A systematic methodology for the techno-economic optimization of Organic Rankine Cycles

This work presents a general and systematic methodology for the techno-economic optimization of Rankine cycles. The proposed superstructure for Rankine cycles allows to reproduce a wide range of cycle configurations, such as cycles with/without regenerator, cycles with single or multiple pressure levels, and cycles integrated with multiple heat sources. The model is integrated with a recently developed methodology capable of optimizing also the arrangement and sizing of the heat exchangers of the plant (heat exchanger network synthesis). This allows to perform a full techno-economic optimization of the entire system. The resulting problem is a challenging Mixed Integer Non Linear Problem (MINLP) which is solved with an ad hoc algorithm. The methodology is applied to two case studies for power cycles with single and multiple heat sources. This work can help engineers identify the right thermodynamic cycle to integrate with an industrial process and design techno-economically optimal Rankine cycles for waste heat recovery from single or multiple heat sources, by considering heat integration and cycle design optimization simultaneously. (C) 2017 The Authors. Published by Elsevier Ltd.

Theoretical and Experimental Study of a Thermally Driven Heat Pump Based on a Double Organic Rankine Cycle

Jonathan Demierre

Nowadays, one of the main alternatives for a more rational use of energy in heating applications is the heat pumping technologies. The market is dominated by two kinds of heat pump systems: the electrically driven vapor compression heat pumps (EHP), which are the most widely used in residential heating applications and the thermally driven heat pumps (TDHP), that are usually based on a sorption process. In this thesis, theoretical and experimental investigations of a concept of thermally driven heat pump, based on the coupling of a vapor compression heat pump cycle and an organic Rankine cycle, are presented. The system is named here as ORC-ORC. The studied concept uses a single stage centrifugal compressor directly coupled to a single stage radial inflow turbine. The shaft is rotating on gas bearings, which allows the system to be oil-free. Like most of the other TDHP's, this system has the advantage to work with a variety of fuels or heat sources like wood pellets, natural gas, solar heat, geothermal heat or waste heat. The concept studied in this work is a gas red system for space heating and domestic hot water production in small residential buildings. A systematic approach has been used to evaluate, in term of energy efficiency, the potential of ORC-ORC systems and to propose optimal design specifications at different levels of the design process. The method is based on the optimization which allows to identify the best configurations at each design step with respect to the designer choices. This approach is divided in three steps. In the first step, a model of the complete system has been developed based on a process integration approach. This step allows to quickly determine whether the system is potentially attractive or not, for given conditions, before going deeper in the design process. The results show that, in general, it is advantageous for the ORC evaporation to be supercritical. In the second step, based on the process integration results and heuristic rules, a suitable system heat exchanger network has been generated and modeled. The predicted values of the coefficient of performance (COP) at design conditions, for the different situations, are in the range 1.30 and 2.19. In a last step, the off-design characteristics of the compressor-turbine unit are considered, in order to evaluate the COP as well as the heating capacity range that can be covered with a specific compressor-turbine unit design. For this purpose, a model of the compressor-turbine unit has been developed. Experimental investigations have been carried out with R134a as working fluid. A first simple test rig has been developed to test the selected ORC evaporator in supercritical conditions. The measurements have allowed to calibrate and to validate an in-house supercritical evaporator simulation tool. An ORC-ORC prototype has been developed and built. Commercially available equipment has been used, except for the compressor-turbine unit that has been designed especially for this application. The targeted operating point, with an HP evaporation temperature of 0 °C and a condensing temperature of 35 °C, has not been reached, because of experimental difficulties which caused some delay. A maximum temperature difference of 20 °C between the HP evaporation and the condensation has been achieved, which corresponds to a compressor pressure ratio of 1.9. Measurements have been performed with compressor-turbine unit rotational speeds up to 171 krpm. The theoretical investigations shows that the proposed ORC-ORC concept is an interesting alternative of thermally actuated heating device for small residential buildings. Concerning the experimental investigations, although a number of problems have been encountered during the tests, it has been demonstrated that the proposed concept is feasible with today's technology.

EPFL2012

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.

EPFL2015

Multi-objective optimization of turbo-ORC systems for waste heat recovery on passenger car engines

Eliott Philippe Guenat, Violette Mounier, Kévin Rosset, Jürg Alexander Schiffmann

Waste heat recovery from passenger car internal combustion engines by means of an organic Rankine cycle (ORC) system is promising for reducing CO2 emissions. In this study, different cycle configurations capable of converting waste heat from both coolant and exhaust gases are investigated based on different working fluid categories. Radial-inflow turbines are considered as expansion devices and corresponding isentropic efficiencies are evaluated based on a preliminary design map accounting for the effect of the pressure ratio. Mechanical losses resulting from the use of a gas-bearing-supported rotor driving a permanent magnet generator are also evaluated. In order to identify the turbo-ORC system design tradeoffs, constrained multi-variable and multi-objective optimizations are performed using an evolutionary algorithm. It is found that the optimal cycle configuration and working fluid depend on the available space in the vehicle and that the condenser is the most critical component for the ORC system integration. In addition, the most suitable working fluids for this application are characterized by (1) a boiling point close to the heat sink temperature, (2) a high critical pressure, and (3) a high molecular weight. The resulting optimal radial-inflow turbines are 10e33 mm in tip diameter and operate at 80 e330 krpm.