In 2008, the 200,000-square-foot (sq ft), 105-bed Woman’s Hospital at Renaissance was added to the east campus of Doctors Hospital at Renaissance (DHR), Edinburg, TX. The facility provides 24 labor and delivery suites, 24 post-surgical patient suites, 48 post-partum suites, a 36-bed newborn nursery, a 28-bed Neo-Natal Intensive Care Unit, five surgical suites, and 14 medical beds.

Like every hospital, DHR is required to have reliable backup power to protect the life and safety of its patients in the event of severe weather or a utility power outage. For the first 10 years of operation, the hospital’s planning team relied on large single-engine units for hospitals and chiller plants-the traditional solution for backup power generators.

The Central Plant for the Women’s Hospital is a 2,400-ton plant consisting of four 600-ton Carrier centrifugal chillers, primary and secondary pumping arrangement, and three 800-ton Marley Quad-Flow cooling towers. The total cooling load on the building is 680 tons.

There are actually two electrical services to this site: the central plant and the emergency service. The central plant is fed by a 2,500 kilovoltampere (kVA) transformer and a 4,000 amp switchboard. Three Generac 750-kilowatt (kW) Gemini Bi-Fuel generator sets (gen sets) feed the emergency side of the central plant. The central plant is divided into two loads, with one chiller on one transfer switch and the other two chillers on another transfer switch. All of the pumping and general power for the plant is on a third transfer switch.

The Texas Department of State Health Services requires that an engine have sufficient fuel to run for 24 hours straight. For hospitals that accept Medicare patients, the Joint Commission on Accreditation of Healthcare Organizations’ (JCAHO) emergency management standards raises this minimum run time to 72 hours.

As the DHR campus grew, its facility managers consulted with a team of design engineers from GPM Engineering, Corpus Christi, TX. In 2003, a sales team from Generac Industrial Power, Waukesha, WI, and Generac’s industrial dealer, WPI (Waukesha-Pearce Industries), San Antonio, TX, visited GPM. Concerned by the hospital’s reliance on single-engine gen sets, GPM engineers saw that Generac’s MPS system combines the output of multiple generators without the need for the space-consuming paralleling switch gear typical of traditional paralleled systems. Redundancy and expandability is built into the system since each gen set features onboard paralleling capabilities making it easy to achieve N + 1 or greater coverage by simply adding modular generators of the appropriate size. The MPS solution is also scalable, allowing electric outputs to be tailored more precisely to current and future requirements. The generator system combines diesel fuel (30 percent) with natural gas (70 percent) in configurations of 600 to 9,000 kW.

DHR demonstrates that modern health-care campuses are increasingly more integrated with centralized mechanical systems that use multi-unit cooling and heating equipment. This approach coordinates well with a multi-source, on-site, paralleled power system.

Because the MPS Bi-Fuel system offered reduced consumption of diesel fuel and extended run times per tank, John Rustick, DHR’s associate administrator and director of engineering, ordered 12 MPS Bi-Fuel generators-two sets of three by 750 kW Gemini Bi-Fuel gen sets, which provide 4,500 kW of backup power for the new Women’s Hospital and its adjacent central cooling plant. As space outside the Women’s Hospital was limited, DHR ordered the Gemini Twin Pack gen set, an option of the MPS system that consolidates two independently powerful in-line engine-based gensets. The gen sets are paralleled on board within a single, weather-resistant, sound-attenuated enclosure. The result is a dense on-site power solution with built-in redundancy in a space up to 40 percent smaller than a large, single engine. Because these U.S. Environmental Protection Agency (EPA)-compliant, side-by-side diesel engines operate in parallel, Gemini offered the Women’s Hospital quiet, built-in redundancy and increased reliability.

Rustick realized that the paralleling system would enable DHR to avoid expensive roll-up rental equipment if a solo unit fails or requires servicing. Oftentimes, one such interruption in power availability will cost more than the installed price of a paralleled system within hours, sometimes minutes, of an inoperable critical load.

Health Care

Bi-fuel (the mixture of natural gas and diesel in the combustion chamber) is especially useful for health-care facilities. This is because a Bi-Fuel system is based on a compression-ignition diesel engine platform. Normally the system runs on diesel fuel and introduces natural gas in bi-fuel mode. Btus from natural gas augments energy from the diesel, which effectively extends the available run time from hours into days. In the case of DHR, the Bi-Fuel system extended the expected run time six times over conventional systems. Each generator has essentially 36 hours straight diesel run time in its tanks, which exceeds the requirements imposed by Texas. The Bi-Fuel option can extend the run time of the engines close to nine days without having to refill the diesel tanks, which meets the requirements of JCAHO.

Bi-fuel technology originated several decades ago, when it was noted that the diesel engine sped up when methane was introduced into a diesel engine’s intake. On a fixed-speed fuel delivery system, such as used in a gen set, introducing natural gas speeds the engine, but controls reduce the amount of liquid fuel to maintain operation at its set speed for 60 hertz. Complex controls must also regulate the gas to a level that is acceptable for maximum efficiency. In this way, the automatic fuel ratio controller serves to leverage the most positive performance characteristics of both diesel and natural gas. When a heavy transient is introduced to the power system, the Bi-Fuel controller immediately injects more diesel fuel, re-establishing the advantages of the high-torque characteristics of a compression ignition engine.

The Truth About N+1

What is described as N+1 during design may actually indicate much higher levels of redundancy when referencing the actual normal load factor. This occurs often in the design of many load schemes. Designers typically use conservative numbers when calculating anticipated loads from a load schedule in a new design to ensure appropriate coverage of equipment is maintained at all times. In a paralleled system such as the one at DHR, smaller blocks of power decrease the vulnerability of the system to the loss of a unit. DHR features six 375-kW blocks of power in each paralleled system at DHR.

The “N” referred to as Normal may demand less than half's capacity. The power blocks in the six-unit system require only two or three to maintain the described load factor. The customer can choose to keep the blocks of power turned on and available for action at all times, giving extremely high levels of redundancy. Or the blocks can be turned off to conserve fuel and hours to get the load focused on a smaller amount of units, which is also healthy for diesel engines that operate best at higher load levels.


A value-added option communicates from load to generator and requests more power capacity at times of higher demands. A chiller that needs to be activated can use its controls to tell the generator control system that it needs to come online. The generator system starts up the predetermined units required to handle a transient surge demand from the chiller motors. The chiller motors use the extra power capacity to come up to speed and then use less power when they are running. This steady-state condition may be enough to automatically turn off extra capacity in the generator system while maintaining the chiller power demand while in a running mode.

When the start signal originates and is sent to the generators, they begin their start up process. Once the first generator achieves stable voltage, it closes the emergency bus. Each generator in the MPS follows suit after that. So if one unit fails to start with a plus (an N+1), the facility still has emergency power.

The Test

Packing winds with gusts over 100 miles per hour, Hurricane Dolly came roaring ashore at Padre Island, TX, on Wednesday morning, July 23, 2008. Causing more than $1 billion worth of damage, the Category 1 storm dumped 16 inches of rain on the Rio Grande Valley and more than 212,000 customers lost power. It was to be the first real test for Rustick’s new investment in standby power. “The Generac engines whirled into action instantly and kept us operating for 14 hours before we got power restored. Not a single venue on campus was without power,” Rustick remarked.

Sixty days later Rustick was rewarded again, but this time the trial was more severe. In September, a main switch gear literally blew up on the main power supply line to the Women’s Hospital from DHR’s electric utility AEG Texas. “Without Generac’s modular platform, we would have had catastrophic problems of biblical proportion at our newest hospital,” Rustic said. “Instead, without a hitch, the Generac units kicked on immediately and ran continuously for 13 straight days. Not a single chiller, critical care unit or operation at the Women’s Hospital was ever compromised. That’s an amazing record!”

With each campus expansion, more Generac systems have been installed. DHR engineers requested that their older, single engine units be replaced by MPS systems. Today, out of a total of 25 engines on the two campuses, only two 2-megawatt engines still remain. n

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Sidebar: Doctor's Hospital at Renaissance

Since opening its doors in 1997, Doctors Hospital at Renaissance (DHR), Edinburg, TX, has grown to be one of the premier health-care providers in the nation. For the past two years, DHR has placed on the list of the 100 Top Hospitals in the nation by Thomson Reuters, a major source of independent health-care business intelligence. The 100 Top Hospitals award is based on three measures of hospital performance: clinical excellence, operating efficiency and financial health, and responsiveness to the community.

Beginning as an outpatient surgical center, DHR today is a huge, 90-acre medical complex--an east and west campus on both sides of a major highway in downtown Edinburg. The complex includes seven different medical centers (a total of one million sq ft); three central cooling plants; and a 506-bed, full-service care facility with a medical staff of over 500 physicians. DHR today provides a broad spectrum of medical and surgical services, including intensive care, obstetrics, day surgery, skilled nursing care, outpatient diagnostic services, cardiology services, bariatric services, oncology services, and behavioral and emergency services.

Sidebar: The Women's Hospital at DHR has been selected one of America's Top 100 Hospitals by Thomson Reuters

The facility provides a full-range of services, including neonatal. Many of the areas are heavily computerized, and they all require reliable power and cooling.

Integrating two facilities was a daunting task, but efforts were rewarded when the system continued to function after two unexpected events.

Generac's Bi-Fuel system allows hospital administrators to rely on extended run times.

The Central Plant for the Women’s Hospital is a 2,400-ton plant consisting of four 600-ton Carrier Centrifugal Chillers, primary and secondary pumping arrangement, and three 800-ton Marley Quad-Flow Cooling Towers. The total cooling load on the building is 680 tons.