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Chiller EIR higher than expected

asked 2019-05-04 07:52:56 -0500

HJOJOA gravatar image

updated 2019-05-06 13:52:11 -0500


In a single air cooled chiller project, REF capacity is 320.7 KW at 35 C outdoor temp and 10C chilled water outlet temp, a set of data was used to produce curves to predict chiller power consumption.

The curve coefficients are (for SI):

C:\fakepath\Chiller coeff.JPG

Using excel, hourly data was manually calculated, at the same time EP simulation was done using same data, checking the variable reports, the hourly EIRFT, EIRFPL, CAPFT and PLR are the same:

Variable report from EP C:\fakepath\ENERGYPLUS REPORT.JPG

Manual calculation C:\fakepath\MANUALLY CALCULATED.JPG

But when i compare the chiller compressor power there is a difference, for example at 17,2 C outdoor temp and 251.23 Kw of cooling requirement, the results are:

In EP result Chiller compressor power is 49.8 Kw

In EXCEL result Chiller compressor power is 32.4 Kw

In Chiller data report (from manufacturer) compressor power is 32.5 at partial load (similar to 32.4 Kw in EXCEL result) C:\fakepath\CHILLER PERFORMANCEM320KW.JPG

When i check i notice that the 49.8 Kw in EP report is for a chiller that produce the maximum cooling capacity at corrected conditions but seems is not reduced by PLR because now is producing partial cooling and not total cooling, if you multiply 49.8 * 0.65 (the actual PLR) the result is 32.39 Similar to Excel manual evaluation.

For sure the EP COP is 5 and according Manufacturer data the COP should be 7.7 at this partial load condition similar to manual calculation.

Can some body explain why the compressor power results in EP is like this (higher) and not reduced by the PLR? this difference produce high annual energy consumption than calculated manually.


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answered 2019-05-06 12:12:35 -0500

Assuming you're using a Chiller:Electric:EIR object, are you specifying a Condenser Fan Power Ratio? If so, the condenser fan energy is being added to the total cooling electricity. If the Condenser Fan Power Ratio is zero, the evaporator energy divided by COP should equal the cooling electricity. Try outputting chiller COP and and see if that matches your manual calculation.

I would also suggest using energy variables [J], rather than power variables [W]. The power variables are averaged over your timestep, whereas the energy variables are the total energy accumulated over the timestep.

Output:Variable,*,Chiller Evaporator Cooling Energy,hourly; !- HVAC Sum [J]
Output:Variable,*,Chiller COP,hourly; !- HVAC Average [W/W]
Output:Variable,*,Chiller Part Load Ratio,hourly; !- HVAC Average []
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Hi, thanks to take time to review and help me, yes the chiller is ELECTRIC EIR, but i have tried different options, Condenser fan power ratio 0 and condenser fan power ratio with value, in every case use the right COP, but the result is the same, the power consumption is higher but not only with manual calculation also higher than manufacturer data, also i noticed when you use fan power ratio the fan power is always constant, can you tell me how to put this value to vary with capacity?

HJOJOA gravatar imageHJOJOA ( 2019-05-07 08:55:17 -0500 )edit

And when you multiply the electric power calculated in EP by partial load, the result is very similar to manufacturer data and manual calculation, i don't know why, also i changed the time steep to 1 to consider your statement about average data, and the result is the same, if you multiply the electric power result in EP by partial load the result is very similar to manufacturer data.

If you taste manual the EP result, multiply Reference capacity/Ref COP * CAPFTPLREIRFT*EIRFPL the result is similar to manufacturer data.

HJOJOA gravatar imageHJOJOA ( 2019-05-07 09:02:40 -0500 )edit

Also i have made a new simulation with HAP and the hourly simulation result is similar to manual calculation

HJOJOA gravatar imageHJOJOA ( 2019-05-07 11:16:40 -0500 )edit

The condenser fan power is always constant if you input a Condenser Fan Power Ratio. The condenser fan power is always equal to the ratio multiplied by the chiller's rated output capacity. A better way to model the condenser fan power is to include this power in the chiller curves. Does your manufacturer's data include condenser fan power? If so, leave the Condenser Fan Power Ratio field blank or zero.

ericmartinpe gravatar imageericmartinpe ( 2019-05-07 11:31:33 -0500 )edit

Going back to my first comment, I was referring to the COP at each timestep, not the rated COP. You can output the COP that the chiller is operating at each timestep using the output variable below. The evaporator energy divided by the COP at a given timestep should equal the chiller electricity (with zero condenser fan power ratio). Compare the COP at each timestep with your manual calculation.

Output:Variable,*,Chiller COP,hourly; !-  [W/W]
ericmartinpe gravatar imageericmartinpe ( 2019-05-07 11:34:19 -0500 )edit

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Asked: 2019-05-04 07:52:56 -0500

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Last updated: May 06