The C++ programming language was chosen for the development of the SOLSTILL model. Each component in this system was programmed as a separate function, which was then linked into the main program. The program connects the various components in a manner specified by the user and then proceeds to mathematically execute the compiled system over the selected time period, solving differential and algebraic equations and facilitating information output. Table 1 shows all functions of this submodel together with their inputs and outputs.

The solar radiation processor function is used in order to convert the solar radiation into a form useable by the solar still. Depending on whether the cover involves one or two slopes and the number of covers used, this function calculates the useful solar energy incident on the still covers.

The standard still function is used to simulate the performance of the conventional natural convection solar still. In this function, the values of the physical parameters of the covers, the basin water and the still liner can be either constant or calculated at each desired time interval for the conditions prevailing at that time, depending on the required level of complexity for the simulation.

The forced still function is used to simulate the performance of the still in the forced circulation mode. Again, the values of physical parameters of the covers, the basin water and the still liner as well as the psychometric properties of the flow air can be either constant or variable, depending on the required level of complexity for the simulation.

Table 1. Summary of functions in SOLSTILL

The dehumidifier and pre-heater functions are used to calculate the conditions of the air and water leaving the coils. Users can also use default values for the coils’ physical parameters or enter their own values. The dehumidifier function is linked to the cooling function, and may be used to simulate the performance of the condenser in case there is no condensate in the coil.

The pump, fan and control functions are used to pre-set the system to run in one of three modes chosen by users; the natural circulation mode, the closed loop forced circulation mode and the open loop forced circulation mode. In the first mode, this function will "switch off” the water pump and the blower. In the second and third modes, the function will run the pump and fan at the flow rates selected by the user. The input air flow to the still will be the ambient air in the open loop mode whereas the exhausted air from the air pre-heater will be circulated to the inlet of the still in the closed loop mode.

The quantity integrator function is used to calculate and display the outputs of each component of the still system at selected time intervals, such as minutes, hourly, daily, weekly, monthly and / or yearly values.