Degree day analysis - a real-life example
Using degree day analysis enables you to account for weather and greenhouse temperature influences when looking at your energy use.
The graph below shows an example of degree day analysis Hornsfield Nursery (one of the GrowSave Focus Nurseries), beginning January 2008. It shows at energy use and degree day information.
(Click on graph to see a larger version)
What does the energy use graph show us?
A close look at the graph shows us that degree days and energy use have closely followed each other (i.e. when the degree days' value increases, the energy use increases accordingly). However, there have been two notable exceptions — in Weeks 2 and 11.
The energy use in Week 2 was higher than expected because the greenhouse had to be heated up to its operating temperature from cold. In Week 11, the energy consumption was also higher than expected — but again this can be easily explained.
During Week 11, the wind speeds were exceptionally high. This caused the ‘storm settings’ to come into use which meant that the thermal screen stayed open during the times when the wind speeds were high. This therefore restricted the screen usage and reduced the energy saving.
Both of these examples show the value of using degree day analysis alongside energy use data. In both of our examples we have been able to explain why energy use is higher than we expected. However, if we had not come up with reasonable explanations we would have needed to investigate further.
Possible causes might have been:
- Is the measuring box correct? — e.g. Is the wick dry or has the fan failed?
- Is the heating system faulty? — e.g. Has a mixing valve stuck or is a temperature sensor faulty?
- Is the thermal screen operating correctly? — Many systems do not measure the actual screen position; they calculate where the screen should be. So is the calculation correct and is the screen really closing fully?
- Are the climate control set points correct?
At current energy prices, saving 10 kWh/m2 is worth around £2,000/Ha, so putting some extra effort into energy saving can quickly pay dividends.