Scroll expander Organic Rankine Cycle (ORC) efficiency boost of biogas engines

The most common energy application for biogas is on-site power generation using gas engine units. The introduction of a bottoming cycle based on Scroll expander Organic Rankine Cycle (ORC) technology within a biogas plant is of high interest since the excess heat produced by biogas engines can be used to produce additional power. This paper describes the retrofit of a plant consisting of a digester fed with green municipal wastes and two biogas engine units (200kWe each). The waste heat from the cooling jacket of the first biogas unit is used in cogeneration for the fermentation process and also for heating the buildings of the facility. The objective of the retrofit project is, in a first step, to convert the excess heat from the cooling jacket of the second biogas unit by means of Organic Rankine cycles. Onsite preliminary tests from the operation of the 7 kWe unit have been done, allowing performances to be measured over a broad range of conditions. Those tests confirmed a reasonable behavior of the ORC scroll expander and the interest of the concept. The measured efficiency of the ORC is about 7% with a heat source at around 90°C (i.e. 40% exergy efficiency).