Solar Power and Irrigation - the near perfect use case

It does not take long to realise just how rapid the pay back on a solar system when you are generating electricity to power a high intensity business process like irrigation, especially if you are replacing an older diesel system with a solar system.

Without going into too much detail, an average diesel pump will consume about 0.3L per kWh, some more, some less, but we can use this as a typical constant to calculate cost.

We have an 18.5kwh irrigation pump which is enough to irrigate 9600 trees (6ha) across four equally sized zones. This means to irrigate Forest Lodge we would consume 5.5L of diesel an hour or at $1.03 (at time of writing) about $5.70.

Likewise, at an average price of 16.88c per kwh (the average cost a commercial user paid in New Zealand in 2010) if you were using an electric pump supplied by the grid you would be looking at $3.12 per hour

I wish I could tell you that after much careful analysis I came to the conclusion that a PV array about 150% the size of the pump is optimal, but reality is we had shed roof space for 23.1 kWh of solar, and so that’s what we have installed to date. It’s not quite sufficient, but it’s not far off.

There are multiple of variables in solar that affect efficiency, including the weather, the season, the angle of the panels considering the season and the temperature. At the height of the day in spring and summer we have been able to achieve 20kwh+ of power generation through our panels, but on average we are generating about 15kw every hour over an 8-hour period (10am to 6pm). The surplus power we generate is used to charge our batteries, and following that we export back to the grid as it's generated, and we draw as needed.

Our solar system cost just over $43,000 NZD which included all installation costs and inverters.  For this simple calculation we will round up to $50,000 which would cover the additional panels to get us to 28kwh (150% of the size of our pump).

At 8 hours average irrigation over 180 days would cost $8200 in diesel at $1.03, rising to $11800 at pre-Covid prices. That’s a payback of between 4 and 6 years if we don’t consider the cost of replacing diesel equipment with electric technology.

How about supplementing electric irrigation that is currently powered by the grid?

Well, if we look purely at the unit cost of a kilowatt then solar is going to take 11 years to pay itself off. That’s still pretty good considering solar panels are now usually guaranteed for 25 years at least 80% efficiency. It’s a long game but it’s worth it if you have access to capital or can leverage the current low interest rates.

Now this isn’t perfect, because sometimes the sun isn't shining, so we need to resort to our batteries and then to the grid. Other times we are exporting power because we are generating more than we need and this is bought back off us at 10% under the spot price at the time we export. For the purposes of this rather unscientific blog post about solar pay-back for horticulture I am simply going to assume the two cancel each other out.

In the graph and logs above, you can see that our irrigation system and other orchard related power consumption was 2.8 MWh, however we produced 3.4 MWh. We still utilised the grid for 0.5 MWh of power when the sun was not shining and our batteries were low, but we also broke even on that by pushing 0.68 MWh back to the grid.

Batteries do help your solar system be considerably more efficient, but you need more use cases than solely irrigation to justify their cost. Right now it’s much cheaper to add more panels than it is to add batteries, meaning that in many cases it's better to use the grid as a metaphorical battery.

If you are in the Central Otago area of New Zealand, hit up Regan and Craig – the local solar experts and Infinite Energy. I have been working with them on solar solutions for agriculture and horticulture and these guys know their stuff!

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