Measuring Energy performance |
| YOUR "SELF MEASUREMENT" PROGRAMME FOR BUILDINGS AND STRUCTURES WITHIN THE UNITED KINGDOM. |
This Fact sheet provides a step by step approach to understanding and using Degree Days, if
you follow the steps you will be able to calculate your buildings energy use more accurately
than if you were to take and analyse consumption alone.
Introduction.
The rate of heat loss from a building is related to the building fabric and the temperature
difference between the inside and outside of a building The greater the temperature difference
the more heat will be lost. It is not possible to directly compare your buildings performance from
consumption data alone as weather conditions vary from month to month and year to year.
Heating degree days are a measure of the severity and duration of cold weather, the
colder the weather in a given month the higher the degree day value. If you take these into
account it is possible to compare one year with another and to determine whether any
initiatives have resulted in energy savings or there are problems with your building that need to
be addressed.
The base temperature used to calculate degree days in the UK is 15.5oC At this temperature
most UK buildings can heat themselves without the need for supplementary heating, due to the
internal gains from occupants and equipment and the solar gains through the building fabric –
walls and windows.
Degree days is a measure of the difference between the baseline and the actual outdoor
temperature multiplied by the number of days.
E.g. the temperature measured hourly records a temperature of 7.5ºC for 48 hours the degree
days totalwould be (15.5 - 7.5) x2 = 16
If the temperature is above the baseline then the degree days are set to zero.
It may not always be appropriate to choose the nearest recording station, as the stations local
environment may not be similar to your own. e.g. You may be located on an exposed site and it
may be more appropriate for you to choose another exposed site to take your degree day data
from.
Once you have decided which station you will be using it is important to always use this
stations information.
How to use degree day information
1. It is necessary to collect fuel use information, usually monthly, and for as far back as possible
2. Collect the degree day data for the same time periods as 1 above
3. Continue to take monthly meter readings – preferably at the same time of day at the end of
every month to coincide with degree day data,
Producing a Performance Line
1. Plot degree days on the x axis and energy consumption on the y axis. Plot the line of best
fit. This line is known as the performance line of a building and the amount of scatter on data
from this line will give an indication of the quality of the data collected. An obvious cause
of high levels of scatter is due to reading meters at different times of the month – e.g. If meters
are read on Monday’s it could be 4 or 5 days from the end of themonth.
2. Any wide scatter should be investigated and the causes identified. The performance line
provides a basis for evaluating historical performance of a building and is used to compare
current performance. Consistent variation from this line indicates that the building is operating
differently and can be used to demonstrate that energy efficiency related activities are having
an effect or thatsomething is going wrong in the building.
3. The most common pattern when comparing degree day to energy consumption can be seen
in the diagram below. The line of best fit might not be a straight line.
4. The line fits the formula y = mx+c where the intercept (c) either intercepts the y axis – non
temperature related energy use. Or x axis – no heat required at 15.5oC, because the internal
temperature can be lower, or that there are heat gains due to equipment / process use.
Producing a Control chart
Control charts can be calculated by determining predicted consumption from the equation y
= mx+c, where m = the slope of the line, x = the degree day value
and c = the intercept with the axis.
All this data can be obtained using standard spreadsheet facilities. Present the data in a
spreadsheet with the following
headings.
1 Date of reading
2 Weekly or daily reading
3 Degree Day data
4 Predicted consumption (kWh) based on y = mx+c
5 Difference (actual predicted) consumption
If you plot the difference in kWh against the time the readings were taken it is
possible to identify at what times the building is not performing in the predicted
manner.
Any negative readings are energy savings as a result of energy efficiency
activity.
1. The next level of chart is the CUSUM (The accumulated sum over a period) which allows
managers to measure th eaccumulated sum of the differences and allows the energy
performance of the building to be monitored. Using a CUSUM chart will allow you to observe
when savings are being made –the line of the chart will be going downward. If consumption is
on the increase it can be spotted by an upward trend in the graph and action can then be taken
to reverse the trend.
2. Once a sustained performance change has been identified it is important to produce a new
performance line and CUSUM chart to include data only from the period after the change. To
plot a CUSUM chart it is best to plot the following data.
Date of reading
Weekly or daily reading
Degree Day data
Predicted consumption (kWh)
based on y = mx+c
Difference CUSUM -(cumulative sum of figures from difference column.)
Estimating Savings using degree days
It is possible to assess monetary savings from activities by using this method. Take the data
prior to the change and plot the consumption against degree days. Then take the data from the
period after the changes and insert into a data table as below:
1 Date of reading
2 Weekly or daily reading
3 Degree Day data
4 Predicted consumption (kWh) based on y = mx+c from chart prior to change
5 Difference Money savings
6 Cumulative savings
FAQ
Where can I get Degree Day information from?
From the met office web site.
Where can I get this service provided for my business or employers buildings?
Email: energy@mojacarnet.com
I can provide a fast easy economical service of weekly energy efficiency performances across
the UK.
| FUEL EFFICIENCY MEASUREMENT FOR THE UK WITH THE "DEGREE DAY" |
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