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Theoretical and Experimental Investigation of a Small-Scale, High-Speed, and Oil-Free Radial Anode Off-Gas Recirculation Fan for Solid Oxide Fuel Cell Systems
Abstract
The Laboratory for Applied Mechanical Design (LAMD) designed, manufactured, and experimentally tested a novel recirculation fan for a 10 kWe solid oxide fuel cell (SOFC). The fan uses oil-free bearings, more specifically herringbone-grooved journal and spiral-grooved thrust gas bearings. The radial inducer-less fan with a tip diameter of 19.2 mm features backward-curved prismatic blades with constant height. Prior to coupling the recirculation fan with the SOFC, the fan was experimentally characterized with air at 200 °C. At the nominal point of 168 krpm, the measured inlet mass flow rate is 4.9 kgh-1, the total-tototal pressure rise 55 mbar, the isentropic total-to-total efficiency 55 %, and the power 18.3 W. This paper compares the experimental data towards a computational fluid dynamic simulation of the full fan impeller and volute suggesting an excellent correlation at the nominal point what validates the numerical approach. However, the heat flows crossing the fan fluid domain, have an increased effect at off-design conditions, thus the experimental results need careful consideration. The fan backface leakage has negligible impact on the measurements.
... Wagner et al. [18] presented a thermally-driven recirculation fan lubricated on dynamic gas bearings, i.e., herringbonegrooved journal and spiral-grooved thrust bearings, which is a good oil-free solution for small-scale turbomachinery according to Gu et al. [19]. Table 1 and Table 2 list the main design and geometrical parameters, respectively. ...
... Before coupling the recirculation fan to the actual SOFC system, Wagner et al. [18] characterized it with hot air at 200 • C. The original design blade tip clearance of 0.05 mm was increased to 0.15 mm by a shim of 0.1 mm thickness. During operation at design conditions, a reduction of the tip clearance, denoted by S a in Table 2, by 0.01 mm due to the shaft motion was measured. ...
... The measurement setup and equipment for the hot air tests is similar to the one described by Wagner et al. [18]. ...
The blade tip clearance loss was studied experimentally and numerically for a micro radial fan with a tip diameter of 19.2mm. Its relative blade tip clearance, i.e., the clearance divided by the blade height of 1.82 mm, was adjusted with different shims. The fan characteristics were experimentally determined for an operation at the nominal rotational speed of 168 krpm with hot air (200 °C). The total-to-total pressure rise and efficiency increased from 49 mbar to 68 mbar and from 53% to 64%, respectively, by reducing the relative tip clearance from 7.7% to the design value of 2.2%. Single and full passage computational fluid dynamics simulations correlate well with these experimental findings. The widely-used Pfleiderer loss correlation with an empirical coefficient of 2.8 fits the numerical simulation and the experiments within +2 efficiency points. The high sensitivity to the tip clearance loss is a result of the design specific speed of 0.80, the highly-backward curved blades (17°), and possibly the low Reynolds number (1 × 105). The authors suggest three main measures to mitigate the blade tip clearance losses for small-scale fans: (1) utilization of high-precision surfaced-grooved gas-bearings to lower the blade tip clearance, (2) a mid-loaded blade design, and (3) an unloaded fan leading edge to reduce the blade tip clearance vortex in the fan passage.
... Although the unit uses a shim (precise shim in Figure 4 (C), the alignment to the micrometer is challenging. Wagner et al. [16] measured the thrust bearing clearance for this unit at ambient conditions and estimated it for nominal conditions (200 • C). Due to the axial motion of the rotor (0.01 mm), the rotor and stator hub misalignment increases from the initial -0.019 mm to -0.029 mm at 175 krpm. ...
... The test rig and measurement equipment used for the experiments, are similar to the ones described by Wagner et al. [16]. Figure 4) and with hot steam (option 2) were performed. ...
... The valve 2 can throttle or unthrottle the fan, which leads to different fan powers (at constant rotational speed). However, at a constant rotational speed, the variation of the fan power is within several watts [16]; hence, the change in turbine pressure ratio is small. The turbine characteristics features, therefore, only one point per speed line. ...
A micro steam turbine with a tip diameter of 15 mm was designed and experimentally characterized. At the nominal mass flow rate and total-to-total pressure ratio of 2.3 kg/h and 2, respectively, the turbine yields a power of 34 W and a total-to-static isentropic efficiency of 37 %. The steam turbine is conceived as a radial-inflow, low-reaction (15 %), and partial admission (21 %) machine. Since the steam is limited in the system (solid oxide fuel cell), a low-reaction and high-power-density design is preferred. The partial-admission design allows for reduced losses: The turbine rotor and stator blades are prismatic, have a radial chord length of 1 mm and a height of 0.59 mm. Since the relative rotor blade tip clearance (0.24) is high, the blade tip leakage losses are significant. Considering a fixed steam supply, this design allows to increase the blade height, and thus reducing the losses. The steam turbine drives a fan, which operates at low Mach numbers. The rotor is supported on dynamic steam-lubricated bearings; the nominal rotational speed is 175 krpm. A numerical simulation of the steam turbine is in good agreement with the experimental results. Furthermore, a novel test rig setup, featuring extremely-thin thermocouples (0.15 mm) is investigated for an operation with ambient and hot air at 220 °C. Conventional zero and one-dimensional pre-design models correlate well to the experimental results, despite the small size of the turbine blades.
... Due to their ability to achieve high rotational speeds and to operate without petroleum-based lubricant, gas-lubricated bearings are a convenient solution to support high-speed rotating machinery, while avoiding process fluid contamination. They are particularly suited for small-scale turbomachinery applications such as fuel cell air compressors [1], fuel cell anode off-gas recirculation fans [2,3], vapor compression cycle compressors [4,5], or organic Rankine cycle turbines [6]. Such bearings are subjected to viscous friction losses in the narrow clearance between the rotating and the stationary parts. ...
... The goal of this study is an experimental validation of windage loss models for gas bearing supported spindles operated in dense gases. The objectives are (1) to review existing models, (2) to build generic models suitable for various flow regimes and (3) to validate the proposed models based on experimental data. ...
... Combining Eqs. (1), (2), and (6), the laminar friction loss over a rotating cylinder is given by ...
Generic models are proposed to evaluate the skin friction coefficient acting on enclosed rotating disks and cylinders under various flow regimes. In particular, a model taking into account the inner radius of the disk is developed. The models are compared with experimental data obtained from coast-down tests of a high-speed spindle supported on gas lubricated bearings, operated in air and in halocarbon R245fa at various pressures. The windage losses are first computed considering state of the art laminar flow loss models in the gas bearings and an experimentally validated laminar-turbulent flow loss model in the air-gap. This reference approach predicts the air data with a good accuracy (deviation less than 5%) but underestimates the organic fluid data by up to 36%. This deviation is considerably reduced (max 6.8%) when applying the proposed multi flow regime loss model for enclosed rotating disks to the thrust bearing. Finally, the proposed laminar-turbulent flow loss model for enclosed rotating cylinders is simultaneously applied to the journal bearings and the air-gap. A peak deviation of 6.5% is maintained among all test cases when setting the critical Taylor number to an artificial value (67) instead of the theoretical value (41.1) characterizing the onset of growth of Taylor vortices. Taking into account the uncertainties on the bearing clearances, as well as on the operating pressure and temperature, a ±10% agreement with the experimental data is obtained.
... Wagner et al. [16] presented the design of a novel AOR fan Table 1 Research (top), pre-commercial (middle), and commercial (bottom) solid oxide fuel cell systems with anode off-gas recirculation via ejector or fan. Non-formatted numbers are directly stated in the reference (measurement), italic numbers are estimated by the authors of the reference with a simulation, bold and underlined numbers are calculated and estimated, respectively, by the authors of this paper with the data available in the reference. ...
... The previously mentioned AOR fan concept introduced by Wagner et al. [16] was coupled to a 6 kW e SOFC system. Instead of using an electrically-driven AOR fan, the new AOR concept is propelled by a 15 mm tip diameter, partial-admission (21%), and low-reaction (15%) steam turbine. ...
... However, the fan with the design tip clearance of 0.05 mm is expected to rise the pressure by 70 mbar at the design AOR mass flow rate of 4.9 kg h −1 and the design rotational speed of 175 krpm. [16] • A manually-operated ball valve at the fan outlet prevents the fluid from bypassing the SOFC stack, e.g., the flow direction of streams 7, 6, and 5 (in Fig. 3) is reversed during the startup phase. This could be replaced with a more simple check valve in a final version. ...
While the global fuel utilization of solid oxide fuel cells (SOFCs) is limited by the stack aging rate, the fuel excess is typically used in a burner, and thus limiting the system electrical efficiency. Further, natural-gas-fueled SOFCs require treated water for the steam reforming process, which increases operational cost. Here, we introduce a novel micro anode off-gas recirculation fan that is driven by a partial-admission (21 %) and low-reaction (15 %) steam turbine with a tip diameter of 15 mm. The 30 W turbine is propelled by pressurized steam, which is generated from the excess stack heat. The shaft runs on dynamic steam-lubricated bearings and rotates up to 175 000 rpm. For a global fuel utilization of 75 % and a constant fuel mass flow rate, the electrical gross DC efficiency based on the lower heating value was improved from 52 % to 57 % with the anode off-gas recirculation, while the local fuel utilization decreased from 75 % to 61 %, which is expected to significantly increase stack lifetime. For a global fuel utilization of 85 %, gross efficiencies of 66 % in part load (4.5 kWe) and 61 % in full load (6.3 kWe) were achieved with the anode off-gas recirculation. The results suggest that the steam-driven anode off-gas recirculation can achieve a neutral water consumption.
... However, this assumption is challenged as turbo-machinery decreases size and increases speed, such as in high-speed turbo-machinery [18,19], in which the thermal energy exchanged between casing to the gas has been evaluated to reach values up to 20% of the mechanical compressor power as suggested by Comerais et al. [20] and by Sirakov and Casey [21]. The underlying reason is an increase in the area to volume ratio of the heat-exchanging components, higher temperature gradients [22], an increase in the fluid velocities enhancing convective heat transfer [12] and increasing temperatures [23]. ...
... The first thermal case study is a Fan-Turbine Unit (FTU) comprising a partial admission radial inflow steam-turbine driving an anode off-gas recirculation fan for solid oxide fuel-cell applications, as described in detail by Wagner et al. [22] and by Wagner et al. [64]. Fig. 14 provides a picture of the FTU rig in Appendix B. Fig. 4(a) shows an axisymmetric view of the cross-section of the FTU, while Fig. 4(b) shows a schematic discretization for thermal modeling highlighting the main components. ...
High-speed turbomachinery is commonly designed to achieve high power densities. Limited space for active cooling results in a challenging thermal management. A thermal modeling approach leveraging modern declarative programming capabilities is presented, yielding an efficient dynamic model capable of real-time simulation while achieving accurate results. These properties enable the inclusion of thermal management strategies in an early stage of the design process. Further, the effect of varying thermal and transport properties of materials and fluids during transient conditions is included and is suggested to yield a high impact on thermal loads and heat evacuation capabilities. The often neglected fluid advection within the system is modeled by integrating a 1D fluid network from MatlabTM SimscapeTM to the thermal model, displaying a significant impact on the temperature estimation for critical parts. The accuracy of the presented model is verified against three gas-bearing supported high-speed turbomachinery experiments for stationary and transient operation.
... Herringbone groove thrust bearings (HGTBs) have broad application prospects in hydraulic machinery, machine tool spindles, and other industrial fields [1][2][3], due to the advantages of good stability, high stiffness, and zero leakage. In the bearing, oil and air are commonly used as lubricative mediums. ...
... Some researchers treat cavitation lubricants as gasliquid mixed fluids. According to the different calculation methods of the gas volume fraction, the cavitation models in the mixed fluid model can be divided into three types: (1) Model based on the R-P equation [17,18]; (2) Equation model based on gas solubility and surface tension of bubbles [19,20]; (3) Model of the Transport Equation based on Gas Volume Fraction [21,22]. Moreover, some researchers used CFD to analyze the cavitation. ...
Due to their excellent stability and zero leakage capability, thrust bearings with herringbone spiral grooves are frequently used in transmission mechanisms. However, the lubrication mechanism of thrust bearings has not been clearly understood and explained, preventing the optimization of the bearing performance. Thus, this paper is devoted to solving this problem by building a three-dimensional finite element flow model. In this model, the change in viscosity temperature is considered using Roelands equation, and the turbulence and cavitation are taken into consideration. Using the established model, the influence of parameters such as spiral angle, groove width ratio, and rotational speed on the cavitation area of the thrust bearing are analyzed. The pressure contour and speed distribution are obtained inside the clearance, as well as the volume fraction of the gas phase at the end face. Finally, according to the analysis results, the optimum structural parameter for the herringbone spiral groove structure is proposed, which enables higher bearing stability and provides a reference for engineering practice.
... Wagner et al. optimized [48] and tested [49] an oil-free steam-driven recirculation fan for the application in solid oxide fuel cell systems. In preliminary tests, the rotor was operated in hot air at 220°C, up to 168 krpm. ...
... This was experimentally demonstrated in Refs. [49,50] for a similar setup. A vertically-mounted rotor was operated up to the design operating speed of 168 krpm without any sub-synchronous vibration. ...
The nonlinear behavior of a rigid rotor supported by herringbone grooved journal gas bearings (HGJBs) was investigated in this study. The two-dimensional narrow groove theory (2D-NGT) was adopted to model the HGJBs. A set of integrated rotor-bearing state equations were built by coupling the rotor motion equations and the bearing Reynolds equation. An implicit integrator with adaptive time step method was used to solve those state equations continuously. Two low-stability HGJBs were implemented to experimentally demonstrate and analyze the appearance of self-excitation motions. The theoretical model was successfully validated by the experimental data on predicting the onset speed of the sub-synchronous vibration of the HGJB-rotor system and the whirl frequency ratio. The predicted limit cycle amplitude increases as the speed increases until the rotor contacts with the bearing surface, which leads to a bearing failure. Forward conical mode dominates the self-excited motion during the whole speed range of self-excited motion. The prediction shows that the HGJB-rotor system can still operate in a stable, even though the rotor is installed vertically, i.e., without static load on the bearings. This is a distinct advantage in comparison to plain bearings. As the static load applies on the bearings increases, the onset speed of sub-synchronous vibration increases as well. For the investigated rotor-bearing system, an increase of the onset speed of sub-synchronous vibration from 36 krpm to 75 krpm is predicted as the static load increases from 0 to 4 times of the rotor weight. This indicates an increased HGJB stability with increased static load. The rotor orbits show complex shapes when the imbalance excitation is considered in the simulation. Both synchronous frequency and whirl frequency are shown in the spectral analysis. Moreover, the speed range of self-excited motion reduces from [38,48] krpm to [40,42] krpm as the imbalance increases from 0 to 40 mgmm.
... Then, the performance of the impeller primarily depends on the shape of its blade profile. Traditional blade profiles are generated following camber-line based methods with thin main-blade and splitter-blade hierarchy, as adopted by Wagner et al. and Ou et al. [43,44]. To further regulate the mass flow rate and facilitate the sharp flow turning required by the FCB design, the authors propose a flow-channel based, thick-blade generation method. ...
... Additional research papers [1,[7][8][9] have been added to the previous literature survey and are summarized in Table 1. ...
Designing a configuration of an efficient solid oxide fuel cell (SOFC) system and operating it under appropriate conditions are important for achieving a highly efficient SOFC system. In our previous research, the system layout of a SOFC system with anode off-gas recirculation was suggested, and the system performance was examined using a numerical model. In the present study, the system operating conditions were optimized based on the system configuration and numerical model developed in the previous paper. First, a parametric sensitivity analysis of the system performance was investigated to demonstrate the main operating parameters. Consequently, the fuel flow rate and recirculation ratio were selected. Then, the available operating conditions, which keep the system below the operating limits and satisfy the desired system performance (Ufuel > 0.7 and ηelec > 45%) were discovered. Finally, optimized operating conditions were suggested for three operating modes: optimized electrical efficiency, peak power, and heat generation. Depending on the situation, the demand for electricity and heat can be different, so different proper operating points are suggested for each mode. Additionally, using the developed model and the conducted process of this study, various optimized operating conditions can be derived for diverse cases.
... State-ofthe-art methane-fueled commercial systems can achieve an electrical efficiency of up to 65% 1 (LHV) at 0.4 A/cm 2 without anode off-gas recirculation, which is one of the current technological trends aiming at increasing the system efficiency. The anode off-gas recirculation can increase the overall fuel utilization [12], reduce or even remove the dependency of fuel reforming on the external water supply [13][14][15][16][17][18]. It can also be applied to the systems with other chemicals. ...
... State-ofthe-art methane-fueled commercial systems can achieve an electrical efficiency of up to 65% 1 (LHV) at 0.4 A/cm 2 without anode off-gas recirculation, which is one of the current technological trends aiming at increasing the system efficiency. The anode off-gas recirculation can increase the overall fuel utilization [12], reduce or even remove the dependency of fuel reforming on the external water supply [13][14][15][16][17][18]. It can also be applied to the systems with other chemicals. ...
The increasing penetration of variable renewable energies poses new challenges for grid management. The economic feasibility of grid-balancing plants may be limited by low annual operating hours if they work either only for power generation or only for power storage. This issue might be addressed by a dual-function power plant with power-to-x capability, which can produce electricity or store excess renewable electricity into chemicals at different periods. Such a plant can be uniquely enabled by a solid-oxide cell stack, which can switch between fuel cell and electrolysis with the same stack. This paper investigates the optimal conceptual design of this type of plant, represented by power-to-x-to-power process chains with x being hydrogen, syngas, methane, methanol and ammonia, concerning the efficiency (on a lower heating value) and power densities. The results show that an increase in current density leads to an increased oxygen flow rate and a decreased reactant utilization at the stack level for its thermal management, and an increased power density and a decreased efficiency at the system level. The power-generation efficiency is ranked as methane (65.9%), methanol (60.2%), ammonia (58.2%), hydrogen (58.3%), syngas (53.3%) at 0.4 A/cm 2 , due to the benefit of heat-to-chemical-energy conversion by chemical reformulating and the deterioration of electrochemical performance by the dilution of hydrogen. The power-storage efficiency is ranked as syngas (80%), hydrogen (74%), methane (72%), methanol (68%), ammonia (66%) at 0.7 A/cm 2 , mainly due to the benefit of co-electrolysis and the chemical energy loss occurring in the chemical synthesis reactions. The lost chemical energy improves plant-wise heat integration and compensates for its adverse effect on power-storage efficiency. Combining these efficiency numbers of the two modes results in a rank of round-trip efficiency: methane (47.5%) > syngas (43.3%) ≈ hydrogen (42.6%) > methanol (40.7%) > ammonia (38.6%). The pool of plant designs obtained lays the basis for the optimal deployment of this balancing technology for specific applications.
Purpose
The water-lubricated hydrodynamic herringbone groove journal bearing (HGJB) is capable of running at high speed. However, when running at a low speed, it suffers from a low load-carrying capacity due to the weak hydrodynamic effect. To overcome this problem, this study proposes a hybrid water-lubricated HGJB and aims to investigate its dynamic characteristics.
Design/methodology/approach
A hybrid lubrication model applicable to the hybrid water-lubricated HGJB is established based on the boundary fitted coordinate system, which considers the turbulent, thermal and tilting effects, and the finite difference method is used to calculate the dynamic characteristics of the hybrid water-lubricated HGJB.
Findings
The result shows that the hybrid HGJB has larger dynamic coefficients and better system stability compared with the hydrodynamic HGJB when running at low speed. Furthermore, the stiffness of hybrid HGJB are mainly governed by the hydrodynamic effect rather than the hydrostatic effect when running at high speed.
Originality/value
The proposed hybrid water-lubricated HGJB shows excellent dynamic characteristics at either low speed or high speed; and the hybrid water-lubricated HGJB has a large load-carrying capacity when running at low speed and has a good dynamic stability when running at high speed.
Peer review
The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-06-2024-0233/
The hydrodynamic herringbone groove journal bearing (HGJB) performs exceptionally well at high speeds but is limited by a low load-carrying capacity, largely due to the lubrication characteristics of water. To address this issue, a hybrid water-lubricated HGJB is proposed in this study. A lubrication model for the high-speed hybrid water-lubricated HGJB is developed, taking into account turbulence, thermal effects, and tilt. A comparative analysis of the static characteristics is conducted between the hybrid HGJB and both the hydrodynamic HGJB and the hybrid plain journal bearing (PJB). The results show that the proposed hybrid water-lubricated HGJB offers significantly greater load-carrying capacity than the conventional hydrodynamic HGJB, particularly during start-up or at low speeds. For example, when the bearing operates at 1,000 rpm with an eccentricity ratio of 0.5, the load-carrying capacity of the water-lubricated hybrid HGJB under a supply pressure of 1.6 MPa reaches 650 N, compared to just 261 N for the water-lubricated hydrodynamic HGJB. Additionally, the hybrid water-lubricated HGJB demonstrates a higher flow rate and lower temperature rise than the traditional hybrid PJB, thanks to the improved pumping effect of the herringbone grooves at high speeds.
For low-emission, small-scale combined heat and power generation, integrating a biomass gasifier with a downstream solid oxide fuel cell system is very promising due to their similar operating conditions in terms of temperatures and pressures. This match avoids intermediate high-temperature heat exchangers and improves system efficiency. However, to couple both systems, a high-temperature and oil-free compressor is required to compress and push the low-density, high-temperature bio-syngas from the gasifier to the solid oxide fuel cell stack. The design and development of this high-temperature, high-speed, and gas-bearing supported compressor is presented in this work. An iterative process involving preliminary design, meanline analysis using commercial tools and in-house models is used for the design, which is then numerically analyzed using computational fluid dynamics. The goal is to achieve a design with a wide operating range and high robustness that withstands extreme working conditions. The 727 W machine is designed to run up to 210 krpm to compress 18.23kg/h of syngas at 350°C and 0.81bar. The centrifugal compressor has a tip diameter of 38 mm and consists of 9 backswept main and splitter blades. The impeller is made of Ti6Al4V and coated to prevent hydrogen embrittlement from the hot and highly reactive bio-syngas. The results obtained from the established models suggest a good concordance with the results from numerical analyses, despite the high temperatures and small scale of this design.
Coupling a biomass gasifier with a solid oxide fuel cell system through a high-temperature syngas compressor holds great promise to achieve low-emission, small-scale combined heat and power, since it reduces the number of heat exchangers and increases the system efficiency. However, due to the demanding operating conditions (high temperatures, toxic and explosive gases), electrical motors are not suitable to drive the syngas compressor. Therefore, a high-speed, small-scale cantilever steam turbine that can valorize the system's waste heat to power the compression is designed and developed. An iterative process involving preliminary design, meanline analysis, commercial tools, and in-house codes is used for the design. The design is then numerically analyzed using computational fluid dynamics. The 2.8 kW cantilever steam turbine with a tip diameter of 21 mm runs up to 210 krpm at temperatures of 525°C while being supported on dynamic steam-lubricated bearings. A low-reaction, full-admission design has been chosen to lower the steam consumption, the axial forces, and the turbine backface leakage. The turbine rotor is made of Ti6Al4V and coated for structural integrity and to withstand high temperatures. Despite the small scale of this design, the results obtained from the established correlations based on large-scale turbines yield a remarkable concordance with the results from the numerical analysis, in particular for the isentropic expansion efficiency prediction.
This thesis presents the results of the design and experimental investigation of a patented 10 kWel solid oxide fuel cell (SOFC) system with a thermally-driven anode off-gas recirculation (AOR) fan, the so-called fan-turbine unit (FTU). The system has the advantage of higher global fuel utilization, and thus higher efficiencies and/or lower local fuel utilization, increasing the fuel cell stack lifetime. Electrical net DC efficiencies, based on the lower heating value (LHV) of 65 % for a global fuel utilization of 92 % are demonstrated with a system simulation and optimization. This correlates to an electrical gross DC efficiency based on the LHV of 69 %. Additionally, the waste heat of the SOFC stack can be used for local cogeneration in heating or cooling applications leading to a net utilization ratio (cogeneration of heat and electricity) of 90 %. Other advantages of this system design include the absence of a water supply, a simplified water treatment system, and the reduction of the evaporator and pump component size, which reduces the system's initial and maintenance costs. A first FTU proof-of-concept was designed, manufactured, and extensively experimentally tested. The radial inducer-less fan with a tip diameter of 19.2 mm features backward-curved prismatic blades with a constant height. Prior to coupling the recirculation fan with the SOFC, the fan was experimentally characterized with air at 200 °C. At the nominal operational point of 168000 rpm, the measured inlet mass flow rate was 4.9 kg/h with a total-to-total pressure rise of 55 mbar, an isentropic total-to-total efficiency of 55 %, and a power of 18 W. Although the consumed fan power is very low, this FTU can increase the net efficiency of a 10 kWel SOFC system by five percentage points. The fan and shaft are propelled by a radial-inflow, partial-admission (21 %), low-reaction (13 %) steam turbine with prismatic blades. This turbine has a diameter of 15 mm and consists of 59 rotor blades with a radial chord of 1 mm and a blade height of 0.6 mm. At the design point, it has a total-to-total pressure ratio of 1.9, an inlet mass flow rate of 2.1 kg/h, and an isentropic total-to-static efficiency of 38 %, yielding a power of 36 W. To the best of the author's knowledge, it is one of the smallest steam turbines in the world. The shaft features one single-sided spiral-grooved thrust and two herringbone-grooved journal gas film bearings. Nominally, these bearings operate with water vapor at temperatures up to 220 °C. The bearings were tested with ambient air, hot air, and water vapor to rotational speeds up to 220000 rpm, suggesting very stable operation (rotor orbit less than 0.002 mm). Within this work, the recirculation unit operated for more than 300 hours. In a final step, the FTU was coupled in-situ to a 6 kWel SOFC system, reaching electrical gross DC efficiencies, based on the LHV, of 66 % in part load (4.5 kWel) and 62 % in full load (6.4 kWel) for a global fuel utilization of 85 %. To the best of the author's knowledge, this was the first time that a steam-driven AOR fan was demonstrated in-situ with an SOFC system.
Heating and cooling will more and more rely on heat pumping in order to enable a more rational use of energy. The further development of domestic and small industrial multi-stage heat pumps with enhanced performance is impaired by the presence of oil mixed with the refrigerant and its migration through the hermetic loop. Oil-free directly driven small-scale turbomachinery has been identified as a technology that will allow a significant performance surge in refrigeration and heat pump applications. A proof of concept compressor unit has been designed, built and successfully tested, demonstrating the technical feasibility of such a system. Directly driven turbocompressors are composed of several components, involving different specialization fields like compressor aerodynamics, bearing and rotordynamic design as well as the electric layout of the motor and its drive. Fragmentation is the commonly used procedure for designing complex and interdisciplinary systems. The splitting into submodules certainly simplifies the design process of the individual components, it leads, however, to conservative designs and makes it more complex to keep track of the mutual interactions between the submodules. Only an integrated and simultaneous design of the complete system ensures an optimum solution. The models of the different components have been linked together to build a global system, enabling its integrated optimization. Compared to a unit that has been designed using fragmented design, the overall efficiency could be increased by 16 points by using the proposed integrated design procedure. In order to be able to predict the system's performance, losses, critical speeds and stability margins the different components have been modeled in a modular and generic way. As the system operates with a vapor close to the saturation line, both the impeller and the gas-bearing models include real gas effects. The bearing model additionally incorporates rarefaction and clearance distortion effects resulting from centrifugal growth and thermal influence. Models for calculating the windage losses occurring in the bearings and in the gap between the rotor and the stator of the electric motor have been introduced as well. In order to enable the calculation of the whirl speeds and the corresponding stability margins, a rotordynamic model has been developed specifically for gas bearing supported rotors. The model has been extended with a forced response module that allows to predict the orbits of the rotor as a function of a given unbalance and vice-versa. A vapor phase test rig for measuring the compressor performance has been built. The oil free gas bearing supported radial turbocompressor with a tip diameter of 20 mm could be tested to speeds up to 210 krpm, reaching pressure ratios higher than 3.2 and powers of 1.7 kW. Internal isentropic compressor efficiencies in excess of 80% have been measured. A detailed comparison between the measured and the predicted compressor map shows excellent agreement.
While the global fuel utilization of solid oxide fuel cells (SOFCs) is limited by the stack aging rate, the fuel excess is typically used in a burner, and thus limiting the system electrical efficiency. Further, natural-gas-fueled SOFCs require treated water for the steam reforming process, which increases operational cost.
Here, we introduce a novel micro anode off-gas recirculation fan that is driven by a partial-admission (21%) and low-reaction (15%) steam turbine with a tip diameter of 15 mm. The 30 W turbine is propelled by pressurized steam, which is generated from the excess stack heat. The shaft runs on dynamic steam-lubricated bearings and rotates up to 175 krpm.
For a global fuel utilization of 75% and a constant fuel mass flow rate, the electrical gross DC efficiency based on the lower heating value was improved from 52% to 57% with the anode off-gas recirculation, while the local fuel utilization decreased from 75% to 61%, which is expected to significantly increase stack lifetime. For a global fuel utilization of 85%, gross efficiencies of 66% in part load (4.5 kWe) and 61% in full load (6.3 kWe) were achieved with the anode off-gas recirculation. The results suggest that the steam-driven anode off-gas recirculation can achieve a neutral water consumption.
To improve the industry benchmark of solid oxide fuel cell (SOFC) systems, we consider anode off-gas recirculation (AOR) using a small-scale fan. Evolutionary algorithms compare different system design alternatives with hot or cold recirculation. The system performance is evaluated through multi-objective optimization (MOO) criteria, i.e., maximization of electrical efficiency and cogeneration efficiency. The aerodynamic efficiency and rotordynamic stability of the high-speed recirculation fan is investigated in detail. The results obtained suggest that improvements to the best SOFC systems, in terms of net electrical efficiency, are achievable, including for small power scale (10 kWe).
Prototype of efficient anode-off gas recycle blowers was developed with a 3D turbo impeller and a DC brush-less built-in motor for high efficiency SOFCs. Performance test of the blowers clarified 26% and 52% total blower efficiency for the type SSR-70 for 10 kW-class SOFC and SSR-140 for 250 kW, respectively. System performance simulation of an SOFC system with the SSR-70 blower elucidated that 59% net AC electrical efficiency can be expected. When the blower efficiency is improved to 50% as of SSR-140, electrical efficiency will become 60%. We will continue developing more efficient recycle blowers.
Domestic heating and cooling will more and more have to rely on heat pumps in order to support a more rational use of primary energy consumption. The heat pump market is mainly dominated by electrically driven vapour compression cycles and by thermally driven sorption processes. The drawback of electrically driven vapour compression cycle is their dependence on an electrical grid and the fact that they increase the winter or summer electricity peak demands. Hence, a thermally driven vapour compression cycle would offer substantial advantages and flexibility to the end user for heating and cooling applications.
This paper presents the investigation of an oil-free Compressor-Turbine Unit (CTU) used for a Thermally Driven Heat Pump (TDHP) based on the combination of a heat pump compression cycle and an Organic Rankine Cycle (ORC). The CTU consists of a radial inflow turbine and a centrifugal compressor of the order of 2 kW each, directly coupled through a shaft supported on gas lubricated bearings. The CTU has been tested at rotor speeds in excess of 200 krpm, reaching compressor and turbine pressure ratios up to 2.8 and 4.4 respectively and isentropic efficiencies around 70%. Comparisons between the experimental data and predictions of models, that are briefly described here, have been carried out. A sensitivity analysis based on the experimentally validated models shows that tip clearance, for both compressor and turbine, and surface roughness of the compressor are key parameters for further improving performance.
The AVL SOFC Auxiliary Power Unit development program has reached significant milestones within the last year. The performance of the system has been significantly increased to 30 % electrical efficiency and up to 4 kW net electrical power. The first successful vehicle demonstration has been performed. Operation on sulfur containing fuels is possible. A commercial product launch is planned for 2016. In parallel also the AVL SOFC CHP system showed significant progress. The system is designed for 5/10 kW electrical power based on electrolyte supported stacks from Plansee/IKTS and a hot gas anode recirculation process. The system is since early 2013 continuously in operation and shows very promising results like an electrical efficiency around 50 %.
Solid oxide fuel cells (SOFC) are currently being developed for a wide variety of applications because of their high efficiency at multiple power levels. Applications for SOFCs encompass a large range of power levels including 1-2 kW residential combined heat and power applications, 100-250 kW sized systems for distributed generation and grid extension, and MW-scale power plants utilizing coal. This paper reports on the development of a highly efficient, small-scale SOFC power system operating on methane. The system uses adiabatic steam reforming of methane and anode gas recirculation to achieve high net electrical efficiency. The anode exit gas is recirculated and all of the heat and water required for the endothermic reforming reaction are provided by the anode gas emerging from the SOFC stack. Although the single-pass fuel utilization is only about 55%, because of the anode gas recirculation the overall fuel utilization is up to 93%. The demonstrated system achieved gross power output of 1650 to 2150 watts with a maximum net LHV efficiency of 56.7% at 1720 watts. Overall system efficiency could be further improved to over 60% with use of properly sized blowers.
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