Maximising the return on investment for any solar installation requires using as much of the power as possible, at the time it is generated, or selling excess power at a reasonable price.
Feedback tariffs have steadily declined and are now down to around 2-5c per kWh, so feeding (selling) back into the grid is not the best use of this investment.
The payback period on a 100kW system purchased upfront for $100,000 if all power was fed back to the grid at 5c/kWh could be around 14 years. In contrast, if all power was used on farm and referenced to 25c/kWh as if purchased from the grid, the payback period could be as low as 3 years.
Utilising all power generated on farm is not possible and there are significant other cost associated with increasing utilisation. However, this example highlights the importance of maximising what is used on farm.
The solar production curve
To achieve maximum solar utilisation, we must reconsider how and when we use power on farm. To do so it is important to understand the solar production curve. Without the use of solar energy, we are used to grid power that is delivered at a constant and dependable rate, 24 hours a day.
Figure 1 – depiction of hypothetical power output from a north facing 50kW solar array over one cloudless daylight period.
Solar is generated in a curve and in daylight hours only, with output increasing as the sun rises and decreasing as it sets of an afternoon (Figure 1). As the seasons change, day length and solar radiation intensity also changes somewhat predictably, while weather systems unpredictably influence output due to cloud cover.
Flattening the solar production curve
Solar panels are usually installed all facing north to achieve maximum solar generation within daylight hours. While this does maximise solar production, it causes a steep power production curve (Figure 1).
A flattened production curve (Figure 2) is often more useful. Generating lower peak power but higher and more consistent power output either side of the daily peak.
One way to achieve this is to spread the panels between easterly, northerly and westerly facing directions. This allows greater power generation either side of the sun reaching the apex of its daily arc.
Bifacial panels that allow the back side of the panel to generate power can also help flatten the production curve.
Sun tracking arrays that move with the sun to both maximise production and flatten the curve are available. However, they can be prohibitively expensive and introduce the complication of serviceable moving parts.
Figure 2.- depiction of hypothetical power output from a tri-directional facing 50kW solar array over one cloudless daylight period
Maximising solar utilisation
To fully maximise solar utilisation there are several management techniques you could consider implementing on farm.
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Multiple uses
Consider uses other than just the primary irrigation pump. Having as many different uses for the power as possible means there is a better chance you can use the power you are generating at any given time.
An array installed at a pumping station will have many occasions when the pump is not running or using all the power generated, the question to ask is what can be done with this excess power? Maybe it can heat the dairy hot water or run a bore pump to fill a dam. Maybe this excess power runs other irrigation infrastructure, or even the milk vat coolers.
It is important to note that currently power is not easily transferred across title boundaries and a lot of farms are made up of several titles. This can require arrays on each title, limiting them to applications on those titles or where a pump can deliver water across the titles.
There are some power companies that allow real time consolidation of power meter data, allowing peer to peer trading of on farm renewables. However, this is still in early stages and only available with a small number of suppliers.
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Daylight power demands
Where possible move all power demands that can access power from the array into daylight hours. In irrigation this means moving from nighttime off-peak pumping to irrigating during the day.
This does introduce new considerations such as the crop effects of irrigation in very hot conditions, and the potential for impacts on water efficiency. It does also mean no getting up in the middle of night because an irrigator has thrown an alarm. All the factors need to be weighed up against the potential for power and emissions savings.
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Smaller loads
Break up large loads into several smaller loads where possible. This makes it easier to utilise smaller amounts of available power and better fill the solar production curve at any given time.
For example, if a fixed spray system normally irrigates 12 sprinklers at a time, set it up with a variable speed drive and the option to use 4, 8, or 12 sprinklers depending on available solar. Or for a hot water service consider installing several smaller elements rather than one large element.It can take some critical thinking, but finding opportunities to break down large loads into smaller loads is important to maximising solar utilisation.
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Variable loads
Look for opportunities to have some variable load power demands. This allows flexibility in the utilisation of power as power production changes in real time, further increasing the ability to utilise power across the production curve.
It can be trickier to find these opportunities, but a clear outlier is a pump filling a storage dam. If we fit this pump with a variable speed drive it can change its speed depending on the available power being generated as filling a storage does not require a constant flow or pressure.
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Power storage
Storing excess solar power to be used when needed can help to maximise utilisation. However, batteries are generally not a financially viable option for storing large amounts of energy.
A 100kWh battery can only store 1 hour worth of solar power from an array producing 100kW and can cost in the region of $100,000. Smaller batteries can help to offset short term fluctuation in production due to cloud cover or be used to run some smaller load applications outside of daylight hours.But there are other ways to store excess power than just batteries. Pumping water into an elevated storage can allow that water to be delivered under pressure due to stored gravitation energy. Each meter of height (meters of head) equals around 1.4psi, so a storage 20m above the irrigation area may deliver water at about 28psi. Even a small amount of elevation allows for storage of energy, even if gravity outflow from the dam is not possible and pumping is still required, the work the pump needs to do may be and a more efficient pump can be used for this purpose.
Heating hot water with excess solar energy for use later is a more short-term storage solution as insulated hot water services hold this heat energy for some time. There are even heat and cold exchange systems to use heated or cooled water for other applications that require heating and cooling.
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Smart automation
Smart automation and scheduling is required to tie all these techniques together. It can shuffle all your power usage demands around in real time to match solar production. These systems need to be able to identify what tasks require energy use, what the priority of these tasks is and also keep track of what is occurring where which is particularly important for managing irrigation intervals. This type of energy system automation is quite complicated to get right but can have outstanding results when it is implemented successfully.
In an irrigation demonstration project conducted at Wilandra Farms in Clydebank Victoria, such a system saw a 24ha fixed spray system achieve 95% of the pumping requirement from solar power with very little excess being fed back into the grid.
The fixed sprays were designed to allow 4, 8, or 12 sprinklers to operate depending on solar production, the bore pump was fitted with a variable speed drive and a storage dam was constructed to allow the pump to run when irrigation was not needed. The pump could also deliver to a larger storage dam on farm that all other irrigation infrastructure can draw from. Finally, the pump could also run an 8 span pivot adjacent to the fixed spray area.
Figure 3 – Wilandra fixed spray irrigation demonstration, solar generation, irrigation and dam pump usage, and dam level over one daylight period.
The data in Figure 3 was collected from this system in one daylight period on an intermittently cloudy day in the middle of the irrigation season. It shows a very high utilisation of the power generated.
As solar generation started to increase, at around 6am the bore pump turns on starting to fill the dam. At around 8 am a set of 4 sprays turns on and the bore pump turns off. As power production increases more sets of sprays turn on and the bore pump kicks back in to pick up and use the excess energy above what the irrigation pump is utilising.
At around 2pm the irrigation cycle is finished, and the irrigation pump turns off. The bore pump then ramps up to utilise all the power available and slows down gradually as production decreases into the afternoon, leaving the storage full at the end of the day and around 95% of the solar produced utilised.
Summary
It can be a complicated and expensive pathway to maximise utilisation of solar installations to operate farm irrigation systems. However, when done well it can be a good return on the solar investment and at the same time inherently reduce the labour inputs around irrigation due to the required automation.
If you would like more information on the demonstration project conducted at Willandra Farms or have other questions around solar utilisation and irrigation, see the links below and or reach out to one of the helpful Agriculture Victoria irrigation extension officers based out of Maffra.
