Issue 30: Towards a Future Greenhouse

Issue 30
September/October – 1996
Story Title: Towards a Future Greenhouse
Author: Roger Fox

ROGER FOX reports on a revolutionary greenhouse structure at the University of Western Sydney, Hawkesbury, which aims to provide a design model which addresses the major problems of protected growing.

The concept of the ‘ideal’ greenhouse varies somewhat according to where in the world you are. But for much of Australia, excellent ventilation is a prime concern in achieving effective climate control all year round.

At the University of Western Sydney, Hawkesbury, a purpose-built greenhouse structure, funded by fertiliser company Scotts Australia, has provided the opportunity to trial the most up-to-date theories on how a greenhouse should be designed. Located in the School of Horticulture, the greenhouse is used for a variety of plant research projects. But primarily, it is the structure itself that is providing the most interesting trial.

The greenhouse was built by the Living Shade company, who were commissioned by Scotts Australia, and was designed by Keith Garzoli from the Australian National University. His brief was to come up with the best research/production house possible.

“The design tackles the main questions of light, temperature, relative humidity and CO2 in the best possible way,” says Richard Clough of Living Shade.

The broad aim for all those involved in the project has been to create a ‘design model’, which other greenhouse builders, technologists and growers may seek to follow.

One of the main requirements of Keith Garzoli’s design, was that roof ventilation should account for 30% of the floor area. Also, to optimize cross-flow ventilation, the width of the house was restricted to 25 metres.

But according to Dr Tony Haigh, Senior Lecturer in the School of Horticulture at the University, ventilation considerations had to be weighed up against solar heat and insulation requirements.

“We wanted to stop the house overheating – so that it’s not letting too much radiation in in summer – but we also wanted to keep the heat in at night and over winter,” Tony explains.

“So we needed to find a way of keeping the heat inside, without also stopping all the light coming in. There are compromises you have to make.”

A range of different horticultural fabrics was used throughout the building. The material on the side walls, for example, varies according to the sunlight intensity – the north and west walls have a 70% reflective material, while the east and south walls have a 40% reflective material, because of their lower sunlight load.

Ventilation
To maximise ventilation in the house, the side walls roll up, from ground to top, when activated by a thermostat set at 19°C. The sawtooth roof is also vented – the vertical sections between each roof span open completely, also controlled by thermostat. The roof vents are activated first, as Tony Haigh explains:

“The roof vents are set to open at a slightly lower temperature than the side walls, so that they start venting heat from inside the house before the side walls start to open. We only have a 1°C differential at the moment.”

Screens
A feature of the house design is the use of both internal and external movable screens, ie. located both above and below the greenhouse roof. The roof itself is made from a clear woven plastic from Ludwig Svensen, which transmits 86% of the light.

The external screen is located directly above the roof of the house and is designed as an ‘external, reflective blanket’, to reduce the radiation heat load in summer (it is 55% reflective).

“We’ve chosen to solve the reflective problem outside, rather than inside the house,” Tony comments.

In hot weather, the external screen closes when the temperature inside gets to 30°C. Prior to this happening, the roof vents will have opened at 18°C and the side walls at 19°C. During winter, the external screen is switched off completely, since it is not required.

The internal screen, on the other hand, is an ‘internal heat blanket’, designed to trap warmth around the plants at night. It is controlled by light, rather than temperature, closing over at nightfall and opening again after sunrise each morning. In effect, it limits the area of the greenhouse that needs to be heated, and thereby saves on energy.

“It keeps the warmth of the house inside,” Tony explains, “and the space above it is a further insulation from the outside – you don’t need to heat that space because all your plants are in the lower space.”

Heat is delivered to the house through ducted pipes, which run underneath the tables. The tables themselves are ‘skirted’, so that the warm air rises slowly through the crop. A return pipe, located near the middle of the house, takes the warm air back to the heater unit, thus making the heater system more economical to run.

The sawtooth roof design was chosen for the house because it offers good air flow and a reduction in the amount of condensation dripping onto the crop. The shape of the span directs any drips that form towards a roof gutter, and indeed the roof span lengths were designed to suit the distance that a drip of water will run before falling. This is an important point, since condensation adversely effects light transmission into the house.

“Any condensation you have on your roof will reduce light transmission, and also bend the light into a different direction, so that it’s no longer coming straight through,” Tony says. “A bubble of water changes the direction of incoming light and so makes it more diffuse.”

Heating
The method of heating the house is one of its most revolutionary features. Heat is generated by a Phase Change Material unit (PCM Unit), which is located beside the house. When the temperature in the house drops below 18°C, the PCM unit starts blowing warm air into the house, through the network of under-table pipes already described.

While there has been some discussion of the use of phase change materials as an economical way of heating greenhouse structures, very few successfully operating systems are in existence. A model greenhouse structure such as this one, provided an ideal opportunity.

The concept of phase change materials as a source of energy is often explained using ice as an example, as Richard Clough explains.

“If you put a block of ice on a stove, you’re putting energy into that ice and it changes from ice to water. But it hasn’t changed the temperature – its still 0°C. Until all the ice is melted, the liquid will not rise in temperature. But it has absorbed all that energy and has changed from ice to water, from a solid phase to a liquid phase.

“We’re doing a similar thing in the PCM unit with crystals. They’re changing from a solid to a liquid, and in so doing they absorb energy or release energy.”

“So you melt it by heating it up during the day, and let it set so it releases heat at night,” Tony adds.

The PCM unit consists of a fan, and baffles which open and close. When it is shut off from the house, the air circulates within the PCM unit itself; when heat is needed in the house, a baffle opens and the air is directed there.

“You’ve got a low cost heating system,” Tony says. “You have to buy the unit and set it up, but to run it is a low cost exercise. You don’t have a constant energy input, such as gas or oil etc.”

A solar booster panel has been added to the PCM unit, and this increases the temperature of the air which is collected, by several degrees Celsius. There is a down-side to the present system, however – it can’t cope with 4 or 5 days of rainy weather, since this exhausts all the available heat.

“It’s ideal in a winter climate that has sunny days and cold nights, such as Sydney often experiences,” Richard comments.

Plant Trials
The greenhouse is used for a variety of plant trials, most of them PhD student projects. One recent project using chilli plants, examined the effects of dividing the root systems of the plants into two, and treating each half with a different watering regime.

The plants were set up with the root system split between two 20 litre grow bags and had half the root system growing into each bag. Each bag could be watered independently.

Three watering regimes were used in the experiment: both sides of the root system watered every day; both sides watered once a week; and only one side watered each day for 7 days, followed by the other side for the following 7 days.

“We did a similar experiment with tomatoes over summer,” Tony continues. “With them we found that the plants having only half their root system watered every day, produced a similar number of fruit to those where the whole root system was watered every day. You also got a similar weight of each fruit, but they had a slightly higher acidity and a slightly higher sugar content, which translates into better flavour.

“The plant responds to the fact that half its root system is drying out, by closing up the stomata in its leaves, and it expends less carbohydrate on making new leaf and stem material, and puts more of that into fruit and flowers.

“It suggests you can water just half the plant, so basically using half the amount of water, and still get similar amounts of fruit produced, as well as improvements in flavour characteristics. Half the root system is saying to the plant, ‘we’re stressed, shut down’, and the other half says ‘we’ve got plenty of water’. So the plant in fact has plenty of water, but it responds to the message of stress by continuing to photosynthesize, but putting a lot more carbohydrate into processes other than making leaves and stems. So you end up with a smaller looking plant, but more carbohydrate available.

“The point of the experiment was to see whether we could use a different system to the Israelis, where they are using high salinity water for their desert sweet tomatoes. The problem is that high salinity leads to a lot of blossom-end rot in tomatoes – we have trialled it here.

“What were trying to do is to get the plant to believe it’s water stressed, when in fact it still has plenty of water.”

Other plant trials at the moment include a study of strawberry nutrition to explore the effects of different levels of potassium (K) on growth, yield and flavour of fruit. This project will go on to look similarly at phosphorus and nitrogen and will then be repeated at elevated CO2 levels.

Work on Acacias, Eucalypts and Blandfordia (Christmas Bells) is also being conducted in the greenhouse.

Future Improvements
A future alteration to the greenhouse which Richard Clough sees as being necessary, is the addition of thrip netting. The emergence of Western Flower Thrip in Australia is making this form of insect protection a more important consideration than before.

Thrip netting would need to be placed over all vent openings, as an inside layer. Unfortunately however, such netting also reduces ventilation by 47% and light by 16%.

“Adding thrip netting would immediately knock off 47% of the ventilation. So on the average greenhouse, you would be in real trouble. It would also knock about 16% of the light out.

“But the point is that in a high ventilating house like this, at least we have a reasonable chance of maintaining correct temperatures.”

“We have also found that if we double the surface area of the thrip netting, we get most of the lost ventilation back. This may mean putting it on in zig zag form.”

By reconciling the need to reflect excessive heat in summer, provide maximum ventilation so that hot air can be disposed of quickly, and retain as much warmth as possible at night and in winter, the greenhouse has achieved quite a technological ‘balancing act’. The opportunity to trial a PCM unit approach to heating has also made it a valuable research opportunity, which could further advance the approach to greenhouse climate control in this country.


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