Thermal conversion with fluorescent concentrators

Solar Energy Vol. 36, No. I, pp. 27-35, 1986

0038-092X/86 $3.00 + .00 © 1986 Pergamon Press Ltd.

Printed in the U.S.A.

THERMAL CONVERSION WITH FLUORESCENT CONCENTRATORS W. STAHL, V. WITTWER, A. GOETZBERGER Fraunhofer-Institut for Solare Energiesysteme, D-7800 Freiburg, West Germany (Received 21 March 1983; revised 1 May 1984; replacement manuscript received 26 July 1985) Abstract--Fluorescent planar collector-concentrators are a new possibility for the conversion of solar energy into thermal energy. The collection and concentration of direct and diffuse radiation is feasible, using a transparent sheet of material doped with a fluorescent dye. The collector offers the advantage of separating the global irradiation into different spectral regions. This geometrical and spectral concentrated light can be converted with adapted highly selective absorbers into high temperature heat. Intensity and spectral region of the sunlight and the selectivity of the absorber determine the thermodynamically possible maximum absorber temperature. A test collector with a fluorescent concentrator area of 0.8 m 2 with an absorber pipe of 3 mm diameter in an evacuated glass tube was built. At a total irradiation of 850 W/m2 on the fluorescent collector surface, a maximum stagnation temperature of 555"C (828 K) was reached. Under diffuse light conditions (150 W/m2), stagnation temperatures above 250"C (523 K) were measured. Thermodynamic calculations, experimental setup and results are given.

1. INTRODUCTION

thermal radiation of the metal. Absorbers of that type can have a high selectivity. This is shown to be very important for the generation of high temperature heat.

The effective conversion of solar energy into heat of different temperatures is one of the main aims of solar energy research. While in the low temperature range, for example domestic hot water, systems with good efficiency are available, this is not so in 2. GENERAL THERMODYNAMIC LIMITS the high temperature range. First experiences with OF ABSORBER TEMPERATURES concentrating systems show that besides the probThe angle ps = 0.54 ° which the sun subtends at lems of exact tracking and loss of the diffuse rathe earth determines the temperature which a black, diation, low long-term efficiencies are measured. nonselective absorber can attain when it can gain Up to working temperatures of 150°C (423 K) evacand lose energy only by radiation. Neglecting the uated tubular collectors with selective absorber earth's atmosphere the Stefan-Boltzmann law for coatings are another possibility without the disadthis radiation exchange states vantages of concentrating devices. A new way of generating high temperature heat is with the flutrT 4 = (sin 2 ps/2)crT~, (1) orescent planar collector concentrator. The principle of the fluorescent concentrator has been described in several papers[l, 2] and conference where tr = 5.67 x 10 -8 W/m 2 K 4 is the Stefanproceedings[3]. A thin transparent sheet of mate- Boltzmann constant. With Ts = 5762 K the maxirial, e.g. P M M A doped with a fluorescent dye ab- mum temperature is T = 393 K (120°C). Geometric sorbs light on the large surface. Emitted fluorescent concentration of the sunlight raises the absorber light is guided by total internal reflection to the temperature proportional to the fourth root of the edges of the sheet. This, the concentration of dif- concentration. Disadvantages are the necessity of fuse as well as direct sunlight, without tracking the exact tracking and the loss of the diffuse component sun, is an important advantage. Goetzberger[4] was of the solar radiation. Ps determines the upper limit the first to discuss the possibility of using the flu- for this concentration[5]. orescent concentrator for thermal energy generaFigure 1 gives the dependence of maximum abtion. The radiation collecting part is separated from sorber temperature on the concentration factor. For the absorber. Due to the geometric concentration, the absorptance the value one independent of the the absorber is small, which results in low thermal wavelength is assumed. Kirchhoff's law states that losses. Specially under low diffuse radiation levels for every wavelength the absorptance is equal to good efficiencies are expected. Due to the intensive the emittance of a surface. So this absorber is also fluorescent light emitted in a narrow wavelength in- a very good emitter of thermal radiation, which reterval new selective absorbers can be used. F o r ex- duces the thermal energy output. Selective absorbample, an interference layer on a polished metal ers cannot overcome Kirchhoff's law but a strong substrate gives a high absorption in a narrow wave- wavelength-dependent absorptance is possible. length range without affecting the low emission of Due to the difference between high surface tern27

28

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w I

w

STAHL

I

T[KI 5000 4000

3000

et al.

f = 0.74 gives a wavelength-independent reduction of solar radiation due to the absorption of the earth's atmosphere, a, and e are the absorptance and emittance of the absorber changing like a step function from high to low values at the wavelength he. Radiation equilibrium, which arises at the stagnation temperature is expressed as Es = E,h.

(5)

A numerical solution of this integral equation gives the dependence of the stagnation temperature of the absorber on the critical wavelength go. For a the! oretical value e = 0, this is shown in Fig. 4 as a 1000 dashed line. In this case T is independent of as. C I ;! C2 While for e = 0 T approaches the temperature of s I , I ! . I . I the surface of the sun as kc--* 0, for e # 0 the 10 IO0 1000 1OO00 maximum stagnation temperature are strongly reduced. Figure 2 shows an example of the depenCOEO dence of the integrals Es (dashed line) and Eth on Fig. I. Maximum absorber temperature versus geometric h~ for e = 0.02 and different absorber temperatures concentration factor Cseo. Indicated are the maximum concentration factors for one and two dimensional con- T; ets was chosen to be 0.95. Each intersection of a curve Eth with the curve Es is a point of radiation centration (cl, c:). equilibrium. As h~ ~ 0 the thermal radiation Eth leaving the absorber approaches the value Eth = perature of the sun and working temperature of the e(rT4 given by the Stefan-Boltzmann law. The maxsolar absorber the spectral energy distributions are imum possible absorber temperature is reached, if well separated. the curves Es and Eth only have one point of conA combination of a selective absorber with geo- tact. This point also determines the value of k¢. So metric concentration of solar radiation can increase the influence of the emittance on the maximum posthe efficiency but the effects are small. The reducsible stagnation temperature with optimized h~ can tion of absorber area due to the concentration re- be evaluated. This is shown in Fig. 3. duces the importance of a low emittance. SeraFigure 3 shows how sensitive the absorber temphin[6] points out that the higher the geometric perature is due to the emittance of the absorber. concentration the less important the emittance is. Table 1 gives the critical wavelength k¢ in addition In unconcentrated sunlight the selectivity of the ab- to the corresponding emittance and maximum absorber is the key for generating heat at temperature sorber temperature. It is essential that hc is not in levels over the domestic hot water temperature the region of the solar spectrum for temperatures range. Again taking only radiation losses into ac- lower than 800 K and therefore does not reduce count, absorbed solar radiation Es and emitted ther- the absorption of solar radiation. Low emittance is mal radiation Eth of the absorber can be written[7] an important requirement for generating high temperature heat with flat plate collectors. Besides selective absorbers having an absorpEs = fo x~ affglE~ dh tance changing in the ideal case like a step function it is also possible to think about absorbers with an + f ~ efflE~ dk W/m 2, (2) absorptance behaving like a delta function. From a theoretical point of view we can have an absorber that absorbs and emits radiation at only one waveEth = o t s E x t h dk length. Planck's radiation formula (4) solved for T(X) gives + f ® 8E~.th dk W/m 2, (3) J~,c Cz 1 with T(R)= h ln(k~__~ + 1) K. (6) C1 W/m e Izm (4) Ex = hS(exp(C2/hT ) _ 1) 2000

being Planck's radiation formula. The solid angle factor fl is given by

1

= ~ (1 . cos p,).

T(h) is the so-called effective temperature of a radiator which absorbs and emits according to KirchhoWs law only at the wavelength k. Taking E~ to be the Planck spectrum of the sun with T, = 5762 K, and corrected with the solid angle factor fl, the atmosphere's absorption factor f and a spectral

Thermal conversion with fluorescent concentrators

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1000

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LLI

500

I

10

;I,C [,urn] Fig. 2. Integrals Es (dashed line) and Eth a s a function of he. e = 0.02. a, = 0.95. Different temperatures T. Intersections of curves Eth with curve E, are points of radiation equilibrium.

concentration c, of the sunlight, we get

wavelength interval AX. T(X) is shown in Fig. 4 for c, = 1, c, = 10 and c, = 100. This spectral concentration can be achieved for example with the C2 1 fluorescent concentrator. The dashed curve from eqn (5) is shown for comparison. It is worth noting In c~fll - 1 + exp C2 that also in the case of absorption in only a small wavelength interval, absorber temperatures are as c,fa high as in the case of absorption from zero wavelength to he. The strong dependence of emitted raAs an approximation, (7) is also valid for a small diation from the wavelength in the " W i e n " region of the Planck spectrum is responsible for the fact that maximum absorber temperature is mainly deT [*C] termined through the long wavelength limit. So abI I I I I I I I I sorber temperatures for the step function or delta T[K! 1000 function absorptance do not differ very much when the long wavelength limit is equal. This is still more

1000 Table 1. Emittance, maximum absorber temperature and corresponding critical wavelength kc (ct, = 0.95)

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1 2 3 t. 5 6 7 8 9 emittance [%] Fig. 3. Possible stagnation temperatures in unconcentrated sunlight in dependence of the emittance e with optimized he. % = 0.95.

e

T[K]

Xc[~m]

0.01 0.02 0.03 0.04 0.05

1036 898 823 772 734

1.05 1.25 1.35 1.47 1.59

0.06

704

1.67

0.07

680

1.75

0.08 0.09 O.10

659 641 626

! .80 1.88 1.93

30

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et al.

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3000

T [K]

m D

2000

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tC, X,[um] Fig. 4. Effective temperature of an absorber, absorbing and emitting only at wavelength ), in sunlight with spectral concentration Cs = l, C, = l0 and C~ = 100. The curve of equation 5 is shown as a dashed line. surprising because there are large differences in the total amount of absorbed radiation. Due to the smaller amount of energy absorbed in the case of the narrow linewidth absorber, the efficiency of such a solar system is reduced. However, using the nonabsorbed energy with another system can lead to good overall efficiencies. The delta function absorptance opens new possibilities for preparing highly selective absorbers. In unconcentrated sunlight, high absorber temperatures are also possible. In the following it will be shown that fluorescent concentrators are an optimal sun powered light source for delta-function absorbers.

mum possible concentration ratio with geometric concentration. Thermodynamically they work like an optical heat pump. The advantage of fluorescent concentrators in combination with adapted highly selective absorbers can be demonstrated by comparison of an optimized fluorescent concentrator system with conventional systems for generating work within a thermodynamic cycle. This is shown in Table 2. Imaging concentrators can compensate for the disadvantage of concentrating only direct light by tracking the sun; the low area efficiency and the inconvenience of tracking remain. The flat-plate

Table 2. Comparison of an optimized fluorescent concentrator system with two conventional systems

3. F L U O R E S C E N T PLANAR C O L L E C T O R CONCENTRATORS

Fluorescent concentrators are absorbers and emitters for relatively narrow wavelength ranges. Different fluorescent dyes can absorb diffuse and direct sunlight in different regions o f the solar spectrum. The emitted light is Stokes-shifted to longer wavelengths. Combining different fluorescent dyes leads to a higher absorption. The emitted fluorescent light is still in a small wavelength interval, because reabsorption o f the dyes is responsible for the fact that the emitted light is that of the dye with the longest wavelength absorption. The energy loss due to the Stokes shift is closely related to basic thermodynamic principles[8, 9]. It can be shown theoretically that fluorescent concentrators could reach higher concentration ratios than the maxi-

1: Two dimensional concentrator system 2: Selective flat-plate collector system (a = 1 for wavelengths 300 < g < 1500 nm) 3: Fluorescent planar coUector-concentrator system (a = 1 for wavelengths 450 < k < 700 rim)

1 2 3

~E

~opt

~

1 1 0.45

0.8 0.8 0.5

0.5 0.8 0.8

T~°C

~th

RE

5~ 200 700

0.9 0.7 0.9

0.6 0.4 0.7

"qr solar absorption efficiency, ~opt optical efficiency, area efficiency, Tw/°C working temperature, ~tb thermal efficiency, "tic carnot efficiency, -q total efficiency

0.22 0.18 0.12

31

Thermal conversion with fluorescent concentrators

system cannot reach working temperatures high enough for good Carnot efficiencies. The spectrally selective fluorescent concentrator can compensate for the radiated energy losses by the improvement of the Carnot efficiency and so is almost comparable with the other systems; there remains the advantage of using the transmitted portion of the solar spectrum, e.g. for low-temperature heat. Thus many other possibilities are offered with the partly transparent fluorescent concentrator, so for example a hybrid system, with a fluorescent concentrator for high temperature generation acting as the cover of the conventional flat-plate collector. Also partly transparent covers of greenhouses can be made from fluorescent concentrators which use a small portion of the solar energy for generating heat or other forms of energy without diminishing the growth of the plants.

1.0 d0.8

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C 0

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0.4

3.1 Energy concentration

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400 500 600 700 ,~ [nm] Fig. 5. Absorption spectrum of the fluorescent dye.

The geometric concentration factor of fluorescent concentrators is defined as the receiving surface area divided by the area of the edge. For a 1 m 2 sheet with a thickness of 3 ram, the geometric concentration factor C is 83. The geometric concentration would be equivalent to the energy concentration if the collector absorbed all the sunlight and guided it to the edges without losses. Absorption efficiency of the solar radiation, quantum efficiency of the dye, the loss cone of nonreflected fluorescent light and the reabsorption of fluorescent light in the sheet determine the optical efficiency of a fluorescent concentrator[10, 11]. Optical efficiency ~opt multiplied by the geometric concentration factor gives the energy concentration factor at

1.0 5

o.8

~ 0.6 "i 0.4 ~.2

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!

,100

500

600

,~ [rim]

m

700

Fig. 6. Emission spectrum of the fluorescent dye.

800

32

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Table 3. Energy concentration for two different fluorescent collector sizes collector size [cml

C,,,o

"qopt

C,r,,,-,y

C,p,ctr~

40 x 40 x 0.3 100 x 100 x 0.3

33 83

0.07 0.05

2.3 4.15

18 32

the edge. The spectral concentration C~ is defined as the ratio of radiation output at the collector edge divided by the solar energy in the absorption interval of the fluorescent dye. The high spectral concentration factors measured justify the factors assumed in the theoretical calculations [eqn (7)]. Measured values for two collectors with the same fluorescent dye (optical properties Figs. 5 and 6) are given in Table 3. The energy concentration multiplied with the solar radiation on the collector surface gives the radiation output at the collector edge. 4. EXPERIMENT

The matrix material of the fluorescent collectors is polymethylmetacrylate (PMMA). In these experiments, a dye having the absorption spectrum given in Fig. 5 was used. The concentration of the dye was chosen so that all the sunlight received at the absorption maximum was in fact absorbed. In total, this dye absorbs about 18% of the sunlight. The emission spectrum of the dye at the edge of the collector is shown in Fig. 6. The experimental setup is shown in Fig. 7. Two fluorescent collectors with a total area of 0.8 m 2

absorb direct and diffuse sunlight. The emitted light can leave each collector at only one edge (the energy concentration factor was about 6). Here is situated an evacuated glass tube (p < 10 -4 mbar) through which the emitted light passes to the absorber arranged inside. A copper pipe (diameter = 3 mm) is used; this was initially highly polished, then coated with a film of copper oxide, the thickness of which determines the position of the absorption maximum due to interference. Figure 8 shows the diffuse reflection of copper, with and without the oxide film. The position of the absorption maximum is adjusted to correspond to the emission of the fluorescent dye. The selectivity of the absorber can be calculated from the obtained stagnation temperatures. Taking into account only radiation losses from the copper tube to the surrounding glass tube the selectivity of the oxidized copper is between 20 and 30. The uncertainty comes from the unknown fluorescent radiation losses between collector edge and absorber and from conduction losses through the vacuum sealing rings. The high selectivity of these interference absorbers was also measured with a TiO2 layer on a highly polished gold surface. The absorptance in the region of the emission of the fluorescent light was 95% but not yet narrow enough. The TiO2 layer with a thickness of about 0.1 ~m has no effect on the low emittance of the gold substrate. A measurement of the thermal emission[12] shows that even in the region of strong absorption of TiO2 from 800 to 400 wave numbers no increase in emission is detected. Measurements of the temperature rise of the copper pipe absorber as a function of time were carried out on the roof of our institute in Freiburg.

°v°cu°t°dsl°?s- II

I

absorber Fig. 7. Experimental set up for thermal conversion with fluorescent concentrators.

Thermal conversion with fluorescent concentrators

33

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t [mini The fluorescent coUector was directed towards the south and tilted 45 °. Figure 9 shows the temperature of the absorber in dependence of irradiation time at a total irradiation of 860 W/m = on the collector surface. A stagnation temperature of 490"C (763 K) was reached. The maximum stagnation temperature reached was 555"C (828 K) (irradiation 850 W m-2). In Figure 10 a measurement taken under diffuse light conditions is indicated. Over 250°C was recorded, with an irradiance of about 150 W/m 2. As a first approximation it can be assumed that a re-

Fig. 9. Temperature rise of the absorber at a total irradiation of 860 W/m2.

duction of the irradiation lowers the stagnation temperature by the fourth root of this reduction. This applies for diffuse as well as direct light received by the collector. The thermal efficiency of the system in these experiments has a value of 15-20% referred to the optical power at the collector edge.

T [ "El

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W. STAHLet al.

34

This low efficiency is a result of radiation losses in the optical coupling of the emitted light to the absorber. A fraction o f the light is internally reflected and trapped within the glass tube, a further fraction reached the absorber only after reflection by the glass tube. A new evacuated tube (with rectangular cross section) is planned, with which these losses can be reduced. Apart from this the thermal efficiency will be improved by a further optimisation of the selectivity of the absorber. 5. SUMMARY

The first experiments on the application of fluorescent collectors for producing heat have shown promising results. The still not optimal emission and absorption properties of the absorber and light losses from collector edge to absorber have been responsible for the fact that the stagnation temperatures have not yet approached the theoretical value. Despite this, stagnation temperatures over 500°C were reached which otherwise can only be obtained with tracking concentrators using only the direct component of the solar radiation. In particular the temperature rise at low diffuse irradiation shown in Fig. 10 is a promising result. New aspects for solar energy utilization in moderate climates can be envisioned. Utilization of the diffuse component of sunlight for production of high grade heat is a possibility not offered by other systems. Fluorescent collector systems need no tracking. The fluorescent collector is inexpensive; its price is essentially that of the matrix material (PMMA). The absorber has small dimensions, little material is required and heat losses are small. The further enhancement of the selectivity of the absorber is one aim of our work. Also the optical coupling of collector edge and absorber has to be improved. Here we hope to increase the system efficiency considerable. Also, a test collector with a fluorescent collector area of some square metres will be built. Nevertheless, it must be mentioned that the efficiency of the whole system at present is too low for a stand-alone application. Stacking of several collectors which are doped with different fluorescent dyes can increase the absorption, and so raise the efficiency. Hybrid systems which use the light transmitted by a single fluorescent collector for other purposes can already be considered for commercial development in the foreseeable future. Acknowledgements--This research was supported by a scholarship from the Alfred Krupp yon Bohlen und Halbach Foundation and by the BMFT under the contract 03E 4428 A. NOMENCLATURE (r Stefan-Boltzmann constant (5.67 × 10 -s W/m -2 K-4) T Temperature (K) p Angle

C E a e 1~ f

Concentration Power Density (W m -2) Absorptance Emittance Solid angle Reduction of the solar radiation due to the earth's atmosphere h Wavelength CI Planck's first radiation constant (2~rhc2 = 3.7405 × 10-16 W m2) C2 Planck's second radiation constant (hc/k = 0.0143879 m K)

Subscripts S Sun th Thermal geo Geometric s Spectral c Critical REFERENCES

I. A. Goetzberger and W. Greubel, Solar energy conversion with fluorescent collectors. Appl. Phys. 14, 123 0977). 2. A. Goetzberger and V. Wittwer, Fluorescent planar collector-concentrators: A review. Solar Cells 4, 3 (1981). 3. V. Wittwer, K. Hei~ller, A. Zastrow and A. Goetzberger, Efficiency and stability of experimental fluorescent planar concentrators (FPC). Rec. 14th IEEE Photovolt. Spec. Conf., p. 760 (1980). 4. A. Goetzberger, Thermal energy conversion with fluorescent collector-concentrators. Solar Energy 22, 435 (1979). 5. J. Fricke and W. L. Borst. Energie, p. 359. Oldenburg Verlag, Miinchen, Wien, (1981). 6. B. O. Seraphin, Spectrally selective surfaces and their impact on photothermal solar energy, solar energy conversion. Solid-State Phys. Aspects 31, 12 (1979). 7. J. A. Duffle and W. A. Beckman, Solar Engineering of Thermal Processes, p. 118. John Wiley, New York (1980). 8. P. Wiirfel, Chemisches Potential yon Licht, Habilitationsschrift, Universitat Karisruhe (1982). 9. H. Ries, Thermodynamic limitations of the concentration of electromagnetic radiation. J. Opt. Soc. Am. 72, 380 (1982). 10. V. Wittwer, K. Heidler, A. Zastrow and A. Goetzberger, Theory of fluorescent planar concentrators and experimental results. J. Luminesc. 24/25, 873 (1981). 11. K. Heidler, Efficiency and concentration ratio measurements of fluorescent solar concentrators using a xenon measurement system. Appl. Opt. 20, 773 (1981). 12. K. Molt, Perkin & Elmer Research Center, Oberlingen, West Germany, private communication.

Convertions thermiques ~t l'aide de concentreurs fluorescent W. Stahl, V. Wittwer, A. Goetzberger Fraunhofer-Institut for Solare Energiesysteme D-7800 Freiburg, W..Germany R~um6mLes collecteurs, concentreurs plans offrent de nouvelles possibilit6s pour la convertion de l'energle soiaire en energie thermique. La r6colte et la concentration des radiations directes et diffuses sont possibles en utilisant une surface transparent, dope6 d'une substance fluorescente. Ces collecteurs offrent I'avantage de s6parer

Thermal conversion with fluorescent concentrators

35

les irradiations globales dans diff6rentes r6gions spec- conduite d'absorption de 3 mm de diam6tre a 6t~ construit trales. Cette lumi6re spectrale, concentr6e et g6om~trique dans un tube de verre sous vide. On a obtenu une tempeut ~tre convertie avec un absorbeur adapt6 ~ haute s6- p~rature de stagnation maximum de 555°C (828 K) avec lectivit6 en une haute temperature. L'intensit~, la r~gion une irradiation solaire de 850 W/m e, sur ia surface du colspectrale du soleil et la s61ection de rabsorbeur d~ter- iecteurfluorescent. Sous des conditions de lumi6re diffuse minent la temp6rature thermodynamique maximum pos- (150 W/m2), la temperature de stagnation de 250°C (523 sible de l'absorbeur. Un collecteur test, avec un concen- K) a 6t6 mesur6e. Les caiculs thermodynamiques, les detreur fluorescent, d'une aire d'environ 0,8 m e, muni d'une scriptions et r6sultats ex6rimentaux son donn6s ci-apr6s.