OpenStudio EnergyPlus - VRF/VRV Basics 2

In this video we will clean up a couple if items we missed in the previous episode that discussed how to model a VRF/VRV system using the generic performance curves from EnergyPlus. Transcript: Hello all. We are back. There are a few cleanup items that we need to do. A couple of things that we missed last episode. One is a major issue that has to do with VRF systems. Let us go take a look at our generic VRF system that we modeled in the last episode. We will take a look at the outdoor unit. One thing that we missed on the last episode was the compressors. The outdoor unit that we are trying to simulate, using the generic performance curves, is this Mitsubishi unit. It it only has one hermetic compressor. We are going to take a look at that. We will scroll down to the compressors for this. The generic model has three compressors in it. The one we are trying to simulate only has one. So, that is going to make a significant difference in the simulation because the Mitsubishi equipment can only turn down so far. The generic equipment, if it had three compressors, it would have a significantly higher turndown. The next part of that is ratio of compressor size to total compressor capacity. We only have one compressor, so that ratio will be 1.0. In contrast, if you had three compressors, one divided by three is is is 33% (0.33). The next correction has to do with the defrost strategy. The default generic defrost strategy is resistive heat. Unfortunately, for whatever reason, the generic defrost capacity is basically zero watts. You can see this right here. It does not make any sense. To correct that, we can simply put autosized here. The next thing that we need to talk about is the big shakeup with VRF systems. This goes back quite a number of years. Let us see. Where do I start. VRF systems were introduced to the USA over a decade ago. AHRI cobbled together a performance rating standard for VRF systems. They borrowed the standard from residential heat pumps. There are a number of performance differences between residential heat pumps and VRF systems. Therefore, the standard was not an appropriate standard to use for VRF. But they used it any way. The VRF systems were looking very promising. They looked like they were supposed to perform really well. Over the years, it started to dawn on people that they were not performing as they should. The VRF/VRV systems were coming up significantly short of of where they were supposed to be. You can see a number of these test cases. These are systems that were installed and measured for performance; on this chart. This is the line where they were supposed to be performing. So, that was one big issue. Another issue; engineers were using these efficiency ratings and comparing them to other types of equipment. This is something you really should not do. It is just not equivalent. These AHRI standards are designed for comparing oranges to oranges. They are not designed for comparing oranges to apples. To continue...people were comparing them to hydronic systems. The hydronics industry started realizing that. They cried foul. They put together a white paper discussing this. They are saying; these these VRF systems are are claiming that they can do this. But, because of the way AHRI 1230 was put together, they are not being tested correctly. The error is about 47%, which is a significant difference. So, the end result of this was that AHRI revised their standard; they revised the testing procedure for these ratings. The result is that the VRF systems are no longer as efficient as they claimed. Their rated efficiency is not as quite as good as as they were originally claimed to be. The federal efficiency standards were revised. Some of the energy codes changed as a result of that. For example, California sort of threw up their hands. They said "just look at the federal efficiency standards". We look at the federal code and make comparisons. We can see that, for heating performance, the VRF systems had an an efficiency drop of about 9%. For cooling it was about 3%. Not quite the 47% that the hydronics industry was claiming. They are obviously that is a little bit biased. So, we are going to adjust this generic system. This generic system was originally modeled back in 2010. As a result, it has that bias included. To adjust for that bias, we need to change these coefficients of performance. For cooling the deduction is actually 3%. We need to reduce it by 3%. The cooling COP for this generic VRF/VRV is uh 3.8. We will multiply that by 97%. So, in reality, the COP is more like 3.686. ... Complete transcript: https: //www.helix-engineers.net/technical-articles