ThermoSolar and photovoltaic hybridization for small scale distributed generation : applications for powering rural health

Abstract

The problem of provisioning a remote health clinic or school with electricity, heating and cooling (trigeneration) is considered from an engineering design and optimization standpoint. A baseline technical-economic review of existing options is performed, and a novel alternative is proposed: micro-Concentrating Solar Power (CSP), featuring an Organic Rankine Cycle (ORC) using repurposed HVAC scroll compressors as expanders. The design of the [mu]-CSP technology is informed by a semi-empirical steady state multi-physics sizing and performance model (SORCE) which predicts system output, efficiency, and specific costs as a function of geoposition. Empirical validation of key mechanical and electrical components is performed to parameterize the model. On a levelized cost basis, ,-CSP is shown to outperform standard equipment for trigeneration applications at remote sites. Scroll expander development is identified as an opportunity for enhanced performance, and a computationally efficient method for selecting optimal thermo-mechanical geometries for a scroll expander is described. Tradeoffs between concentration ratio, power block size and thermal storage are examined, and the key role of thermal capacity in the system is highlighted. A semi-dynamic version of SORCE is developed to support optimization amongst system components in a simulated operating environment including insolation and thermal transients; this offers preliminary insights into control decisions that influence cost and performance, such as timing and power management of ORC operation. Finally, the concept of synergies between concentrating solar photovoltaic (CPV) and CSP architecture is explored. A semi-empirical diode model is developed using experimental data from commercially available a-SI and c-Si solar cells and incorporated into PV-SORCE (where the [mu]-CSP thermal absorber is replaced with a PV heat collection element). Optimization of design parameters influencing figures of merit (system efficiency and specific costs) indicates that an optimal configuration is highly sensitive to the PV properties; as such, further optimization of the hybrid system parameters is recommended. This research also involved lab and field (Lesotho, southern Africa) prototyping of small solar ORC units. Relevant design parameters and further development of the [mu]-CSP concept is discussed in the context of field experiences.