Joseph Ademar Araoz

As part of the many alternatives for the development of new methods for sustainable energy transformation, the Stirling engine is distinguished due to the characteristics of an external combustion engine, with many advantages. These engines can be activated with heat, representing an additional option for the use of renewable energy. The Free Piston Stirling Engine (FPSE) is special due to the elimination of all wearing mechanisms of a typical kinematic Stirling engine. A linear mechanical storage device replaces the crank device of the kinematic one, making possible long operating life, higher efficiency and zero maintenance. This document presents a theoretical model of a Free Piston Stirling Engine (FPSE) based on a dynamical and thermal analysis. As the core of the study, the “first order analysis” of the thermal machine is implemented by taking some ideal assumptions. A technique of evolutionary computation is presented as a feasible solution for finding functional correct design parameters. For the dynamical part, the mathematical model is obtained through energy balances, and then a stability study is implemented to assure the oscillations of the machine. Results include several cases, which offers parameters such as cylinder and rod area, cooler temperature, heater temperature, piston and displacer mass, piston and displacer stiffness, phase angle, frequency and power. It is critical to consider the sensibility of the system, and therefore, it is essential to apply a stability analysis in order to fully understand the behavior of the thermal machine. Results are compared are compared in terms of a calculated error considering first order analysis and the stability study developed with the Schmidt analysis.

The Bolivian government’s concerns that are related to reducing the consumption of diesel fuel, which is imported, subsidized, and provided to isolated electric plants in rural communities, have led to the implementation of hybrid power systems. Therefore, this article presents the performance analysis in terms of energy efficiency, economic feasibility, and environmental sustainability of a photovoltaic (PV)/Stirling battery system. The analysis includes the dynamic start-up and cooling phases of the system, and then compares its performance with a hybrid photovoltaic (PV)/diesel/battery system, whose configuration is usually more common. Both systems were initially optimized in size using the well-known energy optimization software tool, HOMER. An estimated demand for a hypothetical case study of electrification for a rural village of 102 households, called “Tacuaral de Mattos”, was also considered. However, since the characteristics of the proposed systems required a detailed analysis of its dynamics, a dynamic model that complemented the HOMER analysis was developed using MATLAB Simulink TM 8.9. The results showed that the PV/Stirling battery system represented a higher performance option to implement in the electrification project, due to its good environmental sustainability (69% savings in CO₂ emissions), economic criterion (11% savings in annualized total cost), and energy efficiency (5% savings in fuel energy conversion).

Trigeneration or combined cooling, heat and power (CCHP) systems fueled by raw biogas can be an interesting alternative for supplying electricity and thermal services in remote rural areas where biogas can be produced without requiring sophisticated equipment. In this sense, this study considers a performance analysis of a novel small-scale CCHP system where a biogas-fired, 5 kW_el externally fired microturbine (EFMT), an absorption refrigeration system (ARS) and heat exchangers are integrated for supplying electricity, refrigeration and hot water demanded by Bolivian small dairy farms. The CCHP solution presents two cases, current and nominal states, in which experimental and design data of the EFMT performance were considered, respectively. The primary energy/exergy rate was used as a performance indicator. The proposed cases show better energy performances than those of reference fossil fuel-based energy solutions (where energy services are produced separately) allowing savings in primary energy utilization of up to 31%. Furthermore, improvements in electric efficiency of the EFMT and coefficient of performance (COP) of the ARS, identified as key variables of the system, allow primary energy savings of up to 37%. However, to achieve these values in real conditions, more research and development of the technologies involved is required, especially for the EFMT.