Polytunnel Solar Powered Drip Irrigation System

This page introduces the principals of how to create an automatic watering system for a polytunnel on an allotment, or any other environment where there is no mains water supply or electricity.

Polytunnels are hot environments from spring to autumn. On a day when it is only 10 °C outside, if the sun is shining, inside a polytunnel the temperature may be 40 °C, or more. In the heat of summer, the temperature can easily surpass 50 °C, even with all the doors and side panels are open.

Watering plants inside a polytunnel can be a twice daily task on the hottest summer days. This page looks at how to reduce the time and effort of keeping up with watering, by creating a drip irrigation system powered by solar energy. Plants benefit too, by having a consistent watering regime that helps to keep the soil moist, reducing the risk of disease and boosting harvest size.

Watch the video, or scroll to read the video transcript.

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Video Transcript

This video introduces the principals of how to create an automatic watering system for a polytunnel on an allotment, or any other environment where there is no mains water supply or electricity. If you have access to a mains water supply, you may like to see this video instead.

On my allotment there is no mains water supply. Instead, dotted around the plot are water pumps. Watering plants is a manual task of pumping water up from the ground, and walking it to where it is needed.

The problem I wanted to solve was how to create a polytunnel watering drip irrigation system powered by solar energy, to avoid needing to water inside the polytunnel twice daily, and to provide my plants with the optimum amount of water for better harvests.

This video shares the principals of creating a system. I am still learning and making improvements each year.

The solution can be easily adapted to power other electrical devices in any environment that does not have mains power. This is discussed at the end of the video.

I am sharing the solution that works for me, and over the years it has saved me hundreds of hours of work.

Please note that the solution involves electricity and water, a potentially dangerous combination. Always follow the advice provided by equipment manufacturers and qualified professionals before implementing your own solution, and check local regulations regarding working with electricity.

My solution involves:

• Water butts to store water
• A water butt pump to pump water
• A solar panel to provide electricity for the water pump
• A battery to store the electricity generated by solar power
• A timer to control when the water butt pump comes on
• A drip irrigation network to distribute water from the pump to the plants

Polytunnels are hot environments from spring to autumn. On a day when it is only 10 degrees outside, if the sun is shining, inside a polytunnel the temperature may be 40 °C, or more. In the heat of summer, the temperature can easily surpass 50 °C, even with all the doors and side panels are open.

The heat generated inside a polytunnel can be a great benefit for a gardener, by extending the growing season almost all year round. However, this heat has a consequence. To keep plants healthy, a gardener needs to provide water consistently and regularly.

The size of the watering challenge depends both on what is being grown, and how. For people who use a polytunnel mainly for germination in spring, watering is less of an issue. In this instance, seeds may be sown directly in seed beds running along the sides of the tunnel. These seed beds contain a large quantity of earth that helps keep the soil temperature lower for the roots, and will hold water for longer. In spring, a gardener may only need to water two or three times a week.

The way I use my polytunnel creates a much bigger requirement for watering. I like to grow a range of heat loving plants like tomatoes, peppers, aubergine, cucumber, chillies, Inca berries, and tomatillos. If I grew these in beds inside a polytunnel, I would run the risk of building up soil born diseases, as I would be using the soil over and over to grow the same crops. To avoid this, I grow all my plants in big pots and containers. This allows me to recycle the soil every year, and use fresh compost for my plants.

The challenge with this approach is that it creates a big demand for watering. Pots dry out faster than plants grown in the ground, and the crops I grow all mature in summer when the heat is highest. The combination of big plants growing in containers during the hottest days of the year means that watering may be needed twice daily. Infrequent and inconsistent watering at worst can kill the plant, but can also spoil the crop. The plants may set much less fruit, and the fruit they do produce may split or mold, or not swell to full size.

A twice daily watering task is a big time commitment, and it is hard for anyone but a full time gardener to commit to this. Sustaining this level of watering at home is difficult, but even more so when a gardener needs to travel to and from their allotment. Over the course of a whole summer, it is almost certain that a gardener will not be able to do this. Also, gardeners like to go away at the weekend, and on holiday, like everyone else.

My solution to this problem is an automatic watering system, and it has taken me several years to arrive at the solution. Instead of needing to water every day, the system needs to be topped up with water every two to three weeks depending on the weather, and aside from monitoring that everything is working as expected, it can be left to look after itself. I estimate the system saves me about an hour’s work a day, which amounts to 14 hours work over a fortnight. Even better, my plants are healthier too, as they receive water more consistently.

Without access to a tap, the first consideration is how to store water near a polytunnel. Filling the storage solution is a subject for another video, but most allotments have a shared water supply available. Rainwater can also be harvested.

There are two common solutions to storing water: a collection of water butts and barrels, or alternatively intermediate bulk containers, or IBC as they are commonly known.

I really wanted to use an IBC, as second hand containers are relatively cheap and they store a large amount of water, typically around a 1000 litres. If buying second hand, it is important to check what they were previously used for, to avoid any risk of exposure to dangerous chemicals.

Delivery can be a challenge. Whilst IBC’s can be delivered to a home address relatively easily, it can be a little more complicated to deliver them to an allotment, especially an allotment with limited vehicle access.

IBC containers are heavy, even when empty. The plastic storage container is typically surrounded by a steel cage to protect it. The total weight of an empty IBC can be around 50 kg or more, and together with their large size, this makes moving them a two person job.

In my case, rather than solving the logistical challenge of manoeuvering an IBC to my plot, I opted instead for a collection of water butts. I could move the water butts by myself, they fitted in my car, and I have found them easy to empty and clean, when required.

To make them act as one water store, I connected them together using their taps. Each water butt connects to each other by running a hose from their tap, to the tap of the other water butts. By daisy chaining the water butts together in this way, I have effectively one water store holding 600 litres of water.

The next obstacle is how to move the water to the plants. One option is to lift the water store above the height of the plant pots, so that gravity pressure pushes the water through the drip irrigation system.

There are challenges with this approach. The most difficult to overcome is the weight of the water. One litre of water weighs 1 kilogram, or 2.2 pounds, so a full 1000 litre IBC weighs over a tonne, including the weight of the IBC itself.

To use gravity pressure, ideally the IBC would be at least a metre higher than the plants. My observation of allotment holders who have tried this is that their supporting structures last for a year or two, but eventually subside. I put this down to wet ground softening, and moving, in winter months, and the gradual rotting of the wood that is used to hold the structure up.

Other challenges with using height include the work involved in building the storage platform, lifting the IBC container onto it, and filling and refilling the container. Filling the container is more difficult as it will be above head height, and therefore will need a pump or mains pressure to lift the water.

I decided to take a different approach, and store the water at ground level, and use a pump rather than gravity to provide water pressure. For the pump, I had two requirements. The pump needed to be submersible so that it was out of sight, and away from any temptation to steal it, and that the pump would automatically shut off when the water level inside the water butts was too low. The pump needed to shut off to prevent damaging itself, as water pumps are damaged when they work without sufficient water.

The good thing about water butt pumps is that they move water with a good amount of pressure. This is ideal for drip irrigation systems, as it provides more consistent water flow through drippers. If the water pressure is too low, it is common for the drippers nearest the water source to work, and those further away to give out less water, or even no water at all.

The challenge with using a water pump is that they need electricity to work, and my allotment does not provide electricity to plots. I found searching for solar power solutions for camping and caravans helpful, as many people use and store solar energy to provide power for kettles, televisions, music, and navigation systems.

This helped me identify a suitably sized solar panel that could provide electrical energy to store in a leisure battery. To choose the right battery, I got advice from a retailer, but in general a leisure battery (rather than a car battery) is suitable for use with solar panels, with the requirement of constantly charging and discharging, and the type of power they provide. Leisure batteries are a little more expensive than a car battery.

Crucially, the battery provides a big enough energy output, or watts, to satisfy the minimum requirements of the pump.

The final obstacle to overcome was that the leisure battery provides DC, or direct current, whilst my water pump required AC, or alternating current. At the time of purchasing, I could not find a submersible water pump with an automatic shut off that ran off DC power. The solution is a power inverter, that converts DC current to AC current. Inverters commonly have a plug socket to allow AC devices to be plugged in.

Therefore the solution works as follows:

• Solar power charges the leisure battery using DC current
• The battery stores the electric charge for when it is required, and outputs DC current when the pump is switched on
• The inverter converts DC current to AC current to power the pump
  It is worth noting that this solution effectively provides a remote power source that can be put to a wide range of uses, like charging a mobile phone, lights, radio, or security system, indeed anything that has a plug and needs electricity to work. The only constraint is the balance of the amount of electricity generated and stored from the solar panel, versus how much electricity is required by devices using the system.

At the moment I am not using a solar charge controller, but I may upgrade my system to use one, and if I do I will create another video on this, and link to this here and below the video.

A solar charge controller can extend the life of the leisure battery, by optimising the charging of the battery, and ensuring the battery is not consistently undercharged.

If a battery is run consistently with little charge, for example by drawing more power than the solar panel provides, this can damage the battery beyond repair. I know this from first hand experience, and I am now careful not to overuse the battery.

The final part of the solution is the use of a timer. I put this between the battery and the inverter. On my circuit, the inverter and pump are always on, but the timer breaks the circuit. What this means is that it is only when the timer is on, that electricity flows, which will then allow the inverter and water pump to work. The timer runs the pump for about a couple of minutes in the morning and evening, which given the high water pressure, provides enough water to keep my plants happy. Depending on the weather, I adjust the length of time the pump is on for.

An important detail is to include an anti-siphon break. Once water starts flowing through the drip irrigation system, there is a risk that it will continue to flow through the system even after the pump is switched off, by siphoning. My solution is to allow air to enter the system once the pump is switched off, by adding an additional spur at the highest point, located between the water butts and the plants. When the pump is working, water squirts out of the spur, but when the pump is switched off, air is sucked into the system, preventing any more water siphoning through.

Not doing this would risk 600 litres of water siphoning off to the plants all in one go.

The length of time the drip irrigation needs to run depends in part on the type of drippers used. At the moment I am using drippers that allow me to manually control the amount of water flowing through each dripper, by screwing or unscrewing the dripper head.

I’ve been really pleased with the results. The high pressure pump means that I can rely on the drippers to evenly water my plants, and this has resulted in better plant growth and bigger harvests. My water store is large enough to provide water for over two weeks worth of watering. I do regularly visit the polytunnel during the fortnight to check everything is working, but this is a tiny fraction of the time I had previously spent watering the plants, and much less effort.

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