Solar energy storage via a closed-loop chemical heat pipe

Solar Energy Vol. 50, No. 2, pp. 179-189, 1993 Printed in the U.S.A.

0038-092X/93 $6.00 + .00 Copyright © 1993 Pergamon Press Ltd.

SOLAR ENERGY STORAGE VIA A CLOSED-LOOP CHEMICAL HEAT PIPE M. LEVY, R. LEVITAN, H. ROSIN, and R. RUBIN Department of Materials and Interfaces, The Weizmann Institute of Science, Rehovot 76100, Israel Abstract--The performance of a solar chemical heat pipe was studied using CO2 reforming of methane as the vehicle for storage and transport of solar energy. The endothermic reforming reaction was carried out with a reactor packed with a supported rhodium catalyst and heated by the concentrated solar flux from the Schaeffer solar furnace at the Weizmann Institute (Rehovot, Israel). The maximum absorbed power was 8.5 kW. The reforming was run under variable insolation conditions, including partly cloudy days. The flux input was regulated by opening the doors of the concentrator building. The product gas temperature followed a predetermined set point that automatically controlled the flow of reactants to ensure constant composition of the reformer products. The exothermic methanation reaction was run in a multistage methanator filled with the same Rh catalyst and fed with the products from the reformer. High conversions were achieved for both reactions. In the closed-loop mode, the products from the reformer and from the methanator were compressed into separate storage tanks. The two reactions were run consecutively, and the whole process was repeated for over 60 cycles. The overall performance of the closed loop was satisfactory; scaleup work is in progress.

1. I N T R O D U C T I O N

CO + H2 = C + H20

The basic principle behind the solar chemical heat pipe (SCHP) is demonstrated in Fig. 1. The method is based on carrying out a highly endothermic reversible reaction at the solar site, cooling the products to ambient temperatures for storage and transport, and releasing the heat at the consumer site by the reverse exothermic reaction. The original products are recovered and transferred back to the solar site for recycling. The whole process is carried out in a closed loop, and therefore it is environmentally clean as no CO2 or other polluting gases are discharged into the atmosphere. As the same chemical reaction is operated in a closed-loop for many cycles, it has to be simple, highly efficient, and free of any side reactions that cause accumulation of undesirable products. The reaction suggested originally for a chemical heat pipe to be used for transport of nuclear energy was the steam-reforming of methane [ 1]. However, for reasons discussed previously, we preferred the CO2 reforming of methane for solar energy storage [ 2 ] : CH4 + CO2 = 2H2 + 2CO AH = 250 k J / m o l .

(1)

This reaction is always accompanied by the reverse water gas shift reaction: CO 2 + H 2 = CO + H20.

(2)

When the initial feed consists of a mixture o f C O 2 and CH4 with a CO2 :CH4 ratio only slightly above one, the water formed in the reaction amounts to only a few percent and remains in the gas phase. Side reactions that can be problematic are those leading to carbon formation, such as

2CO = C + CO2 CH4 = C + 2H2.

(3)

These reactions, if not kept under control, may eventually lead to blocking of the reactor, and the whole process will be interrupted. Therefore, special attention should be given to any signs indicating the beginning of carbon deposition. 2. PRELIMINARY LABORATORY WORK

The reaction was first studied in the laboratory under well-controlled conditions. The laboratory system is shown schematically in Fig. 2. It comprises a reformer for the forward reaction; a methanator for the back reaction; and the necessary auxiliary equipment such as electric heaters, temperature and flow controllers, a circulation pump, a storage tank, analytical instrumentation, and a computer. The system was flexible enough to enable us to study each reaction separately, as well as both reactions in series and finally both reactions in a closed loop. A search for a suitable catalyst yielded a catalyst consisting of Rh on alumina pellets, produced by the Engelhard Comp. The Rh catalyst proved to be stable under rather drastic conditions of changes in temperatures and flows of reactants. We studied the kinetics of both the reforming and the methanation reactions. We ran the reforming and the methanation separately, then in series, and finally in a closed loop. We operated the closed loop for up to 1,100 cycles with no apparent deterioration in the catalyst activity or accumulation of any side products[3]. 3. THE EXPERIMENTAL SOLAR SYSTEM The reforming reaction was then studied under real conditions in the solar furnace. A number of receiver 179

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reactor configurations were tried, including an integrated sodium reflux receiver/reactor[4]; a directly heated metal reactor [ 5 ]; and a reactor with a transparent window, in which the catalyst was directly illuminated by the concentrated solar flux[6 ]. The directly heated metal reactor proved to be the most convenient for the present closed-loop studies. We connected it to three different methanators [ 7 ] and operated the loop for many cycles. The main components of the solar chemical heat pipe consisted of the following main items: 1. solar furnace; 2. reformer; 3. methanator; 4. feed and storage system; and 5. acquisition and processing equipment.

3.1 Solarfurnace All the reforming experiments were carried out at the Schaeffer solar furnace, shown in Fig. 3(a), in a schematic diagram and Fig. 3(b) as viewed from the air. It consists of a 96-m 2 flat heliostat that follows the sun by computer control and reflects the solar radiation onto a spherical concentrator, 7.3 m in diameter, with a rim angle of 65 ° (Fig. 4). The concentrator is made of 590 trapezoidal concave mirrors arranged in concentric rings. The mirrors were individually focused to a distance of 3.5 m from the center of the sphere by using special clamps. The concentrator is housed in a building having large, mechanically operated doors that are used for regulating the power input into the concentrator. Calorimetric and radiometric measurements showed that the attainable solar concentration ratio was over 11,000 suns and the total power measured was 20 kW.

3.2 Reformer The vertical receiver/reactor used in this work is shown in Fig. 5 (a), a frontal photograph, and in Fig. 5(b) a schematic diagram. The receiver is an aluminum box, insulated from the inside by a 5-cm-thick alumina blanket. The inside dimensions of the box are 30 X 30 X 60 cm. The aperture is a circle, 10 cm in diameter, in the front alumina panel. The reactor was made ofa 24-mm-o.d. (20-mm-i.d.) Inconel 600 tube, 130 cm long, bent in a U shape. One hundred fifteen centimeters of the tube was filled with catalyst pellets, and the rest was filled with alumina pellets of the same size. The catalyst was Engelhard (E4823), 0.5% Rh on ~" alumina pellets. The receiver aperture was placed in the focal plane of the solar furnace. The reactor tube was suspended 18 cm behind the aperture. In order to avoid excessive overheating of the front surface of the reactor tube, two ceramic tubes, 15 mm in diameter, were placed in front of the reactor, and thus all the surfaces of the reactor were protected from direct irradiation by the concentrated solar beam. Using this arrangement, the difference in temperature between the front and the back of the reactor did not exceed 50°C. The total power entering the receiver was regulated by opening and closing the doors of the building housing the concentrating mirror. The rate of flow of reactants was changed in accordance with the desired conversions, and the reaction pressure was maintained by a pressure regulator located at the exit of the reaction system. Due to materials limitations, the maximum wall temperature of the Inconel reactor wall was not allowed to exceed 960°C. The COz :CH4 ratio was normally maintained within the range of 1.1 to 1.3. The gas flows ranged between 1,000 and 11,000 L/h, resulting in Reynolds numbers of 200 to 1,600; under

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these conditions, maximum conversions of 84% were reached. The power input was up to 8.5 kW and the power flux on the reactor wall reached 110 k W / m 2. The performance of the reformer and computer modeling of the results are discussed in detail in [ 5 ]. 3.3

Methanator

The methanator system is shown in Figs. 6(a) and 6 (b). It includes one of the three methanators studied: 1. a single-stage nonadiabatic methanator; 2. a four-stage methanator (three adiabatic and one nonadiabatic); and 3. a six-stage adiabatic methanator, shown schematically in Fig. 6(b). The six-stage methanator is the most advanced version and is recommended for future work. A computer program designed to calculate the temperatures and the compositions of the gases at each stage was developed. The results of the experiments and the calculations were presented earlier [ 71.

3.4

Feed and storage systems

All the gases were stored in cylinders in a covered shed in the back of the solar furnace. The initial feed to the reformer was made by mixing CO2 and CH4 in the right proportions and compressing the mixture into a battery of cylinders. The products from the reformer were compressed into another battery of cylinders. The two batteries were connected via a number of valves for maximum flexibility of operation. A 10-atm safety relief valve and a 12-atm blow-out disk were installed in the feed line. The exit pressure from the reactors was controlled by an automatic pressure control valve, and the inlet pressure was kept constant and was measured by a pressure gauge. Leak detectors for H2, CO, and CH4 were placed in critical areas. They were connected to the central controller, and an alarm signal was issued in case of leak. 3.4.1 Feedcontrol A diagram of the entire system is shown in Fig. 7. The inlet pressure to the system was maintained by pressure regulators. A pneumatic o n /

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Fig. 4. A general view of the spherical concentrator and the reformer positioned on the pneumatic working platform.

off valve was installed on the line of the flammable gases as a safety device. The valve was operated by an air solenoid valve and controlled from the computer keyboard. It could also be manually turned on and off by a switch located in the control room, so that the feed flow could be immediately stopped in case of emergency. Two Micro-Motion mass flow meters, transferred the mass flow rate measurements to two electronic controllers that operated the pneumatic control valves and adjusted the flow rates according to the set point. The two flow control systems were connected electronically as "master and slave," meaning that the set point for the CH4 was always relative to the flow rate of CO2, thus keeping the molar ratio CO2: CHa constant during the reaction. The gases passed through a 5-L vessel that served as a mixer. A premixed feed was used in most of the experiments described in

this paper. The feed flow rate could be kept constant, according to a predetermined set point, or changed automatically to control the temperature in the reactor when fluctuations in the solar input occurred. In practice, we use as a set point the product gas exit temperature (TG), which under given pressure conditions also determines the composition of the gases at equilibrium. By comparing the measured gas temperature and the set point, a proportional signal is sent to the mass flow controller, and the latter changes the flow rate accordingly. During a sample run of 5 h on a partly cloudy day (shown in Fig. 8 ), a relatively constant temperature of the product gas was maintained in spite of the clouds. 3.4.2 Internal circulation--start-stop procedure. Initially, the normal working procedure was to purge the entire system with CO2 before start-up. The re-

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Fig. 5. (a) Frontal view and (b) schematic diagram of the U-tube inconel reactor in the vertical receiver.

former would then be heated by the solar flux until the working temperature was reached. At that stage the CO2 was stopped and the reaction mixture was introduced. The reverse procedure was used when stopping at the end of the experiment or when large clouds moved in. Frequently, such start-stop cycles resuited in loss of products and increase in the CO2 :CHa ratio in the gas mixture. When the flow of reactants was simply stopped and the reformer was allowed to cool off under stagnant conditions, carbon deposition resulted. We therefore introduced a small circulating pump into the system, as shown schematically in Fig. 9. The inlet and outlet valves were closed, and the gases present in the reformer, the pipes, and the 5-L intermediate vessel were circulated continuously as soon as the exit temperature dropped below 600°C due to a passing cloud. Circulation was started only after the flow of reactants was reduced to a minimum value and the furnace doors were opened all the way. After the cloud passed and the working temperature was reached, normal operation was resumed. Presently, this procedure is carried out manually. However, it can be automated without any problem.

3.5 Data acquisition and processing The flow of data is schematically represented in the diagram in Fig. 10. All the temperature readings, flows, pressures, and GC analyses are continuously collected and saved on the computer. Other information that is also displayed on the screen includes alarm signals from the leak detectors, extent of the door openings, and solar insolation readings. The FIX software is used for reading and collecting the variables through the controller. The data are then processed using LOTUS software and later reduced and transformed into tables and graphic presentations. 4. RESULTS AND DISCUSSION In a closed-loop operation of a chemical heat pipe, the two reactions, reforming and methanation, can be run independently of each other. As can be seen in Fig. 7, the reformer and the methanator were connected via the same compressor to two separate storage tanks. The two reactions were run separately in consecutive order. The working procedure was the following: the reformer was first heated by the concentrated solar flux to the appropriate temperature, under circulation of

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directly into the m e t h a n a t o r without prior compressing a n d storage. This is a n accelerated m o d e that enabled us to r u n five to six closed loops a day. The experiments with the four-stage m e t h a n a t o r were r u n in this mode. Two series o f experiments were c a r d e d out with a gas flow of a b o u t 2.5 k g / h . One o f the series t h a t included

. Table 2. Reforming composition of the carbon-containing products

5 Fig. 9. The circulation system around the reformer. FC = flow controller; L = 5-L mixing vessel; R = reformer; P = circulation pump; valves: ( 1) = feed inlet, (2) = product outlet, (3) = inlet to pump, (4) = outlet from pump, (5) = vent.

Loop no. 1

the reacting mixture. W h e n the product gases, at the exit o f the reactor tube, reached a t e m p e r a t u r e close to 850°C, the circulation was stopped a n d the reacting mixture from the storage was fed into the reformer. T h e product gases from the reformer were compressed into a battery of cylinders, up to a m a x i m a l pressure of 25 arm. After the reforming was finished, the reformer was allowed to cool down, u n d e r circulation of the reacting mixture (as described earlier). T h e m e t h a n a t o r tube was heated u n d e r a flow o f CO2 a n d steam, a n d w h e n stationary conditions were reached, the CO2 flow was t u r n e d off a n d a c o n s t a n t flow of reactants was i n t r o d u c e d from storage. T h e reaction mixture was preheated to a t e m p e r a t u r e that would allow the initiation of the m e t h a n a t i o n , namely 350°C. T h e t e m p e r a t u r e along the m e t b a n a t i o n tube or in the various adiabatic reactors was monitored. It rose fairly quickly to 700 ° at the start a n d then dropped gradually to 400 ° at the exit o f the last reactor. At these exit temperatures the CO conversions were usually over 90%. T h e m e t h a n a t i o n products, which consisted mainly of a mixture of CH4 a n d CO2, were compressed into a n o t h e r battery o f cylinders, which concluded the first loop. T h e second a n d further loops started from the m e t h a n a t i o n products a n d followed the same procedure. A series o f nine loops c a r d e d out with the same initial gases, using the single-stage m e t h a n a t o r , is summarized in Table 1. T h e compositions of the reforming a n d the m e t h a n a t i o n products are given separately. F o r easier c o m p a r i s o n of the reforming a n d m e t h a n ation results, only the c a r b o n - c o n t a i n i n g c o m p o u n d s are given in the table. In the reforming experiments the CHa conversions were kept high e n o u g h by adjusting the feed flow a n d the d o o r opening to give a T G higher t h a n 850°C in m o s t cases. In the m e t h a n ation experiments, the C O conversions were also fairly high. It should be n o t e d that the m a x i m a l flow o f feed gases into the m e t h a n a t o r did not exceed 1.4 k g / h due to the size of the m e t h a n a t o r tube, although the reformer was r u n at higher rates. T h e O2 :H4 ratio ( w h i c h reflects the initial CO2 :CH4 ratio) r e m a i n e d quite constant. A n o t h e r way of r u n n i n g the reaction was on-line, where the products from the reformer were introduced

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

%CH4 TG

%CO

%CH4

821 804 803 825 825 821 818 800 782 817 832 811 798 690 682 708 724 724 695 714 732 757 753 694 681 689

78.1 74.7 71.2 75.0 77.1 78.7 77.7 . 73.3 77.1 80.9 75.0 67.2 72.3 70.9 75.5 76.4 74.4 72.6 73.2 78.3 84.5 84.7 70.2 67.4 67.0

5.4 10.7 9.7 7.7 6.3 7.5 8.4 . . 8.8 6.8 4.7 9.2 10.3 9.3 9.3 8.7 7.5 8.7 8.3 8.4 5.7 3.7 2.8 10.7 11.4 11.1

%CO2 conversion 16.4 14.6 19.1 17.4 16.6 13.8 13.9 . 17.9 16.1 14.4 15.7 22.5 18.4 19.9 15.8 16.1 17.0 19.2 18.4 16.0 11.8 12.5 19.1 21.2 21.9

--

73.5 75.4 80.3 82.6 79.9 77.1 76.9 81.7 87.3 75.7 72.4 76.7 74.9 77.6 80.8 75.5 76.7 78.0 85.3 90.4 92.5 73.7 72.7 70.4

O2/H4 1.52 1.31 1.44 1.43 1.53 1.41 1.43 1.42 1.48 1.51 1.51 1.45 1.65 1.29 1.31 1.25 1.33 1.25 1.30 1.28 1.27 1.25 1.34 1.35 1.32 1.30

Methanation in the four-stage methanator 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

398 445 448 421 452 429 428 405 395 412 399 403 401 449 455 454 455 474 378 403 412 419 408 420 406 411

1.5 5.4 5.9 5.6 7.8 7.6 8.8 4.9 8.7 8.2 6.5 8.7 6.1 7.6 9.6 7.9 7.2 7.1 4.4 6.4 8.3 6.3 5.1 5.2 3.9 4.5

40.4 40.4 37.2 38.2 34.9 37.2 35.7 39.2 34.3 37.9 38.9 37.1 40.9 31.5 35.5 36.6 36.8 37.8 38.9 39.4 36.8 38.2 36.6 37.9 41.7 36.5

58.2 54.2 56.9 56.1 57.3 55.1 55.6 55.9 57.0 53.9 54.6 54.2 53.1 60.9 54.8 55.6 55.9 55.1 56.8 54.2 54.9 55.5 58.3 56.9 54.4 59.1

98.1 92.8 92.1 92.7 89.9 90.3 88.7 93.4 88.7 89.9 91.3 87.0 91.0 89.5 86.4 89.5 90.6 90.4 93.9 91.2 89.5 92.6 93.9 92.3 94.3 93.3

TG = temperature of the product gas at the exit.

1.42 1.29 1.43 1.35 1.49 1.34 1.47 1.34 1.53 1.36 1.36 1.44 1.25 1.65 1.48 1.44 1.37 1.30 1.35 1.27 1.44 1.32 1.42 1.35 1.24 1.44

188

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26 loops is s u m m a r i z e d in Table 2. In all cases the composition r e m a i n e d constant, a n d the conversions for b o t h the reforming a n d the m e t h a n a t i o n reactions were quite high.

Table 3 shows a set o f experiments carried o u t with the six-stage adiabatic m e t h a n a t o r . This m e t h a n a t o r was larger a n d could take a flow of 3.5 k g / h . T e n loops were r u n with the same feed, giving good results.

Table 3. Reforming composition of the carbon-containing products Loop no.

TG

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%CH4 conversion

O2/H4

1 2 3 4 5 6 7 8 9 10

825 805 850 803 850 845 862 842 835 750

3.4 3.7 3.8 3.6 3.7 3.7 3.7 3.7 3.8 3.7

58.1 56.1 59.6 57.6 59.2 67.6 78.3 75.0 76.6 68.0

18.4 21.2 18.8 19.6 19.2 13.4 10.5 9.8 8.3 9.8

23.5 22.7 21.6 22.9 21.5 19.0 11.2 15.2 15.1 22.2

63.6 54.8 59.2 54.1 55.7 67.5 74.2 76.1 79.7 76.2

1.2 1.0 1.0 1.1 0.9 1.0 1.1 1.2 1.1 1.3

98.1 91.1 95.3 95.5 95.9 85.1 84.8 91.9 96.2 95.3

1.5 1.1 1.1 1.1 1.0 1.1 1.0 1.1 1.I 1.4

Methanation in the six-stage methanator 1 2 3 4 5 6 7 8 9 10

358 385 366 400 440 390 425 420 470 450

3.4 3.6 3.4 3.7 3.7 2.8 3.1 3.4 3.7 3.8

1.1 5.0 2.8 2.6 2.4 10.1 11.9 6.1 2.7 3.2

TG = temperature of the product gas at the exit.

46.9 46.1 42.7 43.3 41.2 40.7 41.0 40.9 41.2 37.4

51.9 48.9 54.5 54.1 56.4 49.2 47.1 53.0 56.1 59.4

Solar energy storage 5. CONCLUSIONS The closed-loop S C H P was run under a variety of insolation conditions with variable feeds and under a circulating regime for start-ups and shut-downs. More than 60 cycles were carried out, and there was no sign of carbon deposition or deterioration of the catalyst. In most cases the conversions were very high in order to attain high efficiency of chemical storage and transport. The experience in operating a closed-loop S C H P under real solar conditions and the calculations carried out with the computer programs helped in the design of an engineering-scale, 400-kW closed-loop system, which will be operated at the solar central receiver of the Weizmann Institute in Rehovot, Israel. With this unit we hope to determine the overall energy efficiency of the system and to derive engineering data that could not be obtained from the small-scale experiments presented in this paper.

Acknowledgment--This work was supported by a grant from the Ministry of Energy and Infrastructure, Jerusalem, Israel.

189 REFERENCES

1. R. E. Harth and U. Boltendahl, The chemical heat pipe EVA and ADAM, Interdisciplinary Sci. Rev. 6, 221 (1981). 2. D. Fraenkel, R. Levitan, and M. Levy, A solar thermochemical pipe based on the CO2-CH4 ( 1:1 ) system, Int. Z Hydrogen Energy 11, 267 (1986). 3. M. Levy, R. Levitan, H. Rosin, G. Adusei, and R. Rubin, Storage and transport of solar energy by a thermochemical pipe. Proceedings of 4th International Symposium on Solar Thermal Technology--Research, Development, and Applications, June 1988, Santa Fe, NM, 527 (1988). 4. R.B. Diver, J. D. Fish, R. Levitan, M. Levy, E. Meirovitch, H. Rosin, S. A. Paripatyadar, and J. T. Richardson, Solar test of an integrated sodium reflux heat pipe receiver/ reactor for thermochemical energy transport, Solar Energy 48, 21 (1992). 5. M. Levy, R. Levitan, E. Meirovitch, A. Segal, H. Rosin, and R. Rubin, Chemical reactions in a solar furnace: 2 direct heating of a vertical reactor in an insulated receiver. Experiments and computer simulations, Solar Energy 48, 395-402 (1992). 6. M. Levy, R. Rubin, H. Rosin, and R. Levitan, Methane reforming by direct solar irradiation of the catalyst, Energy 17, 749 (1992). 7. R. Rubin, R. Levitan, H. Rosin, and M. Levy, Methanation of synthesis gas in a solar chemical heat pipe (in press).