The present work focuses on the investigation of part-load operating strategies of a large-scale industrial
recuperative Waste Heat Recovery Organic Rankine Cycle (WHR-ORC) equipped with an air-cooled
condenser (ACC). The WHR-ORC includes an intermediate heat transfer oil (HTO) loop that is used
for recovering thermal energy from the high-temperature exhaust gases and supplying it to the working
fluid of the ORC (cyclopentane). The modelling approach is based on an iterative solver considering
the off-design variation of the heat transfer coefficient of the heat exchangers, the turbine sliding
constant and isentropic efficiency as well as the pump motor, turbine generator and ACC fan motor
efficiencies. The part-load model can be used for predicting the electric power output and thermal
efficiency of the ORC under variable heat transfer oil (HTO) mass flow rates, temperatures and ambient
temperatures. Two main operating strategies are considered based on: a) constant superheating degree
and variable turbine inlet temperature and b) variable superheating degree and constant turbine inlet
temperature. In each case, the air mass flow rate in the ACC is optimized in order to maximize the net
power output of the plant, while also taking into account the electricity consumed by the ACC fan
motor. The operating strategy based on variable superheating and constant turbine inlet temperature
(OS1) results in higher overall electric efficiencies compared to an operating strategy based on constant
superheating degree and variable turbine inlet temperature (OS2) at lower HTO flow rates. This happens
because OS1 is associated with higher superheating which promotes the beneficial effect of the
recuperator. However, for the same heat transfer fluid flow rate, OS2 results in higher net power output
than OS1 because it is associated with higher heat recovery in the ORC evaporator. Meanwhile, the
part-load model was used to calculate the optimal condensation pressure of the ORC that corresponds
to different combinations of HTO flow rates and ambient temperatures. For each ambient temperature,
there is a threshold HTO flow rate under which the optimal air flow rate in the ACC is maximized. For
lower HTO flow rates, the optimal air flow rate varies in an approximately linear way with the HTO
flow rate. The threshold HTO flow rate is lower for lower ambient temperatures, for which the tradeoff
between the cycle thermal efficiency and ACC fan power consumption favors the former, contrary
to higher ambient temperatures, for which the higher air flow rates have a less positive effect on the net
power output because of the increased negative influence of the ACC fan power consumption.
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