Small-scale solar photovoltaic (PV) installations are now relatively common, but it is still unusual to see a roof-mounted array as large as the one recently commissioned by the McLaren family at Cronan, Coupar Angus.
With a maximum output of 176kilowatt peak (KWp) it is the match of many wind turbine systems and a very good fit for the broccoli and potato enterprises on the farm.
Ian McLaren and his son, Mike, of M.J. and J. McLaren, did look at wind power, but the low-lying farm location in the heart of Strathmore implied poor wind speeds.
The River Isla is only 1,640ft from the steading, but although it has plenty of volume the lack of a head in this slow-flowing stretch made hydro power a non-viable option.
This left solar power as the front-runner, but the decision to proceed was not a rushed one.
The key consideration, as with any renewable energy project, was to match energy generation and use.
The McLarens’ main electricity use at Cronan is tied to cold storage for 7,500 tonnes of potatoes and chilling of between 15 and 30 tonnes per day of broccoli. There are also up to 15,000 tonnes of potatoes graded on the site annually.
John Forster, of Brechin-based Forster Energy, who oversaw the Cronan project, said: “Much of our work with solar power involves cold stores and poultry units where we are aligning high-power generation with requirements for high-power demand.
“This is the case here, with the cold store power demand highest during periods of sunny weather.
“The broccoli harvest period is spread over the months when solar radiation is high.”
Mike added: “To maintain broccoli quality it is really important for us to remove field heat within an hour of picking and draw temperature down to 2C at 100% humidity.”
With 400 acres to harvest from June to October the energy requirement can be high, so savings can be considerable.
But, as Mike pointed out, all the broccoli, which is marketed through East of Scotland Growers, goes to supermarket buyers and they appreciate the carbon savings achieved by using renewable energy.
Once the decision to go ahead was reached, the installation of the 720 solar panels needed to produce the required power was relatively straightforward.
After consultation with Forster Energy the McLarens chose the south-west facing roof of the broccoli store to host the array, realising that even though it was 45 degrees off facing due south it was the best available site.
The 15-degree roof pitch was suitable, but the McLarens decided to spend an extra £5,000 on bolting the panels direct to the building’s purlins rather than fixing them on to the roof panels.
Mike added: “The roof is made up of laminated panels with an insulated internal layer. The last thing we wanted was for the panels to start to lift during a gale and ruin the roof.”
The panels might look as if they add to the roof loading, but at 15 tonnes in total it is relatively insignificant compared with a snow loading over the same area of more than 200 tonnes.
There are other “extras” in a project that cost £260,000 in total.
Ian spent time with his calculator and worked out that spending £25,000 on a sophisticated control system would ultimately save money. It monitors temperatures from the four cold stores on site and works out which areas require cooling and when.
“It is a case of maximising the use of solar-generated electricity during the day, and making sure that as much of the bought-in electricity as possible is on the cheaper night-time tariff. Taking into account the feed-in-tariff, on the home-generated electricity, should help cancel out the cost of bought-in power,” said Ian.
“We also installed Bradley Cold Store invertors to give us variable speed on the ventilation fans. Running a fan at 80% of normal speed saves 51% of power but still produces 90% of the airflow.”
With 10 fans controlled this way as well as the cold store compressors, the power savings are reckoned to be around 25%.
Ian said this technique could be used in any cold store system regardless of how the electricity is produced. It fits particularly well at Cronan.
Payback on the project is projected to be within seven years, but in September – the first full month of operations – the electricity output was an encouraging 3% ahead of budget.
All solar energy produced attracts a feed-in tariff (FiT) from the UK Government of 10.62p per kilowatt hour (kWh) whether it is used on the farm or exported to the grid for 4.64p per kWh.
For comparison bought-in electricity from the grid costs the McLarens 12p per kWh on the day tariff and 7p per kWh on the night rate.
Mr Forster said: “Typically a solar PV system such as this will produce 825 kWh per kilowatt installed. However this is dependent on location, orientation, roof pitch and most importantly the degree of shading.
“People often think that because we are farther north solar won’t be as effective, but due to longer summer daylight hours it actually works very well.”
The McLarens will continue to gain energy saving benefits from the RV array long beyond the 20-year FiT.
The current FiT of 10.62p per kWh is contractually fixed for 20 years, but there is an on-going digression model which is reviewed quarterly.
“The FiT has dropped over recent years but it was initially set artificially high. The good news is that the equipment costs have reduced dramatically and installation methods are now much more efficient,” said Mr Forster.