Heating Demand and HDD

Heating degree days (HDDs) are supposed to serve as an indicator for heating demand. At first glance, everything seems obvious. HDDs are a smart combination of the temperature difference between the interior of a dwelling and the outside temperature and the number of days where the difference is valid. (See one of our previous articles for technical details).

In an earlier posting we investigated the link between HDD and gross energy consumption. Then the findings were not as convincing as one might expect. An increase in HDD does not necessarily mean that consumption is rising in the same way. On the contrary, in some cases the two parameters might even go in opposite directions.

Surely, primary energy demand is certainly too crude a measure to be strongly correlated to simple variations in temperature. Too many other factors like energy demand for transport purposes and manufacturing come into play. At the end of the day, domestic needs and especially heating constitute just a fraction of the total energy cocktail. Thus it is tempting to go one step further and look into a potential, and hopefully more clearcut relationship between HDD on the one hand and consumption figures for heating purposes on the other.

We performed such an analysis for the case of Austria, covering the years from 2003 till 2010. The source data have been taken from Eurostat and Statistik Austria. The consumption figures refer to three different sources, namely heating, warm water and cooling.

Fig. 1 HDD and energy consumption for heating purposes in Austria, HDD* = HDD/30.

The correlation between the two sets of data is obviously better than in our earlier analysis which was based on gross consumption figures. Nevertheless, there are notable deviations from an ideal scenario which require some interpretation.

Before the economic crisis of 2008 the amount of TWh that went into heating was fairly stable. Up to 2008 and again in 2010 HDDs show much larger deviations from the mean value than the consumption figures. In some years, 2004 and 2008 to be precise, the deviations have a different sign, thus going in opposite directions as can be seen in Fig. 2.

Fig.2 Energy demand for heating and HDD, deviation from mean value.

Both, in 2006 and 2009, consumption figures differ more significantly from their mean value than their respective counterparts in HDD.  However, we may consider this as an exception. In general, energy consumption for heating, cooling and warm water seems to be more inert than the fluctuations caused by weather conditions.

This is good news because it shows that our heating systems are much less sensitive to outside conditions than what we might expect in the first place. On the other hand, it may also indicate that dwellings having a high degree of thermal insulation of which there are many in Austria are less exposed to temperature fluctuations.

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