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CHP supplies the world's largest medical centre

Burns & McDonnell have commissioned a new gas-fuelled CHP system to expand the central utility plant for the Texas Medical Center in Houston, Texas. The new CHP plant adds significant efficiency and security advantages too - as Steve Swinson and Ed Mardiat explain.

On 23 August, 2010, demand on the Texas electricity grid hit an all-time record of nearly 66,000 MW. Electricity prices soared to approximately US$2,200 per MWh, compared to the $50 per MWh average, but Thermal Energy Corp. (TECO) - which provides heating and cooling to institutions in the Texas Medical Center in Houston - was able to avoid extreme price peaks because its new combined heat and power (CHP) plant is now on line.

When the final phase of the master plan implementation is completed, TECO's district energy plant will provide 100 MW of on-site power generation, 160,000 tons (145,000 tonnes) of chilled water, 152,000 ton-hours of chilled water storage and 940,000 pounds/hour (426,000 kg/hour) of steam to the world's largest medical centre, the Texas Medical Center.

The strain on Texas's electrical infrastructure will continue to increase at a rate of more than 2% per year, as the Electric Reliability Council of Texas (ERCOT) predicts an average 10-year load growth of 22.3% from 2009 to 2019, with total system peak demand increasing by 22% over the next decade.

EFFICIENT EXPANSION

In 1978, the Texas Medical Center Heating and Cooling Services Co-operative Association acquired the Texas Medical Center plant, built by the Houston Natural Gas Corporation in 1969.

That association evolved into what is now TECO, formed to realize more cost-effective and energy-efficient ways of supplying steam and chilled water to the Texas Medical Center.

In response to the medical centre's rapid growth, TECO developed a central utility plant master plan in 2006. In 2008, TECO turned to Burns & McDonnell - an engineering, architecture, construction, environmental and consulting solutions firm headquartered in Kansas City, with a regional office in Houston - to validate the master plan's technology selection, life cycle cost and constructability.

Burns & McDonnell reco-mended energy efficient improvements such as the addition of an on-site CHP system to allow TECO to serve the growing utility needs of the Texas Medical Center in a more efficient and environmentally friendly manner.

The CHP system doubles the central plant's operating efficiency to 80% and significantly reduces greenhouse gas emissions and regional air pollutants - decreasing nitrogen oxides (NOx) by 302 tons, carbon dioxide (CO2) by 305,000 tons, and sulphur dioxide (SO2) by 100%.

The CHP system also improves the security, reliability and emergency operation capacity of the utility infrastructure serving the world's largest medical centre. Over the next 15 years this energy efficiency is expected to save TECO nearly $200 million.

In addition to providing the engineering, procure-ment and construction management of the CHP system, Burns & McDonnell also assisted TECO in securing one of nine American Recovery and Reinvestment Act grants from the US Department of Energy for implementing energy-efficient CHP and district energy technology.

This grant, coupled with nearly $30 million in equipment and subcontract savings realized from the current competitive bidding environment, made CHP an obvious choice.

Click to Enlarge
The expansion of TECO's district energy plant features a CHP system, which greatly increases operating efficiency, significantly reduces greenhouse gases and improves the security, reliability and emergency operation capacity - saving nearly $200 million in utility costs over 15 years.

Because of the ongoing medical care and research conducted at the Texas Medical Center, it is essential that the TECO facility stays in continuous operation.

Although the facility is interconnected to the Houston area grid, it also features the ability to operate in island mode if a disaster interrupts power service to the surrounding region. The addition of the CHP system also dramatically increases TECO's capacity when the facility is operating in island mode.

HOW IT WORKS

Natural gas fuels TECO's CHP system to produce both electrical and thermal energy on-site more efficiently than in a traditional central power delivery system.

In a central power delivery system, only a third of every BTU, or unit of energy, consumed, is converted into electrical energy, with the rest going up the stack or cooling tower. By contrast, the on-site CHP energy delivery system converts three-quarters of every BTU consumed either into useful energy within the TECO district energy plant or into thermal energy for the medical centre.

In this district energy plant, a GE LM6000 PD Sprint aeroderivative gas turbine generates electricity with dry-low emissions through the same energy-efficiency, higher-reliability and reduced-emission technology that powers some of today's aircraft.

The hot gas turbine exhaust passes through a heat recovery steam generator, which recovers heat via conductive heat transfer to produce steam.

Chilled water is supplied to the turbine inlet air-chilling coils for increased power production during hot, ambient conditions. The chilled water return from the coils is routed to ancillary CHP equipment for cooling, and the condensate is recovered and used as makeup water for a nearby cooling tower, thereby reducing the site's water demand.

The heat recovery steam generator is a two-pressure steam generator fitted with natural gas-fired burners to add up to 220 million British thermal units (BTU) per hour of additional steam production capacity. Feedwater flows through the exisiting central plant boiler feedwater pumps to the first economizers, where it is heated. A portion of the feedwater then supplies the low-pressure evaporator, and the remainder of the feedwater is fed to the second economizer, which heats the feedwater to near saturation before it enters the high-pressure evaporator. A superheater heats the high-pressure steam to the final, desired temperature.

The steam header pressure produces low-pressure steam, at 250 pounds per square inch (psig) (17.2 barg), and provides saturated steam to the Texas Medical Center. The high-pressure steam, at 400 psig, travels through the existing steam-driven equipment supply headers and conditioning stations.

The HRSG includes a selective catalytic reduction (SCR) system that cuts NOx emissions by 302 tonnes per year and has the capability to install a future carbon monoxide (CO) catalyst for reducing CO emissions. The SCR is sized to meet permitted emission levels at peak fired conditions. Burns & McDonnell incorporated aqueous ammonia equip-ment into the design to serve the SCR system. Aqueous ammonia at 19% ammonia concentration is stored in a 10,000-gallon (37,854-litre) tank with a truck unloading skid and two 100% aqueous ammonia transfer pumps.

OPERATIONAL CONSIDERATIONS

Due to TECO's large power demand and critical role in serving the Texas Medical Center, the district energy plant is served by a customer-owned 138 kV ring bus with two separate utility grid connections. Prior to the installation of the turbine generator, the ring had five positions, and the two utility feeds were separated by a common breaker. When two additional positions were added to the ring, the team took the opportunity to improve reliability by moving one of two existing utility connections, so a single breaker failure would not interrupt both utility connections.

Thanks to the existing redundancy within TECO's facility, Burns & McDonnell completed all of these modifications in a single winter without limiting TECO's capacity to serve the Texas Medical Center's heating and cooling needs.

Once the new chilled water building is completed in April 2011, the turbine will be directly connected to its 13.8 kV distribution switchgear. Eventually, the central plant's demand will far exceed the output capacity of the turbine, so power can be directly consumed by TECO without transformer losses. Power generated in excess of TECO's demand will be exported to the 138 kV ring bus through the facility's two 138 kV/ 13.8 kV transformers to be sold on the open power market.

Click to Enlarge
The addition of an on-site CHP system allows TECO to serve the growing utility needs of the Texas Medical Center in a more efficient and environmentally friendly manner.

With a competitive heat rate of 5000 to 8500 BTU per kW, this CHP system's efficiency is comparable to national and ERCOT generation, so when electricity demand increases its cost, TECO can seamlessly increase its production and sell any excess electricity that it has generated.

To meet TECO's varying seasonal demand for electricity, the district energy plant includes two service transformers with automatic load tap changers that operate in parallel. These transformers automatically adjust tap settings to share the VARs. When the turbine is operating in parallel with these transformers, the tap changers are disabled so that the turbine can control the 13.8 kV bus voltage. Operating on a fixed tap setting with this electrical demand profile would result in a swing of more than 10% in voltage at the 5 kV and lower buses in addition to the variance in the utility voltage.

The on-site CHP system can serve as the primary source of power for TECO during an electricity outage. Should this occur, the 138 kV ring bus will be configured to island the site from both of the utility sources. While islanded, TECO would also operate backup generators in parallel to the gas turbine via the 138 kV ring bus in order to maximize production of steam and chilled water.

The combined capacity of backup generators on-site is about one-third of that of the CHP system. Due to this ratio, when islanded the CHP system's turbine will operate in isochronous mode with the remaining generators in droop mode. Reconnection to the electric grid is accomplished at the 138 kV ring bus by synchronizing either of the two utility wing breakers.

PERFORMANCE

In the district energy plant's first few months of operation, the CHP system's peak demand was 48.1 MW of electrical energy and 129,000 pph of steam during the summer months.

The Texas Medical Center's current demand for steam runs at 100,000 pph during the summer and at 250,000 pph during the winter months. The existing steam-driven chilled water production equipment can consume up to 100,000 pph.

From its conception, TECO has been focused on the economic and reliable generation of utilities to support the life-saving work performed at the Texas Medical Center.

With Burns & McDonnell's assistance, TECO has been able to look at the efficiencies of not only the individual systems, but also the entire site as a whole. This change in focus has allowed TECO to recognize the benefits in terms of energy, cost and the environment that CHP can provide when it is correctly implemented. TECO's new on-site CHP system allows TECO to produce the utilities critical to the Texas Medical Center more efficiently and with less impact on the environment both now and well into the future.


Steve Swinson is president and CEO of TECO in Houston, Texas, US. Ed Mardiat is a principal in the OnSite Energy & Power Group at Burns & McDonnell in Kansas City, Missouri, US. E-mail: emardiat@burnsmcd.com

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