Vincent Lemort

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

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 is designed to test only the performance of the expander. It is made up mainly 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 was originally a compressor designed for heat pump applications and is 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 functions 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 per cent 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 analyse the measured performance of the expander. Finally, a polynomial empirical model of the expander is proposed for fast and robust simulations of organic Rankine cycle systems.

ThermoCycle: A Modelica library for the simulation of thermodynamic systems

This paper presents the results of an on-going project to develop ThermoCycle, an open Modelica library for the simulation of low-capacity thermodynamic cycles and thermal systems. Special attention is paid to robustness and simulation speed since dynamic simulations are often limited by numerical constraints and failures, either during initialization or during integration. Furthermore, the use of complex equations of state (EOS) to compute thermodynamic properties significantly decreases the simulation speed. In this paper, the approach adopted in the library to overcome these challenges is presented and discussed.

Smart grid energy flexible buildings through the use of heat pumps and building thermal mass as energy storage in the Belgian context

The management of electricity grids requires the supply and demand of electricity to be in balance at any point in time. To this end, electricity suppliers have to nominate their electricity bids on the day-ahead electricity market so that the forecast supply and demand are in balance. One way to reduce the cost of electricity supply is to minimize the procurement costs of electricity by shifting flexible loads from peak to off-peak hours. This can be done by offering consumers time-of-use variable electricity tariffs as an incentive to shift their demand. This study provides typologies of smart grid energy ready buildings within the context of the Belgian residential building stock and the Belgian day-ahead electricity market. Typical new residential buildings are considered, equipped with air-to-water heat pumps that supply either radiators or a floor heating system. Five heating control strategies are compared in terms of thermal comfort, energy use, cost, and flexibility. Flexibility is quantified in terms of load volumes shifted and in terms of procurement costs avoided. The first three are rule-based control strategies, whereas the last two are a smart grid-oriented optimal predictive control strategy responding to a time-varying electricity price profile. The results show that the smart grid control strategies allow reduction of procurement costs by up to 15% and the consumers cost by 13%. The flexibility, defined in terms of loads volume shifted, is increased by 3% to 14% with the same thermal comfort. The impact of building insulation level and thermal mass is also evaluated. The flexibility for load shifting is higher when shifting from a low-energy (average U-value of 0.458 W/m2K) to a very-low-energy house (average U-value of 0.152 W/m2K).

Economic Feasibility Study of a Small Scale Organic Rankine Cycle System in Waste Heat Recovery Application

The Organic Rankine Cycle (ORC) appears progressively as a promising solution to recover waste heat energy from thermal processes for electricity generation. A prototype of small-scale ORC has been built and successfully tested at the University of Liege. It uses R-245fa and R-123 as working fluid, and an oil-free scroll compressor adapted to run in expander mode. Thermodynamic model of the system was derived and validated for performance prediction. The validated thermodynamic model is used to optimize the operation of the small ORC in waste heat recovery application (ORC-WHR). For exhaust gases at 180 °C and a mass flow rate of 0.21 kg/s, a maximum net power output of 2 kWe is obtained for an evaporator pressure of 11.84 bar. The cycle thermal efficiency is 8.23 and the recuperation efficiency, 66.32%. Based on the aforementioned conditions, the economic assessment of small scale ORC-WHR was carried out using economic criteria such as levelized electricity cost (LEC), Net present value (NPV) and depreciated payback period (DPP). For a 2kWe ORC-WHR, the specific installed cost is 5775 €/kW with a LEC of 13.27 c€/kWh while for a 50 kWe, the specific installed cost is about 3034 €/kW and the LEC, 7c€/kWh. For an electricity unit price of 20 c€/kWh, the payback period of a 2 kWe system is 6 years while it is 2.5 years for a 50 kWe system. It is concluded from the study that recovering the waste heat by way of ORCs is technically and economically feasible. As recycled energy, waste heat has the same advantages as renewable energy and should benefit from the same legislative conditions (Feed-in-Laws).Copyright

Experimental performance of a piston expander in a small- scale organic Rankine cycle

Volumetric expanders are suitable for more and more applications in the field of micro- and small-scale power system as waster heat recovery or solar energy. This paper present an experimental study carried out on a swatch-plate piston expander. The expander was integrated into an ORC test-bench using R245fa. The performances are evaluated in term of isentropic efficiency and filling factor. The maximum efficiency and power reached are respectively 53% and 2 kW. Inside cylinder pressure measurements allow to compute mechanical efficiency and drown P-V diagram. A semi-empirical simulation model is then proposed, calibrated and used to analyse the different sources of losses.

Evaluation of the Energy Performance of an Organic Rankine Cycle-Based Micro Combined Heat and Power System Involving a Hermetic Scroll Expander

This paper evaluates the performance of an organic Rankine cycle (ORC) based microcombined heat and power (CHP) unit using a scroll expander. The considered system consists of a fuel boiler coupled with an ORC engine. As a preliminary step, the results of an experimental campaign and the modeling of a hermetic, lubricated scroll compressor used as an expander are presented. Then, a fluid comparison based on several criteria is conducted, leading to the selection of R245fa as working fluid for the ORC. A simulation model is then built to evaluate the performance of the system. The model associates an ORC model and a boiler model, both experimentally validated. This model is used to optimize and size the system. The optimization is performed considering two degrees of freedom: the evaporating temperature and the heat transfer fluid (HTF) mass flow rate. Seasonal simulation is finally performed with a bin method according to the standard PrEN14825 for an average European climate and for four heat emitter heating curves. Simulation results show that the electrical efficiency of the system varies from 6.35% for hot water at 65 � C (high temperature application) to 8.6% for a hot water temperature of 22 � C (low temperature application). Over one entire year, the system exhibits an overall electrical efficiency of about 8% and an overall thermal efficiency around 87% without significant difference between the four heat emitter heating curves. Finally, some improvements of the scroll expander are evaluated. It is shown that by increasing the maximum inlet temperature (limited to 140 � C due to technical reasons) and using two scroll expanders in series, the overall electrical efficiency reaches 12.5%. [DOI: 10.1115/1.4023116]

Increasing the efficiency of Organic Rankine Cycle Technology by means of Multivariable Predictive Control

Abstract The Organic Rankine Cycle (ORC) technology has become very popular, as it is extremely suitable for waste heat recovery from low-grade heat sources. As the ORC system is a strongly coupled nonlinear multiple-input multiple-output (MIMO) process, conventional control strategies (e.g. PID) may not achieve satisfactory results. In this contribution our focus is on the accurate regulation of the superheating, in order to increase the efficiency of the cycle and to avoid the formation of liquid droplets that could damage the expander. To this end, a multivariable Model Predictive Control (MPC) strategy is proposed, its performance is compared to the one of PI controllers for the case of variable waste-heat source profiles.

Experimental study of Predictive Control strategies for optimal operation of Organic Rankine Cycle systems

In this paper the performance of Model Predictive Control (MPC) and PID based strategies to optimally recover waste heat using Organic Rankine Cycle (ORC) technology is investigated. First the relationship between the evaporating temperature and the output power is experimentally evaluated, concluding that for some given heat source conditions there exists an optimal evaporating temperature which maximizes the energy production. Three different control strategies MPC and PID based are developed in order not only to maximize energy production but to ensure safety conditions in the machine. For the case of the MPC, the Extended Prediction Self-Adaptive Control (EPSAC) algorithm is considered in this study as it uses input/output models for prediction, avoiding the need of state estimators, making of it a suitable tool for industrial applications. The experimental results obtained on a 11kWe pilot plant show that the constrained EPSAC-MPC outperforms PID based strategies, as it allows to accurately regulate the evaporating temperature with a lower control effort while keeping the superheating in a safer operating range.

Low-order models of a single-screw expander for organic Rankine cycle applications

Screw-type volumetric expanders have been demonstrated to be a suitable technology for organic Rankine cycle (ORC) systems because of higher overall effectiveness and good part-load behaviour over other positive displacement machines. An 11 kWe single-screw expander (SSE) adapted from an air compressor has been tested in an ORC test-rig operating with R245fa as working fluid. A total of 60 steady-steady points have been obtained at four different rotational speeds of the expander in the range between 2000 rpm and 3300 rpm. The maximum electrical power output and overall isentropic effectiveness measured were 7.3 kW and 51.9%, respectively. In this paper, a comparison between two low-order models is proposed in terms of accuracy of the predictions, the robustness of the model and the computational time. The first model is the Pacejka equation-based model and the second is a semi-empirical model derived from a well-known scroll expander model and modified to include the geometric aspects of a single screw expander. The models have been calibrated with the available steady-state measurement points by identifying the proper parameters.

From model validation to production of reference simulations: how to increase reliability and applicability of building and HVAC simulation models

Validation of simulation models appears from a long time as a key issue in order to promote a more intensive and more efficient use of simulation models in the field of building and HVAC simulation. IEA Annex 34/43 originally targeted a number of specific applications where a more advanced validation was required: ground coupling problems, multizone building, shading, day lighting and cooling load interaction, HVAC components and ventilated facades. These validation exercises were built on the large methodological experience obtained in previous projects and address sometimes very fundamental problems of heat transfer in buildings. Consulting engineers and practitioners might see these exercises as a bit too far from their objectives and it is the reason why an additional activity was proposed with the specific aim of producing, based upon the results of the validation of models, a set of reference simulations. These applications cover a range of building types (residential, commercial) and systems (production, distribution emission) and run in a variety of climates. The paper will describe how models dedicated to these applications were developed, starting from validation results, going through the selection and consolidation of simulation hypothesises and ending with a number that might be considered as reference for the concerned applications. The paper will concentrate on models required by a residential building application (multizone building equipped with a heat pump or a condensing boiler, heat emitted by radiators or floor heating systems). Simulations make use of both EES and TRNSYS software and both software are applied in parallel as far as possible in the different applications in order to get a better judgment of their potential advantages and drawbacks. The use of reference simulations in view of qualifying normative methods currently in development in the frame of the European Energy Performance in Buildings Directive is also addressed and demonstrated in the paper.