Diesel rotary vs. Static UPS

David Johnson, HITEC Power Protection


The benefits between static UPS systems and diesel rotary UPS (DRUPS) systems are important

The role of an uninterruptible power supply (UPS) is simple in concept: provide a temporary power bridge to support the load between the loss of the utility and the transfer to diesel generators. Even though the task is easy to define, the best way to achieve the goal is often clouded in obscurity as the benefits between static UPS systems and diesel rotary UPS (DRUPS) systems are debated.

Viewed in its simplest form, the differences between the two systems are based on their method of energy storage. Static UPS systems use batteries similar to automobiles to provide energy required for the ride through period. By contrast, DRUPS systems use a flywheel instead of the batteries to store kinetic energy used for the ride through period.

Oftentimes static UPS systems are selected over their DRUPS counterparts based on their ability to provide cost-effective protection for loads less than 1mW. CIOs are quick to put their stamp of approval on large ticket items waving the “cost effective” banner, but they sometimes forget to look beyond the initial lower price tag and into the total cost of ownership (TCO) benefits (see Figure 1).

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Static UPS systems are selected over their DRUPS counterparts based on day one cost. But DRUPS makes more sense when you look beyond the initial lower price tag and into the total cost of ownership (TCO) benefits.

One of the items lurking behind a misleading, initial low price tag is the fact that static UPS systems are battery-based and require a much larger installation footprint. A traditional battery-based static UPS system is composed of a rectifier, batteries, an inverter, and a transfer switch. A 1 mW system laid out to support the load for five minutes requires nearly 2,000 square feet of space. In manufacturing facilities where every square foot needs to be mapped to a revenue-generating procedure, that kind of space to store electrical equipment can impact the bottom line (see Figure 2).

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Plant space is an important consideration, and a SUPS installation would take up twice the footprint of the DRUPS switchgear shown. The diesel rotary option is also superior because of its ability to handle mechanical and motor loads found in manufacturing applications.

The other static UPS line item that is often left off the balance sheet is that these batteries typically have a lifespan of five to seven years. There is a considerable amount of maintenance and care that must be allocated during this life cycle. At the end of their life, these large energy storage cells cannot be conveniently deposited in landfills, so additional costs are needed to properly dispose of these large battery systems.

On the other side of the debate, DRUPS systems are much kinder to the balance sheet over the long run compared to their static UPS counterparts. A DRUPS solution has no toxic battery chemicals to dispose of and replaces all of the battery-backed, static UPS equipment required to support critical IT loads within a much smaller footprint – up to half as much space.

A deeper inspection of the diesel rotary alternative reveals some impressive accomplishments. DRUPS solutions typically:

•          Integrate multiple traditional backup components into a single system. When utility power is lost, the system retrieves kinetic energy from its flywheel to support the critical load until the diesel engine takes over.

•          Last approximately 30 years, saving any manufacturing plant considerable lifecycle cost compared to traditional UPS systems that last 15 years.

•          Produce clean voltage, which also carries the distinct advantage of being more reliable.

These facts are interesting, but how does each system perform in the field?

A good litmus test is the ability to handle medium-voltage. Static systems can handle up to 600 VAC, but DRUPS systems go from 400 VAC to 35 kV, often times without the use of transformers.  Since static UPS systems only operate at low voltage, the facility manager is limited by breaker capacities to parallel enough of these units to protect a manufacturing plant-sized load.

By contrast, one of the largest DRUPS applications currently in use is a 22-megawatt application that uses 2,000 kW units in parallel at medium voltage.  These units support the operation of a semiconductor fabrication plant, and support both motorized and non-motorized loads. In this application, static units are just incapable of achieving this type of performance objective. 

Let's not forget that static units are hot heads as well! With a static UPS, facility managers must provide cooling for not only the heat rejection from the static UPS unit itself, but also keep the batteries at a cool 77 degrees Fahrenheit required to optimize their otherwise short lifespan. This added cost of cooling is another line item that must be added to the TCO calculation. And the TCO argument becomes even more compelling with a close inspection of the two systems.

A good rule of thumb is that the operating cost difference for systems of the same capacity is approximately 35% year over year.  If the cost to operate a static system and its supporting mechanical infrastructure is XX per year, then the operating cost difference of the two systems over a conservative 20-year period is YY.  When you factor in major overhauls required of the static system, including batteries, fans, and capacitors, as well as a total system replacement required during years 13 and 15, it is easy to understand that a DRUPS unit will save $1.5 million dollars for every 1MW of load supported.

Even though the numbers are very compelling for a DRUPS system, some organizations pinch pennies by getting creative with their UPS installations. One example is to deploy multiple, smaller static UPS systems around an entire plant. Rather than protecting the entire plant with a single UPS, some facilities will single out small portions of the production chain that contain motor pumps or automation process and individually support them with smaller static UPSs. This is a Band-Aid approach to protecting very complex assembly machines across an entire production line, and brings with it several areas of risk. Many times, these plant managers discover that this trial-and-error approach leaves key parts of the assembly line ignored opening the entire manufacturing process to downtime again during the next power outage.

The second problem with this multiple static UPS approach is that having many UPS units is costly to deploy and maintain. This piecemeal UPS placement also actually reduces the overall reliability of the system because each UPS compounds the single point of failure calculation, causing the plant’s reliability to drop. DRUPS solutions simplify power distribution by supporting the entire manufacturing plant at the correct voltage with a single unit.

Another noteworthy UPS installation can be found within a PVC extrusion plant in Malaysia. This facility had to run their standby diesel generators continuously and was spending millions of dollars per month for diesel fuel. Fuel costs were increasing because power quality at the plant was poor and the facility constantly had to run the standby diesel gensets to compensate. After the installation of a new DRUPS, the PVC extruder plant realized a notable decrease in fuel costs and a 10-month payback period. In addition, the facility was able to repurpose their existing standby diesel gensets at another location where power was more reliable. In this situation, there was not an option for static UPS deployment because this type of unit could not handle the load requirements.

Another noteworthy UPS installation example can be found in Puerto Rico. This pharmaceutical site was also experiencing poor power quality throughout the day. Faced with the need to support expanded operations, the facility realized that the majority of the loads were mechanical. The mechanical load requirements automatically disqualified the static UPS solution as an option.

The decision came down to either a 2255 kW or a 2500 kW DRUPS unit. To achieve this capacity, two parallel static systems would be needed to meet the inrush of mechanical loads. The static UPS option would be a 4 or 6-unit installation to match the capabilities of 1 DRUPS system. The diesel rotary option is a superior solution because of its ability to handle the mechanical loads, without interruption, and providing the diesel backup through extended outages.

One way to know if you should consider DRUPS for your plant is through a simple calculation you can do for your facility.  First, you need to know the downtime cost of an outage with all factors taken into account:  production time, labor, material spoilage, and supply chain impact.  Next, you will need to know the total amount of power required to support your load.  If you divide the first number by the second, and it is $200/kW, then you will pay for a DRUPS unit within the next 5 power outages.  If your number is more per kW, your ROI will be even sooner.

Remember, alternatives are available for companies to consider when shopping for UPS technology. Although battery technology is more prevalent, it is not always the best overall choice for the organization. This is an important consideration as manufacturers look to reduce unwanted and unnecessary capital costs and ensure productivity levels. The debate is over: the ROI for a DRUPS system is a much better option than the traditional static UPS solution. As manufacturers begin to understand and evaluate these facts, it's clear that selecting DRUPS technology will improve productivity and decrease expenses.

HITEC Power Protection