Driving up LED efficiency to reduce the costs of artificial sun

Indoor horticulture or Controlled Environment Agriculture (CEA) enables increased yields and more predictable all-year-round growing conditions, allowing crops to be grown closer to where they are to be consumed. Driven in part by growing consumer awareness of the benefits of consuming fresh, locally produced food, this market is predicted to expand at a compound annual growth rate (CAGR) of 10.9% from 2021 to reach a value of over $75 billion1 by 2028. The cost of lighting large indoor areas is leading growers to choose more efficient and more controllable LEDs over conventional lighting systems. However, running costs remain high. Here we consider a new system for powering LEDs that significantly improves efficiency to drive down electricity costs.

Energy costs are a key issue for indoor growers, frequently second only to labor costs. Controlling and reducing electricity is critically important to both commercial feasibility and meeting sustainability targets. When we consider that lighting is one of the biggest consumers of electricity in an indoor growing environment, increasing lighting efficiency can make a big difference to running costs. This is one of the reasons driving the move to replace traditional lighting solutions with LEDs in recent years.

LED lighting in horticulture

LEDs offer longer lifetime, lower energy consumption and lower maintenance costs than traditional horticultural lighting options. They also offer deliver greater control over the light output across different wavelengths. This allows growers to more closely match spectra to the needs of their plants. In addition, luminosity can be adjusted to manage production depending on plant species and growth stage. The availability of smaller LED lights improves control over where the light goes, increasing efficiency and reducing energy consumption. 

Greater efficiency means that LED lights operate cooler than traditional high pressure sodium (HPS) bulbs so they can be placed closer to plants, resulting in increased farming density. Less heat also lowers water consumption, thereby reducing cost. So far, the primary adopters of horticultural LED lighting have been traditional greenhouses where it is used to supplement natural light. However, when used in larger greenhouses ('growing warehouses'), LED lighting can improve control over the nutrient balance and growth cycle of a wide range of crops.

Greenhouses are generally lit from above, illuminating the hall and plants from the ceiling level. Typically, installations would involve fitting LED luminaires in place of HPS lamps, modifying the spectral content of the light. While this approach generally works well, there are some challenges including light being concentrated on certain plants, ensuring uniformity and constant quality of the light spectrum, as well as the generally significant amount of electrical power consumed.

Vertical farming places the lighting much closer to the plants but continues to illuminate from above. The layout brings additional challenges including heat (due to proximity and plants shading each other. The need for uniformity and constant quality of the light spectrum remain a challenge in vertical farming.

Intra-canopy lighting involves lighting from the side or even in-between plants. HPS lighting was too hot for this approach, but more efficient LED lighting now makes it possible. The directionality of light can be a challenge with this approach and, as with lighting from above, growers must ensure uniformity and constant quality of the light spectrum.

Improving the Efficiency of CEA Lighting

Despite the inherent efficiencies of LEDs over their conventional counterparts, the sheer volume of high-power LEDs still makes electricity consumption in lighting a dominant cost for growers. At present, around 100 watts of LED power is needed per square meter and lamps operate between 12 to 18 hours a day. While the exact power requirement depends on a number of factors, including the crop being grown, the power usage from these LEDs can account for well over 50% of the total electricity bill. 

Costs can also increase due to the way in which the energy is taken from the grid. When current and voltage are not in phase, for example, an additional load can be placed on the supply. Known by engineers as Power Factor (PF), this load does no useful work in powering the lighting but is metered, thereby increasing the costs. Then there is Total Harmonic Distortion (THD), which is a measure of how much of the distortion of a voltage or current is due to 'harmonics' in the signal. A lower THD typically means higher power factor, lower peak currents and higher efficiency - all of which are desirable in a power system and beneficial to the application. While these issues may seem removed from the day-to-day running of a CEA facility, many utility companies now use smart meters for large energy users to measure THD and modify billings accordingly.

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Photo Courtesy of iFarm

Source: Agritech Tomorrow