Irrigation with renewable energy: which technology to use?

On agricultural and livestock farms, often far from urban centres and without access to a robust electricity grid (or at all), renewable energies are an economical and sustainable option for energy storage and crop irrigation.

The most suitable renewable technology in these cases is photovoltaics, due to the easy availability of the sun and the efficiency/price ratio. Furthermore, such systems are the perfect alternative to the use of fossil fuel-based electricity generators for agricultural and livestock farms. On the plus side, in most cases, the demand for energy in irrigation or pumping installations tends to have a markedly seasonal profile, usually coinciding with the months of highest solar radiation. Harnessing this solar radiation by means of photovoltaic panels is an efficient, economical and environmentally responsible solution for irrigation and water pumping applications.

Characteristics of farm irrigation with photovoltaic energy:

To meet the irrigation requirements of each crop, there are different ways of implementing photovoltaic technology. In this article we are going to analyse two different modalities: the so-called pumping systems or solar or direct irrigation and the self-consumption systems used for pumping. We will also look at the differences between installing PV panels with a fixed structure or with solar tracking. The suitability of one system and structure or another will depend on the needs of the client and the crop.

Solar trackers

The solar tracker is a type of movable structure of photovoltaic panels, so that the panels are oriented during the day to be approximately perpendicular to the sun’s rays as the sun moves from sunrise to sunset.

There are various types of trackers depending on the movement they produce: single-axis (if they rotate in one direction) or dual-axis (if they follow the sun in all directions so that the surface of the panel is always perpendicular to the sun). This structure allows a better use of solar energy, i.e. a higher production efficiency. On the other hand, it is a more expensive technology with higher maintenance requirements, as it is made up of moving parts and is more complex than fixed structures.

Direct solar pumping

This modality is the most suitable when there is a pond where water can be stored or the irrigation application always coincides with maximum solar radiation and it can be assumed that at certain times or on certain days irrigation will not be possible due to a lack of solar resources. The idea is to take advantage of the moments of photovoltaic production to activate a pump that takes water from a well to store it in a pond, or to pump water directly to irrigate.

Although fixed PV installations are more economical, it may be advisable that the PV system structure is not fixed, i.e. oriented in one direction all the time, but has solar tracking to “follow” the movement of the sun in order to capture as much radiation as possible and maximise production.

The single-axis photovoltaic tracker makes it possible to flatten the production curve. That is to say, to extend solar production slightly in the mornings and afternoons, instead of concentrating it in the middle of the day. To do this, the panels must be oriented so that the tracker rotates from east to west, from sunrise to sunset.

Production graph of a photovoltaic system for direct irrigation of pistachio trees with a fixed structure. The solar radiation curve is the usual one, with the highest photovoltaic production during the central hours of the day. Installation: farm in Belchite (Zaragoza).

Production graph of a photovoltaic system for direct irrigation of vineyards with single-axis trackers. The solar radiation curve flattens out at midday and more energy is produced during the first and last hours of sunshine, thus achieving more hours of irrigation and avoiding the non-use of surpluses during the central hours of the day. Installation: Masatrigos II Farm in Belchite (Zaragoza).

On farms that irrigate for less than 8 hours, a fixed structure can be installed, but for crops that require more hours of irrigation, it is advisable to install solar trackers, since, although the structure is more expensive, the tracker usually takes advantage of 3 to 4 more hours of sunlight.

Regarding the economic factor, the amortisation period is similar for both types of photovoltaic structure, only the initial investment is different. In tracker irrigation, although the initial investment is much higher, there are also greater annual savings in irrigation. However, it should be taken into account that solar trackers generally require more maintenance and there may be more problems due to wind.

Another technology that may be of interest for this application is solar pumps. This is a special type of pump that uses photovoltaic energy directly to pump water. They can be either direct current, alternating current, single-phase or three-phase. In addition, they can be submersible or surface mounted. To manage the energy, optimise the performance of the PV system and ensure the safety and efficiency of the entire system, a controller must be implemented.

If, on the other hand, a conventional pump is already in place, it will be necessary to incorporate a frequency variator that lowers the pump’s starting peak and supplies the pump with the energy in the format it needs, from the electricity generated by the solar panels.

Photovoltaic self-consumption

Is it crucial to always water at the same time? If it is necessary to have greater control over irrigation and it cannot be assumed that irrigation will not take place because there is no solar resource available, or if you wish to have energy available all the time and irrigation does not always coincide with the sun, the most recommendable option is a self-consumption installation, either connected to the grid or in isolated operation with batteries, to provide energy when necessary. In this case, irrigation can be carried out regardless of the low solar resource, either because of bad weather or because it is night time.

If there is an electricity grid close to the farm, it is recommended that the PV self-consumption is connected to the grid. One advantage of this is that, if self-consumption is configured in the form of surplus compensation, all the energy that is produced and not used at the time can be fed into the grid, receiving financial compensation for it in the electricity bill.

In order to optimise the correct use of energy and obtain maximum yields from the photovoltaic system, it is advisable to implement monitoring systems.

Production graph of a photovoltaic system for self-consumption connected to the grid, in surplus compensation mode. When solar radiation is not sufficient, consumption is charged with electricity taken from the grid. When there is an excess of photovoltaic production, the surplus is fed into the grid. This occurs in the central hours of the day, when solar radiation is greater than consumption needs, and at the beginning and end of the day, when energy is not being demanded. Installation: Comunidad de Regantes Belchite (Zaragoza).