How dairy’s solar fence captures energy at peak demand

At Kilmeaden Farm in County Waterford, cows graze either side of a 150m fence that runs north to south the length of one of David Foran’s fields.

This is no ordinary fence but a configuration of bifacial solar panels installed vertically to face east and west to capture energy from the sun when David is milking.

He installed the 27kW system in 2019, with 30% of the cost met by a government grant.

The energy savings he has captured by not importing all of his electricity from the grid recouped his €20,000 (£17,173) share of the investment this year.

See also: 4 ways dairy farmers can cut energy costs

High energy costs in the past 18 months meant the payback period was quicker than the seven years he had initially budgeted for.

David considered different renewable energy options before he settled on the bifacial solar modules. It seemed logical to install a system that matched energy generation with electrical load, he says.

Panels facing east and west

As the sun comes up, it hits the east-facing side of the panel and the power generated by the module quickly increases. When he is milking in the afternoon, the sun in the west falls on the reverse side.

“I like the whole concept of it – it seemed a no-brainer to me,” says David, who produces milk from 160 cows, with milking times generally starting at 7am and 5pm.

Gene Hourihane, of Sunstream Energy Solutions, which supplied the system, says the angle of incidence – the angle of the panel’s surface compared with the sun’s rays – is directly proportional to the incidence of the sun hitting the module’s surface. This means the intensity of light hitting the module is high.

By mounting the bifacial modules vertically on a specially designed frame with one side facing east and the opposite side facing west, the resulting power curve has two peaks with a trough at midday.

Gene says this makes them well suited to dairy farms. “They have high variations in electric loads, especially those batch-milking cows in the morning and evening, with those peak loads typically 10 times greater than midday loads,” he explains.

The conventional approach to solar photovoltaics (PV) is to install modules on a south-facing roof to maximise the energy generated from the midday sun.

But Gene suggests this approach is not ideal for a dairy farmer milking twice a day, as peak solar power and maximum farm electric loads occur at different times.

“Here, a substantial amount of the solar energy is overspilled to the grid at little, or no, value,” he says.

“This can be overcome by diverting the surplus solar energy to battery storage.

However, lithium-ion battery technology – the most popular form of storage – is expensive and has an expected service life of eight to 10 years before performance degradation sets in.”

Economics

Whether a system is ground- or roof-mounted, hydro or wind, the most economically viable way to pay for a renewable energy investment is to consume energy as it is being generated, argues Gene.

When panels are on a roof, the sun hits them at midday, when cows are not going to be in the parlour unless the farm is three-times-a-day milking.

“Yes, this energy can be stored in a battery to recycle, to heat water for wash down or to build an ice bank for evening milking to chill the milk, but this is just shifting the energy use. It is not a perfect match,” he says.

“Every unit that can be consumed directly is one that doesn’t have to be purchased from a supplier or means the farmer doesn’t have to go to the expense of battery storage.”

There was a requirement for David to apply for planning consent for the panels, but he says this was “only a matter of form filling”.

At that time, the planning rules in Ireland allowed an exemption if a ground-mounted solar panel array covered 25sq m or less. That has now increased to 60sq m or less.

David’s system takes up no field area other than a fence running for 150m. He installed the system himself with guidance from Gene.

“Anyone who can put up a fence would be very capable of installing these,” David says.

He chose an exposed field to maximise the benefit from the sun.

“There is no point in putting them in a narrow field that is 50m across with very tall trees either side because the panels will be shaded from the sun,” he says.

An electric fence runs down either side of the panels to protect them from “curious cows”. However, he finds the cows take little notice of them.

The system yields 1,200kWh/kW, which Gene says is slightly better than an optimum south-facing roof system.

David uses as much of the power generated as possible, however, he does export some to the grid because he does not have any battery storage.

“Most of the time, they are working full pelt at milking times. In the winter, we don’t get the benefit from the sun at those times, but neither are we losing out on what they are producing because we sell that to the grid.”

Battery storage

Battery storage is something he might contemplate as technology improves.

“The lifespan of the battery is much shorter than the panels, but I might invest in a battery further down the line if the economics work out. It will depend on the technology and the warranty offered.”

Gene says the panels are more expensive than roof-mounted options, with the cost premium ranging from 30-60%, but being bifacial, they are also the most efficient.

“The front is the strong side and the back is weaker, but it can still achieve 90% of what the front achieves,” he says.

Unlike roof-mounted systems, farmers can also offset the cost by doing some of the installation work themselves, he adds.