14:00
System design and optimization V
Chair: Dr. Sankar Mohan
14:00
20 mins
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EVALUATION OF PINCH POINT SMOOTHING AS A MEANS TO ENHANCE THE POWER PRODUCED IN ORC UNITS WITH VARIABLE TEMPERATURE HEAT SOURCE
Mario Gaia, Claudio Pietra
Abstract: In a subcritical ORC, fed by a liquid stream, the presence of a Pinch Point causes that even a large increase of thermal exchanger surface does not involve a significant increase in thermal power subtracted from the heat source.
A number of solution like adopting hypercritical cycles, multiple evaporation levels or zeotropic mixtures as working fluid can be used to reduce the exergy loss associated with thermal exchange.
Here an alternative solution called by the authors Pinch Point Smoothing is presented, consisting in heating a fraction of the working fluid flow at lower pressure and compressing it to turbine inlet pressure.
The solution features are analyzed for a number of working fluids and cycle organization.
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14:20
20 mins
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AIR COOLER CONDENSER CHALLENGES FOR LOW GRADE WASTE HEAT ORC INSTALLATIONS IN NORTHERN CLIMATES
Doris Weiss, Hank Leibowitz
Abstract: There is increasing global interest in the recovery of energy from low temperature streams from which low-grade waste heat (<100°C) is captured in order to increase the energy efficiency and reduce emissions of various oil and gas and energy facilities. While many sources of this waste heat exist, the challenge to utilize the heat economically still remains, especially in northern climates (above the 49th parallel) where water cooling is not an option.
Currently the Organic Rankine Cycle (ORC) technology is the most promising and much emphasis is placed on developing efficient expanders to operate with new organic refrigerants i.e. R134a and R245fa. This is necessary to allow for larger units that are more economically viable.
However, even the most efficient expander and ORC unit will encounter operational challenges if certain aspects of the process are overlooked and not taken into consideration. Of prime importance in Canadian oil sands operations is the effect of ambient temperatures on air cooler/condenser design as well as the process control of the unit, both of which differ from traditional processes that use cooling water. Common practice has shown that these parameters are often overlooked in pursuit of optimizing the turbine design and cycle performance. Frequently, this results in ORC units that are not operational in northern climates and costly to modify to get them working. For example, temperature swings on the order of 15-20 °C per day are frequently encountered in the spring and fall months. The typical control scheme of a water cooled condenser does not lend itself to these types of temperature swings and many condensers have failed using the cooling water control scheme model. Another issue is the potential to increase power production at low ambient temperatures. While this can be done, it is challenging to design a system that is optimized at both high and low ambient temperatures as the working fluid can create problems within the expander if the temperature is too cold. Furthermore, heat exchanger design can be challenged when the ambient temperature falls below 0°C and the potential for higher power production is realized.
This paper will discuss the various aspects and challenges of the design of low-grade ORC units in order to provide mechanical and process engineers with design considerations to produce workable ORC units in challenging environments.
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14:40
20 mins
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INFLUENCE OF THE CONFIGURATION OF HEAT EXCHANGERS ON THE PERFORMANCE OF ORCs: A FIRST STEP TO A SYSTEM OPTIMIZATION
Daniël Walraven, Ben Laenen, William D'haeseleer
Abstract: An ORC can be split into two levels. The first level contains the cycle parameters (temperatures and pressures) and the second level contains the components (heat exchangers, turbine, etc.). Often, both levels are optimized seperately and the interconnection between these two levels is chosen, based on experience. Pinch point temperature differences, pressure drops, etc. are assumed. This design process will in general not lead to the optimum system configuration. In a system optimization, the two levels of the ORC are optimized together and no assumptions have to be made about the interconnecting parameters.
In this presentation, a simple version of a system optimization is explained. For a given heating and cooling source and for a fixed pump and turbine efficiency, the configuration of the heat exchangers is optimized together with the cycle parameters. Plate heat exchangers with chevron-type corrugations and shell-and-tube heat exchangers are modeled and can be used in the cycle. Single- and double-pressure cycles are investigated and both cycles can be a standard or a recuperated one.
The developed model is based on a previous developed code and this code is extended with heat exchanger models which are taken from the literature. The Bell-Delaware model is used for the shell-and-tube heat exchangers. For single-phase plate heat exchanger the method of Martin is used while the models of Han, Lee and Kim are used for plate-type condensers and evaporators.
The results show that plate-type heat exchangers are, as expected, more efficient than shell-and-tube heat exchangers. It is also shown that adding a second pressure level is useful, but the effect is less than in the case with ideal components. This is because adding an extra pressure level improves the fit between the heat source cooling curve and the working fluid heating curve, but this will also increase the heat transfer surface. So, the pinch point temperature difference has to increase to keep the heat transfer surface constant. A similar effect is seen in the recuperator. The average temperature difference in the desuperheater is often relatively high, so replacing it partly with a recuperator, in which the average temperature difference is mostly lower, will also increase the heat transfer surface.
This model will be further extended with a turbine model, a model for air cooled condensers and a model for mechanical draft water cooling towers. Economics will also be taken into account.
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15:00
20 mins
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MODELING AND SIMULATION OF SOLAR ORC SYSTEM FOR REGIONAL FEASIBILITY STUDY
Taehong Sung, Hyun Dong Kim, Sang Youl Yoon, Kyung Chun Kim
Abstract: This study aims to develop a hydro-thermal model and simulation code of solar ORC (Organic Rankin Cycle) system, and to predict capable efficiency of energy conversion from solar to electrical and thermal energy for a regional feasibility study. Solar ORC system is one of solar power systems including photovoltaic power generation. The hydro-thermal performance of solar ORC system should be addressed for operating conditions with environment variables, working fluids and mechanical components including solar collector, expander, generator, pump, condenser, ducting, refrigerant storage tank, etc. Recent researches have focused on the system configuration, design point simulation and the suitability of various working fluids. However, the system performance strongly depends on operating conditions, especially the ambient temperature and amount of the solar radiation. In our literature review, the available energy efficiency with operating scenario under regional specific conditions has not been fully studied. We are developing a model of solar ORC system and an in-house simulation code for various classes of solar ORC system, for various working fluids, and for different mechanical components. In the seminar, the hydro-thermal model of solar ORC system and simulation results under various regional daily and annual environmental conditions will be presented. The simulation result includes available electrical and thermal energy efficiencies.
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