[TRNSYS-users] Thermal lag in absorption chillers

[Fred Betz]

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 at aeieng.com<mailto:fbetz at aeieng.com>  |  www.aeieng.com<http://www.aeieng.com/>



From: Anand Upadhyay [mailto:Anand.Upadhyay at teri.res.in]
Sent: Friday, April 16, 2010 5:44 AM
To: trnsys-users at 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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