The complete solar hot water system involves heat exchange between three liquids:
- Solar collector fluid. The fluid which is heated in the solar collector (above, fig. 2) and which flows into the heat exchanger unit (see figs. 3 and 4), and back into the solar collector by thermosiphon. Antifreeze (anticongelante) is used as the solar collector fluid because it exchanges heat readily, and to prevent fluid from freezing in pipes and bursting them in winter. Alyssa Grassi and the 2006 project team have both reconsidered the necessity of antifreeze in this design, as Parras winters, while they very occasionally bring snow, are reportedly mild. Additionally, there are exposed pipes at Zaragoza #1 which carry water from the street, and these pipes appear not to have posed any problems.
- Intermediary heat exchange liquid. Water held in heat exchange unit. This intermediary fluid is necessary because of the hazards antifreeze can pose to human health. If, for some reason, the antifreeze circuit were to burst, its fluid would be released into the intermediary heat exchange fluid in the heat exchanger, not into the household flow.
- Household-bound flow.
City water flows into the rooftop system through exposed metal and PVC pipes.
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