A rainy day solution?

As roof real estate for solar is slowly dwindling, the commercial solar designer and installer is peering down. No, not at their feet but at terra firma. Mother Earth.

Unlike a roof mount system where rain striking the surface of the solar array is captured eventually by the gutter system, with a ground mount system rain simply runs off and this is something that we could be collecting - especially at rural properties, where ground mount solar systems tend to be installed.

Erosion and dying vegetation

These ground mount systems take up a large space and the actual surface area of the panels are being rained upon on a regular basis.

What usually happens with this rain is that it drains off the edge of the panels, potentially creating erosion.

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One of the driest continents in the world

In a continent as dry as Australia and with water being the most precious commodity in the world, surely ground mount solar developers should be looking at capturing this precious resource?

As always there is a cost benefit analysis process that must be applied to ascertain the financial viability of a system that will cost more per watt (due to guttering, trenching and pumping costs) than a traditional standard ground mount system with no capacity to capture, redirect and store water.

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Questions must be asked 

If we are to start looking at solar renewable energy systems and how they interact with the environment from a water perspective, we need to ask questions such as:

• How much rain falls per year in a particular area designated for solar?
• How much rainfall can be captured?
• What calculations do we need to know? 
• How can the water be redirected into storage?
• What are the storage options?
• What can the water be used for?

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How much rain falls per year in a particular area?

Australia is one of the driest continents in the world on average, but rainfall can vary tremendously: from Mulka Bore, west of Lake Eyre (SA), with an average annual rainfall of 100mm (4 inches); to Bellenden Ker, North Queensland with 7950mm per year.

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Where are these solar ground mount arrays?

The decision concerning where to locate these ground mount arrays is based on various criteria. One of the more important ones is the amount of sunlight in the area followed by the proximity to electrical infrastructure. As mentioned above, rainfall varies significantly across Australia - for example, the Sunshine Coast Solar Farm in Valdora, near Yandina has an average annual rainfall of 1350 mm, and the Numurkah Solar Farm, located near Shepparton has 537 mm. Annual averages across other regions of Australia are shown in the table below (in mm).

How much rainfall can be captured?

Effectively it is the panel’s surface area that acts as a roof to capture the rain that falls upon it. But how much rain can a solar panel capture? Every 1 mm of rain falling on 1 m2 of surface collects 1 litre. 

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How do you calculate the available surface area of the panel?

Panels are never installed in a flat configuration with ground mount systems; this means you need to determine the horizontal plane in order to calculate how much water can be collected.

And how do we determine this, you might ask? We need to use some good old trigonometry!!

The calculation

Let’s say we have a solar array using 400 watt panels and the dimensions are 2000mm x 1000mm per panel. They are being installed at a 30 degree tilt in a portrait configuration.

Here comes the trigonometry! 

Sin30 x 2000 = 0.5 x 2000= 1000 mm

So we have the hypotenuse which is 2000mm and the opposite (the height which is 1000mm)

We need the adjacent, which is the horizontal plane

• hypotenuse2 = opposite2 + adjacent2, or
• adjacent2 = hypotenuse2 - opposite2

Adjacent2 = 20002 - 10002 = 1732 mm 

We multiply, in this case, by the width of the panel, 1000mm, and we get 1.732m2

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The numbers, continued

We now have most of the information needed:

• Average rainfall of location in question
• Available horizontal plane of panel in m2

Now we need the total area of the ground mount array and we do this by calculating the: 

• (number of rows) x (number of panels per row) x (the area of the horizontal plane of the panel in question)

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Results

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As can be seen from the table above, where the system is installed plays a large role in the amount of water that can be potentially harvested. 

With a 1.140 MW system this can range from 8,944,657 litres in Darwin to 2,557,027 litres in Melbourne (per year). 

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How can the water be redirected into storage?

Effectively there are two main ways the water can be redirected into storage:

1. The use of a guttering system that connects to the solar panel framing system, or
2. Allow the water to flow off the panels into a trench system or similar.

The water can be stored in trenches running near the bottom edge of the panels.

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Storage options

The captured water can be contained within:

• Underground or above ground water tanks, using gravity or a pump system or
• In appropriately lined/prepared trenches positioned at the base of the panels

The trench system is potentially the cheapest as there are no pumps and the excavations could be included as part of the project scope. The cost effectiveness of these options needs to be discussed.

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What can this captured water be used for?

The captured water can be used for the watering of stock and or irrigation of crops grown between or under the panels, see our agrivoltaics video or to periodically clean the solar panels via some kind of spray system, see our solar panel cleaning video.

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Conclusion

Australia is a dry continent and water is a precious resource that should be managed.

Ground mount solar systems can potentially capture and redirect rain into storage and this can be used for stock watering, plant irrigation and panel cleaning. The viability of capturing and storing water from large PV arrays will depend on many factors but one thing is indisputable; water is the most valuable commodity on this planet and should be managed accordingly.