Experimental and numerical study of a micro-cogeneration Stirling unit under diverse conditions of the working fluid

Abstract

In the scenario of micro-cogeneration (or micro Combined Heat and Power, micro-CHP), Stirling units are a promising technology for residential applications because of high total efficiencies, favorable ratios of thermal to electrical powers and lower emissions compared to reciprocating engines and micro gas turbines. 1 The present work covers the experimental and numerical analyses of a natural gas-fired commercial unit capable of generating 8 kW of hot water (up to 15 kW with an auxiliary burner) and 1 kW of electricity. Although featuring in general a low electrical efficiency, these micro-cogeneration units are able to recover heat with a high thermal efficiency, thus qualifying positively in terms of primary energy savings. Moreover, the thermal-to-electrical power ratios of 8-to-1 (and 15-to-1 with the additional burner) are well suited for covering simultaneously the thermal and electrical loads of typical residences in continental climates. ☆This article is based on a short proceedings paper in Energy Procedia Volume 161 (2014). It has been substantially modified and extended, and has been subject to the normal peer review and revision process of the journal. This paper is included in the Special Issue of ICAE2014 edited by Prof. J Yan, Prof. DJ Lee, Prof. SK Chou, and Prof. U Desideri. AbstractMicro-cogeneration Stirling units are promising for residential applications because of high total efficiencies, favorable ratios of thermal to electrical powers and low CO as well as NO x emissions. This work focuses on the experimental and the numerical analysis of a commercial unit generating 8 kW of hot water (up to 15 kW with an auxiliary burner) and 1 kW of electricity burning natural gas. In the experimental campaign, the initial pressure of the working fluid is changed in a range from 9 to 24 bar g -20 bar g being the nominal value -while the inlet temperature of the water loop and its mass flow rate are kept at the nominal conditions of, respectively, 50 °C and 0.194 kg/s.The experimental results indicate clearly that the initial pressure of the working fluid -Nitrogen -affects strongly the net electrical power output and efficiency. The best performance for the output and efficiency of 943 W and 9.6%(based on the higher heating value of the burnt natural gas) are achieved at 22 bar g . On the other hand, the thermal power trend indicates a maximum value of 8420 W at the working pressure of 24 bar g , which corresponds to a thermal efficiency of 84.7% (again based on higher heating value). Measurements are coupled to a detailed model based on a modification of the work by Urieli and Berchowitz. Thanks to the tuning with the experimental results, the numerical model allows investigating the profiles of the main thermodynamic parameters and heat losses during the cycle, as well as estimating those physical properties that are not directly measurable. The major losses turn to be the wall parasitic heat conduction from heater to cooler and the non-unitary effectiveness of the regenerator.

Theoretical investigations of Stirling engine performances for different filling gas properties

“…The described model in Section 2, was applied to predict the performances of the FCSE manufactured by the Whispergen industry. The model follows the dimensions of an engine modeled and experimented with by Valenti et al 43,44 With a pressure range of 5 to 30 bar, nitrogen is employed as a pressurized filling gas. It has a maximum pressure load of 30 bars and a rotating speed of 1500 rpm.…”

Performance Analysis of a Stirling Engine Hybrid Power System

“…Geometric features such as heater, cooler, regenerator configurations, and cylinders capacity [29][30][31][32][33]; • Operating conditions (pressure and rotational speed) and performance characteristics (overall efficiency, power, heat exchangers effectiveness, heat losses, and energy dissipation) [34,35], and other studies have focused on additional parameters such as concentration ratio, hot and cold temperature, working fluid, and absorber temperature in order to analyze a solar dish solution [36][37][38][39]; •…”

Use of Micro-Cogeneration in Microgrids to Support Renewables

“…Simulations of two distinct areas in Italy are investigated for thermo-economic efficiency [8]. A microcogeneration Stirling unit is investigated for various conditions of the working fluid by experiment and simulation [9]. Efficiency and emission properties of a liquid fuelfired vugular burner for micro-cogeneration of thermoelectric power are proposed by an experimental study [10].…”