Worth its salt

Over the past 30 years the number of greenhouses being built has increased steadily due to yields that are typically 10 to 100 times greater than yields achieved outside. Greenhouses also enable high-value crops to be grown out of season. However, despite this increase, there are many places around the world where hot and arid climates and a lack of fresh water make supporting a greenhouse operation very difficult and not very practical.

This didn’t stop Charlie Paton and Seawater Greenhouse Ltd., a designer of growing systems, from creating a greenhouse system that could operate in some of the hottest and driest climates on Earth. A seawater greenhouse provides a low-cost solution to year-round crop production by using seawater and sunlight.

“It’s an idea we have been playing with for a long time — over 20 years,” says Paton, Seawater Greenhouse founder and managing director. “It’s quite counterintuitive; seawater greenhouses in hot, arid places don’t really make sense on the face of it. Everybody knows you can’t use seawater to grow plants because the salt kills the plant. Everybody knows you don’t have hot houses in hot countries. It takes a bit of understanding to get over those objections.”

The seawater greenhouse system took Paton a lot of trial and error. His travel experiences are what inspired him to start toying with the idea.

“I used to spend a lot of time in my youth traveling around the Middle East and North Africa,” Paton says. “I was always perplexed by the fact that these countries — the Sahara Desert in North Africa and the Middle East — were generally surrounded by seawater, but were suffering to a greater or lesser extent from drought.”

Paton began looking for solutions that would make a greenhouse in these climates practical. He had a background in lighting and understood quite a bit about light, color, control, heat, infrared and ultraviolet. He used this knowledge to create solar stills, a simple way of distilling water using the heat of the sun to drive evaporation from humid soil and ambient air to cool a condenser film.

“I spent quite a lot of time trying to develop solar stills to turn seawater into fresh water,” he says. “I learned that I might get a solar still to make four liters of water a square meter of still area. I thought, ‘That’s quite good, four liters a day. How much can we irrigate with that?’”

As it turned out, Paton realized that in many of these hot and arid locations, evapotranspiration occurred at a higher rate than his stills produced fresh water.

“Then I thought, ‘Why not put the plants in the still and redesign the still to suit the plants?’” he says. “So that’s where the idea of the seawater greenhouse came from.”

To date, Seawater Greenhouse has built these unique greenhouse systems in Tenerife, Abu Dhabi, Oman and Australia. Paton says there are very few limitations to the greenhouses and many places that could benefit from them.

The process
The original concept for the seawater greenhouse came about through controlling light and separating it in such a way that you can use the maximum amount of available PAR, photosynthetic active radiation, for the crop cultivation, and use the rest of that energy for desalination.

“In simple terms it’s that photosynthesis is driven by light, but it’s only red light and blue light in the visible part of the spectrum that drives the synthesis,” Paton says. “The conflict is the more light you have for crop growth, the better. The more heat you have the worse. So if you can take the heat out of the light, in other words cool the light, then you can use that heat for some other purpose like distilling seawater.”

The process is as follows: The air going into the greenhouse is first cooled and humidified by seawater, which trickles over the first evaporator. As the air leaves the growing area, it passes through the second evaporator over which seawater is flowing. This seawater has been heated by the sun in a network of pipes above the growing area, making the air much hotter and more humid. It then meets a series of vertical pipes through which cool seawater passes. When the hot, humid air meets the cool surfaces, fresh water will condense as droplets run down to the base where they can be collected. The process imitates the water cycle where seawater heated by the sun evaporates, cools down to form clouds, and returns to the earth as rain, fog or dew.

The conditions inside the greenhouse are cooler than outside temperatures. This is significant because it creates cool, humid conditions in high light areas, which is ideal for plant growth.

“It very much depends on what the local conditions are; temperature, wind speed, wind direction, solar radiation and all those sorts of things,” Paton says. “What we do primarily is spend a lot of time analyzing the meteorological data and the climate conditions and then coming up with a design that is best suited to those conditions. It’s not a one-size-fits-all solution that you can parachute into place. We start from scratch using that accumulated knowledge and the data that we’ve generated to work out what’s the best design.”

The greenhouses are capable of growing a wide range of produce, herbs and flowers.

The company has done projects mainly in parts of Africa and the Middle East.

“You can grow crops like lettuce, tomato and cucumber using a fraction of the water that you’d use otherwise, and they grow well because the high humidity reduces stress,” he says. “In that sense, it’s quite different from the conventional high-tech Dutch greenhouse because the Dutch like to control everything to quite a precise degree with heating, cooling, venting and CO2. We don’t really do any of that. We rely on what the design of the greenhouse will achieve and then simply adjust airflow rate.”

Structurally a seawater greenhouse can be identical to any other greenhouse. The difference between conventional greenhouses and a seawater greenhouse is the use of an evaporator.

“The difference is we have a large area for an evaporator instead of a wall and we choose to make the evaporator as big as possible to take up the wall that’s facing into the wind,” Paton says. “On the opposite wall, we either have fans or we don’t depending on the strength and relativity of the prevailing wind. If it’s very hot and very dry, evapotranspiration might be five, 10 or 15 liters a square meter a day, which is quite high. Even if you irrigate at those rates, if the sun is very strong and the wind is very dry and strong and the temperatures are high, it will be very difficult to grow temper-type crops even with an abundance of water. So not only do we enable crops to grow in places that they wouldn’t grow otherwise, we also enable them to grow using very little water.”

Adoption
Paton says that seawater greenhouses are still in the early stages of being recognized as a widespread solution. However, the evidence to support them in these hot and arid climates is clear.

“As a topic we aren’t quite on the map,” Paton says. “The people who understand greenhouse horticulture tend to be dismissive of it because it is so different from a conventional Dutch greenhouse where people get fixated on CO₂ control and humidity and temperature regulation within a closed environment.”

Closing a greenhouse stops pests from getting in, but also stops CO₂ from coming in, forcing you to inject CO₂. When you inject CO₂, you’re burning gas and producing heat. You’ve got to store the heat somewhere and use it later, and you don’t want to open vents because then you lose the CO₂.

“There are lots of problems that there are some wonderfully elegant, brilliant solutions to, but that kind of thinking is the opposite of what we do,” he says. “It’s vented. Air is coming through all the time. We don’t have any problem with CO₂ depletion because we have an air change every minute, so the CO₂ levels are always high. We don’t have a problem with pests and disease because the saturated salt evaporator is toxic to anything in the air. It’s a very effective method of scrubbing airborne pathogens, disease and contaminants out of the air. So we have a very clean, sterile airflow coming into the greenhouse. Intellectually and practically, it is quite a different approach. The result is the same, but the means of achievement are different.”

Most recently, the company built a system in Australia, which is the company’s first commercial-scale greenhouse.

“We’re getting growth rates that are equivalent to top-end, high-tech, best-practice Dutch cultivation,” he says. “I think we can safely say that in the right place, it is much cheaper to cool a greenhouse with seawater than it is to heat and light one artificially. Most greenhouses in the world are in cool temperate climates. The greenhouses that are in hot, arid places tend not to be used in the summer. They tend to be only used in the winter months. In places like Abu Dhabi and the hottest countries on earth, we can grow tomatoes and cucumbers all year round.”

Seawater greenhouses can be small or large operations and there are next to no limitations to the greenhouses from a design standpoint.

“There are no limitations, but for practical reasons you want to be close to seawater because you don’t want to be constructing kilometers of pipe for one little greenhouse,” he says. “At the same time, there is a lot of merit in the idea of running seawater maybe 50, 100 or 200 miles into the desert region in order to establish agriculture there as long as you don’t contaminate the ground with salt.”

There are large areas of desert where this would be applicable. The major limitation is the cost of pumping, which is related to the height above sea level that you pump to.

“There are large areas in North Africa and the Middle East that are either at sea level or below sea level,” he says. “In the future, especially in the Middle East and North Africa to a large extent, bringing seawater inland will be a very good solution to overcoming food security.”

While Seawater Greenhouse has done projects mainly in parts of Africa and the Middle East, the company has been exploring the idea of expanding its reach and finding additional locations that would greatly benefit from seawater greenhouses. Places such as Yemen, Somalia and Mexico are areas that Paton is optimistic about as future locations the company could break into.

“We have interest in a lot of the countries that are on the edge of being what are called ‘failed states,’ like Yemen and Somalia, and quite a few parts of North and South Africa where drought is a real problem,” he says. “Mexico and the Sonora Desert would be ideal locations,” he says. “We have had a lot of interest from there. That is potentially the best place for us because Mexico does have very good growers. Mexico can overcome all those problems I’ve talked about; politics, culture and the know-how. There are huge areas of desert in some places next to the coast, and at the same time there are other places where there are farmers who are really stressed through shortage of water and drought. Here is a solution that can overcome all of that.”

Gregory Jones is a freelance writer for Greenhouse Management.