Modified vacuum tubes for overheating limitation of solar collectors: A dynamical modeling approach

In this paper are presented two designs of vacuum tubes designed to avoid the overheating of solar collectors. As we discuss, this behavior must be studied by considering the collector's nonlinear dynamic, which is numerically studied by developing a solar-thermal modeling based on a fourth-order approximation for the efficiency function. In this way, the two main heat-losses mechanisms involved can be simulated and so, two kinds of modified vacuum tubes are studied: (a) by increasing its heat convection coefficient and (b) by increasing its infrared emissivity. Therefore, we have calculated their modified efficiencies in order to get a non-overheating collector, in which the maximum design temperature is always kept below 111 °C (for water-in-glass tubes) or 131 °C (for heat-pipe tubes). Then, we have studied the performance of these collectors when they work on low temperatures, showing that the first design of modified vacuum tubes (increasing the convection heat losses) penalizes the collector's performance up to 26%, meanwhile the second design (increasing the infrared heat losses) does not change the collector's performance. Therefore, a new collector based on these tubes could improve its performance on cloudy days by using a greater number of vacuum tubes. In this way, we found that by using 50 standard tubes (instead of 20 tubes) a solar collector based on a 200-l water tank could satisfy the daily household demand of hot water (200 kg@45 °C) even during cloudy days.