Date of Award

Spring 1-1-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Michael Brandemuehl

Second Advisor

Chuck Kutscher

Third Advisor

Moncef Krarti

Fourth Advisor

Harihar Rajaram

Fifth Advisor

Ed Wolfrum

Abstract

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 been compromised by hydrogen permeation from the heat transfer fluid through the absorber pipe. Heat conduction is significantly increased by the presence of hydrogen in the annulus even though its final pressure - 10 to 100 Pa - is sufficiently low that the thermal conductivity and temperature difference alone cannot predict it. Heat conduction for the concentric cylinder receiver geometry at these pressures is in the transition regime, where neither free molecule nor continuum heat conduction solutions apply. Most solutions to transition regime concentric cylinder heat conduction focus on single species, monatomic gases with small temperature difference boundary conditions. Further constraints limiting their applicability to this research include their typical wire-in-tube geometry and assumption of complete thermal accommodation on the outer cylinder surface. Much experimental data focuses on validating these solutions so is similarly constrained. This study measures heat conduction across the annulus of a parabolic trough receiver in the freemolecular, transition, temperature jump, and continuum regimes for argon-hydrogen and xenon-hydrogen mixtures at an absorber temperature of 350°C. Experimental values are predicted successfully by Sherman's interpolation formula and the Direct Simulation Monte Carlo Method. Depending on pressure, heat conduction of hydrogen in the annulus of a receiver can be greater than 500 W/m receiver length and decrease the annual net electricity production of a parabolic trough power plant by more than 50% relative to a plant with evacuated receivers. However, heat conduction can be reduced to 50-100 W/m when hydrogen is mixed with an inert gas such that the molar fraction of the inert gas is 95% or greater. This results in annual net electricity production penalty of 3-7% instead of more than 50%. Assuming 100 Pa of hydrogen in the annulus of a current receiver, the addition of 1900 Pa of xenon or 4900 Pa of argon will effect this reduction while avoiding natural convection in the annulus.

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