A computationally efficient hybrid time step methodology for simulation of ground heat exchangers

Validation of vertical ground heat exchanger design methodologies

“…Firstly, the Handbook equation uses a constant load throughout the course of operation, with a magnitude equal to the average value, plus a peak "block load" for which no guidance is given on how to determine magnitude or duration; this peak block load is also coupled with the part-load factor for the design month to arrive at the value for the monthly thermal pulse. The design tool, on the other hand, uses a monthly time step, with loads input as monthly totals plus a monthly peak for both heating and cooling; Cullin (2008) describes how the peaks are selected in terms of magnitude and duration, while Cullin and Spitler (2011) show that this method of load representation performs very well, predicting design depths within ±7% of what would be predicted using an hourly time step. First, the loads used with the simulation-based design tool were modified to more closely match the Handbook-style load representation by inputting a single average load applied every month plus a single peak load applied for 6 hours.…”

“…In any case, Cullin and Spitler (2011) have shown that going from a pure hourly simulation to a hybrid monthlyplus-peak-hours time step introduces a difference of roughly 7% into the design length over the course of a multi-year simulation. So, then, the simulation-based design tool is essentially as accurate as could be reasonably expected, given the limitations of a non-hourly simulation as well as experimental uncertainty.…”

“…Loads are computed directly from the experimental data using the measured flow rate and temperature differential across the GHX; these loads are computed for each measured data point, and then processed into a monthly total/monthly peak format consistent with the hybrid time step procedure described by Cullin and Spitler (2011). For three of the systems, multiple years of data are available.…”

“…As discussed above, for each system, some portion of the measured load data was chosen to make up the representative year used for the design methods. From these portions of the data, the average total monthly load was used for each month of the year, while the peak load magnitude and duration were determined following the load analysis procedure of Cullin and Spitler (2011) to assure that the selected peak representation matches an hourly simulation as closely as possible. Tables 2 through 5 show the simulation-based design tool load inputsÑboth total for the month and peak, for both heating and coolingÑfor each location; the month number '1' corresponds to the first month of operation, which is not necessarily January in each case.…”

“…Many design and energy analysis tools (e.g., Hellstršm et al, 1997;Fisher et al, 2006;Liu and Hellstršm, 2006) rely on the g-function approach first developed by Eskilson (1987); one of these tools (Spitler, 2000) will be analyzed here. This tool, also described in some detail by Cullin (2008), is a ground heat exchanger simulation tool that operates on a hybrid time stepÑmonthly average plus hourly peak, as described by Cullin and Spitler (2011)Ñand is widely used for system design due to its quick computations compared to other hourly simulation methods.…”