Degree Days in the Platform

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Jordi Garcia

In this article, you are going to learn what degree days are and how the EM Platform can calculate them.

 

Introduction

What are degree days?

"A degree day is a measure of heating or cooling. [...] Weekly or monthly degree-day figures may also be used within an energy monitoring and targeting scheme to monitor the heating and cooling costs of climate controlled buildings" - wikipedia.org

Degree Days in the platform

The EM platform automatically calculates both Heating and Cooling Degree Days for a location, provided you have an outdoor temperature Categories configured, either from the platform or your own.

The system offers you three different parameters relating to the calculation of the Degrees Day: Degree Day Heating, Degree Day Cooling and the sum of both.

Requirements

In order to have Degree Days for a location with the name "Location A", either with the platform's virtual weather station or with your own outdoor temperature sensor, you have to follow these steps:

  1. Configure the Winter/Summer reference temperature in the location. Learn how to configure a location here.
  2. Select the degree day calculation method. Learn how to configure a location here.
  3. Configure the "Outdoor temperature" Category in the location. Learn about Categories here.

How does it work?

By doing both actions previously described, you will enable the automatic calculation of degree days. Degree days are calculated daily from 7:00 to 10:00 GMT+1, so you might need to wait for the process to run before using degree day parameters.

Once the process has run, a new Categories will appear in the Datasource where the temperature Categories is, which will be linked to the location where the reference Categories was assigned (Location A in our previous example) with a name such as "DD Location A".

dexma-degree-day-01.png

Calculation Methods

The platform allows the calculation of Degree Days using three different methods:

  1. UK Met Office: Standard of the UK Met Office.
  2. COSTIC: Standard of the Scientific and Technical Committee of the Climatic Industries of France, also called the Method of Energy Professionals.
  3. German degree days (20/15) based on VDI practice: Standard in Germany to calculate Heating Degree Days, based on VDI practice.

The three calculation methods are detailed below. The formulas used differ for each method.

UK Met Office

This method was developed by the UK Met Office in 1928 (CIBSE, 2006), daily Heating Degree Days (HDD) and Cooling Degree Days (CDD) are calculated based on a comparison of daily minimum and maximum air temperatures with the selected base temperature, taking account of fluctuations of daily air temperature around the base temperature, as well as the asymmetry between daily average temperature and diurnal temperature variations. The UK Met Office equations for computing daily HDD and CDD are shown below (Matzarakis & Balafoutis, 2004) (Spinoni et al., 2018):

For Heating Degree Days (HDD)

dexma-degree-days-001.png

For Cooling Degree Days (CDD)

dexma-degree-days-002.png

This method is widely used, as historically meteorological stations only recorded maximum and minimum daily temperatures for practical reasons.

Where:

  • Tavg is the average temperature 
  • Tmax is the maximum temperature of the day
  • Tmin is the minimum temperature of the day
  • Tbase is the base temperature

 

COSTIC

The Unified Day Degree (Degré jour unifié - DJU) is the difference between the outside temperature and a reference temperature, which allows estimates of thermal energy consumption to keep a building comfortable in proportion to the harshness of winter or the heat of the summer. The usual reference of 18°C was defined by considering that the interior temperature of the premises is 19°C and that the free internal (occupants, lighting, equipment, etc.) and external (solar radiation, etc.) supplies cover the equivalent of 1°C of heat loss. They are therefore divided into heating degree-days (DJC) and cooling degree-days (DJF).

There are two methods of calculating DJU, giving different results: a method called "météo" (Météo-France, 2017) with simple calculation and a method called "Professionnels de l'énergie" or “COSTIC method” with more elaborate calculation (COSTIC, 2020). The COSTIC calculation method is the recommended one by the French government (GEM/CC), 2007 and it’s the following:

 

 Heating degree days (HDD)

  • Tbase for HDD is the winter reference temperature defined for the location.
  • Tmax is the daily maximum temperature from the outdoor temperature reference categories.
  • Tmin is the daily minimum temperature from the outdoor temperature reference categories.

 

Cooling degree days (CDD)

  • Tbase for CDD is the summer reference temperature defined for the location.
  • Tmax is the daily maximum temperature from the outdoor temperature reference categories.
  • Tmin is the daily minimum temperature from the outdoor temperature reference categories.

The result of the calculation for the COSTIC method is rounded to the closest integer,  0,5 being rounded to 1. 

German Degree Days (20/15), based on VDI practice

The German method follows VDI 3807 conventions. It uses the daily mean outdoor temperature and distinguishes between:

  • An indoor reference temperature of 20°C.
  • A heating threshold of 15°C.

A day counts as a heating day only when its mean outdoor temperature is below 15°C. This convention is commonly described as the German 20/15 method.

This method does not require the user to configure reference temperatures for heating and cooling because the thresholds are part of its definition.

Heating Degree Days

Under VDI 3807, the Heating Degree Days for a given day are calculated as:

An important feature of VDI 3807 is that the heating threshold and reference temperature serve different purposes. The 15°C threshold determines whether heating demand is counted, while 20°C determines the temperature difference accumulated on a qualifying heating day.

This method does not account for cooling degree days, so total Degree Days = total Heating Degree Days. Cooling Degree Days are considered = 0 during the year.

Base temperature

As seen above, the calculation of Degree Days requires defining a base temperature for both HDD and CDD. If you are not sure which temperatures to use for your building, keep reading.

Base temperature (Tb) can be physically interpreted as the outdoor temperature at which solar and internal gains offset heat losses (Eto, 1988).

Tb = Desirable indoor temperature - (Mean energy profit / Total conductivity of building )

As can be seen, the base temperature is not easy to calculate. It is different for each building and climatic zone. Anyway, the following table gives different standard values for different countries (Spinoni, Vogt, & Barbosa, 2015):

Country Base temperature Source
Turkey 14-22ºC (HDD) 18-28ºC (CDD) Buyukalaca et al. (2001)
Spain 15ºC (HDD) 21ºC (CDD) Valor et al. (2001)
Greece 10-20ºC (HDD) 20-27.5ºC (CDD) Papakostas et al. (2010)
Canada 18ºC (HDD and CDD) Wibig (2003)
United States 18.3ºC (HDD and CDD) Wibig (2003)
United Kingdom 15.5ºC (HDD) 22ºC (CDD) UK MET Office
France 18ºC (HDD and CDD) COSTIC

Despite these general references, every Energy Manager can use their own base temperatures (per building, per project, etc) according to specific project needs or previous experience.

Cost

This feature is free of charge, you only need to have a temperature meter in your location (physical or virtual).  

 

References

 

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