rollable PV-cell with electric pump
A rollable PV-cell provides the electric motor with power to drive the centrifugal pump.

Solution

The system only requires few components and works in two versions:

The first version can be completely prepared using commercially available components.
Components: rollable PV-cell, inverter, asynchronous motor (230 V AC) with centrifugal pump, electrical leads, two hoses, a Sieve, two mounts for the pump threads.
A rollable PV-cell feeds the asynchronous motor of the centrifugal pump. The DC voltage of the PV-cell(s) (e.g. 24 to 40 V DC) is transformed by an inverter to 230 V AC. If the pump power should be reduced, you can just roll up the PV cell a bit or partly cover it.
Advantage of this system: The 230 V AC can also be used to feed into a home network or to operate other electrical devices.

The second version requires a precise selection of components and more accurate searches. In this configuration you don’t need an inverter.
Components: rollable PV-cell synchronous motor (about 30 V DC) with centrifugal pump, electrical leads, two hoses, a Sieve, two mounts for the pump threads.
The rollable PV-cell directly feeds the synchronous motor (DC) of the centrifugal pump. If the pump power should be reduced, you can just roll up the PV cell a bit or partly cover it.

The downside of the whole concept is that the power of the pump can’t be higher than 600 W with a budget of 1100 €. Therefore the flow is limited to about 3500 l/h. A diesel -powered pump with approximately 3.8 HP has a flowrate of 22000 l/h. Unfortunately the PV-cell-system can’t keep up with such a power. The main Reason are the high costs of PV-cells. Currently the rollable PV cells cost around 150 € per m² and have a minimum capacity of 80 W per square meter. For my calculation I assumed 100 € per m² because this product is expected to be ready for the market in about one year. And we will get it cheaper ordering large quantities.