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Physical model based reliability analysis
In a fuel supply system of a conventional household oil heating system the fuel degrades over time depending on the system temperature, the tank volume and the flow rates. The test fuel is a blend of 80 vol.% conventional domestic heating oil and 20 vol.% fatty acid methyl ester from rapeseed oil feedstock. The fuel will increase the deposits in the fuel supply which lead to system failures. In this paper a model based approach is introduced for the analysis of accelerated life testings. The impact of the tank volume on the failure time is analyzed. A well balanced physical model is set up for stochastic simulation. The model contains short term dynamic and long term transient degradation effects that have an important impact on the system reliability. The degradation effects of the components and the used fuel cause a system failure over time. A system failure is detected when the volume flow of the fuel supply declines. The analysis shows that the typical variances of characteristic parameters lead to a significant distribution of failure time. Furthermore, the functional relation between tank volume as stress factor and failure time is described.
Primary Energy Savings of HT-PEM CHP Units
The electrical efficiency of a combined heat and power (CHP) plant and the relation of power and heat demand have an important impact on primary energy savings. In a target application for domestic energy supply with electrical power up to 100 kW, high temperature proton exchange membrane (HT-PEM) fuel cells promise a higher electrical efficiency in comparison to competing technologies. Due to thermal and mechanical limitations, fuel cells are commonly scaled up modularly. The benefit of a modular concept is that the operating point of each module can be controlled separately in order to optimize the complete CHP plant. In this paper we describe the primary energy savings of a modular CHP plant based on HT-PEM fuel cells by numerical simulation. The demand side is modeled using reference load profiles for 10 multi-family buildings. A system model is set up and the control strategy is presented. With the modular CHP concept primary energy savings up to 20% compared to separated generation (boiler and power plant) could be demonstrated. Furthermore, a major benefit of the modular concept is that the primary energy relation increases only slightly with increasing installed power, due to a stable degree of coverage for electrical demand at higher installed capacities. This characteristic also helps to optimize degradation behavior.DOI: 10.1016/j.applthermaleng.2016.05.055