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Boiler with two stages

asked 2018-02-27 15:23:45 -0500

updated 2018-05-21 21:47:56 -0500

I have a boiler with two stages that has no gas rate modulation besides off and those two stages. What is the proper method to model this? Is the appropriate setting "SequentialLoad" for the Load Distribution Scheme at the "Supply Equipment / Demand Equipment" boundary? I tried that and "Optimal", but got the same water heating energy usage results either way.

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answered 2018-03-02 08:26:57 -0500

That Load Distribution Scheme input field ONLY applies when there are multiple pieces of supply equipment in the loop. For example, with two boilers setting this to "Optimal" will have one boiler operate at it's optimal PLR and the other boiler operate to meet the remaining load. If there's only one boiler, it has to meet all the load by itself, which is why you're seeing the same water heating energy usage results when changing it between "Optimal" and "SequentialLoad".

To model a two-stage boiler in OpenStudio, I would recommend adding two boilers to the supply side. Each boiler would represent a stage, so that you can define the appropriate parameters separately (capacity, efficiency, flow rate, min/max PLR, biquadratic performance curve coefficients, etc.). Now, to have the boilers operate independently (i.e. first-stage boiler runs by itself for low loads and second-stage boiler runs by itself for higher loads), you'll need to do some editing to the EnergyPlus IDF generated by OpenStudio. I say that because you will need to update things that are not exposed in OpenStudio:

  • the PlantEquipmentOperationSchemes input structure that defines what kinds of load a plant loop tries to meet (heating loads, in this case) and min/max heating load ranges.
  • Each min/max heating load range references a PlantEquipmentList object that defines what supply equipment will operate when the system sees a load value within that range.

So, using an example of staging 0 - 50kW and then 50-100kW, your updated PlantEquipmentOperationSchemes input structure will look like:

    HW Loop Operation,       !- Name
    PlantEquipmentOperation:HeatingLoad,  ! Control Scheme 1 Object Type
    Boiler Staging,               !- Control Scheme 1 Name
    AlwaysOn;                     !- Control Scheme 1 Schedule

    Boiler Staging,               !- Name
    0,                                  !- Load Range 1 Lower Limit
    50000,                           !- Load Range 1 Upper Limit  
    Boiler First Stage List,    !- Load Range 1 Equipment List Name
    50000,                           !- Load Range 2 Lower Limit
    100000,                         !- Load Range 2 Upper Limit  
    Boiler Second Stage List;       !- Load Range 2 Equipment List Name

    Boiler First Stage List,    !- Name
    Boiler:HotWater,            !- Equipment 1 Object Type
    Boiler Stage 1;               !- Equipment 1 Object Name

    Boiler Second Stage List,    !- Name
    Boiler:HotWater,              !- Equipment 1 Object Type
    Boiler Stage 2;                  !- Equipment 1 Object Name

    Boiler Stage 1,           !- Name

    Boiler Stage 2,           !- Name

Actual names of the Boiler:HotWater objects will be what you defined in OpenStudio.

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How are the maximum, minimum, and optimal Boiler:HotWater part load ratios used when there is only one boiler? Let's say chose 1, 0.5, and 1; that my efficiency curve doesn't modify efficiency based on load; and that my primary loop pump is constant speed. Should these settings cause the boiler to have full gas rate modulation between 50% and 100% of full fire based on demand? I don't see such variation in the Boiler Gas Rate data. Perhaps my quarter hour time step is just too coarse. I will also try two boilers to avoid full modulation, but need to understand this simpler case.

Bo White's avatar Bo White  ( 2018-03-02 18:43:00 -0500 )edit

When there is only one boiler, EnergyPlus will ignore the optimal PLR because the lone boiler has to meet all hot water demand. The Boiler:HotWater object has a "Boiler Flow Mode" input field that tells the boiler if it can modulate water flow or leaving water temperature. The flow mode then dictates how the boiler should modulate between min / max PLR to meet the hot water demand.

If you ONLY want the boiler to operate at 0% - 50% - 100%, then I would still recommend creating two boilers. Boiler 1 has min / max PLR BOTH equal to 50%, Boiler 2 has min / max PLR BOTH equal to 100%.

Aaron Boranian's avatar Aaron Boranian  ( 2018-03-03 15:33:11 -0500 )edit

Dear Aaron, Thank you for your kind explanation! I would like to ask another question about Load Distribution Scheme. In my model, I have central chiller on supply side and two heat exchangers on demand side. In other different two loops, where each of heat exchangers exists on supply side, there are fan coil units on demand side. In this case, how the indoor load is distributed into two different fan coil units? Is load is distributed only by the schedule or still affected by the load distribution scheme?

dana's avatar dana  ( 2018-03-16 00:06:08 -0500 )edit

@dana if these fan coil units are inside the same thermal zone, then their operation depends on the heating and cooling priority set by the ZoneHVAC:EquipmentList object. In OpenStudio the heating/cooling priority is set by the order that components are displayed in the GUI. You can also apply a measure to set heating/cooling priority. Read more here.

Aaron Boranian's avatar Aaron Boranian  ( 2018-03-16 16:00:35 -0500 )edit

@Aaron Boranian For more detail, eventually one of the fan coil units is connected to Chiller:Electric:EIR and the other one is connected to ThermalStorage:ChilledWater:Stratified. Also, each equipment has a supply water pump, I want to report a total four results, 1. chiller is operated with priority 2. TES is operated with priority 3. chiller is only operated (TES schedule would be plant off) 4. TES is only operated (Chiller schedule would be plant off while occupied hours), after that I will calculate optimization method.

dana's avatar dana  ( 2018-03-16 21:26:40 -0500 )edit

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Asked: 2018-02-27 15:23:45 -0500

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Last updated: Mar 02 '18