[TRNSYS-users] Air source heat pump system validation

David BRADLEY d.bradley at tess-inc.com
Fri Oct 19 11:25:36 PDT 2012


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
>
>
>     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. http://www.dit.ie
>
>
>     _______________________________________________
>
>     TRNSYS-users mailing list
>
>     TRNSYS-users at cae.wisc.edu  <mailto:TRNSYS-users at cae.wisc.edu>
>
>     https://mailman.cae.wisc.edu/listinfo/trnsys-users
>
>
>
> -- 
> ***************************
> 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
>
> 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. http://www.dit.ie 

-- 
***************************
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

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.onebuilding.org/pipermail/trnsys-users-onebuilding.org/attachments/20121019/4dde8eca/attachment-0001.htm>


More information about the TRNSYS-users mailing list