Assessment of Recuperator Materials for Microturbines

Abstract

EXECUTIVE SUMMARYBecause of the changing structure of the electric power industry, new opportunities exist for small-scale power generation technologies in distributed-generation applications. Microturbines (gas turbines that operate in the range of 25 to 500 kW and at a compression ratio of 3 to 6) are prominent among these new technologies. Therefore, the U.S. Department of Energy's Office of Power Technologies (DOE-OPT) is developing a long-range plan to develop ultra-efficient advanced microturbines for distributed power applications.The goal of DOE Advanced Microturbine Program is to raise microturbine efficiency from its current level of 30% to more than 40% by the year 2007, while attaining low emissions and cost of operation. To reach higher thermal efficiencies, an increase in turbine rotor inlet temperatures may be required. Consequently, research and development (R&D) is needed to produce and evaluate the materials that will be able to withstand the higher operating temperatures. Because the recuperator (heat exchanger) will be the largest and possibly the most expensive component in an ultra-high-efficiency microturbine, a review was commissioned to assess recuperator technology with emphasis on materials requirements and fabrication.This review was compiled from a study of the current state of microturbine recuperator technology, a review of the literature published since the early 1970s, and interviews with specialists in the field. It covers three topics: microturbine and heat-exchanger designs, metallic recuperators, and ceramic recuperators.The temperature of the exhaust gas determines the materials requirements for microturbine recuperators. Consequently, R&D is needed to produce and evaluate hightemperature materials for both metallic and ceramic recuperators. Based on this assessment, we concluded that the most effective strategy for recuperator development would be to place immediate emphasis on the cost-effective manufacture of higher-temperature metallic recuperators and to consider the development of ceramic recuperators as a long-term objective.Also, a further exchange of ideas within the discipline would be useful. As part of its assessment of microturbine technology, DOE convened a workshop where goals for 2005 and beyond were compiled for the new generation of microturbines (see Summary of the Microturbine Technology Summit, Orlando, FL, 1998, DOE/ORO 2081. A second, comprehensive workshop entitled "Recuperator Materials for Advanced Microturbines" was held for those involved in microturbine recuperator R&D. Workshop participants discussed technology needs and development priorities for the production of cost-effective recuperators for high-efficiency microturbines.

Thermoeconomic Optimization and Comparison of Plate-Fin Heat Exchangers Using Louver, Offset Strip, Triangular and Rectangular Fins Applied in 200 kW Microturbines

“…Ranges of net power output in microturbines are almost between 5 and 200 kW; however, the net power output range in miniturbines is between 200 and 500 kW [1]. The efficiency of simple cycles of microgas turbines is between about 16% and 20%, and employing the cycles is rather uncommon [2]. In order to increase the overall efficiency of the cycle at constant parameters such as turbine and compressor efficiencies, compressor pressure ratio and turbine inlet temperature, it is beneficial to recover the turbine exhaust heat energy.…”

“…Nomenclaturea ¼ thickness of separation plate, m A ¼ overall heat transfer area, m2 A fin ¼ fin heat transfer area, m2 A w ¼ wall heat transfer area, m2 b ¼ fin height, mC ¼ heat exchanger cost coefficient, W/K c Capital ¼ capital cost, $ c f ¼ profit of heat recovery, $ c Maintenance ¼ maintenance cost, $ c ope ¼ operational cost, $ c purchase ¼ purchase cost, $ c à ¼ ratio of heat capacity rate Cp ¼ specific heat capacity, J/kg K Cost ¼ total cost, $ D h ¼ hydraulic diameter, m f ¼ friction factor f à ¼ inflation rate, % G ¼ mass flux, kg/m 2 s h ¼ convective heat transfer coefficient, W/m 2 K H t ¼ plate thickness, m i ¼ interest rate, % j ¼ Colburn number k el ¼ price of electrical energy, $/MWh k f ¼ price of fuel, $/kg k fin ¼ fluid thermal conductivity coefficient, W/m K k wall ¼ wall thermal conductivity coefficient, W/m K K c ¼ entrance pressure loss coefficient K e ¼ exit pressure loss coefficient L ¼ fluid flow stream length, m L c ¼ cold flow stream length, m L f ¼ fin length, m L h ¼ hot flow stream length, m L l ¼ louver height, m L n ¼ nonflowstream length, m L p ¼ louver pitch, m LHV ¼ fuel lower heating value, kJ/kg m ¼ mass flow rate, kg/s n ¼ recuperator expected life time, year NPV ¼ net present value, $ NTU ¼ number of transfer unit Nu ¼ Nusselt number P ¼ pressure, kPa P o;cycle ¼ cycle pressure outlet, kPa p t ¼ plate pitch, m Q ¼ rate of heat transfer, W R f ¼ fouling factor, m 2 K/W Re ¼ Reynolds number s ¼ fin pitch, m St ¼ Stanton number t ¼ fin thickness, m T ¼ temperature, K T TIT ¼ turbine inlet temperature, K T 0 ¼ reference temperature, K UA ¼ overall heat transfer coefficient, W/K V t ¼ volumetric flow rate, m 3 /s W t ¼ louver length, m x ¼ offset length, m Greek Symbols DP ¼ pressure drop, kPa DT m ¼ corrected temperature, K e ¼ recuperative effectiveness, % g ¼ cycle efficiency, % g comp: ¼ compressor efficiency, %Fig. 13 Final optimum designs based on nondominated sorting concept for crossflow arrangement Fig.…”

Creep Deformation Characteristics of Austenitic Stainless Steel Foils

“…Clean and efficient microturbine system (100 kWe) employs compact, high efficiency heatexchanger or recuperator with thin-foil folded air cell as the primary surface [1]. Current primary surface recuperators are made of AISI 347 stainless steel foils that operate at gas inlet temperatures of less than 650 o C and attain about 30% efficiency [2]. For low-compression ratios such as 5, the efficiency target of greater than 40% is possible with the increase in turbine inlet temperature to 1230 o C, and consequently recuperator inlet temperature to 843 o C. At this elevated temperature level, the steel foils are susceptible to creep failure due to the fine grain size, accelerated oxidation due to moisture in the hot exhaust gas and loss of ductility due to thermal aging.…”