Blazing hot by day but freezing at night

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“In the greenhouse area in Almeria and El Ejido, you often have to follow map coordinates if you want to find your customer,” shares Nelson Perez, Climate Consultant from Svensson for Spain and Portugal. “No street address?” asked a colleague visiting Almeria for a week. “How can the customer not have an address?” Almeria has the highest density of greenhouses in the world – just in Almeria, there are over 32000 ha. Fortunately, a GPS is happy to aim for satellite coordinates, and the customers know to share them when we visit.”

Her visit to Almeria was connected with something that was happening to a greenhouse’s climate at night. “It was as if it was driving along with the brakes on. Something was holding yields back”, Nelson describes. “We knew if we could find a great solution for the grower, we could really help many of our customers. It was about this time we all became night-time heat loss nerds.”

Anyone who is growing through winter in a traditional greenhouse should be a little nerdy about what happens in the greenhouse at night. “Understanding some of the physics just makes the greenhouse work better.”

The team traveled to the heart of El Ejido for one of the trials, where they made a comparison between a greenhouse that had no screen and another one where they placed a high transmission energy screen to fight against heat loss at night. The results were good the first year, but they were about to surprise us even more. Together, we decided to run a second year of trials using a screen that is optimized for daytime growing conditions rather than night-time energy saving – the Harmony 3845.”

Was that really the best choice, Nelson wondered. “After all, keeping the plants warmer at night was the initial goal. Here, we were shooting for a wider range of variables for the greenhouse, so we felt it was worthwhile. Night-time heat loss is full of surprises. I think most of us are still a bit stunned by the amount of physics around it.”

A good start is to get some terms straight. He explains the key terms:

Conduction. “Most of us understand that things lose their heat when they touch something cold and lose heat to that object.”
Convection. “This is when the crop is surrounded by cooler air. This can be caused when cold air infiltrates, or warm air exfiltrates from the greenhouse.”
Radiation. “It can come as a surprise to some that after a very warm day, an open sky at night can lead to substantial infra-red radiation, as energy moves from the warmer crop to the freezing open cosmos.”

“I’m not sure that any of us go around thinking that after a 30°C (86°F) day, a crop could be plummeting near to 0°C because the clouds parted to reveal a clear and open night sky”, he adds. “The physics of radiative heat loss was described by physicists Stefan and Boltzman in the 1880s. They explain why soldiers who had dealt with 40°C (+100°F) and clear skies by day in the first Gulf War woke up in the desert to find ice on their sleeping bags. They were radiating their warmth away to the next closest object: space. “

As an example, he shares what happens without an energy screen in the exterior from the nearest to the far point where the eye can see.”

“We’ve long been curious about lack of performance of some crops in hot regions during the winter period, and these trials were a great chance to explore and find solutions. We saw huge drops in temperature on clear nights with high humidity and no wind. When this happens the inside of the greenhouse can become even colder than the external temperature an event called thermal inversion.”

Without a cloud cover or a screen inside the greenhouse, heat escapes into the atmosphere very fast causing significant energy loss and, in conjunction with poor ventilation and a lack of wind, this can further lower the greenhouse’s internal temperature below the exterior.

The team considered various solutions for this problem during the visit. “We had assumed that an energy screen – designed for insulation – would be best at preventing night-time heat loss, but in fact, something as thin as a piece of paper will stop outgoing radiation, at least for a while. It interrupts the line of sight. You might have experienced this: If you are sitting under a parasol at a barbeque on a hot day and the sun goes down, you might notice a sudden shock of cold when you step out from under the thin sheet of the parasol.”

“In our case, we realized we could use a shade cloth to reflect the energy back inside the greenhouse as evening came on. It would have the added advantage of being able to shade the greenhouse from peak radiation during the day, when temperatures were extreme.”

This was a gamechanger, as the team made the adjustments and prepared for a new trial with their grower. Now, a single Harmony screen would replace the Luxous energy screen in the role of reducing night-time heat loss and the impact of the thermal inversion in the greenhouse.

“When we checked the results some way into the second trial, we gathered some remarkable results. We have written these up in a guide called “Thief in the Night.”

More participation in the profitable winter market
When talking to Hugo Plaisier, Senior Climate Consultant in the Netherlands, about the really encouraging results, Hugo said that ultimately it is their success in taking the chill off these winter nights that makes all the difference. “It is how cold it gets that dictates how long they can participate in the market,” he said. “The screen gives them an added 1 to 3°C of warmth at night and increases the dew point as well,” he added.

This blog is the first publication of the new Warmzones Growers Blog. “For some time, we’ve wanted to share some of our experiences with these extraordinary growers: site visits, some of the data from growing trials, some of their best tips – even just the friendships and banter with customers and partners from Culiacán on the Mexican Pacific coast, to Qingdao on the Chinese Yellow Sea coast,” the Svensson team shares. “We’ll be asking questions like what do you do when your staff are struggling to pick in high temperatures, or your bumble bees go on strike because even they think it’s too hot?”

Click here for the Warmzones Growers Blog

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