LED Lighting & Humidity

Introduction

Closing out our recent series on humidity control, this article explores the effect that light has on humidity and the differences in supplemental lighting choices.

For more information on dehumidification and plant empowerment, see our recent articles (Greenhouse dehumidification – Growsave, Plant Empowerment & Dehumidification – Growsave, Air Movement & Humidity – Growsave, Active Dehumidification – Growsave).

Energy Balance

We know that water must be recruited through the roots of a plant to uptake essential nutrients. In order for water to be recruited, it must be balanced against water loss, mainly via transpiration. And for transpiration to occur, there must be a positive energy balance at the leaf.

There are four ways that energy flows into and out of a plant:

Shortwave radiation – energy transfer via light,
Longwave radiation – energy transfer via heat emission,
Convection – energy transfer via air movement,
Evaporation – energy transfer via water phase change.

At a given temperature, these energy flows will balance, such that an increase in shortwave radiation at the leaf will lead to an increase in evaporation. If insufficient water is available, the temperature of the leaf will increase until the energy balance is restored.

Likewise, when screens are left open overnight, the crop will emit high amounts of longwave radiation to the open sky. Without enough convective energy in the greenhouse to balance this, the temperature of the leaf will drop, balancing with energy gain from water condensation on the leaf. This is one of the reasons why overnight screening is very important for humidity control.

Differences in Lighting

The following table is found in ‘Plant Empowerment – The Basic Principles’ (van Weel et al., 2018). It outlines the energy inputs, outputs, and contribution to plant evaporation from a couple of lighting options (high-pressure sodium (HPS) and light-emitting diode (LED)), normalised by micromoles of Photosynthetically Active Radiation (PAR) delivered. Using data from Philips Signify, radiation to the crop has been split into light and heat.

Of course, individual fixtures will have differing characteristics. These data are typical, meant to outline the differences in options rather than be prescriptive.

	Solar Radiation		HPS	LED
	Clear Sky	Clouded Sky	HPS	LED
PAR output µmol/m².s	200	200	200	200
PAR efficiency µmol/J	2.0	2.7	1.9	3.5
Energy input W/m²	100	75	105	57
Radiation to the crop W/m²	100	75	95	45
as light W/m²	–	–	36	36
as heat (IR) W/m²	–	–	58	6
Convection energy W/m²	–	–	10	12
Contribution to crop evaporation g/m².h	145	108	137	65

Table 1: Influence of different light sources (van Weel et al., 2018)

LED lighting has the highest PAR efficiency, and lowest energy input; due to LED spectra being tuned around PAR, and their high efficiency illumination.

HPS more closely resembles the figures that solar radiation delivers, in terms of radiation to crop and contribution to evaporation. This highlights why some growers have found that LEDs do not perform as expected; they produce light and heat differently to HPS and natural light, therefore, they must be treated as such.

Outcome

The radiation to crop is comprised of radiated heat and radiated light. HPS bulbs deliver much more radiative heat than LEDs, which directly heats leaves. This leads to the large difference in contribution to crop evaporation. Assuming 12 hours of supplemental lighting, to balance the evaporation between HPS and LED, an additional 0.6kWh/m²/day of heat is needed (for as many days as supplemental lighting is employed).

For comparison, given current electricity and gas prices and the figures above, switching from HPS to LED will lead to an electricity saving of £0.12/m²/day, whilst delivering the additional heat requirement via natural gas boiler will cost £0.02/m²/day.

Setting aside the cost savings, decoupling light and heat generation allows for more precise control of the growing environment. Using LEDs rather than HPS should reduce the amount of ventilation required for cooling, which wastes CO₂ additions. As well, LEDs offer longer lifetimes and have the potential for intensity and spectrum variation, which can be tailored to deliver the characteristics required at different points of your crop’s growth.

References