A solar plant monitored for two years in western Senegal lost 0,34 % of its output per day of soiling across the year as a whole, and up to 0,49 % per day in February and March, at the height of the harmattan. Diouf and co-authors draw a conclusion you can use straight away: the optimum washing interval comes down to one pass every 14 days. For a pumping station feeding a plantain plantation, that translates into four rhythms to keep, and nothing more: a look each day, an inspection each week, a panel wash every two to four weeks, and a check of the structure every two to three months.
These rhythms come from the Maintain module of the toolbox on solar powered irrigation systems published by GIZ with FAO, in its version 3.0 of July 2020. They call for no specialised tooling and no technician. They call for regularity.
What plantain asks of the pump in the dry season
FAO puts the water need of a banana crop between 1 200 and 2 200 mm over a full cycle of 300 to 365 days, with a crop coefficient rising to 1,1 at full canopy, and rates the crop as highly sensitive to water deficit. This is a crop that draws water all year, with no stage where you can cut the water off without consequence.
In southern Cameroon, Dépigny and co-authors described plantain cropping systems on sites receiving 2 300 to 3 200 mm of rain a year, spread from March to November, with a dry season from December to February. On farm densities range from 393 to 2 775 plantains per hectare depending on the system, from an associated orchard to a plot grown in pure stand.
Put those two sets of figures side by side and the logic is plain. Rain covers the need nine months out of twelve, then stops dead for three months, at the period when radiation is strongest and dust is heaviest. The pumping station only works hard during those three months, and that is exactly when a breakdown cannot be made up. A plantain bunch that stops filling in January does not start again in March. Maintenance therefore happens now, in the weeks beforehand, not when the pump stops.
If your station is not yet installed, the sizing is settled before the purchase: flow rate, total head and buffer reservoir are covered in our article on sizing a solar pumping system, and the measurements to take on the plot in the one on field surveys before equipping a plot.
The daily check, two minutes at first light
The Maintain module holds it in one sentence: check each day that the system is working. If the pump is not running, the toolbox points to two checks in order, and that order matters because it runs from the most likely cause to the least likely.
- The water source and the pipes. Is there enough water? Any dirt, any blockage? In the dry season the level in a well or a borehole drops, and a strainer left uncovered makes the pump lose suction with no part at fault.
- The electronics. Controller, inverter, connections. A clean cut on a device that normally displays a status is often easier to read than a hydraulic fault.
Record the check in a notebook kept at the pump house: date, start time, and one line whenever something has changed. That notebook is what will let you say, three months later, whether the flow has been dropping for a long time or since the day before yesterday.
The weekly inspection, item by item
Once a week, the toolbox calls for seven items to be reviewed. Always take them in the same order, from the source towards the plot.
- The energy generated by the PV array. Read off what the inverter or the controller displays, at a comparable time of day from one week to the next.
- Pump performance, meaning the actual pumping rate. A flow that erodes week after week tells a story of soiling or wear, not of a breakdown.
- The condition of the water source, and in particular the clarity of the water. Water that starts carrying sand signals filtration trouble and pump wear ahead.
- The condition of the controller and the electronics, enclosure closed, no trace of moisture or insects inside.
- Blockage of the drip emitters, if the network runs on drip. This is the item that costs the most when it is left to run, and the usual causes are gathered in our article on drip irrigation mistakes in market gardening.
- The condition of the storage and the pipework: leaks, level in the reservoir or tank, weeping fittings.
- The condition of the panels and their mounting: soiling, cracks, a loose bolt.
Washing the panels: every 14 days or every four weeks
The toolbox calls for a wash every two to four weeks. The Senegalese study lets you choose within that range instead of guessing, because it measures the soiling rate month by month: 0,33 % loss per day in October, 0,42 % in December, 0,49 % in February and March. In other words, dust builds up half again as fast at the heart of the dry season as it does at the end of the rains.
The practical rule that follows is simple. In the rainy season, rain washes part of the array and a pass every four weeks is enough. In the dry season, and as soon as the air turns dusty, tighten to 14 days. In the study, going from 8 to 20 washes a year cut the total cleaning cost by 31 %, because the energy recovered far exceeds the cost of the extra pass.
The method in the Maintain module comes down to four actions and two prohibitions.
- Clean water, and a little scrubbing with a cloth covered sponge or a soft brush: that is enough to remove the most persistent grime.
- Wash early in the morning or late in the day, when the panels are cool.
- Never spray cold water on a hot panel: the glass can crack.
- Never walk or step on the panels. The cell microcracks this creates cannot be seen and cannot be repaired.
Use the wash as a chance to look at what is growing around the array. A single branch shading one corner of a panel in mid morning penalises the whole chain, because the cells within a panel are wired in series.
Strainer, filter and suction line
When the flow drops and the panels are clean, the problem is almost always upstream of the pump or just behind it. Three points to take in order.
- The strainer. Pull it out, brush it, and check that it stays submerged at the lowest water level of the dry season, not at the level on the day of installation. A strainer resting on the bottom picks up sand, a strainer set too high loses suction.
- The head filter. A drip network is judged on the pressure difference between the filter inlet and outlet. When that gap widens, the filter is loaded: it gets cleaned before the emitters notice, not after.
- The suction line and the fittings. An air leak on the suction makes the pump turn without delivering water, and the symptom looks like an electrical fault. Tighten up, redo a seal, and listen to the pump: a pump drawing air does not sound like a pump under load.
Measure instead of guessing
The GIZ report on solar powered irrigation systems, published in August 2020, is clear on this point: every water pumping installation should be equipped with a monitoring device, at least a water flow meter, and ideally a pressure gauge as well. These are two instruments with no electronics, which need no setting and cannot drift out of adjustment.
Their value is that they turn an impression into a decision. A flow rate read each week says whether the drop is abrupt, so mechanical, or slow, so tied to soiling or wear. A pressure read on either side of the filter says when to clean it. Without those two figures you dismantle at random, and you end up replacing a sound part.
The same reasoning applies to the check every two to three months called for by the Maintain module: vegetation growing back around the array, new shading, fencing in good order, mountings and braces tight. And a full inspection, outside the schedule, after every severe weather event, a wind squall or hail.
Maintenance schedule for a solar pumping station
| Frequency | Operation | What it prevents |
|---|---|---|
| Every day | Check that the pump is running, note the start time, check the source and the pipes then the electronics | A stoppage that lasts several days with nobody aware of it |
| Every week | Energy generated, flow rate, water clarity, controller, emitters, storage and pipework, panels and mountings | A slow drift mistaken for normality |
| Every 4 weeks in the rainy season, every 14 days in the dry season | Wash the panels with clean water, panels cool, and clear the vegetation that casts shade | From 0,33 to 0,49 % of output lost per day of soiling |
| Before the dry season | Strainer brushed and repositioned at the low water level, filter cleaned, suction fittings retightened | Loss of suction or sand ingress at the moment the crop depends on the pump |
| Every 2 to 3 months | Vegetation, shading, fencing, mountings and braces | A panel lifted by the wind or shaded without anyone noticing |
| After a wind squall or hail | Full inspection of the PV array and its mountings | A crack or a torn fixing that gets worse |
For the rest of the crop management and to tell the varieties apart, our article on plantain and dessert banana gives the basic benchmarks.
Having your station checked by JOGOO
JOGOO Agriculture installs and maintains solar pumping stations, irrigation networks and storage works, and trains the farm team on the weekly readings. See the Irrigation & Pumping page for the services, and the Contact page for a visit or a quote for your plantation.
Frequently asked questions
How often should the panels of a solar pump be cleaned?
Every two to four weeks according to the GIZ and FAO toolbox. In a dusty dry season, tighten to 14 days: the Senegalese study by Diouf and co-authors measures up to 0,49 % of output lost per day of soiling in February and March, against 0,33 % in October.
Can the panels be washed in the middle of the day?
No. Washing is done early in the morning or late in the day, when the panels are cool. Spraying cold water on a hot panel can crack the glass.
How much water does a plantain plantation need?
FAO takes 1 200 to 2 200 mm over a full cycle of 300 to 365 days for a banana crop, with a crop coefficient reaching 1,1 at full canopy, and rates the crop as highly sensitive to water deficit.
Which instruments should be fitted on a pumping station?
At least a water flow meter, ideally a pressure gauge as well, according to the GIZ report of August 2020. A flow rate read each week separates a mechanical breakdown from progressive soiling, and the pressure on either side of the filter says when to clean it.
The flow is dropping but the panels are clean: where to start?
With the strainer, then the head filter, then the fittings on the suction line. An air leak on the suction gives the same symptoms as an electrical fault, and a water level falling through the dry season can leave the strainer uncovered.
Should the installation be inspected outside the schedule?
Yes, after every severe weather event. The toolbox recommends a full inspection of the PV array and its mountings after a wind squall or hail.
Sources
- FAO, Irrigation Water Management Training Manual, chapter 3, Crop Water Needs
- GIZ and FAO, Toolbox on Solar Powered Irrigation Systems, version 3.0, July 2020, Maintain module, Implement Maintenance Routines
- GIZ, Solar Powered Irrigation Systems (SPIS), Technology, Economy, Impacts, August 2020
- Diouf M. C., Faye M., Thiam A., Sambou V., 2022, A framework of optimum cleaning schedule and its financial impact in a large scale PV solar plant, a case study in Senegal, EPJ Photovoltaics 13
- Dépigny S., Delrieu Wils E., Tixier P., Ndoumbé Keng M., Cilase C., Lescot T., Jagoret P., 2019, Plantain productivity, insights from Cameroonian cropping systems, Agricultural Systems 168
