10:30
Session 1D: Hybrid systems
Chair: Konstantinos Braimakis
10:30
20 mins
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Exergetic Efficiency of the Ejector Operating Across Ambient Temperature in a Combined Power and Ejector-refrigeration Cycle
Hossein Akbari, Mikhail V. Sorin
Abstract: Exergy analysis became a standard method for efficiency assessment and optimization of thermodynamic systems and their components. An extensive literature review reveals that exergetic efficiency is often open to individual interpretations which leads to several coefficients of exergy efficiency such as the input-output efficiency, the consumed-produced efficiency and fuel-product efficiency [1]. Serious shortcomings of the aforementioned efficiency definitions while applying to the expansion and compression processes operating across or below ambient temperature were revealed by Sorin and Khennich [2]. They have also recently presented a definition of the coefficient of exergy efficiency for components based on the concept of transiting exergy, i.e. the part of the exergy that has been unaffected within an analysed process. The transiting exergy concept provides unique definitions of the exergy produced and exergy consumed in a process operating across or below ambient temperature and the exergetic efficiency as a result of the ratio between these values.
The main objective of this research is to determine the exergetic efficiency of the components of a combined power and ejector-refrigeration cycle with focus on the single-phase ejector operating across ambient temperature and its effects on the overall exergetic efficiency of the system. The system is a combination of Organic Rankine Cycle and ejector-based refrigeration. A fraction of fluid entering the turbine is withdrawn and enters the ejector as its primary flow. The remaining flow in the turbine expands further and is mixed with the outlet of the ejector (the latter is the sum of primary flow and the entrained stream coming from the evaporator) [3]. In this research, the effects of operating conditions of the ejector, i.e. the pressure and mass flow rate of the primary flow, the pressure of the evaporator and the pressure of condenser (back pressure) on the exergetic efficiencies of the ejector and the overall system have been investigated. The link between the efficiency of the overall system and the exergetic (transiting) efficiency of the ejector is established.
[1] D. Marmolejo-Correa and T. Gundersen, “A comparison of exergy efficiency definitions with focus on low temperature processes,” Energy, vol. 44, no. 1, pp. 477–489, 2012.
[2] M. Sorin and M. Khennich, “Exergy Flows Inside Expansion and Compression Devices Operating below and across Ambient Temperature,” in Energy Systems and Environment, vol. 2, InTech, 2018, p. 64.
[3] A. Habibzadeh, M. M. Rashidi, and N. Galanis, “Analysis of a combined power and ejector-refrigeration cycle using low temperature heat,” Energy Convers. Manag., vol. 65, pp. 381–391, 2013.
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10:50
20 mins
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Thermodynamic Analysis of a Two-stage Organic Rankine Cycle with Ejector
Almutaz ballah R Algharbawi, Karsten Hasselmann, Stefan aus der Wiesche
Abstract: It is well known that lowering the expander backpressure is beneficial for the performance of steam and organic Rankine cycles (ORC). Typically, the temperature level of the coolant medium governs the achievable backpressure level, but in several turbine applications diffusers are also applied for lowering the exhaust pressure level.
In the case of small and medium organic Rankine cycles it is in principle possible to achieve significant backpressure level reductions by means of ejectors (or jet pump devices). This approach was recently proposed by Xinguo Li and co-workers. In their so-called EORC concept two independent evaporators were applied, and the vapour from the secondary evaporator worked as primary fluid for the ejector to suck the exhaust from the expander so as to decrease the backpressure. Some minor technical advantages were identified for that EORC, but a thermodynamic analysis indicated a substantial exergy penalty for driving the ejector by means of an independently generated vapour.
With a look to a second-law thermodynamic analysis, it is obvious to drive the ejector by second-stage vapour extracted after a first expansion. Then, a two-stage organic Rankine cycle with ejector results without the need to use a secondary evaporator. Furthermore, the vapour extraction lowers the low-pressure volume flow rate, and a rather compact turbine design including the ejector can be achieved.
In this contribution results of a detailed thermodynamic analysis of the two-stage organic Rankine cycle with ejector are presented. The different cycle concepts were analyzed by means of the commercial process simulation tool EBSILON Professional developed by STEAG. This tool permits the simulation of cycle performance under the assumption of realistic component performance behaviour and fluid properties based on REFPROP data. The ejector performance was modelled in detail, and implemented as sub-routine within the simulation process. The thermodynamic analysis showed promising results for this novel cycle concept in terms of both output capacity and efficiency.
This work was financially supported by the academic exchange program “NRW-Nahost (Israel, Palästina, Jordanien)”. The fruitful discussion about ejector modelling with the development team of EBSILON Professional is also acknowledged.
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11:10
20 mins
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Off-design Analysis of Organic Rankine Cycle Integrated with Proton Exchange Membrane Fuel Cell
Hong Wone Choi, Jin Young Park, Dong Kyu Kim, Min Soo Kim
Abstract: The primary target of this study is to examine performance shift during off-design operation of proton exchange membrane fuel cell (PEM fuel cell). Recently, the organic Rankine cycle (ORC) system has been considered to improve the energy efficiency of the PEM fuel cell system through recovering waste heat from a fuel cell stack. However, due to deterioration of fuel cell or change of operating conditions, a heat source temperature of the ORC system can be varied which influence the performance of the ORC system. A key question of this research is, thus, how the performance of the ORC system changes from design point concerning a variation of temperature and mass flow rate of coolant of the fuel cell system. To investigate the off-design performance of the ORC system, mathematical models of the ORC system and PEM fuel cell are set up under a steady-state regime. The amount of heat played a more significant role in ORC performance when the evaporator size was enough to absorb waste heat of fuel cell stack fully. The performance of PEM fuel cell decreased with a rise of cell temperature from 343 K. But, the ORC system can compensate for the deterioration of PEM fuel cell.
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11:30
20 mins
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Assessing Fuel Consumption Reduction of Revercycle a Reversible Mobile Air Conditioning/ Organic Rankine Cycle System
Luca Di Cairano, Wissam Bou Nader, Florent Breque, Maroun Nemer
Abstract: Regulators are imposing a reduction of fleet fuel consumption to car manufacturers due to the global warming. Among the different technologies able to improve vehicle efficiency Organic Rankine Cycle (ORC) is a promising solution. ORC systems are studied from more than 40 years, but their commercial success is hindered by the compactness and cost requirements of the automotive sector. In the attempt to overcome these limits a reversible Mobile Air Conditioning (MAC)/ORC, called hereafter ReverCycle, is developed by the CES in collaboration with PSA Group. ReverCycle is a compact system able to operate in two different modes: a standard mobile air conditioning system, when a cabin cooling need is required, or an ORC recovering mechanical energy from the waste heat of an engine cooling system.
This paper presents a simulation methodology to assess ReverCycle fuel consumption gain. A system approach is developed with a global light duty vehicle model. The global model allows estimating the yearly working hours for each of the ReverCycle operating modes and to quantify the recovered mechanical energy in ORC mode. By coupling the two results it is possible to provide the fuel consumption reduction for a given climatic region. In order to be as close as possible to real driving conditions the calculation of the waste heat recovery potential is based on a WLTP cycle at different ambient temperatures.
In a temperate zone the MAC activation is limited to 21% of trip occurrences. ReverCycle average fuel consumption reduction is 1.3% with cold start conditions and 2 % with hot start conditions. The reversible MAC/ORC system loses 25% of the ORC waste heat recovery potential due to MAC activation time, but there is an advantage of a significant cost and compactness reduction.
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11:50
20 mins
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Thermo-economic Investigation of a Hybrid Solar/Biomass Multigeneration System for off-grid Communities
Ammar Mouaky, Adil Rachek
Abstract: Multigeneration systems driven by renewables sources are a relevant solution enabling clean, flexible and efficient generation of numerous outputs for different applications. Within this scope, this work aims to evaluate the thermo-economic performances of a renewable energy- based polygeneration system supplying electricity, freshwater, domestic hot water, space heating and cooling requirements for an off-grid community, based on the combination of compound parabolic collectors, a biomass boiler, an ORC and a vapor compression cycle (VCC).
Solar collectors are used to preheat the heat transfer fluid (HTF). During highly solar irradiated off-peak periods, a fraction of the preheated HTF is transferred to charge a thermocline thermal energy storage, with the aim to reuse the stored energy during peak-demand periods; whereas the remaining part of the HTF is sequentially transmitted to the biomass boiler, providing the complementary energy to drive the ORC. Rejected heat from the ORC is recovered through a counter-current heat exchanger to meet domestic hot water and space heating requirement of the community. Depending on the community electricity load demand, part of the turbine energy will be used to drive the VCC (summer period) or a reverse osmosis unit (summer and winter periods), allowing a continuous operation of the system.
A model of the studied system was built in Ebsilon® Professional and simulations were conducted to assess the plant’s thermodynamic and exergoeconomic performances under both winter and summer modes operation. Results showed that the considered system is able to meet the community requirements in winter mode with a solar field contribution of 29.7 %, an ORC exergy efficiency of 38.8 %, a global exergy efficiency of 6.55 % and a total exergetic cost of 31 €/month/inhabitant, whereas in summer mode solar field’s contribution reaches 17.8 %, ORC exergy efficiency is 37.8 %, global exergetic efficiency is 6.43% and the total exergetic cost is 55 €/month/inhabitant.
The obtained results showed that the considered system can be a promising solution to provide basic requirements for isolated communities. Sensitivity analysis will be conducted to assess the impact of main system’s parameters (solar field aperture, storage volume, working fluids) on the plant’s performances.
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12:10
20 mins
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Simulation of a Pumped Thermal Energy Storage Based on a Reversible HP-ORC-system
Bernd Eppinger, Lars Zigan, Stefan Will
Abstract: The use of renewable energy sources such as solar and wind power leads to variations in the electricity supply especially during the night phase. This entails the need for storage facilities to balance the electrical load profile and a storage that does not degrade during the loading and unloading cycles. One solution consists of a pressurized hot water storage tank which is loaded with a heat pump (HP) and unloaded with an ORC (Organic Rankine Cycle). In order to minimise the electrical energy required to charge the storage tank, a waste heat flow at an elevated temperature level (<100°C) is used. By upgrading the waste heat flow through the heat pump and storing the upgraded heat, it is possible to convert it back into electricity via an ORC when it is required and thus to recapture a large proportion of the exergy of the supplied electrical power. In order to minimise the material costs of the system, double use of the components for the heat pump and ORC modes is also considered.
The results of our simulations show the influence of the heat source used, the storage temperature, the storage type (sensible or latent) as well as the selection of fluid and the working machine on the overall process. In particular, the choice of fluid and working machine is decisive for the efficiency of the plant.
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