Finding Commercial Peak Load (CPL)
How to determine the maximum commercial peak load using the Find Loads algorithm in the TRNSYS Tool.
The commercial scheme allows the CPL to be scaled up to match the system’s actual capacity. Unlike the residential schemes — where the load is fixed at 42 MJ/d — a commercial product can claim a much higher load provided it still delivers all water above 45 °C and achieves at least 60 % energy savings. Finding the maximum CPL that still satisfies both criteria is the central modelling task.
How the Find Loads algorithm works
The Find Loads macro automates a binary search across CPL values. It runs two iterations:
Iteration 1 — coarse search. Starting from an initial CPL guess, the macro runs 9 simulations spaced by CPL Scatter #1 (e.g. 5 MJ/d apart) to bracket the crossover point — the CPL at which MDT first drops below 45 °C. After this iteration, the optimal CPL is known to within one scatter interval.
Iteration 2 — fine search. The macro re-centres on the bracketed region and re-runs 9 simulations spaced by CPL Scatter #2 (typically 1 MJ/d) to resolve the crossover to the nearest 1 MJ/d.
As CPL increases, MDT decreases and energy savings increase. A higher load keeps the tank cooler on average (higher COP), but delivers water at lower temperatures near peak — so the binding constraint is always MDT, not savings.
The result is a table of CPL vs. MDT and savings for each zone. The maximum CPL is the highest value where MDT ≥ 45 °C and savings ≥ 60 %.
Example — two-iteration result:
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Iterations 1–2 with CPL Scatter #1 = 5 MJ/d and CPL Scatter #2 = 1 MJ/d. Maximum CPL = 74 MJ/d (MDT = 45.1 °C, savings = 69.8 %).
Running Find Loads
1. Set initial CPL guesses
Enter an initial CPL estimate for each of the three climate zones (Z3, Z4, Z5) in the ComPeakLoad cells of the FAM_COM sheet.
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Initial CPL guesses for each zone.
CPL is typically highest in Z3 (Sydney) and lower in Z4 and Z5, due to warmer inlet water temperatures and higher ambient temperatures in Z3.
2. Set CPL scatter
Configure CPL Scatter #1 and #2 in the FAM_COM sheet.
- For smaller commercial systems (CPL ≈ 60–150 MJ/d): Scatter #1 = 5–10 MJ/d, Scatter #2 = 1 MJ/d.
- For larger systems: Scatter #1 may need to be 50–100 MJ/d. Use judgement based on the expected CPL range.
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Example CPL scatter settings.
3. Click Find Loads
Click the Find Loads button to run the macro. The macro runs 9 simulations per iteration across 3 zones — allow 1–2 hours to complete.
To check progress while the macro is running: click on the Excel sheet, hold Esc, click Debug to pause in the VBA editor. Switch back to Excel and open the CPL_[ModelName] sheet to review completed simulations. Click the green play button to resume.
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4. Verify the result
Once the macro finishes, open the CPL_[ModelName] sheet and confirm the maximum CPL for each zone. Cross-check that the values written back to the ComPeakLoad_Z3–5 cells in FAM_COM match.
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Final CPL solution sheet showing maximum CPL per zone.
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ComPeakLoad cells in FAM_COM updated by the macro.
TRNSYS files for each CPL trial are saved to C:\TRNSYS15\Include&weather\AExxx_Client\CPL_Calibration\. Once Find Loads completes, the final simulation files are copied automatically to C:\TRNSYS15\Include&weather\AExxx_Client\FAM_COM\.
Once CPL is confirmed, proceed to Review Results to verify MDT, energy savings, and certificate quantities in the Com_Results sheet.