Abstract
This paper conducts an analysis study on effect of the heat source flowrate on heat transfer characteristics of a preheater and system performance of a 250-kW level organic Rankine cycle (ORC) system. Refrigerant R245fa is used as working fluid with a constant flowrate of 11.58 kg/s. The operating pressures of the preheater/evaporator and the condenser are 1.265 MPa (i.e., evaporation temperature, TR,eva, of 100 oC) and 0.242 MPa (i.e., condensation temperature of 39 oC), respectively. The analyzed heat source flowrate (mw) is ranging from 9.39 kg/s to 27.39 kg/s with a constant inlet temperature of 133.9 oC. The efficiencies of the used pump, turbine, and generator are set to 90%, 80%, and 90%, respectively. The net power output is 243 kW and the system thermal efficiency is 9.46% under design conditions (mw = 15.39 kg/s). For an off-design heat source flowrate, a new operating pressure of the preheater/evaporator will be chosen to meet that R245fa reaches the saturation liquid state at the outlet of the preheater and the saturation vapor state at the outlet of the evaporator, i.e., without superheating. The results demonstrate that (1) the operating pressure of the preheater/evaporator increases from 0.775 MPa to 1.675 MPa (i.e., TR,eva increases from 79.3 oC to 113.2 oC) with an increase in mw, ranging from 9.39 kg/s to 27.39 kg/s; (2) the higher mw presents the better heat transfer performance of the preheater and the smaller requirement of heat capacity of the evaporator; and (3) the net power output (172 kW to 282 kW) and the system thermal efficiency (7.07% to 10.65%) increase with an increase in mw, especially for mw < 17.39 kg/s.
Highlights
An organic Rankine cycle (ORC), in general, employs the same principle as the steam Rankine cycle but uses organic fluids with a low boiling point as the working fluid, which enables power generation for a low heat source temperature [1]
ORCs are being studied in many contexts, including technical-economical-market surveys [1,3], selection of the working fluid [4,5,6], proof of concept demonstrations [7,8], models for optimal control strategies [9], quasi-dynamic models [10], and running tests of prototypes for ORC systems [11,12]
Because an ORC system provides the heat to power process, the heat exchanger system is a very important component
Summary
An organic Rankine cycle (ORC), in general, employs the same principle as the steam Rankine cycle but uses organic fluids with a low boiling point as the working fluid, which enables power generation for a low heat source temperature [1]. The ORC is considered to be one of the most economic and efficient ways to convert low grade thermal energy, such as geothermal energy, solar thermal energy, waste heat recovery, biomass energy, and ocean thermal energy, to electricity [2]. An evaporating temperature related to the working fluid flow rate and the specific enthalpy change is a crucial parameter in an ORC system. Li et al [13] explored the effect of the evaporating temperature on the system thermal and exergy efficiencies and the net power output of an OCR system. Their results demonstrated that the system exergy efficiency and net power output increases with an increase in the evaporating temperature
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