You’d think that the climate in modern, well insulated greenhouses would be a lot more homogeneous than in days gone by. Nothing could be further from the truth, says climate specialist Bas Knoll, of TNO, the Netherlands. In a project taking several years he is assessing the causes and solutions and is working on a climate model that should offer growers and system developers more understanding.
Greenhouses have by definition a heterogeneous climate, both horizontally and vertically. That also applies to modern, well insulated greenhouses. “The temperature differences increase as the heating becomes more intensive and horizontally this can reach more than 5ºC,” says climate researcher Bas Knoll. “As a consequence the RH can also vary from place to place by 15%. That has implications for the heating system. Actually the differences are the largest in highly insulated, large greenhouses."
To reduce the internal differences growers need to make very local adjustments. That rarely happens. Firstly because equipment such as air vents and heating groups are not sufficiently geared up for that. And secondly because the majority of nurseries have too few data collection points to accurately measure and follow the climate differences.
“This is the direction we need to go in the long term and most growers recognise that,” according to Knoll. “In addition there’s a sharp rise in Next Generation Growing. Here you can take more calculated risks and thereby get more from the crop and at the same time save energy. The condition is that you have the maximum grip on the climate. That is only possible if the differences in temperature and RH inside a greenhouse or area remain small."
Monitor and model
Initiatives to bring the homogenisation of the greenhouse climate to a higher level have resulted in some progress but the whole picture about what works and what doesn’t is far from clear. This was a reason for the Dutch ‘Greenhouse as Energy Source’ project to invite TNO to make a critical evaluation and to develop a simulation model. Its aim is to give growers and suppliers more grip on climate control and the systems involved in that.
Researcher Knoll: “Many whole and half truths are doing the rounds and everyone is struggling with the question what can we do? To clarify that we have to take measurements over a long period in greenhouses to discover when and to what degree climate differences occur, to make the connections and determine the influencing factors. Because it is very difficult for growers at any given moment to see through the interaction of factors there was an urgent need for a simulation model that offered the desired insight and understanding. In addition, there had to be an overview of available and yet to be developed solutions to be able to solve the most important bottlenecks."
These steps are now in motion according to the report ‘More homogenous climate in greenhouses' that TNO published recently. The last step that still needs to be made is the verification and tightening up of the simulation model, as part of a design platform that has gained the abbreviation SIOM (System integration and Optimization Model). Preparations are in full swing.
Cause of temperature differences
Various causes can be the basis for the increasing horizontal temperature differences. Firstly the heating units installed in well insulated greenhouses often have narrower dimensions so are slower and there is a suggestion of higher temperature gradients in parts of low-value heating networks. Also, during the refurbishment of greenhouses the wall heating often remains unchanged.
The second factor is the wind and window vents (see figure 1). Wind that blows over a greenhouse with (partly) open windows always results - due to local over and under pressure - in uneven natural ventilation. Relatively cold outside air comes into the greenhouse furthest away from the windward side (see supply), while nearby on the sheltered, leeward side, warm greenhouse air is removed (see outlet). In addition, the temperature gradient on the roof is barely taken into account.
An equally underestimated point is, according to Knoll, the accuracy of wind and wind direction meters. Often the meters are too low, so that the measurements are influenced by nearby objects, such as high buildings or chimneys. “And an inaccurate measurement leads to inaccurate control of the air vents," says the researcher.
Thirdly, screens and their use make a small contribution. Notorious is the cold dump that occurs by making a gap in the screen but also when the screen is fully closed often a structural localised cold dump occurs through small gaps in the screen. This forms the motor behind internal airflow and temperature gradients. “In addition there is often an imbalance because horizontal screens work dynamically, while the wall screen is permanently insulating,” adds Knoll.
The list of possible causes is easy to expand when other, often crop- or nursery-specific factors are included. Examples are artificial lighting, which are often switched on and off in groups, variations in crops, limitations of the greenhouse and (other) installations, design flaws and so on. In addition, the change to a new cultivation strategy and the installation of new equipment can disturb the precarious climate balance.
To effectively reduce climate differences within a greenhouse or an area two things are essential, argues the TNO researcher. First of all, more data collection points are needed to indeed be able to measure those differences. “I know a gerbera grower who intensively installed sensors but otherwise hasn’t invested anything,” says Knoll. “By better following the internal climate differences and eliminating them as much as possible with minimum means, he says he’s been able to save tens of thousands of euros annually.”
The researcher also says it’s advisable not just to control the greenhouse or even a section but to narrow that down further. “For example, consider varying the window openings in small sections and be more focused on the internal air circulation,” he suggests.
There are many aspects and possible interactions therefore that need to be fathomed out. This comes together in a new simulation model that helps to find the right combinations. When such a climate model is in place and has proved reliable, it offers several advantages, says the researcher: "You can use it to develop improvements and innovations in order to optimise necessary systems in terms of capacity, lay-out and energy-efficiency. You can also use the output of the model as input for improved control and management of the climate.”
This climate model is part of the design platform SIOM. This is certainly not intended to reinvent the wheel on every terrain, but to bring the multitude of existing computing and design models made by different parties under one umbrella and to serve as a platform for their integration. It also uses new information structures and decision support technologies.
“The climate model is currently only used for research. It has been tested in various forms and cases and enthusiasm for it is steadily growing,” according to Knoll. “The next step is to involve external parties in more practical exercises. It is still ‘work in progress’. To see the real benefits of the model we now want to collaborate with greenhouse and equipment designers who want to be out in front.”
A project spanning several years is assessing the causes and possible solutions for climate differences in greenhouses. The results have been taken into account during the development of a design platform, in which many existing design and calculation models have been integrated. This should give both growers as well as builders of greenhouses and equipment more understanding about how to achieve a more homogeneous climate.
Text and images: Jan van Staalduinen