Part 3 – TRNSYS Modelling Guidelines
Division A – Introduction
This Part sets out the TRNSYS Modelling Guidelines, which must be adhered to in using the methods in Parts 1 and 2 of this Schedule. The TRNSYS Modelling Guidelines specify the format and structure required for the TRNSYS deck files, and the default values to be used in certain circumstances.
Division B – TRNSYS Decks
Deck layout
All include files shall be at the top of the deck immediately after the SIMULATION statement and the timestep shall be 0.02 h or less
The chosen time step shall be an exact divisor of the load draw off period.
The TOLERANCE statement shall be 0.005, 0.005 or less for relative tolerances.
Each load draw-off event shall be applied over a period of 0.1 h for small SWHs and ASHPs and 1 h for large SWHs.
Use the template deck structure in the version of the document titled CER template decks published by the Clean Energy Regulator, unless the Clean Energy Regulator approves in writing. Except for the product parameters, the template decks shall be used without modification unless the product has features not covered by the example decks. There are different template decks for small SWHs, ASHPs and large SWHs.
Note: The document ‘CER template decks’ could in December 2016 be viewed on the Clean Energy Regulator’s website (http://www.cleanenergyregulator.gov.au/RET/Pages/Forms and resources/Forms-and-resources-for-manufacturers.aspx).
The include files provided by the Clean Energy Regulator shall not be modified.
All constants shall be towards the top of the deck.
The output, list and other output file names shall be the same as the deck name except for the file type.
The output printers in the example decks shall not be modified. Additional printers may be used if required.
Simulation display
Simulation output: A TYPE 25 printer unit shall be included in the deck. This shall output the following results: the zone, load size, percent energy savings, STC rating and annual energy delivered below 45°C.
Energy balance: A monthly energy balance in a TYPE 28 output unit shall be included in the deck. This shall include a full system energy balance i.e. collector input + boosting + pump input – load – pipe losses – tank losses – energy dumped.
Runtime graph: A runtime graph (TYPE65) shall be included in each deck showing at least:
o Hot water delivery temperature
o Solar collector flow rate
o Solar collector inlet temperature
o Solar collector outlet temperature
o Load
o Similar variables shall be plotted for heat pump water heaters.
Solar collector area
The area specified for the solar collector (absorber, aperture or gross) shall be the same as the area used for the specification of the solar collector efficiency coefficients.
Controller default settings
If the maximum tank temperature setting of the pump controller used for no-load system operation test (clause 7.4.3.2 in AS/NZS 2712:2007) is not specified in the AS/NZS 2712:2007 test report, then a maximum value of 65°C shall be used (Tmax = 65oC and Tmax_reset = 60oC shall be used in the modelling).
For a simple temperature difference pump controller the minimum turn off temperature difference shall be 1 K.
Piping models
All piping shall be modelled using the TYPE 31 pipe routine.
Instantaneous gas heater defaults
If an instantaneous gas booster has not been assessed for electrical power consumption during standby under AS 4552-2005, then a value of 10 W shall be used. If the electrical power consumption during burner operation is not available, then a value of 50 W shall be used.
If the startup heat loss has not been assessed under AS 4552-2005, then a default value of 0.5 MJ shall be used.
AS 3498-2009 requires water heating to minimum of 70oC for a storage solar preheat system with an inline instantaneous gas booster when the solar preheat tank temperature is less than 55oC (clause 7 (j) (vi)). If a gas booster trigger temperature (≥ 55oC) less than the gas booster set point temperature (≥ 70oC) is used, then specifications for the gas booster control logic shall be provided. If specifications for the gas booster control logic are not provided, the trigger temperature used for rating purposes shall be equal to the gas booster set point temperature at all times.
Stratification option for pumped circulation tanks
The default stratification specification for a pumped circulation storage tank shall be “Uncontrolled flow pumped circulation” (AS/NZS 4234:2008 clause 3.7.4 or H3.4). For pump circulation SWHs satisfying the "Low flow criteria" (AS/NZS 4234: 2008 clause 3.7.2 or H3.2) the tank thermal stratification shall be modelled as per AS/NZS 4234:2008 clause 3.8.2 or H4.2, if:
i. for solar preheat tanks, the collector flow return to the tank is in the top 2/3 of the tank; or
ii. for in-tank electrically boosted solar tanks, the collector flow return to the tank is in the top 2/3 of the tank and the solar return is below any electric element; or
iii. for tanks connected to stand alone heat pumps, the heat pump flow return to the tank is in the top 2/3 of the tank.
For booster tanks in large SWHs where the water temperature is boosted with a separate pumped recirculation loop “Uncontrolled flow pumped circulation” shall be assumed unless:
i. the return from the recirculation booster loop to the tank is in the top 2/3 of the tank; and
ii. the recirculation flow rate is below 1 L/min per kW thermal power added by the external booster; and
iii. the booster tank recirculation water turnover rate is less than 1 tank volume per hour.
Effective air temperature
The effective air temperature used to determine solar collector and piping heat loss during night time (horizontal irradiation < 1 kJ/h.m2) shall be Taeff = Ta – (Ta –Tsky)/5, where Ta denotes the ground level air temperature and Tsky the sky temperature as defined by AS/NZS 4234:2008.
The effective air temperature used to determine solar collector and piping heat loss during day time (horizontal irradiation ≥ 1 kJ/h.m2) shall be Taeff = Ta.
Dip tubes
Note for a tank with dip (or snorkel) tube, the heat transfer between the stored tank water and the water flowing inside the dip tube usually is small compared to the heat capacitance rate of the water flowing through the tube. In this case the temperature of the water supplied to the collector inlet pipe (or the load) shall be the tank temperature at the dip (or snorkel) tube inlet. Evidence of any dip (or snorkel) tubes must be included in tank drawings supplied for system audits.
Height parameters for thermosyphon SWHs
Thermosyphon145HeightsDefnV3
Note: If a dip tube is used on the cold inlet port then the measurements Hsep and Hcold are relative to the level of the outlet of the dip tube.
Units
The units system used in TRNSYS is metre, kg, h and kJ. All energy transfer rate parameters must be converted from W to kJ/h (multiply Watts by 3.6 to get kJ/h).
Rounding of numerical results
The printing of STC and Energy Savings % outputs shall use the format F5.1 as specified in the example decks. This means that an STC rating of 20.94 will be rounded down to 20.9 in the printout and this will be taken as STC = 20. An STC rating of 20.96 will be rounded up to 21.0 in the print out and this will be taken as STC = 21.
Division C – Measurement or analysis for TRNSYS modelling parameters
Site specific pump flow rate setting
Pumped circulation SWHs are classified as “controlled flow” or “uncontrolled flow” systems in clause 3.7.2 of AS/NZS4234:2008.
If a pumped SWH is claimed to be a “controlled flow” system, then documentation demonstrating how the flow rate is controlled for every installation shall be provided. The alternative product configurations that shall be detailed in the installation manual are:
i. Flow meter in every installation
If a flow meter is fitted in every installation with an adjusting valve that is not easily accessible by the user then provide a copy of the flow meter specification and the SWH installation manual detailing the flow setting procedure. The pump power use shall be measured as detailed below.
ii. Flow meter used during commissioning
If a flow meter is fitted in every installation and the flow rate set by an adjusting valve that is not easily accessible by the user and the flow meter is removed after commissioning, then provide a copy of the flow meter specification and the SWH installation manual detailing the flow setting procedure. The pump power use shall be measured as detailed below.
iii. Flow rate set by controller
If the pump flow rate is set by a controller based on a real time signal from a flow meter or the temperature rise in the collector circuit or by another real time signal, then provide a copy of the controller logical functions and provide measured flow rate and pump electrical power as detailed below.
iv. Orifice restrictors
If a set of orifice restrictors are used to set the flow rate for installations with different collector loop piping lengths and number of collectors then provide a copy of the installation manual detailing how the appropriate orifice is selected on the basis of the site specific piping lengths and number of solar collectors. The pump flow rate and power use shall be measured as detailed below.
v. Other site specific flow control method
Provide details if applicable.
Measurement and documentation of pump flow rate and power
For small SWHs the collector loop flow rate and pump electrical power input to be used in the method must be measured for a typical system installation as specified in Step 2.h. of the method in Division B of Part 1.
For large SWHs the collector loop flow rate and pump electrical power input to be used in the method may be either measured or calculated for a typical system installation as specified in Step 2.g. of the method in Division B of Part 2. These measurements or calculations shall be carried out for the specific collector array size or recirculation loops that are specified for the product.
Measurements or calculations for products with variable flow rate shall cover the full range of operating conditions.
A report must be prepared setting out the measurement or calculation of pump flow rate and power. The report must include:
i. a description of product configuration tested, including collector model, number of collectors, series or parallel connection, pump model, piping length and diameter; and
ii. a detailed description of the measurement or calculation procedure used, including schematic diagram or photograph of setup; and
iii. (if measured rather than calculated) measuring equipment used; and
iv. (if measured rather than calculated) results for flow rate and pump electrical power input.
The Clean Energy Regulator has published a procedure that may be used to calculate pump flow rate and power consumption for large SWHs, titled Guide for Calculating Pump Flow Rate and Power Consumption for Large Solar Water Heaters. For the avoidance of doubt, use of the procedure is not mandatory.
Note: The guide could in December 2016 be viewed on the Clean Energy Regulator’s website (http://www.cleanenergyregulator.gov.au/RET/Pages/Forms and resources/Forms-and-resources-for-manufacturers.aspx).
Division D - Notes
Pump cycling and simulation stability
For pumped SWHs the pump controller may cycle the pump on/off if the ratio Ton/Toff is too small. See Duffie & Beckman, "Solar Engineering of Thermal Processes", Section 10.4 Controls for more detail [1] and how to calculate the required Ton/Toff ratio to avoid pump cycling.
Modelling an in-tank coil heat exchanger for the solar circulation loop using the Type 60 tank model can lead to convergence problems in TRNSYS 15, particularly for glycol-water mixtures. Using water as the HX fluid instead and/or artificially increasing the collector flow rate may overcome this problem. Changing the heat transfer fluid and/or flow rate will introduce an error, and the increase in flow rate should be kept as small as possible.
Batching multiple TRNSYS rating Calculations
Running multiple TRNSYS decks can be automated by using a Windows batch file. For example, four decks (deckfilenameZ1 to deckfilenameZ4) can be run successively by writing the following lines into a Notepad file (note the spaces) and saving the file with .bat extension.
C:\Trnsys15\trnsys.exe deckfilenameZ1.dck /N
C:\Trnsys15\trnsys.exe deckfilenameZ2.dck /N
C:\Trnsys15\trnsys.exe deckfilenameZ3.dck /N
C:\Trnsys15\trnsys.exe deckfilenameZ4.dck /N
Right-clicking the batch file allows to one to edit it, left clicking or double clicking will run it.
References
[1] Duffie, J. A. and Beckman, W. A., Solar Engineering of thermal processes, 3rd ed., Wiley, New York, 2006.