R. Chacartegui

A New Concept for High Temperature Fuel Cell Hybrid Systems Using Supercritical Carbon Dioxide

Hybrid power systems based on high temperature fuel cells are a promising technology for the forthcoming distributed power generation market. For the most extended configuration, these systems comprise a fuel cell and a conventional recuperative gas turbine engine bottoming cycle, which recovers waste heat from the cell exhaust and converts it into useful work. The ability of these gas turbines to produce useful work relies strongly on a high fuel cell operating temperature. Thus, if molten carbonate fuel cells or the new generation intermediate temperature solid oxide fuel cells are used, the efficiency and power capacity of the hybrid system decrease dramatically. In this work, carbon dioxide is proposed as the working fluid for a closed supercritical bottoming cycle, which is expected to perform better for intermediate temperature heat recovery applications than the air cycle. Elementary fuel cell lumped-volume models for both solid oxide and molten carbonate are used in conjunction with a Brayton cycle thermodynamic simulator capable of working with open/closed and air/carbon dioxide systems. This paper shows that, even though the new cycle is coupled with an atmospheric fuel cell, it is still able to achieve the same overall system efficiency and rated power than the best conventional cycles being currently considered. Furthermore, under certain operating conditions, the performance of the new hybrid systems beats that of existing pressurized fuel cell hybrid systems with conventional gas turbines. From the results, it is concluded that the supercritical carbon dioxide bottoming cycle holds a very high potential as an efficient power generator for hybrid systems. However, costs and balance of plant analysis will have to be carried out in the future to check its feasibility.

A comparison between conventional recuperative gas turbine and hybrid solid oxide fuel cell—gas turbine systems with direct/indirect integration

Abstract Conventional recuperative micro gas turbines have a 30 per cent low heating value (LHV) maximum efficiency at full load. Therefore, if they are to be used in a potential distributed energy scenario, solutions must be developed that increase efficiency. An innovative gas turbine-based technology is the fuel cell — gas turbine hybrid system. This work is aimed at studying how the basic performance of a conventional Brayton cycle changes when heat addition is done at a fuel cell. Two layouts are considered: a direct system where the compressor feeds the fuel cell directly and an indirect system where only heat is transferred between subsystems. Direct and indirect systems have been studied at full and part load, concluding that the efficiency versus pressure ratio curves of hybrid systems change substantially with respect to a traditional gas turbine; part-load efficiency hardly decreases. Maximum efficiency of hybrid systems doubles the efficiency of state of the art micro gas turbine and remains high at part load. Furthermore, the benefit of a certain increase in temperature is higher for hybrid systems than for conventional engines. Finally, a simple economic analysis shows that the total installation and operation/maintenance costs of hybrid systems make them competitive against conventional gas turbines.

Determining compressor wash programmes for fouled gas turbines

Abstract An assessment on the impact of compressor fouling over gas turbine thermodynamic and economic performance is presented. This operational problem is dependent on engine location as it is caused by airborne particles dragged into the engine by the compressor. For the most hazardous locations, performance deterioration can reach 10 per cent/month for power and 5 per cent/month for efficiency, with respect to rated values. Engine washing is required to compensate for these losses. Different approaches made by relevant authors in the past are analysed, showing big contradictions in predicting sensitivity to fouling of engines with different sizes and specifications. This adds complexity to establishing washing programmes based on engine specifications rather than engine location. The conclusion is that a tailored washing schedule must be developed through a trial and error methodology, which is very dependent on the operators experience with similar engines. These concepts are applied to evaluate the performance of a running engine with severe fouling. The impact of hourly variation of electricity price and length of washing on the cashflow of a plant incorporating compressor is assessed, along with an evaluation of the effect of adopting a washing schedule which is far from the optimal or close to it.

Analysis of Intermediate Temperature Combined Cycles With a Carbon Dioxide Topping Cycle

The development of high efficiency solar power plants based on gas turbine technology presents two problems, both of them directly associated with the solar power plant receiver design and the power plant size: lower turbine intake temperature and higher pressure drops in heat exchangers than in a conventional gas turbine. To partially solve these problems, different configurations of combined cycles composed of a closed cycle carbon dioxide gas turbine as topping cycle have been analyzed. The main advantage of the Brayton carbon dioxide cycle is its high net shaft work to expansion work ratio, in the range of 0.7–0.85 at supercritical compressor intake pressures, which is very close to that of the Rankine cycle. This feature will reduce the negative effects of pressure drops and will be also very interesting for cycles with moderate turbine inlet temperature (800–1000 K). Intercooling and reheat options are also considered. Furthermore, different working fluids have been analyzed for the bottoming cycle, seeking the best performance of the combined cycle in the ranges of temperatures considered.Copyright

Performance of biomorphic Silicon Carbide as particulate filter in diesel boilers

Biomorphic Silicon Carbide (bioSiC) is a novel porous ceramic material with excellent mechanical and thermal properties. Previous studies have demonstrated that it may be a good candidate for its use as particle filter media of exhaust gases at medium or high temperature. In order to determine the filtration efficiency of biomorphic Silicon Carbide, and its adequacy as substrate for diesel particulate filters, different bioSiC-samples have been tested in the flue gases of a diesel boiler. For this purpose, an experimental facility to extract a fraction of the boiler exhaust flow and filter it under controlled conditions has been designed and built. Several filter samples with different microstructures, obtained from different precursors, have been tested in this bench. The experimental campaign was focused on the measurement of the number and size of particles before and after placing the samples. Results show that the initial efficiency of filters made from natural precursors is severely determined by the cutting direction and associated microstructure. In biomorphic Silicon Carbide derived from radially cut wood, the initial efficiency of the filter is higher than 95%. Nevertheless, when the cut of the wood is axial, the efficiency depends on the pore size and the permeability, reaching in some cases values in the range 70-90%. In this case, the presence of macropores in some of the samples reduces their efficiency as particle traps. In continuous operation, the accumulation of particles within the porous media leads to the formation of a soot cake, which improves the efficiency except in the case when extra-large pores exist. For all the samples, after a few operation cycles, capture efficiency was higher than 95%. These experimental results show the potential for developing filters for diesel boilers based on biomorphic Silicon Carbide.

Compressor Fouling: A Comparison of Different Fault Distributions Using a “Stage-Stacking” Technique

This work compares different approaches (i.e. fault distribution patterns) that aim to explain how a gas turbine compressor gets fouled and how its corresponding performance map deteriorates. To do so, a series of such maps is first developed based on an updated “stage-stacking” methodology. Then, the maps so developed are affected by fouling using the aforementioned approaches. The results evidence that the lack of consensus with respect to how fouling proceeds is bond to yield engine performances that differ by large. In other words, there is still a lot of work to be done if a general approach to anticipate the quantity and quality of compressor fouling is to be developed. For the moment, all the systematic analyses are specific to the particular site where they have been applied and hence have no generality that can be extended elsewhere. Moreover, the methodologies available allow that a fouled compressor be identified and the distribution be predicted a posteriori by means of adaptive modelling and compressor zooming utilising measurements from the whole engine or by using compressor specific measurements.Copyright

Modeling on/off-design performance of solar tower plants using saturated steam

This works presents a model for the performance prediction of concentrating solar tower plants using saturated live steam at on and off-design. For solar tower plants, the most important and critical element is the steam generator that, in this model, is supposed to produce saturated steam. The solar receiver is thus assumed to comprise several panels, each one of which is formed by a number of pipes. Given some boundary and initial conditions, heat and mass balances for the plant at any rated conditions can be obtained. Off-design operation is then defined by a modified map of the radiative heat flow onto the panels, though other operating conditions featuring new ambient conditions or abnormal operation of the plant can be simulated as well. As a result, a new set of heat and mass balances and performance data — efficiency, steam generation, live steam pressure and temperature, bleed pressures — are obtained. The developed model demonstrates its functionality during the design process in three ways. On one hand it allows to calculate and optimize the performance of the plant at rated conditions, developing a sensitivity analysis of the parameters involved. On the other hand, various off-design conditions can be studied and, consequently, it is possible to carry out a long term — for instance yearly — thermodynamic and economical analysis. Finally, it allows detecting undesirable operating conditions of one or more components that could eventually lead to a not admissible operation of the plant: for example a too high vapor fraction in the steam generator, a too low deaerator pressure, cavitation of pumps or other situations that are not appropriate for the steam turbine.Copyright

Performance Analysis of a 565 MW Steam Power Plant

In this work, a tool to predict the performance of fossil fuel steam power plants under variable operating conditions or under maintenance operations has been developed. This tool is based on the Spencer-Cotton-Cannon method for large steam turbine generator units. The tool has been validated by comparing the predicted results at different loads with real operating data of a 565 MW steam power plant, located in Southern Spain. The results obtained from the model show a good agreement with most of the power plant parameters. The simulation tool has been then used to predict the performance of a steam power plant in different operating conditions such as variable terminal temperature difference or drain cooler approach of the feed-water heaters, or under maintenance conditions like a feed-water heater out of service.Copyright

Parametric Analysis and Optimization of a High Temperature Fuel Cell: Supercritical CO2 Turbine Hybrid System

The integration of high temperature fuel cells — molten carbonate and solid oxide — and gas turbine engines for efficient power generation is not new. Different strategies for integrating both systems have been proposed in the past ten years and there are some field tests being run presently. However, the commercial availability of such power systems seems to be continuously delayed, probably due to cost and reliability problems. The materials used in high temperature fuel cells are expensive and their cost is not decreasing at the expected pace. In fact, it looks as if they had reached stabilization. Therefore, there seems to be agreement that operating at a lower temperature might be the only way to achieve more competitive costs to enter the market, as metallic materials could then be used. From the point of view of conventional hybrid systems, decreasing the operating temperature of the cell would affect the efficiency of the bottoming cycle dramatically, as long as turbine inlet temperature is a critical parameter for the performance of a Brayton cycle. This is the reason why hybrid systems perform better with solid oxide fuel cells operating at 1000 °C than with molten carbonate cells at 650 °C typically. This work presents a hybrid system comprising a high temperature fuel cell, either SOFC or MCFC, and a bottoming Brayton cycle working with supercritical carbon dioxide. A parametric analysis is done where all the parameters affecting the performance of the hybrid system are studied, with emphasis in the bottoming cycle. For the Brayton cycle: pressure ratio, expansion and compression efficiencies, recuperator effectiveness, pressure losses, turbine inlet temperature... For the fuel cell: fuel utilization, current density, operating temperature, etc. From this analysis, optimum operating point and integration scheme are established and, after this, a comparison with conventional hybrid systems using similar fuel cells is done. Results show that, although the fuel cell is not pressurized in the CO2 based system, its performance is similar to the best conventional cycle. Furthermore, if lower operating temperatures are considered for the fuel cell, the new system performs better than any of the conventional.© 2008 ASME

Model of Performance of Stirling Engines

This work presents a detailed model of performance of Stirling engines which is expected to be of interest for the Concentrated Solar Power community. In effect, gas turbines of different types have been proposed for small and medium scale solar applications based on their reduced (even inexistent) water consumption and modularity. In the medium to large scale, conventional steam turbine based plants demand high investment costs as well as high operation costs (mostly due to water consumption). In the small-scale it is the Stirling engine which is generally consider as the prime mover of choice due to its high efficiency at moderate temperatures. In this context, this paper describes a detailed model of performance of Stirling engines. The model includes frictional and mechanical losses, heat transfer within the engine and other features like auxiliary power consumption and applies to both on-design and off-design operation. The validation of all these capabilities is also presented in the text. Hence, the model is expected to provide a valuable tool for individuals who need to assess the performance of externally heated piston engines.Copyright