Re: [TRNSYS-users] (no subject)

Jeff,

First of all, thanks for your response.

I was analysing different parameters of the model. I realised the coefficients for the Nusselt calculation on the heat exchanger of the storage tank are critical.

Nu=C*(Rayleigh)^n

A while ago, I set them as C=0.4 and n=0.25. I did this based on some data I had from my Heat Transfer class in University.

I know the coefficients are by default as: C= 0.6 and n=0.6 . If I set the model with these parameters the problem is solved and the heat pump can operate below 50 (°C)

Nonetheless, now I was starting a little research on the subject, I notice there is plenty of different results in the literature to define C and n for horizontal tubes in natural convection. Now I'm not sure if the cofficients I used are the correct ones, I'm continuing my research on it to properly understand it.

Are the Default values proven to be the best suited?

Kind regards,

Javier

PS: I will attach the Screen Capture you asked. Please, tell me if you need more details or info.

2017-05-08 12:09 GMT-03:00 Jeff Thornton <thornton@tess-inc.com>:

Javier,

Please post a screen capture of your system so we can see the flow connections. I have an idea as to what may be wrong.

Jeff

Sent from my iPhone

On May 6, 2017, at 5:37 PM, Javier Vega via TRNSYS-users <trnsys-users@lists.onebuilding.org> wrote:

Hi TRNSYS users,

I'm experiencing a weird issue. I'm testing a simple system where I have a heat pump (Type 941) and solar collectors (Type 538) coupled to a Vertical Storage Tank via heat exchangers, modelled with Type 534. The tank is connected to a closed loop to a radiant floor.

I have the control system setted this way:

-The collector's circuit works if they can gain energy. ON/OFF strategy
-The Radiant Slab Loop operates if the thermostat of the thermal zone requires it (ON/OFF strategy)
-The heat pump works if the required temperature for the radiant slab loop isn't satisfied by the collector's energy gain. The temperature set for heating is 35 (°C) in the top of the storage tank. ON/OFF strategy

For some reason, when the heat pump is active its operating temperatures are above 60 (°C) at the heat pump's inlet, even if the tank is at 35 (°C). I've checked and the heat exchanger seems to transfer all the heat pump's energy to the storage tank. I don't know what is causing the temperature raise.

Is this what is supposed to happen? Is it a weird simulation unrealistic error?

My heat pump has a nominal capacityof 9 kW. The heat load transfered to the radiant slab is above 7 kW max.

My heat pump's performance map has operating data just to 50 (°C) inlet water temperature. If I force the heat pump to work below this, the heat pump operates less often and the system can't supply enough energy to keep the radiant floor water at 35 (°C).

Here are two pics of the situation. First is the heat pump operating "naturally" at high temperature. Water inlet in the heat pump is in pink. Water temperature to the radiant slab is in blue. Heat output in the radiant slab is in green . Red is for the airnode temperature.

<image.png>

<image.png>

I would appreciate some help, thanks in advance.

King regards,
Javier Vega

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