Effect of working-fluid mixtures on organic Rankine cycle system performance : heat transfer and cost analysis

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.A thermodynamic limitation of single-component working fluids in organic Rankine cycles (ORCs) is the large exergy destruction (and, consequently, useful power loss) associated with evaporation and condensation. Due to their non-isothermal phase-change behaviour, non-azeotropic working-fluid mixtures have shown reduced exergy losses, leading to improved cycle efficiencies and power outputs. These benefits are exclusively observed from a thermodynamic perspective. The present paper considers the effects of selecting such working-fluid mixtures on heat transfer performance, component sizing and system costs compared with those of pure fluids; a mixture of n-pentane and n-hexane is selected. While the fluid-mixture cycles do indeed allow higher efficiencies and the generation of higher power outputs, they require larger evaporators, condensers and expanders; thus, the resulting ORC systems are more expensive than those based on the pure fluids. While a working-fluid mixture (60% n-pentane + 40% n-hexane) leads to the thermodynamically optimal cycle, a pure n-pentane ORC system has reduced costs of 37% per unit power output over the thermodynamic optimum.am201

Thermoeconomic assessment of a spectral-splitting hybrid PVT system in dairy farms for combined heat and power

We investigate the thermoeconomic potential of a solar-combined heat and power (S-CHP) system based on concentrating, spectral-splitting hybrid photovoltaic-thermal (PVT) collectors for the provision of electricity, steam and hot water for processing milk products in dairy applications. Transient simulations are conducted by using a system model with real-time demand and weather data as inputs, taking account of the spectrum-selective features of the PV cells as well as key heat transfer mechanisms that determine the electrical and thermal performance of the PVT collector. Economic performance is also assessed by considering the investment and savings enabled by the reduced electrical and fuel consumption. The results show that incorporating spectral beam-splitting technology into hybrid PVT collectors can be effective in maintaining the PV cells at low temperatures, while at the same time supplying thermal outputs (fluid streams) at temperatures significantly higher than then cell temperatures for steam generation and/or hot water provision. Based on a 15, 000-m2 installed area, it is found that 80% of the thermal demand for steam generation and 60% of the hot water demand can be satisfied by the PVT S-CHP system, along with a net electrical output amounting to 60% of the demand. Economic and environmental assessments show that the system has an excellent decarbonisation potential (1, 500 tCO2/year) and is economically viable if the investment cost of the spectrum splitter is lower than 0.85 of the cost of the parabolic concentrator (i.e., <2, 150 €/m2 spectrum splitter) in this application

A coupled thermal and electrical model of a sheet-and-tube hybrid photovoltaic/thermal (PV/T) collector

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.The goal of this paper is to obtain the (pair of) efficiency curves of a hybrid PV/T collector with a sheet-and-tube design and to evaluate the effect of a non-uniform temperature distribution on the surface of the solar cell on its electrical power output. A 3-dimensional numerical model is developed to estimate the performance of such a collector. The model allows various design parameters of the PV/T to be varied so that the influence of each of these parameters can be studied on the overall system performance both at steady-state and at varying atmospheric conditions. The main parameters considered are the number of glass covers, ranging from an unglazed collector configuration to a double-glazed collector configuration, and the width-to-pipe diameter (W/D) ratio. The results show that, while the thermal efficiency increases with the additional glazing, the electrical efficiency deteriorates due to the higher temperature of the fluid and due to increased optical losses, as expected. The dynamic performance of the PV/T collector and system are also investigated. Results from the dynamic model and also from a simplified quasi-steady state model are reported. The results show that in the case of highly fluctuating incident radiation, e.g. from clouds, the quasi-steady solution can deviate by up to 20% from the dynamic solution in the evaluation of the thermal energy output in the case of low incident radiation with large fluctuations.am201

An investigation of film wavy structure in annular flow using two simultaneous lif approaches

The paper is devoted to development and validation of film thickness measurement techniques in interfacial gas-liquid flows. The specific flow investigated here is that of downwards (co-flowing) annular flow in a vertical pipe, however, many of the observations and findings are transferable to similar flow geometries. Two advanced spatially resolved techniques, namely planar laser-induced fluorescence and brightness-based laser-induced fluorescence, are used simultaneously in the same area of interrogation. A single laser sheet is used to excite fluorescence along one longitudinal section of the pipe, and two cameras (one for each method) are placed at different angles to the plane of the laser sheet in order to independently recover the shape of the interface along this section. This allows us to perform a cross-validation of the two techniques and to analyse their respective characteristics, advantages and shortcomings.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

A two-phase single-reciprocating-Piston heat conversion engine

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.This paper considers an energy-conversion heat-engine concept termed ‘Up-THERM’. This machine is capable of converting low- to medium-grade heat to useful positivedisplacement work through the periodic evaporation and condensation of a working fluid in an enclosed space. These alternating phase-change processes drive sustained oscillations of thermodynamic properties (pressure, temperature, volume) as the working fluid undergoes an unsteady thermodynamic heatengine cycle. The resulting oscillatory flow of the working fluid is converted into a unidirectional flow in a hydraulic load arrangement where power can be extracted from the machine. The engine is described with lumped dynamic models constructed using electrical analogies founded on previously developed thermoacoustic and thermofluidic principles, which are extended here to include a description of the phase-change heat-transfer processes. For some sub-components of the engine, such as the gas spring, valves and the temperature profile in the heat exchangers, deviations from the linear theory are nonnegligible. These are modelled using non-linear descriptions. In particular, the results of linear and non-linear descriptions of the gas spring are compared using three important performance indicators — efficiency, power output and frequency. The non-linear description of the gas spring results in morerealistic predictions of the oscillation frequency compared to direct measurements on an experimental prototype of a similar engine. Owing to its mode of operation and lack of moving parts, the Up-THERM engine does offer a much simpler and more cost-efficient solution than alternative engines for heat recovery and solar applications. The results from this work suggest that this technology can be a competitive alternative in terms of cost per unit power in low-power, small-scale applications, especially in remote, off-grid settings, for example in developing countries where minimising upfront costs is crucial.The research leading to these results has received funding from the 7th Framework Programme of the European Commission, grant agreement number 605826.am201

Hydrodynamic characteristics of harmonically excited thin-film flows : experiments and computations

We present new results from the simultaneous application of Planar Laser-Induced Fluorescence (PLIF) and Particle Tracking Velocimetry (PTV), complemented by Direct Numerical Simula- tions (DNSs), aimed at the detailed hydrodynamic characteriza- tion of harmonically excited liquid-film flows. The experimental campaign spans the Reynolds number range Re = 8 − 320, and three Kapitza numbers Ka = 85, 350 and 1800. PLIF was em- ployed in order to generate spatiotemporally resolved film-height data, and PTV to generate two-dimensional (2D) planar velocity- vector maps of the flow-field underneath the wavy interface. By combining the two optical techniques, instantaneous and highly localised flow-rate data were retrieved, based on which the ef- fect of local film topology on the flow-field is studied in detail. Surprisingly, the instantaneous flow rate is found to vary linearly with the instantaneous film-height, while both experimental and numerical flow-rate data are closely approximated by a simple analytical relationship with only minor deviations. This relation- ship, which is reported here for the first time, includes the wave speed c and mean flow-rate Q, both of which can be obtained by simple and inexpensive methods, thus allowing for spatiotempo- rally resolved flow-rate predictions to be made without requiring any knowledge of flow-field information.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

Integrated computer-aided working-fluid design and thermoeconomic ORC system optimisation

The successful commercialisation of organic Rankine cycle (ORC) systems across a range of power outputs and heat-source temperatures demands step-changes in both improved thermodynamic performance and reduced investment costs. The former can be achieved through high-performance components and optimised system architectures operating with novel working-fluids, whilst the latter requires careful component-technology selection, economies of scale, learning curves and a proper selection of materials and cycle configurations. In this context, thermoeconomic optimisation of the whole power-system should be completed aimed at maximising profitability. This paper couples the computer-aided molecular design (CAMD) of the working-fluid with ORC thermodynamic models, including recuperated and other alternative (e.g., partial evaporation or trilateral) cycles, and a thermoeconomic system assessment. The developed CAMD-ORC framework integrates an advanced molecular-based group-contribution equation of state, SAFT-γ Mie, with a thermodynamic description of the system, and is capable of simultaneously optimising the working-fluid structure, and the thermodynamic system. The advantage of the proposed CAMD-ORC methodology is that it removes subjective and pre-emptive screening criteria that would otherwise exist in conventional working-fluid selection studies. The framework is used to optimise hydrocarbon working-fluids for three different heat sources (150, 250 and 350 °C, each with mcp = 4.2 kW/K). In each case, the optimal combination of working-fluid and ORC system architecture is identified, and system investment costs are evaluated through component sizing models. It is observed that optimal working fluids that minimise the specific investment cost (SIC) are not the same as those that maximise power output. For the three heat sources the optimal working-fluids that minimise the SIC are isobutane, 2-pentene and 2-heptene, with SICs of 4.03, 2.22 and 1.84 £/W respectively

Exploring optimal working fluids and cycle architectures for organic rankine cycle systems using advanced computer-aided molecular design methodologies

The combination of computer-aided molecular design (CAMD) with an organic Rankine cycle (ORC) power-system model presents a powerful methodology that facilitates an integrated approach to simultaneous working-fluid design and power-system thermodynamic or thermoeconomic optimisation. Existing CAMD-ORC models have been focussed on simple subcritical, non-recuperated ORC systems. The current work introduces partially evaporated or trilateral cycles, recuperated cycles and working-fluid mixtures into the ORC power-system model, which to the best knowledge of the authors has not been previously attempted. A necessary feature of a CAMD-ORC model is the use of a mixed-integer non-linear programming (MINLP) optimiser to simultaneously optimise integer workingfluid variables and continuous thermodynamic cycle and economic variables. In this paper, this feature is exploited by introducing binary optimisation variables to describe the cycle layout, thus enabling the cycle architecture to be optimised alongside the working fluid and system conditions. After describing the models for the alternative cycles, the optimisation problem is completed for a defined heat source, considering hydrocarbon working fluids. Two specific case studies are considered, in which the power output from the ORC system is maximised. These differ in the treatment of the minimum heat-source outlet temperature, which is unconstrained in the first case study, but constrained in the second. This is done to replicate scenarios such as a combined heat and power (CHP) plant, or applications where condensation of the waste-heat stream must be avoided. In both cases it is found that a working-fluid mixture can perform better than a pure working fluid. Furthermore, it is found that partially-evaporated and recuperated cycles are optimal for the unconstrained and constrained case studies respectivelyPapers presented at the 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Portoroz, Slovenia on 17-19 July 2017 .International centre for heat and mass transfer.American society of thermal and fluids engineers