• Minecraft: 1.20.1
• Forge: 47.3.10
• Powah: 5.0.8
  I tried to add a new coolant but it produces the same amount of energy that when i used water.
  Fluid goes in the generator and it says coolant - 250ºC but produces the same amount of power that water that as 1ºC

Powah conf
// List of coolant fluids used in the Reactor and the Thermo Generator.
"coolant_fluids": {
"minecraft:water": 1,
"mekanism:hydrogen": -250
},
thanks

I have the same problem playing on a modpack. Hopefully this will help, sorry if it's long, I just wanted to share my findings in hope we can fix the problem

• Minecraft version: 1.21.1
• Modpack: Craftoria 1.16.0
• Powah: 6.1.6

They added Ether Gas from Industrial Foregoing as a coolant for -116ºC and also normal Milk at 2ºC.
I'll join some screenshot of my observations.

Generation with Milk (2ºC) as coolant:

Generation with Water (0ºC) as coolant:

Generation with Ether Gas (-116ºC) as coolant:

We can see Milk which is a hotter coolant actually gives a bonus multiplier of 1.5 compared to Water at 0ºC and the Ether Gas at -116ºC.

I also have a screenshot of the code I tried to check where I suspect the mistake, the formula checks out with the bonus increases from the Milk.

I suspect the error to be within the Math.max function, since the coolant is negative it choose the max between 1.0 and 1.0 + a negative value, explaining the absence of change between Water and Ether Gas and the bonus you get from Milk as (1.0 + 2.0)/2 is equal to 1.5 which is greater than 1.0 and is the bonus I could see in my testing.

Thanks if you take the time to look at this issue and my analysis.

Good luck!