Final Program EXPRES 2012 - Conferences

nighttime, to weather or to the changing seasons, the SPSconcept has the potential to achieve much greater energyefficiency than ground based solar power systems (interms of utilization of fixed capacity) /12/.II.PRESTIGIOUS CONCENTRATED SOLAR RADIATIONTECHNOLOGYSolar technologies are maturing and new opportunitiesare emerging due to continuing improvements in designs.Concentrated solar radiation (CSR) technologies,combining less-expensive optical components with smallarea, highly efficient, and somewhat more expensivedevices to achieve low cost, in general, are on thethreshold of significant developmentThe stretched membrane concentrator cluster andinnovative cavity - heart type high temperature receiverfocally positioned present attractive solution for thermalpower and photo-thermal applications – second award at theDOE and Southern California Edison Solar Two ChallengeDesign competition 1994 KU Lawrence team (/4/, Fig.1).Concentrated solar radiation transmitted by the receiver'scover and adjacent liquid is absorbed by the absorber whichhas a black nonselective coating and is with its integratedliquid passages immersed directly inside the circulatingstream of liquid /4/. Highly specularlly reflective internalskin of receiver walls reduces the complex of radiative,conductive and convective heat losses and results in a moreefficient alternative to highly insulated receiver enclosures.Steady-state energy flux losses can be expressed as a sumof reflective radiation losses and heat-transfer losses:qg= qr+ qtg(3)Radiative losses are given as.( )4 4q = ε⋅σ⋅A ⋅ T − T (W) (4)z p z aConvective q kon and conductive losses q pr are::q kon +q pr = A z (h+k z /δ z )·(T z -T a ) (5)where the receiver's wall temperature, ambienttemperature, receiver wall surface area, wall thickness,thermal conductivity and convective heat transfercoefficient are denoted by T a , T z , , A z , δ z , , k z , and hrespectively. Thus total heat transfer losses are4 4( ) ( δ ) ( )qg = qz + qkon + qpr = ε⋅σ⋅Ap ⋅ Tz − Ta + Az h+ kz z⋅ Tz −Ta(6)1.00.9η0.80.70.60.5CR1002334005006008001000400 600 800T z a)1.00.8η0.60.40.2CR1002334005006008001000Figure 1. Stretched membrane concentrator cluster and focallypositioned high temperature receiver.The receiver's instantaneous thermal efficiency is equalto the ratio of heat supplied to the receiver's workingfluid and used for the sodium evaporation and itsisothermal expansion q krf versus the incident concentratedsolar radiation flux q:qmkrfη = = 1 − ∑ q g qqk=1(1)where q g presents receiver's total heat losses -reflective, radiative, convective and conductive. Theconcentrated solar radiation energy flux incident in thereceiver's plane is a function of the concentrator field areaA g covered by heliostats A h = φ ⋅ A g = φ ⋅ CR ⋅A p , isgiven as:q = I b ⋅ φ ⋅ CR e ⋅ A p ⋅ ρ ⋅ r (W) (2)where I b , CR e , ρ and r intensity of incident directsolar radiation, effective concentration factor, heliostatsfield efficiency and its mirror reflectance, respectively.0.0400 600 800T z b)Figure. 2. Receiver's heat losses (a - radiative losses, b - total heatlosses) dependence on solar radiation concentration factor and thereceiver's temperature.Solar radiation concentration factor and temperatureinfluence on receiver's simultaneous short and long waveradiative, convective and conductive heat transfer losses arepresented on diagrams in Figure 2.1.00.80.6η0.40.2T z500K600K700K800K900K1000K0.00 1000 2000 3000 4000C R eFig. 3 Dependence of the instantaneous thermal efficiency on theeffective concentration factor and receivers wall surface temperature.33