Optoelectronics/Photonics: Controlling Light in Agriculture July 2013
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The US government's National Intelligence Council released its periodic global trends report in December 2012. Global Trends 2030 discusses a wide range of issues that relate to security, including food security. By 2030, climate change may have shifted global weather patterns and resulted in increasing costs for food production. Increasing demands from a rising human population—in combination with a dwindling water table in parts of the world—drives the need for new ways of producing food. Plants that are for food generally grow outside on vast tracts of land. The sun, rain, and soil all contribute to the quality and yield of the crop. Farmers can typically control the quality of the soil to some extent by applying fertilizer—natural or artificial—and augmenting rainfall with irrigation. Some crops grow in large greenhouses that offer the farmer full control over the amount of water, light, and nutrients each plant receives. However, producing artificial light is energy intensive, and gas-discharge lamps and similar technologies do not produce light at optimal wavelengths for plant growth. Can high-brightness LEDs contribute?
Reducing Energy Expenditure
Lighting represents a major expense for businesses of all types, and replacing existing lighting with more efficient technologies is an expensive undertaking. However, given the energy savings, LEDs can—depending on the manufacturer—operate using only 25% of the power of high-pressure sodium lamps. Energy savings can be even more dramatic for greenhouses, where the lighting is on for extended periods during the winter. Moreover, LEDs avoid a key additional cost associated with key applications of sodium lamps in greenhouses: the need to shift heat away from sodium lamps, which can otherwise scorch delicate crops such as tomatoes.
The use of LEDs in agriculture is limited, but research dates back more than 20 years. However, only recently has LED technology matured to the extent that it can replace traditional forms of lighting. One recent study led by researchers at the Department of Horticulture and Landscape Architecture at Purdue University shows that LEDs can result in lower energy costs for farmers who grow tomatoes in greenhouses. Professor Cary Mitchell led the study, which also explores how the position of the lighting affects the fruit yield of tomatoes in greenhouses. High-pressure sodium lamps produce enough heat to damage plants, even when the lamps are overhead. In contrast, LEDs operate much cooler, enabling LEDs to reside quite close to the tomato plants—a practice that can increase incident-light intensity relative to overhead lighting. The key findings from the study are that the sodium-lamp arrangement imposed 403% higher energy costs than the LED arrangement in order to produce the same amount of fruit and that conversion of light energy into fruit biomass is 75% higher for LED lighting that is close to the plants than for overhead-mounted high-pressure sodium lighting. Today, when outside the main growing season (when in winter months), many nations import tomatoes and other salad crops, because the costs of producing the crops locally are not cost-effective. But the reduction in energy costs afforded by LEDs might help farmers produce all year round.
Plants metabolize sunlight into energy, and researchers are exploring which wavelengths of light are optimal for plant growth and crop yields. The ability to modulate LEDs represents another dimension—how long and how fast should LEDs operate?
Certain types of algae are useful for biofuels because of the extracts that derive from them. Fatty acid methyl esters (FAME) are the key component of biodiesel, which proponents of the technology hope will help reduce carbon dioxide emissions from vehicles that usually run on nonrenewable petrochemical sources. Therefore, enhancing the yields of FAME is desirable, and researchers are exploring ways in which different wavelengths of light affect growth rate and yields of algae. Research scientist Probir Das and colleagues from the National University of Singapore compared how different wavelengths affect productivity of a type of algae (Nannochloropsis sp.) that is naturally in Singapore's coastal waters. Exposure to green LED light produced the greatest weight of biomass but not the greatest volume of derived FAME. Instead, in comparison with exposure to red, green, or white light, exposure to blue LED light led to the greatest quantity of FAME.
A major advantage of LEDs over incandescent- or discharge-lamp technologies is the ability to modulate each LED at different frequencies without appreciable delays in achieving full brightness. Effectively, the lighting designer has enhanced control over the exact amount of light, together with the ability to optimize the modulation of the LEDs to deliver maximum energy efficiency (shorter "on" cycles result in higher efficiency). Researchers from the Faculty of Textile Science and Technology at Shinshu University in Japan are exploring ways to exploit LED modulation to optimize plant growth. In this research, a pulse-wave-modulation system mixes red and blue LEDs to explore how Arabidopsis thaliana grows in response to different quantities and mixes of red and blue light. The key takeaway from this research is that the grower can alter photosynthesis in plants with different mixes of modulated red and blue LED light. The research is in the early stages, but it is possible that different plants respond to different mixes of light and that specialized LED-lighting systems might enhance plant growth.
Food spoilage can occur at any point along the chain of production, on account of poor harvesting conditions, ineffective refrigeration, and other causes. Some food crops are more vulnerable to spoilage than are others, such as strawberries. To ensure optimal condition, growers harvest strawberries when they are nearly ripe, pack them securely, and deliver them to retail outlets within a short time. In strawberries and other delicate fruit crops, fungal spores are present; they germinate after the fruit has been harvested. Farmers and logistics operators can use hermetically sealed containers with partial atmospheres made up of nitrogen and oxygen to expose the strawberries to brief bursts of UV light to deactivate fungal spores.
Current UV lamps still mostly use mercury-discharge technology. Mercury-discharge lamps rely on materials that are toxic to the environment, are inefficient, and do not work well at the low temperatures inside refrigerated trucks in use to transport strawberries and other produce. The solution, proposed by Steven Britz at the US Department of Agriculture's Food Components and Health Laboratory is to use UV LEDs. Dr. Britz is working with Sensor Electronic Technology—a leading developer of UV LEDs—to develop a new system for prolonging the life of fresh strawberries. In one experiment, chilled strawberries that were exposed to moist air and UV light from LEDs were still fresh on visual inspection and retained a bright red color. By contrast, strawberries in the same conditions, but without exposure to UV light, were covered in mold growth. Also, according to Dr. Britz, greenhouse windows can filter natural UV light that strawberries need to produce important nutrients (antioxidants), and UV-LED illumination may compensate for this deficiency. Dr. Britz does not currently have a commercial product but suggests that a practical system might consist of special UV-resistant cases inside home refrigerators that house UV LEDs and switch on only when the refrigerator door is closed. The UV protection would prevent unnecessary UV exposure to users.
Numerous opportunities exist for LED developers to diversify away from the competitive high-brightness illumination market and into agricultural technology markets. Food production and security of resources are pressing issues for governments now and increasingly so in the future as pressures increase. Optimizing food-crop yields is one side of the equation, and many questions remain about how specific wavelengths affect different crops. Although LEDs are robust, the driver systems are generally not. Therefore, ruggedized enclosures that are resistant to ingress by water and dust are essential and represent further opportunities for LED developers.
Although progress in UV LEDs continues, few developers are working in the field. Given the very large market potential for visible LEDs in illumination, the gap in UV-LED development is not surprising. But progress in UV LEDs promises the potential for preserving food and disinfecting water supplies, among other applications.