OpenStudio WSHP Condenser Loop Cooling Tower Issues

I am working on a OS model that has zone WSHP in a residential high rise (500,000 sf). On the condenser loop there is boiler and a cooling tower keeping the loop temp between 50-90F. The loop is set up with the boiler and cooling tower is series with a SP of 50F after the boiler and a SP of 90F after the cooling tower.

When I use a variable speed cooling tower the cooling tower doesn't keep the condenser loop below 90F and I get a ton of cooling hours not met. I have used all three variable speed cooling towers in OS (default, Cool Tools, and York) and I get the same result.

I have had this issue before and have been able to just use a single speed cooling tower and the loop is kept at the setpoint and my unmet hours go away. For some reason with this model, when I use a single speed cooling tower I get the following error :

* Severe * CoolingTower:SingleSpeed "1 SPD COOLING TOWER". Tower performance input method requires valid nominal capacity.

The nominal capacity is not an editable option for the cooling tower in OS. I know I can write a script to hard enter the nominal capacity, but I also want to know why this isn't working. I also am curious why the variable speed cooling towers are not working. Has anyone run into these issues? Does anyone have an idea on how to fix them?

Thanks

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TaylorRoberts

asked 2014-10-20 09:58:44 -0500

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Hey Taylor, I was having the same problem a few months ago. In my case, the problem had to do with the condenser loop being sized incorrectly.

Take a look at your design temperature parameters for the loop. In particular, the "Design Loop Exit Temperature" and "Loop Design Temperature Difference".

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Lincoln

answered 2014-10-20 11:16:24 -0500

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Hey Lincoln, do you know what you set the delta T to? I have it at 10R right now and the design loop exit temp to 75F and I am still having the same issue.

Thanks

TaylorRoberts (2014-10-20 15:16:33 -0500) edit

10 R seems reasonable. Do you have a setpoint manager on the loop exit node? Have you tried a fluid cooler?

Here's another idea if you're still at a deadend. Try placing the boiler and cooling tower in parallel instead of series and using a "Setpoint Manager Scheduled Dual Setpoint" on the loop exit node. This is how I've modeled WSHPs in the past but it was before OS had the dual setpoint manager so I had to create a script to inject the necessary E+ objects. Now that OS supports the dual setpoint manager it may be possible to do it without having to write a script but I haven't tried yet

Lincoln (2014-10-20 17:27:02 -0500) edit

Hey Lincoln, I have used a fluid cooler and it does work, which will be my backup if I can't get the loop working. I will try setting it up in parallel with the dual setpoint manager and see if that works.

TaylorRoberts (2014-10-22 09:01:33 -0500) edit

Setting the cooling tower and boiler in parallel with the dual setpoint manager did the trick.

Thanks Lincoln

TaylorRoberts (2014-10-22 09:14:18 -0500) edit

@TaylorRoberts for your final setup did you have a setpoint manager on the boiler and cooling tower as well as the dual setpoint on the outlet node? I have a very similar set up as yours except I have a heat exchanger with a HW loop to keep the condenser loop warm enough, but my temperatures seem to be going out of range.

pflaumingo (2015-04-14 18:03:01 -0500) edit

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I don't think you want your loop temp to be 75F, and you'll need a larger delta T than 10R ... this is the loop temperatures, not the space temperatures (space design temps are assigned elsewhere). What you need to assign is the temperature of the LOOP exit temperatures... probably in the range of 55F since it's a cooling loop, and a delta T of 20-40 F.

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Benjamin

answered 2014-10-20 16:21:58 -0500, updated 2014-10-20 16:23:26 -0500

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Hey Benjamin, according the the mechanical schedule for this project the WSHP are sized with a 50F entering water temperature and a 44F leaving water temperature during heating. During cooling a 80F entering water temperature and a 90F leaving temperature. So it has a 6 degree delta during heating and a 10 degree delta during cooling. It is also a condenser loop, not a cooling loop.

TaylorRoberts (2014-10-20 16:55:10 -0500) edit