled processes to shut down and be re-booted. This can lead to thousands of dollars in lost productivity and scrap.

The VDC is used wherever mission-critical power is required. All companies that depend on power to keep their businesses operational are potential users of the technology. These could be companies in the financial, banking, securities, and insurance sectors as well as companies that provide IT services such as internet data centers, web-hosting and co-location providers. In addition, companies that provide essential services like hospitals, telecommunication, radio / TV broadcasting are also potential clients. The one thing that all these clients are seeking is a higher level of reliability and performance of their critical power systems. The VDC can provide this and also show how it can be financially beneficial while also being “friendlier” to the environment.

Most standard generators can start and accept load within 6 seconds of receiving the start signal. The VDC configurations can accommodate UPS backup times in a mission-critical application from 15 seconds up to 1-2 minutes. In various industry studies such as the IEEE Gold Book, genset start reliability for critical and non-critical applications was measured at 99.5%. For applications where the genset is tested regularly and maintained properly, the reliability is increased even higher. 80% of the time that a genset fails to start is because of failure of the battery being used to start the generator. Just monitoring or adding a redundant starting system can remove 80% of the non-start issues.

Many customers are under a false sense of security from having 10 or 15 minutes of battery runtime. They assume that if the generator does not start they will be able to have a chance to correct the issue. As many mechanics will testify, if the genset does not start on the first one or two cranks of the engine, it will take much longer than 10-15 minutes to diagnose and repair the genset. The main concept here is therefore to improve the start-up reliability of the system. This can be done in several different ways; some of these are briefly described below (for deeper analyses on genset integration with UPS please refer to: The Chloride Academy, “Advanced Module Eight: Integration With Diesel Generation Systems”.)

Test the generator regularly (once per week).
Monitoring and remote diagnosis of main parameters (battery voltage, lubricating system, fuel level and so on).
Positive pressure fuel line.
Coolant heating.
Use of redundant system.

There are basically two categories of flywheels commercially available in the market

High Speed (RPM) Flywheels

angular velocity 30 – 60 krpm (potential limit up to 100krpm)
much lighter flywheels for high power needs (energy stored through higher spinning velocity)
full magnetic levitation
the flywheel has lower periodic maintenance
smaller footprint and lighter weight
easy commissioning, start up and shutdown

Low Speed (RPM)

angular velocity <10krpm
energy for high power needs heavy steel flywheels (Heavy and bulky)
periodic maintenance and replacement to the mechanical bearings
high amount of parasite energy losses
requires special concrete slab specifications for installation

Another comparison can be made from different high speed (RPM) flywheel solutions that utilize a flywheel made from carbon fiber. While both VYCON and other carbon fiber systems use high speed, the materials used for the flywheel design are different as well as the design of the motor/generator. VYCON utilizes standard, aerospace grade 4340 steel. The material properties are very well known, available from numerous suppliers and this material is used in many high-speed rotating applications. Most important is the integrity of the material can be measured through core samples and ultrasound to assure it complies with the application specifications. Carbon Fiber flywheels typically use miles of carbon-fiber that is wound on a spindle with an epoxy resin that is provided from a single source. Imperfections in the process and gaps between the fibers could lead to an “unbalancing” of the wheel over time due to the stresses applied as the wheel is spun from high RPM to low RPM and back again, which occurs during every discharge event. Once the carbon fiber flywheel becomes unbalanced, the entire flywheel module must be replaced… a very costly and time-consuming process. The VYCON flywheel has been used not only in UPS applications, but also in high-cycling, regenerating applications like in electric motors for electric rail. These applications call on the flywheel to be charged and discharged sometimes 20 times per hour. These applications prove the robustness of utilizing aerospace grade steel as the preferred flywheel material.

The other difference is in the motor generator configuration. VYCON utilizes a permanent magnet type motor generator. The benefit of this is twofold; one benefit is higher efficiency of the motor generator when charging and discharging. This allows the high cycling capability of the VYCON flywheel. The second is that the flywheel can generate its own power to maintain the flywheel levitation even if control power is lost or a failure occurs in the power electronics. Other flywheel manufacturers use a synchronous reluctance motor that cannot self-generate power if a failure occurs in the power electronics. Thus the unit requires a back-up supply from a small UPS to provide power to the magnetic bearings if this occurs. Using a battery UPS to protect your battery-less flywheel solution does not seem to make sense. In addition, if all power is lost to the magnetic bearings or if a failure occurs to the magnetic bearing controller, the rotating flywheel will touch down on the bearings at full speed. The flywheel will come to a stop, but it will no longer be suitable for use, it must be replaced completely. By contrast with the VYCON flywheel, the back-up ceramic bearings can support several drops at full speed and recover normally.

The VDC product requires minimal maintenance. Because the flywheel utilizes a full levitation system with no mechanical bearing, there is nothing in the flywheel module that needs to be maintained. The VDC also utilizes a “medical grade” vacuum pump to maintain a high vacuum level in the flywheel chamber. Because all materials located in a vacuum environment “out-gas” vapor molecules over time, these vapor molecules must be captured and removed in order to keep the vacuum at the appropriate level in the flywheel chamber. The VDC gathers these vapor molecules in the mineral oil of the vacuum pump. Typically this mineral oil will need to be replaced after about 1 year of operation as the mineral oil has absorbed these contaminants. Replacing the mineral oil “regenerates” the vacuum. The VDC vacuum regeneration procedure takes about 10 minutes to perform and the unit does not have to be taken off-line, thus is available to function if called upon. Because the “out-gassing” rate decreases over time, the frequency of needing to regenerate the vacuum by changing the oil is less. The estimated life of the vacuum pump itself is approximately 8-10 years.

Flywheel systems that use a molecular vacuum sleeve instead of an external vacuum pump, also face the issue of “out-gassing” of the flywheel components. This vacuum sleeve system directs the vapor molecules and contaminants from the flywheel chamber into an upper chamber where they are absorbed and collected in getter or desiccant materials, much like the external vacuum pumps collects these in the mineral oil. Typical within 12-18 months of operation, the vacuum of the these systems must also be regenerated as the getters have been saturated and thus the vacuum sleeve is less effective in maintaining the vacuum. Unlike a 10 minute operation with the changing of the mineral oil on the VDC unit, the regeneration of the molecular sleeve based vacuum system unit requires the unit be taken off-line for 6-8 hours as an external vacuum pump must be attached to the flywheel and the getters/desiccant materials are heated up using a probe. The heating allows the getters / desiccants to release the vapor molecules which are then removed by the external pump. During this time the unit is not available to protect against an outage. Because the other flywheel systems are liquid cooled and the inner housing is immersed in liquid with power and control cables fed through, it is susceptible to leaks over time. If this occurs, the vacuum cannot be regenerated in the field and the flywheel module will need to be replaced….again expensive and time-consuming.

Besides the 10-minute vacuum regeneration on the VDC, the only other required maintenance is the replacement of standard DC caps at around 7-8 years of operation. This is standard maintenance on all flywheel systems that utilizes a bi-directional converter.

The recharge time after a complete discharge is factory adjustable per application requirement. For UPS applications the recharge time default is approximately 12 minutes. The current is limited by the current available from the UPS rectifier. Since after an outage, the UPS is typically fed by the standby emergency diesel generator, the recharge current is set for an approx. 12 minute re-charge so not to over-stress the diesel engine as typically even if the utility returns, the diesel will typically power the UPS for 10 to 15 minutes to assure a stable utility supply. If the UPS and diesel engine are properly sized, the recharge current can be increased to reduce the time needed to re-charge. While recharging, if another event were to occur, the VDC will provide whatever energy is available instantaneously even though it is not fully charged.

Unlike the VDC, other high speed flywheels that use a synchronous reluctance generator don’t have the capability to self-generate an emergency back-up supply in the event of a power conversion (IGBT) failure. They will rely on the DC bus of the rectifier or need another uninterrupted source. This can be from the output of the UPS if batteries are paralleled with the flywheel on the DC bus, but if the flywheel is the only energy storage on the UPS, then the back-up must be fed from another battery-based UPS. The scenario is as follows: A flywheel is used on the DC bus of a UPS. The magnetic bearing controls obtain power from three redundant sources, the DC bus from the rectifier of the UPS, the self-generated DC from the output of the IGBT converter (when flywheel is discharging) and the back-up power from the output AC of the UPS that the flywheel is being used. If the utility fails and then a failure occurs on the flywheel IGBT converter then the rectifier DC bus is not available due to the utility outage, the output of the flywheel IGBT is not available as the IGBT has failed and since the flywheel cannot provide DC power to the UPS, the output of the UPS inverter is no longer available so even the back-up supply is missing. This will result in the flywheel losing its magnetic levitation, dropping on the touch-down bearings at full-speed and then needing to be completely replaced.

This is why VYCON has chosen a permanent magnet motor-generator technology as it can self-generate critical power to the magnetic levitation controller even with an IGBT failure. The unit can even go down on its back-up bearings at full speed several times and recover completely without damage to the flywheel system. While the VYCON VDC does require an AC input from the output of the connected UPS, it is used to power aux equipment and it is not required for the survivability of the system.

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