Your thermodynamic concerns are well taken, and it is certainly not practical to recover the low temperature heat from a typical cooling tower at a thermal power plant to make useful energy. However, that only highlights the problems with a paradigm that starts by maximizing power output.

Our approach when designing a typical “topping cycle” cogen plant is to maximize efficiency and operating margins by making a bit less power than one might make in a power-only approach so as to leave high value residual heat in the tail exhaust that can be more fully used. Note that this gets substantially more useful energy out since the customer who needs high grade heat needs the full heat of vaporization (effectively, the lumber mill/chemical plant/etc. becomes a cooling tower, using the full ~1000 Btu/lb available rather than just the few degrees of latent heat.)

In a waste heat recovery facility, we do the reverse, taking high grade heat that is being dissipated from a manufacturing facility to generate heat and/or power that would otherwise require the use of additional fossil fuel.

However, an energy conversion system capable of operating cost-effectively from such a heat source could capture much of the 40% heat normally dissipated by the cooling system. My company has such a technology, but the political and societal roadblocks that stand in the way of full scale demonstration and commercialization are immense. I’m sure the Casten father/son team
are well aware of such a situation!

Another viable application for low-grade heat is to dry out the woodchip fuel. Humidity in this form of biomass is notoriously high and has a significant impact on the heat rate of the facility.

Alan Belcher.