Pergamon PII: SOO38492X (96)00084-97

Solar Energy Vol. 58, No. 4-6, pp. 191-195, 1996 Copyright 0 1996 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0038-092X/96 %15.00+0.00

DEVELOPMENT OF A SOLAR RECEIVER FOR A HIGH-EFFICIENCY THERMIONIC/THERMOELECTRIC CONVERSION SYSTEM

H. NAITO, Y. KOHSAKA, D. COOKE and H. ARASHI Faculty of Engineering, Tohoku University, Aramaki, Sendai 980-77, Japan

(Communicated by Lorin Vant-Hull)

Abstract-Solar energy is one of the most promising energy resources on Earth and in space, because it is clean and inexhaustible. Therefore, we have been developing a solar-powered high-efficiency thermionic/thermoelectric conversion system which combines a thermionic converter (TIC) with a thermo- electric converter (TEC) to use thermal energy efficiently and to achieve high efficiency conversion. The TIC emitter must uniformly heat up to 1800 K. The TIC emitter can be heated using thermal radiation from a solar receiver maintained at a high temperature by concentrated solar irradiation. A cylindrical cavity-type solar receiver constructed from graphite was designed and heated in a vacuum by using the solar concentrator at Tohoku University. The maximum temperature of the solar receiver enclosed by a molybdenum cup reached 1965 K, which was sufficiently high to heat a TIC emitter using thermal radiation from the receiver. Copyright 0 1996 Elsevier Science Ltd.

1. INTRODUCTION

Direct highly efficient conversion from solar thermal energy to electricity is considered to be one of the promising approaches for the use of energy both on Earth and in space in the future. The topping temperature of the conversion system must be kept as high as possible in order to obtain high-efficiency conversion. As therm- ionic conversion occurs at a high temperature (approx. 1800 K), a solar-powered conversion system that utilizes a thermionic converter as a topping system has potential for a high conver- sion efficiency. Combining a thermoelectric con- verter (TEC) with a thermionic converter (TIC) as a bottoming system, high-efficiency conver- sion close to 40% was reported by Eguchi et al. (1994) and Nikolaev (1995).

The TIC emitter is heated up to 1800 K using concentrated solar radiation as the heat source. As the concentration distribution of solar radia- tion is not uniform at the focal plane of the solar concentrator, concentrated solar radiation cannot be used to directly heat TIC emitters that require uniform temperature distribution. Thermal radiation from a cavity-type solar receiver can uniformly heat a TIC emitter. A graphite cavity-type solar receiver was devel- oped for this purpose. This receiver is enclosed by a molybdenum (MO) cup that can act as a reflector of thermal radiation from the receiver or as a TIC emitter.

In the present article, the design of the cavity- type solar receiver and the experimental results of heating TIC emitter are reported.

2. SOLAR-POWERED THERMIONIC/THERMOELECTRIC

CONVERSION SYSTEM

Figure 1 shows a schematic drawing of the solar-powered conversion system. Concentrated solar radiation irradiates an inner wall of the cavity-type solar receiver, which is placed at the focal point of the paraboloidal mirror. The cavity is constructed of graphite, because of its high absorption coefficient for solar radiation and its high thermal conductivity. The graphite receiver is enclosed by an MO cup, which can be used as a reflector or as a TIC emitter depending on its thermal emissivity.

If the MO cup possesses functionally graded thermal emissivity properties, it can be used directly as a TIC emitter. In the present case, the inside wall of the MO cup has high thermal emissivity, and the outside wall has low emissiv- ity. High thermal emissivity corresponds to a high absorption coefficient in metal, so the MO cup effectively absorbs thermal radiation emit- ted from a graphite receiver and can be heated

Thermal radiation fran solar receiver

Fig. 1. Schematic view of solar-powered conversion system.

191

192 H. Naito ef al.

to a high temperature. The low emissivity of the outside wall decreases thermal radiation heat loss from the MO cup.

An MO cup with both sides optically polished has low emissivity, and acts as a reflector of thermal radiation from the receiver. Using an MO cup of this type, a graphite receiver is effectively heated to a high temperature without thermal radiation heat loss through the outside of the receiver. A TIC emitter is heated by thermal radiation from the graphite receiver through a small hole in the MO cup, as shown in Fig. 1. The emitter surface opposite the graph- ite receiver is coated with TIC, which has a high absorption coefficient, in order to allow heating to a high temperature. Thermal stability of the TIC plasma spray coating is realized because of a gradual change in composition between TIC and MO. Thus, this apparatus is an application of the functionally graded material method.

The TIC produces electric power by emitting thermoelectrons at an emitter with a temper- ature of approximately 1800 K and moving them to a collector with 1100 K. Thermal energy released from the collector is transferred to a high-temperature side of the TEC using a heat pipe. Therefore, this heat can be used again to heat the hot junction of the TEC. Thus, the combined TIC/TEC system has potential for high-efficiency conversion.

3. SOLAR RECEIVER SUBSYSTEM

3.1. Solar concentrator at Tohoku University The large solar concentrator at Tohoku

University was designed and constructed as a solar furnace capable of achieving temperatures up to 4000 K for the crystal growth of materials with high melting points and for high-temper- ature physics. The solar concentrator consists of a heliostat and a paraboloidal mirror. The heliostat tracks the Sun and continuously reflects solar radiation parallel to the optical axis of the fixed paraboloidal mirror. The parab- oloidal mirror, 10 m in aperture and 3.2 m in focal length, is a mosaic composed of 181 mirror segments, each of which is accurately polished into a portion of the paraboloid. Thus, an accurate Sun image close to the theoretical value of 3.2 cm in diameter is formed at a focal point. The maximum concentration ratio on the focal plane reaches 40,000 (Kamada, 1965).

3.2. Concentration ratio on inner surface of cylindrical cavity-type solar receiver

A conical beam with an angle of 2l, reflected at off-axial point B on the parabolic mirror forms an elliptic image on a focal plane, as shown in Fig. 2. In this figure, F and f represent the focal point and the focal length, respectively, and to is the half-angle subtended by the Sun. The short radius s and the long radius r of the ellipse are expressed as

2f50 s= 1 +cose

2fhl r=(l+cosB)cose

(1)

(2)

where 8 is the angle between the optical axis (AF) and the line BF. The open end of the cylindrical cavity-type solar receiver is set on the focal plane, so the radius of the open end of the receiver must coincide with r in order to efficiently capture the concentrated solar radia- tion. Therefore, the aperture of the solar receiver depends on 0, the rim angle of the parabolic concentrator. The concentration ratio along the line HI (parallel to the optical axis of the parabolic concentrator) on the inner surface of the cylindrical receiver is shown in Fig. 3. As a representative point, we consider point C. The parabolic concentrator is rotationally symmetric about its optical axis. Therefore, we consider an annular mirror band with an infinitesimally small width, which contains point B, as shown in Fig. 2. If point C is contained within the Sun image projected by the conical beam reflected at B onto the inner surface of the cylindrical receiver, the solar radiation reflected at B con- tributes to irradiate point C. To obtain the concentration ratio, we first sum the contribu- tion from the portion of the annular mirror at

I_ __ (2 .L__.__ r 1 i

S

Fig. 2. Elliptic image on focal plane: to, half-angle sub- tended by the Sun; 6, rim angle; f; focal length; r, radius.

Thermionic/thermoelectric conversion system 193

reflected by portion of paraboloidal mirror

H C I Focal plane

Fig. 3. Concentration ratio on inner surface of cylindrical receiver.

constant 0, on which reflected conical beams irradiate point C, and then integrate 8 until the corresponding r coincides with the radius of the open end of the solar receiver.

The concentration ratio distribution as a func- tion of the radius of the open end of the solar receiver or the corresponding rim angle of the parabolic concentrator is shown in Fig. 4. The maximum concentration ratio does not substan- tially increase when the radius of the open end of the solar receiver exceeds 2.5 cm. A radius of 2.5 cm corresponds to a parabolic concentrator with a rim angle of 45”. Parabolic concentrators with this rim angle are utilized in space because of their high concentration ratio (Winston, 1991). Taking these points into consideration, we developed a cylindrical cavity-type solar receiver with an open end radius of 2.5 cm.

0 10 20 30 40 50 60

Distance frcfn the focd plane @Ml)

Fig. 4. Concentration ratio distributions as a function of

4. EXPERIMENTAL

In order to achieve homogeneous temperature distribution, the solar receiver is constructed of graphite, which has a high absorption coefficient for solar radiation and high thermal conductiv- ity. The graphite solar receiver is maintained in a vacuum chamber equipped with a quartz glass window through which the concentrated solar radiation irradiates the inner wall of the graphite receiver. A schematic cross-sectional drawing of this cup is shown in Fig. 5. A wall of the vacuum chamber is cooled by circulating water. The graphite solar receiver enclosed by an MO cup with a gap of 3 mm is placed on porous ceramic with low thermal conductivity in order to avoid heat conduction loss from the receiver into the wall of the vacuum chamber. The open end of the graphite solar receiver is situated on the focal plane of the parabolic concentrator. The length and wall thickness of the graphite solar receiver are 40 mm and 10 mm, respectively. The temperature distribution on the outer side wall and the closed back wall of the graphite solar receiver being exposed to concentrated solar irradiation is measured using type-R (PtRh-Pt) thermocouples, and data are acquired using a computer-controlled data logger.

5. RESULTS AND DISCUSSIONS

The measured temperature distributions of the outer side wall and the back wall of the graphite solar receiver without an MO cup are expressed as functions of the incident solar energy in Fig. 6. The temperature distribution is nearly homogeneous because of high thermal conductivity and the multi-reflection effect of the thermal radiation in the graphite cavity receiver. The attained maximum temperature of 1700 K governed by an energy balance between

Concentrated solar radiation

Fig. 5. Schematic cross-sectional drawing of graphite solar radius of open end of solar receiver. receiver.

194 H. Naito et al.

2ooo i 1500

I

cz z ii! 1000 -

ii I-

500 -

??? ? ? ? ? ?o--i- O---Y

??.O-_ -13

A------ O----O -0

A---A--~ -----O

----A

OoL-- J 20 40

Distaxefruncpnerd dsidetil (m-n)

2000 ,

I

1500 b -------o------o k

z 4 O-------O

$ 1000 1 A-------A

-A- Pi,=2.2kW -o- Pi,=2.7kW -II-- Pi,=5.4kW

Fig. 6. Temperature distributions on outer side wall and back wall of graphite solar receiver: (a) outer side wall; (b) back wall.

the incident solar energy and the heat loss by thermal radiation from the receiver is not suffi- cient for our TIC.

By enclosing the graphite receiver in a metal cup with high reflectivity, heat loss by thermal radiation from the outer surface of the receiver can be suppressed, and a higher temperature can be obtained. The temperatures of the receiver and metal are estimated as functions of the incident solar energy by solving the following simultaneous equations. The following equations are derived from energy balances at the graphite receiver and at the metal cup, respectively.

W&t, - T&l + Acs (l/4 + (&/AM) U WMI) - 11

@-:a, - T&J + Acb (l/E) + (l/E& - 1

4Tlta, - Th) Ac8 (l/4 + (&/AM) {( ~/EM,) - 11

4T:a, - T&J + Acb (l/E) + (l/E& - 1

&‘?, - T&d •I- AM (~/EMMI) + (AM/AA{(~/E) - 11

4% - T:a,) + AMb (l/E& + (l/E) - 1

(3)

(4)

where: Pi, = incident solar energy; A,i = inner

surface area of the metal cup; A,, = side wall area of the cavity; Acb = back wall area of the cavity; AM = side wall area of the metal cup; AMb = back wall area of the metal cup; Teal = cavity temperature; TM = metal cup temper- ature; c1= absorptivity of the cavity; E = emissiv- ity Of the Cavity; EM0 = emiSSiVity Of the Outer

surface of the metal cup; and eMI = emissivity of the inner surface of metal cup. The obtainable temperatures of the solar receiver and metal cup are expressed in Table 1 as functions of emissiv- ity for the metal cup.

Figure 7 shows the measured temperature dis- tributions of the outer side wall and back wall of the graphite solar receiver enclosed by an MO cup with a thermal emissivity of 0.16. The temperature exceeded 1800 K, and reached a maximum of 1965 K at the back wall of the graphite receiver. This temperature is sufficient to heat a TIC emitter by thermal radiation from a graphite solar receiver, as shown in Fig. 1. The fact that the measured temperature of the back wall is higher than that of the side wall is attributed to the difference of the concentration ratio, which is higher at the back wall than at the side wall. The average measured temperature

Table 1. Calculated temperature

%l1r GO

0.16, 0.16 0.85, 0.16

0.16, 0.16 0.85, 0.16

Pi, (kW

8.6 8.6

5.3 5.3

LI (K) K, (K)

2002 1799 1987 1932

1774 1594 1760 1712

Thermionic/thermoelectric conversion system 195

2000, I 2000,

ol.. I , ,

0 20 40 0 10 20 30 - Distmcefromopnend Distancefruncenter

cfsidetil(mn) dtackvdI(mn)

Fig. 7. Temperature distributions on outer side wall and back wall of the graphite solar receiver enclosed by MO cup: (a) outer side wall; (b) back wall.

of the receiver is about 1760 K, which is matched with the calculated value shown in Table 1.

By increasing the emissivity of the inner sur- face of the metal cup that encloses the graphite solar receiver, the temperature of the metal cup is increased, as shown in Table 1. Therefore, by employing a metal cup constructed of function- ally graded material (FGM) in which the ther- mal emissivity gradually decreases from the inside to the outside, the TIC emitter is heated more effectively by concentrated solar radia- tion. The development of such materials is in progress.

6. CONCLUSIONS

A cylindrical cavity-type solar receiver con- structed of graphite was developed. The receiver was heated in a vacuum using solar radiation concentrated by the solar furnace at Tohoku University. The maximum temperature of the solar receiver enclosed by an MO cup was

1965 K. This temperature level is sufficient for the combined TIC/TEC system, which uses con- centrated solar radiation as its heat source. The TIC emitter can be uniformly heated to a tem- perature of 1800 K using thermal radiation from a graphite solar receiver.

Acknowledgements-The authors wish to thank M. Chiba for skillful technical assistance. This work was supported in part by Special Coordination Funds from the Promoting Science and Technology Agency, Japan.

REFERENCES

Eguchi K., Hoshino T. and Fujiwara T. (1994) Performance analysis of FGM-based direct energy conversion system for space power applications. Proc. of 3rd Int. Symp. on Structure and Functional Gradient Materials. (Lau- sanne Polytechnique Press, Lausanne), pp. 619-625.

Kamada 0. (1965) Theoretical concentration and attainable temperature in solar furnaces. Sol. Energy 9, 39-47.

Nikolaev Y. V. (1995) Design concepts and materials of space energy conversion systems with nuclear and solar heat sources. Proc. of Japan-Russia-Ukraine Int. Work- shop on Energy Conversion Materials, pp. 117-190.

Winston R. (1991) Nonimaging optics. Scientijic American, 52-57.