Retro­fitting agri­v­oltaics on existing struc­tures

Agri­v­oltaics are solar panels which are installed above or along­side agri­cul­tural produc­tion – and the bene­fits can be trifold, gener­ating energy while poten­tially improving plant produc­tivity and reducing emis­sions. In the UK, a Farming Inno­va­tion Programme study, funded by Inno­vate UK, is being run to explore the possi­bility of agri­v­oltaics instal­la­tions being retro­fitted to existing green­houses.

Having previ­ously conducted research into the effect of agri­v­oltaics onto leafy crops like basil and spinach, the Univer­sity of Green­wich received funding to test retro­fitting on a glasshouse and poly­tun­nels for straw­berry growth. Further funding has now been given to roll out a larger instal­la­tion exploring more soft fruit types.

The univer­sity is working in collab­o­ra­tion with Polysolar to explore the use of these retro­fitted solar panels at Hugh Lowe Farms, a commer­cial soft fruit farm in Kent. They are currently using vari­a­tions of semi-trans­parent, flex­ible, and grey rigid panels across the farm and univer­sity sites to explore the effects on energy produc­tion, plant produc­tivity and carbon emis­sions. Addi­tion­ally, the farm staff are being trained to install and move the photo­voltaic panels as required.

Dr Elinor Thompson is a photo­syn­thesis researcher and reader/assistant professor at the Univer­sity of Green­wich. She spoke to The Furrow about the initial find­ings from the programme.

Is it easier to retrofit panels on existing green­houses and poly­tun­nels than to create the whole struc­ture anew?

It is cheaper for farmers to retrofit panels on existing build­ings rather than going through the expense of starting from scratch. Retro­fitting also allows us to get dual yield of crop and elec­tricity from existing land instead of building desig­nated solar struc­tures on agri­cul­tur­ally useful land which prevents crops being grown there. All of the photo­voltaics in our trials are retro­fitted to the existing growth house struc­tures without adap­ta­tion of tunnels or green­houses.

What is the envi­ron­mental impact of the panels on the crops?

The addi­tional shade that the panels bring is thought to improve the water use effi­ciency of the crop growth system. Soft fruit can struggle in the summer with too much direct sunlight and there is usually excess light for photo­syn­thesis for plants, even in the UK. Although there is some differ­ence in humidity and temper­a­ture, this seems to cause small changes at points in the day but not a big change in a poly­tunnel setting.

With some extra shade, there are obvi­ously changes on a cellular level within the plants, but we have not yet seen any adverse effects on growth or plant produc­tivity; the fruit yield has not been impacted. We are measuring how the water effi­ciency is affected by the shade and we are also in the process of assessing which plants can benefit most and are most suited to the system.

We can see changes in photo­syn­thesis at a mole­c­ular level, depending on the colour and amount of shade from the panels. More severe effects can occur with some colours of light and partic­ular crops, so it is likely that some crops would be suit­able for growth with partic­ular colours or panels, and others not suit­able at all.

What are the bene­fits of energy produc­tion in the system?

There is a finan­cial benefit derived from the system. The energy produced could be sold to the grid, used to offset power use directly at the farm, or enhance the use of elec­tri­cally powered automa­tion. We are eval­u­ating the poten­tial of the agri­v­oltaics elec­trical yield to reduce power use, but this will happen as the project progresses.

The amount of power used varies throughout the year, with peak use in summer when the farm is all systems go. This is largely as a result of water pumps, but includes irri­ga­tion, picking robots, main­te­nance of surrounding land, all powered vehi­cles and machinery. There is an increasing reliance on automa­tion in soft fruit produc­tion. Picking robots, UV robots, crop sensors and elec­trical vehi­cles are all utilised and need power.

The costs asso­ci­ated with solar energy are reducing all the time so it will hope­fully become more of a cost-effec­tive power. We will be able to put instal­la­tion vs return calcu­la­tions in the research papers when we have eval­u­ated the contrasting instal­la­tion patterns we are testing on poly­tun­nels.

There is also a choice avail­able for farmers over whether or not to use a battery, as battery tech­nology develops. Part of the projects’ aims have been to explore battery tech­nology as well as the best PV mate­rials. There will be specific require­ments and settings where it is useful to have a battery, and battery tech­nology is improving all the time, but at present it is just simpler and safer to have a solar power instal­la­tion with a connec­tion to the grid.

How much energy is the system producing?

Using flex­ible panels, the yield will be depen­dent on the area of roof covered by the panels, which is in turn dictated by the crop below. A conser­v­a­tive esti­mate is that the current instal­la­tion produces approx­i­mately 130MWh per hectare on an annual basis. That is roughly a quarter of the yield from a hectare of solar farm, but you do not have the asso­ci­ated land loss or new plan­ning issues. It is hard to calcu­late real life yield as layout, connec­tions and exact loca­tion might make a differ­ence, and that is what we are working out. We are using fixed panels rather than moving ones, as the simpler the system the better.

What effect do tinted solar panels have?

We are using some semi-trans­parent photo­voltaic panels which only allow some colours of light to reach the plants, whilst using the other wave­lengths for elec­tricity gener­a­tion. In our previous project we found that tinted solar panels resulted in higher protein levels in leafy crops; extra red light changed the nutri­tional char­ac­ter­is­tics of the plants. Tinted panels can there­fore tailor the nutri­tional content of crops. We are inves­ti­gating the effects on more crop types and measuring the solar yield that can be achieved.

How do poly­tun­nels work in the system?

Poly­tun­nels are cheaper than glasshouses and are becoming more and more in demand. Some people find them an eyesore, so installing solar panels could be a solu­tion. The idea is that the addi­tion of flex­ible photo­voltaics breaks up the reflec­tive and light-coloured appear­ance of the poly­tun­nels from a distance. This allows them to offer an aesthetic improve­ment as well as a produc­tive one. This system is there­fore a prag­matic way of improving produc­tivity of the whole system, including both energy and plant produc­tion.