- Energy Saving
Relative humidity or humidity deficit?
In recent years, the accepted norm for growers of edible crops such as tomatoes, cucumbers and peppers has been to control humidity based on humidity deficit (HD). This is because conventional wisdom suggests that maintaining a minimum HD promotes transpiration, which, in turn, ensures optimised plant development and growth.
In contrast, ornamental crop growers typically use relative humidity (RH). This is because RH theoretically gives a better indication of the impending risk of condensation (and the subsequent possibility of disease formation), and historically ornamentals growers have been more concerned about disease control than maximum growth.
However, with energy prices rising, growers of all crops have become more concerned about providing reliable disease control at an acceptable cost.
As a result, the question that is now being asked by edible crop growers is, “Should I control humidity based on RH”?
Using RH rather than HD at an edible crops nursery
Two of the GrowSave Focus Nurseries (Red Roofs Nursery and Eric Wall Ltd) chose to challenge the conventional thinking for edible crops and use RH for humidity control in preference to HD.
At first this might be seen as a bold move. However, this is an approach that many edibles growers in Holland are already using, which begs a number of questions including:
- If Dutch growers are using it, why shouldn’t we?
- Does plant performance suffer at the expense of disease control?
- Will I save energy?
The following information gives some background on the use of RH and why it might be the preferred approach when energy saving is a key consideration.
Both Red Roofs Nursery and Eric Wall Ltd controlled to a maximum RH of around 85% to 90%. This range of settings was chosen to provide good disease control at acceptable heating pipe temperatures / energy inputs.
So how does this compare in HD terms, especially where growers using HD based control typically try to achieve a minimum of 3.0g/m3 during the day and 2.0g/m3 overnight?
The graphs below show the results from Red Roofs Nursery on two days. On the first day, the light levels were excellent and on the second day, light levels were significantly lower.
Results from a high light level day
This graph shows that the humidity was below the RH set point, although it was not excessively low. For the majority of the time, the resulting HD was also above the minimum setting that might be used by a grower using HD control. What is particularly striking is that the HDs were particularly good throughout the daytime period because of solar gain.
Results from a low light level day
This second graph shows an interesting contrast because, although the control of RH was again satisfactory, HD levels were significantly lower. This highlights that, had ‘conventional’ HD control been used, the energy consumption would have been appreciably higher, since higher pipe temperatures would have been needed for most of the day.
Evidence provided by the two Focus Nurseries highlighted that using RH in preference to HD provides significant potential for energy savings. This is particularly the case on dull days, when acceptable RH levels are maintained at significantly lower heating pipe temperatures than those needed for conventional HD control.
Doubts still remain that plant environment is not optimised on dull days and yields will ultimately fall. However, commercial evidence from growers in Holland and from Eric Wall Ltd (who have been using RH control for a number of years) suggests that these worries are unfounded. This is because growers using RH are achieving good yields and excellent disease control with reduced energy inputs.