Using 'degree days' to see how the weather is affecting your energy use
We all know that colder weather means we use more energy for greenhouse heating. But how do we know if any increases in energy use are just down to the weather, or perhaps to other factors making us less efficient? This is where a technique known as 'degree days' can help.
What is a degree day?
The difference between the outside temperature and a structure's internal temperature is the biggest factor affecting how much energy is required for heating the structure – and this underlying fact is the basis of the calculation method called degree days (degree C days).
To explain this further, let’s consider the example of a greenhouse which has been heated to an average temperature of 20 degrees C when the average outside temperature was 10 degrees C.
- The difference between the inside and outside temperatures was: 20 degrees C - 10 degrees C = 10 degrees C
- If these conditions lasted for 1 day, we would have 10 degrees C x 1 day = 1 degree C day
- Similarly if they lasted for 1 week, we would have 10 degrees C x 7 days = 7 degrees C days
Using degree days
By calculating the number of degree days over two periods, we can compare how much the heating energy demand has changed. Again, let’s consider our example:
Day 1:
- Average greenhouse temperature = 20 degrees C
- Average outside temperature = 10 degrees C
- Degree days = (20-10) x 1 day = 10 degrees C days
Compared to ...
Day 2:
- Average greenhouse temperature still 20 degrees C
- Average outside temperature drops to 5 degrees C
- Degree days = (20-5) x 1 day = 15 degrees C days
These calculations show that Day 2 had a degree day figure 5 degrees C days higher than Day 1. So theoretically the energy use on Day 2 should be 50% more than on Day 1.
Degree day data: real-life example
We carried out work at the tomato growers R&L Holt, concentrating on saving energy through better screen operation.
In order to know whether or not our work was effective, we collected lots of background data. This included greenhouse temperature and humidity and weather data (temperature, radiation and wind speed).
By using this data, we were able to calculate the average inside and outside temperature data on a day-by-day basis. This information enabled us to calculate the degree day figures for the site for each day. Daily figures were then added together to derive a weekly figure.
This graph shows how the weekly degree day figures had changed for Holts at the beginning of the season.
(Click on graph to see a larger version)
Energy Use Data
We also took weekly energy meter readings, so that we could track the week-by-week energy use of the site. The following graph shows the energy use so far this year.
Figures were calculated based on total gas use per unit of greenhouse area (kWh/m2). The graph below now includes week 6 (w/c 4th Feb).
(Click on graph to see a larger version)
Look closely at the graph and you will see that we have included an extra line in yellow. This is the overlaid weekly degree day line for the site.
What is interesting is that the shape of the energy use graph compared to the degree day line. Because the lines are of a similar shape — i.e. when the degree-day line goes up, the energy use increases in a similar proportion — this indicates that changes in energy use are in line with changes in weather.
The only exception was in week 2, when the greenhouse was heated up from cold before the crop went in.
Detailed analysis
We have already said that the degree day data for Holts showed the correct trends when compared to energy use, but carrying out more detailed analysis can give us a better insight into how energy efficiency is changing.
To do this we need to make some slightly more complex calculations that work out the energy use per degree-day (kWh/m2/degree C day).
At first sight this might look daunting, but this calculation allows us to easily see the effect of changes that have been made. The graph opposite below the kWh/m2/degree C day data for the Holts site for each week of 2008.
Again, if we ignore week 2 (because of the special circumstances that week) we can see that the amount of energy being used / degree day was decreasing.
This confirms that the changes we are making to screen operation are actually saving energy!
At the stage of the project shown in the graphs, a few more weeks' data would have been needed to confirm how effective the changes really were. But the good news is that the changes look to be having the right effect.
(Click on graph to see a larger version)
This more detailed analysis can also help in making other assessments, such as how simple changes — e.g. growing your crop 1 degree C warmer — might affect the amount of energy used.
Assuming that figure of 0.1 kWh / m2 / degree C day taken from the above graph for Holts is representative; we can calculate how much energy would be saved by growing the crop 1 degree C colder for 1 week. This is done as follows:
1 degree C colder x 0.1 kWh / m2 / degree C day x 7 days = 0.7 kWh / m2 per week
At a gas cost of 2p/kWh, the cost of this change can be calculated to be £140/Ha per week for the Holts site.
To get figures for your nursery, you will need to do an analysis identical to the one described above.
So start collecting greenhouse temperature, weather and energy use data now and carry out the analysis for your nursery — and you will soon start getting better information on how energy efficient you really are.