Gas Turbine Technology: Recuperator breakthrough

James H. Watts
IR Energy Systems, Portsmouth, NH, USA

Small to medium size (20 kW to 2 MW) gas turbine engines configured in simple cycles typically offer lower heating value (LHV) efficiencies that are very low: in the low 20s or even in the teens. Competing technologies produce electricity at much higher efficiencies than this. So to be economically justifiable, microturbines typically need to reach a benchmark level of around 30 per cent LHV.

Figure 1. IR has developed its own recuperator for the PowerWorks microturbine
Click here to enlarge image

Gas turbine designers today can call upon a wide variety of design alternatives to raise both engine and overall system efficiencies. However, microturbine designers face relatively stiff system complexity and cost constraints. System improvements available to larger engines such as combined cycles are too costly to include in a microturbine.

Microturbines cannot afford to use the latest generation of expensive materials capable of handling very high operating temperatures, and some advanced design techniques, such as integral cooling of turbine blades, are not practical for microturbines. Microturbine designers must minimize both first cost and maintenance costs over the full life cycle of the machine.

The efficiency of the turbomachinery components (compressors and turbines) in the engine of microturbines strongly affects engine efficiency and might be improved with better fluid dynamic design. However, such improvements would only be incremental and would represent a relatively small overall system gain as long as the turbomachinery components are well matched to the operating requirements of the microturbine in the first place.

Designers can also employ more aggressive engine operating conditions, like raising the temperature of the gases entering the turbine inlet or increasing the pressure ratio of the engine. Either of these can result in substantial system improvements but at a high cost to engine component life. Raising the temperature is especially detrimental due to the creep-life and stress constraints of the materials used in microturbines. Microturbine inlet temperatures in current designs vary between 1600à‚°F and 1850à‚°F (871à‚°C and 1000à‚°C).

Increasing the pressure ratio also has the negative effect of raising stresses in the system, again reducing engine life. An additional disadvantage is the need for higher fuel delivery pressures which then raises the power needed by the fuel gas booster (for example, required in at least half of the US natural gas service area). This would bring a noticeable parasitic loss to the system.

Critical impact

The most promising way to raise engine efficiency is to recover heat by using a regenerator or recuperator as part of the engine cycle. Indeed, a recuperated cycle has been adopted by all of the microturbine designers, although the degree of heat recovery varies considerably. Figures 3 and 4 summarize the performance impact in balancing turbine inlet temperature, pressure ratio, and recuperator effectiveness for a theoretical gas turbine design.

Notice the critical impact of recuperator effectiveness. The chart on the left (Figure 3) shows how a recuperator effectiveness of only 85 per cent would require a theoretical turbine inlet temperature of 871à‚°C and a pressure ratio of 3:1 to ensure an engine efficiency of 30 per cent LHV. Of course, a practical engine with realistic losses would not perform as well. Therefore, temperatures of 927à‚°C or even 982à‚°C are likely to be required.

However, with a 91 per cent effective recuperator, a much greater performance margin is available. A practical engine operating at 871à‚°C turbine inlet temperature and a pressure ratio of about 3:1 would likely suffice to reach the benchmark efficiency. An engine running under these more conservative conditions will enjoy a significantly longer life and reduced maintenance costs.

Figure 2. Impact of recuperation on simple cycle engines
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Unfortunately, many microturbine designs do not include a recuperator in this effectiveness range. Why? Recall that many microturbines have evolved from hybrid vehicle and aerospace applications. They were designed to be compact and relatively lightweight, taking advantage of the potential power/weight superiority of a gas turbine engine. Recuperators are relatively large and heavy components where more effectiveness means more surface area and thus more weight and volume. A designer thus must compromise recuperator effectiveness if engine weight/volume limits are to be met.

However, the commercial building and industrial market opportunities for today’s microturbines are stationary applications. Within reasonable limits, it does not matter how heavy the system is. Therefore, a microturbine designed specifically for stationary applications is free to incorporate a very effective recuperator.

However, recuperator life and cost represent another difficulty. Although simple in concept, the thermal gradients and thermal cycles associated with applying recuperators in a gas turbine application have proven to be very difficult to manage, and over the years the practical application of recuperators has been both limited and generally unsuccessful. Producing a recuperator that can reliably withstand these has generally meant high unit costs. Ingersoll-Rand (IR) has focused on this problem during the course of its microturbine development.

Breakthrough technology

Originally, IR planned to find and adapt an existing recuperator to work with the PowerWorks engine (see “Recuperation makes for efficient cogeneration”, PEi October 1998, Vol. 6 Issue 8, p30-33). Considering all of the recuperators currently available at that time, IR purchased and tested each manufacturer’s technology and design, but they all failed on some significant criteria. In every instance, the PowerWorks specifications called for major 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, IR decided to develop its own improved recuperator technology.

Figure 3. Efficiency vs specific power for a recuperated engine at 85 per cent effectiveness
Click here to enlarge image

Therefore, IR began work on the novel recuperator now used in the PowerWorks engine in the mid-1990s. The resulting breakthrough technology has been engineered with the design latitude of a conventional plate-fin heat exchanger and the ability to vary the size to meet performance requirements. To overcome the durability and fatigue problems of conventional plate-fin design, the unique construction of the PowerWorks recuperator core provides exceptionally long life under even the harshest thermal transients. Structurally, the design behaves more like a primary-surface type recuperator with its associated high durability but without its inherent deficiencies. The PowerWorks recuperator has good flexibility to fit a wider range of applications and does not suffer from ‘creep’ problems.

Figure 4. Efficiency vs specific power for a recuperated engine at 91 per cent effectiveness
Click here to enlarge image

The overall physical package offers a compact size and low part count. The folded fin surface configuration yields a high heat transfer rate and area to raise effectiveness. The internal structure is arranged to easily accommodate thermal strain and cycling and is quite durable. It offers a high cyclic life and creep rupture capability and is corrosion resistant. Preload requirements are low and the structure is geometrically stable under operating pressures over the design life.

The resulting low cost and exceptional durability are important enabling factors that allow the recuperator to be used in the PowerWorks microturbine. The design has been thoroughly tested for thousands of hours including extreme thermal cycling. For example, one endurance sequence for a military gas turbine engine programme subjected a test recuperator to 1500 hours of peak temperature testing at exaggerated differential temperatures well beyond those experienced in a PowerWorks microturbine. The test sequence included 500 rapid thermal cycles between operating temperatures that were again, well beyond those required for a PowerWorks system. At the conclusion of the test, the recuperator showed no measurable or visible degradation.

“Derived from military applications where performance requirements are very challenging, the IR recuperator technology is exceptional,” said Steven I. Freedman, an energy systems expert and former executive scientist at the Gas Research Institute (now the Gas Technology Institute). “Combining the recuperator with the other design features, Ingersoll-Rand has created a unique microturbine line that offers performance versatility and reliability that extends beyond simply generating electricity.”

Field tests

Continuing its microturbine development programme, Ingersoll-Rand Company is installing and operating several PowerWorks systems in diverse test sites in a field test programme that started in late 2000. These systems are providing electricity and heat to facilities, using the heat in a variety of applications. The field test sites represent a wide spectrum of commercial and industrial activities and are located throughout the USA.

Designed to help satisfy increasing power needs by producing electricity at the point of consumption, the PowerWorks microturbine comprises a versatile design that not only generates electricity for base consumption or peak shaving but also generates heat for other applications.

Figure 5. PowerWorks cycle diagram
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The PowerWorks’ industrial quality allows the microturbine to work 24/7 for long periods with low maintenance. This can quickly make the PowerWorks microturbine the least expensive electricity generating option over the life of the machine for a facility. PowerWorks is thus an excellent fit for continuous duty applications such as those found in cogeneration opportunities.

Useful heat is extracted from PowerWorks microturbines using a hot water heat exchanger fully integrated into its exhaust system. This built-in cogeneration ability allows overall system efficiencies to approach 80 per cent depending on the temperature of the inlet water.

For example, a community centre (skilled nursing facility) located in NY state is using an IR PowerWorks microturbine to generate hot water that provides most of the facility’s Domestic Hot Water (DHW) needs. The microturbine was installed as a complementary power source to help reduce the centre’s use of electricity from the public grid. Currently, the microturbine generates up to 70 kW of electricity from the PowerWorks unit and leverages the microturbine’s versatile, patented design to generate heat for the 60 000 square-foot (5600 m2) facility’s domestic hot water system.

Another microturbine provides grid-parallel electricity to a 90 000 square-foot (8400 m2) public ice skating facility containing two ice rinks (one NHL-sized for hockey and the other Olympic-sized). Heat is captured to provide hot water to regenerate a desiccant wheel. The desiccant system acts to dehumidify the facility, which improves ice quality and occupant comfort. In addition, the hot water is available for tasks such as ice melting, DHW, and for the ice-resurfacing machine.

Refining performance

Industrial facilities are also included in the field test programme. For example, a PowerWorks microturbine is providing power and heat for a refinery in Pennsylvania. The electrical power produced by the system helps reduce the base load of the facility. Heat captured from the microturbine exhaust preheats feed water for process boilers. This is an excellent cogeneration application since the overall system should achieve over 70 per cent efficiency.

Another microturbine is being installed in a greenhouse located in Colorado. The system provides baseload electricity for the facility. In addition, the microturbine heat helps preheat water that is used to water plants. Studies have shown that preheating ground water to temperatures around 60à‚°F to 70à‚°F (15à‚°C to 21à‚°C) has a significant benefit on plant growth as opposed to using the original cold water. The greenhouse uses at least 90 000 gallons (24 000 l) of ground water per day through a system of storage tanks and boilers. The PowerWorks microturbine makes a significant contribution in heating the water that correspondingly reduces the amount of natural gas used by the boilers.

PowerWorks microturbines connect directly to the electrical distribution system of these and other facilities in the field test programme to provide high quality electricity. The distinctive free power turbine configuration allows PowerWorks systems to use reliable rotating generators to produce electricity. This is the same technology used by utilities to produce power for the grid and it allows PowerWorks systems to similarly provide clean, reliable power.

Many commercial buildings and industrial facilities are not supplied by the high-pressure natural gas typically required by microturbines. Therefore, a fuel gas booster is required. PowerWorks systems can incorporate a fully integrated booster based on IR’s in-house screw compressor technology. Already used in thousands of critical industrial applications, this highly reliable, proven technology actually offers a design life well beyond the 80 000 hours targeted for PowerWorks microturbines.

Microturbines possess broad market potential as a cost-effective independent power source for facilities such as supermarkets, small industrial factories, hotels, schools, utilities, hospitals, office buildings and multifamily homes. Analysts foresee a $20 to $30 billion market for distributed power generating devices, which includes microturbine technologies.

PowerWorks microturbines were developed with the assistance of the Gas Research Institute, the Southern California Gas Company, and the New York Gas Group. PowerWorks microturbines are part of IR’s Independent Power Sector, which focuses on identifying, developing and marketing alternative power and energy management solutions.

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