Changes from Slack Convo 5/11/23

Changes made to the NFP based on comments/conversation that took place over Slack on 5/11/23.

design/FY2023/NFP-EarthTube-1DEnhancement.md

@@ -5,17 +5,20 @@ Earth Tube 1-D Conduction Enhancement
 
  - April 2023
  - Revision Date (Version 2): May 11, 2023
+ - Revision Date (Version 3): May 12, 2023
 
 
 ## Justification for New Feature ##
 
 It has long been known that the earth tube model in EnergyPlus, though an important addition to the program, has one critical assumption that potentially affects the accuracy of the earth tube results.  The assumption is that the presence of the earth tube and any heat transfer from the air passing through it to the ground is negligible.  While this is a common assumption for many first principle models in the literature (these models generally all use the model used in EnergyPlus to determine the temperature at a particular depth), it does not take into account any changes that the heat rejection from the outside air to the ground itself.  It is not currently known how much of an impact this will have, but it would seem at least plausible that the ground around the earth tube would be impacted and that this would affect the potential outlet temperatures that can be achieved and thus the amount of cooling that is possible.  The goal is to provide a more accurate assessment of these systems using EnergyPlus.
 The improvement of the earth tube model will be done in stages.  The first step is to handle the impact of the earth on the ground surrounding the earth in a single dimension (with the soil depth being that dimension).  Future work would potentially handle heat transfer axially along the length of the tube, the impact of more than one earth tube and the impact that adjacent tubes have on each other, and any time lag due to the air passing through the earth tube.
 
-## E-mail and Conference Call Conclusions ##
+## E-mail, Slack, and Conference Call Conclusions ##
 
 Technicalities Meeting (5/3/23): team review the document briefly and stated the desire for this to get additional feedback in the next technicalities meeting.  Two initial requests from the team included modifying the proposal to factor in the impact of the soil at the depth of the earth tube since this would be more realistic of the situation (as in, the earth tube in not infinite in width) and to contact John Nelson who has expertise in earth tube modeling and has requested improved earth tube simulation capabilities in EnergyPlus in the past.  This update includes a modification of the finite difference scheme to address the concern regarding what is happening at the depth of the earth tube itself.  In addition, GitHub was used to contact John Nelson and that conversation is contained in the comments for the GitHub issue regarding earth tubes (https://github.com/NREL/EnergyPlus/issues/6627).  John suggested that the model at some point include the ability to model temperature variation axially along the earth tube (something already logged as a potential future enhancement) and also to provide some improved controls where air might bypass the earth tube and use heat recovery in certain situations to save the earth tube’s ability to cool.  This is an interesting idea but may also be fairly complicated because it may deal with specific control algorithms and bridging between the zone and air loop simulations.  Such a concept is different from what is currently in EnergyPlus and there doesn’t appear to be another model in E+ that bridges with air flow at the zone and air loop level.  Such a concept is definitely beyond the scope of this initial enhancement.
 
+Slack Discussion (5/11/23): Neal Kris brought up two issues in this NFP.  First, there is the issue of the infinitely wide domain which is unrealistic.  Second, there is the question of what air temperature is being used for heat transfer between the earth tube and the air passing through it.  The second issue will be resolved by adding the discussion of the effectiveness-NTU heat exchanger model that is being used.  This will be similar in the assumptions as the heat exchanger algorithm used in the low temperature radiant system model (solid side with a “constant” temperature and a fluid side which has a temperature which varies throughout).  The first issue will be addressed by modifying the solution domain to have a user-defined width with adiabatic boundary conditions on either side.  In reality, at some distance from the earth tube there will be negligible heat transfer toward/away from the earth tube.  Making this a user-defined input parameter will allow the width to be studied.
+
 ## Overview ##
 
 The earth tube model in EnergyPlus uses an established equation for determining the temperature of soil below grade.  This equation is published in the literature and is documented in the EnergyPlus Engineering Reference.  In EnergyPlus, this equation for temperature as a function of distance is integrated over the diameter of the earth tube to come up with an average temperature that is then used as the soil temperature with which the earth tube interacts.  While this is useful and is a common assumption in much of the existing literature, it does not take into account any impact that the earth tube has on the surrounding soil in any direction.  The model simply assumes that the earth tube does not impact the local soil temperature.
@@ -30,15 +33,25 @@ Furthermore, research in this area seems to at least give the impression that th
 
 ## Approach ##
 
-Overall, the approach will be to add a new 1-D heat transfer model that will run as a separate option from the existing model.  The reason for this is two-fold: execution speed and comparative accuracy.  First, the new model will potentially be slower than the existing model.  It may be that users do not wish to have their run times increase and at this time it is uncertain how much the run time will be impacted give the solution technique that will be used, particularly as additional dimensions of heat transfer get added later.  Second, it would be helpful to know how much the solution was impacted by the new enhancement of 1-D heat conduction.  This will allow test cases to be run and comparisons made to deduce when the use of the new enhancement will be necessary and how much the soil temperature is impacted.  This might also give the team useful information on how important additional solution dimensions will be.
+Overall, the approach will be to add a new 1-D heat transfer model that will run as a separate option from the existing model.  The reason for this is two-fold: execution speed and comparative accuracy.  First, the new model will be slower than the existing model.  It may be that users do not wish to have their run times increase and at this time it is uncertain how much the run time will be impacted given the solution technique that will be used, particularly as additional dimensions of heat transfer get added later.  Second, it would be helpful to know how much the solution was impacted by the new enhancement of 1-D heat conduction.  This will allow test cases to be run and comparisons made to deduce when the use of the new enhancement will be necessary and how much the soil temperature is impacted.  This might also give the team useful information on how important additional solution dimensions will be.
+
+The 1-D heat conduction model itself will be implemented using a 1-D implicit finite difference scheme.  The solution dimension will be bounded by a temperature just below grade at the top of the solution space and another temperature boundary condition at a depth below the earth tube.  The temperatures at these upper and lower boundaries will be set using the existing equation from the literature for undisturbed soil temperature.  At the sides of the solution space, adiabatic conditions will be assumed.  The user will be given the option to specify the overall thickness of the solution region, but there will only be heat transfer vertically (none horizontally since it will be adiabatic on the “sides”).
 
-The 1-D heat conduction model itself will be implemented using a 1-D implicit finite difference scheme.  The solution dimension will be bounded by a temperature just below grade at the top of the solution space and another temperature boundary condition at a depth below the earth tube.  The temperatures at these upper and lower boundaries will be set using the existing equation from the literature for undisturbed soil temperature.  The finite difference grid will include an upper portion from the upper temperature boundary to the node just above the earth tube and also a lower portion from the node just below the earth tube to the lower boundary condition.  The node at the earth tube itself will also be modeled as an “all soil” node but also include a connection to the earth tube itself.  In a one dimension model like the one proposed for this enhancement, it is more realistic to model the level at the earth tube to be soil because in all directions there will be soil and not an entire layer of air/earth tube.  Thus, this earth tube node will capture the conditions of the soil at that level, and a heat exchanger model that connects to the  temperature of the earth tube node will be used, and checks will be made to insure that the heat loss of the air is equal to the total heat gain of the soil.
+The finite difference grid will include an upper portion from the upper temperature boundary to the node just above the earth tube and also a lower portion from the node just below the earth tube to the lower boundary condition.  The node at the earth tube itself will also be modeled as an “all soil” node but also include a connection to the earth tube itself.  In a one dimension model like the one proposed for this enhancement, it is more realistic to model the level at the earth tube to be soil because in all directions there will be soil and not an entire layer of air/earth tube.  Thus, this earth tube node will capture the conditions of the soil at that level, and a heat exchanger model that connects to the  temperature of the earth tube node will be used, and checks will be made to insure that the heat loss of the air is equal to the total heat gain of the soil.
 
 The solution for this finite difference model will use an implicit scheme.  Implicit schemes are known for being inherently stable.  Since it is possible that the number of nodes may be small and thus the nodes large, it will be important to avoid potential stability issues in the solution.  The user will be given some flexibility in the input to specify the number of nodes above and below the earth tube.  While some reasonable limits will be applied to those inputs to avoid too few or too many nodes, the control of the number of nodes will also help during the testing and verification process to make sure that the model is producing results that make sense and are believable.  An example of the node layout is shown in the figure below.
 
 ![earth_tube_solution_space_diagram](earth_tube_solution_space_diagram.png)
 <p style="text-align: center;"> Figure 1. Earth Tube 1-D Model Solution Space Node Diagram.</p>
 
+The heat transfer between the soil and the air flowing through the earth tube will be calculated using an effectiveness-NTU heat exchanger formulation.  Because the soil temperature does not vary axially along the earth tube in this model, the effectiveness for a heat exchanger equation in this situation where one side is at constant temperature and the other side (air side) varies in temperature is:
+
+\begin{equation}
+\varepsilon  = 1 - {e^{ - NTU}}
+\end{equation}
+
+This is similar to the reasoning used in the low temperature radiant system models where similar assumptions are made for the embedding material and the fluid being circulated through the tubing.  The effectiveness will then used to calculate the outlet air temperature based on an inlet air temperature, soil temperature, and other conditions of the situation (flow, thermal properties, etc.).  The overall heat exchange between the soil and the air is calculated using this effectiveness, and there is no need to determine the "average" temperature of the air as it passes through the earth tube.  The heat exchanger model defines the heat exchange using the effectiveness and the theoretical maximum heat transfer between the air and soil.
+
 Another important point regarding this model is that unlike the existing model which only calculates the performance of the earth tube when the earth tube is actually operating, the enhancement 1-D model will need to calculate the response of the soil even when the earth tube is not running, whether that is due to no cooling being needed, the earth tube is turned off, or it being not the appropriate season for the earth tube to operate.  So, while the existing model simply skips the earth tube code when it is “off”, the enhanced model will have to calculate node conditions all the time.  This again could add to the execution run time as the earth tube calculations will have to be made all of the time.  This will naturally be impacted by the number of nodes selected by the users.  At this time, it is uncertain how much time this model will add to execution time.  Testing will be done with the new model to help identify some benchmarks and provide some helpful guidance to users via the Engineering Reference documentation.
 
 This work will also include modernization of the earth tube code based on recent standard practices implemented by the EnergyPlus development team.
@@ -191,6 +204,9 @@ This parameter sets the dimensionless distance above the earth tube for the solu
 \paragraph{Field: Earth Tube Dimensionless Boundary Below}\label{field-earth-tube-dimensionless-boundary-below}
 This parameter sets the dimensionless distance below the earth tube for the solution space.  The maximum value is 1.0, and the minimum value is 0.25.  This parameter is interpretted in a similar fashion as the previous parameter where the depth of the solution space below the earth tube is determined by the maximum distance above the earth tube (earth tube depth minus diameter).  This allows the user to have different thickness for the modeled portion of the ground above and below the earth tube.  The default value for this parameter is 0.25 (the minimum value).
 
+\paragraph{Field: Earth Tube Dimensionless Solution Space Width}\label{field-earth-tube-dimensionless-solution-space-width}
+This parameter sets the dimensionless width of the solution space horizontally as a function of the earth tube radius as defined in the main earth tube input syntax.  The maximum value is 20.0, and the minimum value is 3.0.  The default value for this parameter is 4.0 which means that the width of the solution space is four times the radius.  In other words, this would include soil one radius length beyond the edges of the tube on either side of the earth tube.
+
 An IDF example:
 
 \begin{lstlisting}
@@ -226,7 +242,8 @@ EarthTube:Parameters,
   5,                !- Earth Tube Nodes Above
   3,                !- Earth Tube Nodes Below
   1.0,              !- Earth Tube Dimensionless Boundary Above
-  0.5;              !- Earth Tube Dimensionless Boundary Below
+  0.5,              !- Earth Tube Dimensionless Boundary Below
+  4.0;              !- Earth Tube Dimensionless Solution Space Width
 
 \end{lstlisting}