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Re: [EnergyPlus_Support] Geothermal system in closed loop



There is a model for ground source heat exchangers in EnergyPlus. Here 
is some information from the Engineering Reference.


      Plant Loop Deep-Ground-To-Water Vertical U-Tube Field Heat Exchanger

This model (Object: GroundHeatExchanger:Vertical) documentation is 
derived from the M.S. thesis of Arunachalam Murugappan, which is 
available on the Oklahoma State University web site 
http://www.hvac.okstate.edu/. Eskilson (1987) Yavuzturk and Spitler 
(1999) developed the long and short time response factors respectively, 
which are used in determining the borehole temperature responses. 
Response factors are infinite series of numbers, which relate the 
current value of a variable to past values of other variables at 
discrete time intervals. These response factors are referred as 
/g/-functions. The variable time-step model presented here uses both 
long time-step /g/-functions and short time-step /g/-functions to 
predict the boreholes response to short term fluctuations in the load.


        Long Time-Step Response Factors

Eskilson developed /g/-functions for various borehole configurations. He 
plotted the g function curves for different sets of borehole spacing to 
borehole length ratio typically for .05, 0.1, 0.15, 0.2, 0.3 and ? (? 
represents the single borehole configuration. All the plots were for the 
ratio of 0.0005 between the borehole radius and the borehole length . 
For any other radius a simple relation between the two radii as given by 
Eskilson (1987) can be used. Eskilson gives the /g/-function curves for 
38 different configurations.

Figure 188 shows the /g/-function plotted against the non-dimensional 
time defined as , for various configurations of vertical boreholes with 
/B/H/ ratio of .1 along with single borehole. It is seen from this 
figure that the thermal interaction between boreholes increases with 
time and with the number of boreholes in field.

Figure 188. Short Time Step g Function Curve as an Extension of Long 
Time Step g Function Curves for Different Configuration of Boreholes 
(Eskilson 1987, Yavuzturk 1999).

The /g/-functions developed by Eskilson are valid only after time 
estimated by Eskilson as . This time varies from 3-6 hours for a typical 
borehole field. This is because the analytical line source model, based 
on which the Eskilson?s model was developed, does not give a prompt 
increase in borehole wall temperature at . It gives acceptable results 
only after the non-dimensional times of . But to model short time 
responses of a borehole we need response factors, which can give 
accurate results down to minutes.


        Short Time-Step Response Factors

Yavuzturk and Spitler (1999) developed short time step response factors 
using a transient, two-dimensional, implicit finite volume model on a 
polar grid.

The circular u-tube pipe in the ground loop heat exchanger was 
approximated as a pie sector of equivalent perimeter. A constant heat 
flux for the heat transfer form/to U-tube, a zero heat flux in the 
angular direction and a constant far field temperature in the radial 
axis makes up the three boundary condition and undisturbed far field 
temperature as the initial condition. The numerical model accounts for 
the thermal resistance due to individual borehole elements; such as 
resistance of the pipe and grout material and the convection resistance 
due to the heat transfer fluid in the pipes. The long time step 
/g/-functions discussed in the previous section do not account for such 
effects.

The short time-step /g/-functions are the same for different borehole 
configurations. This is because there is no thermal interaction between 
the boreholes for times less than 200 hrs during which the short 
time-step /g/-functions apply. So it is suggested to use the short 
time-step /g/ function for time steps in the range of 2.5 min and 200 
hours and the long time-step /g/-functions for time steps longer than 
200 hours. The /g /function for any time can be found by linear 
interpolation between the bounding known values.


        Development of the Variable Short Time Step Model

The variable time step model was developed as an extension of the model 
presented by Yavuzturk and Spitler (1999). The variable, short time step 
model uses a similar algorithm and extends it to accommodate sub-hourly 
responses, variable time steps and explicit equations to calculate the 
outlet fluid temperature of the ground loop heat exchanger.

The uniform time-steps model developed by Yavuzturk and Spitler (1999) 
is able to pre-calculate all the g-functions at the beginning of the 
simulation. The variable time-step model on the other hand must 
calculate the /g/-functions when the borehole response calculation for 
each time step is carried out. For every time step a different set of 
/g/-functions is needed in the variable time step model as the time at 
which the /g/-function is to be applied for the past loads changes for 
each time-steps.

This is made clear from the illustration in Figure 189, which shows a 
simulation in progress. The boxes with numbers represent the sub-hourly 
loads. The time (in hrs) at which these loads occurred are shown by 
solid arrows above the respective load boxes. The right-most solid arrow 
gives the current simulation time, which is 3.31 hrs. The times given 
below the boxes, pointed by dashed arrows, are the time at which the 
/g/-functions are to be estimated and applied to the respective sub 
hourly loads (boxes) for the current time.

For example, let us take the sub hourly loads 1,2 & 3. These loads 
occurred at 0 hrs, 0.16 hrs & 0.28 hrs. The response of the borehole 
temperature for the current time step is calculated by applying the 
/g­-/functions at 3.15 hrs, 3.03 hrs & 2.5 hrs respectively. Thus to 
calculate the present borehole temperature, the sub hourly loads 1-12 
are superposed using the corresponding /g/ functions at times given by 
the dashed lines. This gives the borehole temperature at hr 3.31. 
However, for the previous time step, which occurred at 3.15 hrs, the 
g-functions for the loads 1, 2 & 3 are at 2.99 hrs 2.87 hrs and 2.42 
hrs, and the over all response is obtained by superposing the loads 1-11.

Thus for each time step since the time step increments are not uniform 
we need to store the simulation times at which these time-steps 
occurred, and calculate corresponding /g/-functions at each time-step.



Pregas Srinivasan wrote:
>
> Hi,
>
> I am designing and analyzing a geothermal closed loop vertical pipes 
> system. My company uses Trace, but it doesnt have the capacity to 
> calculate the geothermal pipe length and simulate the conditions based 
> on the soil characteristics. I was wondering if energy plus can do 
> such analysis? Any inputs/help is appreciated.
>
> Thanks
>
> Pregas Srinivasan
> Thompson Consulting engineers,
> Newport News, VA
>
> 

-- 
Richard A. Raustad
Senior Research Engineer
Florida Solar Energy Center
University of Central Florida
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