Confront heat stress or save energy

Growers are curious to know whether they would save money in regard to greenhouse heating during wintertime upon using a high-pressure sodium (HPS) lighting system rather than a light-emitting diode (LED) system. We explain that it is not feasible to provide more lighting as a useful heating source with the increased amount of electricity power for lighting, so using HPS fixtures, rather than high-efficient LEDs, is wasting energy within the greenhouse/indoor farm. HPS might also deliver some heat stress and cause abnormal fruit growth at a higher intensity than 200 µmol·m^-2·s^-1.

The greenhouse industry across Canada, including Alberta, has been continuously challenged by the market to provide high-quality, off-season products at good prices, despite climatic obstacles — primarily an inadequate amount of sunlight during winter months. That is why currently, vegetable production in Canada is being supplemented by imports from the United States and Mexico, adding up transportation costs.

Artificial light is an essential energy source for vegetable production in greenhouses at high latitudes where lack of natural sunlight severely limits plant growth from mid-autumn to early spring. For instance, during fall and winter months in North America, vegetables such as cucumber and tomatoes are commonly grown under HPS supplemental lighting. Supplemental lighting with HPS lamps is commonly practiced in Canada to increase crop productivity during off-season production. However, HPS lamps waste a large portion of energy and can be a safety concern due to high-voltage requirements. Moreover, the use of HPS lamps can produce symptoms of edema in ornamental and vegetable crops.

An alternative to the HPS lamp is the high-intensity LEDs, which have higher Photosynthetic Photon Flux (PPF) efficiency (indicator of light intensity), longevity and a lower safety concern. If LEDs are designed and operated successfully, the greenhouse growers will be able to save up to 46 percent on energy costs (data will be shown in a future issue of Produce Grower). The Canadian greenhouse industry could benefit from new, energy-efficient and cost-effective LED lighting approaches to obtain maximum year-round production.

Cucumbers and tomatoes are the most common greenhouse vegetables produced in Canada, specifically Albertan greenhouses, and as they require more light intensity, they have been selected for this research project. Available evidence comparing HPS and LED lighting systems has been controversial, prompting further studies to provide more accurate information for growers. In small-scale studies, HPS lamps were shown to consume 40 percent more electricity than LED lights to achieve the same PPF over the canopy, suggesting that HPS lights are energy-inefficient_1,2. However, in a larger-scale study, HPS lights were shown to save 44 percent more energy when compared to older generations of LED lights₃. Further studies have revealed that artificial lighting systems, including LED lights, can create problems with crop growth and physiology, possibly due to insufficient tuning of crop cultivation under these lit conditions₄. These problems seem to focus on plant phenology, plant load and the influence of LED light quantity and quality on microclimate and plant morphogenesis.

In the current study, we have compared HPS and LED lighting systems side-by-side to address the aforementioned issues arising in large-scale commercial greenhouses under Alberta’s climatic condition (Fig. 1). To assess the effect of light supply on leaf, flower and fruit microenvironment as well as plant phenology, we recorded sunlight intensity received at plant canopy levels in calendar weeks under glass and poly glazing materials. To determine the microenvironment and percentage of photosynthetically active radiation (PAR) available inside the greenhouses, we measured light intensity from sunlight, HPS, LED and their combinations (hybrid overhead and intra-lighting), using WatchDog Plant Growth Stations (10-second interval) with 16 Li-Cor quantum sensors. Also, we recorded temperature, relative humidity and dew point (15-second interval) with Onset Temp/RH loggers inside the plant canopy for each experimental unit (Fig. 2).

Part 1. Plant microenvironment

One basic question for many greenhouse growers is whether temperatures rise within the plant canopy upon using lighting systems; HPS lighting systems compared to LED or no supplemental light might significantly save energy for them or adversely affect their crops. They need to determine if this heat would be useful, and whether there would be any savings for the cost of greenhouse heating during the wintertime when using an HPS rather than an LED system.

HPS lamps release plenty of direct heat due to a low energy efficiency for converting electricity to photons at the PAR range. As demonstrated in Fig. 3A, heat is mostly produced by a high peak in the HPS spectrum within the far-red band of 820 nm. Far-red photons at 820 nm are not activating chlorophylls (chl a, b, c) and other phytochromes to stimulate photosynthetic activity and plant yield improvement. Far-red photons also contain a large amount of heat₅. The question remains: Would the far-red photons produced by HPS lamps provide useful heat to be a heating source during winter months and save energy within the greenhouse?

We tried to understand the actual distribution of heat at different locations of the plant canopy in side-by-side comparisons of HPS, LED, HPS plus LED and control conditions in commercial scale greenhouses. In Fig. 4, it has been illustrated how the temperature and humidity of the plant canopy are greatly influenced by supplemental lighting, particularly during the winter months, when day lengths are short and vents generally stay closed. We collected data for the top, middle and bottom of the plant canopy with four replications.

The recorded minimum and maximum temperatures under sunlight (control) were 16.8^o C in the night and 22^o C during the day. Under the same condition, the minimum and maximum temperatures were 16.5^o C in the night and 24^o C in the day using HPS alone and HPS plus LED treatments. The greater temperature fluctuation under the HPS lights might be because of far-red radiation produced by the HPS lamps. These temperature fluctuations may cause a thermal stress for the plant (due to sharp consecutive rise and fall of temperature at upper plant leaves). It also means that non-uniform leaf temperatures increase the likelihood of fruiting disorders caused by high temperatures.

When temperatures were compared between the HPS and LED treatment groups, the HPS lights were found to warm up plant tissue more than the LED lights only at the top of canopy (top five leaves). Temperature, and consequently humidity and dew point, were quite similar in HPS, LED and HPS plus LED treatments in the middle and bottom of the plant canopy (Fig. 4).

In general, the majority (>98 percent) of electrical energy that enters an indoor farm or greenhouse via a lighting system will ultimately be converted to heat, regardless of the energy use efficiency of the lighting fixture. Heat can be released directly from the light fixture, such as in the case of an HPS lamp, or indirectly, when light is reflected by the plant tissue or other objective within the greenhouse; less than 2 percent of the energy from light is absorbed by chlorophyll and converted into hydrocarbons (sugars) through photosynthesis. Therefore, it is not feasible to provide more light as a useful heating source with the greater amount of electricity power which is used for the lighting of additional lamps. This leads to energy wasting within the greenhouse when using an HPS fixture compared to an LED fixture. All growers should know that the electricity power is not a cost-effective choice for heating a greenhouse. Moreover, in the cloudy days of spring or summer while the indoor temperature rises to above the set point (hot but dark days), it may be necessary to continuously run the supplemental lighting. On those days, growers are challenged to eliminate the heat released by HPS lighting. As a result, some growers must install additional air fans with extra electricity consumption in order to reduce the heat stress on their plants. Though, if they select the right LED fixture with significantly lower power consumption then there should not be any heat stress concerns. For more details on energy saving in your greenhouses, follow the second chapter of this article in a future issue of Produce Grower.

Saeid Mobini, Ph.D in Horticulture, was a greenhouse specialist, Crop Research and Extension Division in the Alberta Ministry of Agriculture and Forestry in Canada. He is currently the master cultivator at SugarBud Craft Growers. mobinisaeid@gmail.com

References:
1. Wojciechowska, R., et al. (2015) Effects of LED supplemental lighting on yield and some quality parameters of lamb’s lettuce grown in two winter cycles. Scientia Horticulturae, 187, 80-86. 2. Nelson, J. A., & Bugbee, B. (2014). Economic analysis of greenhouse lighting: light emitting diodes vs. high intensity discharge fixtures. PLoS One, 9(6), 99010. 3. Pinho, P., et al. (2013). Dynamic control of supplemental lighting intensity in a greenhouse environment. Lighting Res. Tech. 45:295–304. 4. Dueck, T., et al. (2011). Growth of tomatoes under hybrid LED and HPS lighting. Paper presented at the International Symposium on Advanced Technologies and Management Towards Sustainable Greenhouse Ecosystems: Greensys2011 952. 5. Nelson, J. A., & Bugbee, B. (2015). Analysis of environmental effects on leaf temperature under sunlight, high pressure sodium and light emitting diodes. PLoS One, 10(10), 0138930.