Issue 25: Open Roof Greenhouse

Issue 25
November/December – 1995
Story Title: Open Roof Greenhouse
Author: Roger Fox

The issue of inadequate ventilation in greenhouses is a topical one in Australian horticulture. At one NSW nursery a fixed greenhouse roof has recently been replaced with two fully opening screens.

Good ventilation, as greenhouse growers are constantly being reminded, is paramount under Australian conditions. The advantages which protected cropping offers during the cool periods of the year is sometimes countered by the disadvantages of having an excessively hot environment at other times.

Because the natural movement of hot air is up, the roof of the greenhouse is a ‘prime site’ when it comes to vent openings. But what if one could go further than just vents, and open up the entire roof when required? This approach is already being used in some greenhouses around the world, and was recently introduced to Avondale Nursery at Glenorie, NSW, where a fixed roof structure has been replaced by two movable screens. The system uses Ludvig Svenson (LS) screens and has been installed by the Living Shade company, Australian distributors for the LS screen range.

Essentially, the principle is quite simple. One of the screens (the lower one) is a thermal screen, providing warm greenhouse conditions; the upper screen is a shade screen. Having these two alternative ‘rooves’, which can be used singly or together, gives one quite a range of climatic options inside the growing environment.

Duncan Herbert, Living Shade’s sales engineer, who designed the greenhouse conversion, describes it as a ‘retrofit’. That’s to say, they removed the walls and roof of an existing structure and replaced them, rather than constructing a new building from scratch. But Duncan envisages that the system could be provided in kit form in the future.

“This is a retrofit of an existing building,” he explains, “but we envisage that we would be able to supply this in a kit form, so the grower could go along with a post-hole digger, drill the foundations, put the posts in, put the concrete in and put up the steel work.”

So how does the system work in practice? The two screens operate independently, each with its own motor, computer controller and thermostat. The lower screen is a waterproof plastic thermal screen, while the upper screen is a solar shade screen. The temperatures at which the screens open or close are pre-set via the thermostat controls, which are wired back to the control units. At Avondale Nursery, the system is set so that the thermal screen opens when the temperature exceeds 20°C, and the shade screen closes when the temperature rises above 25°C. Duncan explains a typical scenario:

“For example on a summer morning, at around 9am, it may be 22°C, so the bottom (thermal) screen will be open. The shade screen will also be open, so we’ll be getting maximum light and perfect growing conditions.

“Then when the temperature rises above 25°C, the top screen will close. We’re still letting maximum light in, but we’re taking the sting off the plants, because the aluminium in the screen reflects the heat.”

During the day therefore, the air temperature within the greenhouse will dictate the position of the screens. At night, however, the system behaves differently.

“At night both screens are closed,” Duncan explains. “At 6 in the morning the top (shade) screen opens to allow light to come in. So in our night function we have time overriding temperature. Whereas during the day, temperature dominates, controlled by the thermostats inside the house.”

It is possible to manually override the thermostats and also the motors in case of a blackout.

The overhead thermal screen used in this greenhouse system, QLS Ultra, has been specially designed for this purpose. It is a thermal screen fabric which has been laminated to make it waterproof.

“QLS is an existing fabric that has been converted on a plastic laminating machine. They’ve used a thermal screen and laminated it, so we’ve got basically a plastic thermal screen, which gives us the waterproof quality. It’s 100% waterproof and has a 40% thermal coefficient. It diffuses light and has about an 84% light transmission.”

One of the great advantages of this type of computerised environmental control is that it operates independently of people. For a large commercial nursery like Avondale, where staff work Monday to Friday, it means that plants can still enjoy optimum conditions on week-ends, even if no-one is around.

Having only recently installed the system at Avondale, Duncan still intends to make a few subtle refinements, particularly to the computer control area. Prime among these is the need for pauses in the computer’s reaction to temperature changes.

“You actually need a system that step-functions it,” Duncan says, “so that at 20°C the screen opens 25%, pauses for 15 minutes and then re- analyses the temperature. If it’s too cold, it will close up; if it’s still heating up it will open half way, analyse the temp again, and so on. So over a period of an hour, the screen will open up fully.”

Another aspect of the new greenhouse is the use of different screen fabrics on different walls of the house. On the south end wall, a clear fabric has been used, with no aluminium, so that no light is reflected. On the north end wall, a more reflective screen fabric has been used because of the higher radiation levels which this aspect experiences.

The greenhouse floor consists of concrete paths and walkways, with gravel under the tables. The greenhouses are used for raising hardy nursery tubestock (mainly shrubs) and irrigation consists of pop-up spray jets. As a result, much water is sprayed around the greenhouse, but this is collected via underground drainage pipes and channeled back to a central dam. In addition, the dual screen roof includes a gutter system, so that all rainfall can be collected and stored. It is, as Duncan describes it, very “EPA friendly”.

“All the water drains back into a central drainage system and goes into a dam, and that’s an advantage of this roof and gutter system. Every drop that falls on these rooves is collected, so with regard to EPA restrictions, we’re well ahead. We’re catching run-off, we’re redirecting and storing it, and we’re using it again in feeding our plants. Its quite environmentally friendly,” he says.

Duncan believes that there’s another advantage to using gravel and concrete as flooring. It acts, he says, as a natural energy bank, absorbing heat during the day and radiating it back into the greenhouse at night. The reflective screen roof then prevents it from escaping.

“So we keep a warmer environment in here during the night time, without having to heat,” he says.

In fact, Avondale Nursery has no heating or cooling systems, but either could be incorporated with the dual screen system if required. It depends of course on one’s climate, crop and production expectations.

Elsewhere at the Avondale Nursery complex are a number of Poly-tunnel houses which, being low in height, experience ventilation and overheating problems. A proposed improvement here would be the installation of either an internal screen, to reflect some of the heat, or more ideally an external screen. The latter has the advantage of reflecting much of the heat before it reaches the house interior.

Now that the dual-screen, open-roof system is up and running, Duncan Herbert would like to see it become the basis of a full scale analysis of this type of technology.

“I’d like to see a tertiary student take up the open roof system as a project, and do some physical testing- perhaps as part of a pHD study. It could provide a full analysis of soil temperature and air temperature patterns, fabric quality, plant growth rates and so on,” he says.