Producing work from waste heat is technically problematic. It doesn't really matter if you have the best engine design in the world, although your odds get worse if you don't.
Every thermodynamic cycle has a theoretical peak efficiency (which can never be reached in the real world) whereas the peak theoretical efficiency for any thermodynamic engine is described by the Carnot cycle. This can be expressed as (Tin-Tout/Tin) where the inlet gas temperature (Tin) and exhaust gas temperature (Tout)are given in absolute temperature (any temperature scale where zero degrees is at absolute zero).
And there's the rub. Waste heat implies that the heat was already used in some other process and this is what is left over. By definition, it means the gasses are much cooler than they were before being used to perform the primary work. So, we're starting with a more diffuse source of energy. At the same time, there is no practical way to extract more work in such a way that the exhaust gasses are that significantly cooler because we have to exhaust to the ambient environment.
From the above we see that, in the large majority of applications, a waste heat engine is going to have relatively low efficiency because the difference between absolute temperature in and out is much smaller than in most prime movers. Combine this lowered efficiency with the fact that the gasses powering the WHE have relatively low available energy since the "good stuff" was already extracted by the prime mover and it's easy to see that WHE output will be small compared to that of the primary system.
This low output is perfectly acceptable IF the total WHE price, operating expenses, maintenance, associated plant expenditures, depreciation and so on yield a lower overall cost per unit of output than that of the prime mover --- including its expensive fuel. In plain English, the WHE has got to be cheap, simple, reliable, durable and automatic. It is hard to argue that your expenses will be very minor when you start using multiple cylinders, a boiler, feed pumps, condenser and so on. It's just my opinion but,to me,it appears that WHE systems either have to be a simple add on (such as a single stage exhaust turbine bolted to the manifold) or be sized to recover energy from large systems so that economy of scale works in its favor.
This is another instance where I question if anyone did an impartial analysis or even developed a comprehensive business plan. It would appear that a WHE has to meet pretty stringent parameters to pay for itself and we can assume most businesses will want a detailed explanation of how savings are generated. For the life of me, I can't figure out how to make any Rankine cycle system competitive at the kinds of horsepower contemplated.
