Entropea Labs (United Kingdom)

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.

<div class="section abstract"><div class="htmlview paragraph">Modern heavy duty diesel engines can well extend the goal of 50% brake thermal efficiency by utilizing waste heat recovery (WHR) technologies. The effect of an ORC WHR system on engine brake specific fuel consumption (bsfc) is a compromise between the fuel penalty due to the higher exhaust backpressure and the additional power from the WHR system that is not attributed to fuel consumption. This work focuses on the fuel efficiency benefits of installing an ORC WHR system on a heavy duty diesel engine. A six cylinder, 7.25ℓ heavy duty diesel engine is employed to experimentally explore the effect of backpressure on fuel consumption. A zero-dimensional, detailed physical ORC model is utilized to predict ORC performance under design and off-design conditions. The ORC model includes a detailed exhaust gas heat exchanger model and a thermodynamic ORC submodel to explore the effect of recovering various amounts of waste heat on ORC thermal efficiency under the same engine load and speed conditions. This study focuses on maximum engine power conditions where the engine exhaust gas and temperature are maximized. The results show that increasing the heat exchanger surface area leads to higher heat recovered at the expense of higher exhaust backpressure and higher WHR system weight, as the <i>ΔT</i> between the fluids approaches zero. At the same time, the weight increase of the heat exchange is illustrated as the main parameter that limits the ORC system design in vehicular applications. Finally, the optimum heat exchanger length is a trade-off between exhaust backpressure, the required net ORC power and weight increase.</div></div>