Simulation on a Novel Solar High-temperature Thermochemical Coupled Phase-change Reactor

T.T. Ma; Y.Z. Zhu; H.J. Chen; Y. Ma; L. Yang

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Energy Procedía 69 (2015) 471 - 480

International Conference on Concentrating Solar Power and Chemical Energy Systems,

SolarPACES 2014

Simulation on a novel solar high-temperature thermochemical coupled phase-change reactor

T. T. Maa, Y. Z. Zhua*, H. J. Chena, Y. Maa, L. Yangb

aJiangsu Key Laboratory of Process Enhancement and New Energy Equipment Technology, School of Mechanical and Power Engineering,

Nanjing Tech University, Nanjing 211816, China. bSchool of Environment, Nanjing Tech University, Nanjing 211816, China.

Abstract

Solar high-temperature thermochemical process is a promising concept to produce hydrogen as well as basic chemical materials by concentrated solar energy. An important feature of this technology is the design of a satisfactory reactor. A novel solar high-temperature thermochemical coupled phase-change reactor based on a special-shaped high-temperature heat pipe (SHHP) receiver is presented. The SHHP integrated with phase-change heat transfer, temperature leveling of heat pipe and heat plate (flat plate heat pipe) separates the upper reaction chamber and the lower solar absorber. In this manner, radial temperature gradient in absorber and axial temperature gradient in reaction chamber will be lowered, thus to enhance safety and thermochemical conversion efficiency of the solar reactor. A three-dimensional model of the reaction chamber coupling heat transfer with nitrogen as working fluid instead of reactants is developed to optimize geometry configurations. The temperature distribution of the reactor wall and the working gas are presented. The impact of the inlet/outlet configurations and arrangement of heat pipes in heat plate are investigated. The results show that different inlet/outlet positions has significant influence on the thermo-fluid behavior, and the existence of the heat pipes on heat plate enhances the heat transfer in reaction chamber.

Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG Keywords: Solar energy; High-temperature thermochemical reactor; Heat pipe (heat plate); Thermal performance

* Corresponding author. Tel.: +86 13505199993 E-mail address: zyz@njtech.edu.cn

Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG doi: 10.1016/j.egypro.2015.03.054

1. Introduction

Concentrated solar energy can be used to provide the heat necessary to drive high-temperature endothermic chemical reactions for renewable fuel production including direct thermolysis of water, reduction of metal oxides or hydrogen production for thermochemical water splitting cycles, and gasification of biomass, coal or other carbonaceous materials to produce synthesis gas [1-3]. Both solar energy and fossil resources (including water or biomass) are upgraded through this integrated process [4-6]. The solar high-temperature reactor is one of the keys to this process. Most of the reactors are derived from solar cavity-type collectors (also known as solar absorbers or receivers) commonly used in solar tower or dish concentrating power generation. Cavity-type receiver [7] is basically an enclosure designed to effectively capture the incident solar radiation entering through a small opening (the aperture). Directly irradiated reactors [2, 8-10] provide a very rapid and efficient means of heat transfer directly to the reaction site. However, the reactors require high thermal shock resistance since it is directly subjected to severe thermal shocks that often occur in applications with transient high-flux irradiation. Overheating or "hot spots" in partial region [11, 12] can be easily found because of nonuniform fluxes, which further lead to reactants or catalyst sintering. Another drawback when working with reducing or inert atmospheres is the requirement for a transparent window, which is a critical and troublesome component in high-pressure or severe gas environments.

One solution is an indirectly irradiated reactor [5, 6, 13, 14] with the introduction of either absorbing tubes or a separate cavity. With this arrangement, the inner cavity serves as the solar receiver, radiant absorber, and radiant emitter. Thus, the safety and reliability of the reactor can be improved. But the indirect heat transfer between absorber cavity, reactants and intermediate heat transfer fluid results in large thermal gradients inside the reactor that can lower system thermal efficiency and chemical conversion.

The high temperature metal heat pipe with excellent heat transfer characteristic is an approach to the indirect solution, while maintaining an indirectly irradiated reactor [15-18]. In this manner, liquid metal contained in an evacuated chamber evaporates under the effect of concentrated solar radiation impinging on solar-heated surface of the containment in absorber cavity. The metal vapor condenses on the reactor tubes or separate wall in the reaction chamber and energy is transferred to the reactants. Condensed liquid then flows back to the wick-covered evaporator surface under the influence of gravity. Since the metal is in a saturated state, temperatures within the reactor are uniform. Therefore, thermal stresses are minimized.

A novel solar reactor integrated with heat pipe technology is proposed to conduct high-temperature thermochemical reactions. In order to improve the design of the solar reactor, the present study focuses on investigating the effects of different geometrical parameters such as inlet/outlet configurations and arrangement of heat pipes in heat plate on the thermal behavior in reaction chamber.

2. Solar high-temperature thermochemical coupled phase-change reactor

A schematic of the solar high-temperature thermochemical coupled phase-change reactor [19] configuration is depicted in Figure 1. It features two cavities in series, with the upper cavity functioning as the reaction chamber, and the lower cavity as the solar absorber. These two cavities are separated by a special-shaped high-temperature heat pipe (SHHP). The upper, an insulated cylindrical cavity with inlet/outlet ports for reactants and products respectively. The lower comprises an aperture with a quartz window mounted on a pair of flanges, and inlet/outlet ports for an inert SHHP-cooling/window-cooling carrier gas. The solar flux concentration may be further augmented by incorporating a compound parabolic concentrator (CPC) in front of the aperture. The SHHP is a heat exchanger which consists of one cylindrical heat plate (flat plate heat pipe) and more heat pipes, as shown in Figure 2. The inner cavity of SHHP is an evacuated chamber containing sodium. In operation, concentrated sunlight heats the front circular plane (evaporator surface or solar-heated surface) of heat plate in absorber, and vaporized sodium. Sodium vapor condensed on the rear circular plane of heat plate and heat pipes (condensing surfaces) in reaction chamber, releasing latent heat and returning to the absorber by the capillary pumping action of wick structure. A groove wick with triangular cross-section and metal felt were chosen to form a combined wick for the SHHP.

Fig. 1. Conceptual schematic of solar high-temperature thermochemical coupled phase-change reactor.

Fig. 2. Structure schematic diagram of special-shaped high-temperature heat pipe.

Concentrated sunlight enters the reactor at its bottom. It then passes through a water cooled CPC, where the light is concentrated further before it goes through a gas cooled quartz window and into the lower cavity. The SHHP absorbers the solar radiation and transfers the heat to the upper cavity. Particles of reactants, conveyed in a flow of inert gas, are continuously injected into the reactor's upper cavity, absorb the heat and start the chemical reaction. At last, the chemical products exit the cavity via outlet port. When the temperature of the solar-heated surface is out of range, the inner cavity of SHHP may be subject to high pressure and damaged. In order to avoid this, the inlet gas flow in the absorber can be regulated to take away part of heat from solar-heated surface, or increasing the inlet reactants flow to absorb more heat from condensing heat surfaces.

The solar high-temperature thermochemical coupled phase-change reactor has presented several following advantages compared to traditional reactors. The radial temperature gradient in absorber can be lowered and overheating problem can be eliminated that results in a longer lifetime, because liquid sodium is distributed over the solar-heated surface by wick structure, which compensates for uneven flux that is delivered by the concentrator. The excellent heat transfer characteristics of evaporating and condensing sodium result in uniform temperatures throughout the chamber and transferring energy directly and efficiently to the reactants by heat pipes extending into the reaction chamber. Separation of the absorber and the reaction chamber; allows independent optimization of both components. Therefore, the solar high-temperature thermochemical coupled phase-change reactor can improve the system thermochemical conversion efficiency and safety.

3. Numerical simulations

Since the sodium in SHHP is in a saturated state when the solar high-temperature thermochemical coupled phase-change reactor is in operation, the temperature profile of the SHHP chamber can be regarded as uniform. In this paper only the thermal behavior of the reaction chamber is considered for optimizing geometry configurations. A three-dimensional CFD heat transfer model of reaction chamber is developed in a commercial software Ansys14.5. The simplified schematic and dimensions of the studied reaction chamber which is being fabricated is shown in Figure 3. Some configurations are modified later to investigate the impact of different geometrical parameters such

Fig. 3. Simplified schematic and dimensions of the reaction chamber.

The different inlet and outlet positions in reaction chamber reaction cavity can lead to different velocity field within the chamber and also different thermal performance. Three kinds of inlet/outlet configurations are established as shown in Figure 4. These configurations are: (a) a side inlet with the horizontal plane at 30° located at the bottom and a horizontal outlet located at the top; (b) a normal inlet on the top surface and a horizontal outlet located at the bottom; (c) a tangential inlet located at the bottom and a tangential outlet located at the top. The geometry and dimensions for each case are the same as shown in Figure 3 except section of the inlet and outlet positions.

(a) Case (a) (b) Case (b) (c) Case (c)

Fig. 4. Different inlet/outlet configurations used in this study.

For the reference of arrangements of tubes in heat exchanger, The impact of the arrangements of heat pipes on heat plate on the performance of the reactor is investigated. Three different arrangements are shown in Figure 5. These configurations are: (d) without heat pipes; (e) heat pipes are in triangular arrangement on the heat plate, with one in center and the other three distributed uniformly on a circle; (f) heat pipes are in square arrangement on the heat plate, with four distributed uniformly on a circle. The geometry and dimensions for each case are the same as shown in Figure 3 except the section of heat pipes.

(a) Case (d) (b) Case (e) (c) Case (f)

Fig. 5 Different arrangements of heat pipes on heat plate used in this study.

Since N2 is chosen as the carrier gas and the reactants are in powder form, the flow field of them should be substantially consistent with each other in the reaction chamber. Only the structure factors is to be considered in this study. N2 and 310S stainless steel is selected as the heat transfer working fluid instead of reactants and cavity material in the modeling, respectively. The radiation heat transfer is considered inside the reaction chamber, and DO (Discrete Ordinate) model is employed. N2 is considered to be nonparticipating medium for the radiation exchange. The inner cavity walls are assumed to be diffuse gray surface, and the absorption coefficient is equal to the normal emissivity, 0.76. All the simulations is set at the following conditions: N2 at ambient conditions enters at constant flow rate of 2.93*10-4 kg/s, and leaves through the outlet which exposed to the ambient conditions. The thermal conditions of condensing surface is set as temperature input, 1033K, the other walls are assumed to be adiabatic. A combination of the SIMPLE algorithm for pressure velocity coupling and a second order upwind scheme for the determination of momentum and energy is included in the model. The convergence criteria is set to 2*10-4.

Fig. 6. Non-uniform unstructured grid.

To ensure that the results obtained by the numerical study are independent of the computational grid, grid independence studies are carried out based on average nitrogen temperature in the chamber. For the computations, a non-uniform tetrahedral grid is employed. Based on the grid independence test, the mesh size of 3 mm is selected for the model. Figure 6 shows the grid employed in this study.

4. Results and discussion

In the following we present a parametric analysis of the impact of different parameters on the thermo-fluid behavior inside the reaction chamber. Mainly from four aspects to analyze the performance of the reactor, the distribution of temperature distribution at the reactor wall, temperature distributions, velocity pathlines and circumferentially-averaged temperatures of nitrogen gas in reaction chamber. The results are based on the simulations conducted using the model described above.

4.1 Different inlet/outlet configurations

Figure 7 shows the temperature distribution at the inner surface of the reactor wall for different inlet/outlet. Case 3 show higher temperatures and lower temperature gradient than other two cases. Case (a) shows lowest values of the wall temperature, and the largest temperature gradient is about 300 K. In case (b), the upper and lower sections of the reactor wall show approximately 200 K higher than the middle part, and the average temperature in the reactor wall is 960.7 K, which is 14 K greater than case (c).

(a) Case (a) (b) Case (b) (c) Case (c)

Fig. 7. Temperature distribution at the inner surface of the reactor wall for different inlet/outlet configurations. The colorbar is in Kelvin.

The velocity pathlines and temperature distributions of the nitrogen gas inside the reaction chamber is shown in Figure 8 and Figure 9, respectively. The temperature contours show that the temperature distribution in horizontal planes is not uniform and some obvious low temperature areas can be seen in case (a) and case (b). The stronger velocities and their higher variability are responsible for this variation in the temperature field. The comparison for velocity and temperature fields confirms this trend.

(a) Case (a) (b) Case (b) (c) Case (c)

Fig. 8. Velocity pathlines of the nitrogen gas for three cases. The colorbar is in m/s.

(a) Case (a) (b) Case (b) (c) Case (c)

Fig. 9. Temperature distributions of the nitrogen gas in different horizontal planes

In case (a), the flow enters the chamber with the horizontal plane at 30° at high speed and meets the heat plate and heat pipes at the bottom. And then the velocity magnitudes becomes low at the upper section.

In case (b), the flow enters from the top and exits through a side outlet. The velocity contours in Figure 8 (b) show that the flow forms a jet as it enters the reaction chamber. A high-speed zone is formed along the center axis in upper section which results in short residence time, inadequate heat exchange and the low temperature in this region.

In case (c), the flow enters from the tangential inlet at the bottom and exits from the tangential outlet at the top. The velocity field for case (c) in Figure 8 (c) shows that the flow in the bottom inlet plane accelerates along the receiver wall as it enters, with the swirling pattern over the circumferential length. The temperature fields for case (c) show significant variation in the temperature magnitude near the walls which is likely due to the higher velocity magnitude variability in the velocity field. Swirling flow patterns can increase the gas motion path and enhance the convection heat transfer between the chamber wall and working fluid, which is helpful for chemical reaction [20].

1100 -

" —■— case (a)

1050 - —case (b)

- —A— case (c)

1000 - t

750 —1 ' 1 '—'—1—'—' 1 ' 1—' 1 ' 1—1 0 20 40 60 80 100 120 140 160

Height /mm

Fig. 10. Circumferentially-averaged temperatures of the nitrogen along different reactor height for different inlet/outlet configurations.

Circumferentially-averaged temperatures of the nitrogen along different reactor height for different inlet/outlet configurations are shown in Figure 10. Although the temperature of the nitrogen gas at the upper half section of the reactor is almost in the same trend for different configurations, the plots shows that the variations of gas temperature along the height of lower half section is different. The largest gas temperature in the reactor are observed for the case (b) and case (c) at the bottom, which is about 1000 K, 65 K higher than case (a). Note that a sharp drop and rise in the gas temperature observed for case (b), about 163 K. And for case (a) and case (c), the temperature

distributions in different heights are more uniform than case (b), with the largest temperature gradient about 34 K and 72 K, respectively. Furthermore, the temperatures in reactor for case (c) is on average 30 K higher than case (a). To sum up, case (c) shows better thermal performance than other cases.

4.2 Different arrangements of heat pipes in heat plate

Figure 11 shows the wall temperature distributions for different configurations. For case (e) and case (f), the upper half section of the reactor wall is more uniform and higher than the lower section, and obvious lowest values of the wall temperature at the lower section are observed. The average temperature of the wall is almost identical for case (e) and case (f), about 962 K. Case (d) shows relatively uniform temperature field over all the reactor wall, and the average temperature is 953 K.

(a) Case (d) (b) Case (e) (c) Case (f)

Fig. 11 Temperature distribution at the inner surface of the reactor wall for different arrangements of heat pipes on heat plate

The velocity pathlines and temperature distributions of the nitrogen gas inside the reaction chamber is shown in Figure 12 and Figure 13, respectively. The velocity pathlines show that the existence of the heat pipe strengthens the turbulent flow in reaction chamber and increases the residence time, which relatively improves the heat transfer effect. The temperature in different horizontal planes for case (e) and case (f) are higher than case (d), and the temperature distributions in horizontal planes are not uniform. The comparison for velocity and temperature fields confirms this trend. The higher velocity variability is responsible for this variation in the temperature field. The nitrogen outlet temperature of case (e) and case (f) is about 951 K, which is 33 K higher than case (d).

(a) Case (d) (b) Case (e) (c) Case (f)

Fig. 12. Velocity pathlines of the nitrogen gas for different arrangements of heat pipes on heat plate.

(a) Case (d)

(b) Case (e)

(c) Case (f)

Fig. 13. Temperature distributions of the nitrogen gas in different horizontal planes.

Circumferentially-averaged temperatures of the nitrogen along different reactor height for different arrangements of heat pipes on heat plate are shown in Fig. 14. In case (d), the average nitrogen temperature is gradually increasing with the increasing of the height, the highest temperature of 923 K, maximum temperature difference about 55 K. The temperature distributions for case (e) and case (f) show similar trends. With the increasing of height, the average nitrogen temperature varies between 920 K and 960 K. Due to the existence of the heat pipe, the heat exchange area increases, and the average nitrogen temperature of case (e) and case (f) are significantly higher than case (d). From the above, the case (d) (triangular arrangement) and case (f) (square arrangement) are better in thermal performance than case (d) (without heat pipes). And there is little difference between the case (e) and case (f). Maybe the arrangement of heat pipes on heat plate needed to be coupled later with different inlet and outlet positions to investigate the thermo-fluid behavior under different configurations in view of the above results.

& 880 E

■ ♦ '4 * ^ . î > • '

_ —• ■ ■—■—■—■

.__■^^ _ H

—■— case (d)

—•— case (e)

—*— case (f)

0 20 40 60 80 100 120 140 160 Height /mm

Fig. 14. Circumferentially-averaged temperatures of the nitrogen along different reactor height for different arrangements of heat pipes on heat

plate.

5. Conclusions

Heat pipe solar reactors (receivers) offer an efficient alternative to transfer heat from the focus of tower and dish collector to the reactants. A novel solar high-temperature thermochemical coupled phase-change reactor, featuring two cavities, of which the upper one is functioning as reaction chamber and the lower one as solar absorber, is designed. Main component is a special-shaped high-temperature heat pipe integrated with phase-change heat transfer, temperature leveling of heat pipe and heat plate, which separates the two cavities. The excellent heat

transfer characteristics of SHHP can ensure the system safety and high thermochemical conversion efficiency.

A three-dimensional CFD heat transfer model of reaction chamber is developed to improve the solar reactor design. The thermal performance of the reaction chamber is investigated for three different inlet/outlet positions and three different arrangement of heat pipes in heat plate. The configuration with nitrogen coming from the bottom tangential inlet and leaving from the top tangential outlet shows better thermal performance. The existence of the heat pipes on heat plate enhances the heat transfer in reaction chamber. Either the triangular arrangement or square arrangement can be chosen. And as for the final choice, further detailed investigation on the coupling between both geometrical parameters is needed.

Acknowledgements

This research has been founded by National Natural Science Foundation of China (Grant NO. 55105192; Grant No. 51105192).

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Simulation on a Novel Solar High-temperature Thermochemical Coupled Phase-change Reactor Academic research paper on "Chemical engineering"

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