Available online at ScienceDirect. Energy Procedia 110 (2017 )

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1 Available online at ScienceDirect Energy Procedia 110 (2017 ) st International Conference on Energy and Power, ICEP2016, December 2016, RMIT University, Melbourne, Australia Power generation from low grade heat using trilateral flash cycle Md Arbab Iqbal*, Mahdi Ahmadi, Farah Melhem, Sohel Rana, Aliakbar Akbarzadeh, Abhijit Date School of Engineering, RMIT University, Melbourne 3000, Australia Abstract Though the world is suffering from energy crisis, a large amount of low grade heat (temperature lower than 100 o C) is being wasted for the lack of availability of cost effective technology. Recovery of waste heat is drawing the attention of the researcher around the world and the recovery of energy from low grade heat which has a temperature lower than 100 o C is still a big challenge. In our current work, a concept has been developed to produce electricity using a generator coupled with a Pelton turbine. Though solar pond is considered as the heat source for this conceptual design, it can be replaced by any other heat source like geothermal heat, industrial waste heat etc. Unlike Organic Ranking Cycle (ORC), heat will be added in a single phase working fluid of low boiling temperature like iso-pentane under high pressure to increase the heat transfer efficiency. Afterward, saturated liquid iso-pentane would be passed through a convergent-divergent (CD) nozzle and a high velocity jet of mixture of liquid and vapor from the nozzle exit will run the turbine. Finally, the mixture from the turbine exit would be condensed to liquid and recycled to the heat exchanger by a high pressure pump Published 2017 by Elsevier The Authors. Ltd. This Published is an open by access Elsevier article Ltd. under the CC BY-NC-ND license ( Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. Keywords: waste heat recovery, electricity generation,organic ranking cycle, trilateral cycle; 1. Introduction Not only the depletion of traditional non-renewable energy sources i.e fossil fuel, coal etc. but also the growing concern about the environmental changes forcing the researchers to search for alternative sustainable sources of energy as well as develop system for waste energy recovery. Gas turbine and steam turbine are the most common electricity * Corresponding author. Tel.: ; fax: address: s @student.rmit.edu.au Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. doi: /j.egypro

2 Md Arbab Iqbal et al. / Energy Procedia 110 ( 2017 ) generation system over the ages which are on average 20% and 40% efficient respectively. Combined cycle power plants are the most efficient system which can have energy efficiency up to 65%. However, there is still a big amount of loss of energy because of the technological limitations. If we consider a 500 MW combined cycle power plant having 65% thermal efficiency, there is still a loss of about 260 MW of energy and if we can recover only 1% of this waste energy, we can easily power a small town. Nomenclature CD ORC TLC Convergent-divergent Organic Rankine cycle Trilateral cycles Though modern cutting edge technology improved the efficiency of power generation system, there is still limitation of recovery of waste heat, specially the low grade heat having temperature lower than 100oC. For the recovery and conversion of low grade heat energy, organic Rankine cycles (ORC) and trilateral cycles (TLC) can be considered [1]. In TLC the liquid is heated up at constant pressure to its boiling point and then performs a flash expansion through a convergent-divergent (CD) nozzle during which it delivers power. If the end point of the expansion is in the wet vapour region the process is represented in the temperature vs entropy (T-s) diagram approximately by a triangle and hence this cycle is called trilateral cycle. Fig. 1 represents a typical comparison between ORC and TLC and it is clear from the figure that in TLC, heat is transferred in a single phase working fluid which is more efficient than multiphase heat transfer like ORC. Fischer [2] observed high heat transfer efficiency for TLC which is definitely more important for the improvement of system efficiency especially for low-grade heat recovery system. Smith et al. [3] also reported the TLC system as 14-85% more efficient over ORC. a) b) Entropy (kj/kgk) Entropy (kj/kgk)

3 494 Md Arbab Iqbal et al. / Energy Procedia 110 ( 2017 ) c) ORC d) TLC Pinch point 0% Heat transferred (%) 100% 0% Heat transferred (%) 100% Fig. 1: a) T-s diagram for ORC, b) T-s diagram for TLC, c) Working fluid phases in ORC, d) Working fluid phases in TLC Basically, the TLC has the same components as the Rankine-cycle engines but unlike the Rankine cycle, it does not evaporate the working fluid during the heating phase; instead expands it, from the saturated liquid condition, as a two-phase mixture. The choice of a suitable working fluid for a given application, the feasible operating conditions/ parameters and the system configuration design, are the most essential features of any thermal system design [4-6]. With the application of a suitable high molecular working fluid, the TFC power plants could recover and convert heat energy efficiently from renewable thermal sources as well as from the by product (waste heat) of numerous nonrenewable sources into mechanical or electrical power. The high molecular mass allows efficient exploitation of nonisothermal single-phase heat sources for heat recovery-to-power generation, which allows the net output work of the TLC in a wide range of power capacities [7, 8]. The working fluid adopted for the study is iso-pentane because of its good thermo-physical properties (e.g. relative high critical temperature and pressure), low-cost, and a boiling point slightly above room temperature. It displays a strong positive slope on the T s diagram and its saturated liquid expansion tends to dry out at temperatures slightly exceeding 453 K [9]. More so, iso-pentane is a dry fluid, whose thermo-physical properties are well-suited for lowgrade heat recovery to-power generation. 2. Proposed system design In the proposed system, solar pond is considered as the source of low grade heat energy. The system consists of iso-pentane as working fluid, high pressure pump to achieve required pressure to get saturated liquid iso-pentane at high temperature, a plate type heat exchanger for transferring heat to the working fluid, an isentropic CD nozzle to create high speed flash of wet vapor of iso-pentane, an impulse turbine (i.e Pelton turbine) to produce mechanical power using the jet from the nozzle, a generator to produce electricity from the turbine power, a condenser to condense the iso-pentane vapor which would be pumped to the heat exchanger to repeat the cycle. A schematic of the system has been presented in figure 2.

4 Md Arbab Iqbal et al. / Energy Procedia 110 ( 2017 ) System working principle Fig. 2: Schematic of the proposed system setup In the proposed electricity generation system, saturated iso-pentane would be pumped to the heater where it would be heated up under constant pressure to the saturation temperature. Afterward, saturated high temperature iso-pentane would be expanded through a horizontal CD nozzle at isentropic condition which will create a high speed jet of wet vapor of iso-pentane. The jet from the nozzle will rotate the turbine coupled with the generator which will produce electricity. Finally, the vapor would be condensed to liquid in the condenser and the process would be repeated by pumping the liquid back to the heater or heat exchanger. 4. Theoretical performance analysis Let us consider, the hot water of Th temperature from the solar pond with a flow rate of mhw is passing through the heat exchanger having effectiveness of Ɛh where the iso-pentane is to be heated up. If Pl is considered as the initial condenser pressure and Tpl is the corresponding saturated liquid iso-pentane temperature in the condenser, the hot isopentane temperature, Tph can be defined as, (i) Maximum heating power can be defined as, (ii) Here, C pw is the specific heat of liquid hot water. The maximum iso-pentane flow rate can be defined as, (iii)

5 496 Md Arbab Iqbal et al. / Energy Procedia 110 ( 2017 ) Here, C pp is the specific heat of liquid iso-pentane. The isentropic quality at the nozzle exit can be defines as, (iv) Here, S f,i, S f,e and S fg,e are the entropy of the fluid at the nozzle inlet, the entropy of the saturated liquid at nozzle exit conditions and the entropy difference of saturated liquid and vapour at nozzle exit, respectively. The isentropic enthalpy at the nozzle exit can be defines as, (v) Here, h f,e and h fg,e are the enthalpy of the liquid fluid at the nozzle exit and the enthalpy difference of saturated liquid and vapour at nozzle exit, respectively. Now, the isentropic nozzle exit velocity (neglecting inlet velocity and change in potential energy) can be given by, (vi) The force developed by the nozzle can be given by, (vii) If, the process is not isentropic, the isentropic efficiency of the nozzle can be defined as, (viii) Here, h e,a is the actual enthalpy at nozzle exit when the nozzle is not completely isentropic. For non-isentropic nozzle, the actual quality at the nozzle exit can be given by, (ix) Now, the actual nozzle exit velocity (neglecting inlet velocity and change in potential energy) can be given by, (x) The actual force developed by the nozzle can be given by, (xi) 5. Analytical performance analysis Case 1: If we consider hot water of 80oC is flowing through the heat exchanger having effectiveness of 95% at a flow rate of 1kg/s to heat up the iso-pentane to run the system. The pressure inside the condenser is 1.5 bar and corresponding saturated liquid iso-pentane temperature is 39oC. Now the isentropic performance of the system can be obtained by using equations (i) to (vii) and the thermodynamic properties of iso-pentane and presented in table 1.

6 Md Arbab Iqbal et al. / Energy Procedia 110 ( 2017 ) Table 1: Performance of the proposed TLC system Case 2: In real practice, it is not possible to develop a nozzle which is completely isentropic; therefore, if we consider a nozzle of 35% isentropic efficiency, the system performance would be changed and it is presented in table 2 using equation (viii) to (xi) (all other conditions are same as case 1). Table 2: Performance of the proposed TLC system considering nozzle isentropic efficiency Hot Iso- Maximum Iso- Isentropic Isentropic Max Maximum Isentropic Max Pentane temp Pentane flow rate Quality at Exit velocity ( o heating (kw) Force (N) C) (kg/s) Nozzle exit (m/s) Hot Iso- Maximum Iso- Non-isentropic Non-isentropic Maximum Non-isentropic Pentane temp Pentane flow rate Quality at Exit velocity ( o heating (kw) Force (N) C) (kg/s) Nozzle exit (m/s) Conclusion From table 1, it is clear that the proposed system can develop a significant amount of force (9.9 N) if we use a heat source of 80 C. Further increase of temperature can develop more power. Furthermore, as it is not possible to construct a nozzle of 100% isentropic efficiency and if the nozzle efficiency is considered as 35%, the same system can generate about 6.17 N of force (table 2) which is quite reasonable for power generation. Therefore, we can consider the proposed system for power generation from low grade heat sources. The amount of the power, developed by the system is highly dependent on the isentropic efficiency of the nozzle. The design of nozzle for iso-pentane is the most crucial part of this study and that is the next part of this research. References [1] Lai, N.A. and J. Fischer, Efficiencies of power flash cycles. Energy, (1): p [2] Fischer, J., Comparison of trilateral cycles and organic Rankine cycles. Energy, (10): p [3] Smith, I., N. Stosic, and C. Aldis, Trilateral flash cycle system a high efficiency power plant for liquid resources. Proceedings World Geothermal Congress 1995, 1995: p [4] Dincer, I. and H. Al-Muslim, Thermodynamic analysis of reheat cycle steam power plants. Fuel and Energy Abstracts, (4): p [5] Chen, H., et al., A supercritical Rankine cycle using zeotropic mixture working fluids for the conversion of lowgrade heat into power. Energy, (1): p [6] Sahin, A.Z., et al., Special Issue: Thermodynamic Optimization, Exergy Analysis, and Constructal Design. Arabian Journal for Science and Engineering, (2): p [7] Invernizzi, C., P. Iora, and P. Silva, Bottoming micro-rankine cycles for micro-gas turbines. Applied Thermal Engineering, (1): p [8] Baral, S. and K.C. Kim, Thermodynamic Modeling of the Solar Organic Rankine Cycle with Selected Organic Working Fluids for Cogeneration. Distributed Generation & Alternative Energy Journal, (3): p [9] Smith, I.K. and R.P.M. da Silva, Development Of The Trilateral Flash Cycle System Part 2: Increasing Power Output With Working Fluid Mixtures. Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy : p