Paolo Iora

Thermodynamic Analysis of Integrated Molten Carbon Fuel Cell–Gas Turbine Cycles for Sub-MW and Multi-MW Scale Power Generation

This paper investigates the thermodynamic potential of the integration of molten carbon fuel cell (MCFC) technology with gas turbine systems for small-scale (sub-megawatt or sub-MW) as well as large-scale (multi-MW) hybrid cycles. Following the proposals of two important MCFC manufacturers, two plant layouts are discussed, the first based on a pressurized, externally reformed MCFC and a recuperated gas turbine cycle and the second based on an atmospheric MCFC, with internal reforming integrated within an externally fired gas turbine cycle. Different levels of components quality are considered, with an analysis of the effects of variable pressure ratios, different fuel mixture compositions (variable steam-to-carbon ratio) and turbine inlet temperature levels, together with potential advantages brought about by an intercooled compression process. The analysis shows interesting effects due to the peculiarity of the mutual interactions between gas turbine cycle and fuel cells, evidencing the importance of a careful thermodynamic optimization of such cycles. Results show the possibility to achieve a net electrical efficiency of about 57-58% for a small plant size (with a difference of 1.5-2 percentage points between the two layouts), with the potential to reach a 65% net electrical efficiency when integrated in advanced cycles featuring high-efficiency, large-scale equipment (multi-MW scale cycles).

Simulation of Intermediate-Temperature SOFC for 60%+ Efficiency Distributed Generation

This work proposes a process simulation of high efficiency intermediate-temperature (660–730°C) SOFC systems for promising applications in the foreseeable future distributed power generation sector. Two case-studies have been considered: the kW-scale unit proposed by Ceramic Fuel Cell Limited (CFCL), which reaches up to 68% stack DC efficiency, and the FuelCell Energy (FCE) SOFC system, where a 65% DC efficiency has been verified on a 10 kW module. Both systems can be applied to distributed generation, yielding 60%+ net electric efficiency (LHV basis) from natural gas at small scale.This study aims at calibrating the two considered SOFC balance of plants with the Politecnico di Milano in-house software GS. Throughout a zero-dimensional model of the complete system a validation of the manufacturer’s claimed performance is possible. The general module configuration is made up of a natural gas pre-treating processor, a SOFC stack, an anodic spent fuel combustor and a waste heat recovery system for CHP applications. A pre-reforming adiabatic reactor has been proven to be an efficient choice to reduce the higher hydrocarbon chains content in the fuel stream and therefore to lessen the burden on the anodic channel, especially in terms of solid carbon deposition. The fuel is then pre-heated and, in the FCE case-study, mixed with the anodic outlet recycle; this last solution is regarded as of utmost importance for the attainment of the high overall fuel utilisation (≈80–85%) factors necessary to reach the proposed high efficiency targets, as well as to provide the steam required by the internal reforming process.Both the considered fuel cell systems performance have been verified and their extremely high efficient operation proven, according to those reported by their manufacturers.In addition to the process simulation, the work lays the foundations for a more thorough SOFC stack modelling throughout a 2D in-house developed software. This analysis gives valuable insights on the geometry characterisation and on the flow arrangement, as well as on their effects on cell internal temperature and composition profiles. In particular, the proposed analysis focuses on the case of a planar cross-flow arrangement, representative of the latter of the two case-studies. The understanding of the internal behaviour of the systems provides useful information to optimise the cell performance and design.Copyright

Closed versus open cycle energy recovery from solid oxide fuel cells

Abstract A computer model of a solid oxide fuel cell (SOFC) was implemented with a number of operational options to evaluate the best use of the cell waste heat. The simplest solution for heat recovery is represented by adopting a pressurized fuel cell with the eventual expansion of the exhaust stream in a gas turbine. Other alternatives feature the adoption of various closed cycle recovery systems: a standard steam cycle, an inter-refrigerated helium cycle, and an advanced binary cycle. Assuming a fuel energy input of 100 MW, the overall efficiency of a hybrid cell was computed, which was much higher than that of a small capacity (100 kW) existing plant (67 versus ∼53 per cent). Such high efficiency hybrid SOFC systems were taken as the reference to evaluate the performance of alternate options. Reclaiming the cell waste heat by means of a subcritical steam cycle yields a performance similar to that of a hybrid system. Similar results are obtained resorting to a closed helium cycle. Advanced binary cycles, using either potassium or cesium as working fluids, were extensively investigated in view of achieving a superior overall performance. The hot, clean stream exhausted by the fuel cell is recognized as an ideal heat source for a binary plant. The SOFC-binary cycle system was fully optimized yielding top efficiencies ∼74 per cent and an additional power output of ∼45 per cent of the cell rating (30 per cent in the case of the hybrid system). The excellent efficiency of a binary cycle combined with the large amount of oxygen that is available in the cell exhaust flow suggests that a supplementary firing of the fuel could improve the overall merit of the system. Burning 75 per cent of the additional natural gas, for example, reduces the total efficiency to a level of that of a hybrid system but makes available an extra power 1.35 times greater than the fuel cell capacity. Pushing further this concept, the fuel cell could become an auxiliary equipment of a binary cycle.

What Fraction of the Fuel Consumed by a Heat-and-Power Cogeneration Facility Should Be Allocated to the Heat Produced? Old Problem, Novel Approach

The question of what fraction of the fuel consumed by a cogeneration plant is to be allocated to either the heat or the electricity is still open, leading to some arbitrariness in the quantification of the economic value of the different cogenerated goods and of the subsidies often granted to such facilities. In this work, we first evidence the drawbacks of the conventional allocation methods such as Incremental Electricity-Centered Reference (IECR), Incremental Heat-Centered Reference (IHCR) and Separate-Productions Reference (SPR), in that they use fixed partial primary energy factors chosen by some authority to represent the reference efficiencies of heat and /or electricity production technologies that can be different from the local energy portfolio. Here we propose a slightly more elaborate, but self-consistent method whereby the allocation is adaptive and self-tuned to the local energy scenario by sharing the fuel savings on the basis of the average primary energy factors for electricity and heat in the given local area including the cogeneration facility of interest. We call it the Self-Tuned Average-Local-Productions Reference (STALPR) method. We finally show by means of a representative case study that the classical methods might provides unfair, distorted figures that become increasingly important as cogeneration gains higher fractions of the energy market in a given local area.Copyright

WHAT FRACTION OF THE ELECTRICAL ENERGY PRODUCED IN A HYBRID SOLAR-FOSSIL POWER PLANT SHOULD QUALIFY AS 'RENEWABLE ELECTRICITY'?

Hybrid power production facilities, based on the integration of renewable resources into conventional fossil-fuel-fired power plants have gained a growing interest during the past decades due to a world-wide continuous increase of shares of the renewable sources into the electricity generation market. In fact, in spite of the variable nature of most of the renewable sources, the hybrid configuration may provide a more economic, sustainable, and reliable use of the renewables in all load-demand conditions compared to renewable single-resource facilities. Nonetheless, the question of what fraction of the electricity produced in such facilities is to be considered as generated from renewables, still remains not fully addressed. This implies that there is space for some arbitrariness in the quantification of the share of the produced electricity to be qualified for the subsidies granted to renewable electricity, as normally prescribed by most of the policies that promote the applications of renewable primary energy resources. To overcome this problem, in this work we first define the classical Single-Resource Separate-Production Reference allocation method (SRSPR) usually considered by the regulators which is based on reference partial primary energy factors that must be chosen by some authority as representative of the performance of the (best available or representative average single-resource) power production technologies that use the same renewable resource and the same fossil fuel as the hybrid facility. Then we propose a Self-Tuned Average-Local-Productions Reference allocation method (STLAPR) whereby the electricity allocation fractions are based on the energy scenario of the local area of interest that includes the hybrid plant itself. We compare the two methods for a case study consisting on the renewable-to-fossil allocation of the power produced in an Solar-Integrated Combined-Cycle System (SICCS) with parabolic trough solar field. It turns out that the differences between the classical SRSPR and the STLAPR method become significant as the hybrid facilities take on a sizable fraction of the production of electricity in the local area.Copyright

Thermofluid-Dynamic Analysis of Circular-Planar Type Intermediate-Temperature Solid Oxide Fuel Cells

This work presents a computational thermofluid-dynamic analysis of circular-planar type intermediate-temperature solid oxide fuel cells (SOFCs), based on the Hexis design. A single cell, representative of the average conditions of a real stack, is simulated in detail considering the real anode and cathode channel design, featuring an array of square pegs supporting the interconnection layers. The analysis is developed starting from cell operating data assumed from real test experimental information for an anode-supported SOFC with a 100 cm 2 active area, fed with pure hydrogen, and is extended to different reactant flow rates and generated heat flux power densities to evidence a generalized correlation for the thermofluid-dynamic behavior of the fuel cell under variable operating conditions. Aiming to provide a set of general results for the calculation of the heat transfer coefficient, which is applicable for the purpose of a complete thermal and electrochemical finite volume analysis, the simulation calculates local temperature distributions depending on radial and angular positions. The fluid-dynamic analysis evidences the existence of preferential flow paths and nonuniformity issues of the gas flow field, which may affect significantly the cell performances, and indicates possible cell design improvements.

Flare gas reduction through electricity production

ABSTRACT This article analyzed the potential energy recovery from rather small quantities of associated gas (<2000 m3/h), where the on-site electricity generation within the oil extraction field may represent a cost-effective solution as an alternative to flare combustion. Various power plant technologies were considered and compared from both the economic and avoided CO2 emissions points of view. It turned out that adopting a scheme with non-derated internal-combustion engines (ICE) fed by treated gas, and partial gas flaring, the most cost-effective result was obtained, showing a payback time of about 5 years and an internal rate of return (IRR) of 42.2%.