Colin F. McDonald

Recuperator considerations for future higher efficiency microturbines

First-generation microturbines are based on the use of existing materials and proven technology, and with low levels of compressor pressure ratio and modest turbine inlet temperatures, have thermal efficiencies approaching 30% for turbogenerators rated up to 100 kW. For such small machines the goal of advancing beyond this level of performance is unlikely to include more complex thermodynamic cycles, but rather will be realised with higher turbine inlet temperatures. Advancing engine performance in this manner has a significant impact on recuperator technology and cost. In the compact heat exchanger field very efficient heat transfer surface geometries have been developed over the last few decades but further improvements perhaps using CFD methods will likely be only incremental. Automated fabrication processes for the manufacture of microturbine recuperators are in place, and on-going developments to facilitate efficient higher temperature operation are primarily focused in the materials area. Based on the assumptions made in this paper it is postulated that in the 100 kW size the maximum thermal efficiency attainable for an all-metallic engine is 35%. To achieve this the recuperator cannot be designed in an isolated manner, and must be addressed in an integrated approach as part of the overall power conversion system. In this regard, temperature limitations as they impact the recuperator and turbine are put into perspective. In this paper there is strong focus on recuperator material selection and cost, including a proposed bi-metallic approach to establish a cost-effective counterflow primary surface recuperator for higher temperature service. If indeed there is a long-term goal to achieve an efficiency of 40% for small microturbines, it can only be projected based on the utilisation of ceramic hot end components. Alas, the high temperature component that has had the minimum development in recent years to realise this goal is the ceramic recuperator, and efforts to remedy this situation need to be undertaken in the near future.

Low-cost compact primary surface recuperator concept for microturbines

Abstract By the year 2000, microturbines in the 25–75 kW power range are projected to find acceptance in large quantities in the distributed power generation field, their major attributes include low emissions, multifuel capability, compact size, high reliability and low maintenance. For this type of small turbogenerator, an exhaust heat recovery recuperator is mandatory in order to realize a thermal efficiency of 30% or higher. The paramount requirements for the recuperator are low cost and high effectiveness. These characteristics must be accomplished with a heat exchanger that has good reliability, high performance potential, compact size, light weight, proven structural integrity, and adaptability to automated high volume production methods. In this paper, a recuperator concept is discussed that meets the demanding requirements for microturbines. The proposed stamped and folded metal foil primary surface recuperator concept has as its genesis, a prototype heat exchanger module that was fabricated as part of an energy research program in Germany over two decades ago. This novel heat exchanger approach was clearly ahead of its time, and lacking an application in the late 1970s was, alas, not pursued and commercialized. Based on this earlier work, a further evolution of the basic concept is proposed, with emphasis placed on the following: (1) minimization of the number of parts, (2) use of a continuous fabrication process, (3) matrix overall shape and envelope flexibility (annular or platular geometry), (4) ease of turbogenerator/recuperator integration, and (5) a later embodiment of a bi-metallic approach, towards the goal of establishing a compact and cost-effective recuperator for the new class of very small gas turbines that are close to entering service. For a representative microturbine, an annular recuperator would have only five basic parts. The matrix cartridge would be essentially a plug-in component, analogous to an automobile oil filter element. In this paper, the important role that the recuperator has on turbogenerator performance is discussed, together with a summary of the early prototype heat exchanger development. The major requirements, features and cost goals for a compact primary surface recuperator for microturbine service, are also covered.

The utilization of recuperated and regenerated engine cycles for high-efficiency gas turbines in the 21st century

Abstract In the gas-turbine field ‘simple-cycle’ engines (compressor + burner + expander) have been dominant across almost the full spectrum of power-generation and mechanical-drive applications. Paced by aerodynamic and materials-technology advancements, efficiency values have progressed significantly over the last five decades. However, to reduce specific fuel consumption further (by say a step change of 30–40%) and to reduce emissions significantly, more-complex thermodynamic cycles that include the use of exhaust-heat-recovery exchangers are necessary. Clearly, there are discrete applications where the use of recuperators or regenerators will find acceptance on a large scale, an example being for gas turbines rated at less than about 100 kW for hybrid automobiles and small generator sets. The role that recuperators and regenerators can play in future gas turbines is put into perspective in this paper. Innovative engineering concepts will be required to meet the demanding high-temperature operating environment and low-cost requirements, and these will essentially necessitate the utilization of ceramic-composite heat-exchanger configurations that are amenable to large-volume manufacturing methods.

Recuperated gas turbine aeroengines. Part III: engine concepts for reduced emissions, lower fuel consumption, and noise abatement

Purpose – This paper seeks to evaluate the potential of heat exchanged aeroengines for future Unmanned Aerial Vehicle (UAV), helicopter, and aircraft propulsion, with emphasis placed on reduced emissions, lower fuel burn, and less noise.Design/methodology/approach – Aeroengine performance analyses were carried out covering a wide range of parameters for more complex thermodynamic cycles. This led to the identification of major component features and the establishing of preconceptual aeroengine layout concepts for various types of recuperated and ICR variants.Findings – Novel aeroengine architectures were identified for heat exchanged turboshaft, turboprop, and turbofan variants covering a wide range of applications. While conceptual in nature, the results of the analyses and design studies generally concluded that heat exchanged engines represent a viable solution to meet demanding defence and commercial aeropropulsion needs in the 2015‐2020 timeframe, but they would require extensive development.Research...

Gas turbine recuperator renaissance

Abstract In recent years it is true to say that for industrial gas turbines, recuperators have not found acceptance, and even the large increases in fuel prices in the 1970s failed to stimulate their utilization to a noticeable degree. For defense applications, however, where minimizing the total volume and weight of the engine and fuel inventory, together with having a low specific fuel consumption characteristic at part power (e.g. battle tank and marine propulsion) are paramount, recuperated engines are being used in ever increasing numbers. With earlier impediments now overcome (i.e. structural integrity, reliability, cost, etc.), technology spin-offs from defense programs should result in a renewed interest in recuperated engines for a wide spectrum of applications, and this is the major theme of the paper. The author would like to dedicate this paper to Prof. A. L. London of Stanford University on the occasion of his 75th birthday, and to give recognition of his significant technical contributions over many years, towards the realization of recuperators for many gas turbine applications.

Recuperated gas turbine aeroengines, part I: early development activities

Purpose – Interest is currently being expressed in heat exchanged propulsion gas turbines for a variety of aeroengine applications, and in support of this, the aim of this paper is to evaluate the relevance of experience gained from development testing of several recuperated aeroengines in the USA in the late 1960s.Design/methodology/approach – Technology status, including engine design features, performance, and specific weight of recuperated propulsion gas turbines based on radial and axial turbomachinery, that were development tested in the power range of about 300 to 4,000 hp (224 to 2,984 kW) is discussed in Part I.Findings – A successful flight worthiness test was undertaken in the USA of a helicopter powered solely by a recuperated turboshaft engine and this demonstrated a specific fuel consumption reduction of over 25 percent compared with the simple‐cycle engine. However; in an era of low‐fuel cost, and uncertainty about the long‐term structural integrity of the high‐temperature heat exchanger, f...

Compact buffer zone plate-fin IHX : The key component for high-temperature nuclear process heat realization with advanced MHR

Based on ever-demanding environmental considerations, it is projected that, before the middle of the 21st Century, nuclear power will play an important role in the non-electric energy sector, particularly in the generation of hydrogen in a large quantity by means of high-temperature nuclear process heat (PH). The modular helium reactor (MHR) is the only type of nuclear plant that has this high-temperature capability. The key component in the primary circuit of the PH-MHR is the intermediate heat exchanger (IHX) since this facilitates transfer of the nuclear thermal energy to the process plant in a secondary helium loop. With an operating temperature of up to 1000°C, and perhaps even higher, the IHX requirements are demanding, the foremost being integrity (e.g. leaktightness), high compactness for integration in the steel pressure vessel and high performance. The major theme of this paper is that, for commercial PH-MHR service in the 21st Century, the IHX will be of plate-fin construction and be of a double-barrier construction to obviate both helium leakage and tritium diffusion into the secondary loop. This advanced type of IHX will replace first-generation units which are based on a single-barrier, low surface compactness, tubular, helical-bundle geometry, and are essentially a carry-over from steam generator technology used in earlier gas-cooled reactors. The use of very efficient offset fin surface geometries will give a compact IHX assembly that is compatible with installation in a steel vessel for a nuclear process heat source rated perhaps as high as 1200 MW(t). Included in this paper are discussions in the areas of IHX geometry/construction, materials considerations, development required, and the applicability of existing technology bases.

Ceramic Recuperator and Turbine — The Key to Achieving a 40 Percent Efficient Microturbine

Based on the use of state-of-the-art component technologies and the use of existing metallic materials, achieving an electrical efficiency anywhere near 40 percent in low pressure ratio recuperated microturbines is proving elusive. Current microturbines, rated at say 100 kW, operate with efficiencies approaching 30 percent. Advancing this to an upper level of about 35 percent is projected based on the ability to operate at turbine inlet temperatures greater than 1100C, and the utilization of a higher cost superalloy recuperator. This paper puts into perspective the challenge of trying to achieve 40 percent efficiency for small recuperated turbogenerator designs with radial flow components; the major constraints being associated with stress limitations in both the turbine and recuperator. Various publications (issued by both industry and the Government) often mention an efficiency goal of 40 percent for small gas turbines of this configuration, however, it needs to be recognized that the means to achieve this are beyond current high temperature metallic component capabilities. To achieve this “goal” necessitates increasing the operating temperature of the turbine and recuperator above 1100C and 800C respectively. Such advancements are projected to be technically and cost-effectively achievable by utilizing ceramic components, which with a dedicated development program, could perhaps become a reality in less than a decade to meet both future distributed power generation needs and defense applications, and be in concert with ever-demanding conservation goals and reduced emissions.Copyright

Microturbine/Fuel-Cell Coupling for High-Efficiency Electrical- Power Generation

Microturbines and fuel cells are currently attracting a lot of attention to meet future users needs in the distributed generation market. This paper addresses a preliminary analysis of a representative state-of-the-art 50 kW microturbine coupled with a high-temperature solid-oxide fuel cell (SOFC). The technologies of the two elements of such a hybrid-power plant are in a different state of readiness. The microturbine is in an early stage of pre-production and the SOFC is still in the development phase. It is premature to propose an optimum solution. Based on today’s technology the hybrid plant, using natural gas fuel, would have a power output of about 389 kW, and an efficiency of 60 percent. If the waste heat is used the overall fuel utilization efficiency would about 80 percent. Major features, parameters and performance of the microturbine and the SOFC are discussed. The compatibility of the two systems is addressed, and the areas of technical concern, and mismatching issues are identified and discussed. Fully understanding these, and identifying solutions, is the key to the future establishing of an optimum overall system. This approach is viewed as being in concert with evolving technological changes. In the case of the microturbine changes will be fairly minor as they enter production on a large scale within the next year or so, but are likely to be significant for the SOFC in the next few years, as extensive efforts are expended to reduce unit cost. It is reasonable to project that a high performance and cost-effective hybrid plant, with high reliability, will be ready for commercial service in the middle of the first decade of the 21st century.While several microturbines can be packaged to give an increased level of power, this can perhaps be more effectively accomplished by coupling just a single gas turbine module with a SOFC. The resultant larger power output unit opens up new market possibilities in both the industrial nations and developing countries.Copyright

The key role of heat exchangers in advanced gas-cooled reactor plants

Abstract The use of helium as a nuclear reactor coolant has been successfully demonstrated in plants built and operated in the U.K., U.S.A., and Germany. Following the pioneering proof of principle plant, two small power plants were operated for several years and this led to the construction of two commercial power stations. For the next generation of gas-cooled reactors new criteria have been developed, namely, the plants will be smaller, simpler, safer and of lower cost. The base case Modular High-Temperature Gas-Cooled Reactor (MHTGR) utilizes existing technology to offer a tried and proven power generating plant using a conventional steam turbine power conversion system that could be in utility service just after the turn of the century. The capability of the MHTGR to operate at very high temperatures will be exploited early in the next century in the form of advanced variants to meet the needs of the power generation and process industries. A key component in the MHTGR is the heat exchanger, since this is where the reactor thermal energy is transferred to the prime-mover or process system. This paper addresses the various roles that heat exchangers will play in advanced MHTGRs, recognizing that the requirements for the steam cycle, gas turbine (direct- or indirect-cycle), and process heat reactor are unique. Topics include thermodynamic considerations, differing configurations, and construction types; materials (metals, composites, ceramics); germane technology bases; and advanced heat exchanger technologies.