Numerical study of heat loss from a non-evacuated receiver of a solar collector

• CFD model on heat losses from air filled receivers with different annular gaps.
• Non-uniform temperature distribution on receiver pipe i.e. sinusoidal and square wave.
• Comparison with uniform temperature case.
• Critical radius ratio (RR).

Heat loss from receivers of solar collectors is a major reason for drop in their efficiency. Receiver pipes enclosed in glass tubes with evacuated annulus show considerable reduction in heat losses. However, manufacturing and maintenance costs for such receivers are high. An inexpensive alternative is a similar receiver with non-evacuated annulus. This paper presents a numerical study of heat loss from a non-evacuated receiver typically used in parabolic trough collectors, generating moderate temperatures and designed particularly for process heat applications. In order to come closer to the realistic situation, rather than assuming uniform temperature distribution on it, receiver pipe temperature has been assumed to be varying along the surface. Sinusoidal and square wave functions are employed in modeling, since actual temperature distributions on solar receiver pipes are combinations of these two functions. Main goal of the paper is to optimize the design of the non-evacuated solar receiver for minimum heat loss, by using CFD technique. Also effects on heat loss from receivers due to different parameters like average temperature (Ta) of the pipe, non-uniformity in the temperature (α) along its surface, hour angle (γ), denoting position of the sun in the sky and radius ratio (RR) of radius of receiver pipe to that of outer glass tube have been studied. It is seen that as non-uniformity in temperature distribution increases in both types of temperature distribution, heat losses from receiver pipes decrease up to 10%. Also as hour angle increases from 0° to 90°, heat loss decreases by 20% in case of sinusoidal temperature distribution and 24% in case of square wave temperature distribution. The effect of radius ratio (RR) on heat loss has been studied. In present study, we found out that 1.375 is critical radius ratio for which heat losses from receiver are minimum