Recuperation Makes for Efficient Cogeneration
A new range of microturbines using recuperation technology has been developed. These turbines are ideal for cogeneration systems and other industrial applications. A key feature is the use of recuperation to yield high efficiencies at modest temperatures and pressure ratios.
James H. Watts
Northern Research and Engineering Corporation
Commercial cogeneration systems below 500 kW typically employ reciprocating engines, which have long been a limiting factor in the development of a strong cogeneration market. Relative to gas turbine engines, piston engines incur high installation costs and demonstrate a much higher frequency of maintenance. This has been a serious impediment in small engine markets, which demand appliance-like reliability and infrequent service intervals.
A significant development programme is now underway by NREC, a wholly-owned subsidiary of the Ingersoll-Rand Company, in alliance with the Southern California Gas Group, The New York Gas Group, and the Gas Research Institute.
The result is a new line of small, low NOx, recuperated gas turbine products known as PowerWorks. They include complete packages for cogeneration and air compression, plus modules to power vapour compression chillers and other types of refrigeration.
The development of small gas turbines in this power range for stationary applications has been underway since at least 1980. The Gas Research Institute (GRI) sponsored a programme to develop a small gas turbine in a 50 kWe cogeneration package called the Advanced Energy System (AES).
By 1990 however, GRI had dropped the AES programme citing serious product cost problems, the lack of a technical service infrastructure to support the sophisticated aerospace product, and a significant capital cost impediment. GRI then began funding a much more compelling engine concept that would provide a reliable, low maintenance gas turbine system that could be permitted in stationary applications. SoCal Gas later joined GRI and the result of this programme was the PowerWorks product.
A critical component within each product is NREC`s patent-pending recuperator technology that recovers turbine exhaust heat and uses the energy to preheat the engine`s combustion air. The recuperated gas turbine cycle yields exceptionally high engine efficiencies at modest temperatures and pressure ratios.
Originally NREC planned to find and adapt an existing recuperator to work with the PowerWorks engine. Considering all of the recuperators currently available at that time, NREC bought and tested each manufacturer`s technology and design, but they all failed some significant criteria. In every instance, the PowerWorks specifications called for significant improvements in existing recuperator efficiency, durability, size, or cost. Since there was no practical way to overcome all of these design deficiencies in any single commercially available unit, NREC decided to develop its own improved recuperator technology.
In comparison to other designs, NREC`s recuperator has a much higher strain tolerance and a vastly superior thermal-cycle endurance limit. Tests have also validated comprehensive finite-element transient models as well as critical specifications for pressure loss, thermal effectiveness, and durability. An NREC recuperator recently completed a rigorous test programme based on military-mission profile requirements. The prototype met or exceeded all design parameters.
In addition to PowerWorks product applications, the recuperator can be sized in several ways to fit a large number of engines and to meet their particular performance requirements. Most other designs are limited to the niche application for which they were developed. The desire for new recuperator technology from both gas turbine engine manufacturers and users has predominantly been driven by the need for greater durability and lower maintenance, while others also want improved performance, or a smaller package.
The recuperator is well positioned to meet the cost and performance requirements of gas turbine engine manufacturers who need more power and greater efficiency to take advantage of emerging opportunities in distributed power generation. These distributed generation applications require a much higher level of power, efficiency, and durability than is typically provided by low cost 500 kW and smaller engines used as standby or peaking units.
The recuperator has the efficiency, reliability, durability, and affordable cost needed in the 1 MW to 5 MW and greater power ranges. Because the recuperator is well suited for distributed power generation, many manufacturers are now evaluating lower pressure-ratio recuperated engines of higher pressure intercooled and recuperated engines in this size range.
Each system uses a high reliability gas turbine engine as the prime mover and offers various options for heat recovery. The simple “ruggedized” turbocharger-based design offers reduced maintenance and lower operating costs compared to reciprocating engine-driven systems. By employing industrial components like turbochargers, NREC is able to offer cost-effective packages that are better adapted to the rigours of stationary applications than aerospace – or vehicle-derived microturbine designs.
“Unlike single-shaft design microturbines, all PowerWorks engines are designed exclusively for stationary mechanical drive applications,” says Peter Baldwin, president of NREC. Therefore the PowerWorks engine can directly drive a variety of shaft loads with or without a gearbox depending on the application. This can include electrical generators, screw compressors, or centrifugal devices such as compressors or pumps. The thermal load can also be adjusted depending on the degree of recuperation incorporated in the engine for a particular application. The system can thus support a variety of thermal applications such as providing domestic hot water, steam, or driving absorption cooling systems.
PowerWorks engines have been designed from scratch to provide the ruggedness and high performance required in the tough environment of the plant floor or utility room. The systems are designed to achieve 80 000 hours of life, achieved through:
The use of turbocharger components featuring pressurized lube-oil systems consistent with industrial best practice
– A two-turbine configuration which reduces stress by splitting work output between a “gasifying” turbine and a free power turbine
– A conservative 870 degreesC (1600 degreesF) turbine inlet temperature
– The use of breakthrough technology recuperators that overcome the durability and fatigue problems of conventional plate-fin designs with a unique core construction that provides exceptionally long life under even the harshest thermal transients.
The engine is also designed to offer lower operating costs than reciprocating engine-driven systems and lower noise levels. PowerWorks engines are designed for an 8000-hour Mean Time Between Forced Outage (MTBFO).
The PowerWorks packages are equipped with patented lean premix, dry low-NOx combustion systems that meet southern California`s stringent emissions standards (SCAQMD). NREC`s advanced combustion technology for recuperated gas turbine engines reduces NOx emissions to below 9 ppmv at all operating conditions. This is a remarkably low value, even for modern gas turbine engines employing selective catalytic reduction systems or water injection.
In a cogeneration system, the free power turbine of the engine drives a low speed induction generator to produce electricity. Synchronous generation models will be offered later for off-grid operation and power factor correction.
Ranging in size from 70 to 250 kW with electrical efficiencies of more than 30 per cent HHV, overall cogeneration efficiencies of more than 80 per cent HHV are possible. Waste heat, available at nominally 400 degreesF (204à‚°C), can be used to generate either steam or hot water.
The waste heat recovery subsystem is completely packaged within a sound attenuating enclosure and requires no dump radiator to maintain operation through extended periods when no thermal load is present. The complete package is designed for quick and efficient installation in a wide range of commercial and industrial environments.
In a chiller system, the power turbine directly drives a vapour-cycle compressor for standard air conditioning applications. Matching between power turbine and the custom HFC-134a compressor section provides good part-load efficiencies. The chiller systems are designed to deliver between 100 and 400 RT of chilled water and yield typical Indicated part load value (IPLV) coefficient of performances (COP) approaching 2 (HHV) under ARI conditions down to 35 per cent load.
The high-speed power turbine drive results in a compact and therefore economical compressor impeller. The refrigerant impeller for the 100 RT chiller is only 3.9 inches (99 mm) in diameter and 7.1 inches (180 mm) for the 400 RT chiller. As shown in Figure 6, the chiller configuration uses the same heat exchangers and other basic components of a conventional chiller system. The gas turbine engine and refrigerant compressor stage fit in the same space that would be reserved for a conventional motor and compressor.
In the longer term, the ruggedness and efficiency of the engine has created new hybrid opportunities for advanced energy systems. The PowerWorks engine has been integrated with a dish solar collector to create a hybrid system. Both a combustor and the solar collector can be used to provide the heat needed to power the basic engine. The combustor is only fired when insufficient solar energy is available, such as at night or on cloudy days.
Similarly, a hybrid system that combines a gas turbine engine and a fuel cell boosts overall efficiency and is expected to be a clean, highly efficient, and reliable way to produce electricity.
Fuel cells represent very clean and efficient power-generation technologies. However, the high first-cost and operational problems associated with fuel cells have limited commercial and industrial applications.
But the many benefits of fuel cells, and especially hybrid gas-turbine/fuel-cell systems, have recently driven the technology toward potentially more practical and cost-effective solutions that will help to develop new markets.
In a typical hybrid gas-turbine/fuel-cell power plant, the fuel cell would contribute about 75 per cent of the electric output, while the gas turbine provides the remaining 25 per cent. Overall, the combined efficiency is greater than either system alone and can be as high as 65 per cent. Many issues are still to be resolved with this intriguing approach however, including size versus yield, longevity, and integration.
NREC currently has built five engines in different configurations reflecting the various applications. These systems are undergoing endurance tests in NREC`s facilities. Together these engines have operated for thousands of hours under a variety of loads and reached tested performance levels very close to product targets.
After four generations of development, NREC anticipates selling commercial production units in the year 2000. Field testing will begin in 1999.
Figure 1. A 70 kWe PowerWorks microturbine
Figure 2. PowerWorks application matrix
Figure 3. Impact of recuperation on simple cycle gas turbine engines
Figure 4. Cogeneration package cycle diagram
Figure 5. Chiller system cycle diagram
Figure 6. Chiller system packaging