In general the throughput efficiency of fluid couplings is about the same as eddy current couplings, and there are several well done efficiency comparisons to ECC based drive systems vs VFDs out there. As a GROSS generalization the savings are there, especially if the cost of a cooling system for the heat of FC or ECC system is factored in. People often forget that if the slip coupling systems are in a conditioned space. In a VFD, you are looking at roughly 3-10% efficiency losses worst case, with a slip clutch system it can be as high as 50% losses at 60% speed. That's dramatically different, and the losses are heat that has to go somewhere. How much time you spend at the different flow rates however can make a world of difference in overall efficiency of the system as a whole.
But even if the VFD saves energy it may take a few more years to realize the Return On Investment than many bean counters (accountants) are willing to wait. Most will want an ROI of 18 months or less. If comparing a VFD against a throttling valve with flow rates concentrated between 50-90% most of the time, that usually works out well. But here you are just comparing one speed control method vs another, so the only efficiency gain is the difference in losses. The VFD will likely still win out based on energy costs eventually, but if the FC is already in place and you do not need to replace it for some other reason, then it might be difficult to justify on that alone. One study I saw on ECC systems (again, similar eff to fluid couplings) showed a savings of around $200k in 5 years going with VFDs, but at an installed cost of $295k. The bean counters likely laid that guy off long before that ROI went positive...
That said, I am doing a project right now wherein dozens of ECC drive systems are being replaced by VFDs, and the energy savings WILL pay for itself, because the ECC drive must be replaced ANYWAY due to failures and inaccessibility of replacement parts. So in this case the relatively low ROI on the efficiency improvements is only factored against the DIFFERENCE in cost between the VFDs and new replacement ECCs, in which case the ECCs will actually cost almost the same as the VFDs. This is partly because they will have to use soft starters now to start the AC motors, whereas the old system is so old they were "grandfathered" to allow Across-the-Line starting. They can no longer get the necessary variance from the utility to do so.
Bottom line, you have to look at EVERYTHING, not just the energy efficiency.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
