Demand not met - Parameters

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Posts: 1
Joined: Sun Nov 05, 2017 6:45 pm

Mon Nov 06, 2017 7:57 pm

Hello everybody,
We are a group of ETH students and we face some problems understanding some aspects of the software, regarding our energy demand. Specifically, after running a simulation we get the message "Energy demand is not met". Although, we followed all the suggestions over the changes the software prososes to us the demand cannot still be met. We run two different systems and we compared the Qdem with the Quse.
In the first one we tried to make the use bigger than the demand but even when this was accomplished, we still got the same message. What other parameters should we check in this case to meet the demand?
Also with a gas boiler of 90%efficiency and a small fraction of electricity form grid, we get a system performance of 1.13. Is that possible?
Additionally, on the second system layout the program does not show the message and the demand seems to be met, although Qdem>Quse annually and it is stated in descriptions that in this case there is an energy defifit and the desired temperature is not met.
Finally, we would like to know which graphs would you suggest us to check. We tried to compare the set point temperatures (as one of the proposals was to check this) with the actual reached temperature, but we could not find the latter.
Attached you can find some screenshots of the simullations we run and it was not met. Thanks, in advance.
23432803_10213659551061165_1634007345_o2.png (204.96 KiB) Viewed 38 times
23336435_10213659545941037_1979878837_o2.png (244.92 KiB) Viewed 38 times

Lars Kunath
Posts: 36
Joined: Thu Mar 02, 2017 2:35 pm

Tue Nov 07, 2017 9:46 am

Dear all,

first of all it is important how you define the load (Qdem). I assume you are using hourly consumption profiles or the degree hour model.

In both cases Polysun gets the load data from a second tool such as TRNSYS... The data you are uploading in Polysun are just values for the energy demand (no information about the house, heat capacity of the house, size, building standards...) So the problem is that you are giving static values in a dynamic simulation. E.g. you are changing with these profiles the demand from one hour to the next hour very rapidly, but the simulation can not react with the same speed. If the simulation is too slow it sums up the missing energy as energy deficit. If it brings more energy to the system as required it has no influence on the energy deficit. In reality it is not a big deal because there you can
easily except a little swinging of the temperature in a room and you have always a certain heat capacity which even reduce this swinging.

The warning "Energy demand not met" appears (in the most cases) whenever you have an energy demand larger 10% or when you do not bring at more than 10% of the time enough energy to the system.

I assume in your case the second condition will apply, but it is just a problem when you have a reasonable difference between Qdem and Quse.

To check what is going on you can go to the graphical evaluation and under building you can plot energy to the system and energy demand. both curves should behave in a similar way. (set point temperatures are not suitable for a comparison since you gave an energy demand to Polysun and not a temperature)

If there is a large difference between the curves you will either have to make sure that the energy comes to your system (check if you have a warm storage tank) or if there are troubles to get the energy to the building (in the heating loop controller you can use a temperature shift).

I hope this helps you.

BR Lars

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