High-Efficiency Small-Scale Combined Heat and Power Organic Binary Rankine Cycles

Costante Invernizzi,Nadeem Sheikh

doi:10.3390/en11040994

Costante Invernizzi, Nadeem Sheikh

Open Access

https://doi.org/10.3390/en11040994

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Abstract

Small-CHP (Combined Heat and Power) systems are generally considered a valuable technological option to the conventional boilers, in a technology developed context. If small-CHP systems are associated with the use of renewable energies (biomass, for example) they could play an important role in distributed generation even in developing countries or, in any case, where there are no extensive electricity networks. Traditionally the considered heat engines for micro- or mini-CHP are: the gas engine, the gas turbine (with internal combustion), the steam engine, engine working according to the Stirling and to the Rankine cycles, the last with organic fluids. In principle, also fuel cells could be used. In this paper, we focus on small size Rankine cycles (10–15 k W ) with organic working fluids. The assumed heat source is hot combustion gases at high temperature (900–950 ∘ C ) and we assume to use only single stages axial turbines. The need to work at high temperatures, limits the choice of the right organic working fluids. The calculation results show the limitation in the performances of simple cycles and suggest the opportunity to resort to complex (binary) cycle configurations to achieve high net conversion efficiencies (15–16%).

Highlights

A sharp rise in global energy demand and ambitions to curtain global greenhouse gas emissions have led to various innovative alternatives in energy sector
The need to work at high temperatures, limits the choice of the right organic working fluids
Choosing appropriate working fluid for the operation of OrganicRankine Cycle (ORC) is essential for its thermo-economic viability, [1,2,3]

Summary

Introduction

A sharp rise in global energy demand and ambitions to curtain global greenhouse gas emissions have led to various innovative alternatives in energy sector. As the engines working according to the Rankine cycles convert external heat into mechanical work, we assumed, as a reference example, the availability of a mass flow rate of syn-gas of about. The selected working fluids are common refrigerants, the maximum temperatures are rather low (though the maximum temperature of the gaseous heat source is high) or, on the contrary, the considered Rankine cycles have a substantial superheating, often with volumetric expanders, [24]. In this work we assume as calculation hypotheses the use of only one stage (axial) turbine, to minimise the mechanical complexity of the expander, and to resort only to saturated thermodynamic cycles that, fixed the maximum operative temperature, have potentially the better thermodynamic efficiency. The efficiency of a hypothetical gasifier (or wood combustor) is not taken into account because we focused on the ORC engine

The Working Fluids

The Cycle Performances

Findings

Conclusions