Floating solar feasibility on irrigation reservoirs

Spain irrigates from tens of thousands of artificial reservoirs, and almost all of them are empty on top. Floating panels on that water produce electricity where it is already consumed and, on the way, cover up the evaporation that carries off part of the stored water every summer. elSOLución estimates both of those for one specific reservoir, free and without an account.

What floating solar on an irrigation reservoir is

A floating photovoltaic installation (FPV) is a solar array built on pontoons moored to the bank or the bed. On an irrigation reservoir, a balsa de riego, the idea is unusually clean: the water body is artificial, still, regular in shape, and belongs to a single owner, who is usually the same party paying the electricity bill for the pumping.

Against ground mount, floating brings three concrete advantages. It takes no farmland, which in many districts is the scarce resource and the reason solar parks meet local opposition. The water cools the modules from below, and a cooler module yields a little more. And the shade cuts evaporation from the reservoir, which is a real economic benefit wherever water is bought or allocated by quota.

In exchange the design is more demanding: modules sit flatter to cut wind uplift on a structure that is not pinned to the ground, a strip of shoreline has to stay clear for mooring and maintenance, and the walkways between blocks take up surface that produces nothing.

Why Spain, and why irrigation reservoirs

The combination that makes this case interesting is hard to find elsewhere: high irradiation, water that is scarce and expensive, and a private irrigation infrastructure spread very widely. Spanish irrigation reservoirs are also where the consumption is. The grower who fills the reservoir by pumping is often a mid-sized electricity consumer with a daytime, summer-weighted demand profile, exactly when the sun is producing.

That changes the arithmetic completely. A kWh exported to the grid earns the wholesale price; a kWh that avoids a purchase is netted off the retail price, which is several times higher. So the tool gives not one result but two mutually exclusive scenarios, self-consumption and grid export, each with its own investment and its own payback period.

How the feasibility is worked out

The estimate is built in layers, and each one uses data for the reservoir's own location wherever it exists.

The platform that actually fits

The first step is geometric, not energetic. A shoreline strip is taken off the reservoir outline, growing with the size of the water body from 3 metres on small reservoirs to 20 metres on those over 20 hectares. What is left is tiled with rectangular blocks of real 2.10 by 1.13 metre modules, separated by maintenance walkways and, every second block, by a wider service walkway so that everything stays reachable. The platform is turned to follow the reservoir's longer side and centred on the water.

You can pick between three coverage levels, from the most conservative to the most that fits. Capacity comes from the platform area actually drawn, at roughly 1 MWp per hectare of platform. Per hectare of platform rather than of reservoir matters: the water given up to moorings, setbacks and access carries no capacity on it.

The generation, month by month

Output does not come from a rule of thumb but from PVGIS, the photovoltaic performance service of the European Commission's Joint Research Centre, queried at each reservoir's own coordinates. It is asked for the monthly yield of a 1 kWp array under the floating design assumption: 12 degrees of tilt, flatter than ground mount because of wind uplift and self-shading; facing south; and 14% system losses.

The year is those twelve months summed, never an annual figure divided. That is why the estimate tells a January from a July, and why the monthly chart in the app has a shape.

The water that stops evaporating

Open-water evaporation comes from ERA5-Land, the Copernicus climate reanalysis, as a ten-year monthly normal in the reservoir's own grid cell. The whole surface loses that depth of water each month; over the area the platform covers, that loss is cut by around 85%. The saving is the difference, valued at the price of irrigation water.

The money

The investment does not use a fixed price per watt. A cost curve is applied that falls with project size: a small installation spreads the same fixed job over fewer watts, so it is built dearer per watt. That is what makes the payback period depend on the size of the reservoir rather than on the assumed prices alone.

For the self-consumption scenario, knowing how much is generated is not enough: what matters is how much is used. The model simulates a year hour by hour, twelve representative days, crossing the PVGIS daily generation shape with the P3.0TD consumption profile published each year in the BOE for the access tariff an irrigation site typically sits on. It then tries batteries from 0 to 3 kWh per installed kWp and keeps the one leaving the greatest net benefit over its life, if any of them does.

Payback is walked year by year rather than divided: generation fades with array degradation while the water saving and the running costs hold. Every figure is in real euros of today, with no prices escalated into the future, so a payback period is a real-terms one.

Who this is for

• Installers and engineering firms who need to filter quickly which reservoirs in a district are worth a visit.
• Irrigation communities who want a first number before commissioning a study.
• Reservoir owners with a pumping electricity bill, wondering whether the roof of water they own is worth anything.

When the map finishes estimating a reservoir, it also lists the floating solar installers active in that province, ordered by how strong the evidence is that they have done this kind of project: a completed plant first, then a project under way, then a technology reference.

Try your reservoir

Search for your municipality, select the reservoir on the map, and you will see the platform drawn on the water with its capacity, its monthly generation, the water it stops from evaporating, and the two routes to a return. No account, no sign-up.

These figures are preliminary estimates based on public data and on documented design and price assumptions. They are not an engineering study nor financial advice, and should not be the only basis for an investment decision.