operating air-to-water heatpump at low temperatures ...?

asked 2015-09-26 05:34:59 -0500

I would like to simulate an air-to-water heatpump for DHW and heating with inlet air = outdooor air at temperatures ranging from appr. -20°C up to +25°C. For this purpose I tried WaterHeaterHeatPump with user-defined performance curves by DataTable:TwoIndependentVariables objects for biquadratic approximations of (relative) heating capacity and COP as functions of outdoor air temperature and water inlet temperature.

However, I did not succeed in making it run without fatal error messages, such as temperature(s) out of range etc.

Maybe the current air-to-water-heatpump object does not allow at all operating conditions with outdoor temperatures below freezing point ?

Is there any work-around ? Are there any plans to implement suitable air-to-water heatpump models for cold climates in future releases ?

answered 2015-09-28 13:16:31 -0500

aparker

8107 ●4 ●18 http://www.openstudio.net/

The EnergyPlus.IDD file currently defines the minimum compressor operation temperature as 5C . (This file lives in the same directory as the EnergyPlus executable). If I were you, the first thing I'd try is modifying this file to a lower value that is more realistic for your particular heat pumps. I think this minimum limit was established before cold climate heat pumps. I have encountered the same issue with air source heat pumps.

  WaterHeater:HeatPump,
   ...
   N4 , \field Minimum Inlet Air Temperature for Compressor Operation
   \type real
   \units C
   \minimum 5
   \default 10
   \note Heat pump compressor will not operate when the inlet air dry-bulb temperature
   \note is below this value.

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Comments

THANKS, modifying the IDD settings as suggested worked. However, simulations produce huge numbers of warnings: Calculated outlet air relative humidity greater than 1...(fatal error when autocalculating water flow rate, air flow rate)

After adjusting rated air volume flow rate and sensible heat ratio:

*minimum inlet air temperature ...must be greater than or equal to 5C.
*Rated air volume flow rate per watt ...is out of range...
*Air volume flow rate per watt ... is out of range
*full load outlet air dry-bulb temperature < 2C

ANY MORE RECOMMENDATIONS ?