PROBLEM
Modeling Building Heating & Cooling System:
Ground Source Heat Pump (GSHP) = Geothermal Heat Pump (Overview at bottom)
with Software: Trane Trace 700 in 2012
SOLUTION
STEP 4: Select the cooling plant and click the Cooling Equipment tab at the bottom of the Create Plants screen.
STEP 5: Choose the ground-source heat pump that best matches the target performance.
STEP 7: Assign the Backup Boiler as the Backup Heat Source. A cooling tower is assigned automatically.
The thermal storage capacity represents the amount of fluid in the entire system, including the borefield, for the ground source heat pump (GSHP) and water source heat pump (WSHP) systems. If the size of the well field has not been determined, users are encouraged to enter a value that approximates the expected size.
5
10 144
15 96
20 72
25 58
30 48
STEP 11: Apply your changes.
STEP 12: Click on the Controls button at the right side of the screen.
STEP 13: Click on the Cooling Plant and Geothermal Controls button at the right side of the screen.
STEP 14: Select one of the TLoop Ent Bldg options in the Plant Controls - Geothermal Loop editor. This field defines the method TRACE 700 uses to compute the monthly ground loop temperatures.
Vertical Bore
For information on importing custom geothermal fields, click here.
STEP 15: Close all of the controls editors.
STEP 16: Click on the heating equipment tab at the bottom of the Create Plants editor.
STEP 17: Select a boiler.
Note: Trace requires a backup heating source for all water source heat pumps.
STEP 18: Enter the full-load energy consumption for the circulator pump.
STEP 19: Close the create plants editor and assign the heating and cooling coils to their respective plants.
GROUND SOURCE HEAT PUMP (GSHP) OVERVIEW
ADVANTAGES & DISADVANTAGES
Disclaimer: Content above sent from Trane Trace Support in 2012
Modeling Building Heating & Cooling System:
Ground Source Heat Pump (GSHP) = Geothermal Heat Pump (Overview at bottom)
with Software: Trane Trace 700 in 2012
SOLUTION
STEP 1: Pick Water Source Heat Pump as the system type. Then click Apply to save your entries.
STEP 2: On the Fans tab, select Hydronic in heat pump fan and enter 0.5 for the static pressure.
Next, describe the cooling and heating plants represented by the GSHP system and backup boiler.
STEP 3: Drag the appropriate icons from the Equipment Category section to define each plant. Rename the cooling plant as Ground Source Heat Pump and the heating plant as Backup boiler.
Note: Do not remove thermal storage from the tree. The water loop is modeled as a special thermal storage type in TRACE 700. Removing the thermal storage eliminates the water loop from the simulation.
Note: A boiler will still need to be input for a GSHP model in TRACE 700. (Refer to the "TRACE Requirement: Cooling Tower and Backup Heat Source" section below.) However, if the water loop is properly sized (input as the thermal storage capacity), the minimum and maximum condenser operating temperatures are correct, and the load profile is realistic, the boiler may not activate.
STEP 4: Select the cooling plant and click the Cooling Equipment tab at the bottom of the Create Plants screen.
Note: The ability of the heat pump to produce heat is defined within the Heat Recovery section of the Operating Mode table on the Cooling Equipment tab in Create Plants. A value must be entered
for both the Capacity and Energy rate for the Heat recovery operating mode:
STEP 6: Enter the full-load consumption of the primary chilled-water pump which serves the common water loop.
Loop temperature for a ground source heat pump system is typically maintained by the heat pumps attached to the system. As some heat pumps demand cooling and others demand heating, the associated addition of heat to the loop may offset the subtraction of heat from the loop eliminating the need for a cooling tower or backup heating source. However, if the system demands more heating or cooling capacity than the loop can provide a cooling tower or backup heating source will be required to maintain loop temperature.
STEP 7: Assign the Backup Boiler as the Backup Heat Source. A cooling tower is assigned automatically.
The thermal storage capacity represents the amount of fluid in the entire system, including the borefield, for the ground source heat pump (GSHP) and water source heat pump (WSHP) systems. If the size of the well field has not been determined, users are encouraged to enter a value that approximates the expected size.
5
10 144
15 96
20 72
25 58
30 48
WshpTairMin = WDDB, deg F
WshpTairMean = Yearly average OADB, deg F
DryBulbAvemo = Monthly average OADB, deg F
WshpEwtMax = WshpTairMean + 0.8*(WshpTairMax - WshpTairMean)
WshpEwtMin = WshpTairMean - 0.4*(WshpTairMean - WshpTairMin)
*** These equations based on Figure 3.13 from the IGHSPA manual.
WshpACoefClg = WshpEwtMean - WshpBCoefClg * WshpTairMean
WshpBCoefHtg = (WshpEwtMean -WshpEwtMin) / (WshpTairMean - WshpTairMin)
WshpACoefHtg = WshpEwtMin - WshpBCoefHtg *WshpTairMin
WshpEwt = WshpACoefClg+WshpBCoefClg*
ELSE
WshpEwt = WshpACoefHtg+WshpBCoefHtg*
END IF
When Custom is selected the user must create a custom geothermal schedule within the Schedules Library which Trace uses to specify the monthly loop water temperature.
When vertical bore is selected the ground loop is configured as a vertical borehole-type ground loop heat exchanger and modeled using an algorithm described by Yavuzturk and Spitler. (See Yavuzturk, C., J.D. Spitler. 1999. "A Short Time Step Response Factor Model for Vertical Ground Loop Heat Exchangers." ASHRAE Transactions. 105(2): 475-485.) The characteristics of the bore field must be previously defined in an Energy-Plus formatted file (IDF) exported from the GLHE-PRO 4.0 program. (GLHE-PRO is distributed by the International Ground Source Heat Pump Association through their web site.)
Note: TRACE requires both a cooling tower and a backup heating source when modeling ground source heat pumps.
In TRACE, the loop temperature is maintained between the Design enetering Condeser temp. and the minimum condenser operating temperature (typically 40°F). Thess temperatures are entered on the Options tab of the ground source heat pump in the Cooling Equipment Library:
If the ground loop temperature exceeds the design condenser entering temperature (85°F in this example) the cooling tower, located downstream of the ground loop, is started to reject the excess heat. The tower will operate until the loop temperature is drops below the design entering temperature. To reduce or eliminate the use of the cooling tower raise the condenser design entering temperature.
If the loop temperature drops below the minimum condenser operating temperature, 40°F in this example, the backup heat source is enabled. The backup heat source will operate until the loop temperature rises above minimum condenser operating temperature.
The cooling tower and backup heating source serve as indicators that additional heat rejection or backup heati is required during the simulation. If the Equipment Energy Consumption Report shows energy consumption by either the cooling tower or backup heating plant, users should review the inputs for the condenser temperatures, the capacity of the heat recovery portion of the heat pump, and the capacity of the loop itself.
STEP 8: Set the Reject condenser heat field to Ground loop.
When the Reject condenser heat field of the Create Plants - Cooling equipment tab filed is set to "Ground loop", TRACE 700 will automatically create an export file during the ENERGY calculation which contains the cooling plant's total monthly cooling and heating loads in addition to the monthly peak loads. These loads are those seen by the ground source heat pump equipment and not the loads seen directly by the ground loop.
The export file is a text file that is saved in the same location as the project (TRC) file, with the same filename as the project, but with a .GT1, .GT2, .GT3, or .GT4 file extension where the number represents the corresponding alternative.
EXPORT FILE FORMAT:
Clg/Htg Consumption (kBtu),TotalMonthlyClgLoads(1-
Clg/Htg Demand (Btuh), PeakMonthlyClgLoads(1-12),
The export file can be imported into GLHEPRO, GLD, or any other program, used to size ground wells for GSHP systems that allows data to be imported from a geothermal export file.
STEP 9: Select one of the GLHE options with the appropriate wellfield entering and leaving water temperature difference in the Thermal Storage Type field.
Note: All water source heat pumps must have a Heat Pump Loop Storage tank specified in the Thermal Storage "Type" field.
When a thermal storage type is specified, TRACE 700 generates a thermal storage report for ground-source heat pumps which is available after calculating the file in the Energy Consumption section on the Analysis tab. The Thermal Storage report provides an hourly profile for ambient conditions and plant-level cooling loads, as well as heat pump loads, heat pump energy consumption, and condenser-loop temperatures for hot and cold storage applications.
STEP 10: Enter the capacity of the loop (including the well fields) in the Thermal Storage section of Create Plants:
Here is a way that geothermal heat pump system capacitance can be estimated, assuming the Heat Pump Cooling Design DeltaT (HPCDDT):
For each ton of cooling in an hour, 12,000 Btus of heat will be rejected to the loop:
Q = M x Cp x DeltaT
Using the Cp for water (1 Btu/lb-degF):
12,000 Btu/hr (per ton) = Y gallons (per ton) x 8.34 pounds / gallon x 1 Btu / lb-degF x HPCDDT deg F
Solving for Y yields:
HPCDDT Gallons/ton
There are two library values in TRACE, but they may not be sufficient in size for the particular application. Users are encouraged to run the simulation again once the capacity has been determined. This process may require several simulations before an adequate value is determined.
G
round Loop Design Software
TRACE 700 was not designed to size well-fields for Ground Source Heat Pumps but there are several programs that can perform these calculations.
GLHEPRO is developed as an aid in the design of vertical borehole-type ground loop heat exchangers used in geothermal heat pump systems. For more details, visit http://www.hvac.okstate.edu/
Ground Loop Design (GLD), a Windows·-based geothermal HVAC software design package created by Gaia Geothermal is now available for system designers. For more details, visit http://www.GEOCLIP.com.
Note: These programs are not affiliated with, or supported by, Trane.
STEP 11: Apply your changes.
STEP 12: Click on the Controls button at the right side of the screen.
STEP 13: Click on the Cooling Plant and Geothermal Controls button at the right side of the screen.
STEP 14: Select one of the TLoop Ent Bldg options in the Plant Controls - Geothermal Loop editor. This field defines the method TRACE 700 uses to compute the monthly ground loop temperatures.
Ground Loop Temperature Calculations
TRACE is able to compute the ground loop temperature using several methods: IGSHPA , Custom or Vertical Bores.
IGSHPA (International Ground Source Heat Pump Association)
Calculates the average monthly ground loop temperature (GeoEwt, the temperature of the loop fluid entering the building after circulating in the ground) as a function of the selected weather location per the algorithms noted in Figure 3.13 of the IGHSPA manual. These values are generally representative of a shallow ground loop heat exchanger but should be used with extreme caution because the actual GeoEwt is a strong function of the soil conditions, the ground heat exchanger configuration, building loads and many other factors.
When the IGSHPA method is selected TRACE does not explicitly calculate ground-coupled heat transfer for its GSHP model. Instead, TRACE calculates an average monthly ground loop temperature (the temperature of the loop fluid entering the building after circulating in the ground) as follows:
a) At the start of the energy simulation, the following values are calculated:
*** Air temperature range
WshpTairMax = SDDB, deg F
*** Ground loop temperature range
WshpEwtMean = WshpTairMean
*** These coefficients are used to estimate the initial loop water temperature entering the building after circulating in the ground.
WshpBCoefClg = (WshpEwtMax- WshpEwtMean) / (WshpTairMax -WshpTairMean)
b) For each month, this initial ground loop temperature is then given by:
IF (DryBulbAvemo >= WshpTairMean) THEN
c) WshpEwt is not allowed to fall below the TcondDsnMin defined in the Cooling Equipment library (a mixing valve that mixes warmer return building loop WSHP water with the cooler entering ground-coupled loop water would maintain this value when necessary)
The storage tank size represents the thermal capacity of the loop fluid which affects how much the loop temperature in the building increases or decreases each hour depending on how much heat is added/removed from the loop. This hourly in-building loop temperature is used to determine the hourly performance of the heat pump. This algorithm will then iterate to determine the steady state loop temperature/heat pump performance since the two are dependent on each other.
CustomVertical Bore
For information on importing custom geothermal fields, click here.
STEP 15: Close all of the controls editors.
STEP 16: Click on the heating equipment tab at the bottom of the Create Plants editor.
STEP 17: Select a boiler.
Note: Trace requires a backup heating source for all water source heat pumps.
STEP 18: Enter the full-load energy consumption for the circulator pump.
STEP 19: Close the create plants editor and assign the heating and cooling coils to their respective plants.
GROUND SOURCE HEAT PUMP (GSHP) OVERVIEW
A common variation of the water-source heat-pump system uses earth as both a heat source and a heat sink. Known as a ground-source (GSHP) system, this variation takes advantage of the relatively constant 45°F to 65°F temperatures that exist 20 to 30 feet below the surface. When buried in the ground, the high-density thermal plastic pipe acts as a heat exchanger. During the cooling season, it transfers the heat absorbed by the heat pumps to the ground for storage. When heating is needed, the heat exchanger recaptures the heat from the ground and returns it to the building.
If properly designed, an applied GSHP system does not require a cooling tower. Aesthetically, this means that all of the heat from the building can be rejected without any visible sign of a cooling system. A boiler is also unnecessary if the heat pumps can satisfy the entire heating load, which saves initial cost and floor space.
ADVANTAGES & DISADVANTAGES
- GSHP systems are more expensive to install, but less expensive to operate, than conventional water-source heat-pump systems. Perform a life-cycle cost analysis to determine the economic viability for a particular application.
- Installation requires excavation, trenching, or boring by a qualified contractor.
- Ground-coupled loops can be installed in a horizontal, vertical, or spiral configuration. Available land, soil conditions, and excavation costs will determine the appropriate choice for a given application.
Disclaimer: Content above sent from Trane Trace Support in 2012
REFERENCES
Tutorial from my Penn State Architectural Engineering Thesis Research
Alex Quercetti
Trane Trace Support