[BLDG-SIM] Citi Multi Variable Refrigerant System

Wed Aug 22 11:40:31 PDT 2007

Hello again,

I recieved some great information from Sam Mason who is an environmental designer at Atelier Ten Consulting Environmental Designers in New York. He researched the City Multi for his Masters. Thank you Sam for your help! 

Here it is:

Dana,

The City Multi was the topic of my master's project and I ran into several 
problems trying to the model the system. My conclusion was that
using eQUEST, it is not possible to model the system for a few reasons.
There is also a lack of information from Mitsubishi that makes modeling
tough.

First, with regards to each indoor unit, setting a constant EIR (like
you mentioned doing) does not allow for the variation in performance due
to temperature changes in the space and outdoors. If you look in the
engineering manual, there are power and capacity relations for each unit
at various temperatures. These corrections need to be incorporated for
each indoor unit and this may be possible using ms-dos batch files.

Second, there is no way to account for the free-energy BC controller
operation in terms of refrigerant transferred to/from zones and the
effectiveness at which the refrigerant used. Even though the coil loads
would be known for each zone from DOE2, the state and temperature of the
refrigerant would be needed to figure out how much energy is available
fro space conditioning in other zones.

Third, DOE2 is steady-state modeling software, but this system is hugely
dynamic. From my monitoring of the system, I found that the compressor
cycles at various frequencies over the course of one minute, thus
changing the capacity of the system hundreds of times an hour. Energy
Plus might be a better tool because it can run at 15 minute intervals.
Fourth, Mitsubishi does not supply the part load curves for the
compressor operation. This means there is no way to accurately model
the unloading and part load performance of the compressor. In addition,
it is an inverter driven and not a hard start compressor, so the part
load performance should be quite good.

These systems are going to become
more popular and already are in certain areas. I have found one paper
referring to a module for energy plus that models these types of
systems, but I cannot get a hold of the author. I also know that energy
soft has been working on developing a module for their software, but I
do not know the status.

Sam
--
Sam Mason
Environmental Designer
Atelier Ten
Consulting Environmental Designers
45 East 20th Street, 4th Floor
New York, NY 10003
T +1 (212) 254-4500
F +1 (212) 254-1259
E sam.mason at atelierten.com
W www.atelierten.com 

-----Original Message-----
From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com]On Behalf Of Dana Troy
Sent: Wednesday, August 22, 2007 8:29 AM
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] Citi Multi Variable Refrigerant System

Hello all,

We have recently performed an analysis on a building that is potentially going to incorporate the City Multi system. After many hours of going through eQUEST, I devised a method of modeling it that I would like to run past you all for any thoughts/ideas. 

First, for those not aware of the problems, here is a background of the systems:

The City Multi system consists of essentially three parts: the condenser, the BC controller, and the heat exchanger unit at each zone. Refrigerant is the only property used for heat exchange, i.e. there is no water running from the condenser to the controller to the zone units. Each zone unit is essentially a fan coil, in that there is no compressor work at the zone, only fan energy. The BC controller is essentially a heat exchanger which gives the City Multi system a large advantage by being able to transfer heat, say from the south face of a building, to the north face, which is in the shade. All of the individual zone units connect to the BC controller and the BC controller distributes the refrigerant accordingly. If there is not enough free energy being generated by the system, the compressor kicks in and supplies or rejects heat from the loop to meet the demand. 

To model this in eQUEST, which has been suggested before, the water source heat pump system should be selected in order to account for the free transfer of heat from zone to zone. There are, however, several problems with this. One is that pumping energy is required to pump the water throughout the building. The second problem is that at each zone, each heat pump runs a compression cycle in order to supply or reject heat to or from the zone to the water loop. The third is that a boiler and cooling tower take the place for the condenser in the City Multi system. Each system requires a make-up air unit which can be modeled the same in each simulation.

Now here is what I would like you all to review: 

These are some of the steps I went to in order to make the water source heat pump as much like the City Multi as possible.

*	Made the boiler and electric boiler (to simulate the electric compressor) and gave it the appropriate heating EIR that was specified in the Citi Multi specification manual. I gave the fan the same EIR as the cooling side of the compressor in the same manual. 

*	I made a spreadsheet to subtract out the pumping energy (only from the water loop! Not the DHW loop!) to simulate the suction of the refrigerant through the building in the City Multi system. I had to do this in the BEPS and the BEPU report which made it kind of a hassle to find the energy cost savings. 

*	For each heat pump, I set the EIR equal to 0.0001 to simulate the fan coil in the City Multi system. Therefore, whatever heat is transferred to or from the water loop is only shown with the fan energy from blowing the air over the coils. IF this is not a proper way to do it, then what should the EIR be? It cannot be the EIR from the condenser, because that has nothing to do with the free heat transfer energy at the zones. 

What do you guys think about this approach? I figure that since this is a tough problem that other people have encountered as well, I might as well use this huge resource that has many great minds to come to a solution. 

Please, any feedback would be greatly appreciated. 

Regards, 

Dana Troy
Energy Analyst 

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Portland, OR 97204-2640
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