Effect of the transient energy input on thermodynamic performance of passive water-in-glass evacuated tube solar water heaters

Abstract

In this work the effect of the energy input time distribution on thermodynamic performance of a Water-in-glass Evacuated Tube Solar Water Heater (WGET-SWH) is studied, including thermo-hydraulic, first law and second law analysis. Geometrical model was based on a commercial WGET-SWH with nominal capacity of 40 L and 8 evacuated tubes. Simulations were carried out for four different transient energy inputs and 10°, 20°, 27° and 45° collector tilt angles. If tilt angle and total energy input are fixed, the energy input function with larger rates produced larger velocities and temperatures. Nevertheless this temperature increment is negligible for any practical purpose and the increment of kinetic energy is not large enough to affect stratification. Thus cumulated energy rather than energy input rate is significant for systems performance. Several second law indexes are proposed to assess stratification in the storage tank. The internal stratification number was used to compare the performance of WGET-SWH and active energy storage systems, showing that the former can achieve higher levels of stratification at low tilts, even comparable to those obtained in active systems with stratification promoters. The novelty of this work is to assess the influence of the transient energy input on the thermodynamic performance and stratification of a passive solar water heater.