What is the cost of humidity control?
Using heat to control humidity in a greenhouse gives the advantages of a good yielding and a disease-free crop. But with energy prices rising, many growers are left wondering whether humidity control is something they can really afford.
With this in mind, the specialist growers Coletta & Tyson took the decision to save on energy costs by minimising their active humidity control. This step meant the exposure of their crop to potential disease problems ... so was it the right decision? We carried out research to find out.
Comparing humidity strategies
The size and layout of Coletta & Tyson’s Millbeck Nursery provided us with the unique opportunity to compare the performance of two identical greenhouses operating under different humidity control strategies. We carried out side-by-side comparisons of the following two approaches:
- Optimised humidity control - With this approach, we used the heating and ventilation set-points previously discussed in the articles Spark spread - what is it and what does it mean? and How to avoid condensation.
- No active humidity control - The graphs below show the heating pipe temperatures and the climate in the two greenhouses on the same day last week.
Greenhouse with humidity control (click to see larger version)
Greenhouse without humidity control (click to see larger version)
If we look at the night period, we can see that the greenhouse with humidity control had an average Relative Humidity (RH) of 85% compared to 90% in the ‘no control’ greenhouse. However, to achieve this, the pipe temperature in the humidity control house was 10 degrees C hotter.
So what did humidity control cost?
Over the period from 16:00hrs to 08:00hrs:
- Active humidity control required heating for the entire 14 hours at an average pipe temperature of 35 degrees C.
- No humidity control used heat for 12½ hours at an average pipe temperature of 21 degrees C.
We can use this information with data about the greenhouse temperature, heating pipe length and diameter and boiler efficiency to calculate the heat used in each scenario. On this night, the active humidity control used 0.7 kWh/m2 more energy, costing an additional 1.75p/m2. Based on the plant density of 9 plants/m2, the cost of humidity control on this night was therefore 0.2p/plant.
However, don’t assume that humidity control costs 0.2p/plant every night. This was a worst case night, with mild temperatures; so humidity control was particularly difficult. On a colder night, more heat would have been needed to maintain the temperature in the ‘no humidity control’ block and the differences between the treatments would have been equalled out.
When assessing the value of humidity control, we have to remember the potential advantages, which include:
- Reduced grade outs / rejects.
- Labour saving due to less picking over.
- Fewer fungicide applications.
- Better shelf life.
So, before you rush out and change your humidity control settings to save costs, remember the additional work and costs that you might incur if you go too far.
Scenario: With humidity control:
- Energy consumption = 0.9 kWh/m2
- Cost based on gas @ 2.5p/kWh (73p/therm) = 2.25p/m2
Scenario: No humditity control:
- Energy consumption = 0.2 kWh/m2
- Cost based on gas @ 2.5p/kWh (73p/therm) = 0.5p/m2