Glasshouse LEDs save greenhouse gasses

While outdoors in the summer there is no real need to provide plants with artificial lighting, it is increasingly used to extend the growing day and growing season inside greenhouses.

It is not necessarily the efficacy of LEDs that is the advantage, as the high-pressure sodium (HPS) lamps currently used can deliver up to 150 lm/W, but the ability save power by only delivering wavelengths useful to plants – green, for example, reflects off plants.

Strockbridge Technology Centre (STC) is a plant growing research lab in Yorkshire.

“The most efficient way to drive photo synthesis is red light. You need a small amount of blue and in some situations you need far-red light,” STC science director Dr Martin McPherson told Electronics Weekly.

STC is one of many horticultural research establishments around the world trying to work out exactly what spectrum is needed when for what plants in what situation.

“We are experimenting to work that out,” said McPherson. “We have a mixture of lights, and a close working relationship with Phillips, and we’re working with other lighting manufacturers. They are producing some interesting lights.”

Philips, owner of LED firm Lumileds, makes a range of LED-based growing lamps, branded GreenPower, and is honing its products based on feedback from plant research customers.

The latest GreenPower product puts out 50µmol (see below) of useful light from 23W of electricity. “Its predecessor used 32W, and it still produces the same 50µmol output,” said Philips.

Its grown lamps now come in seven different spectrum options. “With these we offer the best combinations of spectrum, intensity, moment of lighting, uniformity and positioning to steer specific plant characteristics such as compactness, colour intensity and branch development,” said the firm.

One of its recent introductions, dubbed Far Red, is optimised to promote flower formation and rooting, and includes some white light for people working around the plants.

So, how good is LED plant lighting?

McPherson says it is early days. He is certain it will be popular, but the jury is still out on how effective it will be, which lights will win in which situations and when the commercial tipping point will be.

Information from lighting manufacturers, he said, suggests growers are seeing 30-60% energy savings over HPS illumination and are expecting more as the technology improves.

STC’s experiments are under way, with a team of photo-biologists using research lighting modules with two sets of LEDs – one red and one blue – whose intensity can be set independently to vary the spectrum.

It has a greenhouse-scale tomato growth comparison with HPS lighting (see photo, purple against yellow respectively) – using, for those interested, Sunstream midi plum variety.

“The first tomatoes from Stockbridge’s research facility were harvested in time for Christmas and have been very well received,” said Nigel Bartle, board member of the East Yorkshire Local Food Network.

STC also has a series of ‘multi-layer’ trials (see photo) – for intensive horticulture where racks of plants are grown one above the other in trays. One potential application for multi-layer horticulture is in warehouse (rather than greenhouse) growing environments. Intended for growing crops in towns, these warehouses are sometimes called ‘city farms’ or ‘urban farms’.

HPS is unsuitable for multi-layer growing. “You can’t put it too close to the crop, it’s too hot. You have to keep it one metre away,” said McPherson. Here, fluorescent tubes are incumbent, and Philips is offering LED-based growing lamps shaped like fluorescent light fittings.

Stockbridge Technology Centre is an independent, not-for-profit agricultural and horticultural technology-transfer organisation, wholly owned by the UK horticultural industry. 70% of it work is commercial research. It was once a government research centre.

An aside: µmol

Watts is a measure of the quantity (radiant flux) of electromagnetic radiation leaving a light source. To convert this into the amount of visible light emerging, the human eye’s spectral response is taken into account and the new unit is ‘lumens’ – which like watts is an energy/second term. ‘Lux’ also takes the eye spectrum into account, and is the brightness you get if a lumen is spread evenly over a square metre.

Photosynthetic activity in a plant is proportional to the number (rather than energy of) of useful photons landing on chlorophyll. The ‘lumens’ equivalent for plants – the metric which plant-aware people use to take into account chlorophyll spectral response as well as the number of photons leaving a lamp is µmol, and it is a measure of ‘photosynthetically active radiation’ (PAR).

To take this a stage further, the ‘brightness’ of plant-useful light falling on an area is the ‘photosynthetic photon flux’ (PPF) per second, and this is measured in µmol/(m2.s) – for the chemically aware, the mol here is indeed related to Avagadro’s number – it is the number of photons needed to activate a mol of chlorophyll

To bring this back down to earth, for a plant person with a standard light meter:

plant-useful light falling [in µmol/(m2.s)] = lm/m2 [lux] / constant

The division constant depends on the spectrum of the light source, and is roughly 20 for incandescent bulbs and 80 for HPS lamps.