David,
Thank you very
much for your response a couple of days ago. I have been
considering your advice and trying to make some changes in
my simulation models. However, I have not been successful. I
did identify the major problem, as you have explained, once
the controller trigger the ON function, the heat pump runs
without limiting the useful energy required for the thermal
load, this results a massive energy overproduced especially
for summer months. I am trying to write an equation to
reduce the flowrate in order to prevent energy been overly
produced. The whole validation I am carrying out against is
a system without a buffer tank, hence I will have to set up
an air source heat pump system without a buffer tank even
though I understand it is more accurate to do so. In your
experience, how would you do if you are in my situation?
Also come back
to the solar thermal system. I remembered that you
recommended flat plate solar collector type539 as it is the
most robust collector as you mentioned. I have tried it, it
worked pretty good. However, I would think evacuated tube
solar collector type538 is also very robust and accurate,
however the result I am getting is significantly larger than
expected. Then I tried the simple version Type71 evacuated
tube collector, it does give me decent result. The results
difference between Type538 and Type71 is about 33% which I
think it is huge. Would you recommend Type538 for general
use even though I do think this type is quite sophisticated
from what I have seen in Mathematic description in Tess
model?
I have one
final question, where could I get information for the
capacitance of collector? I have tried varies manufacture
website, none of them has given this value. If I leave the
default value, is this going to affect my result by much?
Thank you very
much for your help.
Zhe Li
Zhe Li,
I look at similar energy balances quite often so I suspect
there is not a systematic problem but something wrong in the
simulation. There are a few things to look at.
First, make sure that you have some thermal capacitance in the
liquid loop. This will usually take the form of a buffer tank.
The tank should have enough volume to store at least one
timestep of liquid at whatever flow rate your pump is running.
Second, look at the controls on your flow loop. Type941 is
basically an ON/OFF device; it does not meet a set point
temperature but simply puts its entire current capacity into
cooling down (or heating up) the liquid loop. Under low load
situations, this can dramatically overheat or overcool the
liquid loop; the tank above should help this a lot.
Third, make sure that you are using a reasonably short
timestep (1 or 5 minutes). this will allow your system to be
controller appropriately.
When you look at the energy balance, you have to also look at
the beginning and ending temperature of the liquid loop. If it
is significantly different than it was at the simulation
start, you have to account for stored energy in your balance.
Kind regards,
David
On 10/16/2012 08:42, Zhe Li wrote:
Dear TRNSYS users,
Thanks for your time.
I am currently performing a simple
validation of an air source heat pump system. The air-source
heat pump is Type941, and I am using a defined thermal load
(Type682) with an input file (Type9). However, the results I
am getting for the Total Heat Transfer to Liquid from output
option in air source heat pump component is between 2 and 3
times higher than the Heating Load Met from output option in
thermal load. I cannot think of any reason why the generated
energy is significantly higher than the load met. I would
understand if the generated energy is a bit higher than the
load met as there might be heat losses from connected pipes.
Could anyone give me some advices? This
is very important for me to continue my research project.
Thanks very much in advance.
Zhe Li
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David BRADLEY
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Thermal Energy Systems Specialists, LLC
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