Is there a standard way to track PUE for Liquid Cooled Workloads?

Liquid-cooled products each DC operator offers are still in a very early concept stage. Adopting a certain way of measuring and reporting liquid-cooled PUE is difficult. 

PUE impact will vary by liquid solution type, and several key parameters are at play that determine it. The first one is whether the server fans are accounted for in the IT load: 100% for air-only cooled IT, ~10-20% for direct-to-chip liquid cooling (DLC), and 0% for immersion cooling. The industry never tracked the server fan power component in the past, so it is not something you can deduct to get accurate like-for-like reporting.

Then, normally, direct-to-chip cooling has Cooling Distribution Units (CDU) that contain the pumps and liquid-to-liquid heat exchangers. There is a question as to who owns the CDU, if it is the data center operator, it could be easier to add it to the non-IT power PUE component whilst the opposite could be true if it is a custom-made liquid cooling solution and the Customer brought in their own CDU. 

There is another liquid-cooled DLC solution that has a liquid-to-air heat exchanger rejecting heat in the data hall, and then you have CRAH/CRACs to take it out. It is rather inefficient and expensive to install it in data halls just to end up meeting again the air flow management challange, but it seems to exist as an interim solution in the liquid cooling adoption. This liquid-to-air solution is more likely to have onboard pumps that are included as part of the IT load.

The liquid cooling SLA temperatures will determine whether you'd can connect your CDU to the chilled water loop or preferably directly to the condenser water loop to save yourself CAPEX.

All of these factors in the liquid cooling solutions add complexity to the PUE reporting, and if these solutions are in a shared data center, some blended PUE metric likely has to be developed to be used in chargeback models and reporting processes. Alternively, you'd have to track two PUEs - one for air and one for liquid-cooled IT. The liquid one excludes power contribution by some components such as CRAHs, Sec. Pumps, Chillers, Mech UPS based on the case scenario.

I trust this stirs up some good debate