Steady and transient operations of small-scale ORC-based power unit for low-grade thermal sources

Waste Heat Recovery (WHR) from exhaust gases of Internal Combustion Engines (ICE) is one of the most significant opportunities to reduce carbon dioxide emissions in the transportation sector. This is particularly useful because the category, at European defined a fine for any gram of CO2 which exceed the specified limits, redefining in this way the value of a low-carbon technology. In ICE applications, the main critical aspect is the intrinsic time-varying nature of the upper thermal source, which forces the ORC-based units to work frequently in off-design condition. This requires the recourse of reliable control systems and a careful choice of the design specifications of components. A key role is played by the expander which it must ensure to deal with highly variable flow rates and inlet thermodynamic conditions with the eventuality to receive two-phases flow when the high temperature thermal source is not enough to guarantee a full vaporization. Hence, to address all these issues, positive displacement machines are generally preferred to the dynamic types (radial or axial turbines). Scroll expanders among the other types appear to be the most suitable candidates, thanks their reliability, compactness, high efficiency and ease of starting. In this paper a scroll expander has been tested inside a test rig which allows to evaluate the performance of a 1.5 kW mechanical ORC-based power unit fed by the exhaust gases of a 3L turbocharged diesel internal combustion engine represented by an Iveco F1C. As working fluid, R245fa was selected. The results are presented for different steady working condition observing a maximum plant efficiency and power respectively equal to 4% and 500 W. Also transient analysis were performed observing that the ORC unit presents a relatively low start up time (370 s) thanks to the capability of scroll machine to work with two phase working fluid.