A 300 m borehole with grid backup:
the arbitration in the Alentejo
In southern Portugal, an Alentejo farm draws from a deep aquifer: the borehole goes down to 300 m and the water settles at 290 m while pumping, and it is that measured dynamic level the study uses. The need is 65 m³ a day, and the tank already exists. The grid reaches the substation, and the farmer wants it as backup, because he knows winter will not make the 65 m³ on sunlight alone. That requirement changes everything: it takes the only truly hybrid range in the catalogue out of the running, and it sets two very different ways of bringing alternating current into a solar pump against each other.
When depth commands everything
Sixty-five cubic metres a day is a modest need. A dynamic level 290 m down is another matter: with the lift and the friction losses, LE LAB computes 311 m of total dynamic head. At that head, the energy needed for the same cubic metre is more than twice that of a 120 m borehole, and the catalogue narrows fast.
The input data
The irradiation reference city is Beja, in the heart of the Alentejo. The study enters the dynamic level of 290 m measured while pumping in the 300 m borehole, lift 5 m, 120 m of pipe, existing tank, and the backup power box on yes. It is that last answer which will tip the arbitration.
The summary before calculation, with the grid backup requested and the tank already in place.
The selected solution: a permanent-magnet motor on a drive
LE LAB selects a GRUNDFOS SPE 18-35, a 7.1-inch pump with a permanent-magnet synchronous motor, driven by a 37 kW RSI solar drive. It accepts 24 m³/h and 350 m of head: at 311 m it stays below its head ceiling, and its operating point settles at 16.6 m³/h, the most it gives at that head. The array counts 44 panels of 630 Wp, that is 27 720 Wp, wired as four parallel strings of eleven panels in series. Average production settles at 72.0 m³ a day.
The solution at a glance. The seasonal range is stated: from 47.1 m³ a day in December to 97.8 in August.
What depth does to the catalogue
At 311 m of head the selection tightens abruptly. The LORENTZ PSk3 range, the only natively hybrid one in the catalogue, tops out at 240 m: it does not reach down here. The PSk2 does, but with the pump end that climbs highest in its range, a C-SJ30-35 whose maximum head is 330 m, and it then asks for 48 925 Wp, close to twice the selected array. The asynchronous SP reaches it too: the smallest one that manages it needs 28 840 Wp, and the SP 77-20 climbs to 81 900 Wp. That leaves the SPE, whose range goes up to 450 m and which holds the point with the smallest array in the catalogue.
| Solution | Photovoltaic array | The study's verdict | Alternating-current input |
|---|---|---|---|
| GRUNDFOS SPE 18-35 + RSI 37 kW | 27 720 Wp · 44 × 630 Wp | selected, average production 72.0 m³/d | drive terminals, with source changeover |
| GRUNDFOS SP 9-79 + RSI | 28 840 Wp | covers the need, with a larger array | drive terminals, with source changeover |
| LORENTZ PSk2-40 C-SJ30-35 | 48 925 Wp | covers with overproduction: 78 m³/d for 65 requested | SmartPSUk2 converter to be added |
| GRUNDFOS SP 77-20 + RSI | 81 900 Wp | covers the need, with an array close to three times larger | drive terminals, with source changeover |
| LORENTZ PSk3 | out of reach: the range stops at 240 m of head | ||
The PSk2 demands 48 925 Wp where the SPE asks for 27 720, close to twice the photovoltaic array. This is no longer a matter of appreciation, it is a difference in kind: at 311 m the LORENTZ pump end approaches its range limit, given at 330 m, while the SPE, given at 350 m on this model and up to 450 m across the range, keeps more margin.
Why backup is not a comfort here
Production runs from 47.1 m³ a day in December to 97.8 in August. The summer months go well beyond the need, but January, February, November and December fall below. On a deep borehole, winter cannot be caught up by reasonably enlarging the array: you would have to size on the least favourable month and pay all year for an array calibrated for December. The farmer chose the other route, keeping a right-sized array and letting the grid make up the grey weeks. That is exactly the role of backup.
Four months fall below the line of the need. That is where, and only there, the grid takes over.
Three ways of bringing alternating current into a solar pump
The word backup covers three very different architectures, and the solar catalogue offers all three. Telling them apart is the real technical subject of this file.
Here the hybrid range does not go down to 311 m, and the separate-converter architecture imposes 48 925 Wp instead of 27 720. The SPE on an RSI drive therefore wins on the photovoltaic array, and its alternating-current input connects without a converter, a source changeover being enough. On a shallower borehole and with no backup requested, the ranking reverses, as this Andalusian orchard shows, where the integrated controller comes first.
What the drive imposes around it
A submersible motor driven by a drive is not wired like an integrated controller. The diagram generated by the study carries the mandatory items of this architecture: the sine-wave filter between drive and motor, and the dry-running sensor. On this range, the motor's internal temperature sensor is disabled as soon as the pump is fed by a drive, and an external probe takes over.
The installation diagram generated by the study, with the items proper to a pump on a drive. The pump sits at 292 m, two metres below the dynamic level, in the 300 m borehole.
Depth chooses the family, backup chooses the architecture
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