Can LED fixtures outperform HPS/HID agricultural lighting systems? Maybe. Sometimes. It depends. But now there is an entirely different type of LED out there that may affect the answer to this common question.

The new entry in the agricultural lighting contest is the remote phosphor LED, a fixture that indirectly creates various colors of light from blue LEDs. Remote phosphor technology has made its way into high-end architectural and commercial lighting over the last few years, but has never been used in greenhouse and grow lighting, until now. Here’s how it stacks up against other forms of lighting.

Conventional LEDS vs. HPS

A study published in June 2014 by plant scientists at the University of Utah entitled Economic Analysis of Greenhouse Lighting: Light Emitting Diodes vs. High Intensity Discharge Fixtures compared the most efficient HPS and conventional LED fixtures head-to-head to determine the lowest cost-per-micromol of photosynthetically active radiation (PAR) photons per year in several different greenhouse applications. A micromol or μmole is simply a term describing a quantity of photons.

Anyone spending thousands of dollars to equip a greenhouse should check out the Utah study. It found the best 1,000-W, double-ended HPS lights and the best conventional LED fixtures available at the time could produce photons in the photosynthetic range with equal electrical efficiency, producing between 1.66 and 1.7 μmoles of PAR photons per joule. Both light fixtures far outperformed the mogul-base, single-ended HPS fixtures, which have efficiencies of only 1.02 μmoles per joule, and the fluorescents, which came out at 0.95 μmoles per joule.

If electricity was the only consideration for growers, mogul-based HPS fixtures could be replaced with either the best double-ended HPS fixtures or the best LED fixtures, and you could call it a day.

Either of these replacements would consume half the electricity of older HPS fixtures, and both are equally effective and would cost far less to own and operate over a five-year period. However, there are many factors involved in choosing between HPS or LEDs for supplemental or primary indoor garden lighting, such as bulb replacements, HVAC costs, spectrum choice and the initial capital cost.

The Utah study weighed all of these and found that the only situation in which LED lighting was less costly than the equally efficient double-ended HPS over a five-year period was when it was used in smaller greenhouses with wide aisles requiring highly directional light. In all other situations, double-ended HPS fixtures were more economical because of their lower initial capital cost. But that was then.

Remote Phosphor LEDs

Shortly after the Utah study was published, a new LED technology emerged in agricultural lighting—the remote phosphor. In short, the remote phosphor uses high-energy LED lights to excite a phosphorescent coating. The coating then emits light at various color frequencies. Remote phosphor LED agricultural lights promise a higher fixture efficiency of up to 2.4 μmoles per joule.

We have all seen phosphors before. They are used to make the white coating on the inside of CFL lighting tubes and the yellow coating on many white LEDs designed for residential lighting. The coating absorbs high-energy photons and re-emits them in another color, losing some energy to heat. Unfortunately, the coating reduces efficiency. A high proportion of scattered photons that do not go straight out bounce back into the diode and are re-absorbed. The color conversion produces heat that further decreases the efficiency and lifespan of the diode. Cool white fixtures made with blue LEDs and a phosphor are only about 33% efficient.

So what exactly is a remote phosphor LED and how is it different? The phosphor is made “remote” by moving the coating away from the LED to a lens several inches away. The phosphor is energized by the uncoated blue LED. As it turns out, this simple change makes an enormous difference. It enables manufacturers to shape a “mixing chamber” that directs the photons upward where they have a far better chance of striking the phosphor head on and leaving the fixture. The result is a more-than-30% increase in emitted light. When the phosphor is moved away from the diode, heat created

by the color conversion is too far away from the diode to affect it. In larger fixtures, there is so little heat at the diode that it can be passively cooled—no fan required. Color mixing and light uniformity improve and the fixture becomes smaller, cooler and lighter.

A remote phosphor fixture can generate any spectrum a grower requires. The 55%-efficient blue LEDs can produce any light desired, from blue all the way to far red, with no spikes or gaps, and minimal losses to heat. The fixture loses 15-30% of that light to the lens and phosphors.

Depending on how much blue manufacturers decide to convert to longer wavelengths, total fixture wall-plug efficiencies remain above 40% and can reach as much as 45%. This is far superior to conventional LED and HPS fixtures, which do not exceed 35%. In addition, the light from remote phosphor fixtures is highly directional and does not require a reflector hood. Like conventional LEDs, the fixtures can be mounted much closer to the plant canopy without harming the plants.

Annualized Costs of Running LEDs in a Growroom

Let’s see what happens when we plug test results for remote phosphor LED fixtures into the University of Utah’s calculations. Remember that the Utah study used a five-year life for LED and HPS fixtures, which is 43,800 hours of continuous use. The decision to measure useful life at five years stemmed from the shorter life of fixtures made up of various colored diodes and using active cooling fans. However, high-quality blue LEDs in a large remote phosphor fixture are projected to last 96,000 hours or more before experiencing a 10% drop in output—and they don’t need fans.

The only other operating part in a well-constructed remote phosphor fixture is a power supply. Good ones are rated at 200,000 hours MTBF (mean time between failures) and are easily and inexpensively replaced. In short, a well-constructed remote phosphor fixture should not fail or degrade after five years, and will probably last much, much longer. Regardless, I am going to use use five years for comparison, not because the fixtures will degrade at that time, but because I believe that’s a realistic budget window. I also hope and expect that rapid efficiency improvements in blue LEDs will make them obsolete in that time.

Let’s look at how a 200-W remote phosphor fixture compares with the best HPS and LED fixtures over a five-year period at eight hours per day and 11 cents/kWh (approximately 30,000 hours of use). The Utah study compared this for three different capture angles of 68, 100 and 180 degrees for each type of fixture. Generally speaking, the higher angles apply to larger greenhouses with relatively narrower aisles, where little light is lost to the sides and to the aisles. The 68-degree angle would be appropriate for any small installations with single tables and wide aisles, as would be used in greenhouses used for retail displays. The 100-degree angle represents something in between.

Using the Utah calculations for the 100-degree angle, I’ve worked out the total cost of ownership for each type of fixture over a five-year period to produce a photosynthetic photon flux (PPF) of 1,000 μmoles of photons per second for eight hours a day.

400-W HPS: It takes 2.4 of the best 400-W HPS fixtures to produce a PPF of 1,000 μmoles. The five-year total cost of operation for these fixtures is $4,036.

1,000-W HPS: It would take 0.57 of the best 1,000-W HPS fixtures to do the same job. The five-year total cost of ownership would be $2,207.

Conventional LEDs: It would take 1.53 of the best LED fixtures and the cost would be $2,943.

Remote Phosphor: It would take 2.08 200-W remote phosphor fixtures to produce a PPF of 1,000 μmoles of photons per second. The fixtures come in at a five-year total cost of $2,036.

The remote phosphor light has a lower total cost of ownership compared to every other type of fixture in the Utah study, making it 41.2% more efficient than the best HPS or LED fixtures. But what happens if we operate the various fixtures longer than eight hours per day or if the electric rate is higher than 11 cents/kWh? Remote phosphor lights are the most electrically efficient and at such high efficiencies, operating costs become more sensitive to electric rates and hours of use. The savings rise substantially.

The remote phosphor LED fixture is not the only fixture to exceed 1.7 μmoles per joule. There are now double-ended HPS lights that exceed 2.0 μmoles per joule. A lot has happened since Utah’s June 2014 study. Nowadays, either high-efficiency HPS fixtures or the remote phosphor LED fixtures can cut energy use by more than half compared to older mogul-base HPS lighting.

What Does the Future Hold for LEDs in Horticulture?

So what’s in store for the future for remote phosphor LED lighting? I expect larger sizes to bring down the costs-per-mole of lighting. I expect blue LEDs to become more and more efficient, eventually approaching their theoretical maximum of about 75%, with potential remote phosphor electrical efficiencies of 90%, and I expect sensors on each light to dim or brighten the system in response to ambient light, perfectly maintaining the desired daily light integral.

Electricity is not the only factor when it comes to selecting an agricultural lighting system. Growers must also factor in fixture cost, labor, desired spectrums, heat and light distribution, HVAC, weight, shading, maintenance costs, longevity and reliability. Ultimately, it is the cost of plant growth that matters, and I believe remote phosphor lighting wins on all of these counts. In the end, the best lighting source is the one that provides the highest yield and best quality at the lowest cost to the grower.