Effective temperature glide of zeotropic mixtures for Lorenz cycle power generation

The temperature glide of zeotropic mixtures has usually been treated as a modest, efficiency-improving property: a glide of several kelvins, matched to an appropriate design point of a cycle that already operates. This study takes a different view and regards the temperature glide itself as a design feature of the cycle. We introduce the notion of an effective temperature glide, namely a glide large enough to follow the temperature drop of the heat source, and examine how such a glide, designed in advance through the boiling point difference of the mixture, improves the operating limits of small-scale power generation from sensible heat sources below 373 K, which organic Rankine cycles (ORCs) using pure working fluids cannot handle because of the rapid temperature drop. A Lorenz cycle with a vapor–liquid separator was modeled under constant heat transfer area constraints and compared with an R245fa ORC for hot water at 373 K and flow rates of 15,000, 7000 and 3000 kg/h. To isolate the role of the glide, three n-butane-based binary mixtures were chosen so that the standard boiling point difference of the pair increases systematically, from n-butane/n-pentane (36.6 K) through n-butane/ethanol (78.3 K) to n-butane/1-propanol (97.7 K) (the hydrocarbon/alcohol pairs themselves have scarcely been examined as ORC working fluids). At 7000 kg/h, the operable mixture conditions delivered 12.2–16.1 kW, exceeding the ORC by 8.9–43.8 % under identical equipment constraints. At 3000 kg/h no baseline condition could operate; widening the composition and pressure range, however, revealed an operable point (n-butane/1-propanol = 0.4/0.6 at 400 kPa) that produced 6.52 kW while the source temperature fell by 56.3 K. At this point the heat source leaves the evaporator colder than the cooling water leaves the condenser, so a Carnot-type reference between the terminal temperatures is not even defined; to the authors’ knowledge, power generation by a zeotropic-mixture cycle in this Carnot-undefined regime has not been reported before. A first-order second-law assessment attributes the enhancement not to a higher conversion efficiency but to a larger capture of the source exergy, with second-law efficiencies of 22–44 %. These results indicate that the glide is effective when it spans the temperature drop of the source; because the glide magnitude is determined by the standard boiling point difference of the pair, this condition can be checked from the vapor–liquid equilibrium before any cycle simulation. The vapor–liquid equilibrium of the mixture therefore provides a practical a-priori criterion for working fluid selection in low-grade heat recovery.