T. Sánchez

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 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

Influence of Design Parameters on the Performance of a Multistage Centrifugal Compressor for Supercritical Carbon Dioxide Applications

The development of the supercritical Carbon Dioxide power cycle has relied on parallel tracks along which theoretical and experimental works have successfully complemented each other in the last few years. Following this approach, intensive work on the development of critical components has enabled the demonstration of the technology in small-scale test loops. The next step in the roadmap is scaling-up the technology in order to bridge the gap to commercialisation. To this aim, not only is it necessary to demonstrate that the cycle works, but it is also mandatory to rise component (and system) efficiencies to levels comparable with competing technologies. In this process, assessing the impact of the main design parameters on the efficiency of turbomachinery is deemed crucial.The present work is a follow-up to others presented by the authors in previous years where preliminary analysis on centrifugal compressor design combining tools of different levels of fidelity were used. Nevertheless, whilst these presented guidelines to design the main compressor successfully, this new piece of research presents how the design space of the unit is affected by the characteristics of the working fluid. A review of past research is first presented to evidence that the design space is largely influenced by the particular behaviour of the working fluid close to the critical point. Then, design maps are presented for different operating conditions (cycle heat balance), showing that their shapes change substantially depending on compressor inlet pressure and temperature. Also, a comparison of these maps confirms that the design regions enabling high efficiency can be substantially reduced depending on the inlet/outlet thermodynamic states. Finally, conclusions are drawn regarding optimal intervals for the main design parameters involved in the process.© 2015 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

Alternative Approach to Determining the Preferred Plant Size of Parabolic Trough CSP Power Plants

The share of Concentrated Solar Power plants in power generation has increased significantly in the last decade due to the need to develop and deploy clean technologies that help reduce the carbon footprint of the power generation industry and, at the same time, are less voracious in terms of fossil fuel consumption. As a governmental support to promote the installation of solar plants, different incentives are found in most countries: complementary rates to the market price of electricity (premium), tax credits, financial support, long term power purchase agreements and, in general, other mechanisms that are generally grouped in a “feed-in tariff” that should ideally be more demanding (stringent) over time. The objective of these measures is to make this technology competitive in the mid/long term. At the same time, and in order to distribute these economical resources as fairly as possible, governments have usually limited the power output of those power plants benefitting from these incentives, as a means to prevent oligopolies that would eventually stop technology evolution while concentrating on preserving market conditions. This has led to the common 50 and 80 MW limits that exist in Spain and the USA respectively. As a consequence, OEMs and EPCs have focused on developing reliable and cost-effective CSP plants of these sizes, especially 50 MW. This work is based on unrestrained regulatory or market scenarios, with the aim of finding out which plant size yields the best efficiency at the lowest cost of electricity (COE). In other words, the objective is to establish the plant size of interest for power producers and consumers, should CSP facilities compete in the same market conditions as conventional fossil-fuel plants. The work begins by reviewing briefly the origins of the usual constraints applied to CSP plants. Then, a survey of existing literature dealing with the issue of technical and economic CSP optimization is presented, with a special focus on the work by B. Kelly from Nexant Inc. Taking this work as reference, a model of performance of parabolic trough plants developed in Thermoflex environment to put forth strong project specific feature of CSP facilites. Thermal storage and natural gas hybridization are included among the key design parameters.Copyright

A Design Strategy for Supercritical CO2 Compressors

The international scientific community researching the supercritical carbon dioxide power cycle has already developed the first turbomachinery designs, which are currently operating in reference laboratories worldwide. Nevertheless, the performance of this equipment is still quite far from the target values yielding fairly disappointing system efficiencies (65% vs. 80% total to total efficiency for radial compressors). In the light of these past results, the thermal Power Group (GMTS) at the University of Seville, Spain, has been researching the SCO2 cycle during the last few years. Hence, after researching elementary diffusion processes and some basic features of the system, the authors are now in the process of developing guidelines for compressor design based on one-dimensional codes developed in-house and CFD analysis. The results stemming from both approaches are presented in this paper showing that the rather simple 1D model is able to produce a fairly good model which can then be tuned with a more complex and computationally expensive 3D CFD code. The entire approach is presented in this paper, from the initial reference value for the key design parameters through the 1D code and to the multi-dimensional tool. The results of the two latter approaches are compared in detail in this paper.Copyright