Numerical investigation of the energy performance of a solar micro-CHP unit

Abstract

This article presents the development of a numerical model of a micro-CHP prototype fueled by concentrated solar power. This micro-CHP unit converts concentrated solar energy into electricity and heat by coupling an assembled parabolic trough collector with an oil-free single-cylinder steam engine operating according to the Rankine cycle. The main originality of this study is the modeling of a large solar heating system (46.5 m2 aperture area), including a two-axis solar tracking system, and its integration into a dynamic thermal simulation code in order to evaluate its potential for building application, which is the objective of this work. The quasi-static modelling of the elements forming the primary conversion loop, allowing the integration of the system into a dynamic thermal simulation code, is also an innovation. In addition to limitations such as payback period, system effectiveness, technological issues (e.g. high pressure, hot steam water) and the availability of solar energy resources, the deployment of this technology suffered from the lack of a reliable model to predict both performance and energy production. In order to evaluate its potential, an experimental facility was modeled using Trnsys© and Matlab© software. Analytical models were developed for each element of the thermodynamic cycle. The numerical results obtained from these models were compared with experimental results. For a full day of operation, electricity and heat production at nominal conditions (1.2 kW and 15 kW, respectively) were satisfactorily estimated by the model. Once validated, the complete model was used to estimate the production potential of a solar micro-CHP unit located in La Rochelle, France (latitude 46°2′ North, longitude 1°1′ West). For an annual direct solar irradiance of 1241 kWh/m2, a yearly production of 11 671 kWh of heat and 1110 kWh of electricity was predicted. A parametric study then highlighted the impact of location and flow rate on the micro-CHP unit performance. Moreover, the two-axis solar tracking system appears to constitute an important advantage when the performance is measured over an entire year.