Essentially a double-click on a symbol in the variant will open an info dialogue box that allows you to modify the features of the relevant components. Within this dialogue box the user will be able to modify all features for the use of such components (for example the orientation of collectors). Features directly relating to components (for example type of collector) may be defined over the catalogue. A double-click on the catalogue symbol will open a catalogue dialogue box that allows the user to change the relevant catalogue values. In the Light level version of Polysun a simple click in the catalogue allows the user to select a different catalogue entry. Starting from the Professional level version the software allows the user to create a new entry with the desired values.
In most cases solar systems are used as a back-up to conventional heating systems for water heating purposes. Whilst usually idle in the summertime auxiliary heating systems meet in the winter the larger share of the heating demand.
Auxiliary electric heaters are often times only used to reduce the use of boiler burners in the summertime. When a solar system is installed auxiliary electric heaters are usually dropped. In fact as electric power is expensive if compared to heating oil and gas users try at least to take advantage of more convenient off-peak electricity (this can be defined in the dialogue box). In Polysun auxiliary electric heating is implemented as an integral part of the tank whilst auxiliary heating with heating oil and gas appears as a separate component.
In Polysun operating hours and controllers may be defined just like in actual heating systems to the great possible extent. Controllers are used for this purpose (see below).
The amount of heated energy is defined as the «heated volume» times the difference between the temperature of hot and cold water (times the heating capacity of the fluid). The volume is defined by the overall content of the tank and the layer on which the electric heater is located.
Auxiliary heating may be controlled by means of two temperature sensors that may be placed freely over the electric heater. Cut-in and cut-off temperatures may be set separately (cut-in temperature should be, however, lower than the cut-off temperature). In the example the required hot water temperature is 50°C. Cut-off temperature should always be at least a few degrees higher than the former so that even after a few hours tank temperature will not drop too low. Finally electric heating will only come on when an outside temperature higher than 17°C is detected and, therefore, in the summertime.
Two different models allow to calculate the efficiency value of collectors. The standard model is used for flat-plate or tube collectors whilst the efficiency value of unglazed collectors is calculated by means of the unglazed-collector model. The latter will not be reviewed here; for further details please see the Polysun user manual.
|Eta0 laminar||"Eta0 laminar" is the efficiency value of a collector operating at room temperature and in laminar flow conditions. Values of Eta0 laminar up to and of a2 refer to the aperture area of the collector and are determined at a radiation intensity of 800 W/m2. |
|Eta0 turbulent||The efficiency value of a collector operating at room temperature and in turbulent flow conditions. |
|A1 (without wind)||A1 coefficient for flat-plate and tube collectors measured with no wind|
|A1 (with wind)||A1 coefficient for flat-plate and tube collectors measured in normal ventilation conditions|
|A2||A2 coefficient for flat-plate and tube collectors|
|Dynamic heating capacity||Value computed pursuant to EN 12975-2|
|Nsis-Axis||The orientation (pipe curve at a 90° horizontal or vertical elevation) for tube collectors. Mostly irrelevant in case of flat-plate collectors. |
|IAM-Modell||The "Ambrosetti Model" is used to interpolate different flat-plate collectors. Tube collectors are interpolated by means of cubic spline. |
|Angle factors||IAM data are read over a table|
|Volume||Measured value of volume fluid volume in the collector inclusive of collector tube. |
|Internal diameter||Internal diameter of heat transfer pipes in the collector (used for pressure drop calculation purposes). |
|>Single pipe length ||The length of a single heat transfer pipe in the collector (used for pressure drop calculation purposes)|
|Parallel piping||Number of parallel pipings in the collector (used for pressure drop calculation purposes)|
|Pipe roughness||Roughness factor relating to the inner side (used for pressure drop calculation purposes)|
|Linear from factor||The form factor of a pipe ranges based on bend radius between 1-1.5. The factor for rectilinear pipes is 1. |
|Friction factor||The friction factor refers to pressure drops in branchings, valves, etc. If not measured it will be set on zero |
|Test flow rate||Fluid flow rate during a test. In l/h and collector.|
Polysun was designed so as to allow users to create the desired system layout by means of the available components. The first step of the simulation foresees hence a careful analysis of the system layout. The process includes in the sequence the identification of fluid domains and subsequently the identification of fluid loops.
Definition: The term Fluid refers to the fluid that circulates the different components and transports energy. As fluids often contain different substances Polysun includes a catalogue of main fluid components showing the pure substances (like, for example, water, ethylenglycol, propylenglycol) as well as a fluid mixture catalogue showing the fluids that are actually employed (like, for example, potable water, ethylene mixture, propylene mixture).
Definition: A fluid domain is a continuous hydraulic area permeated by a common fluid. A system typically consists of several different fluid domains. System components belonging to a defined fluid domain are circulated by the same fluid.
Definition: A fluid domain consists of one or more fluid loops. A fluid loop always includes a flow-rate producer. Results are given for fluid loops (not fluid domains).
The climatic characteristics of central Europe require solar systems to be able to withstand below freezing temperatures. This makes it impossible, for example, to produce the required amount of hot water directly in the collector. If pure tap water should freeze in the collector its expansion would destroy it. Normal tap water also shows a further disadvantage in that it may calcify the collector over the time.
For the heat transfer fluid to conform to the above-mentioned requirements the use of normal water mixed with a certain amount of glycol will be required. For this purpose in many cases ethylenglycol (for example Antifrogen L) or propylenglycol are employed as a frost protection agent. As the fluid circulates in a closed loop calcification problems are generally not an issue. As to the mixing ratio the following aspects will need to be considered:
- The heating capacity of the fluid decreases as glycol concentration grows
- Viscosity increases in case of higher glycol levels (pressure drop issue)
- The freezing point sinks as glycol concentrations grow
- The boiling point increases as glycol concentrations grow
- Possible chemical processes, especially in case of transition of different metals, should be carefully taken into account
- The fluid's resistance to heat should be taken into account
Starting from a certain concentration freezing fluids no longer cause problems as they do not freeze into ice crystals but acquire instead a gelatinous-granular structure. An explosive effect may no longer intervene. The above effect may intervene starting from a volume proportion of 33 % (propylenglycol) or 38 % (ethylenglycol). Polysun allows the user to define glycol concentrations.
Three-way valves play a crucial role when it comes to the analysis of fluid loops. Three-way valves share the inflowing fluid-flow in a fixed proportion. The position of the mixing valve is adjusted by the controller. Based on the situation in the schematic system diagram the three-way valve alternatively distributes inflowing fluid-flows or brings together two separate fluid flows. The adjusted output is marked with x. If the signal of the controller is logically one the x output will be completely open. Both situations are schematically outlined in the following graph: