Direct solar production of medium temperature hot air for industrial applications in linear concentrating solar collectors using an open Brayton cycle. Viability analysis

Abstract

Medium temperature heat required by industrial processes worldwide is mostly provided consuming fossil fuels. Among renewable energy alternatives, concentrating solar collectors can provide medium temperature heat at decreasing costs, either for medium and small scale applications. Several high energy-demanding processes need hot air in the range 150–400 °C (drying, curing, dehydration, thermal treatments). For such applications, the direct air heating inside linear concentrating solar collectors is investigated in this work. Although is not a common practice, direct heating reduces installation costs and maintenance, eliminating the need for heat transfer fluid and heat exchanger. The theoretical analysis developed indicates its feasibility despite the high pumping power consumption. An innovative system using Brayton cycle is proposed and analyzed. An automotive turbocharger is used to compress inlet air, with a compression ratio from 1.5 to 4, before solar heating up to the technology limit of 600 °C, thus reducing and even avoiding pumping power consumption. Hot air expands through the turbine recovering the compressing power, holding at the outlet suitable temperature for industrial usages in the range of 300–400 °C. No mechanical power at the shaft is expected; instead, turbocharger freewheeling enables to drive air through the collectors without auxiliary energy consumption, configuring a compact solar installation. Numerical results provided support the viability, showing the performances and critical parameters.