Multi-vane expanders: Geometry and vane kinematics

A multi-vane expander (MVE) offers considerable promise as a prime mover for organic Rankine-cycle engines utilising solar energy or waste heat to provide the power input.¹ Therefore, mathematical models describing an expander's behaviour have been constructed; these models are applicable to a wide range of MVEs. For two existing designs, the geometrical characteristics and the vane kinematics have been evaluated using the computer sub-routines composed.The results indicate that the neglect of the vanes' thickness when evaluating the geometrical characteristics of the expanders can lead to considerable errors in their predicted volume expansion ratios, especially when numerous vanes and small inlet angles are employed. The expected effects of the geometrical parameters of the expanders on their performances are discussed.     

Multi-vane expanders as prime movers for low-grade energy organic Rankine-cycle engines

The behaviour of the prime mover in a low-grade energy Rankine-cycle engine is one of the most important factors affecting the overall system performance.A survey of published data concerning various types of expander in use in Rankine engines and an analysis based on the concept of similarity shows that, for the low power outputs, positive-displacement expanders have potential advantages when compared with turbines. However, the multi-vane expander (i.e. the MVE) offers considerable promise as the most appropriate prime mover for organic Rankine-cycle engines utilizing solar energy or waste heat as energy inputs, especially in the developing countries. The features and characteristics of the appropriate MVE's are discussed, together with the operational problems remaining to be solved.     

Selection of operating conditions and optimisation of design parameters for multi-vane expanders

Multi-vane expanders possess many advantages as prime movers for organic Rankine-cycle engines utilising low-grade heat as the energy inputs. The performances of these devices can be improved considerably by the proper selection of the operating conditions for existing units and by optimising their designs proposed for specified applications. Computer programs, composed for predicting the behaviours of multi-vane expanders, were employed to ascertain the most suitable operating conditions for two existing designs. A simple formula for selecting the optimal working pressure ratios of the expanders, using R-113 as the working fluid, is presented. Optimal values of the design parameters of the expanders were determined for selected operating conditions. The performances of the optimised designs of the considered expanders are relatively insensitive to changes in the operating conditions of the machines in the vicinity of their optimised values. This is a desirable feature of prime movers in Rankine engines utilising fluctuating low-grade energy sources, such as solar energy.     

Optimal design and operating conditions for a multi-vane expander

Multi-vane expanders (MVEs) possess several advantages over turbines and other positive-displacement machines (i.e. reciprocating or screw expanders) as prime movers for low grade energy engines, particularly for systems with low (< 10 kW) power outputs. The performances of such devices can be improved considerably by controlling their operating conditions and by optimising their designs. A suite of computer programs developed for simulating a MVE's behaviour was employed to predict the most appropriate operating conditions and design for a non-circular MVE. A prototype of the optimised expander design was built and operated. It exhibited isentropic efficiencies in excess of 73 per cent at rotational speeds up to 3000 rpm. This is comparable with those of the most advanced rival prime movers for the considered ranges of speed and power output.     

Rankine-cycle systems for harnessing power from low-grade energy sources

Rankine-cycle systems, each employing a single organic compound as the working medium, are the most commonly used units for converting low-temperature heat into mechanical work. The performances of these systems have been analysed and simulated. The composed interactive computer programs (in BASIC) for predicting the properties of the candidate organic fluids, and for evaluating the behaviour of the simple and regenerative Rankine-cycle units, are listed. The accuracies of the estimated values of the thermodynamic properties have been assessed. Samples of the performance characteristics predicted, employing the routine developed, are given. The programs should help facilitate (i) choosing the most appropriate working fluid from amongst those considered, as well as (ii) predicting the optimal design and operating conditions of a proposed system for a particular application.     

Influences of vane design and lubricant on a multi-vane expander''s performance

Multi-vane expanders possess advantages as prime movers for organic Rankine-cycle engines stimulated by low-grade heat, particularly for low-power output applications. The behaviours of the vanes in these devices influence radically their overall performances. In this study, the effects of the percentage of the lubricating oil mixed with the working fluid, the arrangement of the pressurising slots machined into the vanes and the vane material upon the performance of a multi-vane expander are investigated. The experimental results indicate that the performances and operation reliabilities of multi-vane expanders can be improved considerably by optimising these design and operational variables.     

Multi-vane expanders: Vane dynamics and friction losses

Mechanical friction and viscous drag represent major causes of power loss in multi-vane expanders. So a mathematical model has been developed to describe these loss phenomena. The constructed dynamic model was employed to investigate the loss of contact between the vanes and the stator-cylinder, which results in significant internal leakage losses. Two existing designs of multi-vane expander were considered in detail when using R-113 as the working fluid. The composed computer sub-routines were used to predict the effects of the different design parameters and operating conditions on the mechanical efficiencies of the expanders.

The predictions obtained indicate that most of the frictional power loss in a multi-vane expander occurs due to the rubbing of the vanes against the stator-cylinder. Either the operating conditions for an existing expander can be controlled, or the optimal design parameters of a proposed expander for a particular application can be selected, in order to maximise the appropriate multi-vane expander's mechanical efficiency.     

Multi-vane expander performance: Breathing characteristics

The throttling of the working fluid flow at the inlet and exhaust port openings of multi-vane expanders leads to power losses. Therefore, a mathematical model has been developed to analyse their breathing characteristics. In particular two existing designs of multi-vane expanders, using refrigerant 113 as the working fluid, were considered. The results obtained indicate that multi-vane expanders exhibit easy-breathing behaviours. Improved breathing characteristics can be achieved by the wise selection of operating conditions for any existing design or by optimising the design parameters for a specific application.     

Energy performance and numerical optimization of a screw expander–based solar thermal electricity system in a wide range of fluctuating operating conditions

This paper provides fundamental principles to study the thermodynamic performance of a new screw expander–based solar thermal electricity plant. While steam turbines are generally used in direct steam generation solar systems without admitting fluid in two-phase conditions, steam screw expanders, as volumetric machines, can convert thermal to mechanical energy also by expanding liquid-steam mixtures without a decline in efficiency. In effect, steam turbines are not as competitive as screw expanders when the net power is smaller than 2 MW and for low-grade heat sources. The solar electricity generation system proposed in this paper is based on the steam Rankine cycle: Water is used as both working fluid and storage, parabolic trough collectors are used as a thermal source, and screw expanders are used as power machines. Since screw expanders can operate at off-design working conditions in several situations when installed in direct steam generation solar plants, studying expander performance under fluctuating working situations is a crucial issue. The main aim of the present paper is to establish a thermodynamic model to study the energetic benefits of the proposed power system when off-design operating conditions and variable solar radiation occur. This entails, first and foremost, developing overexpansion and underexpansion numerical models to describe the polytropic expansion phase, which considers all the losses affecting performance of the screw expander under real operating conditions. To assess the best operating conditions and maximum efficiency of the whole power system at part-load working conditions under fluctuating solar radiations, parametric optimization is then improved in a wide range of variable working conditions, assuming condensation pressures of water increasing from 0.1 to 1 bar, under an evaporation temperature rising from 170°C to 300°C.     

Multi-vane expanders: Internal-leakage losses

Secondary flows of the working fluid via internal leakage paths in an MVE contribute significantly to its efficiency reduction. So the major leakage channels were indentified, and a mathematical model for predicting the flow rates via them has been developed. The behaviours of two existing designs of MVEs, using R-113 as the working fluid, were considered. Computer sub-routines were composed and employed to predict the effects of changes in the design and operating variables on the leakage characteristics of the expanders.The predictions indicate that the major internal-leakage flows, in the two MVEs considered, occur past the tips of the vanes. By maintaining continuous contact between the tips of the vanes and the stator cylinder, leakage efficiencies of over 75% are expected for the non-circular expander. The predictions also show by how much the leakage characteristics can be improved by running an existing expander at its optimal operating conditions. Alternatively for a proposed application, there will be an optimal choice of design parameters for an MVE so that the maximum efficiency can be achieved.     

A new analytical model for temperature predictions

A new analytical model for predicting surface air temperature as well as (solar-derived) heat energy variations is described. This model, which can be used for global or regional heat/temperature predictions, is much simpler than general circulation models (GCMs) and requires relatively less computer power to the extent that it can be implemented on a simple personal computer. The usefulness and accuracy of the new model are ascertained through sample predictions made for equatorial regions from 1982 to 2009. The new model offers a cheaper alternative for implementing global and regional predictions which have been almost exclusively done through GCMs. Fortunately, this cheaper alternative is easily affordable even by individual departments/universities in developing countries which have been unable to implement locally any of the current GCMs due to the latter's high computer power demand and intricacy.     

Multistate Wind Energy Conversion System Models for Adequacy Assessment of Generating Systems Incorporating Wind Energy

Wind energy is considered to be a very promising alternative for power generation because of its tremendous environmental, social, and economic benefits. Electrical power generation from wind energy behaves quite differently from that of conventional sources. The fundamentally different operating characteristics of those facilities, therefore, affect the power system reliability in a manner different from that of the conventional systems. This paper is focused on the development of suitable models for wind energy conversion systems, in adequacy assessments of generating systems, using wind energy. These analytical models can be used in the conventional generating system adequacy assessment utilizing analytical or Monte Carlo state-sampling techniques. This paper shows that a five-state wind energy conversion system model can be used to provide a reasonable assessment of the practical power system adequacy studies, using an analytical method, or a state-sampling simulation approach.     

Performance analysis of a solar-powered/fuel-assisted Rankine cycle with a novel 30 hp turbine

The subject of this analysis is a novel hybrid steam Rankine cycle, which was designed to drive a conventional open-compressor chiller, but is equally applicable to power generation. Steam is to be generated by the use of solar energy collected at about 100°C, and is then to be superheated to about 600°C in a fossil-fuel fired superheater. The steam is to drive a novel counter-rotating turbine, and most of its exhaust heat is regenerated. A comprehensive computer program developed to analyze the operation and performance of the basic power cycle is described. Each component was defined by a separate subroutine which computes its realistic off-design performance from basic principles. Detailed predicted performance maps of the turbine and the basic power cycle were obtained as a function of turbine speed, inlet pressure, inlet temperature, condensing temperature, steam mass flow rate, and the superheater's fuel consumption rate. Some of the major conclusions are: (1) the turbine's efficiency is quite constant, varying in the range of 68.5–76.5 per cent (75 per cent at design) for all conditions, (2) the efficiency of the basic power cycle is 18.3 per cent at design, more than double as compared to organic fluid cycles operating at similar solar input temperatures, at the expense of adding only 20 per cent non-solar energy. This, combined with the fact that actual organic Rankine cycles operate typically at temperatures above 140°C, predicts that this system would be economically superior by using less than half of the collector area and by also using less expensive collectors.     

A novel comprehensive procedure for determination of optimum operating conditions for cleaner energy production system

Cleaner energy production system such as direct alcohol fuel cells (DAFCs) are considered as an alternative source for generating cleaner energy. Studies based on design of catalysts, electrodes design, proton exchange membrane, and flow field were conducted for improving its performance characteristic such as power density. However, the less focus was paid on determining the operating conditions considering the uncertainties that will result in an increase of power density of DAFCs. Therefore, the present work proposes a novel comprehensive procedure involving experimental study and evolutionary approach of genetic programming (GP) in formulation of robust power density models for DAFCs. Two uncertainties such as the selection of objective function and variations in measurement of operating conditions are incorporated in framework of GP. The power density models incorporate the formulation of new objective function in GP that will result in higher accuracy of the models. Experiments performed on DAFCs validate performance of the models. Simulation profiler is then generated for models to verify its robustness in uncertain operating conditions. The inferences on relationships between power density and operating conditions for DAFCs are made by surface analysis of the models.     

Performance modelling and analysis of double-pass packed-bed solar air heaters: an analytical framework

In this paper, the thermal performance of double-pass packed-bed solar air heaters is investigated theoretically. A suitable computer program is developed for the analytical solution of the mathematical model which involves the energy balance equations for the different components of the air heaters. Three different characteristics of the double-pass packed-bed solar air heaters are considered for the analysis. The results obtained from the analytical models are compared with the experimental model data obtained from the previous works and showed that good agreement is achieved.     

Modelling and testing of a hybrid solar‐biomass ORC‐based micro‐CHP system

Expanders employed recently in organic Rankine cycle (ORC)‐based systems suffer from key problems including excessive working fluid leakage, thermal losses, low isentropic efficiency and high cost. The majority of the units available in the market are for medium and large‐scale applications (>100 kW) with no commercial micro‐scale expanders available and applicable for ORC units for residential and building applications. Moreover, the majority of the studies conducted on ORC expanders employed HFC and HCFC working fluids which have high global warming potential leading to negative environmental impacts. In this study, a micro‐scale CHP system based on the ORC technology is theoretically and experimentally investigated to provide the thermal needs and part of the electrical demands for residential applications. An innovative design for a hybrid ORC‐based micro‐CHP system is proposed using a biomass boiler and a solar concentrator to run the CHP system providing more reliable and clean operation compared to conventional natural gas‐driven units. The micro‐CHP system employs a new type small‐scale scroll expander with a compact design, integrating the generator and the turbine in a single unit. A numerical model was developed using the Engineering Equation Solver (EES) software to simulate the thermodynamic behaviour of the ORC unit predicting the thermal and electrical performance of the overall CHP system. In addition, an experimental setup was built to test the whole ORC–CHP system performance under different conditions, and the effect of various operational parameters on the system performance has been presented using an environmentally friendly HFE7100 working fluid. The maximum electric power generated by the expander was in the range of 500 W at a pressure differential of about 4.5 bars. The attained expander isentropic efficiency was over 80% at its peak operating conditions with no fluid leakage observed. Being mass‐produced with low cost in the automotive industry along with the high isentropic efficiency and the leakage‐free performance, the proposed compact scroll expander represents a potential candidate to be used in the development of micro‐scale ORC–CHP units for building applications. Copyright © 2013 John Wiley & Sons, Ltd.     

氟利昂引射式制冷机的性能研究

本文研究了引射式制冷机的性能,当工质采用氟利昂时,可以利用低位热能来制冷,编制了计算机仿真程序,分析了状态参数对制冷机性能的影响。计算结果表明这一装置适合于空调系统。     

Applying water cooled air conditioners in residential buildings in Hong Kong

The objective of this study is to conduct a realistic prediction of the potential energy saving for using water cooled air conditioners in residential buildings in Hong Kong. A split type air conditioner with air cooled (AAC) and water cooled (WAC) options was set up for experimental study at different indoor and outdoor conditions. The cooling output, power consumption and coefficient of performance (COP) of the two options were measured and calculated for comparison. The experimental results showed that the COP of the WAC is, on average, 17.4% higher than that of the AAC. The results were used to validate the mathematical models formulated for predicting the performance of WACs and AACs at different operating conditions and load characteristics. While the development of the mathematical models for WACs was reported in an earlier paper, this paper focuses on the experimental works for the AAC. The mathematical models were further used to predict the potential energy saving for application of WACs in residential buildings in Hong Kong. The predictions were based on actual building developments and realistic operating characteristics. The overall energy savings were estimated to be around 8.7% of the total electricity consumption for residential buildings in Hong Kong. Wider use of WACs in subtropical cities is, therefore, recommended.     

An improved thermal and electrical model for a solar photovoltaic thermal (PV/T) air collector

In this paper, an attempt is made to investigate the thermal and electrical performance of a solar photovoltaic thermal (PV/T) air collector. A detailed thermal and electrical model is developed to calculate the thermal and electrical parameters of a typical PV/T air collector. The thermal and electrical parameters of a PV/T air collector include solar cell temperature, back surface temperature, outlet air temperature, open-circuit voltage, short-circuit current, maximum power point voltage, maximum power point current, etc. Some corrections are done on heat loss coefficients in order to improve the thermal model of a PV/T air collector. A better electrical model is used to increase the calculations precision of PV/T air collector electrical parameters. Unlike the conventional electrical models used in the previous literature, the electrical model presented in this paper can estimate the electrical parameters of a PV/T air collector such as open-circuit voltage, short-circuit current, maximum power point voltage, and maximum power point current. Further, an analytical expression for the overall energy efficiency of a PV/T air collector is derived in terms of thermal, electrical, design and climatic parameters. A computer simulation program is developed in order to calculate the thermal and electrical parameters of a PV/T air collector. The results of numerical simulation are in good agreement with the experimental measurements noted in the previous literature. Finally, parametric studies have been carried out. Since some corrections have been down on thermal and electrical models, it is observed that the thermal and electrical simulation results obtained in this paper is more precise than the one given by the previous literature. It is also found that the thermal efficiency, electrical efficiency and overall energy efficiency of PV/T air collector is about 17.18%, 10.01% and 45%, respectively, for a sample climatic, operating and design parameters.     

Modeling and Dynamic Performance of a Line-Commutated Photovoltaic Inverter System

Interest in utility-interactive photovoltaic (PV) inverter systems has increased over the past decade and numerous central-station PV systems have been installed. It is anticipated that as PV system costs decrease, residential systems will be installed in increased numbers. Although a substantial amount of literature is available concerning the design, protection, safety, economics, and operating experience of residential and central-station PV systems, little information is available regarding their dynamic electrical characteristics and the computer modeling of these systems. Moreover, most of the available literature concerning modeling and/or dynamic performance focuses either upon the long-term dynamic behavior as it affects power system scheduling or upon the steady-state harmonic characteristics. In recent work, highly detailed computer models of a representative set of PV systems have been developed and several of these models have been verified by comparison with system measurements [1, 2]. However, the models described in [1, 2] are more complex than necessary for large-scale power system studies in which the fast switching transients associated with the dc-to-ac inverter are of little concern and only the slower cycle-to-cycle behavior of the PV system is of interest. In fact, it is not possible to incorporate these detailed models into conventional transient stability programs due, in part, to the very small time-step requirements associated with these models. In this paper, a three-phase line-commutated utility-interactive photovoltaic inverter system is investigated. A schematic diagram of the selected PV inverter system is depicted in Fig. 1.