Anand,
The only thing I can think of to simulate a thermal lag is to
use Type661, Delayed Inputs, beyond just using a really long pipe that has more
than 1 time step worth of mass inside it.
Use this with caution as Type661 is really meant to be for
control purposes and not for heat transfer.
You can have heat transferred to the absorption chiller and then
use type661 to delay the output for any number of time steps. Unfortunately,
that leaves it to you to figure out how many time steps are appropriate.
I hadn’t thought of this before, but the delay is probably
due to the fact that as additional heat is delivered to the absorption chiller
more refrigerant (water or ammonia depending on the type of chiller) is boiled
off and then goes through the condenser and evaporator before providing the
cooling benefit. It would make sense that there is a lag as the thermal inertia
of the system would matter. You might be able to estimate this by determining
the flow rate of the refrigerant and the pipe length between the desorber
(generator) and the evaporator. That would get you in the ball park, maybe.
You might also want to consider the time it would take to boil
off the additional refrigerant. If you have a vessel with refrigerant at a
certain condition and you wish to evaporate a certain mass, and you have a heat
input rate you should be able to estimate it. This is complicated by the fact
that as you are boiling off refrigerant more refrigerant is entering the system
from the previous time step.
Typically, absorption chiller calculations like this are done
with EES rather than TRNSYS. After creating the EES model, you can use TRNSYS
to call the EES model.
Good luck,
Fred
|
Fred Betz PhD.,
LEED AP
Sustainable Systems Analyst
|
|
AEI |
AFFILIATED ENGINEERS, INC.
5802 Research Park Blvd. | Madison, WI 53719
P: 608.236.1175 | F: 608.238.2614
fbetz@aeieng.com | www.aeieng.com
|
From: Anand Upadhyay
[mailto:Anand.Upadhyay@teri.res.in]
Sent: Friday, April 16, 2010 5:44 AM
To: trnsys-users@engr.wisc.edu
Subject: [TRNSYS-users] Thermal lag in absorption chillers
Dear
all,
I need to model a single stage absorption chiller based cold storage system.
When interacting with the manufacturer of the chiller, I came to know that
there is an appreciable thermal lag in the absorption chiller machines. The
manufacturer told me that with a varying heat source temperature, the
absorption chiller wont respond to the changes in source conditions fast
enough. Note that I plan to use solar thermal energy (how water) as my heat
source.
Hence my queries are as follows:
1. Does Type 107 in TRNSYS, take into consideration this thermal lag ? I
referred to the documentation but I am still confused.
2. Alternatively is there any other Type which models the thermal lag, and
which I should buy ?
3. Otherwise any suggestions on how I can model this thermal lag in the chiller
?
Hope some of you have already encountered this problem (and reached a solution
!!!).
Looking forward for your help
-
Regards,
Anand Upadhyay
The Energy & Resources Institute (TERI), New Delhi
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