Stirling Engines for Distributed Low-Cost Solar-Thermal-Electric Power Generation

Abstract

Due to their high relative cost, solar-electric energy systems have yet to be exploited on a widespread basis. It is believed in the energy community that a technology similar to photovoltaics, but offered at about $1/W, would lead to widespread deployment at residential and commercial sites. This paper addresses the feasibility study of a low-cost solar-thermal electricity generation technology, suitable for distributed deployment. Specifically, we discuss a system based on nonimaging solar concentrators, integrated with free-piston Stirling engine devices incorporating integrated electric generation. We target concentrator collector operation at moderate temperatures, in the range of 120°C to 150°C. This temperature range is consistent with the use of optical concentrators with low-concentration ratios, wide angles of radiation acceptance which are compatible with no diurnal tracking and no or only a few seasonal adjustments. Therefore, costs and reliability hazards associated with tracking hardware systems are avoided. This paper further outlines the design, fabrication, and test results of a single-phase free-piston Stirling engine prototype. A very low loss resonant displacer piston is designed for the system using a very linear magnetic spring. The power piston, which is not mechanically linked to the displacer piston, forms a mass-spring resonating subsystem with the gas spring, and has a resonant frequency matched to that of the displacer. The design of heat exchangers is discussed, with an emphasis on their low fluid friction losses. Only standard low-cost materials and manufacturing methods are required to realize such a machine. The fabricated engine prototype is successfully tested as an engine, and the experimental results are presented and discussed. Extensive experimentation on individual component subsystems confirms the theoretical models and design considerations. Based on the experimental results and the verified component models, an appropriately dimensioned Stirling engine candidate is discussed.