Experimental study of two phase loop thermosyphons for hybrid solar systems

•Two loop thermosyphon associated in series and parallel configurations are studied.•In series association, the thermosyphon condenser is coupled to the evaporator of the other.•In parallel association, two independent evaporators are connected to a common condenser.•Solar intermittency for hybrid systems operating in a typical day was studied. The use of thermosyphon technology to improve the efficiency of solar collectors, in thermal energy applications, has received literature increasing attention. The absence or decrease of solar radiation during nights or cloudy days can be compensated by another energy source, in the so-called hybrid solar systems. In the present paper, these systems, composed by two loop two-phase thermosyphons with independent evaporators, arranged in series and in parallel, are studied and compared between themselves for the first time in the literature. In the series configuration, the backup energy source feeds the evaporator of the auxiliary loop, which condenser is thermally connect to the evaporator of the main loop, also fed by the solar energy. In the parallel arrangement, both loops deliver the heat to a common condenser. These systems are tested under steady state conditions, keeping one or both evaporators active, representing the single (solar) or hybrid operation. The thermal resistance for both configurations are compared. Results of experiments simulating the operation throughout a typical day, including solar intermittency, are shown. In the solar intermittence tests, the backup evaporator, in the parallel arrangement, is able to compensate for solar absence 47% quicker, on average, when compared to the serial configuration. For the daily operation tests, the serial configuration is able to maintain the vapor temperature levels stable, when operating under different heat input rates between evaporators, while, in the parallel arrangement, vapor temperatures and thermal resistances varied significantly along the test.