Cooling Degree Days: using both wet and dry bulb temperatures 1

[MAZA128]

I have not been able to find any literature, but does anybody know if it is possible to calculate CDD using a combination of wet and dry bulb temperatures.

I ask this because I have projecting cooling demand for Delhi India, and comparing current cooling degree days when just dry bulb temps underestimate the continuation of cooling demand throughout the monsoon season, as the peak temperatures reduce but humidity remains high resulting in high demand in reality, whereas using just the wet bulb temps underestimates the demand during the peak summer season.

I combined the two using as I was using the Discomfort Index (Thom) and out of curiosity I used these the calculate the CDD (DI = (0.5* wet bulb temps)+(0.5*dry bulb temps)) and these appear to show a clear continuation in cooling demand in both peak summer and the monsoon period which is much more realistic of what actually happens.

Any thoughts welcomes!

 

[DRWeig]

There are many ways to factor humidity into your cooling degree-days forumla. Yours seems to work and match the load, so I vote stick with it. Another way is to calculate outdoor enthalpy-days and re-work the degree-days equation to cope with it.

Best to you,

Goober Dave

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[317069]

Can you give an example how did you apply the equation mentioned in your post (DI=0.5wb+0.5db) in order to calculate CDD

 

[MAZA128]

I simply used the DI in the normal CDD equation,

so hourly CDD = (DI - base temperature)/24

I know this is not an established method, but doing just wet bulb temps underestimates the effect of peak summer temperatures.

 

[317069]

Thank you but can you explained by numbers

 

[MAZA128]

How exactly do you want an example, its simply the DI value calculated from the wb and db input into the CCD formula, a base temperature of 22 degrees C is used. These hourly CDD are then summed then divided by 24 to give daily CDD then summed over a year of month to give those CCD values respectively.

 

[317069]

I mean why do you want to calculate CDD over a year, is it related to an energy study or to an HVAC design, if you are able to know hourly db&wb temperature over a year then you should have data tables that give you CDD,
from google: "A cooling degree day is every degree that the mean temperature is above 65 degrees during a day. So, if the high temperature for the day is 95, and the minimum is 51, the average temperature for the day is 73. That would be 8 cooling degree days (73-65)."

 

[MintJulep]

Following on DRWeig's thought:

Calculate the enthalpy using wet and dry bulb temps. Then calculate DB temperature that results in equivalent enthalpy.

 

[MAZA128]

I am modelling cooling energy demand for Delhi India, under climate change temperature projections for a climate change mitigation and adaptation plan.

However, as said, CDD are normally measured using dry bulb temps but this hugely underestimates cooling demand during the monsoon season as the humidity is an important factor.

In relation to calculating enthalpy, could you point me in the direction of a website/paper that outlines how to calculate that? Thanks

 

[MintJulep]

http://www.engineeringtoolbox.com/enthalpy-moist-air-d_683.html