The complete flax stem, which contains shives and technical fibres, has the potential to reduce the cost, energy consumption and environmental impacts of the composite production process if used directly as reinforcement in a polymer matrix. Earlier studies have utilised flax stem as reinforcement in non-bio-based and non-biodegradable matrices not completely exploiting the bio-sourced and biodegradable nature of flax. We investigated the potential of using flax stem as reinforcement in a polylactic acid (PLA) matrix to produce a lightweight, fully bio-based composite with improved mechanical properties. Furthermore, we developed a mathematical approach to predict the material stiffness of the full composite part produced by the injection moulding process, considering a three-phase micromechanical model, where the effects of local orientations are accounted. Injection moulded plates with a flax content of up to 20 V% were fabricated to study the effect of flax shives and full straw flax on the mechanical properties of the material. A 62% increase in longitudinal stiffness was obtained, resulting in a 10% higher specific stiffness, compared to a short glass fibre-reinforced reference composite. Moreover, the anisotropy ratio of the flax-reinforced composite was 21% lower, compared to the short glass fibre material. This lower anisotropy ratio is attributed to the presence of the flax shives. Considering the fibre orientation in the injection moulded plates predicted with Moldflow simulations, a high agreement between experimental and predicted stiffness data was obtained. The use of flax stems as polymer reinforcement provides an alternative to the use of short technical fibres that require intensive extraction and purification steps and are known to be cumbersome to feed to the compounder.

Fluctuations in incoming solar energy adversely affect the temperature stability within solar receivers, leading to a decrease in thermal efficiency. Therefore, it is essential to design a control system with the capability to maintain quasi-steady temperatures inside the reactor consistently throughout the day. This study introduces a dual-actuator control technology to regulate the temperature within a high-temperature cylindrical cavity-type gas receiver. The actuating system comprises two primary components. The first component involves a variable aperture mechanism, executed through a rotary mechanism made of stainless steel. This mechanism features seven holes of fixed diameters arranged in a half-circle configuration. The rotary mechanism is powered by a stepper motor regulated by a feedback control system. The second actuator is a mass flow controller (MFC) responsible for meticulous adjustment of the inlet gas flow directed toward the solar receiver. The direct normal irradiance (DNI) is simulated using a 10 kW high-flux solar simulator (HFSS) with a variable power supply ranging from 80 to 200 A. This setup enables the simulation of different operational conditions. The dual-actuator method concurrently adjusts both gas flowrate and aperture size. While utilizing each of these methods individually can achieve reasonable temperature control performance, the hybrid approach leverages the strengths of both control methods, resulting in a significant improvement in the temperature regulation performance of the solar receiver. Two control strategies, namely, proportional integral (PI) and model predictive control (MPC), were implemented to regulate the temperature inside a cavity-type gas receiver. Experimental tests indicate that the incorporating the dual-actuator controller is a promising technique, and its application can be extended to include additional parameters for utilization in a multi-input multi-output (MIMO) control system.

The substantial energy provided by the sun is a promising substitute for traditional heat sources in various industrial applications. However, the transient nature of solar energy still poses a significant challenge to its widespread utilization. This work presents a methodology for regulating the temperature within a solar receiver by dynamically adjusting incoming sunlight through the aperture using a controlled iris mechanism. The performance of this technique is experimentally compared with the gas flowrate control method, which is typically used in industry. The proposed control system, grounded in the physical model of the solar receiver, underwent experimental testing under varying conditions, including different gas flowrates, simulator power levels, and aperture sizes. The collected data were then analyzed to estimate a simplified model of the solar receiver. A model predictive controller (MPC) is implemented using the model estimations, and its performance was assessed by tracking two set points (335 and 325 °C) over a period of 2 h. The experimental testing of both control systems indicates the superiority of iris mechanism over gas flowrate controller in terms of robustness, settling time, and smoothness. A hybrid control system utilizing both aperture size and gas flowrate is also developed and tested during the operation of the solar receiver via computer simulations.