Hi all. I am looking at the differences between different default HVAC systems in honeybee and am wondering why I am getting these results:
ideal cooling load: 153 mwh
VAV district chilled water cooling load: 501 mwh
VAV chiller cooling load: 79 mwh
Other than changing the default HVAC, there are no other building changes. I would have expected the district chilled water cooling load to be more similar to the ideal cooling load, if it is measuring the energy used in the coil, or more similar to the vav chiller load if it is measuring the energy in the district chiller. Either way, 501 mwh seems to be outside of the realm of possibilities. Can anyone help explain why this may be occuring?
“VAV with reheat” systems are one of the most energy intense types of HVACs that are still used today and a lot of it has to do with the “reheat” part. With these systems, you often expend a lot of energy cooling down the supply air stream (including both outdoor air and recirculated air) only to “reheat” it back up again to ensure that it is not too cold for the thermostat setpoint needs of any particular room. This whole process provides really good humidity control and sometimes this is critical for buildings like clean rooms or data centers but, for a lot of buildings, it’s pretty wasteful. Another reason why the industry likes VAV systems is that they are good at dealing with high peak cooling loads (eg. those resulting from all-glass facades) and they can serve spaces with different heating/cooling needs using the same central ducts (eg. cooling the glassy space blasted by the sun at the perimeter while reheating the supply air of core spaces that need heating at the same time). But again, a lot of this is wasteful as the vast majority of building’s don’t need all-glass facades.
The better engineers in our industry will usually include some helpful controls in VAV systems with sensors that only run the cooling coil for dehumidification when it is needed. And, if you serve spaces with different load profiles using separate VAV AHUs, you can sometimes get VAVs to perform pretty well. But they’re usually never as good as a DOAS system where the conditioning of the ventilation stream is handled separately from the space heating/cooling loads. Particularly when there is good heat recovery on the DOAS ventilation air stream, it’s pretty much always going to beat out a VAV in terms of energy performance.
Long story short, this is why the VAV cooling is much higher than just the ideal air cooling load. The reason why the chiller is a lot less than the district chilled water is that you are looking at the chiller electric energy rather than the thermal energy supplied by chilled water. A good chiller can have a COP as high as 5 of 6 so that’s why the electric energy is around a 6th of the thermal energy.