Heating degree day
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Heating degree day (HDD) are quantitative indices designed to reflect the demand for energy needed to heat a home or business. These indices are derived from daily temperature observations, and the heating requirements for a given structure at a specific location are considered to be directly proportional to the number of HDD at that location. A similar index, cooling degree day' (CDD), reflects the amount of energy used to cool a home or business.
HDD are defined relative to a base temperature - the outside temperature above which a building needs no heating. The most appropriate base temperature for any particular building depends on the temperature that the building is heated to, and the nature of the building (including the heat-generating occupants and equipment within it).
For calculations relating to any particular building, HDD should be selected with the most appropriate base temperature for that building. However, for historical reasons HDD are often made available with base temperatures of 65°F, 18°C, or 15.5°C - base temperatures that are approximately appropriate for a good proportion of buildings.
There are a number of ways in which HDD can be calculated: the more detailed a record of temperature data, the more accurate the HDD that can be calculated. However, most HDD are calculated using simple approximation methods that use daily temperature readings instead of more detailed temperature records such as half-hourly readings. One popular approximation method is to take the average temperature on any given day, and subtract it from the base temperature. If the value is less than or equal to zero, that day has zero HDD. But if the value is positive, that number represents the number of HDD on that day.
HDD can be added over periods of time to provide a rough estimate of seasonal heating requirements. In the course of a year, for example, the number of HDD for New York City is around 5,000 whereas that for Barrow, Alaska is over 20,000. Thus, one can say that, for a given home of similar structure and insulation, four times the energy would be required to heat that home in Barrow than in New York.
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[edit] Example of Use
HDD provide a simple metric for quantifying the amount of heating that buildings in a particular location need over a certain period (e.g. a particular month or year). In conjunction with the average U-value for a building they provide a means of roughly estimating the amount of energy required to heat the building over that period.
Method
One HDD means that the temperature conditions outside the building were equivalent to being below the temperature required for thermal comfort inside the building by one degree for one day. Thus heat has to be provided inside the building to maintain thermal comfort. The rate at which heat needs to be provided is the rate at which it is being lost to the environment. The rate at which heat is being lost to the environment for one degree temperature difference is simply the U-value of the dwelling (as calculated by averaging over all components) multiplied by the area of the thermal envelope of the dwelling. For a ten-degree temperature difference it is ten times this amount. If we multiply the rate at which the building is losing heat by the time (in hours) over which it is losing heat we get the amount of heat lost in Wh (or BTU) and, therefore, the amount of heat that has to be provided by the heating sources. To convert Wh to kWh we simply divide by 1000. Therefore if the area of the dwelling’s thermal envelope (walls, roof and floor) is A, its average U-value is U and the number of degree days is D then the amount of heat required in kWh to cover the period in question is just:
A×U×D×24/1000
where the factor of 24 is needed to get the value in kWh rather than kWdays or simply BTUs in the case of US units.
The same method is used (except you don't need to divide by 1000) for US units with the result being in BTUs rather than in kWh.
[edit] Problems
Calculations using HDD have several problems. Heat requirements are not linear with temperature,[1] and heavily insulated buildings have a lower "balance point". The amount of heating and cooling required depends on several factors besides outdoor temperature: How well insulated a particular building is, the amount of solar radiation reaching the interior of a house, the number of electrical appliances running (e.g. computers raise their surrounding temperature) the amount of wind outside, and individuals' opinions about what constitutes a comfortable indoor temperature. Another important factor is the amount of relative humidity indoors; this is important in determining how comfortable an individual will be.
Another problem with HDD is that care needs to be taken if they are to be used to compare climates internationally, because of the different baseline temperatures used as standard in different countries and the use of the Fahrenheit scale in the US and the Celsius scale almost everywhere else. This is further compounded by the use of different approximation methods in different countries.
[edit] Conversion
To convert ºF HDD to ºC HDD: ºC HDD = (5/9) x (ºF HDD)
To convert ºC HDD to ºF HDD: ºF HDD = (9/5) x (ºC HDD)
Note that, because HDD are relative to a base temperature (as opposed to being relative to zero), it is incorrect to add or subtract 32 when converting degree days from Celsius to Fahrenheit or vice versa.
[edit] See also
[edit] References
- ^ Valor, E.; Meneu, V., Caselles, V. (2001). "Daily Air Temperature and Electricity Load in Spain". Journal of Applied Meteorology 40 (8): 1413–1421. doi:.
[edit] External links
Sources of free HDD data:
- Degree Days.net (worldwide) - daily/weekly/monthly HDD and CDD for locations worldwide, in Celsius or Fahrenheit
- ECI Oxford (UK only) - daily/weekly/monthly HDD and CDD for 200+ UK stations, in Celsius
- weatherDataDepot (North America only) - monthly HDD and CDD for US and Canada, in Fahrenheit
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