[TRNSYS-users] Energy Balance for solar cooling system

[David BRADLEY]

Safwan,
  It is always a VERY good idea to do an energy balance and if it is 
balancing to within 1kWh over a month then I think you can have great 
confidence that your model is correct (of course that doesn't 
necessarily mean that the model reflects reality but it is a good first 
step).

2- I found there was no way to calculate cooling load while running the 
simulation, so I calculated it separately. any idea?

this question gets to the heart of how TRNSYS is different from a lot of 
other building modeling tools. Many tools calculate a space load and 
then try to meet that load with the HVAC system model. It is difficult 
in this case to write an energy balance because you have to make the 
assumption that the HVAC system can deliver exactly the amount of energy 
required by the load at all times. While TRNSYS can operate in that 
manner, you can also use it to get a lot closer to the way that a 
building actually operates. To do so you turn off the automatic heating 
and cooling in TRNBuild and allow the building temperature to float. 
Then you implement a thermostat model and decrease the simulation time 
step to something on the order of 1 minute. You then implement your HVAC 
system and control it (determine whether fans, pumps, and equipment) 
should be ON or OFF based on your thermostat's output. The HVAC system 
outputs (air flow rate, temperature, and RH) are inputs to VENTILATION 
types defined in TRNBuild. You can then do your energy balance using the 
actual amount of energy that was transferred by the equipment to the zone.

3-When I come to (pipes, duct and fan loss) some of cold pipe will gain 
heat instead of lost it, so should I add it to heat losses or subtract it?

you just need to be consistent about the sign of the terms in your 
energy balance. In a water loop you might define energy into the loop as 
positive (an energy gain) and energy out of the loop as negative (an 
energy loss). If the liquid in a pipe is hotter than the ambient then it 
will lose energy to the ambient and you take that term to have a 
negative sign in your balance. If the liquid is colder than ambient then 
it will gain energy from its surroundings and you would take that as a 
term with a positive sign.
regards,
  David

On 09/21/2015 08:48, Safwan Kanan wrote:
> Dear David,
>
> I doing a simulation of a solar cooling system( collector, absorption 
> chiller and building) and everything seems fine but I want to do an 
> Energy balance for the whole system. So I calculated as Energy in 
> (Useful heat from collector+Electricity consumed in the whole system 
> such as pumps ,fan,chiller and tower fan+Building cooling load which I 
> calculated separately assuming unlimited cooling power) and for Energy 
> out ( heat subtract in cooling tower+pipes, duct and fan losses).
>
> I found there was around one kWh difference during July of the second 
> year simulation time.
>
> My questions are:
>
> 1-Do you think that's correct? if not correct me, please.
> 2- I found there was no way to calculate cooling load while running 
> the simulation, so I calculated it separately. any idea?
> 3-When I come to (pipes, duct and fan loss) some of cold pipe will 
> gain heat instead of lost it, so should I add it to heat losses or 
> subtract it?
> Any idea I will appreciate that.
>
> I look forward to hearing from you as soon as possible.
>
> Regards,
>
> Safwan Kanan
>
> PhD student at the University of Manchester
> Manchester
> M13 9PL
> UK
>

-- 
***************************
David BRADLEY
Principal
Thermal Energy Systems Specialists, LLC
22 North Carroll Street - suite 370
Madison, WI  53703 USA

P:+1.608.274.2577
F:+1.608.278.1475
d.bradley at tess-inc.com

http://www.tess-inc.com
http://www.trnsys.com

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