LELABAPP
Case study

Solar first, generator as backup:
the hybrid system in Chad

In the Abéché region, in eastern Chad, a borehole supplies a town and its livestock with 100 m³ per day. The existing generator is to be kept as backup, starting automatically when solar output is no longer sufficient. The study looks at this hybrid architecture and at the coverage provided by the existing tanks.

Abéché region · Chad Need: 100 m³/day, year-round 50 m borehole Existing reservoirs: 95 m³ Generator kept as backup NGO-led project
The project

The need, as the NGO expresses it

Water for a Sahelian town is not negotiable: residents, passing herds, small dry-season market gardening. The network relies on a water tower and tanks totalling 95 m³, filled until now on diesel. The association's goal is not to remove the generator: it is to make it fall silent. Solar as the main source, the generator as automatic backup, with no human intervention.

100 m³/daythe need of the town and its livestock, year-round, rainy season included
50 mthe water depth in the borehole, plus the tower and 200 m of pipe: 72 m of total head
95 m³the existing reservoirs, a little under one day's need
1 generatoralready on site and kept: the NGO's request is a genuine hybrid system, not a replacement

Chad is proven ground for solar pumping: documented installations have been running there for over ten years, including on larger supply systems than this one, often led by NGOs and international solidarity associations. In the Sahel, a kWh produced from diesel costs 3 to 4 times a solar kWh: every hour of sun used is an hour of generator run time saved.

Step 1

The input data in LE LAB

A few minutes of input. The solar reference city is Abéché, at the heart of the project region. Borehole at 50 m, delivery head of 15 m to the tower, 200 m of pipe: the total head is calculated automatically, 72 m. And one answer carries more weight than the others: electrical backup, yes. It is this answer that steers the study toward a hybrid solution.

Study summary in LE LAB: Abéché, 100 m³ per day year-round, depth 50 m, delivery head 15 m, 200 m of pipe, calculated TDH 72 m, electrical backup yes, water reserve 95 m³

The summary before calculation: nine answers are enough, the total dynamic head is calculated for you.

Step 2

What LE LAB proposes: a 100% hybrid solution

The chosen solution is a LORENTZ PSk3-15 with the C-SJ17-11 end, and the tile announces it on its own: "100% hybrid". The solar array has 17 panels of 515 Wp in a single string, for 8,755 Wp, with an average production of 110.5 m³ per day, from 83.1 m³/day in August to 128.0 in February. The single string is not a whim: the PSk3 controller operates at high voltage and the array is adjusted by whole string. The pump is rated for 25 m³/h and 90 m at most: at the project's head, 72 m, it stays inside its curves.

Solution proposed by LE LAB: LORENTZ PSk3-15 C-SJ17-11 pump shown as 100% hybrid, 8755 Wp solar array with 17 panels of 515 Wp, average production 110.5 m³ per day

The solution at a glance: the "100% hybrid" label is carried by the pump itself.

What "hybrid" really means with a PSk3. The controller does not switch from one source to the other: it combines them. Priority goes to solar power, and when water demand requires it, generator power tops it up watt by watt, automatically; the SmartStart option starts the generator on its own, even in the middle of the night if the water setpoint calls for it. Compared with so-called "dual-supply" systems, which only switch, LORENTZ reports 60 to 70% savings. And the enclosure is built for the Sahel: IP66, operation up to 60°C ambient, active temperature management, remote monitoring through the LORENTZ app and platform, valuable for a remote site.
Step 3

Production, month by month: the Sahelian sun carries the dry season

Over sixteen years of the region's PVGIS solar history, nine months stay above the 100 m³ line and three fall below: June at 94.2 m³ per day, July at 84.8 and August at 83.1, at the heart of the rainy season, for the worst August observed at 5.0 kWh/m²/day. The rainfall curve tells Abéché's climate: 0 mm in January, 181 mm in August. Solar alone carries the dry season; during the rains, the generator takes over.

Estimated monthly production against the need of 100 m³ per day: nine months above the line, June, July and August below, critical month August at 83.1 m³ per day, rainfall from 0 mm in January to 181 mm in August

The critical month is shown: August, 83.1 m³/day for 100 required. The blue line is monthly rainfall.

Daily output for a typical day in August: hour-by-hour flow rate, peak at 15.2 m³/h, 88.2 m³ produced, 88% of the need

The typical August day peaks at 15.2 m³/h for 88.2 m³ produced, 88% of that day's need.

Step 4

The 95 m³ reservoirs, and what is left to the generator

LE LAB simulates the water reserve day by day over sixteen years of PVGIS weather history. With the existing 95 m³, 313 of 365 days are fully covered by solar alone. The app flags the coverage as insufficient: 52 days a year on average remain incomplete, concentrated from June to August, at 74% of the need, an annual shortfall of 1,341 m³ out of 36,525, or 3.7%. The worst episode of the sixteen years runs from 9 May to 30 August 2012, 114 days of low sun. That is exactly the generator's job.

Autonomy and water reserve block: 313 of 365 days covered with the 95 m³ reservoirs, insufficient coverage flagged, 52 incomplete days a year delivering 74% of the need, missing volume 1,341 m³ out of 36,525 per year, benchmark 811 m³

The benchmarks show the price of all-solar: 811 m³ of reservoirs instead of the 95 m³ in place. No one pours that concrete when a generator is already there.

The hybrid system turns this trade-off into intelligence. Covering those days with storage alone would mean taking the reservoirs from 95 to 811 m³; the app also notes that an array enlarged by about 6%, 18 panels instead of 17, would leave around 24 incomplete days a year with the current reserve. Covering them with the generator costs a few hours of diesel during the rainy season, triggered automatically by the controller. The generator that used to run all year becomes a backup that starts on the rainy-season days. That is the exact answer to the NGO's request: solar first, the generator as backup, and no one to send on site to switch over.
Step 5

Manufacturer curves, at the real operating point

The pump's Q/H curves, plotted from official LORENTZ data, with the project's head highlighted: 18.5 m³/h at 72 m. The C-SJ17-11 accepts up to 25 m³/h and 90 m: at this head the pump stays inside its curves, and it is the available solar power that sets the flow, not the pump.

Manufacturer curves for the PSk3-15 C-SJ17-11 pump: flow according to available power by head, operating point 18.5 m³/h at 72 m

Flow according to available power, by head. The yellow curve is the project's, at 72 m.

What this case shows

The tool frames, the trade decides

100% hybridthe PSk3 controller combines solar and generator watt by watt, priority to solar, automatic backup start
52 days/yearwhat is left to the generator once solar is in place, concentrated from June to August: a few days of diesel instead of a year of jerrycans
16 yearsproduction and the reservoirs are tested day by day over sixteen years of the Sahel's PVGIS solar history
Honestythe insufficient coverage of solar alone during the rains is shown, the 114-day episode of 2012 is named, the enlarged array is quantified

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