But I think you're right, the autosize is not that accurate. By changing the water rate and temperature, I've been able to meet the demand temperature in all the zones.
The peak demand at startup is caused by choosing a throttling range which is to low (0.5°C) as I have noticed now, this makes the radiant system to fast, which is not realistic as it is supposed to be a slow system.
Thank you for thinking with me.
Are you using radiant system? I found hard time to meet the demand using radiant system, maybe thats why you see demand increases with your capacity. You can check your room temperature, I am pretty confident your room temperature cannot meet thermostat setpoint during design day. In theory, if your system can meet the demand, no matter what capacity you use, your demand should stay the same. Demand is basically the room load, which is separated from system side.
So back to your question, the way EnergyPlus size radiant system is not robust and it cannot meet the room load if weather is too cold. My sugestion is to use a conventional air system to calculate the demand and use that as your radiant system sizing parameter and do the radiant system sizing by the manufacture software.
HJ
On 8/25/2014 3:22 PM, Geert Bellens geert.bellens@xxxxxxxxxxxxxxxxxxxxxx [EnergyPlus_Support] wrote:
I do know the difference between demand, capacity and consumption. That's not the question. I do have to improve my English :)
The question is rather: how to prevent peaks in the demand, especially when you don't use autosizing.
Increasing the manually set capacity, makes the the peaks increase also.
After playing a little with the parameters of the floor heating, I've seen that changing the throttling range has a high impact on these peaks.
Changing the default setting of 2°C (which appeared to me as high) to 0.5°C, does makes a big difference. The peak demand is very high...
Geert
By default, the demand IS capacity. This is not the power it consumed, but how much cooling or heating the device can output. You dont need to use 100% as efficiency to get this value since it is reported in the HTML summary report. I think you mixed the concept of capacity with the energy consumption.
HJ
On 8/25/2014 2:32 PM, Geert Bellens geert.bellens@xxxxxxxxxxxxxxxxxxxxxx [EnergyPlus_Support] wrote:
Richard,
Thank you for the response, but I've expressed myself in a wrong way I think.
I don't want to calculate the consumption, but the 'demand', so efficiency is put at 100%
I've noticed that using autosize, sometimes gives me a total heating capacity which is to low.
If I manually set a higher capacity, the heating load is met.
However, raising the boiler capacity again and again, raises the heat demand at startups, getting higher (but smaller) peaks.
These peaks seems to be to high and not realistic.
I also see that at startup (in the morning) the floor heating, asks a HVAC heating rate which is sometimes twice as high as expected....
Are these peaks for real? I really can't decide to take these short peaks (neither at zone level or building level) as design load....
Geert
Richard Raustad RRaustad@xxxxxxxxxxxx [EnergyPlus_Support] schreef op 25/08/2014 18:08:
How can I determine what the real power demand is ? Enter the real equipment efficiency.
Regarding sizing, you can use the sizing parameters object to set an averaging window for sizing. This will average out the startup peaks you see yet still meet the load. In the example below, try using 4, 8, and 12 timesteps for the averaging window (1, 2, and 3 hours) to see what impact that has on sizing and hours set point not met.
Timestep,4;
Sizing:Parameters,
1.0, !- Heating Sizing Factor
1.0, !- Cooling Sizing Factor
; !- Timesteps in Averaging Window
On 8/25/2014 11:52 AM, geert.bellens@xxxxxxxxxxxxxxxxxxxxxx [EnergyPlus_Support] wrote:
I usually simulate a building with a 100% efficient boiler and chiller.
If I use autosize for the capacity of the boiler or chiller, the capacity is sometimes to low.
If I fill in the capacity manually, .... I can fill in what I want.
How can I determine what the real power demand is ?
Especially for capacity peaks at startup (morning for heating, afternoon for cooling) the manually choosen capacity, will influence the duration of the startup peak.
This leads to discussion with my client ofcourse. the software is intended to determine the peak load, but I can choose it myself....
Sometimes these peaks are also extremely high and short in duration. Is there a limiting aspect I maybe have forgotten ? (pump , or other part of the heating/cooling loop)
Geert
-- Richard Raustad Senior Research Engineer Florida Solar Energy Center 1679 Clearlake Road Cocoa, FL 32922 Ph: (321)638-1454 http://www.fsec.ucf.edu/en/ Electric Vehicle Transportation Center http://evtc.fsec.ucf.edu/
__._,_.___
Posted by: Geert Bellens <geert.bellens@xxxxxxxxxxxxxxxxxxxxxx>
Primary EnergyPlus support is found at:
http://energyplus.helpserve.com or send a message to energyplus-support@xxxxxxxx
The primary EnergyPlus web site is found at:
http://www.energyplus.gov
The group web site is:
http://groups.yahoo.com/group/EnergyPlus_Support/
Attachments are currently allowed but be mindful that not everyone has a high speed connection. Limit attachments to small files.
EnergyPlus Documentation is searchable. Open EPlusMainMenu.pdf under the Documentation link and press the "search" button.
__,_._,___