Modelagem dinâmica de um sistema solar termossifão usando coletores atmosféricos de plástico

Abstract

This work presents the dynamic model of a thermosyphon solar system consisting of two PVC double walled absorber plate collectors of 1,44 m2 of area and 120 I polypropylene vertical storage tank. The model was validated with experimental data obtained in Uberlandia, Brazil, latitude 18°55’23”S, during winter time (August and September, 2003) in clear days. The storage tank was filled up with water the night before the test and the temperature readings were recorded for the next day’s test. Temperatures were recorded (in five reservoir levels, at collectors entrance and output), as well as weather conditions like solar irradiation on collector plate plane and environmental temperature. The system developed uses recycled material, usually found in Brazilian cities, and hydraulic material commercially available, normally used in cold water residential systems. The maximum hot water temperature obtained is limited to 60°C in order to avoid fails in the PVC junctions and in other materials used in the system. The dynamic model developed is a simple one that consists of two parts: the energy balance of the solar collector plates and the stratified tank. Mathematical model of the absorber plate describes output temperature as a function of radiation, mass flow rate and thermal losses. Partially stratified tank model (Duffie & Beckman, 1980) was adopted. Model approaches the thermal stratification of the tank by assuming that the tank consists of N fullymixed volume segments. A number of 10 nodes were chosen. An initial estimated value of flow rate is used to evaluate the temperature distribution around the thermosyphon loop. An estimate of the thermosyphon head may be found based on relative positions of the tank and collector. The flow rate is that which balances the thermosyphon buoyancy forces with the frictional resistance in the flow circuit. The results show good agreement between simulated and experimental results. The validated simulation model is then used to found the best relationship between collector area and capacity of storage tank for known locations. The preliminary studies presented show the systems potentialities when used as domestic heating system in low-income habitational groups.