Abstract
Positive displacement machines have been identified as appropriate expanders for small-scale power generation systems such as Organic Rankine Cycles (ORCs). Screw expanders can operate with good efficiency in working fluids under both dry and two-phase conditions. Detailed understanding of the fluid expansion process is required to optimise the machine design and operation for specific applications, and accurate design tools are therefore essential. Using experimental data for air expansion, both CFD and chamber models have been applied to investigate the influence of port flow and leakage on the expansion process. Both models are shown to predict pressure variation and power output with good accuracy. The validated chamber model is then used to identify the optimal volume ratio and rotational speed for experimental conditions.
Highlights
There is currently significant interest in reducing global greenhouse gas emissions from industrial processes, which alone account for almost 26% (275 Mtoe/yr) of Europe’s energy consumption [1].Studies looking at global thermal energy availability [2] have shown that about 52% of the primary energy consumption is currently being rejected as waste heat
High efficiency can be achieved by matching the screw expander’s built-in volume ratio, v, to the volumetric expansion of the fluid in the process; the maximum value of v is limited due to geometrical constraints of the screw rotors, increased filling losses due to the decreasing size of the inlet port and the decreasing mass flow rate for a given machine size. The influence of these different factors makes performance prediction and optimisation of screw expanders essential when considering their use in Organic Rankine Cycles (ORCs) systems
A range of inlet pressures between 1.5–3 bar was investigated at an inlet temperature of 75 °C, with expander rotational speed ranging fromfrom the expander rotational speed ranging
Summary
There is currently significant interest in reducing global greenhouse gas emissions from industrial processes, which alone account for almost 26% (275 Mtoe/yr) of Europe’s energy consumption [1]. High efficiency can be achieved by matching the screw expander’s built-in volume ratio, v , to the volumetric expansion of the fluid in the process; the maximum value of v is limited due to geometrical constraints of the screw rotors, increased filling losses due to the decreasing size of the inlet port and the decreasing mass flow rate for a given machine size The influence of these different factors makes performance prediction and optimisation of screw expanders essential when considering their use in ORC systems. For single- or two-phase conditions, the validated model allows for evaluation of maximum efficiency maps as a function of built-in volume ratios at different pressure ratios This will be demonstrated for the air expander considered in this paper.
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