[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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>
>     -- 
>
>     ***************************
>
>     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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>
>     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  <mailto:d.bradley at tess-inc.com>
>   
> http://www.tess-inc.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
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