A dynamic model for the efficiency optimization of an oscillatory low grade heat engine

Abstract

Abstract.A simple approach is presented for the modeling of complex oscillatory thermal-fluid systems capable of converting low grade heat into useful work. This approach is applied to the NIFTE, a novel low temperature difference heat utilization technology currently under development. Starting from a first-order linear dynamic model of the NIFTE that consists of a network of interconnected spatially lumped components, the effects of various component variables (geometric and other) on the thermodynamic efficiencies of the device are investigated parametrically. Critical components are highlighted that require careful design for the optimization of the device, namely the feedback valve, the power cylinder, the adiabatic volume and the thermal resistance in the heat exchangers. An efficient NIFTE design would feature a lower feedback valve resistance, with a shorter connection length and larger connection diameter; a smaller diameter but taller power cylinder; a larger (time-mean) combined vapor volume at the top part of the device; as well as improved heat transfer behavior (i.e. reduced thermal resistance) in the hot and cold heat exchanger blocks. These modifications have the potential of increasing the exergetic efficiency of the device by 50% points, corresponding to a 3.8% point increase in thermal efficiency.

Optimizing the Non-Inertive-Feedback Thermofluidic Engine for the Conversion of Low-Grade Heat to Pumping Work

“…The approach taken in modeling the NIFTE device is explained in detail in references [5][6][7][8][9], where these models are also validated against a range of experimental data from an early-stage NIFTE water-pump prototype [4,11,12]. Here, we summarize the main points of this approach.…”

“…• C [5]) between the heat source and heat sink, while having significantly simpler and smaller heat exchangers owing to the higher heat transfer coefficients associated with phase change (compared to single-phase forced convection).…”

“…In previous efforts of modeling the NIFTE device [4][5][6][7][8][9], the authors developed a set of spatially lumped and linearized submodels, with each submodel describing separately the dominant thermal and/or fluid process identified as taking place in a particular compartment or subsection of the NIFTE device. After using electrical analogies, as is the convention in this field, the resulting differential equations governing the important processes undergone in each of the spatial regions of the NIFTE were represented by passive electrical components, such as resistors, capacitors, and inductors.…”

“…In references [4][5][6][7][8][9] these parameters were evaluated for the geometric configuration and fluids used in an early NIFTE water-pump prototype presented in references [4, 11, and 12], and are denoted here as the "nominal" values of these parameters. Subsequently, individual parameters were perturbed within a reasonable range from their nominal values while all other parameters were kept constant, in order to investigate the influence of each parameter on the operation and performance of the system, especially the oscillation DHX models, where k j is the feedback gain for model " j," R i is a resistance, C i is a capacitance, L i is an inductance, P i is a pressure and U i is a volumetric flow rate.…”

“…Notably, they feature no or few mechanical moving parts. The particular type of thermofluidic oscillator investigated herein is known as the "Non-Inertive-Feedback Thermofluidic Engine" (NIFTE) [4][5][6][7][8][9][10], and is a two-phase oscillator within which the working fluid exists simultaneously in the liquid and vapor phases [4,11,12]. The cyclic evaporation (during heat addition) and condensation (during heat rejection) of the working fluid induces sustained thermodynamic (pressure, volume, temperature) oscillations, giving rise to oscillatory fluid motion, which is output as useful hydraulic work.…”