Surface roughness impact on the heat loss of solar vacuum heat collector elements (HCE)

Heat losses from line focus solar collectors, such as parabolic troughs and Linear Fresnel Reflectors (LFR), are reduced by using selectively coated vacuum heat collector elements (HCE). The main parameters governing convective and radiative heat losses are the metal tube's outer surface radiative emittance and the vacuum level between tube and glass envelope. We investigate the effect of surface roughness on tube emittance as function of substrate roughness. Through a series of experiments, using several surface polishing techniques, such as grinding processes and electro polishing, a good correlation between tube surface roughness and heat loss was obtained. We show the limitations of representing the surface roughness by only using the roughness average (Ra) or the root mean square (RMS) roughness (Rq), especially when an electro polishing process is involved. A better metric is the roughness slope (1/Er) or RMS slope (Rδq) in addition to the roughness height (Rz) for which a good correlation to heat loss is demonstrated. While the electro polishing process is shown to reduce emittance and therefore heat losses, less costly and simpler changes in the grinding process can lead to a similar result. Recommendations for metal tube roughness yielding minimum heat loss are presented.