How to specify where the water main connects in a plantloop?

asked 2018-01-16 15:46:24 -0600

mldichter
2575 ●2 ●11

updated 2018-01-26 17:40:07 -0600

BEGIN SECOND EDIT
Made progress on figuring this out. By setting an unusual water mains temperature in the Site:WaterMainsTemperature object so we could look for that temperature on various nodes, water heaters replenished on the use inlet node of a WaterHeater:Mixed in a plant loop. This seemed to imply that I was unable to specify which plantloop would be replenished with water.

After my multiple plantloop configuration failed, I found the WaterUse:Storage objects. They seems to be designed for storage of rainwater or graywater. However, the water in a WaterUse:Storage object can be used to replenish plantloops before a plantloop replenishes from the water main. The idea is to have water from the PVT panels eventually make it's way into the WaterUse:Storage object, then the water heater plantloops in the model will use the hot water from the WaterUse:Storage before drawing from the water main. This is not the intended use of this object, but it should perform as I need it.

I will report back if it works or not and hopefully answer this question.
END SECOND EDIT

I'm adding photovoltaic thermal panels and a hot water storage tank to a model. The PVT panels heat the water in the heat storage tank, and the heat storage tank supplies hot water to the water heaters.

I have a concern about where cold water from the water main is being put into the plant loops though. I want only the hot water storage tank connected directly to the water main and none of the water heaters connected directly to the water main. This would force the water heaters to replenish water from the hot water storage tank, which itself is connected to the water main. Otherwise, the heat in the hot water storage tank may not be used completely if a water heater replenishes water directly from the cold water main.

The problem is the water main seems to be implicitly connected to the plantloops in energyplus. How can I model where the water main physically connects? Or is energyplus smart enough that I don't have to worry about this and the water heaters will always replenish from the hot water storage tank, even though the water heaters can replenish from the water main?

Here is the link to the model I made. The base was DOE_StripMall and I added in all the stuff (I think) I needed for the PVT.

https://drive.google.com/file/d/171Qp...

Edit:
Here is a sketch of what I currently have. (There are no heat exchangers. All water is mixed together.) As you can see, there are an awful lot of connections to main because each plantloop is implicitly connected. I am concerned that the plantloops connecting WaterHeaters to their uses are replenishing water directly from main rather than replenishing from the PVT heat storage tank. image description

Here is a simplified picture of what I would like to do. How to model this setup in ... (more)

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answered 2018-03-14 13:31:23 -0600

mldichter
2575 ●2 ●11

updated 2018-03-14 13:34:36 -0600

The answer is you can't specify where the water main connects in a plantloop. The water main is connected to the outlet node of WaterUse:Connections objects. It's more like the WaterUse:Connections objects change the water temperature of used water to the temperature of the water main. This goes with the general conservation of everything model in energyplus, which is water volume in this case.

As for making sure the water heaters used water from the PVT heat storage water tank instead of water from the main, I found a reasonable way to do it.

I added a second plant loop to the PVT heat storage water tank with a bunch of WaterHeater:Mixed objects that never heat their water. These heat transfer tanks are inserted before the outlet node of the demand side of each plantloop that has WaterUse objects in it. By connecting like this, the cold water from the main is run through a heat transfer tank before going to the supply WaterHeater:Mixed supply for that loop. Not the cleanest solution, but it works.

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answered 2018-01-18 02:32:02 -0600

Avi
4320 ●2 ●11

updated 2018-01-19 02:01:49 -0600

I suggest reading the Engineering Reference section about Water Systems.

The Water Use Equipment through the Water Use Connections if connected to Plant Loop will internally account for make up water to compensate for water going down the drain or used. So you can imagine the return line as actually being connected to the mains supply. It is easier to view that in the OpenStudio GUI.

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So in the above diagram the Pump is fed with mains water the left Water tank gets the mains water and the right Instantaneous heater water tank will get blended water coming from the tempering valve and from the left Water tank. As you can see, the order of the components in your loop will define what would be the temperature of the water supplied to the different components in your model.

Specifically for Solar Thermal I would suggest to follow the examples of E+ ShopWithSimplePVT.idf located in the ExampleFiles folder of the EnergyPlus installation.

You can certainly follow the PVT example just create separate Plant Loop for each heater;

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@Avi "I suggest reading the Engineering Reference section about Water Systems."
Yes. I've read that thoroughly. There was quite a learning curve to make my first plantloop. Considering in the linked IDF file that I was able to successfully add PVT panels and a heat storage tank to a model, connect those PVT panels to the heat storage tank in a plantloop, and connect 7 existing WaterHeater:Mixed objects in another plantloop, it's not the most useful information.

mldichter ( 2018-01-18 14:32:16 -0600 )edit

@Avi "The Water Use Equipment through the Water Use Connections if connected to Plant Loop will internally account for make up water to compensate for water going down the drain or used."
I am aware that water is added to a plantloop when water is lost down the drain, my question is, Can I control which plantloop is replenished with water? Since all the plantloops are connected, I would like the water heaters not to be directly replenished from the water main but instead replenished from the PVT heat storage water tank.

mldichter ( 2018-01-18 14:34:16 -0600 )edit

@Avi "Specifically for Solar Thermal I would suggest to follow the examples of E+ ShopWithSimplePVT.idf located in the ExampleFiles folder of the EnergyPlus installation."
This is good advice and where I started learning. However, the ShopWithSimplePVT is much simpler in that there is only one WaterHeater:Mixed which is directly connected to the PVT panels and also all the water uses in the model. There is no choice which plantloop the water main replenishes water because there is only one plantloop that loses water to the drain.

mldichter ( 2018-01-18 14:37:25 -0600 )edit

@Avi I don't understand. I have already successfully added a plantloop containing all the PVT panels and the PVT heat storage water tank AND I have already successfully added a plantloop that connects ALL the WaterHeater:Mixed objects to the PVT heater storage water tank. Why would creating additional plantloops in the model ensure that the water main is used to replenish exactly ONE of the plantloops?

Please read ALL the words in the question and comments before editing your answer or commenting.

mldichter ( 2018-01-19 12:12:25 -0600 )edit

@mldichter you are asking great questions that make me think. Yes I think that also in your second model you are clear of problems. About your first (current) model I am not sure, but I think it will not work as you need it.