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<p>Jan,</p>
<p> I cannot count how many times I have been in similar
situations!</p>
<p> A couple of thoughts about how I would debug the issue.
Probably you have already tried most of these things but hopefully
there will be some little trick in there.</p>
<p>- I would first put in a Type65 that shows the temperature and
flow rate leaving every component in the system.</p>
<p>- force the system to be only in mode 1 or in mode 2 and see if
the problem is only happening in one mode or another. If the
problem goes away when you force the mode then the issue is likely
in the mode decision.</p>
<p>- make sure that you have some thermostat models in the system to
determine control signals. Thermostat models have hysteresis built
into them where equations do not and the problem might be hiding a
convergence issue.</p>
<p>- when your system is running in direct heating mode you don't
have any thermal capacitance. The heat pump could be supplying hot
to the load, which isn't removing enough energy from the stream,
which causes the return water temperature to go up at the heat
pump inlet, which then adds more heat to the stream and it runs
away on you. Part of the solution might be to modify your heat
pump performance data file so that the heat pump's capacity drops
off if the inlet temperature gets too high. As a side note, this
is a problem in real life too! We had a heat pump directly feeding
some radiators and when the room was near its setpoint, the
radiators weren't taking much heat out of the loop, the return
water temperature went up and tripped a safety on the heat pump!</p>
<p>- try breaking the loop somewhere and watching inlet and outlet
temperatures and flow rates. There is a very good chance that the
problem will go away when you do this but it might give you a hint
about the source of the problem.</p>
<p>kind regards,</p>
<p> David</p>
<p> <br>
</p>
<div class="moz-cite-prefix">On 12/9/2022 7:09 AM, Böttner, Jan via
TRNSYS-users wrote:<br>
</div>
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<p class="MsoNormal"><span lang="EN-US">Hi everyone,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I have built a model
with a heat pump that can operate in two different modes:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Mode 1: The heat pump
directly supplies the heating cycle<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Mode 2: The heat pump
supplies a storage tank and the heating cycle is supplied
through the tank.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">The supply temperature
for my heating cycle, as well as the return temperature and
the mass flow are calculated in advance and supplied through
the Input file.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">The system looks like
this:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"></span><img
style="width:5.5729in;height:3.1354in" id="Grafik_x0020_1"
src="cid:part1.tq08TDV0.B0FNTE5l@tess-inc.com" class=""
width="535" height="301"><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal"><u><span lang="EN-US">So what does the heat
pump do?<o:p></o:p></span></u></p>
<p class="MsoNormal"><span lang="EN-US">Mode 1: Incoming and
outgoing mass flow are the same, determined by the input
file. The incoming fluid temperature is the same as the
outgoing temperature of the Heating cycle. This temperature
Is raised by 5K and the outgoing temperature of the heat
pump is the same as the incoming temperature of the Heating
cycle. These temperatures are also determined by the input
file, calculated in advance.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">The diverter and mixer
control signals are set by the heat pump. So this means 0
for the diverter and 1 for the mixer on the heat pump site,
0 for the mixer and 1 for the diverter on the heating cycle
site. The bypass flowrate on the heating cycle site should
be 0, as the supply temperature is already correct. So the
control signal “Fraction to heatsource” from the valve to
the diverter should be 1.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Mode 2: The storage tank
should be supplied until it reaches a setpoint (55°C).
Therefore the maximal mass flow (2000 kg/hr) runs through
the heat pump and is heated with the maximum Power.
Meanwhile the heating cycle needs to be supplied with water
from the storage tank, Supply temperature, mass flow and
return temperature are determined by the input file. Delta T
is again 5 K.
<br>
The water from the storage tank has a higher temperature
than the target for the heating cycle and needs to be cooled
with the bypass flow. The control signals are set opposite
as in mode 1 and the control signal for the bypass diverter
is set by the tempering valve (Fraction to heat source =
0…1).<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><u><span lang="EN-US">What is my problem?</span></u><span
lang="EN-US"><br>
It doesn’t work. I have huge temperature spikes (up to
20.000°C) in my streams and I don’t know where they come
from.<br>
In mode 1 this results in too high temperatures coming in to
the heat pump, 5K are added and the temperature arriving on
the heating cycle site are too high, so that the bypass
starts mixing in “cold water” (which is already higher than
the target temperature). But in mode 1, the bypass should
never be letting through fluid and it should be one large
outer circuit.
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I have tried to
calculate the control signal for the bypass diverter within
an equation as follows:
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">ctrl_signl_bypass =
GE(fraction_to_heatsource,heating_cylce_Diverter_ctrl_signl)*
fraction_to_heatsource +
GT(heating_cylce_Diverter_ctrl_signl,
fraction_to_heatsource)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">This might be incorrect
as well. But I have tried a lot of different combinations
and the result is always wrong (only in different ways
</span><span style="font-family:"Segoe UI
Emoji",sans-serif" lang="EN-US">😃</span><span
lang="EN-US">)<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I would be so grateful
if anyone could help me, as I’m stuck here since days, tried
a lot of stuff and nothing seems to help.<br>
I have the feeling, the system can’t cope with the two
different circuits and the inconsistent mass flows (which
varies, as the delivered energy isn’t constant but delta T
is always 5K).<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">If you have any
questions feel free to ask, the whole system is even more
complex and I tried to break it down as good as I could.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Best regards,<br>
Jan<o:p></o:p></span></p>
</div>
<br>
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<pre class="moz-signature" cols="72">--
***************************
David BRADLEY
Principal
Thermal Energy System Specialists, LLC
3 North Pinckney Street - suite 202
Madison, WI 53703 USA
P:+1.608.274.2577
<a class="moz-txt-link-abbreviated" href="mailto:d.bradley@tess-inc.com">d.bradley@tess-inc.com</a>
<a class="moz-txt-link-freetext" href="http://www.tess-inc.com">http://www.tess-inc.com</a>
<a class="moz-txt-link-freetext" href="http://www.trnsys.com">http://www.trnsys.com</a></pre>
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