Experimental analysis of a micro-scale organic Rankine cycle system retrofitted to operate in grid-connected mode

Abstract

This work presents the retrofitting of a micro-scale organic Rankine cycle system originally designed for off-grid operation, modified to operate in grid-connected mode. Quantitative and qualitative performance comparisons of the grid-connected and off-grid connected organic Rankine cycle system are made based on experimental data of an organic Rankine cycle test rig. The operating strategies of the two systems are discussed in detail. The organic Rankine cycle test rig has a nominal power output of 1 kW with R245fa as the working fluid, a scroll expander as the expansion machine and steam as the heat source. Initially, the experiments were performed in off-grid operation mode with a self-excited alternating current generator. Subsequently, the organic Rankine cycle test rig was retrofitted for grid-connected mode, replacing the alternating current generator with an induction motor, regenerative variable frequency drive and grid connection. The rig modification was carefully done in such a way that the expander rotational speed was not fixed by the grid frequency, offering an additional control parameter for operational control optimisation. The modified system is able to supply 1.162 kW of gross electric power to the grid while a net electric output of 0.967 kW is measured. The same system is able to generate 1.016 kW (gross) and 0.838 kW (net) electric output in grid-connected mode. The net thermal-to-electric conversion efficiency at peak power generation is 7.36% and 4.66% in grid-connected and off-grid operating mode, respectively.