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Re: [EnergyPlus_Support] Comparison between E+ and TRNSYS
>From a member of the EnergyPlus development team . . .
---------------------
Your email really has two questions that are separate issues, so I
will try to answer those separately even though they may seem
somewhat related.
Ground heat transfer is one of the more difficult problems to address
in building simulation. EnergyPlus relies on the use of an outside
program to calculate an equivalent "ground temperature" at the
exterior side of any surface that is in contact with the ground.
This ground temperature is a monthly value. This value establishes
the outside boundary condition (temperature) for a particular surface
in contact with the ground. There are other methods of ground heat
transfer being investigated for incorporation into EnergyPlus. One
of those methods might be similar to what is discussed in the ASHRAE
HoF.
The difference in the conduction transfer functions does not surprise
me because it is highly likely that the calculation method in TRNSYS
differs from that in EnergyPlus. This may result in a different
number of flux history terms which can change the coefficients
significantly. In addition, it is likely that the TRNSYS
coefficients were calculated for a one-hour time step whereas the
EnergyPlus coefficients are for whatever time step the user has
defined. This would also result in vastly different values for the
coefficients. I am unfamiliar with PREP and do not have much
knowledge of TRNSYS, but here is some background information on the
calculation of CTFs in EnergyPlus:
In EnergyPlus, CTFs are calculated using a state-space method
(summarized in John Seem's Ph.D. dissertation, University of
Wisconsin-Madison 1987). In this method, a finite difference like
grid is applied to the surface description (construction) and linear
algebra is used to solve for the CTFs. In this method, the number of
history terms for temperature and flux are the same, something that
differentiates it from some of the other methods such as the Laplace
transform approach to solving for CTFs. The advantage of the state-
space method is the relative flexibility and stability at time steps
of less than an hour as well as the opportunity to expand the method
to include moisture transport or heat sources/sinks embedded in the
construction. More information should be available in the EnergyPlus
engineering documentation.
I hope this information was of assistance and answered your
questions.
------------------
On 17 Dec 2002, at 6:18, skalogir@xxxxxxxxxxxxxxxx wrote:
> I managed to model my first building with EPlus and tried to compare
> the results with those obtained by modelling the same building with
> TRNSYS. I found some large differences between the two models which I
> think are due to the ground heat losses (by increasing the ground
> temperature the difference decreases) and the TFC's estimated by E+.
>
> In TRNSYS the ground losses were calculated by considering the
> perimeter heat losses as suggested in ASHRAE H/B of Fundamentals. How
> these losses are estimated in E+?
>
> The second difference concerns the TFC. In TRNSYS these were estimated
> with the routine PREP and the TFC's max. values of 16.5 and 18.3 for
> the ext. wall and roof respectively were obtained and used in the
> model. The factors given by E+ (eio file) are as follows:
>
> Ext. Walls:
> 35.6, -143.4, 240.2, -217.7, 116.5, -37.7, 7.3, -0.787, -0.412,
> -0.702, 0.167
>
> Roof:
> 49.8, -92, 52.4, -10.1, 0.577, -0.937, 0.398, -0.337, 0.183
>
> These values are much different than those obtained with PREP and I
> wonder why, as the same construction is used for both.
>
> Yours help would be greatly appreciated.
>
> Wishing you all Merry Christmas and Happy new year,
>
> Soteris
> Kalogirou
>
>
>
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