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The photochemical heat pipe
$o/ar Energy Vol. 21, pp. 87-92 © Persamoa Press Ltd., 1978. Printed in Great Britain
0038-092xf'/810801-0~7/$02.00/0
THE PHOTOCHEMICAL HEAT PIPE Be CARLSSON and GUNNAR WETTERMARK Department of Physical Chemistry, The Royal Institute of Technology, S-100 44 Stockholm 70, Sweden
(Received 4 June 1977; in revised form 27 February 1978; received for publication 14 March 1978) Abstract--The performance of a solar collector system for high temperature heat delivery based on a photochromic reaction is discussed. The system consists of a non-focusing collector and a reactor integrated into a flow system. In the collector, kept close to ambient temperature, the chemical potential of the photochromic system is increased through an endothermic photochemical reaction and is used to drive the reverse thermal reaction taking place in the reactor at a high temperature. No separation of the photoproducts is involved. Accordingly, the highest temperature at which heat can be delivered from the reactor is determined by the maximum attainable photostationary state in the collector and not, as in a conventional flat-plate collector, by heat-loss from the collector to the surroundings. Accordingly, the highest temperature at which heat can be delivered from the reactor is determined by the maximum attainable photostationary state in the collector and not, as in a conventional flat-plate collector, by heat-loss from the collector to the surroundings. The functioning of the device is exemplified by calculations for a model system utilizing the photodissociation of gaseous nitrosylchloride. The results show that it should be possible to build a system which, on a clear day, delivers about 100 W heat at temperature above 200°C for each m 2 collector area. A tenfold reduction in the radiation flux density of the incident light will only slightly reduce output efficiency.
INTRODUCTION
Solar energy technologies which require comparatively high temperatures are usually based on focusing collectors. Low temperature applications on the other hand usually employ a flat-plate collector where the sun's radiation is directly converted into heat through absorption by a "black" surface behind a glazing. The latter kind of collector is mechanically simpler than focusing collectors which as a rule require suntracking. The highest useful temperature of the flat-plate collector is determined by the heat-loss to the surroundings. Even with evacuated windows and selectively coated surfaces it is difficult to maintain high efficiency when reaching a temperature beyond 100°C of that of the ambient. An indirect way of obtaining high temperatures while keeping the temperature of the collector down and thus reducing heat loss may be to employ a heat pump. Another, perhaps more direct way, is to use a photochemical collector. The characteristics of such a device are described here and exemplified with a specific photochemical system, namely the photodissociation of nitrosylchloride.
transferred to a storage medium via the heat exchanger HI. To minimize heat loss during transfer from the reactor to the solar collector, the flow of the reaction mixture into the reactor is heat exchanged with the flow out of the reactor by the heat exchanger H3. Heat directly generated in the solar collector itself is transferred away by the heat exchanger He. THERMODYNAMICPERFORMANCEOF AN IDEAL PltOTOCHEMICALCOLLECTOR
For the reaction A--,B in the solar collector, the change in the Gibbs free energy (G) can be expressed as AG(T,) = AG°(TI) + RTm. In (aa/aA)
(2)
where a,~ and aa are the activities of A and B. Accordingly, the maximal temperature TM at which heat can be delivered from the reactor is determined by
aalaA = K(TM )
(3)
where K(TM)=thermodynamic equilibrium constant at TM. Thus, as AG(TM)= 0 it follows from the GibbsHelmholtz equation that
GENERALPRINCWLES
The solar-collector system is based on a reversible photochemical system which may be written as
(4)
AG(T,)/T,=fr~UAH/T2dT.
hv
A,
~ B.
(1)
Assuming yields
The basic principles of the system are illustrated in Fig. 1. In the solar collector the endothermic photochemical reaction A
hv
AH(T)=AH(Tm)+ACp(T- Tin) eqn
AG(T,) = AH(T,)[(T~
~B occurs at a temperature, T~, which can
- T,)/TM ]-
- (TM - T , ) / T M
be kept low. From the solar collector the reaction mixture, enriched on B, is transferred to a reactor where the reverse exothermic reaction B--}A takes place at a temperature T2, which can be considerably higher than T,. From the reactor, high temperature heat can be
ACpT,[ln
].
(4)
TM/T,
(5)
The temperature dependence of AH is usually small. Thus, assuming complete independence aG(T,) = aH[(T~
87
- T,)/T~
1.
(6)
B. CARLSSONand G. WETTERMARK
88
Ill
hu
Solar Col[ector
(TI)
Reactor
TI
:/VV Heat Exchanger
Heat Exchanger
H2
H1
Fig. 1. A photochemical solar collector system for high temperature heat delivery based on a photochromic system of the type A ~ B.
Equation (6) is also the work obtainable if the heat that is generated in the reactor from the back reaction is used to feed an ideal heat engine working between TM and Y, Thus, in the ideal case, the photochemical solar collector works in a way analogous to a heat pump with an efficiency equal to that of the Carnot cycle.
cules formed per photon, has been determined to be about 2 for activating light of wavelengths from UV up to about 630 nm [4]. The primary photochemical step has been suggested to be
nWMANDSON THE ~OTOC~SaC~ SYS'mM TO summarize the results from above the system (1) must fulfil the demands of being endothermic (AH > 0) and reversible (AG > 0 at T2). Moreover, to obtain high temperatures a necessary property is AH ~, AS as TM ~ AH/AS (see eqn (6)). However, additional demands must be placed on the system. Specifically, the photochemical properties of the system must be such that the majority of the solar spectrum can effectively be used. This means, firstly, that the absorption of solar radiation must be high for the components of A but low for the components of B. Secondly, the majority of the energy of a photon absorbed must be converted into "chemical" energy, i.e. O" AHINA must be of the same order as the photon energy (0 = quantum yield, NA = Avogadro's number). Finally, the kinetics of the reverse reaction have to be such that the thermal reaction rate can be kept slow in the solar collector and fast in the reactor.
which requires an energy of 158 Id/mol[5] ( A - 752 nm). The absorption spectrum of gaseous NOCI is given in Fig. 2(13), with data taken from work reported by Goodeve and Katz[6]. As seen, the absorption in the visible region is rather weak and limits the possibilities of using long-wavelength visible light for activation. Gaseous NOCI is also in thermal equilibrium according to
THE NOCI SYSTEM
hv
2NO + C12.
(7)
The quantum yield, defined as the number of NO mole-
hv
, NO + CI
2NOCI k~32 2NO+C12 (AH° = 76 kJ).
(8)
(9)
The dissociation reaction has been experimentally found to be of second order and the association reaction of third order. General characteristics of the NOCI system are listed in Table 1. As is seen the system fulfils most of the demands specified above and, consequently, provides a suitable working model for the photochemical solar collector.
THE NOC! S Y ~
To provide example of a possible system for the photochemical solar collector outlined above, the photochromism of NOCI(g) will be considered. The NOC1 system was chosen as basic experimental data for its thermochemistry and photochemistry are available in the literature. Previous schemes for the utilization of NOCI in energy conversion have involved the separation of the photoproducts [ 1-3]. • Gaseous NOCI is photocbemically dissociated according to the reaction: 2NOCI ~
NOCI
IN RELATION TO THE INCIDENT SOLAR RADIATION
The light absorption ability of NOCI vanishes for wavelengths longer than about 660 nm. The spectrum of the solar radiation reaching the earth's surface varies with the incident angle and atmospheric conditions, therefore, for our calculations we have chosen to use the solar spectrum from 350--660nm as observed outside the earth[7] (see Fig. 2(a)). The lower limit has been set equal to 350 nm in order to account for scattering losses in the atmosphere of the earth. The dotted curves in Fig. 2(a) illustrate the maximum portion of solar radiation that can be converted into "chemical" energy by the NOCI system. Two cases are shown, one where total absorption by NOCI is assumed, the other where absorption takes place in a I0 cm layer
The photochemicalheat pipe Table I. Subject field of
Demands he A ~
E2 {M-Icrn4l
on
Example:
B
2NOCI(g)
Thermo-
^H
dynamics:
^G > 0 at T 2
0
AG = 0 when T2>>TI, aS/~H
~.e.
chemistry:
Molar
(25°C)
AG ° = 42 kJ
(25°C)
30 ~
(p=l arm)
absorption
coef-
NOCI:
cA high
for
around
but
NO,
20
part of the photon must
energy,
per Einstein)
~.e. small
ponds
The
reverse
(B b)
the solar and
reaction
must be
fast
slow
in
decomposed from UV
to a w a v e l e n g t h
to
of 0
for s t e e r J n E
reverse
reaction:
the t e m p e r a t u r e
in the teat-
catalysts
'
'
400
the
and
" ~
~ 500
N
~
' ~ 6O0
Wavelength {nrn}
use of
sure d e p e n d e n c e ,
tor
300
blethods
collector
\
~ -- 2
1574 nm _ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kinetics:
!
AH°-~/2 corres-
630 r ~ .
- ~-AH
yield
(moles of NOC1
"chemical"
al-
%0
,q
t
Absorption
negligible
Quantum
be conver-
,%, /
',
up to
660 nm.
Cl2:
most
ted into
NA.h~
Absorption
radiation
energy
1,5
i"
~G O = 0 for T2--600 K
¢B small
Major
--0-
• 2NO+C12
hu ~
~H O = 76 kJ
small
ficient solar
E;2 (M-Icm-11
-0-
(~S O = 122 J/K) (25°C) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photo-
89
Fig.2(b).
pres-
use of
in the r e a c t o r
of NOCI held at the pressure of one atmosphere. The symbols used in the figure legend and in the following are: We.~ = radiant flux density in the wavelength interval A--,A + dA; AA, As lower and upper wavelength limits (350 and 660nm respectively); 4, = quantum yield for dissociation of NOC1; AHN = enthalpy difference between NOCI and NO+½CI2 (at Ti); h=Planck's constant; u=Hght frequency; ex =molar absorption coefficient of NOCI at the wavelength A; d = optical path length. The solar radiation, curve A, yields a radiant flux density, We, within the interval AA--'AB, which We ), (Wlm2nm)
Fig. 2. (a). ( ): Spectraldistributionof the radiantflux density,W~, of solarenergybetween 350 and 600 nm as observed outsidethe earth(A).(-----):The maximum fractionof the radiantfluxdensitythatcan be convertedintochemicalenergy by the dissociationof NOCI(g) assuming B = totalabsorption (W,,~ .:p.AI-ININAh~,),C=absorption in a 10cm layer at a pressureof I atm, temperature40°C ((W.,~/NAhU).~ •AH~v(I10-'~xdx(?°IRr))). (b).Absorptionspectrumof gaseousNOCI at a pressureof I arm (T = 22°C)(datafrom Ref.6). represents 516W/m 2 as compared to a value of 1353 W/m2 for the solar constant. With total absorption by NOCI, curve B, 168 W/m2 can be converted into chemical energy. In a solar collector with a 10 cm path length and NOCI pressure of 1 arm, 360W/m" is absorbed and the photochemical yield becomes 108 W/m2. Increasing the path length to 20 cm gives an 8 per cent higher yield. THE NOCISYSTEMIN A PHOTOCHEMICAL SOLARCOLLECTOR In the following the behaviour of the NOCI system is discussed in relation to the solar collector illustrated in Fig. I. All gases are treated as ideal. The solar collector as well as the reactor are considered to be ideal tank reactors. Only steady-state solutions are studied which means that the reactions in the solar collector and the reactor can be treated as being both isothermic and isobaric.
2.0.
1.5
1.C
The solar collector
..
1 68W/m2
_/_
......
For the solar collector the basic mass balance equation can be written:
B__. Fo " Aa = V, • (Po/RT,) " ~ - /
0.5
{lo)
/ -._s
/ •
108Wlm2/',
~6o
5(]0 Fig. 2(a).
/
/
~ Wuvele~:Jt~ 600 (nm)
where Fo = molar flow at the degree of dissociation a = 0; Aa = a , - a = with a,, a2=dissociation degree of NOCI at the outlet and at the inlet of the solar collector respectively; V, = volume and T, = temperature of the solar collector; Po = pressure of NOCI at a = 0.
90
B. CARLSSONand G. WETrERMARK
The change in the degree of dissociation in NOCI arises from both photochemical and thermochemical reactions. For the photochemical reaction, (see, for example eqn (8)), the kinetics can be described by the following expression, assuming the light absorption of NO and CI2 to be negligible d[NOCI] _ A 1r A8 dt V, J~, (W,.~/huNA). 4,{1
different values of Po. Figure 4 presents the functions S(a~,T1), {S(aI, TO+G(aI, TO} and G(a2, T2) for selected parameter values (see eqns (11) and (12)). The plots are based on equilibrium and rate constants evaluTemperature (*C) 500
•
A
- 10-'~a ' (v°mr°(I-*')} dA = S(al, TI)
(11)
where A, = illuminated area of the solar collector. The thermochemical reactions can be described as (see eqn (9))
Po=_aim ~
t.O0.
/'"
300.
~
• .
•
~
20Q
d[N~3Cl] = k2[NOCl]2 - k3[NO]2[C12]
•
= - k3(Po/RTO 3 "~1 (O~l 3 _ K=.eo(l - al) 2)
100i
/ /
/-
(12)
= G(al, TO.
The constant K,.vo is related to the thermodynamic equilibrium constant Kv as
°~,°
~ ' 0 3 .
P.=_l.atm
arm
.~'~'J
.-~°
.t
02~
023 0~5 Thermal Diss. Degree
027
Fig. 3. The relation between the temperature and the degree of dissociation of NOCI(g) at thermal equilibrium, a~, for three different values of Po.
K,,,vo = 2k2/k3. (Po/RT) -~ = 2K~,/Po= a,3/(1 --a~,) 2 (13) Reaction Rate. 103.(mot/s)
where ae = the degree of dissociation at thermal equilibrium. The final expression for the mass balance in the solar collector is thus: Fo" Aa = V~{S(a,, T,)+ G(al, T,)}.
(14)
3:1
The heat exchanger H3, (see Fig. 1), is assumed to be ideal in the sense that the temperature of the gas is the same both when entering and leaving the solar collector. Furthermore, it is assumed that all thermal heat that is evolved in the collector can be transferred away by the heat exchanger H2 (see Fig. 1).
\
\ '\
Fo" Aa = V2" G(a2, Y2) Qnl = - V2" G(a2, T2)' AHn
",..
\
The reactor For the reactor the mass balance as well as the heat balance can be written directly as -
(il
\\
"\.
,
\
I I t
(15) (16)
where V2 = volume of the reactor; Qu~ = heat transferred away by Hi (see Fig. 1). Equation (16) accounts for the net heat loss from the reactor in the heat exchanger H3, ideally ACe" Fo" Aot(Y2-Ti), where ACe is the difference in heat capacity between NOC1 and its dissociation products. Note that the heat generated in the reactor is - - V2" G(ot2, T2)" {AHN + A C p ( T 2 - T0} as AHr~ refers to the enthalpy change as T~, see above. PERFORMANCE OF THE SOLAR.COLLECTOR FOR SELECTED PARAMETER VALUF~
The position of the thermal equilibrium is illustrated in Fig. 3 where a, is plotted vs temperature for three
0.1
0,3
0.5
0.7
Diss. Degree
Fig. 4. The reaction rate, -(d[NOCl]/dt), vs the degree of dissociation, a. Two levels of W, are assumed, 516 and 103 W/m2 (350--660 nm). A = I m2. Function
Po
S(ahTO+G(aI, TO latm
V
We
400C 516W/m 2 103 W/m2 S(al, TO+G(abYO 0.latin l m 3 40"C 516W/m2 (- . . . . . ) 103 W/m2 S(al, TI) 40"C 516W/m 2 (. . . . . . ) PoV = 0.1 atm m3 103 WIm2 - G(a2, T2) 1 arm 0.1 m3 100*C ( ) 0.1 m2 200°C 0.01 m3 300"C (-----)
0.lm 3
T
Curve a b c d e f g h i
The photochemical heat pipe
91
Energy Output (W/m 2) _V2.~ 100
-- ~ - - - -
-V2\
"~ ~
Fo (mot/s) .0.06 ----~'~
~
-Po=0.1; 3 otto
\ "~-~Po =1;lotto
/-.V2:" ~°
0.04
V~=O,Olm32," .. " , . --z
0.03
5G CL02 001
100
200
300
Temperature 0 LO0 (°C)
Fig. 5. Energy output QH~ from the reactor vs the reactor temperature, Tz. Solid lines; ( ) Collector: A t = l m 2, Po= latm, V, =0:-lm3, T1=40°C. Reactor: Po=l alto. ~.:'~ ~ '.i.,~ : ~...... ) Collector: A==lm 2, Po = 0.1 arm, V~= I m3, T~= 40"C. Reactor: Po = 3 atm. W, = 516 W/m~(359--660rim); Aa -*0 (Fo~®) for the curve labelled I,'2--*®.Aa = 0.025 in all other cases. The right ordinate yields the flow rate, Fo, for the latter cases. V2 is indicated on the graphs. ' ated from the experimental data in Refs. [8] and [9], namely: k3 = exp (11.29- 2.47 x 103/T)
K=~o= 1/Poexp(15.35-9.15x 103/T) (atm, K).
(17)
Note that the assumed illumination area is 1 m 2. Generation of energy in the reactor can only take place when al > - , at 7'2. The maximal attainable temperature in the reactor is thus determined by the degree of dissociation which can be obtained in the photostationary state in the collector. For equal values of Po in the collector and reactor, Po = I atm, V~ = 0.1 m 3 (d = 10cm) this maximal temperature is about 220°C, am(max) = 0.31 = ae (220°C), when W, is 516 W/m 2 (350660 nm, see Figs. 3 and 4). With a lower radiant flux of the incident light this temperature decreases, but the intensity dependence is small. A five-fold reduction in W, (to 103 W/m2)-results in 180°C (m = a, = 0.18). The reason for the small intensity dependency is that S(a~,T~) is proportional to We whereas G(am,T~) contributes very little when a, is far from its value at the photostationary state. It is obvious that higher temperatures can be reached by reducing the thermal association reaction taking place in the collector. This can be accomplished by reducing either the temperature or the pressure in the collector. Cooling is a comparatively inefficient means of accomplishing the first objective. For example the thermal reaction rate is only decreased by a factor of about 1.7 when the collector temperature is lowered to 10°C (from 40°C) at Po = I atm. Pressure and volume are of course tWben optimizing the parameters it should be remembered that the amount of light absorbed is not a linear function of pressure and volume at the levels discussed here. Reducing the pressure from 1 to 0.Satm in a 0.1m 3 collector (d= 10cm) results in a reduction of the association reaction rate by a factor of 8 but reduces light absorption by about only 8%.
interrelated in that the volume will have to be increased when the pressure is lowered in order to keep the light absorption the same. At Po = 0.1 atm and V1 = 1 m 3 (the amount of light absorbed being the same as in the example above) the thermal association reaction becomes less critical. The curve of S(a,, T~)+G(a~, T,) follows more closely that of $(a,, T~) and the decrease in the reaction rate occurring with an increase in the dissociation degree is thus essentially due to the reduction in light absorption.t The intersection of a curve for - G ( a 2 , T2) with a curve for S(a,, T,)+ G(a,, TI) represents a steady-state solution for the total system including the collector and reactor in the ideal case when Fo--,Aa, i.e. Aa-~0. Accordingly, the maximum energy output, Q,~(max), from the reactor at different reactor temperatures T2 is obtained by multiplying S(m, T,)+ G(a,, T~) with AH~, as for this case a~ = a2(Fo~oO)and a2 = ae(T2)(V2--*oo). The curves labelled V 2 ~ = in Fig. 5 show QN~(max) for two situations, one with equal and the other with different pressures in the collector and reactor, where in the latter a Compressor stage is introduced. In the ideal case when all work during expansion is utilized for compression only the net work, ACp (T2- Tl)Fo" An, has to be added. This becomes negligible for the cases selected here. The other curves of Fig. 5 show Q,, at different volumes of the reactor for some real cases with Aa set equal to a constant value of 0.025. They were determined from plots of the type shown in Fig. 4. As is seen from Fig. 5, the performance of a system with the solar collector at low pressure and the reactor at high pressure is distinctly superior to that of a system working with the same pressure in the collector and reactor. Another means of steering the association reaction and thus deeping the reactor volume down may be to employ catalysts. Several are known, for example active carbon, glass woll and aluminium-oxide[10]. The
• 92
B. C~u.ssoN and G. WEyr~
gas flows involved, Fig. 5, are not technologically unreasonable.? They were obtained through application of eqn (15). Finally it should be stressed that an ideal model has been used in the above calculations in order to obtain the upper limit for the efficiency of a photochemical solar collector system based on the dissociation of NOCI. DISCUSSION
The above calculations show that it should be possible to construct a high-temperature solar-heat system by using a reversible photochemical system (photochromic reaction) in the collector process. The photodissociation of NOCI(g) appears to be suitable for such an application. The properties of the solar-collector system outlined above differ in many respects from those of the conventional flat plate collector system. The main advantage of the latter collector is that almost the entire solar spectrum can be converted into heat. The photochemical collector can not make use of long wavelength light. However, the photochemical collector can, in principle, be combined with a flat-plate solar-collector surface and, consequently, enable utilization of solar energy in the infrared region (see below). The efficiency of a fiat-plate solar collector decreases almost linearly as the temperature of the collector increases, due to heat loss to the surroundings. Furthermore, the maximal temperature difference between the collector and its surroundings is roughly proportional to the radiant flux density of the solar radiation involved. This strongly limits the use of the fiat-plate collector as a tool for high-temperature heat delivery. For the photochemical collector, the temperature of the ?The gas flow in terms of volume will depend on degree of dissociation, pressure and temperature (a~=0.25, Po = l arm, TI = 40°C and al = 0.40, Po = 0.1 arm, TI = 44rC correspond to 78 and 1.4 x 103Ilmin, respectively).
collector can be kept low without decreasing its efficiency and high-temperature heat is still obtained from the reactor. The attainable temperature is set by the photostationary state in the solar collector, which means that if the thermal reverse reaction can be effectively suppressed the collector will ideally work just as efficiently at low light intensities. Of course, non-ideal behaviour, particular in the case of heat exchanger H3, will limit efficiency at higher temperatures. The heat exchanger He acts as a cooling device to suppress the thermal association reaction in the solar collector. However, if the working temperature can be set high enough the heat passing He may be utilized for heating purposes. At the assumed temperature of 40°C it may, for instance, be used for space heating. Furthermore the radiation passing through the photochemical collector can be collected. Placing a conventional fiatplate collector behind the photochemical collector is one way of accomplishing this goal.
REFERENCF~ 1. R. J. Marcus and H. C. Wohlers, Solar Energy 5, 44, 121 (1961). 2. O. S. Neuwirth, J. Phys. Chem. 63, 17 (1959). 3. E. Findl, W. B. Lee, J. D. Margerum and W. E. McKee, SAE J. 84 (1960). 4. G. B. Kistiakowsky, J. Am. Chem. Soc. 52, 52 (1930). 5. C. R. Bailey and A. B. D. Cassie, Proc. Roy. Soc. A, 145, 336 (1934). 6. C. F. Goodeve and S. Katz, Proc. Roy. Soc. A, 172, 432
(1939). 7. J. A. Dutiie and W. A. Beckman, Solar Energy Thermal Processes (The NASA (1971) Standard Spectral Irradiance), p. 6. Wiley-Interscience,New York (1974). 8. C. M. Beeson and D. M. Yost, J. Chem. Phys. 7, 44 (1939);J. Am. Chem. Soc. 61, 1432 (1939). 9. I. Welinsky and H. A. Taylor, J. Chem. Phys. 6, 466 (1938). 10. L. J. Beckham, W. A. Fessler and M. A. Kise, Chem. Rev. 48(3), 319 (1951).
Ramman--Se examinan los resultados de un sistema de colector solar para el suministro de calor a alta temperamra, basado en una reacci6n fotocr6mica. El sistema consta de un colector no focalizante y un reactor integrado en un sistema circulante. En el colector, que se mantiene pr6ximo a la temperatura ambiente, el potencial qufmico del sistema fotocr6mico aumenta mediante una reacci6n fotoqufmica endot6rmica y se destina a impulsar ia reacci6n t~rmica inversa que se produce en el reactor a elevada temperatura. No impfica separaci6n de los fotoproductos. Por consiguiente, la m~txima temperatura a la que el reactor puede suministrar calor viene determinada por el m~ximo estado fotoestacionario alcanzable en el colector y no, commo occure en un colector convencional de pinca plana, por las p6rdidas t~rmicas al exterior del colectur. Para ejemplificarel functionamiento de este dispositivo pueden verse los c~lculos de un sistema modelo que utiliza la fotodisociaci6n de cloruro de nitrosilo gaseoso. De los resultados se desprende que seria posible construir un sistema que en un dfa despejado suministre alrededor de 100W de energia calor/fica a temperatures superiores a los 200°Cpor cada m2 de superficie del colectur. Una reducci6n a la d6cima paste de la densidad de flujo de radiaci6n de la luz incidente afectara la capacidad s61oligeramente. R~sum#--La rSalisationd'un syst~me de capteurs solalres pour la distribution de chaleur ~ haute temperature bas~e sur une r~action photochromique est discut~e ici. Le syst~me est constim8 d'un capteur non convergent et d'un r~acteur integr6s darts un syst~me circulatoire. Darts ie capteur, malntenu pros de la temperature ambiante, une r~action photochimique endothermique augmente le potentiel chimique du syst~me photochromique, entra~nant ia r~action chimique inverse qui se produit darts le r~acteur ~thante temp&ature. Cela n'implique aucune s~paration des photoproduits. Par cons6quent, ia temp6rature maximale a laquelle la chaleur peut ~tre distribu6e par le r~acteur est d~termin~epar l'~tat photostationnaire maximal pouvant ~tre atteint clans le capteur, et non, comme dans ie cas de capteur~ plans traditionnels, par les d~perditions de chaleur du capteur vers renvironnement. Le fonctionnement du dispositif est illustr~par un exemple comportant les calculs pour une maquette faisant appel ~tla photodissociation de chlorura de nitrosyle gazeux. Les r~sultats montrent qu'il est possible de construire un syst~me qui par temps clair, distribuerait une puissance caiorifiqued'environ 100W ~tdes temperatures sup6rieures 200°C pour chaque m~tre carr~ de capteur. Une r~duction de 90% de la densit~ du flux lumineux incident n'entra~ne qu'une I~g~rediminution du rendement.
0038-092xf'/810801-0~7/$02.00/0
THE PHOTOCHEMICAL HEAT PIPE Be CARLSSON and GUNNAR WETTERMARK Department of Physical Chemistry, The Royal Institute of Technology, S-100 44 Stockholm 70, Sweden
(Received 4 June 1977; in revised form 27 February 1978; received for publication 14 March 1978) Abstract--The performance of a solar collector system for high temperature heat delivery based on a photochromic reaction is discussed. The system consists of a non-focusing collector and a reactor integrated into a flow system. In the collector, kept close to ambient temperature, the chemical potential of the photochromic system is increased through an endothermic photochemical reaction and is used to drive the reverse thermal reaction taking place in the reactor at a high temperature. No separation of the photoproducts is involved. Accordingly, the highest temperature at which heat can be delivered from the reactor is determined by the maximum attainable photostationary state in the collector and not, as in a conventional flat-plate collector, by heat-loss from the collector to the surroundings. Accordingly, the highest temperature at which heat can be delivered from the reactor is determined by the maximum attainable photostationary state in the collector and not, as in a conventional flat-plate collector, by heat-loss from the collector to the surroundings. The functioning of the device is exemplified by calculations for a model system utilizing the photodissociation of gaseous nitrosylchloride. The results show that it should be possible to build a system which, on a clear day, delivers about 100 W heat at temperature above 200°C for each m 2 collector area. A tenfold reduction in the radiation flux density of the incident light will only slightly reduce output efficiency.
INTRODUCTION
Solar energy technologies which require comparatively high temperatures are usually based on focusing collectors. Low temperature applications on the other hand usually employ a flat-plate collector where the sun's radiation is directly converted into heat through absorption by a "black" surface behind a glazing. The latter kind of collector is mechanically simpler than focusing collectors which as a rule require suntracking. The highest useful temperature of the flat-plate collector is determined by the heat-loss to the surroundings. Even with evacuated windows and selectively coated surfaces it is difficult to maintain high efficiency when reaching a temperature beyond 100°C of that of the ambient. An indirect way of obtaining high temperatures while keeping the temperature of the collector down and thus reducing heat loss may be to employ a heat pump. Another, perhaps more direct way, is to use a photochemical collector. The characteristics of such a device are described here and exemplified with a specific photochemical system, namely the photodissociation of nitrosylchloride.
transferred to a storage medium via the heat exchanger HI. To minimize heat loss during transfer from the reactor to the solar collector, the flow of the reaction mixture into the reactor is heat exchanged with the flow out of the reactor by the heat exchanger H3. Heat directly generated in the solar collector itself is transferred away by the heat exchanger He. THERMODYNAMICPERFORMANCEOF AN IDEAL PltOTOCHEMICALCOLLECTOR
For the reaction A--,B in the solar collector, the change in the Gibbs free energy (G) can be expressed as AG(T,) = AG°(TI) + RTm. In (aa/aA)
(2)
where a,~ and aa are the activities of A and B. Accordingly, the maximal temperature TM at which heat can be delivered from the reactor is determined by
aalaA = K(TM )
(3)
where K(TM)=thermodynamic equilibrium constant at TM. Thus, as AG(TM)= 0 it follows from the GibbsHelmholtz equation that
GENERALPRINCWLES
The solar-collector system is based on a reversible photochemical system which may be written as
(4)
AG(T,)/T,=fr~UAH/T2dT.
hv
A,
~ B.
(1)
Assuming yields
The basic principles of the system are illustrated in Fig. 1. In the solar collector the endothermic photochemical reaction A
hv
AH(T)=AH(Tm)+ACp(T- Tin) eqn
AG(T,) = AH(T,)[(T~
~B occurs at a temperature, T~, which can
- T,)/TM ]-
- (TM - T , ) / T M
be kept low. From the solar collector the reaction mixture, enriched on B, is transferred to a reactor where the reverse exothermic reaction B--}A takes place at a temperature T2, which can be considerably higher than T,. From the reactor, high temperature heat can be
ACpT,[ln
].
(4)
TM/T,
(5)
The temperature dependence of AH is usually small. Thus, assuming complete independence aG(T,) = aH[(T~
87
- T,)/T~
1.
(6)
B. CARLSSONand G. WETTERMARK
88
Ill
hu
Solar Col[ector
(TI)
Reactor
TI
:/VV Heat Exchanger
Heat Exchanger
H2
H1
Fig. 1. A photochemical solar collector system for high temperature heat delivery based on a photochromic system of the type A ~ B.
Equation (6) is also the work obtainable if the heat that is generated in the reactor from the back reaction is used to feed an ideal heat engine working between TM and Y, Thus, in the ideal case, the photochemical solar collector works in a way analogous to a heat pump with an efficiency equal to that of the Carnot cycle.
cules formed per photon, has been determined to be about 2 for activating light of wavelengths from UV up to about 630 nm [4]. The primary photochemical step has been suggested to be
nWMANDSON THE ~OTOC~SaC~ SYS'mM TO summarize the results from above the system (1) must fulfil the demands of being endothermic (AH > 0) and reversible (AG > 0 at T2). Moreover, to obtain high temperatures a necessary property is AH ~, AS as TM ~ AH/AS (see eqn (6)). However, additional demands must be placed on the system. Specifically, the photochemical properties of the system must be such that the majority of the solar spectrum can effectively be used. This means, firstly, that the absorption of solar radiation must be high for the components of A but low for the components of B. Secondly, the majority of the energy of a photon absorbed must be converted into "chemical" energy, i.e. O" AHINA must be of the same order as the photon energy (0 = quantum yield, NA = Avogadro's number). Finally, the kinetics of the reverse reaction have to be such that the thermal reaction rate can be kept slow in the solar collector and fast in the reactor.
which requires an energy of 158 Id/mol[5] ( A - 752 nm). The absorption spectrum of gaseous NOCI is given in Fig. 2(13), with data taken from work reported by Goodeve and Katz[6]. As seen, the absorption in the visible region is rather weak and limits the possibilities of using long-wavelength visible light for activation. Gaseous NOCI is also in thermal equilibrium according to
THE NOCI SYSTEM
hv
2NO + C12.
(7)
The quantum yield, defined as the number of NO mole-
hv
, NO + CI
2NOCI k~32 2NO+C12 (AH° = 76 kJ).
(8)
(9)
The dissociation reaction has been experimentally found to be of second order and the association reaction of third order. General characteristics of the NOCI system are listed in Table 1. As is seen the system fulfils most of the demands specified above and, consequently, provides a suitable working model for the photochemical solar collector.
THE NOC! S Y ~
To provide example of a possible system for the photochemical solar collector outlined above, the photochromism of NOCI(g) will be considered. The NOC1 system was chosen as basic experimental data for its thermochemistry and photochemistry are available in the literature. Previous schemes for the utilization of NOCI in energy conversion have involved the separation of the photoproducts [ 1-3]. • Gaseous NOCI is photocbemically dissociated according to the reaction: 2NOCI ~
NOCI
IN RELATION TO THE INCIDENT SOLAR RADIATION
The light absorption ability of NOCI vanishes for wavelengths longer than about 660 nm. The spectrum of the solar radiation reaching the earth's surface varies with the incident angle and atmospheric conditions, therefore, for our calculations we have chosen to use the solar spectrum from 350--660nm as observed outside the earth[7] (see Fig. 2(a)). The lower limit has been set equal to 350 nm in order to account for scattering losses in the atmosphere of the earth. The dotted curves in Fig. 2(a) illustrate the maximum portion of solar radiation that can be converted into "chemical" energy by the NOCI system. Two cases are shown, one where total absorption by NOCI is assumed, the other where absorption takes place in a I0 cm layer
The photochemicalheat pipe Table I. Subject field of
Demands he A ~
E2 {M-Icrn4l
on
Example:
B
2NOCI(g)
Thermo-
^H
dynamics:
^G > 0 at T 2
0
AG = 0 when T2>>TI, aS/~H
~.e.
chemistry:
Molar
(25°C)
AG ° = 42 kJ
(25°C)
30 ~
(p=l arm)
absorption
coef-
NOCI:
cA high
for
around
but
NO,
20
part of the photon must
energy,
per Einstein)
~.e. small
ponds
The
reverse
(B b)
the solar and
reaction
must be
fast
slow
in
decomposed from UV
to a w a v e l e n g t h
to
of 0
for s t e e r J n E
reverse
reaction:
the t e m p e r a t u r e
in the teat-
catalysts
'
'
400
the
and
" ~
~ 500
N
~
' ~ 6O0
Wavelength {nrn}
use of
sure d e p e n d e n c e ,
tor
300
blethods
collector
\
~ -- 2
1574 nm _ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kinetics:
!
AH°-~/2 corres-
630 r ~ .
- ~-AH
yield
(moles of NOC1
"chemical"
al-
%0
,q
t
Absorption
negligible
Quantum
be conver-
,%, /
',
up to
660 nm.
Cl2:
most
ted into
NA.h~
Absorption
radiation
energy
1,5
i"
~G O = 0 for T2--600 K
¢B small
Major
--0-
• 2NO+C12
hu ~
~H O = 76 kJ
small
ficient solar
E;2 (M-Icm-11
-0-
(~S O = 122 J/K) (25°C) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photo-
89
Fig.2(b).
pres-
use of
in the r e a c t o r
of NOCI held at the pressure of one atmosphere. The symbols used in the figure legend and in the following are: We.~ = radiant flux density in the wavelength interval A--,A + dA; AA, As lower and upper wavelength limits (350 and 660nm respectively); 4, = quantum yield for dissociation of NOC1; AHN = enthalpy difference between NOCI and NO+½CI2 (at Ti); h=Planck's constant; u=Hght frequency; ex =molar absorption coefficient of NOCI at the wavelength A; d = optical path length. The solar radiation, curve A, yields a radiant flux density, We, within the interval AA--'AB, which We ), (Wlm2nm)
Fig. 2. (a). ( ): Spectraldistributionof the radiantflux density,W~, of solarenergybetween 350 and 600 nm as observed outsidethe earth(A).(-----):The maximum fractionof the radiantfluxdensitythatcan be convertedintochemicalenergy by the dissociationof NOCI(g) assuming B = totalabsorption (W,,~ .:p.AI-ININAh~,),C=absorption in a 10cm layer at a pressureof I atm, temperature40°C ((W.,~/NAhU).~ •AH~v(I10-'~xdx(?°IRr))). (b).Absorptionspectrumof gaseousNOCI at a pressureof I arm (T = 22°C)(datafrom Ref.6). represents 516W/m 2 as compared to a value of 1353 W/m2 for the solar constant. With total absorption by NOCI, curve B, 168 W/m2 can be converted into chemical energy. In a solar collector with a 10 cm path length and NOCI pressure of 1 arm, 360W/m" is absorbed and the photochemical yield becomes 108 W/m2. Increasing the path length to 20 cm gives an 8 per cent higher yield. THE NOCISYSTEMIN A PHOTOCHEMICAL SOLARCOLLECTOR In the following the behaviour of the NOCI system is discussed in relation to the solar collector illustrated in Fig. I. All gases are treated as ideal. The solar collector as well as the reactor are considered to be ideal tank reactors. Only steady-state solutions are studied which means that the reactions in the solar collector and the reactor can be treated as being both isothermic and isobaric.
2.0.
1.5
1.C
The solar collector
..
1 68W/m2
_/_
......
For the solar collector the basic mass balance equation can be written:
B__. Fo " Aa = V, • (Po/RT,) " ~ - /
0.5
{lo)
/ -._s
/ •
108Wlm2/',
~6o
5(]0 Fig. 2(a).
/
/
~ Wuvele~:Jt~ 600 (nm)
where Fo = molar flow at the degree of dissociation a = 0; Aa = a , - a = with a,, a2=dissociation degree of NOCI at the outlet and at the inlet of the solar collector respectively; V, = volume and T, = temperature of the solar collector; Po = pressure of NOCI at a = 0.
90
B. CARLSSONand G. WETrERMARK
The change in the degree of dissociation in NOCI arises from both photochemical and thermochemical reactions. For the photochemical reaction, (see, for example eqn (8)), the kinetics can be described by the following expression, assuming the light absorption of NO and CI2 to be negligible d[NOCI] _ A 1r A8 dt V, J~, (W,.~/huNA). 4,{1
different values of Po. Figure 4 presents the functions S(a~,T1), {S(aI, TO+G(aI, TO} and G(a2, T2) for selected parameter values (see eqns (11) and (12)). The plots are based on equilibrium and rate constants evaluTemperature (*C) 500
•
A
- 10-'~a ' (v°mr°(I-*')} dA = S(al, TI)
(11)
where A, = illuminated area of the solar collector. The thermochemical reactions can be described as (see eqn (9))
Po=_aim ~
t.O0.
/'"
300.
~
• .
•
~
20Q
d[N~3Cl] = k2[NOCl]2 - k3[NO]2[C12]
•
= - k3(Po/RTO 3 "~1 (O~l 3 _ K=.eo(l - al) 2)
100i
/ /
/-
(12)
= G(al, TO.
The constant K,.vo is related to the thermodynamic equilibrium constant Kv as
°~,°
~ ' 0 3 .
P.=_l.atm
arm
.~'~'J
.-~°
.t
02~
023 0~5 Thermal Diss. Degree
027
Fig. 3. The relation between the temperature and the degree of dissociation of NOCI(g) at thermal equilibrium, a~, for three different values of Po.
K,,,vo = 2k2/k3. (Po/RT) -~ = 2K~,/Po= a,3/(1 --a~,) 2 (13) Reaction Rate. 103.(mot/s)
where ae = the degree of dissociation at thermal equilibrium. The final expression for the mass balance in the solar collector is thus: Fo" Aa = V~{S(a,, T,)+ G(al, T,)}.
(14)
3:1
The heat exchanger H3, (see Fig. 1), is assumed to be ideal in the sense that the temperature of the gas is the same both when entering and leaving the solar collector. Furthermore, it is assumed that all thermal heat that is evolved in the collector can be transferred away by the heat exchanger H2 (see Fig. 1).
\
\ '\
Fo" Aa = V2" G(a2, Y2) Qnl = - V2" G(a2, T2)' AHn
",..
\
The reactor For the reactor the mass balance as well as the heat balance can be written directly as -
(il
\\
"\.
,
\
I I t
(15) (16)
where V2 = volume of the reactor; Qu~ = heat transferred away by Hi (see Fig. 1). Equation (16) accounts for the net heat loss from the reactor in the heat exchanger H3, ideally ACe" Fo" Aot(Y2-Ti), where ACe is the difference in heat capacity between NOC1 and its dissociation products. Note that the heat generated in the reactor is - - V2" G(ot2, T2)" {AHN + A C p ( T 2 - T0} as AHr~ refers to the enthalpy change as T~, see above. PERFORMANCE OF THE SOLAR.COLLECTOR FOR SELECTED PARAMETER VALUF~
The position of the thermal equilibrium is illustrated in Fig. 3 where a, is plotted vs temperature for three
0.1
0,3
0.5
0.7
Diss. Degree
Fig. 4. The reaction rate, -(d[NOCl]/dt), vs the degree of dissociation, a. Two levels of W, are assumed, 516 and 103 W/m2 (350--660 nm). A = I m2. Function
Po
S(ahTO+G(aI, TO latm
V
We
400C 516W/m 2 103 W/m2 S(al, TO+G(abYO 0.latin l m 3 40"C 516W/m2 (- . . . . . ) 103 W/m2 S(al, TI) 40"C 516W/m 2 (. . . . . . ) PoV = 0.1 atm m3 103 WIm2 - G(a2, T2) 1 arm 0.1 m3 100*C ( ) 0.1 m2 200°C 0.01 m3 300"C (-----)
0.lm 3
T
Curve a b c d e f g h i
The photochemical heat pipe
91
Energy Output (W/m 2) _V2.~ 100
-- ~ - - - -
-V2\
"~ ~
Fo (mot/s) .0.06 ----~'~
~
-Po=0.1; 3 otto
\ "~-~Po =1;lotto
/-.V2:" ~°
0.04
V~=O,Olm32," .. " , . --z
0.03
5G CL02 001
100
200
300
Temperature 0 LO0 (°C)
Fig. 5. Energy output QH~ from the reactor vs the reactor temperature, Tz. Solid lines; ( ) Collector: A t = l m 2, Po= latm, V, =0:-lm3, T1=40°C. Reactor: Po=l alto. ~.:'~ ~ '.i.,~ : ~...... ) Collector: A==lm 2, Po = 0.1 arm, V~= I m3, T~= 40"C. Reactor: Po = 3 atm. W, = 516 W/m~(359--660rim); Aa -*0 (Fo~®) for the curve labelled I,'2--*®.Aa = 0.025 in all other cases. The right ordinate yields the flow rate, Fo, for the latter cases. V2 is indicated on the graphs. ' ated from the experimental data in Refs. [8] and [9], namely: k3 = exp (11.29- 2.47 x 103/T)
K=~o= 1/Poexp(15.35-9.15x 103/T) (atm, K).
(17)
Note that the assumed illumination area is 1 m 2. Generation of energy in the reactor can only take place when al > - , at 7'2. The maximal attainable temperature in the reactor is thus determined by the degree of dissociation which can be obtained in the photostationary state in the collector. For equal values of Po in the collector and reactor, Po = I atm, V~ = 0.1 m 3 (d = 10cm) this maximal temperature is about 220°C, am(max) = 0.31 = ae (220°C), when W, is 516 W/m 2 (350660 nm, see Figs. 3 and 4). With a lower radiant flux of the incident light this temperature decreases, but the intensity dependence is small. A five-fold reduction in W, (to 103 W/m2)-results in 180°C (m = a, = 0.18). The reason for the small intensity dependency is that S(a~,T~) is proportional to We whereas G(am,T~) contributes very little when a, is far from its value at the photostationary state. It is obvious that higher temperatures can be reached by reducing the thermal association reaction taking place in the collector. This can be accomplished by reducing either the temperature or the pressure in the collector. Cooling is a comparatively inefficient means of accomplishing the first objective. For example the thermal reaction rate is only decreased by a factor of about 1.7 when the collector temperature is lowered to 10°C (from 40°C) at Po = I atm. Pressure and volume are of course tWben optimizing the parameters it should be remembered that the amount of light absorbed is not a linear function of pressure and volume at the levels discussed here. Reducing the pressure from 1 to 0.Satm in a 0.1m 3 collector (d= 10cm) results in a reduction of the association reaction rate by a factor of 8 but reduces light absorption by about only 8%.
interrelated in that the volume will have to be increased when the pressure is lowered in order to keep the light absorption the same. At Po = 0.1 atm and V1 = 1 m 3 (the amount of light absorbed being the same as in the example above) the thermal association reaction becomes less critical. The curve of S(a,, T~)+G(a~, T,) follows more closely that of $(a,, T~) and the decrease in the reaction rate occurring with an increase in the dissociation degree is thus essentially due to the reduction in light absorption.t The intersection of a curve for - G ( a 2 , T2) with a curve for S(a,, T,)+ G(a,, TI) represents a steady-state solution for the total system including the collector and reactor in the ideal case when Fo--,Aa, i.e. Aa-~0. Accordingly, the maximum energy output, Q,~(max), from the reactor at different reactor temperatures T2 is obtained by multiplying S(m, T,)+ G(a,, T~) with AH~, as for this case a~ = a2(Fo~oO)and a2 = ae(T2)(V2--*oo). The curves labelled V 2 ~ = in Fig. 5 show QN~(max) for two situations, one with equal and the other with different pressures in the collector and reactor, where in the latter a Compressor stage is introduced. In the ideal case when all work during expansion is utilized for compression only the net work, ACp (T2- Tl)Fo" An, has to be added. This becomes negligible for the cases selected here. The other curves of Fig. 5 show Q,, at different volumes of the reactor for some real cases with Aa set equal to a constant value of 0.025. They were determined from plots of the type shown in Fig. 4. As is seen from Fig. 5, the performance of a system with the solar collector at low pressure and the reactor at high pressure is distinctly superior to that of a system working with the same pressure in the collector and reactor. Another means of steering the association reaction and thus deeping the reactor volume down may be to employ catalysts. Several are known, for example active carbon, glass woll and aluminium-oxide[10]. The
• 92
B. C~u.ssoN and G. WEyr~
gas flows involved, Fig. 5, are not technologically unreasonable.? They were obtained through application of eqn (15). Finally it should be stressed that an ideal model has been used in the above calculations in order to obtain the upper limit for the efficiency of a photochemical solar collector system based on the dissociation of NOCI. DISCUSSION
The above calculations show that it should be possible to construct a high-temperature solar-heat system by using a reversible photochemical system (photochromic reaction) in the collector process. The photodissociation of NOCI(g) appears to be suitable for such an application. The properties of the solar-collector system outlined above differ in many respects from those of the conventional flat plate collector system. The main advantage of the latter collector is that almost the entire solar spectrum can be converted into heat. The photochemical collector can not make use of long wavelength light. However, the photochemical collector can, in principle, be combined with a flat-plate solar-collector surface and, consequently, enable utilization of solar energy in the infrared region (see below). The efficiency of a fiat-plate solar collector decreases almost linearly as the temperature of the collector increases, due to heat loss to the surroundings. Furthermore, the maximal temperature difference between the collector and its surroundings is roughly proportional to the radiant flux density of the solar radiation involved. This strongly limits the use of the fiat-plate collector as a tool for high-temperature heat delivery. For the photochemical collector, the temperature of the ?The gas flow in terms of volume will depend on degree of dissociation, pressure and temperature (a~=0.25, Po = l arm, TI = 40°C and al = 0.40, Po = 0.1 arm, TI = 44rC correspond to 78 and 1.4 x 103Ilmin, respectively).
collector can be kept low without decreasing its efficiency and high-temperature heat is still obtained from the reactor. The attainable temperature is set by the photostationary state in the solar collector, which means that if the thermal reverse reaction can be effectively suppressed the collector will ideally work just as efficiently at low light intensities. Of course, non-ideal behaviour, particular in the case of heat exchanger H3, will limit efficiency at higher temperatures. The heat exchanger He acts as a cooling device to suppress the thermal association reaction in the solar collector. However, if the working temperature can be set high enough the heat passing He may be utilized for heating purposes. At the assumed temperature of 40°C it may, for instance, be used for space heating. Furthermore the radiation passing through the photochemical collector can be collected. Placing a conventional fiatplate collector behind the photochemical collector is one way of accomplishing this goal.
REFERENCF~ 1. R. J. Marcus and H. C. Wohlers, Solar Energy 5, 44, 121 (1961). 2. O. S. Neuwirth, J. Phys. Chem. 63, 17 (1959). 3. E. Findl, W. B. Lee, J. D. Margerum and W. E. McKee, SAE J. 84 (1960). 4. G. B. Kistiakowsky, J. Am. Chem. Soc. 52, 52 (1930). 5. C. R. Bailey and A. B. D. Cassie, Proc. Roy. Soc. A, 145, 336 (1934). 6. C. F. Goodeve and S. Katz, Proc. Roy. Soc. A, 172, 432
(1939). 7. J. A. Dutiie and W. A. Beckman, Solar Energy Thermal Processes (The NASA (1971) Standard Spectral Irradiance), p. 6. Wiley-Interscience,New York (1974). 8. C. M. Beeson and D. M. Yost, J. Chem. Phys. 7, 44 (1939);J. Am. Chem. Soc. 61, 1432 (1939). 9. I. Welinsky and H. A. Taylor, J. Chem. Phys. 6, 466 (1938). 10. L. J. Beckham, W. A. Fessler and M. A. Kise, Chem. Rev. 48(3), 319 (1951).
Ramman--Se examinan los resultados de un sistema de colector solar para el suministro de calor a alta temperamra, basado en una reacci6n fotocr6mica. El sistema consta de un colector no focalizante y un reactor integrado en un sistema circulante. En el colector, que se mantiene pr6ximo a la temperatura ambiente, el potencial qufmico del sistema fotocr6mico aumenta mediante una reacci6n fotoqufmica endot6rmica y se destina a impulsar ia reacci6n t~rmica inversa que se produce en el reactor a elevada temperatura. No impfica separaci6n de los fotoproductos. Por consiguiente, la m~txima temperatura a la que el reactor puede suministrar calor viene determinada por el m~ximo estado fotoestacionario alcanzable en el colector y no, commo occure en un colector convencional de pinca plana, por las p6rdidas t~rmicas al exterior del colectur. Para ejemplificarel functionamiento de este dispositivo pueden verse los c~lculos de un sistema modelo que utiliza la fotodisociaci6n de cloruro de nitrosilo gaseoso. De los resultados se desprende que seria posible construir un sistema que en un dfa despejado suministre alrededor de 100W de energia calor/fica a temperatures superiores a los 200°Cpor cada m2 de superficie del colectur. Una reducci6n a la d6cima paste de la densidad de flujo de radiaci6n de la luz incidente afectara la capacidad s61oligeramente. R~sum#--La rSalisationd'un syst~me de capteurs solalres pour la distribution de chaleur ~ haute temperature bas~e sur une r~action photochromique est discut~e ici. Le syst~me est constim8 d'un capteur non convergent et d'un r~acteur integr6s darts un syst~me circulatoire. Darts ie capteur, malntenu pros de la temperature ambiante, une r~action photochimique endothermique augmente le potentiel chimique du syst~me photochromique, entra~nant ia r~action chimique inverse qui se produit darts le r~acteur ~thante temp&ature. Cela n'implique aucune s~paration des photoproduits. Par cons6quent, ia temp6rature maximale a laquelle la chaleur peut ~tre distribu6e par le r~acteur est d~termin~epar l'~tat photostationnaire maximal pouvant ~tre atteint clans le capteur, et non, comme dans ie cas de capteur~ plans traditionnels, par les d~perditions de chaleur du capteur vers renvironnement. Le fonctionnement du dispositif est illustr~par un exemple comportant les calculs pour une maquette faisant appel ~tla photodissociation de chlorura de nitrosyle gazeux. Les r~sultats montrent qu'il est possible de construire un syst~me qui par temps clair, distribuerait une puissance caiorifiqued'environ 100W ~tdes temperatures sup6rieures 200°C pour chaque m~tre carr~ de capteur. Une r~duction de 90% de la densit~ du flux lumineux incident n'entra~ne qu'une I~g~rediminution du rendement.