What most users of Energyplus do not understand, is how Energyplus treats cycling equipment. If a boiler operates for 30 minutes, and then turns off for 30 minutes, and continues this timing for the whole day, that operation is equivalent to a 50% duty cycle. This also means that the boiler has twice the capacity as the load. What Energyplus does is simulate this boiler operation during each time step. So the boiler is maintained at 80C, and the boiler cycles at 50%. This same modeling technique is used for many equipment models in Energyplus.
Yes, this is different than the real world where the boiler temperature would change before the boiler turns back on, but Energyplus tries to simplify the problem and report the energy use of the boiler, not the cycling characteristics.
You can model the real boiler using the EMS system to turn on the boiler, say at 70C, and then turn off the boiler, say at 80C. There is an example in the Troubleshooting section at the help desk web site (http://energyplus.helpserve.com) under Energyplus Simulations then choose Controlling HVAC Equipment using a thermostat dead band. This relates to AC equipment, but can be used to model any HVAC equipment.
P.S. Dr. Li knows how a boiler works.
On 1/18/2012 9:48 AM, Tooran T wrote:
thanks Dr.Li for your answering,Is that what happens when you turn on a tap in your bath room? The boiler comes on immediately. The fire stops when the tap is turned off. This is the way the inline heater works.
but it is not Combination boiler that it works by taking cold water straight forward the mains supply and heating as you need it. when you turn on a tap, water drawn from the mains , heated in the boiler and comes straight out of your tap piping hot.it is traditional boiler .A central heating system consists of all the pipe work and radiators that are connected to the boiler. The boiler provides the heat but, it's the pump (Circulator) that moves the heated water from the boiler through the pipe work to the radiators, and back to the boiler for re-heating.the system is controlled by two setpoint that one of them is on return water pipe from radiator and another is on outlet pipe from the boiler .the pump works till the return water temperature of radiator reachs 70°C. Then it stops until the water temperature get lower than 70°C once again.the boiler works at its full capacity till the outlet water temperature of boiler reachs 80°C. Then it stops until the water temperature get lower than 80°C once again.thanks
From: YuanLu Li <yli006@xxxxxxxxxxx>
To: EnergyPlus_Support <energyplus_support@xxxxxxxxxxxxxxx>
Sent: Wednesday, January 18, 2012 4:30 PM
Subject: RE: [EnergyPlus_Support] on/off boiler
Is that what happens when you turn on a tap in your bath room? The boiler comes on immediately. The fire stops when the tap is turned off. This is the way the inline heater works.
A portable radiator heater has a thermostat switch to turn on and off the supply. There is no circulation pump.
What is missing in your simple (basic) assumption? If you have a small room, the boiler temperature may not change.
====================
A storage tank works the way you have described, between the charging cycles.
Dr. Li
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: t.t111ir@xxxxxxxxx
Date: Wed, 18 Jan 2012 07:44:53 +0000
Subject: [EnergyPlus_Support] on/off boiler
I am trying to model a very basic water heating system . The system consists of:
1) Boiler
2) Circulation pump
3) Radiators
in each time during which the boiler is turned on, the boiler operates at the full load, delivering the amount of energy equal to its design power.
In fact, what I want is to simulate the oscillations of the air temperature of the room because of the radiator i have. the boiler works at its full capacity till the outlet water temperature of boiler reachs 80°C. Then it stops until the water temperature get lower than 80°C once again.
i want to know could i model this ? and help me about it please.
thanks
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