FCU change to VAV, space cooling increase 30%?

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Hi all,

Lately I am modelling an office & hotel complex through
eQUEST with energy conservative strategies added one by one to test the energy
saving effect. The project location is a city in China with hot summer and warm
winter.

Base case:

Water side

Central water cooled chiller loop for office and hotel;

Air cooled heat pump for hotel guest room heating;

Air side

FCU + SZR (OA ventilator) for both office and hotel;

FCU ( fan power 0.0003kW/cfm) /SZR ( fan power 0.0009kW/cfm) 

When I switch the office air side system from FCU to VAV
(fan power 0.0011kW/cfm), it is found that though the office fan power
consumption decreased roughly 35%, the office space cooling power consumption
increased for 32% as well as the cooling tower and chilled water pumps. As a
result, the office equipped with VAV consumes overall 4% more electricity than
the office with FCU.

I suspected before that higher fan power of VAV results in
more temperature increase through fan and hence more space cooling requirement.
But after manually change the delta T through fan, the effect is not very
obvious that the space cooling increase proportion drops from 32% to around
28%.seems it is not the main cause.

Before this I thought VAV is generally more energy
efficient than Fan coil especially for office building but am confused now.
Hope to get your advice in this issue. Many thanks!

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