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A new detailed transient model for PVT-water collectors is presented. The model uses a nodal approach based on the electrical–thermal analogy using RC networks to model the thermal response of the PVT collector. Unlike models found in the literature, the RC network is directly used to simulate the PVT collector in an electronic circuit simulator. Heat flow is modelled in one dimension only and average temperatures are used for the fluid and absorber plate in order to calculate the transient thermal output of the collector. A discussion on the correct calculation of the average temperatures for the collector and possible approximations is presented. The model was used to simulate a PVT collector coupled to a domestic hot water load, in order to study the daily performance of a PVT-water system and its transient behaviour. Results show that the proposed solution can achieve extremely good agreement with experimental data, and that the model could be used for performance analysis and product design. [ABSTRACT FROM AUTHOR]

Summary: An assessment of the potential for dew water to serve as a potable water source during a rainless season in a humid tropical climate was carried out in the Pacific islands of French Polynesia. The climate of these islands, in terms of diurnal and seasonal variations, wind and energy balance, is representative of the climate of the tropical Atlantic and Pacific oceans. Measurements were obtained at two characteristic sites of this region; a mountainous island (Punaauia, Tahiti Island) and an atoll (Tikehau, Tuamotu Archipelago). Dew was measured daily on a 30° tilted, 1m2 plane collector equipped with a thermally insulated radiative foil. In addition, an electronic balance placed at 1m above the ground with a horizontal 0.16m2 condensing plate made of PolyTetraFluoroEthylene (Teflon) was used in Tahiti. Dew volume data, taken during the dry season from 16/5/2005 to 14/10/2005, were correlated with air temperature and relative humidity, wind speed, cloud cover and visible plus infrared radiometer measurements. The data were also fitted to a model. Dew formation in such a tropical climate is characterized by high absolute humidity, weak nocturnal temperature drop and strong Trade winds. These winds prevent dew from forming unless protected e.g. by natural vegetal windbreaks. In protected areas, dew can then form with winds as large as 7m/s. Such strong winds also hamper at night the formation near the ground of a calm and cold air layer with high relative humidity. As the cooling power is lower than in the Mediterranean islands because of the high absolute humidity of the atmosphere, both effects combine to generate modest dew yields. However, dew events are frequent and provide accumulated amounts of water attractive for dew water harvesting. Slight modifications of existing rain collection devices on roofs can enhance dew formation and collection. Dew harvesting thus appears as an attractive possibility to provide the local population with a complementary – but on occasion, essential – water resource. [Copyright &y& Elsevier]

From the total spectral emissivities determined for clear skies in a preceding paper their directional values are obtained. The Bliss formulation still used for this doing is justified. The results point out the inefficiency of a large band around the horizon for cooling purposes. As an application, a natural focusing system to obtain dew is explained. [Copyright &y& Elsevier]