Advanced microturbine technology with an intercooled and recuperated (ICR) cycle can deliver efficiency and low emissions at a competitive price. Now in the testing stage, ICR microturbines will be the foundation for the next generation of clean and efficient distributed generation, writes David Dewis.
|ICR350 – 350 kWe NGMT (generator omitted)|
Microturbines are clean, quiet, simple and reliable. They have service intervals of around 8000 hours and few moving parts. Reciprocating engines are more efficient and cost less but have higher emissions. Despite their complexity and higher part count, the ubiquity of reciprocating engines distorts perception, and microturbines are considered more complex. Reality, however, does have a say, and reciprocating engines, with their many sliding surfaces, intermittent combustion and exhaust-contaminated oil require frequent service that is only mitigated with an extensive service infrastructure.
Historically, microturbines have employed the recuperated Brayton cycle, selected for its ability to deliver reasonable efficiency at modest turbine inlet temperatures (TIT). The efficiency gains are realised by transferring energy from the waste exhaust gases to the combustor inlet air, thereby reducing fuel consumption to achieve a given TIT. Using common materials, peak efficiency is attained at a pressure ratio of 4:1. Implemented correctly, recuperation cuts fuel consumption in half. Unfortunately, these low pressure ratio turbines suffer from poor specific power (kW/mass flow of air), increasing not only the size, but also weight and cost.
The ICR350 overcomes these impediments by adopting an intercooled and recuperated cycle (ICR). The cycle is augmented with a variable-area turbine nozzle and an elevated firing temperature. This is a well-known configuration for larger turbines, where power density and high turndown efficiency are desired. The Rolls Royce WR-21, a 25.2 MW aero-derivative gas turbine used in Type 45 destroyers, is the most prominent example of an ICR turbine. In this application a 30% reduction in fuel consumption is achieved compared to alternative gas turbine arrangements.
|A fully instrumented ICR350 undergoing testing to verify power, efficiency and emissions|
ICR turbines achieve meaningful recuperation at high compressor pressure ratios by rejecting heat between stages. Final exit temperature is therefore dependent on the temperature rise associated with the pressure ratio of the last stage only. This resulting higher pressure increases available turbine expansion and its associated temperature drop, allowing a higher firing temperature for a given recuperator material. Efficiency and low emissions are maintained over a wide operating range by using a variable area turbine nozzle, placed upstream of the power turbine, to control mass flow.
Microturbines emerged from the US Department of Energy’s (DOE) support for distributed generation and the economics of on-site power with waste heat recovery, also known as cogeneration or combined heat and power (CHP). While progress has been significant, microturbines have been hampered by comparatively high cost and low electrical efficiency. In order to be successful and grow, the microturbine industry demands innovation, building on the advantages of microturbines and solving the cost and efficiency conundrum. Lower cost and improved efficiency are imperative if microturbines are to claim market share from incumbent technologies. Microturbines are simpler, cleaner, quieter and require less maintenance, but that is only relevant when price and efficiency compare favourably, supporting a reasonable return on investment.
Microturbines have benefitted greatly from reliability improvements since their first introduction, and mean times between failures (MTBF) greater than 16,000 hours are not uncommon. Likewise, improved aerodynamics, build process refinements and reduced turbomachinery losses have increased electrical efficiency. Manufacturing cost reductions are also occurring. Increased volume, design simplification and new manufacturing processes are all contributing. While these efforts incrementally advance the state of the art, a major shift in both cost and efficiency is needed to join the renaissance that gas engines are exhibiting, buoyed by the recent shale gas boom.
The ICR350 realises transformative cost reductions by utilising producible turbocharger-like modules in an ICR configuration optimised for both cost and efficiency. Turbochargers are mass-produced and benefit from low-cost automotive manufacturing practices. In addition, the more expensive HP hot-end components are reduced in size by the lower volumetric flow rates associated with the increased pressure. The resulting cost and weight reductions are well beyond those achievable on microturbines that rely solely on recuperation.
Parameters for several engine configurations are tabulated in Table 1. A market analysis performed under the Advanced Microturbine Systems programme indicates that the ICR350’s efficiency improvement doubles its market size. Cost and efficiency improvements will therefore appreciably advance the market potential for microturbines, leading to higher sales volume which in turn leads to a benefit from economies of scale.
How it works
The ICR350 is characterised by its patented close-coupled, transverse axis construction and recuperator integrated combustor. The construction deviates from the typical concentric shaft arrangement, but offers the most efficient coupling of turbochargers and turbo-expanders in series. The twin turbocharger modules for the gasifier and the turbo-expander for power take-off provide two stages of compression and three stages of expansion. While conventional microturbines operate at a pressure ratio of about 4:1, the two compressor stages produce a pressure ratio of 15:1 at maximum power.
|Figure 1: ICR350 schematic diagram|
Another differentiator, and the main contributor to the ICR350’s superior power density, is the high firing temperature, enabled by the use of ceramic HP turbine gas path components. These components have been carefully designed and developed in collaboration with Oak Ridge National Labs and Kyocera. Particular attention was given to the turbine wheel, which has been thoroughly characterised for the duty cycle and designed for robustness using rules employed on Kyocera’s production turbocharger products. Unlike ceramic wheels of the past, which had features similar to their metallic counterparts, it has short blades, rugged blade geometry and operates at a very low tip speed.
The ICR350 is best described as a turbocharged microturbine with intercooling and a free power turbine. Its increased pressure ratio and high firing temperature combine to yield significant improvement in specific power, reducing the size of all components. The recuperator realises one of the largest benefits, and for comparable power levels it is nominally one-fifth the size and weight of those employed in other microturbines. Despite its elevated TIT, the gas temperature entering the recuperator is below that of other microturbines — a benefit of the high expansion ratio, and one that allows the use of low-cost alloys.
The state of the art
The engine has been in development for four years, having been subjected to several iterations of rigorous design and analysis, with several resulting from changes to power rating while others optimised based on operational profile. The initial hardware was fabricated from commercially available turbochargers. Although heavily modified, these parts were sufficiently robust to withstand the component and module tests used for design verification and validation of analytical tools and models.
Second-phase testing focused on the gas producer and power take-off modules. Tested independently, they offer a greater level of control, aiding in the full mapping of the turbomachinery. For the gasifier module, the power turbine module was replicated by backpressure using a valve in the exhaust duct. The power module was tested using vitiated air, supplied using a compressor combined with a duct burner. The data from these component and module tests resulted in modifications and improvements to the hardware, which is now undergoing testing to verify power, efficiency and emissions.
The next steps
While in-house testing does much to eliminate infant mortality, real reliability growth begins with field trials. These trials allow for product hardening, a critical intermediate step before production release. Other engine programmes have proven time and again that new applications result in different failure mechanisms that harm MTBF. Field trials provide the environment to thoroughly vet the product and, by careful attention to detail, improve application reliability.
|Economies of scale can be realised faster with less capital intensity by focusing only on the engine|
The first installation is for Encana Corporation’s gas processing facility in Grand Prairie, Alberta, Canada, and will host two engines. One will run continuously and the other intermittently. Side-by-side testing minimises extraneous variables, offering significant advantages when performing failure investigations. More demanding test sites will be commissioned after achieving the desired reliability. These other sites will be selected based on market size, geographic region and potential for rapid penetration.
Unlike other microturbine companies, ICRTec is focused on the engine only, our core competency. Established companies with proven abilities and good customer relations are responsible for sales, packaging and service. By segmenting the business in this manner, economies of scale can be realised faster with less capital intensity, allowing controlled growth and the potential for rapid production ramping.
David Dewis is senior vice president of power systems at ICRTec