“Recovering thermal energy from turbine exhaust to directly fire an absorption chiller could revolutionize onsite energy production,” says Ron Fiskum, technical manager for the DOE’s Office of Distributed Energy. “This is the largest system in the world to use exhaust heat from a natural gas–fueled generator as the only fuel source for a chiller that provides air conditioning.” The DOE and Oak Ridge National Laboratory teamed with Austin Energy, a municipal utility, to engage Burns & McDonnell, who developed, installed, and is testing a modular distributed energy system at the Domain in Austin, TX, a multi-use complex that includes retail, residential, and industrial space.

Recycling waste heat to produce steam that drives a generator or runs a chiller is not new. What is new is a chiller of this size that is fueled by waste heat alone—without any supplemental fuel. By design, the full thermal output of the 4.5-MW natural gas–powered Solar Turbines combustion turbine that closely matches the capacity of the 2,500-ton Broad chiller (see Figure 1). Beginning in fall 2004, testing of this integrated energy system (IES) has verified fuel efficiency of over 80%, based on the higher heating value (HHV) of natural gas. Compare this combined electrical and thermal efficiency to a nationwide average efficiency of 32% for electricity generated by central power plant technology and it is easy to understand why this technology is generating a lot of excitement. Ed Mardiat, Burns & McDonnell’s director of CHP development, says, “Because of the advancements made while developing and implementing this project, the design team received an Engineering Excellence Award from the Texas Council of Engineering Companies.”

Broad Absorption Chiller installed at the Domain in Austin, TX.

System Simplifies Installation
As well as being highly efficient, the IES incorporates a modular design. “The modular design is adaptable to various capacity, space, and grid interconnection requirements. By thoughtfully considering equipment, electrical, and piping layouts, the system was not only easier to construct, it will be easier to replicate,” says Tom Pierson, president of Turbine Air Systems Ltd. The modular system design allows the major components to be pre-manufactured and delivered to the site on skids, resulting in a reduced overall construction cost and schedule.

Using lessons learned from the first project, the DOE—through ORNL and Austin Energy—have again teamed with Burns & McDonnell to build another IES to meet the needs of the new Dell Children’s Hospital which is under construction in Austin, TX. This second system is configured with the state-of-the-art 4.3-MW Solar Turbines Mercury 50 recuperated turbine coupled to a heat recovery steam generator and a 1,000-ton two-stage Trane Horizon absorption chiller (see artist’s rendition). Onsite power will supply a micro-grid to assure power availability even if the grid becomes unreliable.

“The combustion turbine and heat recovery absorption chiller modules came almost completely assembled from the manufacturers,” explains Rod Schwass, a program manager at Burns & McDonnell. The natural gas–fired turbine generates electricity that can be used onsite and/or exported for use by the local utility or regional transmission authority. “With an aggregate electrical load of over 15 MW at the project site, the turbine is routinely base-loaded,” says Cliff Braddock, Austin Energy’s director of energy business development.

Condenser water pump and control enclosuare module.

Diverter valve with bypass stack and duct to absorption chiller.

The advanced Broad double-effect heat recovery absorption chiller uses two stages of internal heat recovery to improve efficiency and can be configured to produce hot and chilled water simultaneously. The combustion inlet cooling module is also packaged and uses a portion of the chilled water (200 to 300 tons) to cool the combustion turbine inlet air to improve electric generator output during high ambient temperature days and to improve overall system efficiency. The remaining chilled water output serves the site’s cooling load by supplying a district chilled-water system. The chiller exhaust stack is a separate module and also packaged on a separate skid.

Schwass further describes the design, “A diverter valve and a stack are mounted on a pre-fabricated skid between the turbine and the chiller to modulate the flow of exhaust through the chiller, controlling the amount of chilled water produced by the system. The diverter valve may be completely shut, diverting all exhaust out through the stack during periods when the owner/operator desires to run the turbine only. The natural gas compressor unit is also pre-fabricated on a packaged module. At sites where high pressure natural gas is available, it would be possible to further simplify this modular design by eliminating the natural gas compressor skid.”

Solar Turbines natural gas turbine generator installed at the Domain.

Output-Based Emission Standards Demonstrated
Currently, California and Texas are the only US states that have NOx emission standards for onsite, distributed generation units. The Texas Commission on Environmental Quality developed a standard permit with output-based standards to expedite the permit approval process for onsite generation. Output-based emission standards allow the system owner to take credit for use of the thermal energy produced by a distributed energy unit. When this thermal energy is used to perform useful work—such as absorption chilling, hot water heating, steam production, or desiccant-drying—it offsets the use of centrally generated electricity, resulting in reduced central plant emissions.

The current east Texas standard for distributed generation units operating more than 300-h/yr and in operation prior to January 2005, such as the Domain IES is 0.47-lb NOx/MWh (the standard for units that began operation after January 2005 is 0.14 lb NOx/MWh). The emission rate for gas turbine electricity generation averages 0.67-lb NOx/MWh without a catalyst. The calculation method takes credit for electric and recovered thermal energy to result in a permitted emission rate of 0.24 lb NOx/MWh. The emission credit for this innovative IES is an important method for recognizing the value of highly efficient use of clean natural gas fuel.

Preliminary Performance Assessment
“The project’s data collection methods reflect the long-term monitoring protocol recently promulgated by the Association of State Energy Research and Technology Transfer Institutions (ASERTTI),” explains project manager Jim Teigen of Burns & McDonnell. The gross efficiency of the IES (without ancillary loads) was calculated on the basis of data collected and plotted against both turbine and chiller output while the IES was tested under part-load conditions (see Figure 2).

The mathematical correlation between electric and chiller output versus total system efficiency is represented by R2 values with a value of “one” being a perfect correlation. Gross IES efficiency correlates well with chilled water output (see sidebar).

During full-load conditions, when the IES is producing chilled water near its rated capacity, overall system efficiency is in excess of 80%. As an example of how the chiller output drives gross efficiency, data taken on March 23, 2005, at 4:36 p.m. was investigated. The energy content of the natural gas input and inlet cooling for the turbine was 54.5 MMBtu/h HHV, the turbine output was 4,342 kW, and the chiller output was 2,638 tons, resulting in a gross efficiency of 85.3% HHV (or 94.6% based on the lower heating value of natural gas.)

IES Promotes Successful Onsite CHP Projects
The success of these two packaged CHP energy plants is a strong indicator that, with lower initial costs and much higher efficiencies, a gas-powered generation system can still be competitive even when using more expensive fuel. Braddock indicates that this new approach to onsite generation may be competitive even when compared to conventional power plant technology. “If you convert all of the plant’s energy outputs into kWh’s and look at its heat rate, you will discover that these plants are capable of heat rates near 5,000 BTU/kWh, a rate below the best combined cycle power plants. And when you look at the financial analyses, you will find that thermal products provide more economic value than the electricity” according to Braddock.

Proponents of CHP—onsite generation that recycles thermal energy—are very proud that the Texas Council of Engineering Companies has selected this project for special recognition. By replicating such projects, all of us will enjoy a positive effect on our environment as well as an enhancement of our energy security. The challenge moving forward is to use lessons learned from these projects in communicating the possible benefits for their buildings of packaging similar systems to architects and building owners.

JAN BERRY, Oak Ridge National Laboratory, is a member of Distributed Energy’s Editorial Advisory Board.

DE - July/August 2005