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Cogeneration: Gas Turbine Multitasking OPEN ACCESS

[+] Author Notes
Lee S. Langston

Professor Emeritus of Engineering, University of Connecticut

Lee S. Langston is a former editor of the ASME Journal of Engineering for Gas Turbines and Power and has served on the IGTI Board of Directors as both Chair and Treasurer.

Mechanical Engineering 134(08), 50 (Aug 01, 2012) (1 page) doi:10.1115/1.2012-AUG-4

Abstract

This article describes the functioning of the gas turbine cogeneration power plant at the University of Connecticut (UConn) in Storrs. This 25-MW power plant serves the 18,000 students’ campus. It has been in operation since 2006 and is expected to save the University $180M in energy costs over its 40-year design life. The heart of the UConn cogeneration plant consists of three 7-MW Solar Taurus gas turbines burning natural gas, with fuel oil as a backup. These drive water-cooled generators to produce up to 20–24 MW of electrical power distributed throughout the campus. Gas turbine exhaust heat is used to generate up to 200,000 pounds per hour of steam in heat recovery steam generators (HRSGs). The HRSGs provide high-pressure steam to power a 4.6-MW steam turbine generator set for more electrical power and low-pressure steam for campus heating. The waste heat from the steam turbine contained in low-pressure turbine exhaust steam is combined with the HRSG low-pressure steam output for campus heating.

Here, at the University of Connecticut in Storrs we have a gas turbine powered 25MW cogeneration power plant that serves the 18,000 student campus. It has been in operation since 2006 and is expected to save the University $180M in energy costs over its forty-year design life.

In a conventional power plant about one-third of the input energy is converted to useful electrical power, while two-thirds is thrown away as waste heat to rivers, the sea or the atmosphere. Cogeneration – also called combined heat and power (CHP) – is the simultaneous conversion of chemical energy of a single fuel source (e.g. natural gas) to produce useful energy (e.g. electricity) and propitious heat (e.g. boil a liquid for heating/cooling). Cogeneration in the form of municipal power and district heating has been utilized in Europe for many years[1]. In more recent times, as its efficiencies and operating temperatures have risen through research and technology, the gas turbine has become the prime mover for cogeneration.

The heart of the UConn cogeneration plant consists of three 7 MW Solar Taurus gas turbines burning natural gas, with fuel oil as a backup. These drive water-cooled generators to produce up to 20-24 MW of electrical power distributed throughout the campus. All three have been run in a base load mode, since 2006. In 2011, each, with about 41,000 hours of reliable operation, were removed for refurbishment and replaced with rebuilt Taurus gas turbines.

Gas turbine exhaust heat (at about 900 deg F) is used to generate up to 200,000 pounds per hour of steam in heat recovery steam generators (HRSGs). The HRSGs provide high pressure steam (600 psi) to power a 4.6 MW steam turbine generator set for more electrical power, and low pressure steam (125 psi) for campus heating.

The waste heat from the steam turbine contained in low pressure turbine exhaust steam is combined with the HRSG low pressure steam output, for campus heating. Thus, careful waste heat engineering and management allows three energy usages (gas turbines, steam turbine and campus heating) for only one unit of gas turbine fuel.

During the warmer months, when heating is only needed for some campus kitchens and laboratories, the low pressure steam (from both the steam turbine and the HRSGs) is used to power low pressure steam turbines which drive three refrigeration compressors to supply up to 6300 refrigeration tons of chilled water. The chilled water is distributed to campus buildings for air conditioning. A small part of the chilled water output can also be used to cool hot day inlet air to the gas turbines, thereby maintaining high gas turbine efficiency (nominally 34% at 59 deg F inlet) and electrical power output. Thus careful waste heat management provides campus cooling and the maintenance of high electrical power needs during hot summer months.

The plant, described in more detail in [2], [3] and [4], cost $81M, a figure which included modifications to existing utility facilities. An estimate of its true cost is about $50M, or $2000/kW. It includes a class room which is used for instruction for not only engineering students, but also students from other university disciplines. Thus far, two engineering graduate students (one PhD [5], one Masters [6]) have carried out research on the plant's many interacting control systems.

The Second Law of Thermodynamics requires that a power plant must reject heat, no matter how efficient. Roof mounted cooling towers provide UConn's means of heat rejection, but use substantial amounts of the University's fresh water supply. Facilities are now being put in place to use the University's treated waste water (currently being pumped to the local Willimantic River) in the cooling towers.

UConn's cogeneration thermal efficiency is about 80%. A cogeneration plant's thermal efficiency (more accurately called an energy utilization factor) is calculated as the sum of the electrical power output and the useful heat produced divided by the fuel energy supplied. Other measures show that under a demand rate of 25 MW of electricity and 200,000 pounds/hr of steam, UConn's cogen plant uses only 52% of the fuel consumed by conventional non-cogeneration means. Careful utilization of gas turbine power plant waste heat yields a large payoff for the user, and for the environment.

References

Horlock, J.H., 1997, Cogeneration - Combined Heat and Power (CHP), Krieger.
Langston, Lee, 2006, “Campus Heat and Power,” Mechanical Engineering Magazine, December, pp. 28– 31.
“Conservative Design Assures Top Operational Flexibility, Reliability”, 2007, Combined Cycle Journal, Fourth Quarter, pp. 71–74.
“Cogeneration: Clean Power at UConn”, 2011, < http://www.youtube.com/watch?v=RSeSG7qQK-0>, December 4.
Howard, Rachelle R., 2009, ”Automated Autocorrelation Function Analysis for Detection, Diagnosis and Correction of Underperforming Controllers”, PhD Dissertation, University of Connecticut.
Burns, Joseph W., 2011, “Applied Control Strategies at a Cogeneration Plant”, Masters Thesis, University of Connecticut.
Copyright © 2012 by ASME
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References

Horlock, J.H., 1997, Cogeneration - Combined Heat and Power (CHP), Krieger.
Langston, Lee, 2006, “Campus Heat and Power,” Mechanical Engineering Magazine, December, pp. 28– 31.
“Conservative Design Assures Top Operational Flexibility, Reliability”, 2007, Combined Cycle Journal, Fourth Quarter, pp. 71–74.
“Cogeneration: Clean Power at UConn”, 2011, < http://www.youtube.com/watch?v=RSeSG7qQK-0>, December 4.
Howard, Rachelle R., 2009, ”Automated Autocorrelation Function Analysis for Detection, Diagnosis and Correction of Underperforming Controllers”, PhD Dissertation, University of Connecticut.
Burns, Joseph W., 2011, “Applied Control Strategies at a Cogeneration Plant”, Masters Thesis, University of Connecticut.

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