Reprint ed from ENGINEERING FOR POWER, Vol. 100, No. 1, Januar y 1978 W. S. Duff Solar Energy Applications Laboratory, and Department ol Mechanical Engineering, Colorado State University, Fort Collins, Colo. Introdu ction Performance Analysis of a Pancake Absorber-Helical Heat Exchanger for a Solar Concentrator This paper presents an analysis of the performance of a pancake absorber wit h a helical heal exc hanger for use with point focus solar concentrating collectors. Th e principal dif- ference between this type of absorber and most other concentrating solar collector absorb- ers is that the intensity of co nc entrated radiation monotonically increases fr om the inl et to the outlet of the heat exchanger tube. A performance expression for this absorber type and ra.diation distribution is deriued and u ed to calculate thermal performance for seuer- al different absorber designs. Th ermal performance for these design. is then calculated using a performance anal ysis based on the simplifying assumption that the heat exchang- er tub e is uni f ormly irradiated and the two pe rformances are compared. The co mpari son indicat es that the differe nt radiati on distribution assumptions lead to di ff ere nt absorber design choices. In particular, the si mplif yi ng assumption ouerstates thermal losses for increases in absorber size . A heat exchanger tube in fl at plate co ll ectors and a single ab- so rber-heat exchanger tube in line foc us olar coll ectors is exposed to a uni fo rm radiation intensity along its length. Even fo r the heat exchanger tube wound around a sp he ri cal absorber in a point focus co ll ector, the assumption of uniform radiation along the length of the tube may be a reasonable approximation. In this case there is often a cy li cal variation of radiation in tensity consisting of many cycles and the integrated effect may be reasonably app rox im ated by assuming constant irradiation along the tube length. However, the uniform radiation in tensity assumption is not adequate for a pan cake absorber with a he li ca l heat exchanger tube. Contr ibu ted by the So lar Ener gy Division for publication in the JOURNAL Or ENGi EER ING F'OR POWER. Manuscript received at ASME Headquarters March 14, 1977. This paper illu st rates the performance of the pancake absorber- heat exchanger under va ri ous design cond itions. In addit ion, a com- parison is made with the results of an analysi based on the simplif yi ng Al = the area of one s id e of the pan ca ke ab- sorber Ap = aperture area of the concent rator Cp = specific heat of the heat transport fluid d = diamete r of the heat exchanger tube F = defined by equat ion (3) F' = defined by equatio n (5) F" = defined by equation (2) g = measure of spread of the solar r adiat ion intensity distribution ho = convection coeffici ent between the tube wal l and the circulat ing fluid Io = intensity of the direct compone nt of 178 I VOL 100, JANUARY 1978 solar radi at ion, energy rate per unit area K lin = thermal conduct ivity of the heat ex- changer tube and fin material ri1 = ma s flow rate of the circulating fluid Qu = tota l rate of useful energy gain qu = local useful rate of ener gy ga in per unit area T 0 =ambient temper ature Tb (x) = heat exchanger tube or fin base temperat ure at point x tr.n =ab orber fin thickness Tr(x) = fluid temperature in the heat ex- change r tube at point x T; 0 =inlet fluid temperature of the heat ex- changer tube Tm = mean fluid temperature Tout = outlet fluid temperature of th e heat exchanger tube U1, = absorber-heat exchange r overa ll loss coe ffi cie nt x = distance from th e inl et along the heat exchanger tub e a = ab orpta nce of absorber < = emitta nce of absorber t = lengt h of the heat exchanger tube Pave = average reflectance of re fl ect ing con- centrato r (o r transmittance of lens) r = transmitta nce of any absorber covers Transactions of the ASME