The city of Austin has a vision to become the clean energy capital of the world. Austin Mayor Will Wynn and Austin Energy General Manager Juan Garza are changing the manner in which power is provided to this vibrant, growing city. The latest expression of this vision will provide clean, efficient energy for a massive hospital complex.

Austin Energy (Austin, Texas, U.S.) and Seton Healthcare Network (Austin) forged a business alliance that culminated in an on-site combined heat and power (CHP) energy system serving the new Dell Children's Medical Center in Austin. The US$18 million on-site hybrid CHP plant has been operational since April 2006, and generates more than 100% of the hospital's energy requirements. The excess electricity is exported to the utility grid, and the excess thermal energy (chilled water cooling and steam heating) is distributed through a network of underground pipes to neighboring buildings.

The CHP system is the backbone of a district energy system serving the redevelopment area with the hospital at its core. By aggregating loads of nearby customers, Austin Energy can optimize the plant efficiencies and economies of scale. Two independent electrical feeders provide redundant backup for the on-site generation. This is one of the first hospitals in the state of Texas to feature the capability to use its on-site energy system as a primary source for electricity and use the grid as a backup.

THE ENERGY CENTER

Austin Energy selected the engineering firm Burns & McDonnell (Kansas City, Missouri, U.S.) to provide the equipment and turnkey installation of the energy center. The team selected a Solar Turbines Mercury 50, a recuperated natural gas combustion turbine, as its prime mover because of its competitive simple cycle heat rate and low emissions. The turbine produces up to 4.6 MW of on-site generation at a simple cycle heat-rate efficiency of 38%, while producing less than 5 ppm NOx emissions without any need for after-treatment.

The exhaust from the combustion turbine is ducted through a bypass diverter valve to a heat recovery steam generator (HRSG) that produces up to 13,000 lb (5900 kg) per hour of saturated steam. The HRSG produces enough steam for hospital process heating and a nominal 900 tons (3165 kW) of chilled water from a Trane two-stage absorption chiller. By recycling the exhaust heat into useful thermal energy, the overall CHP system efficiency can reach a remarkable 75%.

The overall system efficiency (or heat rate) is better than other traditional fossil-fuel power plants, including combined-cycle gas turbine power plants. This was important for Austin Energy in determining dispatch strategy for the Mercury 50 and for providing economic justification for owning and operating the energy plant.

The CHP system's guaranteed NOx emissions of 5 ppm without catalyst and low emissions (including CO2) exceed Texas Commission on Environmental Quality standards permitting threshold for CO2, NOx and SO2 for this area of Texas, which has been designated by the U.S. Environmental Protection Agency as being close to noncompliance to standards for air pollution.

In addition to the CHP system, the energy plant includes multiple pre-engineered modular components that create an integrated, highly efficient system. Those combined components include: a 1500-ton (5275 kW) electrical duplex centrifugal packaged chiller plant complete with cooling tower and condenser water pumps; a 20,000 lbs/hour (9072 kW-hour) natural-gas-fired stand-by packaged boiler; a deaerator, surge tank and blow-down separator; primary and secondary chilled water pumps; a 1500-kW emergency “black start” diesel generator; an 8000-ton-hour (28,134 kW-hour) chilled water thermal-energy-storage tank with packaged pump skid; and chemical treatment systems. The stand-by boiler and electric chillers enable both scheduled and unscheduled downtime of the prime mover components.

The new CHP energy center produces high-quality and reliable thermal energy (chilled water cooling and steam heating) to serve nearby buildings (six buildings on the cooling system as of 2007).

SPECIAL CONSIDERATIONS

The Medical Center energy plant operates in parallel with the grid under normal circumstances, but its ability to island will enable the hospital to seamlessly continue its healthcare services in the event of a natural disaster or other unexpected grid problems. Recent disasters have demonstrated that people — even healthy people — will seek sanctuary at hospitals during extended power outages, believing that hospitals will maintain the ability to function during such times.

Unlike a typical CHP facility serving an industrial client, the Dell Children's Medical Center's CHP system meets or exceeds regulatory requirements of the Texas Department of State Health Services for healthcare facilities to have one of their two sources for electrical services located on site to ensure life-safety power systems are never off line for more than 10 seconds. To assure compliance to this requirement, while also providing a means to “black start” the combustion turbine, Austin Energy installed a traditional diesel backup generator that could be used in the unlikely event that both utility grid connections and the combustion turbine are simultaneously tripped.

Because of the sensitive nature of the hospital's electronic equipment, the power source must be very stable. Although the CHP does not include energy storage for a true uninterruptible source, by keeping the combustion turbine on-line and tied to the utility grid, the loss of either the grid source or the turbine does not affect the hospital, thus providing the needed stability. This also allows Austin Energy the flexibility to export excess capacity from the turbine generator to other customers.

INTERCONNECT DESIGN

Burns & McDonnell engineers designed the CHP interconnects so they would not significantly increase the existing short-circuit current available at the existing substations that feed the facility. This limit could not be exceeded without risking connections to other customers or damaging distribution feeder conductors, substation transformers and safety relay settings. In addition to short-circuit current concerns, the engineers also had to minimize the line-to-ground fault currents within the CHP's electrical system. Given that Austin Energy's existing distribution system is solidly grounded, three winding, wye-delta-wye transformers were installed at the point of demarcation between the utility grid and the CHP's electrical system. This allowed the CHP to use resistive grounding to protect the on-site turbine generator while still being able to supply the solidly grounded utility distribution system.

These 12.47-kV transformers connect the energy center to two independent substations on the power distribution grid. Austin Energy's fault-current studies have demonstrated that the isolation transformers have reduced the available fault current on those lines, thereby assuring safety of the distribution feeders.

For the CHP plant to have grid interoperability, the project engineers designed the interface between the plant and the grid to eliminate excessive tripping of the on-site generator whenever the existing grid feeder voltage sags (or swells) as a result of weather, grid congestion or other issues on the Electric Reliability Council of Texas grid, which carries 85% of Texas' electricity load.

Another challenge for the project was to make the grid and the on-site energy system reliable and safe under varying loads, including both maximum and minimum demand periods from the hospital, as well as during times of maximum demand on the grid. Therefore, the Mercury 50 package features a synchronous generator that has variable power factor, automatic synchronization, voltage trip, current trip and ground fault detection. All of these steps protect the turbine generator as well as the CHP's distribution system to the hospital, but it does not detect events on the utility's side of the demarcation point.

To meet these needs, Burns & McDonnell CHP project engineers coordinated with Austin Energy's distribution engineers to determine the best way to interface the protection systems of the distribution network to the CHP. Their solution was to install Schweitzer Engineering Laboratories' (Pullman, Washington, U.S.) distribution feeder relays with a network interface module on each main service breaker. This allows the protective relays in the existing substations to directly communicate to the relays on the main breakers. During a network power event, the substation's system can disconnect the CHP from the grid and prevent it from reconnecting until the system is stable.

Using devices from the same manufacturer for the balance of Austin Energy's distribution system not only allowed for direct communication between the devices, but it also helped the utility's energy control center operators, who already understood the operation of the protection systems. During the summer of 2007, the CHP plant will operate continuously making chilled water, steam and power, thereby enabling Austin Energy to address optimization features of the plant. Over time, the CHP will benefit Austin Energy's grid due to improved power factor, feeder voltage support and alleviation of grid congestion.

A PLATINUM SHOWPIECE

The special features of the Dell Children's Medical Center and its on-site CHP system have made a statement regarding sustainability and protecting the environment. The high energy efficiencies and miniscule emissions, coupled with the hospital's use of environmentally friendly materials from around central Texas and across the state during construction, will likely result in this site being the world's first hospital to receive Platinum Status under the Leadership in Energy & Environmental Design program sponsored by the U.S. Green Building Council. In addition, the Greater Austin Chamber of Commerce's Keep Austin Beautiful program selected the CHP energy center to receive the 2007 award in the “industrial” category. This annual award recognizes noteworthy projects based on aesthetics and significant clean environment contributions.

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Clifton Braddock is director of Energy Business Development for Austin Energy, the city of Austin's municipally owned electric utility. A certified energy manager, Braddock is responsible for development of distributed generation and district energy projects that enable the electric utility to provide total energy services directly to its end-user customers. He is active in ASHRAE, Association of Energy Engineers, International District Energy Association and United States Combined Heating and Power Association. He holds a BSEE degree from Louisiana Tech University and a MBA degree from Louisiana State University.
cliff.braddock@austinenergy.com

Edward Mardiat, DBIA, principal, is the director of CHP Development. He has more than 25 years of design and project management experience, focusing his efforts over the past 12 years in the areas of marketing and project development of on-site energy projects. Mardiat helps industrial, commercial and institutional clients understand the effect of utility deregulation on their facilities, how to mitigate fuel pricing risk and how to maximize demand-side savings.
emardiat@burnsmcd.com

Eric Putnam is a registered professional engineer in 13 states and has been practicing consulting engineering for pharmaceutical, telecommunications and industrial clients for 13 years. Putnam's experience includes the design, startup and commissioning of low- and medium-voltage power systems throughout the country. In addition, he has designed and programmed the controls systems for power-generation systems and building control systems. Putnam holds a BSEE degree from the University of Missouri — Kansas City.
eputnam@burnsmcd.com

INTERCONNECTION GETS EASIER

PECO's outage management process begins with either a customer call, a last-gasp from a meter or a message from the Interactive Voice Response system. An Outage record is created and is sent to the OMS system. SCADA Events are sent directly to the OMS system. A dispatcher reviews the outage record and assigns it to an appropriate crew. When power is restored, the event is closed and validated with the power-up message from the meter.

The IEEE has recently completed its standard for connecting distributed resources with electric power systems (Standard 1547TM, 2003). Similarly, the U.S. Federal Energy Regulatory Commission and the Public Utility Regulatory Policies Act of 1978 have been updated to enable interconnection of distributed electric resources into the electric power system. The result is that current prevailing standards, rules and local utility interconnect criteria are opening the door for on-site energy systems, while assuring the safety of linemen, protecting the integrity of electrical distribution equipment and maximizing reliability of power-delivery systems.