DesignBuilder are putting our EnergyPlus UI software through
the CIBSE TM33 tests and have come across some issues which are confusing us.
1. In
order to match our model with the models used to generate the TM33 acceptable
min/max range output values we have been fixing external and internal
convective heat transfer coefficient data to CIBSE values using idf data such
as:
SurfaceProperty:ConvectionCoefficients,
S_16_0_0_0,
Outside,
Value,
19.87,
,
Inside,
Detailed,
,
;
From reading the documentation I can't find any
indication that this particular data affects radiation heat transfer, but a
look at the results for reports for "Surface Ext Rad To Air Coeff"
and "Surface Ext Rad To Sky Coeff" indicates that radiation to
external air and sky is zero when using the
SurfaceProperty:ConvectionCoefficients data. So I'm not clear about
whether we've missed something or is there an error in software implementation
or documentation? The name of the SurfaceProperty:ConvectionCoefficients
suggests that it affects only convection.
I note the documentation on page 18 of the i/o ref states:
SurfaceConvectionAlgorithm:Outside
Field: Algorithm
Six exterior convection models may be
selected in EnergyPlus. The simple convection model applies heat transfer
coefficients depending on the roughness and windspeed. This is a combined
heat transfer coefficient that includes radiation to sky, ground, and air.
(my italics)
So given that SurfaceConvectionAlgorithm:Outside data covers
both convection and radiation, isn't the name
"SurfaceConvectionAlgorithm:Outside" misleading? Given our results, I
also suspect that this may also apply to use of
SurfaceProperty:ConvectionCoefficients data. Could you confirm this please?
2. The
sky cover and opaque sky cover values in the epw file don't seem to have an
effect on simulated results. Could you let me know under what circumstances
this data is used in simulations?
3. In
order to ensure that we model the radiation to the sky should we use the
equation on p99 in the engineering reference to define field Horizontal_IR
(In weather file) based on cloud/sky cover data?
Thanks for any help you can give.
Andy
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