EVALUATION OF THERMAL PERFORMANCE OF WATER-FILLED POLYETHYLENE TUBES USED FOR PASSIVE SOLAR GREENHOUSE HEATING

The relationship between the size of tube circumference and the amount of energy collected and released per tube linear metre was positive parabolic. The efficiency of water tubes in releasing heat energy expressed as the proportion of energy released to that collected per collection-release energy cycle varied according to radiation and temperature conditions. Under unfavourable conditions, of low radiation intensity followed by low minimum air temperature, the superiority of big tubes over small ones was clear. The daily heat energy balance of tubes was usually negative, i.e., the amount of energy released was greater than that collected during a given collection-release cycle. Under favourable conditions, of high radiation levels followed by mild minimum air temperatures, the release efficiency was usually slightly higher in the small tubes, with usually positive daily energy balance in all tubes, i.e., the energy released was less than the energy collected during the cycle. Under intermediate conditions, these efficiencies differed slightly or were equal for all tube sizes with either positive or negative daily energy balances.

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