Pumping system preliminary design

See also: Sizing of a pumping PV system: general considerations.

After defining the Location, the System button displays a dialog where you are asked to define:

On the right panel: the plane orientation.

NB: the button Show optimization opens a little tool which shows the winter yield according to the plane orientation. For stand-alone or pumping systems, the plane orientation should usually be optimized according to the worst conditions, i.e., for winter irradiance.

On the left panel:

• The Water needs (in yearly, seasonal or monthly values).
• The nominal head at which it should be pumped (level difference between water outlet and source surface).
• The diameter and length of pipes (optional, for eventual friction losses).
• A pump technology (centrifugal for rather low heads, positive displacement for high heads).
• An array-pump coupling strategy, which strongly affects the system performances.

Now you can open the "Results" which ask to choose:

• either the tank volume, or the autonomy of the system in days. These parameters are coupled, according to the daily needs of water.
• the "Loss of Load" probability (P LOL), i.e., the time fraction during which the operator will accept that the needs are not met (empty tank).

These parameters lead to the determination of the array nominal power (i.e., the installed STC power according to the manufacturer specifications), and the pump nominal power required. These are very rough estimations, as the pumping system performances are strongly dependent on the pump technology, head, flowrate, as well as the electrical matching between pump and PV array.

The first result graph shows the potentially available solar energy, along with the user's water and energy needs.

The second one (available by the speed buttons on the left) gives the average filling state of the tank, and the missing water (P LOL) monthly distribution.

The table holds all monthly values, including an eventual required back-up energy.

Finally, the rough economic evaluation gives an idea of the investment and water price.

You can now play with the parameters and immediately see the results.

You can print a report, or store graphs and tables in the clipboard to export it to another software.

You can also save your project, and load another one for immediate comparisons.

Computation

PVsyst performs a very simplified simulation, which runs over one year in daily values.

The evaluation of the available irradiance on the collector plane uses the Monthly Meteo tool algorithms, which calculate irradiation's monthly averages on the basis of instantaneous data for one day per month.

This is not sufficient to manage the water storage balance evolution from day to day, and the effective use of solar incident energy. Therefore the program generates a random sequence of 365 days, according to the algorithms of Collares-Pereira¹, renormalised to the monthly sums, for calculating the daily balance from day to day, and the PLOL.

This simulation is repeated with different array and pump size arrangements, until matching the input requirements (namely the desired PLOL). The program is able to propose:

• the pump(s) size (power),
• the PV array nominal power,
• a rough estimate of the investment cost and the cost of water pumped.

If necessary, the pre-defined parameters used (array, system matching and pump efficiency, etc.) are user-modifiable through the menu option "Preferences"/"Edit Hidden Parameter".

Of course, this early layout proposition should be asserted by a detailed simulation, using real commercially available components, and taking all system features into account in an hourly modeling.