THE USE OF NATURAL OR ARTIFICIAL LAKES AND PONDS IN HVAC PLANTS IN ITALY

摘要

Cooling systems extract heat from a cold zone and dissipate heat to a "hot sink". This sink usually takes the form of cooling towers. The thermal processes that take place inside the tower and the calculation procedures for the design of these cooling towers are well known. Many manufacturers today supply standardised products with quite low costs, which leads to a wide use of these appliances. In some cases an alternative to cooling towers, which assures a lower visual, acoustic and contaminant impact, is a natural or artificial lake or pond. It follows that, when applicable, lakes and ponds are clearly more acceptable to designers than ordinary cooling appliances. Lakes and ponds dissipate heat mainly through their free water surfaces through a combination of heat and mass exchanges between hot water and external dry air. Many factors can influence this phenomenon and most of these cannot easily be explained analytically or simply controlled (because of the stochastical changes in the boundary conditions). By making suitable hypotheses it is possible to represent the behaviour of lakes and ponds with a numerical simulation model. The simplified hypotheses are: unstratified lake (i.e. uniform water temperature) and constant surface level. Heat and mass transfer are described through theoretical and experimental relationships obtained by a state-of-the-art review. This model provides an easy-to-use design tool that is suitable for both architects and engineers. The model output is represented by the hourly time profile of the lake water temperature and the details of heat and enthalpy fluxes. Some test cases located in different parts of Italy have been analysed in the present work. This work will underline how "natural" systems are influenced by climate conditions in Italy. Finally, a sensitivity analysis has been developed in order to focus the influence of the various parameters and boundary conditions on the lake performances, regarding heat flux dissipation.