The thermodynamic system generally studied to get information about heat engine cycles .Heat engine is the device that convert heat into work and part of heat into surroundings .It work on the principle of 1st and 2nd laws of thermodynamics which states that “energy can neither be created not be destroyed but it can be transferred into another form” and “Heat energy can’t completely converted to work as there is some heat loss in surroundings .The thermodynamic cycle is a continuation process under which on which heat engine work .This cycle takes heat from source and converted to work .Rankine cycle and organic rankine cycles both are thermodynamic heat engine cycle in which steam or organic fluid is heated to vapors state then expands in the turbine .Here turbine or expander is work generating device and boiler or evaporator is heat consuming device .
It is the thermodynamic cycle that is used to convert heat into work .It is close loop cycle that uses water (steam) as the working fluid .
Working cycle of rankine cycle shown above .There are four major components of cycle. Boiler ,steam turbine , condenser and pump .In boiler , high pressure water from pump is converted to superheated steam having high enthalpy .Boiler containing multiple tubes from where water flows , heated is produced in boiler by using coal ,oil or natural gas .Superheated steam then passes through steam turbine which rotates the turbine cause reduce in enthalpy of steam .After turbine ,mixture of water and steam enters into condenser in which steam is completely condensed . Condensation process of steam is done to pump the condensate back to boiler as pumping of steam is more difficult than pumping water .Then again water enters in boiler and this cycle continues .Power output obtained from turbine work which further used for electricity generation in generator .Heat is supplied to boiler for steam generation .Some part of turbine work is used to power the pump (Huber, McLinden , 2013) .
ORGANIC RANKINE CYCLE
Steam rankine cycle is uneconomical to use because it need source temperature above 400 Deg C and very low pressure requirements due to which size of components becomes very large and system efficiency becomes very less . Steam rankine cycle upgraded to organic rankine cycle which can be used for lower temperature range in small scale .Organic rankine cycle was developed by two physicist Lucien Bronicki and Harry Zvi Tabor in late 1950. Lots of scientist tried experiment on organic rankine cycle and obtain superior results from using heat input from biomass with recuprator system .It is also considered that efficiency of organic rankine cycle is high in winters as compared to from summers .This is because in summers , the temperature of working fluid also high , so working fluid having less heat carrying capacity but during winters , working fluid temperature is less , due to which difference between heat source temperature and working fluid becomes high therefore fluid having more heat carrying capacity , this increase the efficiency of cycle .This phenomena works on daily basis as well , during the day the temperature of atmosphere increases during sunrise due to which temperature of fluid also increases which reduce the efficiency but at sunset time temperature of atmosphere again decreases due to which heat carrying capacity of fluid increases so efficiency also increases .The efficiency of organic rankine cycle is highly depend on the effectiveness of heat exchanger used in it .As fluid exit from the expander have some heat left in it , this heat can be utilized to heat the working fluid enters in the evaporator .More will be the heat transfer occurs therefore less head need to be provided to working fluid in evaporator therefore for same power output , heat input becomes less so efficiency improves . Organic rankine cycle use geothermal, biomass and industrial waste heat as the heat source to evaporate the working fluid .Working fluid for this cycle are organic fluids which are better than steam for working in low temperature (Macián, Sánchez, 2013). The working fluid for organic ranking cycle are organic fluids like R-245fa ,R-134a .Although process for organic rankine cycle is similar to ordinary stream rankine cycle but some components are changed. There are six major components in this cycle. Evaporator ,Expander , condenser , recuperator, storage tank and pump (Broek, Dewallef, 2013).Organic rankine cycle can also use dry fluid coupled with regenerator to increase the cycle efficiency .As dry fluids do not form liquid vapour mixture (2 phase) after expansion in expander , due to this temperature of dry fluid is comparatively high at exit of expander from other working fluid used which help the liquid fluid before entering in evaporator .So a heat exchanger system should be assembled between condenser and expander to exchange heat between hot vapour exit to expander and liquid enter in evaporator and hence increase the efficiency .The ORC can be widely used in small scale power plants ,industrial system and furnaces where waste heat recovery need to be done .
Model of ORC
The organic rankine cycle is upgradation of steam rankine cycle as the boiler is replaced by evaporator, high pressure organic fluid from pump is converted to vapours having high enthalpy .Evaporator is act as heat exchanger where geothermal, biomass and industrial waste heat is extracted by organic fluid flowing through multiple tubes in evaporator. Vapours of organic fluid then passes through expander (turbine) which rotates the turbine cause reduce in enthalpy of organic fluid .After expander , partial mixture of liquid and vapor enters into recuperator where heat is exchanged with pumped organic fluid and rest of mixture of liquid and vapor enters in condenser where mixture of liquid and vapor are completely liquefy (Quoilin ,Lebrun2010) .Condensation process is done to pump the condensate back to evaporator as pumping of vapor is more difficult than pumping of liquid .Then again organic fluid enters in evaporator and this cycle continues .Power output obtained from expander work which further used for electricity generation in generator .Heat is supplied to evaporator for vapor generation .Part of expander work is used to power the pump. Storage tank contains working fluid which is used to recover any loss in working fluid due to frequent evaporations (Oralli, E. 2010).The ORC cycle is more efficient than ordinary steam rankine but cycle efficiency can be further improved by considering the superheating and sub cooling phenomena .In superheating , the working fluid is heating far above it vaporization point so that it may get extra heat before entering to turbine .In sub cooling ,after expansion in turbine the working fluid is cooled below its limit so that it may easily gain the heat
CALCULATIONS
Specific conditions for the ORC: the temperature of the renewable energy sources is between 100 o C and 150 o C; environment temperature 25 o C; working fluid R245fa; power produced 1 kW
All the enthalpy values are considered from pressure enthalpy chart corresponding to its temperature (Imran, Lee,2015) .
Initially the working fluid at room temperature of 25 deg C
T1 =25+273 =298 k
Enthalpy
After pump work Enthalpy
In evaporator heat exchanged and temperature reach to T3 =150 +273 =423 K
After evaporator heat supplied Enthalpy
Consider that in expander, temperature reduced to T4 = 70 +273=343 k
After expander, Enthalpy
EFFECT OF WORKING FLUID PROPERTIES
DESIRABLE FLUID PROPERTIES
EFFECT OF TEMPERATURE
Conclusion
This is concluded from the report that organic rankine cycle is more effective steam rankine cycle for small scale systems and for less temperature range , but for large scale system it is difficult to evaporate large quantity of organic fluid with evaporator .If the evaporator heat exchange from organic heat source increases then the efficiency of system also increases .
References
Chen, H., Goswami, D. Y., & Stefanakos, E. K. (2010). A review of thermodynamic cycles and working fluids for the conversion of low-grade heat. Renewable and Sustainable Energy Reviews, 14(9), 3059-3067.
Quoilin, S., Lemort, V., & Lebrun, J. (2010). Experimental study and modeling of an Organic Rankine Cycle using scroll expander. Applied Energy, 87(4), 1260- 1268. doi: 10.1016/j.apenergy.
Macián, V., Serrano, J. R., Dolz, V., & Sánchez, J. (2013). Methodology to design a bottoming Rankine cycle, as a waste energy recovering system in vehicles. Study in a HDD engine. Applied Energy, 104(0), 758-771.
Freeman, J.; Hellgardt, K.; Markides, C(2016). Working Fluid Selection and Electrical Performance optimization of a Domestic Solar-ORC Combined Heat and Power System for Year-Round Operation in the UK. Appl. Energy 186, 291–303
Orosz, M., Mueller, A., Quoilin, S., & Hemond, H. (2009). Small Scale Solar ORC system for distributed power. Proc. of the SolarPaces Conference.
Oralli, E. (2010). Conversion of a Scroll Compressor to an Expander for Organic Rankine Cycle: Modeling and Analysis. M.A.Sc. MR71355, University of Ontario Institute of Technology (Canada), Canada.
Lemmon, E.W.; Huber, M.L.; McLinden(2013), M.O. NIST Standard Reference Database 23: Reference Fluid Thermodynamic and Transport Properties-REFPROP; version 9.1; National Institute of Standards and Technology, Standard Reference Data Program: Gaithersburg, MD, USA.
Bao, J.; Zhao, L. (2013) ,A review of working fluid and expander selections for Organic Rankine Cycle. Renew. Sustain. Energy Rev., 24, 325–342.
Quoilin, S.; Broek, M.; Declaye, S.; Dewallef, P.; Lemort, V. (2013), Techno-economic survey of Organic Rankine Cycle (ORC). Renew. Sustain. Energy Rev., 22, 168–186.
Mohammad, U.; Imran, M.; Lee, D.; Park, B (2015). Design andd Experimental Investigation of a 1 kW Organic Rankine Cycle System Using R245fa as Working Fluid for Low-Grade Heat Recovery From Steam. Energy Convers. Manag., 103, 1089–1100 .
Collings, P.; Yu, Z.(2014), Modelling and Analysis of a Small-Scale Organic Rankine Cycle System with a Scroll Expander. In Proceedings of the World Congress on Engineering, London, UK .
Andreasen, J.G.; Larsen, U.; Knudsen, T.; Pierobon, L.; Haglind, F.(2014), Selection and Optimisaion of Pure and Mixed Working Fluids for Low Grade Heat Utilisation Using Organic Rankine Cycle. Energy 73, 204–213.
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