Degree Days in DEXMA

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In this article you are going to learn what are Degree Days 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 device configured, either from the platform or your own.

The system offers you three different parameters relating to the calculation of the Degrees Day: Degrees Day Heating, Degrees 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" Reference Meter in the location. Learn about Reference Meters 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 device will appear in the gateway where the temperature device is, which will be linked to the location where the reference device 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 two 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.

The two 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 temperature because of 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 device.
  • Tmin is the daily minimum temperature from the outdoor temperature reference device.

 

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 device.
  • Tmin is the daily minimum temperature from the outdoor temperature reference device.

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

Base temperature

As seen above, the calculation of Degree Days requires to define 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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