[TRNSYS-users] Air source heat pump system validation
Zhe Li
zhe.li at dit.ie
Tue Oct 23 07:31:48 PDT 2012
David,
Thank you very much for your suggestion. I did put a water tank in between
the load loop and the heat pump. The result I am getting is great. However,
I remembered you mentioned that your company is developing a multi-stage
air-water heat pump component. I think this is very applicable for my case.
I am wondering if this component has been finalised? If it is, would be
possible if I can obtain this component. As even though the simulation
result is very good, I am still not setting up the system as identical as
the actual system (the system I want to set up) is. I may encounter some
difficult questions over the system set up in my final thesis defence.
I asked about the mathematical description of Type941 air-water heat pump. I
think this is still in the writing stage. I am wondering that when this
mathematical description will be released, would be possible if I can obtain
a copy of this description as soon as it is complete? This is very important
for my PhD thesis, it is not convincing without a full detailed maths
description in the thesis.
Please find attached two examples of solar thermal systems using identical
parameters with collector type538 and type71. The sum of Useful Energy Gain
from type538 and type71 is 2669.41kWh and 1876.44kWh respectively. I don’t
understand why the results are significantly different. Hope you could give
me some advice and direction.
Thank you so much for all your help.
Zhe Li
From: David BRADLEY [mailto:d.bradley at tess-inc.com]
Sent: 19 October 2012 19:26
To: Zhe Li
Cc: trnsys-users at cae.wisc.edu
Subject: Re: [TRNSYS-users] Air source heat pump system validation
Zhe,
This is a problem with actual single stage heat pumps, not just a
simulation problem. Single stage heat pumps cannot really modulate their
output power so under low load conditions, they can overheat a fluid loop.
One effective way of solving the problem is to put a water tank in between
the load loop and the heat pump. It requires an additional pump but then the
load loop draws water from and returns water to the tank. When the tank
temperature drops, the heat pump loop turns on and heats the tank back up
again. Under low load conditions, the load slowly draws the tank temperature
down and the heat pump stays off.
I am not sure what the problem with Type538 might be. I'd be interested if
you could send me a project showing the difference in result between the
two.
As for the collector capacitance, often collector manufacturers will tell
you the liquid volume of the collector. You can calculate a capacitance from
the volume and from the specific heat of your working fluid. Without any
better knowledge, though, we use a value of about 10 kJ/m2 of collector
area.
Best,
David
On 10/18/2012 09:35, Zhe Li wrote:
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
From: David BRADLEY [mailto:d.bradley at tess-inc.com]
Sent: 16 October 2012 17:04
To: Zhe Li
Cc: trnsys-users at cae.wisc.edu
Subject: Re: [TRNSYS-users] Air source heat pump system validation
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
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
Tá an teachtaireacht seo scanta ó thaobh ábhar agus víreas ag Seirbhís
Scanta Ríomhphost de chuid Seirbhísí Faisnéise, ITBÁC agus meastar í a
bheith slán. http://www.dit.ie
This message has been scanned for content and viruses by the DIT Information
Services E-Mail Scanning Service, and is believed to be clean.
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***************************
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
Tá an teachtaireacht seo scanta ó thaobh ábhar agus víreas ag Seirbhís Scanta Ríomhphost de chuid Seirbhísí Faisnéise, ITBÁC agus meastar í a bheith slán. http://www.dit.ie
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