[TRNSYS-users] Air source heat pump system validation
David BRADLEY
d.bradley at tess-inc.com
Wed Oct 24 12:52:42 PDT 2012
Zhe,
The incidence angle modifiers (IAMs) that are described in
.\Examples\Data Files\Type71-EvacuatedTubeSolarCollector-IAMData.dat and
which are being used by Type71 are quite a lot different from the IAMs
that are described in .\Tess Models\SampleCatalogData\Bi-Axial
IAMs\Typical_EvacTube_IAM_2d.dat and which are being used by Type538. I
think that if you alter one of the files to match the IAMs in the other,
you will end up with a much more similar result between the two.
Kind regards,
David
On 10/23/2012 09:31, Zhe Li wrote:
>
> 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 <mailto: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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> --
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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 <mailto:d.bradley at tess-inc.com>
>
>
>
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>
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>
> Tá an teachtaireacht seo scanta ó thaobh ábhar agus víreas ag
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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 <mailto:d.bradley at tess-inc.com>
>
> http://www.tess-inc.com
> http://www.trnsys.com
>
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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
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