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HPWH warning when using Create Typical Building From Model OpenStudio measure

asked 2020-01-15 11:06:33 -0500

pajordan's avatar

updated 2020-01-15 11:33:21 -0500

I've just used the "Create Typical Building From Model" OpenStudio measure on BCL and set the Service Water Heating Source option to HeatPump; after applying the measure I noticed this funny warning:

Warning: [openstudio.model.Model] Simple but crappy workaround to represent heat pump water heaters without incurring significant runtime penalty associated with using correct objects.

Does anyone have more insight into this warning, i.e. what is the workaround being used and what would be the "correct" objects to use? The measure added an OS:WaterHeater:Mixed object with a Part Load Factor Curve Name of HPWH_COP_2.8.

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answered 2020-01-15 16:07:42 -0500

updated 2020-01-15 16:09:05 -0500

The correct way to model in-unit HPWH is with a stratified tank model that exchanges air with the zone it is in. Until the most recent release of EnergyPlus, the stratified tank model was agonizingly slow, making it nearly unusable for the sort of parametric analysis one may want to do with the "create typical building" measure. The most recent EnergyPlus version is a little better, and we may update openstudio-standards to use the stratified tank model in the future.

The workaround in openstudio-standards is to instead model a HPWH as an electric water heater, but adjust the part load efficiency curve with a constant efficiency equal to rated annual HPWH COP. So it runs as fast as a basic mixed electric water heater, but accounts for the annual COP difference. This is fine for most modeling projects. However, if you want to model the dehumidification, cooling, and heating impacts of a HPWH in your zone, you need the full stratified tank model. There is a separate residential BeOpt measure which adds it if you are interested.

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Much thanks for the info!

pajordan's avatar pajordan  ( 2020-01-15 16:21:03 -0500 )edit

Correction: the most recent EnergyPlus version is a lot better. The latest stratified tank model is 10x faster than the original model.

shorowit's avatar shorowit  ( 2020-01-15 22:06:31 -0500 )edit

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Asked: 2020-01-15 11:06:33 -0500

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Last updated: Jan 15 '20