5. Estimate System Size and Costs
Proper sizing of the components of an SPIS is critical, since an SPIS with insufficient capacity will not satisfy the farmers’ needs and an over dimensioned system will induce unnecessary operation and capital costs. Negligence of the sustainable water yield of water sources may result in water shortage and a depletion of water resources, thus having negative impacts on the farm budget and the environment. It is therefore very important to be in close contact with the farmer during the planning phase and to inform him about the advantages and limits of SPIS.
The required size of the PV generator can be estimated using the following parameters:
- daily crop water requirement Vd [m3/day]
- total pumping head HT [m]
- mean daily global solar radiation G for the design month [kWh/m2day].
A simple arithmetic formula that takes the individual system component efficiencies into account can be used to estimate the required solar-generating peak power
P peak [Wp].
Example: It is calculated that crops in an irrigation system require 30 m³/d and field observations confirm that water needs to be pumped up 50 meters from a borehole to a reservoir. From the NASA website it becomes clear that the daily total global irradiation at the location of the farm is 5 kWh/m²day. According to this equation, a 2400 Wp PV generator is required.
The DESIGN – Pump Sizing Tool (Excel-based worksheet) can be used to determine the approximate solar generator size, which serves as guideline when engaging with SPIS technology suppliers.
The approximate cost of the planned PV system can be calculated by multiplying the country-specific average system cost [currency/kWp] and the calculated PV generator power (P peak).
The final design of the PV pump and irrigation system should be left to experienced system integrators who use computer-based system sizing and simulation tools such as COMPASS, WinCAPS and PVSYST, HydroCALC, GESTAR (See Further Reading, Links and Tools at the end of the Module).
Following this procedure, the principal analytical steps to support decision-making should be completed. The technical, agronomical and financial aspects of the possible SPIS configuration (and alternatives) should now be available.
- Required PV generator size;
- Pre-selection of motor/pump unit;
- Motor/pump characteristics;
- Layout of water distribution system;
- Daily course of solar irradiation and water flow;
- System cost estimate;
- System cost parameters;
- Suitability checklist / evaluation.
- Daily crop water requirement Vd [m³/day];
- Total pumping head Ht [m];
- Mean daily global solar radiation G for the design month [kWh/m²day];
- Country-specific costs of PV pump [Currency/kWp].
- Agricultural service providers;
- Experienced system integrators.
- A commercial software solution that integrates design for the PV pump and the irrigation system is currently not available on the market.
- SPIS usually have to be oversized to meet these peak demands, resulting in a fairly low degree of system utilization.