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Mohammed A. Al-Weshahi et al Working fluid selection of low grade heat geothermal Organic Rankine Cycle (ORC) phase composition; however, the zoetrope shows different composition at the two phases (Basaran, A and Ozegener, L 2013). The azoetrope refrigerants are indicated by R5xx while the zoetrope refrigerants are named as R4xx. In addition some researchers divided the ORC refrigerants based on their phase state when leaving the ORC turbine: wet, isotropic and dry. The wet refrigerants have a negative slope of saturated vapor curve and in general they have a low molecular mass such as ammonia (R717). The isentropic fluids have a vertical saturated vapor line with a moderate molecular mass such as R123, R141b and R600. The dry fluids own a positive slope of saturated vapour line with high molecular mass such as R601a and R601 (Li, T et al, 2012; Qiu, G, 2012). Many studies have been carried out to investigate the fluid selection in the ORC cycle. Tchanche et al. studied 20 different refrigerants in the ORC cycle powered by hot water gained from solar energy. The study revealed that R134a was the most suitable for such application and R152a, R600, R600a and R290 show an attractive performance with safety concerns. Nine pure refrigerants were evaluated as the working fluid in the ORC energized by vehicle engine waste heat by Wang et al. The study found that R11, R141b, R113 and R123 produced slightly higher performance; however, putting in mind safety concerns; R245fa and R245ca were seen as the most environmental-friendly working fluids for such application. Mago and Luck examined the heat recovery from micro turbine exhaust using the ORC with four different dry refrigerants; R113, R123, R245fa and R236fa. The study concluded the best performance was achieved using the R113 and the worst when R236fa was used (Mago, P.J and Luck, R 2012). Another study used a multi-function objective optimization method to select the best ORC refrigerant among 13 working fluids. It found that R123 is the best working fluid when the source temperature is in the range of 100-180°C; however, R141b was the optimal when the temperature was higher than 180°C (Wang, E. H et al 2012). Aljundi also compared different ORC working fluids and the result revealed that n-hexane was the best working fluid whereas the R227ea was the worst. The study claimed the hydrocarbons could be the future working fluids for geothermal and waste heat applications. This study aims to use the previously validated model of ORC unit powered by low temperature geothermal to assess another 25 refrigerants from different categories (HCFC, HCF, HC and mixtures) to select the best candidates. Throughout this comparison the modelled existing ORC unit equipment design data were maintained constant. 2. Methodology The studied ORC unit in this study consists of four main components: evaporator, turbine, condenser and refrigerant pump. It necessary to emphasize in this study the evaporator (energized by hot water from geothermal source) represents both preheater (moves the refrigerant state from sub-cooled to saturated liquid) and evaporator (where the isothermal process occurs by changing the refrigerant from saturated liquid point to saturated vapor) Thermodynamically, the system is analyzed with assumption: as control volume at a steady state with negligible potential and kinetic energy effects. Consequently the mass and energy balance are obtained by: ∑ṁi ∑ṁe ( ̇̇ ORCunitnetpower( ̇ )isthegrosspower( ̇ )after subtracting the refrigerant pump power consumption ( ̇ ): ̇̇̇ ORC thermal efficiency (energetic efficiency) ( ) is defined as net power divided by the heat input to the evaporator ̇ : ̇ The ORC heat exchangers (evaporator and condenser) are analyzed using effectiveness (ε and Number of Transfer Units (NTU). ̇ ∑̇ ∑̇ where UA and conductance and smaller heat capacity rate of the fluid that pass through the heat exchanger. Evaporator and condenser effectiveness were defined as (Herold, K. E et al 1996): TT TT 3. ORC modelling and validation The previously developed model (Al-Weshahi, M. A et al, 2014) for an existing 250 kW ORC unit utilizing the heat from an underground hot spring in Chena, Alaska (Table 1) (Alaska Energy Authority Chena Power, 2007) using the IPSEpro refrigeration library (SimTec, 2005) was used to assess the studied refrigerants. Fig. 2 represents the original ORC unit state points on the R134a refrigerant p-h diagram. The process from 1 to 2 represents the expansion process in the turbine of the slightly superheated vapor to the vapor-liquid state at the turbine exit. The point 2 to 3 describes the constant-pressure heat rejection process in the condenser while 3-4 shows the refrigerant pump compressing the refrigerant to the operating pressure of the preheater. The process 4 to 1 shows the constant pressure heat addition in the preheater and evaporator. The ε are heat exchanger overall 7 | International Journal of Thermal Technologies, Vol.4, No.1 (March 2014)PDF Image | Working fluid selection of low grade heat geothermal Organic Rankine Cycle
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