To balance it out, I think the basic heat exchanger should pollute half as much as the boiler (so 15/m). Otherwise, having clean energy that cheaply that early seems overpowered.

I do see your point of view, but I actually feel that the mod is balanced well enough as it is, all things considered.

To my mind, this mod primarily fulfills an aesthetic purpose. It deliberately provides green energy early on, which feels nice and is also useful for certain challenge runs. And, just as important, it remains competitive with (and can thus generally replace) vanilla solar panels + accumulators. It also combines well with coal backup power, and it just looks much nicer than its alternative, to me at least.

Regarding your suggestion, making the basic heat exchanger pollute feels plain wrong to me. And even if it creates some seemingly reasonable middle-ground between coal and vanilla solar power, this will greatly diminish the relative usefulness of the thermal solar complex already in the early mid-game, which runs completely counter to the goal of this mod. Besides, the thermal panels still take up a lot of space, and manufacturing them binds up a lot of the extraction/production capacity that is likely to be available in the early-game, where managing the amount and location of pollution and clearing a few nests isn't that difficult anyway, so I don't think its particularly overpowered. The real, practical benefits seem quite small to me, and it doesn't take that long until you can unlock and start producing solar panels either.

Since early green power is a goal of the mod, it has to depart a bit from the intended progression of the base game. It's a choice. That being said, it might be a bit overpowered how the mod allows you to produce green energy around the clock with the simple inclusion of steam storage. It's still "better" than the original mod, which didn't even need that, but it could be considered a weak point of the mod, and I don't really know how to address it. Maybe combining it with some other mod (if it exists) that makes fluids and gases dissipate heat over time could help make things more balanced, but there are definitely various potential problems with that.

Anyway, those are my immediate thoughts on the matter. What do you think?

I understand what you mean. The thermal panel set-up takes up a lot less space than solar panels and accumulators, though, making it technically easier than solar power. It’s easier to set up since it takes up less space, meaning you don’t need bots to set it up easily, and it produces a lot more power.

The normal heat exchanger doesn’t pollute, and it’s unlocked with only blue science. Assuming thermal solar power works with the vanilla heat exchanger, the basic one polluting 15–30/m would be fine as it’d only affect the pre-blue science early game.

If it doesn’t work with the vanilla heat exchanger, you could make the basic one pollute and add a second tier that doesn’t pollute, unlocked with blue science.

But if you don’t want to do this, that’s understandable.

I think your analysis of the power output is quite a bit off. I carefully balanced the thermal solar complex to be just slightly more space efficient than solar + accu + substations. To be more accurate, 27 panels with a nominal power rating of 105 kW should produce maybe ~1822 kW at its maximum (= 64.2% max. efficiency) and maybe ~960 kW on average during a full day cycle on Nauvis (= 33.8% avg. efficiency) That amount can also be produced consistently with ~23 solar panels and ~20 accumulators. Try to place that down along with substations and see how the sizes of the footprints compare!

I don't want the Basic Heat Exchanger to pollute, because it realistically makes no sense, and the thermal complex would lose an important aspect of long-term parity with solar + accu. Having two variants could be a solution, but... I don't want to make tiers just for that either.

By the way, if you just want an early-game renewable source of energy, which also pollutes as a side effect, you could instead consider using some sort of greenhouse mod (such as Krastorio 2 Greenhouse + Wood Pyrolysis, another mod of mine) and burn the wood for power! With the default settings, it should be about twice as space-efficient as solar + accu, but still pollute nearly as much as pure coal power (about 86.5%, assuming that the greenhouses in turn absorb all the pollution that they can). You could still use the thermal solar complex, but impose a rule on yourself about not storing steam for power, increase the nominal output somewhat in the settings, and only supplement power during the night with a Boiler that burns wood from a greenhouse, or coal. Just some ideas that you might consider experimenting with.