Recently, I investigated the use of a variable refrigerant flow system with ground loop condensing for a school HVAC renovation. For the building in question, the ground loop was estimated to cost $405,000, and the savings produced would be expected to show an 8-10 year payback in order to be considered. I used Mitsubishi equipment as the basis for this exercise, and Trane Trace 700 as the energy modeling tool.
Trace has some limitations for modeling the use of ground source with VRV systems. The limitation is that each condensing unit can’t be modeled individually. All VRF cassettes are assigned to one cooling plant, which is the VRV condensing unit, and the condensing unit is assigned to a ground loop. Trace won’t allow the assignment of multiple condensing units to a ground loop. What this means is that the equipment profile used in the model for the condensing unit is a blend of the efficiencies and performances of all of the condensing unit sizes. So, with the air cooled VRV, if you had assigned VRV cassettes totaling 20 Tons to a condensing unit, you would typically assign a 20 T VRV condensing unit equipment library member to the system. But using the ground source, you would use the generic condensing unit equipment library members. Since we were at the schematic design level, this was not considered a major limitation and provided a good approximation of energy use.
I assigned all the VRV cassettes to one condensing unit, which happened to be around 300 T, and used the generic VRV geo library member.
Some definition of terms:
Library member – how Trane Trace stores the information used to calculate the power consumption of a piece of equipment. The library member has the full load efficiency values, the unloading curves and the ambient modification curve, which represents how the power use changes with water temperature.
Full load efficiency values – This is comprised of three values for VRV equipment: the full load kW per ton of cooling, the MBH of heating per ton of cooling capacity, and the kw per ton utilized in heating mode.
Unloading curves – how the power consumption changes based on whether the unit is operating at 100% of it’s full load capacity, 90%, 80% and so on
Ambient modification curves will be discussed in detail below.
So let’s compare ground source VRV with air cooled VRV. Here is a table comparing the power consumption values, or the unloading curves. The numbers below would modify the power consumption. For example, at 30% full load, the air cooled equipment in cooling mode will use 15.48% of full load power.
|Percent load||Air cooled cooling||Water cooled cooling||Air cooled heating||Water cooled heating|
In almost all cases, the power consumed by the water cooled VRV is less than the air cooled VRV (assuming the same full load kW for the air and water cooled equipment).
So why did my energy model show only a small savings in annual energy usage for the water cooled equipment?
To answer this, we need to look at the second modification to power that the energy model program does, which is the ambient modification. The ambient modification means something a little different, depending on whether you are talking about water cooled or air cooled equipment.
Ground source (water cooled) ambient modification.
For cooling, if your water temp is between 50-60, which it generally is for ground source, you find 50 degrees F on the X axis, and looked at the dashed line for capacity in the body of the graph, and find your input on the y axis, which is 0.7 of full load input. On the heating side, if your water temp is between 50-60, your input goes from around 0.9 to 1.0.
As the engineer from Mitsubishi informed me, the temperatures for the ground loop itself should be entered into Trace, if you are not working with the output file from a ground loop design package. This ensures that the program uses the correct efficiency values based on weather conditions.
Figure 1: Water Cooled Ambient Modification
For air cooled systems, the ambient modification curves represent how much less power is consumed at a lower outside temperature. Trace uses this value to modify the power consumption curve, so, for example, if the program calculates that the outside temp is 65, the calculated power consumed for the equipment will be (power consumed) multiplied by (ambient modification at 65 degrees).
Let’s look at heating first. The water cooled unit is going to be performing along the 50 degree inlet water temperature, so the ambient modification is going to be about 1.24, that is, 1.24 times power will be required, and about 0.95 of full load capacity will be available. The air cooled unit, with a temp entering the evaporator of 68 degrees will require less than 1.0 of full power, as long as the air temp is 40 or above. And below 40, the multiplier tops out at a little over 1.1. So, although the part load power consumption is better for the water cooled, the air cooled performs better in colder weather condensing.
On the cooling side, here is the air cooled ambient modification.
The water cooled unit (refer to Figure 1) will perform somewhere along the curve between 55 and 60 degrees inlet water temperature in the summer, so the ambient modification will be 0.68-0.7. The air cooled unit ambient modification, depicted above in Figure 3, will probably be on the 75 degree temp entering evaporator, and will be approximately 0.78 at 60 degrees and 0.92 at 80 degrees. So the air cooled unit will perform worse in the summer than the ground source water cooled unit does.
The results of my energy model were that the water cooled VRV system saved about $3,000 in annual energy costs, which was about 3% of total utility costs. Obviously, this was not a compelling reason to spend $405,000. The reason why the savings was so little is that the building was a school, and did not have many operating hours during the summer. So the considerable efficiency advantage in cooling did not produce much savings. In heating mode, from the consideration of ambient modification, we can deduce that the air cooled system may or may not be less efficient, depending on the number of hours at the percentage load, since the water cooled ambient modification negated some of the power consumption benefit. The ground source VRV did show a very small savings in heating mode, but not much.
Thus, for this building, the ground source VRV system did not show a payback.
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