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Abstract The Organic Rankine Cycle can be applied to convert low temperature heat to electrical power using organic working fluids. Recently, a new generation of working fluids has been introduced with almost no Ozone Depletion Potential and significantly smaller Global Warming Potential, compared to currently used refrigerants. R1233zd-E is a promising low-GWP (global warming potential) alternative to R245fa, a widely used fluid in ORC (Organic Rankine Cycle) systems. This paper analyzes the applicability of the new fluid as drop-in replacement for R245fa in existing systems and compares system parameters such as cycle efficiency and power output. To this end, the influence of the process parameters mass-flow rate, condensation temperature and expander rotational speed is investigated experimentally for both fluids. The test rig used has an electrical heater as a heat source and a scroll compressor as an expander. As a conclusion, R1233zd-E can be used as a substitute for R245fa in existing ORC systems. In addition to the advantage of having a much smaller GWP, the use of R1233zd-E may lead to higher thermal efficiencies. Comparing the highest achieved thermal efficiency, R1233zd-E performs 6.92% better than R245fa. However, comparing the maximal gross power output, R245fa performs 12.17% better than R1233zd-E.

Abstract Herein a novel path is analysed for its economic viability to synergize the production of biomethane and dimethyl ether from biogas. We conduct a profitability analysis based on the discounted cash flow method. The results revealed an unprofitable process with high cost/revenues ratios. Profitable scenarios would be reached by setting prohibitive DME prices (1983–5566 €/t) or very high feed-in tariffs subsidies (95.22 €/MWh in the best case scenario). From the cost reduction side, the analysis revealed the need of reducing investment costs. For this purpose, we propose a percentage of investment as incentive scheme. Although the size increase benefits cost/revenues ratio, only the 1000 m3/h biogas plant size will reach profitability if 90% of the investment is subsidized. A sensitivity analysis to check the influence of some important economical parameters is also included. Overall this study evidences the big challenge that our society faces in the way towards a circular economy.

Abstract Local energy markets are a promising way to involve prosumers in the electricity system and activate demand-side flexibility. In this paper, we develop a modeling framework consisting of an optimization for prosumer home energy management systems embedded in a local energy market simulation. To enable an integrated assessment, the local market is linked to a central spot market. For each prosumer, we consider the individual flexibility potential, home storage systems, and demand response of electric vehicles. Using this model, we analyze the costs and benefits of a local market for prosumers in an energy system with a high share of renewable energy. We compare the systemic effects and the potential economic benefits of this local market for prosumers to pure self-consumption and prosumer participation in a central spot market. Applying our model to a case study of 480 prosumers, we show that a system including a local market is beneficial compared to self-consumption from a systemic perspective. However, allowing prosumers to participate directly in a central spot market is more profitable and facilitates the system integration of renewables.

Abstract The influence of artificial ageing of gas diffusion layers (GDLs) on the cell performance was investigated using high resolution synchrotron radiography. State-of-the-art GDLs of the type SIGRACET® SGL 25BC were aged for 0 h, 16 h and 24 h in a hydrogen peroxide solution before they were assembled in the fuel cells. In-operando radiographic measurements were combined with voltage and contact angle measurements. Correlations between applied ageing conditions, GDL water saturation and cell performance were revealed. Hereby, all cell operating conditions were tested several times to estimate the reproducibility of in-operando radiographic fuel cell measurements. Water films at the GDL-membrane and at the GDL-flow field interfaces were found and attributed to MPL cracks and large pores in the GDL structure. The combination of these cracks and pores are assumed to play a crucial role for blocked gas paths, leading to an undersupply with reactants and an increased humidification of the membrane. It is shown that water agglomerations directly impact the membrane resistance. We assume that the hydrophobicity of the fibers inside the GDL is more important for the cell performance than water agglomerations at the membrane-GDL interface.

Abstract In designing energy supply systems, designers should heighten the robustness in performance criteria against the uncertainty in energy demands. In this paper, a robust optimal design method using a hierarchical mixed-integer linear programming (MILP) method is proposed to maximize the robustness of energy supply systems under uncertain energy demands based on a mixed-integer linear model. A robust optimal design problem is formulated as a three-level min-max-min MILP one by expressing uncertain energy demands by intervals, evaluating the robustness in a performance criterion based on the minimax regret criterion, and considering relationships among integer design variables, uncertain energy demands, and integer and continuous operation variables. This problem is solved by evaluating upper and lower bounds for the minimum of the maximum regret of the performance criterion repeatedly outside, and evaluating lower and upper bounds for the maximum regret repeatedly inside. Different types of optimization problems are solved by applying a hierarchical MILP method developed for ordinary optimal design problems without and with its modifications. In a case study, the proposed approach is applied to the robust optimal design of a cogeneration system. Through the study, its validity and effectiveness are ascertained, and some features of the obtained robust designs are clarified.

Abstract Cost efficient power generation from low temperature heat sources requires an optimal usage of the available heat. In addition to the ORC (Organic Rankine Cycles), cycles with ammonia and water as working fluid show promising results regarding efficiency. Due to their non-isothermal phase change, mixtures can adapt well to a liquid heat source temperature profile and reduce the exergetic losses. In this analysis thermodynamic calculations on the layouts of two existing ammonia–water cycles are compared: a geothermal power plant based on a Siemens’ patent and a modified lab plant based on a patent invented by Kalina (KCS-34). The difference between the two cycles is the position of the internal heat recovery. Cycle simulations were carried out at defined boundary conditions in order to identify optimal operation parameters. For the selected heat source of 393.15 K (hot water) the ammonia mass fraction between 80% and 90% results in the best performance in both configurations. In general, the layout of Siemens achieves a slightly better efficiency compared to the KCS-34. Compared to an ORC using R245fa as working fluid, the exergetic efficiency can be increased by the ammonia/water based cycles by approximately 25%.

Abstract Methane from biomass is a well suited renewable energy carrier with a wide range of applications. The main technologies for its production out of biomass are biochemical conversion from the upgrading of biogas and thermochemical conversion by gasification and methanation. Presently there exists no methodology to compare the process alternatives for methane production from biomass. This paper investigates a comprehensive evaluation method based on a multi-criteria analysis. Due to the comparable well developed biomethane market in Europe, compared to other regions in the world, the study area was restricted to Europe. The weighting of the different criteria is carried out in two rounds as a pair-to-pair comparison of the criteria by experts from different technology fields in a Delphi-Survey. As a result, the prioritisation can be used to classify the biomass conversion technologies to convert biomass to biomethane. According to the weightings given by experts, the two criteria energy efficiency and production costs are of great importance compared to the other criteria.

Abstract Biomass can be converted thermo- and bio-chemically to solid, liquid and gaseous biofuels. In this paper, energy, exergy and exergoeconomic analyses are applied to a biomass integrated post-firing combined-cycle power plant. The energy and exergy efficiencies of the cycle are found to be maximized at specific compressor pressure ratio values, and that higher pressure ratios reduce the total unit product cost. Increasing the gas turbine inlet temperature and decreasing the compressor pressure ratio decreases the CO 2 mole fraction exiting the power plant. The exergoeconomic factor for the biomass integrated post-firing combined-cycle power plant at the optimum energy/exergy efficiency is 0.39. This implies that the major cost rate of this power plant configuration is attributable to the exergy destruction cost rate. Increasing the compressor pressure ratio decreases the mass of air per mass of steam in the power plant, implying a reduction in the gas turbine plant size. Increasing both the compressor pressure ratio and the heat recovery steam generator inlet gas temperature increases the capital investment cost compared with the exergy destruction cost. However, increasing the gas turbine inlet temperature decreases this ratio.