Testing and Preliminary Modelling of a 2.5 kW Micro-CHP SOFC Unit

The experimental activities, carried out at the Laboratory of Micro-Cogeneration (LMC) of the Department of Energy at Politecnico di Milano are hereby outlined in relation to the testing of four 2.5 kWel AC SOFC-based micro-CHP units developed by SOLIDpower S.p.a. The novelty of the work consists in carrying out a complete thermodynamic and environmental performance characterisation of the studied commercial system in a third-party laboratory. The main objectives of the experimental campaign have been the investigation and assessment of the electric and heat recovery performances in different cogeneration thermal power demand loads. The generator has been tested in five different thermal loads, whilst operated at full electric load, in order to simulate the coupling with thermal appliances of diverse nature. The cogeneration water inlet temperature has been varied from 20°C (as in more complex cogeneration systems which may envisage a thermal storage and additional pre-heating section) to 50°C (as for district heating purposes or heating of sanitary water). Each measurement has been acquired with a redundant approach for statistical purposes aiming to the reduction of uncertainty and to guarantee procedure robustness. Moreover, the design point experimental characterisation has been supported by an overall process calibration and simulation performed by means of an in-house software (GS), developed at the Department of Energy. Each component has been modelled using a 0D approach, such that the required mass and energy balances of the plant can be compared with those obtained from the experimental activity. In conclusion, the overall performances have met the expectations, being characterised by a net electric efficiency of approximately 39% and a total efficiency which may overcome 95%.

Fuel Cell Temperature Control With a Pre-Combustor in SOFC Gas Turbine Hybrids During Load Changes

The use of high temperature fuel cells, such as Solid Oxide Fuel Cells (SOFCs), for power generation, is considered a very efficient and clean solution to conservation of energy resources. Especially when the SOFC is coupled with a gas turbine, the global system efficiency can go beyond 70% on natural gas LHV. However, the durability of the ceramic material and the system operability can be significantly penalized by thermal stresses due to temperature fluctuations and non-even temperature distributions. Thermal management of the cell during load following is therefore very critical. The purpose of this work was to develop and test a pre-combustor model for real-time applications in hardware-based simulations, and to implement a control strategy in order to keep cathode inlet temperature as constant as possible during different operative conditions of the system. The real-time model of the pre-combustor was incorporated into the existing SOFC model and tested in a hybrid system facility, where a physical gas turbine and hardware components were coupled with a cyber-physical fuel cell for flexible, accurate, and cost-reduced simulations. The control of the fuel flow to the pre-combustor was proven to be very effective in maintaining a constant cathode inlet temperature during a step change in fuel cell load. After imposing a 20 A load variation to the fuel cell, the controller managed to keep the temperature deviation from the nominal value below 0.3% (2 K). Temperature gradients along the cell were maintained below 10 K/cm. An efficiency analysis was performed in order to evaluate the impact of the pre-combustor on the overall system efficiency.

Investigation of Water Transport Within a Proton Exchange Membrane Fuel Cell by Diffusion Layer Saturation Analysis

Diffusion layer saturation analysis (DLSA) is introduced in order to further investigate water transport within the cell. The analysis relies on two separate experimental processes. First, an ex-situ investigation of the relative humidity of the gas streams and their resulting pressure drop is performed. Next, multiple variables of the cathode and anode gas streams are manipulated in-situ to create an evaporative driving force to remove water out of the porous layers. Multiple gas stream settings are investigated as well as the temperature set point of the cell. The ex-situ pressure drop data is used to infer how much water is being removed from the system and to begin to estimate an overall water balance. Multiple cathode and anode GDL configurations are tested in order to further investigate initial saturation conditions of the GDLs. By coupling the voltage results to the calculated water removed from the system, it can be seen which configurations had low initial voltage due to GDL oversaturation. The potential for DLSA both as a diagnostic tool and an investigative technique into multiphase flow in the porous layer is demonstrated.

Multi-Functional Electrolyte for Thermal Management of Lithium-Ion Batteries

The high thermal conduction resistances of lithium-ion batteries severely limits the effectiveness of conventional external thermal management systems. To remove heat from the insulated interior portions of the cell, a large temperature difference is required across the cell, and the center of the electrode stack can exceed the thermal runaway onset temperature even under normal cycling conditions. One potential solution is to remove heat locally inside the cell by evaporating a volatile component of the electrolyte. In this system, a high vapor pressure co-solvent evaporates at a low temperature prior to triggering thermal runaway. The vapor generated is transported to the skin of the cell, where it is condensed and transported back to the internal portion of the cell via surface tension forces. For this system to function, a co-solvent that has a boiling point below the thermal runaway onset temperature must also allow the cell to function under normal operating conditions. Low boiling point hydrofluoroethers (HFE) were first used by Arai to reduce LIB electrolyte flash points, and have been proven to be compatible with LIB chemistry. In the present study, HFE-7000 and ethyl methyl carbonate (EMC) 1:1 by volume are used to solvate 1.0 M LiTFSI to produce a candidate electrolyte for the proposed cooling system. Copper antimonide (Cu2Sb) and lithium iron phosphate (LiFePO4) are used in a full cell architecture with the candidate electrolyte in a custom electrolyte boiling facility. The facility enables direct viewing of the vapor generation within the full cell and characterizes the galvanostatic electrochemical performance. Test results show that the LFP/Cu2Sb cell is capable of operation even when a portion of the more volatile HFE-7000 is continuously evaporated.

Lightweight System Design Optimization of High Reliability Compact Air Independent PEMFC Power Systems for Aerial and Space Vehicles

Demand has increased for high reliability mobile power systems for space and aerial vehicles in military, scientific, and commercial applications. Batteries have traditionally supplied power in these applications, but the desire to extend mission duration and expand vehicle capabilities would require an energy density increase that is difficult for batteries to achieve. The use of pure hydrogen and oxygen reactants with high efficiency membrane electrode assemblies and novel design concepts for the fuel cell stack bipolar plates and balance of plant (BOP) components has the potential to meet the desired system energy density. This paper reviews subsystem and integrated testing of a lightweight PEM fuel cell system design for implementation into an aerial vehicle or space mission. The PEM fuel cell stack is designed for optimum efficiency at 2 kWe of power during standard operation with the capacity to provide over 5 kWe of continuous power. The passive flow control and water management subsystems provide the gas flow and humidification necessary for efficient operation and remove excess water produced by the stack under all operating regimes. Work is in progress to test the fully integrated system under expected operating conditions for potential lightweight PEMFC applications.

An Optimal Predictive Control of 0.75 kW PEM Fuel Cell Cogeneration With Home Appliances for Efficient PV Utilization

This paper proposes an optimal predictive control of 0.75 kW PEM fuel-cell cogeneration with home appliances. This paper also models fuel cell system for design and operation evaluation of building equipment based on actual measurement of residential fuel cell system on sale. As one application of constructed model and proposed control method, this paper discusses concerning home EMS for efficient PV utilization.

Simulation of Model Predictive Control for a Fuel Cell/Gas Turbine Power System Based on Experimental Data and the Recursive Identification Method

A Model Predictive Control (MPC) strategy has been suggested and simulated with the empirical dynamic data collected on the Hybrid Performance (HyPer) project facility installed at the National Energy Technology Laboratory (NETL), U.S. Department of Energy, in Morgantown, WV. The HyPer facility is able to simulate gasifier/fuel cell power systems and uses hardware-based simulation approach that couples a modified recuperated gas turbine cycle with hardware driven by a solid oxide fuel cell model. Dynamic data was collected by operating the HyPer facility continuously during five days. Bypass valves along with electric load of the system were manipulated and variables such as mass flow, turbine speed, temperature, pressure, among others were recorded for analysis. This work was developed by focusing on a multivariable recursive system identification structure fitting measured transient data. The results showed that real-time or online data is a viable means to provide a dynamic model for controller design. The excursion dynamic data collected between the setup changes of the experiments was processed off-line to determine the feasibility of applying an adaptive Model Predictive Control strategy. One of the strengths of MPC is that it can allow the designer to impose strict limits on inputs and outputs in order to keep the system within known safe bounds. Two identification structures, ARX and a State-Space model, were used to fit the measured data to dynamic models of the HyPer facility. The State-Space identification was very accurate with a second order model. Visual inspection of the tracking accuracy shows that the ARX approach was approximately as accurate as the State-Space structure in its ability to reproduce measured data. However, by comparing the Loss Function and the FPE parameters, the State-Space approach gives better results. The MPC proved to be a good strategy to control the HyPer facility. The airflow valves and the electric load were used to control the turbine speed and the cathode airflow. For the ARX/State Space models, the MPC was very robust in tracking set-point variations. The anticipation feature of the MPC was revealed to be a good tool to compensate time delays in the output variables of the facility or to anticipate eventual set-point moves in order to achieve the objectives very quickly. The MPC also displayed good disturbance rejection on the output variables when the fuel flow was set to simulate FC heat effluent disturbances. Different off-design scenarios of operation have been tested to confirm the estimated implementation behavior of the plant-controller dynamics.

Synthesis and Performance Evaluation of an S-POSS Based PBI Electrolyte for High Temperature PEM Fuel Cell Applications

In this paper, using patented nano-additive based polymer synthesis technology, a novel approach to the design and fabrication of high temperature proton exchange membrane (PEM) has been developed. The presence of sulfonated octaphenyl POSS (S-POSS) in a PBI-PA (polybenzimidazole-phosphoric acid) membrane results in a 40–50% increase in conductivity at 120–200$deg relative to non-sulfonated silica or POSS control fillers at comparable weight percent filler loadings and PBI molecular masses, and also relative to unfilled PBI-PA membranes. In addition, the presence of S-POSS and silica both result in physical reinforcement of the membrane and increased its modulus and mechanical integrity, but only S-POSS offers the benefits of both increased conductivity and increased modulus. Isophthalic acid and 3,3’-diaminobenzidine (DAB) were polymerized in the presence of polyphosphoric acid (PPA) and S-POSS nanoadditive, and the degree of polymerization was monitored by viscosity and torque change measurements. Molecular mass was determined by inherent viscosity measurements of samples removed from the reaction solution. Membranes were prepared by casting the reaction solution and allowing PPA to hydrolyze to PA under ambient conditions. The membranes were characterized for acid content, in-plane conductivity, tensile modulus and shear modulus, and were roll-milled to achieve the desired thickness for membrane electrode assembly (MEA) fabrication.

Electrohydrodynamic-Jet Deposition of Pt-Based Fuel Cell Catalysts

Fuel cell electrodes are traditionally fabricated by hand-painting or spraying a catalyst ink onto a gas diffusion electrode or membrane. However, electrodes prepared via these techniques do not always have a uniform distribution of catalyst. Recently, electrohydrodynamic-jet (e-jet) printing has been developed as a method to deposit a variety of chemical and biological materials with excellent precision for various applications in electronics, biotechnology, and microelectro-mechanical systems. Here we demonstrate e-jet deposition of Pt-based fuel cell catalysts as a technique for achieving uniform catalyst distribution on microelectrodes. E-jet deposition is studied as a function of applied potential, and at 450 V, printed catalyst lines are very uniform at ∼10 μm in width. For electrode areas less than 1 mm2, deposition times are on the order of a few hours, which compares well with traditional hand-painting deposition times. Uniform catalyst distribution is important to reducing catalyst loading, and the deposition technique presented here shows significant possibility to produce electrodes with high uniformity.

Robust PID Controller Design of a Solid Oxide Fuel Cell Gas Turbine

The performance of a 300 kW Solid Oxide Fuel Cell Gas Turbine (SOFC-GT) pilot power plant simulator is evaluated by applying a set of robust Proportional Integral Derivative (PID) controllers that satisfy time delay and gain uncertainties of the SOFC-GT system. The actuators are a fuel valve (FV) that models the fuel cell thermal exhaust, and a cold-air (CA) valve which bypasses airflow rate from the fuel cell cathode. The robust PID controller results for the uncertain gains are presented first, followed by a design for uncertain time delays for both, FV and CA bypass valves. The final design incorporates the combined uncertain gain parameters with the time delay modeling of both actuators. This Multiple-Input Multiple-Output (MIMO) technique is beneficial to plants having a wide range of operation and a strong parameter interaction. The practical implementation is presented through simulation in the Matlab/Simulink environment.