Testing and modelling of a novel oil-free co-rotating scroll machine with water injection

Efficient compressed air energy storage requires reversible isothermal compression and expansion devices. The isothermal compression and expansion processes can either be approached by several stages with intercooling or by the more convenient injection of a liquid, often water. While volumetric machines are readily available for dry processes the compression and expansion of a gas with the presence of liquid is still problematic. The concept of a co-rotating scroll has been identified as a promising technology to cope with the presence of liquid. The current paper discusses the first experimental results of an oil-free co-rotating scroll prototype tested in expansion mode on a wide range of rotational speeds, varying water injection flow rates and with different nominal flank clearances. A maximal overall isothermal efficiency of 34% and a maximum output power of 1.74 kWelwere measured with this first prototype, providing the proof of the technical feasibility of the oil-free co-rotating scroll expander concept. The experimental data indicate a positive effect of water injection suggesting good heat transfer behaviour between the water and the air in the individual chambers, which is a result of the relatively long residence time compared to other volumetric concepts. The experimental sensitivity analysis yields a strong dependency of the machine performance on both the nominal flank clearance and on the injected water rate. The analysis through a semi-empirical model suggests the inversion of a classical trend, i.e. the increase in total leak-age area with rotor speed. This is resulting from the centrifugal loads acting on the flanks and deforming them to produce increased radial and flank clearances. The injection of water is suggested to significantly decrease the leakage. A deterministic reduced order model of the co-rotating scroll expander was devel-oped in order to better understand the governing phenomena within the machine and to provide design guidelines for further prototypes. A novel leakage model takes into account for the structural deformation of the flanks and the scroll involutes as a result from the rotor speed. By means of this comprehensive thermodynamic model, mechanical power, mass flow rate and exhaust temperature were predicted within a range of ±12% and ±4 K respectively compared to experimental data. The calibrated model sug-gests an achievable isothermal efficiency of 87% for an improved co-rotating scroll concept, thus offering promising perspectives not only for compressed air storage, but also for wet expansion in Absorption Power Cycles, trilateral flash cycle and Organic Rankine Cycles.

Innovative isothermal oil-free co-rotating scroll compressor-expander for energy storage with first expander tests

Daniel Favrat, Angel Iglesias

The development of an efficient isothermal turbine and compressor is essential for the realization of a small-scale compressed air energy storage (CAES). This article presents the theoretical development of an oil-free co-rotating scroll air compressor and turbine working with water injection to make the operations of expansion and compression as isothermal as possible. First experimental results in expander mode are shown. The theoretical performance is predicted with the help of a mathematical model using the equations of energy and mass conservation and the equation of state. This model takes into account the effects of water injection and volumetric losses. The experimental prototype is an oil-free scroll air compressor with the distinctive feature of having two mobile involutes working in synchronized co-rotation one relative to another. The prime-mover is an electric motor driving the two scrolls with two synchronizing belts. Water injection in the housing intends to provide a quasi-isothermal compression. The same device is used as an isothermal expander by supplying high-pressure air with water when it rotates backwards in expander mode, the electric motor acting then as a generator. Expected improvements to a standard scroll compressor and expander are a better volumetric efficiency and a greater power density due to a higher rotational speed of the scrolls, thanks to their symmetrical masses. The isothermal processes increase also the overall performance. (C) 2014 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2014

Release kinetics of volatiles from smectite clay minerals

Pascal Clausen

The focus of this thesis was on the mechanisms of release of volatile molecules from smectite clay minerals. The weight loss was monitored by thermogravimetry, differential calorimetry, and mass spectrometry under isothermal conditions and under ramp temperature conditions. Modeling of the clay-volatile release systems was performed with the help of finite element method calculations at the macroscopic level and ab initio calculations at the molecular level. A first comparison between finite element method simulations and experimental data for the evaporation of bulk volatile liquids under a convective gas flow showed the calculations to be in good agreement with experiments. Results of the simulations were used to develop a semi-analytical model explaining the dependence of the evaporation on the total pressure, the carrier gas flow, the temperature, and material constants. In particular, its temperature dependence could be approximated to good accuracy by an Arrhenius-type equation derived from the semi-analytical model. Differential calorimetry measurements of the heats of vaporization showed equilibrium conditions at the surface of the liquids to be satisfied. The same approach was extended to the release of model volatiles (water, ethanol, ethyl acetate and toluene) from smectite clays. At high coverage, the release was found to be close to that for the bulk liquids. Its decrease with time followed the behaviour observed in the respective curves of the gas/condensed phase partition coefficients, the equilibrium desorption isotherms. Equilibrium condition at the surface of the sample was evidenced by a comparison between the measured heats of vaporization and the equilibrium desorption isotherms. The differences observed in the measured equilibrium desorption isotherms of the volatile on the smectite clays could be rationalized by the use of ab initio calculations of the binding of the volatiles on the surface of a sodium smectite clay. At low coverage, the differences could be attributed to their differences in binding energies with the clay counter ions, which could be explained in terms of the chemical nature of the interacting species. At high coverage, the differences could be related to the properties of the bulk liquids, because high coverage corresponds to high activities of the volatiles on the clay samples. The differences observed in the measured rates of release of the volatiles from the smectite clays could be rationalized by the use of finite element calculations and the semi-analytical model developed for the bulk liquids, taking as input parameters the measured equilibrium desorption isotherms, that determines the volatile gas phase concentration at the surface of the sample. The slower rates of release of ethanol and ethyl acetate from the smectite clay, compared to water, could be explained from their differences in their equilibrium desorption isotherms from the clay, though diffusion effects also played a minor role. However, the slower rate of release of toluene, compared to water, could only be explained by its slow diffusion in the smectite clay. Diffusion effects were found to be enhanced with an increase in the size of the aggregate, such that, in addition to the binding strength of the counter ions, the swelling capacity of the clay, allowing the clay to clump, was also found to be an important factor for the controlled release of volatiles from the clay. Ion-exchange of the clay with lithium cations was found to be optimal in terms of ionic strength of the cation and swelling capacity, as compared to other metallic cations and small organic cations. This clay modification was tested on the release of malodorous compounds determined in cat urine by mass spectrometry and sniffing experiments. The smectite clay modified by ion-exchange with lithium cations showed improved controlled release of volatile organic compounds found in cat urine, compared to the sodium smectite, and improved clumping capacity compared to the calcium smectite. The calcium smectite clay used by Nestlé Purina could be modified by column exchange with lithium cations in high quantities. However the potential toxicity of the lithium is unknown. Additional information was obtained by ab initio calculations on the interaction of water with the smectite. The calculations of the binding of one to six water molecules on the surface of a sodium clay showed the water molecules bind to the cation and form hydrogen bonds between each other and with the clay oxygens, leading to the formation of different configurations for the same number of water molecules on the clay surface. With an increase in the number of water molecules on the surface of the clay, intermolecular interaction between the water molecules became gradually more important than sodium-water interactions. The binding energy was also found to be dependent on the location of the charge substitution in the clay (tetrahedral or octahedral) and to the formation of hydrogen bonds between the water molecule and a basal oxygen linked to a tetrahedral replacement. With an increase in the number of water molecules bound to one counter ion, the sodium counter ion was found to move away from the clay surface. The dynamic calculations of water molecules in the clay interlayer showed both the water molecules and the cations to be mobile. The average diffusion coefficient of the water molecules was reduced compared to that of bulk water, due to restrictions imposed by both the clay platelets and the counter ions. The structure of the water molecules showed to be similar to that of bulk water with a preferential orientation of their hydroxyl group almost perpendicular to the clay surface. The difference in the state of order of the water molecules around the sodium cation was interpreted as a symptom of hysteresis as observed in the measured equilibrium sorption isotherm.

EPFL2009

Integration of Organic Rankine Cycles for Waste Heat Recovery in Industrial Processes

Matthias Bendig

Developing a methodology that allows identifying maximumelectricity production with the help of Organic Rankine Cycles (ORC) from the waste heat of an industrial process, at the lowest specific cost, without jeopardising the increase of the industrial process’s thermal efficiency. Such is the goal of this thesis. In order to reach this goal, a software tool which is able to identify the most suitable cycle for a heat source regarding electricity production and cost efficiency is developed and explained in detail. Threemain areas of research can be identified, the definition of waste heat, the analysis of experimental data with the help of data reconciliation and the identification of a suitable working fluid and operation parameters of an ORC for any given heat source. A method for identifying, characterising and quantifying the available waste heat, which can be converted into a useful form, for any industrial system, is presented. A distinction is made between avoidable and unavoidable waste heat. Combined pinch analysis and exergy analysis is used to characterise the waste heat potential of a given process. Based on the study of two industrial processes, it will become clear how the constraints of a waste heat analysis influence the outcome and the potential for the integration of ORCs. The studies illustrate how the increase in energy efficiency and degree of heat recovery and integration of a process can be contradictory to the production of electricity with ORCs. The concept of data reconciliation is needed for the measurements, collected from two ORC demonstrators. Apart from "classical" data reconciliation, a new method is presented, which consists in including parameters as virtual measurements and time dependency of measurements. This increases the redundancy of the system and thus the overall accuracy of the reconciled values. A methodology, capable of choosing the designpoint, a suitable working fluid and a cycle configuration, for the lowest specific investment cost while at the same time maximising the electricity output, for a given process environment is developed. The new methodology uses a multi objective optimisation (master optimisation) andMixed Integer Linear Programming (MILP) problem (slave optimisation). A novel approach of combining multi objective nonlinear master optimisations with single objective linear slave optimisations is introduced, increasing the reliability of the algorithm.