In this paper we present a dynamic three-dimensional volume element model (VEM) of a parabolic trough solar collector (PTC) comprising an outer glass cover, annular space, absorber tube, and heat transfer fluid. The spatial domain in the VEM is discretized with lumped control volumes (i.e., volume elements) in cylindrical coordinates according to the predefined collector geometry; therefore, the spatial dependency of the model is taken into account without the need to solve partial differential equations. The proposed model combines principles of thermodynamics and heat transfer, along with empirical heat transfer correlations, to simplify the modeling and expedite the computations. The resulting system of ordinary differential equations is integrated in time, yielding temperature fields which can be visualized and assessed with scientific visualization tools. In addition to the mathematical formulation, we present the model validation using the experimental data provided in the literature, and conduct two simple case studies to investigate the collector performance as a function of annulus pressure for different gases as well as its dynamic behavior throughout a sunny day. The proposed model also exhibits computational advantages over conventional PTC models-the model has been written in Fortran with parallel computing capabilities. In summary, we elaborate the unique features of the proposed model coupled with enhanced computational characteristics, and demonstrate its suitability for future simulation and optimization of parabolic trough solar collectors.

A new parabolic trough receiver design is tested. In this design, the annulus of the receiver is bifurcated such that the half facing away from the parabolic mirror, and receives minimal concentrated sunlight, is filled with an insulating material, whereas the half receiving the majority of the concentrated sunlight is allowed to be filled with air. By insulating the outward facing half of the annulus, heat loss by radiation is minimized. In the mean time, heat loss by natural convection due to the presence of air in the lower half of the annulus is expected to be significantly subdued, since the hotter air will be closer to the heat collection element, which is at a generally higher position than the glass envelope. Experimental tests were performed on roof-mounted troughs which utilize receivers with air-filled annuli. The system consists of two identical but independent rows. The receivers in the first row have normally air-filled annuli, while the receivers in the second row have annuli that are half-filled with an insulating material and half-filled with air. The results have shown that the thermal performance of the modified receiver was indeed superior to conventional receivers with air-filled annuli.

The annulus of a parabolic trough receiver is normally evacuated to prevent heat conduction between the internal absorber pipe and the external glass envelope. In the past, this vacuum has sometimes been compromised by hydrogen permeation from the heat transfer fluid through the absorber pipe. Heat conduction, and consequently receiver thermal loss, can be significantly increased by the presence of hydrogen in the annulus. Supplying receivers with inert gases in the annulus, or injecting receivers with inert gases after the vacuum has been compromised, could mitigate these heat losses. This study measures parabolic trough receiver heat conduction in the transition, temperature jump, and continuum regimes for argon-hydrogen and xenon-hydrogen mixtures at an absorber temperature of 350°C. Test results show that small heat loss increases over evacuated values are associated with the 95% inert gas/5% hydrogen mixtures and that from a performance perspective gas-filled HCEs would likely induce a 1–3% plant revenue decrease relative to evacuated receivers, but would protect against hydrogen-induced heat loss as long as there was sufficient quantity of inert gas in the annulus. Sherman’s interpolation formula predicted the inert gas and 95% inert gas/5% hydrogen mixture test results within experimental and model uncertainty, but did not accurately capture the larger hydrogen molar fraction test results. The source of this discrepancy will be further investigated.

Continuous improvement of integrated circuitry has allowed for the development of small, sophisticated portable electronics and microelectromechanical systems (MEMS) for a wide range of applications. Compared to the electronics and other system components, the batteries powering small electronics and MEMS are large and heavy. Thus, smaller and lighter power systems are required to advance future products. Electricity for small systems may be supplied by miniature heat engines, which transform chemical energy of fuel into thermal energy, kinetic energy and electricity with the use of combustors, turbines and generators. Combustion at small scales is challenging because system heat losses to the surroundings are large and flow residence times are short. Heat recirculation can be used to improve combustion performance by reducing these heat losses and preheating reactants prior to ignition. Practical heat recirculation systems must be small to keep the overall system volume and mass small. The objectives of this study were: (a) to investigate heat transfer in miniature combustors, and (b) to identify effective means of reducing heat loss from small combustors. The analyses indicated that axial conduction through the combustor wall and radiation across the preheating annulus were the most significant pathways for heat loss from the system. Several design improvements, including extended surfaces and porous inert media (PIM) were analyzed. A design featuring PIM in the annulus with a gap between the PIM and outer wall was the most effective method of reducing system heat loss.

NREL has fabricated a parabolic trough receiver thermal loss test stand to quantify parabolic receiver off-sun steadystate heat loss. At an operating temperature of 400°C, measurements on Solel UVAC2 and Schott PTR70 receivers suggest off-sun thermal losses of approximately 370 W/m receiver length. For comparison, a receiver from the field with hydrogen in its annulus loses approximately 1000 W/m receiver length. The UVAC2 heat loss results agree within measurement uncertainty to previously published data, while the PTR70 results are somewhat higher than previously published data. The sensitivity of several receiver performance parameters is considered and it is concluded that differences in indoor and outdoor testing cannot account for the difference in PTR70 thermal loss results.