Dynamic performance estimation of small-scale solar cogeneration with an organic Rankine cycle using a scroll expander

Small-scale solar thermal cogeneration shows promise as an effective way to get increased benefit out of a given solar availability, since it does not waste potential during summer after the water capacity is heated. In this paper a scroll expander is tested in a small organic Rankine cycle (ORC) and used to calibrate a static expander model. Validation of the scroll expander model shows agreement generally within 10% for the shaft power, 5% for the rotational speed and 6 K for the exhaust temperature, with some outliers at very low pressure ratios. This calibrated model is then incorporated into a larger dynamic model of a solar thermal cogeneration system, designed for some larger dwelling unit or small commercial establishment that requires a larger volume of hot water. An annual simulation is conducted using a collector area of 50 m2, and the scroll expander shows a maximum isentropic efficiency of 59% while the ORC efficiency is 3.47%. The total energy produced is 1710 kWh and the hot water available is on average 2540 L/day. The maximum instantaneous power that can be produced by the system is 676 W, and it is possible to shift the time period that the system is producing power to match the peak demand period by adjusting the solar store volume.