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I am designing a solar thermal collector system in EnergyPlus with conventional boiler as a back up system

asked 2021-02-07 17:26:50 -0500

anonymous user


updated 2021-03-29 08:56:46 -0500

Hello I am designing a solar thermal collector system in EnergyPlus with conventional boiler as a back up system

In the example files of EnergyPlus in the Water Heater Mixed it says that

the instantaneous Water Heater uses electricity, but the fuel type in my case should be fuel because i am using the conventional boiler as a back up, is this right?

And also why in the Water Heater Mixed there is a Storage tank with electricity as a fuel? (when we are using only heat from the solar collectors) why does it says electricity? Thank you

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answered 2021-02-08 02:24:15 -0500

What is the WaterHeater:Mixed?

The I/O reference guide is your friend. Start by reading the documentation for the WaterHeater:Mixed object. What you will learn from it:

  • The WaterHeater:Mixed models a storage tank. Meaning an object that holds a volume of water, and that can be connected to up to two loops: a Source Side Loop and a Use Side Loop
  • The WaterHeater:Mixed can also have an internal heating element
  • The field Heater Fuel Type controls the fuel type used by the internal heating element. That can be electricity, or Natural Gas, or whatever
  • If you set the Heater Maximum Capacity to zero, you are modeling a plain storage tank, with no internal heating element.

Loop connections

Typically the WaterHeater:Mixed (WH) is placed on the supply side of the Use Side Loop, and the Use Side Loop has the "load" (eg: WaterUseConnections) on the demand side. This will be your case too here.

In most applications, the WH is placed on the demand side of the Source Side Loop. In the case of solar thermal applications, there is some gray area on whether the solar collector should be on the demand side or the supply side of the source side loop, and the E+ examples seem to indicate that it should be the opposite of the usual, and the loop is controlled by availability managers instead: See

Back up boiler: WaterHeater:Mixed or dedicated Boiler:HotWater?

As far as whether you want to model your boiler back up via the internal heating element of the WaterHeater:Mixed or via a dedicated Boiler:HotWater (and zero-out the internal heating element turning the WH into a plain storage tank), that is up to you and your project. If your boiler in real life is an atmospheric gas boiler, then it's fine to use the WaterHeater:Mixed as the backup. If you need to model part load efficiency (for a forced-draft boiler or a condensing boiler), the Boiler:HotWater will allow you to input curves to characterize the performance better.

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Thank you for the reply It was really helpfull

But why in the Water Heater Mixed in the Storage tank field there is electricity as a fuel? (when we are using only thermal heat from the solar collectors) why does it says electricity??

I cant choose solar/thermal energy because is not in the options of fuel type field

anonymous userAnonymous ( 2021-02-11 06:09:18 -0500 )edit

The fuel concerns the internal heater only. It is not relevant to what is feeding the tank via the source side

Julien Marrec's avatar Julien Marrec  ( 2021-02-11 07:00:52 -0500 )edit

i am designing direct solar thermal collector for DHW, can you please look at this:

Water Mixed: Name: Storage Tank From Solar Collector for DHW; Tank Volume (m3): 12; Set Point temperature schedule: DHW water heater temperature schedule setpoint; Temperature Difference (delta): 5 ; Maximum temperature limit: 82 ; Heater control type: CYCLE ; Heater maximum capacity: zero

and it says Heater fuel type: Electricity (and this is the part that I'm asking why in the storage for heating DHW from the solar thermal collector it is still mandatory to choose a fuel)? Please let me know Thanks

anonymous userAnonymous ( 2021-02-11 08:34:31 -0500 )edit

Heat Fuel Type is a required field. So you have to provide a value. As lon as you set Heater maximum capacity: zero, then it does not matter at all.

Julien Marrec's avatar Julien Marrec  ( 2021-02-12 08:27:56 -0500 )edit

Thank you very much i understand it now

Another question if possible, I ran the system with one solar thermal collector (Flat plate) for a DHW system in a building

I put only one Thermal Collector as a tryout and I want to ask this

In the output HTML tables results in the On-Site Thermal Sources table it says:

Solar water thermal heat in GJ (this is the heat that is produced by the solar thermal collector), and in the percent heat field it says 100%, but what does this 100% percent heat refers to?

What does 100% mean in this table?

Thank you

anonymous userAnonymous ( 2021-02-14 12:23:38 -0500 )edit

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Asked: 2021-02-07 17:26:50 -0500

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Last updated: Feb 15 '21