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Heat Pump Water-to-Air Curves - error on coefficient calculation

asked 2018-09-13 12:51:00 -0500

mespinoza's avatar

updated 2018-09-13 18:46:16 -0500

In order to create the cooling performance coefficients for a WSHP, I used the Parametric Spreadsheet, as suggested here by Kent Beason. I added the cooling performance data from the manufacturer for the specific heat pump (GEV-024) and followed the instructions for the spreadsheet.

However, the values in the coefficient calculator tab were not reasonable, resulting in negative capacities and errors >100%. The instructions don't give any feedback on what to do if these errors are large. Has anyone experienced this or know what could be causing these results?

The manufacturer data assumes entering air at 80.6°F DB/66.2°F WB and a constant airflow for all - do I need more variability in the data? Also, the spreadsheet does not have R-410a as an option for refrigerant - could this cause issues with the calculation?

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answered 2018-09-13 14:38:43 -0500

updated 2018-09-13 14:47:42 -0500

Regressing manufacturers data requires "sets" of data at varying conditions (e.g., inlet air dry-bulb and wet-bulb temps and inlet water temps). If only a single inlet air dry-bulb and wet-bulb temp is used then the coefficients for some terms will be 0 and the resulting performance curve will be valid only at that specific dry-bulb and wet-bulb temperature. You need to find more data at other inlet air dry-bulb and wet-bulb temperatures.

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The source-side temperature (EWT) does vary, but the return air temperature from the room does not. The performance data available from the manufacturer is for ASHRAE test conditions. In order to get other data points, we would need to contact the manufacturer to see if they also tested the equipment at non-standard conditions (which is not typical). Is there another way to generate reasonable performance curves from standard published data?

mespinoza's avatar mespinoza  ( 2018-09-13 15:21:39 -0500 )edit

You might try finding other WSHP data and compare to the data you have. Plot capacity/power vs entering water temperature at discrete coil inlet air wet-bulb temperatures. A full data set will have multiple curves representing capacity vs water temp at various inlet air wet-bulb temps. If your data looks similar to the more robust data set you can use that data set. If not, then you might be able to expand your data set based on this comparison (this is risky since you don't actually know how performance changes for your system). I suggest contacting the manufacturer, they should have data.

rraustad's avatar rraustad  ( 2018-09-14 07:58:10 -0500 )edit

answered 2021-04-21 17:16:10 -0500

After doing some testing on this spreadsheet today and then coming across this post, I think the main issue is that it is still calculating coefficients for linearly dependent (constant across the data set) variables, as opposed to leaving those variables out of the matrix calculations and setting the related coefficients to 0 as Richard said above (which is what should happen). This, in turn, causes all of the coefficients be erroneous since they are solved for simultaneously.

So what this means, as Richard implies, is that this spreadsheet is limited to scenarios where data is available with variation (linearly independent) in all relevant variables, which are: Qtot - Twb, Tw, Vair, Vw; Qsens - Tdb, Twb, Tw, Vair, Vw; Power - Twb, Tw, Vair, Vw. Therefore, if any of your variables are linearly dependent, the coefficient sets that are produced will not be realistic, as the OP saw in error comparison.

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Asked: 2018-09-13 12:51:00 -0500

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Last updated: Apr 21 '21