Results and Findings
Extensive modelling of the fuel cell based CHP system had been carried out, both for the baseline configuration and including future design improvements such as replacement of the Pressure Swing Absorber with a selective membrane based system.
Firstly, different alternatives to the baseline plant configuration to improve the system performance and to implement innovative technologies, such as selective membranes separators. Three different alternatives have been considered to enhance the CHP performance: (i) Improving the electrical efficiency of the prime mover (i.e. the fuel). (ii) Improving the thermal integration of the power plant subsystems, in particular of the fuel processing section. (iii) Exploring the integration of H2-selective membranes used as separation unit, or integrated in the water gas shift reactor, or integrated in the reforming side of the natural gas reformer reactor. Moreover, the possibility to exploit the low grade cogenerated heat to produce a cooling effect is explored through the modeling of a half-effect absorption chiller. The results show that increasing the nominal power of the fuel cell has a positive impact on plant performance. However, careful economic evaluation is needed to analyze possible cost increases. The thermal integration of the reformer improves the plant performance without any drawbacks and should be adopted. Selective membranes integrated in the reactors (and not used as a standalone purification unit) allow significant efficiency improvements and plant complexity reduction, in particular if the membranes are integrated in the reforming side of the reformer reactor. The half-effect absorption chiller can be directly integrated in the AutoRE CHP system and can exploit all the low grade (i.e. 75°C) thermal energy produced, although with a coefficient of performance lower than 0.5. Full details of the study are available to download as D4.2 report.
Secondly, performance of a trigeneration plant (CHCP) based on the AutoRe cogeneration (CHP) unit on real energy management scenarios has been studied. The characteristics of the complementary elements of the plant (e.g. boilers and chillers) have been taken from the literature. The work was separated in to two distinct parts. In the first part, the performance of the alternative prime mover configurations utilizing a simplified energy management environment was assessed. Then, in the second part, the techno-economic behaviour of the AutoRe cogeneration unit in different buildings and climatic conditions was extensively studied. Specifically, five energy demand profiles illustrative of various commercial buildings combined to five climatic conditions have been considered. Thermal, chilling and electrical loads were assumed to vary on an hourly frequency. In addition, two different cost levels for electricity and natural gas: one representative of the European market and one of the United States market have been used. A custom designed optimization methodology to outline the optimal control strategy for each combination of building and climate accounting for the part load performance of the CHCP plant has been utilised. Full details of the study are available to download as D4.6 Report.
Thirdly, the impact of two innovative technical solutions: (1) the advanced fuel processor designs, and (2) of thermally driven cooling machine has been assessed. The impact has been quantified taking into account environmental and economic performance of the AutoRE 50 kWe CHP system integrated in a building environment. The energy demand of a mid-rise apartment in different climates, considering European energy costs and emission factors as considered. Outcomes of the analysis on the advanced fuel processor configurations have shown that:
the baseline AutoRE CHP power plant ensures energy cost reductions in all the climatic conditions. The CO2 emissions are reduced in cold and moderate climates.
The CHP power plant with selective membranes integrated in the reformer guarantees additional cost reduction in the range 7.8% to 15%, and an additional CO2 reduction between 3.6% and 11%, with respect to the baseline CHP system.
Selective membranes as purification unit or integrated in the water gas shift reactor ensure an additional cost reduction with respect to the baseline CHP always lower than 3% and a further CO2 emissions reduction always lower than 4%.
In addition, a sensitivity analysis to the minimum set point of the membrane reforming CHP power plant has been performed. It was found that the economic and environmental performance of the energy system are affected by less than 6% and 2% respectively, by increasing the minimum set point from 10% to 50%. The results of the analysis on thermally driven cooling machine have demonstrated that the utilization of half-effect absorption chillers in the AutoRE CHP power plant boosts the environmental and economic benefits for all the considered scenarios. It has likewise been demonstrated that the utilization of the absorption chiller reduces the imbalance between the results obtained for the different scenarios (i.e. climates); albeit economic and environmental benefits associated to distributed generation are strongly influenced by the energy context. Full details of the study are available to download as D4.7 report.
Additional modelling has also been undertaken to look at fuel cell degradation. This latter modelling activity has leveraged on the results of a previous project (SAPPHIRE) which also targeted a CHP fuel cell system. Fuel cell degradation mechanisms have been identified and described and some mitigation and diagnostic techniques have been proposed. The main degradation mechanisms were identified such as loss of the active electrochemical surface area due to platinum catalyst particle growth, catalyst support corrosion or adsorption of contaminants and membrane chemical and mechanical degradation. Some diagnostic methods have also been proposed. The usual diagnostic methods such as polarization curves, electrochemical impedance spectroscopy, and cyclic voltammetry require a relatively long time (several hours). They are therefore not applicable for diagnostics while the fuel cell is operating, unless simplifications are made, for example monitoring cell or stack voltage at certain current, or electrochemical impedance measurement at certain frequencies. Recently, in another FCH-JU project (Giantleap) FESB investigated the possibility of using the low frequency intercept in the Nyquist diagram plot of the electrochemical impedance spectroscopy measurement results. Also, mechanisms of catalyst degradation occurring in the fuel processor have been studied, where it has been concluded that reformer catalyst degradation may be caused by coke formation, poisoning (for example with sulfur) or catalyst sintering either by particle migration or Ostwald ripening. In either case, the loss of catalyst surface area results in incomplete reactions; thus less hydrogen is generated resulting in higher CO content in the reformate. It has also been concluded that as the fuel cell stacks degrade, operation at constant net power output (50 kW) would require operation at currents higher than the nominal current. It should be checked whether the auxiliary equipment, namely the reformer, the air compressor, and the cooling subsystem (pump and heat exchanger), could support operation at higher currents.