A comparison of piston, screw and scroll expanders for small scale Rankine cycle systems

in Proceedings of the 3rd International Conference on Microgeneration and Related Technologies (2013, April)

This paper aims at helping the designer of micro-scale Rankine Cycle heat engines to best select the expander among piston, screw and scroll machines. The first part of the paper presents a state of the ... [more ▼]

This paper aims at helping the designer of micro-scale Rankine Cycle heat engines to best select the expander among piston, screw and scroll machines. The first part of the paper presents a state of the art of these three technologies of positive displacement machines. The technical constraints inherent to each machine (rotational speed, pressure ratios, maximum temperatures, volumetric expansion ratios, etc.) are listed and the performance mentioned in the open technical and scientific literature is presented. The second part of the paper deals with the modeling of such expanders. Different simulation models are proposed: black-box, grey-box and white-box models. These three categories of modeling are specifically adapted to different purposes: design of the expander, design of the micro-CHP system, and dynamic simulation/control of the CHP unit. The last part of the paper presents a graphical methodology of selection of expansion machines and working fluids based on operating maps. It is stressed that the selections of both the expansion machine and working fluid should be conducted simultaneously. [less ▲]

Développement Durable et Augmentation des Besoins Energétiques: état des lieux et enjeux pour le futur

Conference (2013, January 24)

Variable Renewable Energy integration and the balancing challenge: Flexibility assessment in Belgium

Poster (2012, November)

The world is facing a historical increase in energy demand and energy consumption. Variable Renewable Energy (VRE) sources are considered as a solution to both environmental issues and energy demand, even ... [more ▼]

The world is facing a historical increase in energy demand and energy consumption. Variable Renewable Energy (VRE) sources are considered as a solution to both environmental issues and energy demand, even though they present high variability and low predictability. Variability and uncertainty are familiar aspects of all power systems. However, the increase of the share of renewable sources leads to new needs in terms of flexible resources. These resources can be provided by four different means: 1. Dispatchable power plants (i.e. with ramp up and ramp down capabilities) 2. Storage systems, mainly in the form of pumping stations. 3. Grid interconnections between countries 4. Demand side management. The present work focuses on the Belgian transmission grid. In order to assess its flexibility, both the technical flexible resource available and the flexibility requirement are evaluated in the considered area and the Present Variable Renewable Energy penetration Potential is calculated. [less ▲]

Advances in ORC expander design

Conference (2012, September 13)

DESIGN OF A SMALL-SCALE ORGANIC RANKINE CYCLE ENGINE USED IN A SOLAR POWER PLANT

in Proceedings of Heat Powered Cycles Conference 2012 (2012, September)

Under the economical and political pressure due to the depletion of fossil fuel and global warming potential, it is necessary to develop more sustainable techniques to provide electrical power. In this ... [more ▼]

Under the economical and political pressure due to the depletion of fossil fuel and global warming potential, it is necessary to develop more sustainable techniques to provide electrical power. In this context, medium and large scale Organic Rankine Cycle solar power plants appear to be a promising solution because of their good efficiency, robustness and acceptable economical probability. However, only a few ORC solar power plants are in operation today, but technical literature indicate that several demonstration projects are under development. The spreading out of that technology requires, among others, a good knowledge of control strategies of the entire plant comprising the field of parabolic trough collectors, the ORC engine and thermal storage systems. The present project aims at designing, building and testing a small scale ORC solar power plant (a few kWe) in order to define and optimize control strategies that could be applied to larger systems. The paper presents the design step of the solar power plant, and more specifically the ORC engine. This design is defined based on simulation models of the ORC engine and on the experience gained in the Thermodynamics laboratory in the field of testing small-scale ORC systems. The design accounts for technical limitations such as allowed operating ranges and technical maturity of components. The paper first presents the architecture of the envisioned solar plant. The choice of the different technologies of components is justified. Simulation models of components (scroll expander, plate and fin-and-tubes heat exchangers, etc.) and of the whole system are then presented. Based on those models, parametric studies are conducted in order to optimize the sizing and the operating conditions of the system and to select the most appropriate working fluid. Also, the relevance of using two expanders in series is discussed and the control of the ORC engine is investigated. Simulations indicate that ORC engine efficiency close to 12% can be reached for evaporating and condensing temperatures of 140°C and 35°C. [less ▲]

Working fluid selection and operating maps for Organic Rankine Cycle expansion machines

in Proceedings of the 21st International Compressor Conference at Purdue (2012)

Fluid selection for the Organic Rankine Cycle has been the object of an abundant literature. Most of the scientific publications focus on the cycle thermodynamic efficiency in order to select the best ... [more ▼]

Fluid selection for the Organic Rankine Cycle has been the object of an abundant literature. Most of the scientific publications focus on the cycle thermodynamic efficiency in order to select the best candidate. However, other thermodynamics properties, such as molar mass, or vapor density condition the whole design of the cycle, and its cost. For example, the molar mass influences the number of stages required in the case of an axial turbine; the volume ratio between expander supply and exhaust conditions the possibility to use a volumetric expander (whose internal volume ratio is limited); the vapor density at the expander exhaust determine the size of the expander, and of the condenser; etc. This paper considers a whole range of ORC applications, in terms of power (from the kW-scale to the multi-MW plants), heat source temperature (from 90°C to more than 300°C) or heat source nature (solar, biomass, waste heat recovery, geothermy, etc.). For each of these applications, a screening of the available fluids is performed, and their thermodynamics performance are compared with respect to the foreseen application. A detailed analysis of the most common expansion machines is then conducted, by comparing their respective operating maps for each fluid and for each application type. The considered expansion machines are the radial-inflow turbine, the screw expander, and the scroll expander, since they are the most widely used in commercial applications and/or in scientific literature. [less ▲]

Sustainable energy conversion through the use of Organic Rankine Cycles for waste heat recovery and solar applications

Doctoral thesis (2011)

This thesis contributes to the knowledge and the characterization of small-scale Organic Rankine Cycles (ORC). It is based on experimental data, thermodynamic models and case studies. The experimental ... [more ▼]

This thesis contributes to the knowledge and the characterization of small-scale Organic Rankine Cycles (ORC). It is based on experimental data, thermodynamic models and case studies. The experimental studies include: 1. A prototype of small-scale waste heat recovery ORC using an open-drive oil-free scroll expander, declined in two successive versions with major improvements. 2. A prototype of hermetic scroll expander tested on vapor test rig designed for that purpose. The achieved performance are promising, with expander overall isentropic effectivenesses higher than 70% and cycle efficiencies comparable or higher than the typical efficiencies reported in the scientific literature for the considered temperature range. New steady-state semi-empirical models of each component are developed and validated with the experimental data. The global model of the ORC prototype allows predicting its performance with a good accuracy and can be exploited to simulate possible improvements or alternative cycle configurations. Dynamic models of the cycle are also developed for the purpose of evaluating the system's reaction to transient conditions. These models are used to define and compare different control strategies. The issues of cycle optimization and fluid selection are treated using the steady-state semi-empirical models. The thermodynamic optimization of such cycles is first demonstrated by practical examples. Furthermore, three different methods for fluid selection are proposed, investigated and compared. Their respective advantages and fields of application are described. Finally, two prospective studies of small-scale ORC systems are proposed. The first one is a solar ORC designed for the rural electrification of remote regions in Africa. This prototype aims at competing with the photovoltaic technology, with the advantage of generating hot water as by-product. The second prospective study deals with the recovery of highly transient heat sources. Advanced regulation strategies are proposed to address the practical issues inherent to such systems. These strategies are compared with the state-of-the-art strategies and show a non-negligible potential of performance improvement. [less ▲]

Development of a waste heat recovery orc prototype using an oil-free scroll expander

Conference (2011, September 22)

The world is facing a historical increase in energy demand and energy consumption. As consequence the conventional fossil fuels are depleting faster with an inherent pollution causing severe damages to ... [more ▼]

The world is facing a historical increase in energy demand and energy consumption. As consequence the conventional fossil fuels are depleting faster with an inherent pollution causing severe damages to our environment. Renewable energy sources are considered as a solution to both environmental issue and energy demand. At the same time a lot of waste heat is witnessed in processes in industries. Our objective is to contribute to the development of ORC systems, that appear to us as a good solution to recover this wasted heat. In such waste heat applications, depending on the heat source flow rate and temperature, electrical power output can be as low as a few kilowatts. In this power range, there is no cost effective expansion machine available on the market. On existing prototypes, expansion devices are usually retrofitted volumetric compressors originally designed for refrigeration or air compression applications. Air compressors have the advantage to handle higher inlet temperature but tightness is often an issue in ORC application since the fluids used have a non negligible environmental impact. This paper presents the development of a small-scale WHR ORC unit at the Thermodynamic Laboratory of the University of Liège: the prototype uses a scroll expander, plate heat exchangers, a diaphragm piston pump and a liquid receiver. This system was tested with different working fluids (R123, R245fa and HFE7000) and a thermal efficiency close to 8% was obtained for a net output power of about 2 kWe. The specificity of the proposed prototype is the absence of lubrication: in order to avoid oil circulation in the ORC loop, an oil-free scroll expander is developed. This expander is originally an air scroll compressor that was modified using a magnetic coupling to ensure tightness. The experimental results highlight the good efficiency of the device, despite a relatively high internal leakage due to absence of lubrication. The necessity of using magnetic coupling is also justified by comparing the experimental results with previous ones obtained using mechanical sealing. [less ▲]

Rural electrification through decentralized concentrating solar power: technological and socio-economic aspects

(2011, September)

This paper presents the development of small-scale solar Organic Rankin Cycle for rural electrification in remote areas of Lesotho. It is subdivided in two parts. The first part deals with the success ... [more ▼]

This paper presents the development of small-scale solar Organic Rankin Cycle for rural electrification in remote areas of Lesotho. It is subdivided in two parts. The first part deals with the success conditions of decentralized rural electrification projects. Through a literature survey, different guiding principles and recommendations are formulated. The second part of the paper describes the proposed system. This system is designed in agreement with the above recommendations. [less ▲]

Experimental characterization of a hermetic scroll expander for use in a micro-scale Rankine cycle

in Proceedings of the Institution of Mechanical Engineers, Part A, Journal of Power and Energy (2011)

This paper presents the results of an experimental study carried out on a prototype of a hermetic scroll expander, integrated into a gas cycle test rig, whose working fluid is HFC-245fa. This system was ... [more ▼]

This paper presents the results of an experimental study carried out on a prototype of a hermetic scroll expander, integrated into a gas cycle test rig, whose working fluid is HFC-245fa. This system was designed to test only the performance of the expander. It is mainly made up of a scroll compressor, a scroll expander, a heat exchanger and a by-pass valve. The latter is used to adjust the pressure ratio imposed to the expander. The expander is originally a compressor designed for heat pump applications and characterized by a nominal power input of 2.5 kWe. Performance of the expander is evaluated in terms of isentropic effectiveness and filling factor as function of the main operating conditions. The study also investigates the impact of oil mass fraction on the expander performance. Maximum overall isentropic effectiveness of 71.03% is measured, which is partly explained by the good volumetric performance of the machine. Using the experimental data, parameters of a semi-empirical simulation model of the expander are identified. This model is used to analyze the measured performance of the expander. Finally, a polynomial empirical model of the expander is proposed for fast and robust simulations of ORC systems. [less ▲]