Findings to improve the performance of a two-phase flat plate solar system, using acetone and methanol as working fluids

An indirect two-phase water heating solar system was tested using acetone and methanol as working fluids. The working fluid circulates in a closed circuit that extends from the solar collector to a coil heat exchanger in the thermo tank. The working fluid evaporates in the solar collector and condensates in the thermo tank coil. This Phase Change System (PCS) prevents freezing, scaling, corrosion, and fouling; these advantages increase the lifetime of the system. The objective of this work is to characterise the performance of the PCS using different filled fractions of acetone and methanol, with two kind of initial pressures (atmospheric pressure, and partial vacuum), in order to find the appropriate conditions for a good performance of the system. For this purpose, the useful heat was determined, as well as the increment of temperature in the water of the thermo tank, and the experimental efficiency. Results are compared to a witness conventional Domestic Solar Water Heating System. The witness has the same characteristics (materials and dimensions) than the PCS, except for the coil heat exchanger presented in the PCS. The instrumentation set throughout the system includes temperature sensors, pressure transducers, a pyranometer and an anemometer, that permit to characterise and understand the performance of the system under different working conditions. By knowing the phenomenology of the working fluid in the closed circuit, the stratification profile of the water in the thermo tank, and the thermal performance of the solar collector, projections to improve the PCS can be formulated. The performance of the system is the result of several variables working together in combination: the working fluid, filled fraction, partial vacuum, coil length, as well as the working and ambient conditions. The appropriate combination of these variables is investigated to improve the performance of the PCS. Experimental results showed that the partial vacuum conditions at the beginning of the test led, as expected, to an improved performance of the PCS. Tests were carried out under the actual field conditions of Temixco, México.

► A phase change system to solar heat water is studied using acetone and methanol. ► Partial vacuum reduce fluid boiling point and improve the system’s performance. ► Acetone exhibits better performance than methanol. ► A solar system that prevents freezing and corrosion has been developed.