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Hi TRNSYS users,
I have used a variable speed water to air heat pump to provide 4 lumped buildings with enough air flow to keep their temperature within the comfort zone. I used diverting valve from the TESS Hydronics library with 4 outlets and the fraction outlet of each exit is proportional to the volume of the building. The outlets are closed when the associated building reaches the comfort temperature. The problem that I have is that the last building always has a higher temperature which is not within the comfort zone and the heat pump is not operating at variable compressor speed.
How can I fix the problem?
With regards
Shams
When one or more of your outlets are closed, the total fraction out of all of the ports (as you've described it) will be less than 1.00. When this happens, Type 646 automatically sets the last outlet port as needed so that the total fraction out of all of the ports equals 1.00. You need to adjust the fractions when one or more ports are closed so the total fraction out of all of the ports still equals 1.00 (e.g., from 0.25 out of all ports to 0.333 out of 3 ports and 0 out of the last port).
ShamsoddinGhiami Topic starter
28/05/2024 1:03 pm
Thanks for your response @a_weiss,
I was thinking about adding an extra outlet to the main diverter with the flow rate fraction of "1-the other output fractions" and connecting it to the inlet. However, I receive convergence error.
Is there any way to resolve the problem?
DavidBradley 28/05/2024 1:07 pm
@shamsoddinghiami In addition to the answer from @a_weiss, some of the variable speed heat pump models ask you to specify the outlet temperature that you want them to provide. The compressor then adjusts its speed (to adjust its cooling/heating capacity) in order to provide those desired temperatures. If you set a very low desired cooling air supply temperature (or a very high desired heating air supply temperature) then the heat pump won't operate in a variable speed mode because it will continue to try and supply those high and low supply temperatures.
DavidBradley 28/05/2024 1:25 pm
@shamsoddinghiami it sounds a bit like you're modeling something like a variable air volume system (if I am correct that the diverter is on the air side of the heat pump). There is a model of a VAV air handler in the standard v18 library. It takes heating/cooling loads-based approach rather than watching thermostats. If you want to continue using the thermostat based approach that you've started then you may need to base the decision as to whether an outlet is open or closed on the zone temperature from one time step prior. You'll definitely want to use a short timestep in this case (at most 1 minute).
@DavidBradley, you're right. The diverter is on the air side of the heat pump and 4 thermostats measure the temperature of 4 and the signal is processed based on an equation to activate the heat pump if any of thermostats have signal 1. The outlets are also controlled by the thermostat signals and building volumes. My time steps are also small (30 Sec). I want to do the modelling using the heat pump rather than using a VAV. Can you please give me a hint on how to correlate it while the heat pump performance is not affected.
@shamsoddinghiami I am sure that you will have to adapt the following a bit but here is how I often proceed when dealing with variable speed flow loops that have diverters in them.
Let's say you have three zones (Z1, Z2, and Z3), each of which has a thermostat and each of which requires a certain constant flow when the thermostat is calling. Let's also say that there is a variable speed fan on the loop so that only the amount of air required by the zones with active thermostat calls is being transferred.
I'd make an equation block that looks like the following
mRtd_Z1 = 1000 ! zone 1 requires 1000 kg/h of air when its thermostat is active.
mRtd_Z2 = 2000 ! zone 2 requires 2000 kg/h of air when its thermostat is active.
mRtd_Z3 = 1500 ! zone 3 requires 1500 kg/h of air when its thermostat is active.
mRtd_Fan = mRtd_Z1 + mRtd_Z2 + mRtd_Z3 ! the fan's rated flow rate is the sum of the flow rates required by the zones.
mWant_Z1 = mRtd_Z1*y1 ! zone 1 wants its rated flow (1000 kg/h) when its thermostat signal is calling.
mWant_Z2 = mRtd_Z2*y2 ! zone 2 wants its rated flow when its thermostat signal is calling.
mWant_Z3 = mRtd_Z3*y3 ! zone 3 wants its rated flow when its thermostat signal is calling.
mWant_total = mWant_Z1 + mWant_Z2 + mWant_Z3 ! the total amount of air needed at any given time.
fFan = mWant_total/MAX(0.001,mRtd_Fan) ! this is the normalized rate at which the fan should run. Its value (0 to 1) is passed to the fan's control signal. The MAX(0.001,mRtd_Fan) just avoids a division by zero when the simulation is starting.
f_Z1 = mWant_Z1/MAX(0.001,mWant_total) ! this is the fraction of the total air flow rate that should go to zone 1. When only zone 1 is calling, the value of f_z1 will be 1. If all three zones are calling, it will be 1000/4500.
f_Z2 = mWant_Z2/MAX(0.001,mWant_total)
f_Z3 = mWant_Z3/MAX(0.001,mWant_total)
David
ShamsoddinGhiami Topic starter
05/06/2024 7:46 pm
Thanks @davidbradley for your reply.
Is there a way to also control the air flow rate from variable speed heat pump (Type 793) so that it does not work on 0 or maximum rated value? The control signal of the heat pump only accepts 0 and 1 value. It therefore results in higher air mass rate provision to the fan which cannot handle that high amount of air flow rate and only provides part of the whole mass to the zones. Is there a way to control the speed of the heat pump or the air flow rate of the HP
@shamsoddinghiami The last two inputs that Type793 requires are the supply air set point temperatures in heating and cooling mode. Essentially the component adjusts the device's present capacity (by adjusting the compressor speed) to meet the desired air temperature. It does not adjust the air flow rate but instead sets it to the value of parameter 24.
ShamsoddinGhiami Topic starter
10/06/2024 11:17 pm
Thanks @davidbradley for your elaborate explanation.
Referring to my previous comment, the mass flow rate does not balance in the system as the fan only takes some of the HP's air flow rate and the rest vanishes in the system. It not only results in high energy dissipation, but it also results in warning, consequently end up in receiving error due to the number of allowed warnings exceeded. Since I need to make the system work in this way, are there any other components in TRNSYS that can substitute Type 793?
DavidBradley 12/06/2024 11:38 pm
@shamsoddinghiami I think it is very important to figure out where the air is disappearing to; it shouldn't be. It might seem quite basic but I usually put a Type65 on every flow loop in my system and I plot the temperature leaving each device on one axis and the flow rate leaving each port of each device on the other. Whenever I have faced disappearing (or appearing) mass in a system it has been because of an accidentally mixed up connection that I made. Type65 is a great place for finding them. Make sure you expand the x-axis so that only a week at a time are displayed on it.
David
