What are fluids and how are they used in Polysun?

Polysun has been designed in such a way that an arbitrary construction of solar systems is possible based on predefined components. Therefore, the first step of the simulation is to analyze the structure of a system. For this purpose, first the fluid domains are identified in order, and then the fluid circuits.

Definition: Fluid is the liquid that flows through the components and transports energy. Since the fluid often contains different substances, in Polysun there is the basic fluid catalog with the pure substances (such as water, ethylene glycol, propylene glycol), and the fluid mixture catalog with the fluids used in reality (such as drinking water, ethylene mixture, propylene mixture).

Definition: A fluid domain is a contiguous, hydraulic area with a common fluid. A plant typically consists of several fluid domains. The plant components belonging to a fluid domain have the same fluid flowing through them.

Definition: A fluid domain consists of one or more fluid circuits. In a fluid circuit there is always exactly one flow generator. Results are determined for fluid circuits (not for fluid domains).

The climatic conditions in Central Europe make it necessary for solar systems to withstand sub-zero temperatures. This makes it impossible, for example, for the required hot water to be produced directly in the collector. If pure tap water freezes in the collector, it destroys the collector by its expansion. Normal tap water also has the disadvantage that the collector would calcify over time.

To make the heat transfer fluid suitable for the above-mentioned requirements, normal water is mixed with a certain amount of glycol. In many cases, ethylene glycol (e.g. Antifrogen L) or propylene glycol (e.g. Antifrogen N) is used as antifreeze for this purpose. Since the fluid circulates in a closed circuit, the scaling problem is not acute. Various considerations must be taken into account when mixing ratios:

• The heat capacity of the fluid decreases with increasing glycol concentration
• The viscosity increases with higher glycol content (problem of pressure loss)
• The freezing point decreases with increasing glycol content
• The boiling point increases with increasing glycol content
• Possible chemical processes, especially at transitions of different metals, have to be considered
• The heat resistance of the fluids must be considered

Above a certain concentration, freezing fluid no longer causes problems because it no longer freezes like ice (crystalline) but acquires a sulky-grainy structure. It can no longer develop an explosive effect. From a volume fraction of 33 % (propylene glycol) or 38 % (ethylene glycol), the above-mentioned effect occurs. The glycol concentration can be defined in Polysun.