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DHW Recirculation in EnergyPlus

asked 2018-02-08 09:14:02 -0600

Amedeo's avatar

updated 2018-05-21 21:47:07 -0600

Hi guys, I have to model a DHW loop with a recirculation system in EnergyPlus but I encountered a big problem while modeling this situation. Below you can see the features of my problem:

1 - Real situation:

  • Water from the mains at 10°C enters a stratified tank, being heated to 70°C;

  • Water from the recirculation loop at 55°C bypasses the tank, then is mixed with water at 70°C and 10°C to obtain the requested flow at 60°C for the users.

2 - E+ model:

  • (V) Requested flow for the users is still at 60°C;
  • (V) Water outlet temperature from the tank is still 70°C;
  • (X) Water inlet temperature is between 10°C and 55°C because E+ mixes recirculation and mains flows just after the users component to have a correct mass balance on the demand side of the loop.

Recirculation and mains flows mixing before the inlet node of the tank creates a problem for me because I want to evaluate water temperature in all the tank's layers and obviously the inlet water temperature greatly affects this aspect of the system. So I ask you if there is a way to model the DHW loop without having this mixing of the two flows of water.

EDIT: to help understanding my situation here it is the plant scheme I have to model:

  1. The recirculation loop is essentially the orange rectangle with inlet temperature of 60°C and outlet one of 55°C (heat loss is present due to considering non adiabatic pipes);
  2. Instead of what I have said before (my fault) water from the recirculation loop doesn't always bypass the tank, in fact it can be reheated if necessary to mantain the setpoint temperature of 60°C for the users.

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@Amedeo Could you explain what is the Recirculation loop consist of? Maybe diagram will clarify it better?

Avi's avatar Avi  ( 2018-02-11 03:01:39 -0600 )edit

@Avi The recirculation loop is a plant's feature to guarantee istantaneous hot water to the farthest users from the tank. In fact in absence of recirculation the further you are away from the tank the more you have to wait to have hot water (because it have to go through all the pipes between the tank and the user). You can see in the diagram I uploaded in the post that in presence of recirculation loop hot water must only go through the derivation when requested with a consequent reduction of waiting time.

Amedeo's avatar Amedeo  ( 2018-02-14 04:20:32 -0600 )edit

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answered 2018-02-14 03:30:03 -0600

This is a non answer, saying you can't do it (hope to be proven wrong)...

You might be tempted to use the TemperingValve object. Place your stratified tank and your tempering valve on two parallel branches on the supply side of the plant loop, and you should only have two parallel branches, no bypasses are allowed.

Your WaterHeater:Stratified has a setpoint temperature of 70°C (mind the deadband too), while the supply outlet node of the plant loop has a SPM:Scheduled at 60°C.

The problem is that - as your correctly identified yourself - the cold water will always be introduced at demand side outlet of the plant loop (source: here, looking at source code it's actually the outlet node of the WaterUse:Connections object that gets the update).

I have never tried setting the WaterUse:Connections outlet node name to something else than a node on the same branch as its inlet node, but I'm pretty sure that won't work.

As far as I know, there is no way to divert all cold water from the mains directly into your tank.

I assume your situation is similar to one where your Stratified tank is being heated by a condensing boiler on the source side and you want to capture the lower temperatures at the bottom of the tank to properly account for a lower return temperature to the boiler (I know I do too!), but currently you can't out of the box (sure, if you invest enough time into EMS you might get to something that does the job...). This would make a good user voice request, so I suggest you check if there is one already, if not add it and provide the link here, I for one would surely upvote it.

Note: It goes without saying, but modeling the recirculation loop has an effect only if you use heat dissipation on it (a pipe that isn't adiabatic), and more importantly force the plant loop to circulate a given amount of water at all times and not just the appropriate flow for the load corresponding to the water equipment on the demand side (or you better have a SPM on the supply outlet that is higher than the temperature requested at the water equipment).

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Thanks for you answer! As you correctly said recirculation loop has effect only in presence of non adiabatic pipes and that's my case, because I have to model the DHW distribution on a cruiser ship where there are long distances between the tank and the users. About modifying the WaterUse:Connections nodes I also believe it won't work because E+ is very strict about his use and conception of nodes. Probably, as you said, a solution is to work with EMS but I think it takes a lot of time to do it. I didn't know about user voice requests, I will give it a try, thanks!

Amedeo's avatar Amedeo  ( 2018-02-15 03:41:26 -0600 )edit

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Asked: 2018-02-08 09:03:02 -0600

Seen: 680 times

Last updated: Feb 15 '18