Experimental investigation into the thermal performance of a solar steam generator based on spray cooling heat transfer and porous silicon carbide ceramic

$h in$

Enthalpy of water inlet, kJ/kg

$h out$

Enthalpy of water outlet, kJ/kg

$m ̇$

Mass flow of water, kg/s

$Q$

Solar radiation from simulator, kW

$T f 0$

Outlet steam temperature before shutdown, K

$T f 1$

Outlet steam temperature after the sudden shutdown of the solar simulator, K

$T 0$

Ambient temperature, K

$Δ h$

Enthalpy difference, kJ/kg

Greek symbols

$θ$

Dimensionless temperature

$η$

Thermal efficiency of steam generator

$τ$

Time, s

Hydrogen energy is expected to play a major part in the supply of energy in the future because of the multiple sources available for it, its high energy density, and environmental friendliness.¹ The production of hydrogen from renewable energy based on electrolysis is regarded as an important technique because of its nonpolluting emissions. Hydrogen production based on solid oxide electrolysis cells (SOECs) can reduce the polarization-induced overpotential and accelerate the reaction at the electrodes to a greater extent than other techniques of electrolysis and, thus, has a higher efficiency of energy conversion.^2–4 

The SOEC coupled with solar energy is an important means of producing hydrogen.^5–9 Zhang et al. proposed a high-temperature steam electrolysis-based system for hydrogen production that was driven by solar energy and modeled its thermodynamics and electrochemistry.¹⁰ Lin et al. conducted a techno-economic analysis of a solar power-driven high-temperature electrolysis system for the production of hydrogen and syngas,¹¹ and Schiller et al. experimentally examined a system consisting of a solar simulator, a steam generator, a steam accumulator, and an SOEC.¹² The results showed that the generator could yield a maximum rate of flow of steam of 5 kg/h in the presence of solar energy and attained a rate of steam conversion of 70% when part of this steam (0.58 kg/h) was mixed with 10% hydrogen in a stack of 12 electrolysis units at a power of 2 kW and a current density of –1.25 A cm⁻² for electrolysis at 770 °C. Zhang et al. proposed a reactor by integrating a volumetric heat absorber and a tubular SOEC.¹³ The results of calculations based on a model of it showed that the SOEC played an important role in enhancing the efficiency of the reactor when it was operated at 760–920 °C. The efficiency of the reactor could be improved by increasing the incident power and the mass fraction of steam and reducing the velocity of flow at the inlet. This integrated reactor could improve the efficiency of energy conversion by 2.63%–30.21% compared with a separate structure.

The SOEC is usually operated at a high temperature of 700–1000 °C.¹² The high-temperature steam generated by using solar energy can reduce the consumption of electrical energy associated with electrolysis in the SOEC to improve its efficiency of energy conversion. Prevalent research in the area has focused on a combination of the SOEC and solar energy for hydrogen production. Many researchers have also investigated solar steam generators from other perspectives.^14–19 Ben-Zvi et al. proposed a solar tube-based steam generator by dividing the location of the heat pipe in it into an evaporative part and a superheated part. The results of simulations showed that the generator could produce superheated steam at a temperature of 550 °C and a pressure of 150 bars.¹⁴ Houaijia et al. developed a solar tube-based heat absorber that could produce steam at a temperature of 700 °C and can be used for electrolysis at high temperatures.²⁰ Pye et al. proposed a solar tube-based steam generator as well and used experiments to show that it could produce superheated steam at 560 °C for a DNI of 1052 W/m².²¹ Swanepoel et al. experimentally investigated solar collectors, and the results showed that the fluid attained 861 W of heat at an intensity of solar radiation of 757 W/m² and an outlet temperature of 343 °C when the total area of incidence of the absorber was 2.7 m² and the rate of water flow was 0.294 g/s. The absorber had a thermal efficiency of 42%.²² Indira et al. designed and built a hybrid system for using photovoltaic/thermal solar energy and validated its model through simulations and tested its experimental performance.²³ The results showed that its maximum electrical efficiency was 4.86% and maximum thermal efficiency was 40% when the intensity of solar radiation was higher than 1000 W/m².

Although researchers have proposed many types of solar steam generators, few experimental studies have been devoted to generators that can produce high-temperature steam at around 700–1000 °C owing to their limited capacity for heat transfer. The introduction of the porous material to solar heat absorbers is among the most effective methods for improving their heat transfer-related performance.²⁴ Therefore, the authors of this study propose a solar steam generator with a porous structure and spray cooling technology. Few studies in the area have systematically investigated the influence of different parameters (irradiation power, rate of flow, and structural factors) on the performance of porous heat absorbers, where this is critical for optimizing them. We tested our proposed steam generator through experiments, and the results showed that it can produce steam from water at a temperature of 800 °C. The generator has prominent advantages in terms of producing high-temperature steam compared with the steam generators proposed in Refs. 20 and 21. We also examined its thermal performance based on several parameters to provide reference values for its optimization.

A diagram and photographs of the high-temperature steam generation platform are shown in Figs. 1 and 2. The experimental system consisted of a solar simulator, a high-temperature steam generator, a high-pressure spray system, and a data acquisition system. Concentrated solar energy from the solar simulator was irradiated through the quartz glass onto the porous ceramic heat absorber to heat it during the experiment. The high-pressure spray system at the back-end provided high-pressure water that flowed through the nozzle and atomized the water into small droplets to be sprayed on the back surface of the heat absorber to form steam. The steam then exchanged heat with the high-temperature heat absorber under the action of a difference in pressure to form superheated steam. The parameters of the system—the temperature, pressure, and rate of flow—were measured and collected by the data acquisition system, details of which are listed in Table I.

1. The solar radiation used in the experiments was drawn from a solar simulator that consisted of 12 short-arc xenon lamps, each with a maximum electrical input power of 7 kW. The power of each xenon lamp could be adjusted from 80% to 100%.

2. The steam generator was the central component of the entire experimental system, and consisted of a porous ceramic heat-absorbing body, a thermal insulation shell, quartz glass, a high-pressure spray system, and a supporting structure. The porous ceramic heat absorber was a cylinder with a diameter of 80 mm, and was installed in the central cavity of the generator. The quartz glass had a diameter of 115 mm and a thickness of 15 mm, and was installed at the front of the generator. The insulation shell was composed of refractory bricks and insulation wool, and was designed to reduce the loss of heat of the generator due to conduction to the environment during the experiment. The high-pressure spray system consisted of a submersible pump, a high-pressure spraying machine, and a micronozzle with diameter of 0.1–0.2 mm. Deionized water was passed through the submersible pump and the high-pressure sprayer in turn, and was atomized under the action of the nozzle to form small liquid droplets.

3. The data acquisition system collected the temperature, pressure, and rate of flow of the steam generator during the experiment. Four thermocouples installed on the generator were used to measure the temperatures at its inlet, its back-end, the outer wall, and the outlet. An infrared thermometer was used to measure the temperature on the surface of quartz glass. Three pressure sensors were installed on the steam generator to measure the pressure at the inlet of water, back-end pressure, and front-end pressure. The flow of water at the inlet was measured by a gear flow meter. Detailed information on the measuring instruments is provided in Table II.

4. The porous ceramic heat absorbers were all cylinders with a diameter of 80 mm. The parameters of the porous ceramic materials are given in Table III.

5. Before testing the thermal performance of the steam generator, we measured the distribution of its incident energy density by using the corresponding measurement system.²⁵  Figure 3 shows the experimental results when all four short-arc xenon lamps of the solar simulator were turned on at a power of 90%.

Steps of the experiments were as follows:

1. Start the software to acquire data on the steam generator. Collect information on its flow, temperature, and pressure.

2. Turn on the solar simulator to start generating solar power.

3. Turn on the CMOS camera to observe the location of the spot of light. It overlaps with the front surface of the porous silicon carbide ceramic heat absorber, as shown in Fig. 4. The steam generator delivers good performance at this location. The experimental test can then proceed.

4. As the power of the incident irradiation increases, so does the temperature of the steam generator. Once the simulator has run for about 20 min, turn on the high-pressure spray system and adjust the rate of flow. Then, start tests 1, 2, 3, and 4, which are described below.

5. Once the experiment has been completed, turn off the simulator, the data acquisition system, and the high-pressure spray system, and copy the experimental data.

Test 1. This was used to assess the effects of the incident irradiation power on the performance of the steam generator. The rate of water flow at the inlet was kept constant, and the experimental data were obtained by increasing or reducing the number of irradiating lamps and regulating their power ratio (80%–100%) when the system reached a steady state under different incident irradiation powers. The steady-state results of different tests were compared. When the primary variables of the system (temperature of steam at the outlet of the generator and pressure inside the generator) no longer changed by much for longer than 5 min (changes of less than 3% in the temperature of steam at the outlet and 1% in the pressure at the measurement point inside the generator), the system was considered to be in a steady state.

Test 2. This was used to test the influence of the rate of water flow at the inlet on the thermal performance of the steam generator. The rate of water flow at the inlet was set to the specified value, and was not adjusted again until the system had returned to a steady state. Tests were carried out at rates of water flow at the inlet of 0.92, 1.52, and 4.68 l/h under a constant power of the incident irradiation.

Test 3. This test was used to evaluate the robustness of the steam generator to fluctuations in irradiance. The conditions of a sudden drop in irradiation were simulated by turning off the irradiation lamp once the steam generator had reached a steady state. Curves of the reduction in temperature at various measurement points 255 s after the simulator had been turned off were used to examined how well the steam generator could withstand fluctuations in irradiation.

Test 4. According to the operating conditions of tests 1, 2, and 3, the experimental designs of different porous heat absorbers were used to investigate the effect of the structural parameters of the porous materials on their performance.

The process of heat exchange with water in the steam generator can be divided into three stages: the preheating stage, the evaporation stage, and the superheating stage. The preheating and the superheating stages of heat exchange can be viewed as single-phase flows, while the evaporation stage can be considered to be heat exchange occurring owing to two-phase gas–liquid flow. When the water was in the preheating stage, most of the liquid phase had not been completely converted into steam. Figure 5 shows the variations in the temperature at the outlet of the steam generator over time when material A was used, once the solar simulator had been turned on. The temperature at the outlet of the steam generator gradually increased as heat transfer proceeded. When the incident irradiation power was 6 kW, the rate of water flow at the inlet was 0.92 l/h, and the temperature of water at the inlet was 39 °C, the steam generator produced superheated steam with a temperature of 800 °C.

Figures 6–9 show the variations in the temperatures at the outlet over time and the average distributions under different irradiation powers of the solar simulator when the flow of water was constant, the water pressure at the inlet was 0.3 MPa, and its ambient temperature was 15–31 °C. They show that the outlet temperature of water increased with the incident irradiation power, as did the temperature of the porous ceramic material. This led to greater heat exchange between the porous ceramic material and the environment, including the volume of radiation exchanged between the porous ceramic material and quartz glass, and convective heat exchange between the material and internal water. The increase in convective heat exchange led to greater heat absorption by water, which increased the temperature of the steam being exported. When water was in the evaporation stage, the process involved a significant increase in the coefficients of heat absorption and heat transfer compared with those in the preheating stage. The temperature of water in the evaporation stage and that of steam at the outlet in the superheating stage thus increased significantly.

The thermal efficiency of a steam generator is an important indicator of its thermal performance. We used the following formula to calculate its thermal efficiency:

Based on Eq. (2), Fig. 10 shows the distribution of the instantaneous thermal efficiency of the steam generator using material B under different irradiation powers of the solar simulator when the rate of water flow at the inlet was constant. It shows that when the steam generator had reached a steady state, the greater the power of solar irradiation was, the lower was its thermal efficiency. When the incident irradiation power was 4.6 kW, water flow at the inlet of the steam generator fluctuated during heat exchange and led to large fluctuations in the corresponding instantaneous thermal efficiency. When the system had reached a steady state, its thermal efficiency also became stable. When the incident irradiation power was 2.3 kW, the system had reached a steady state, and the outlet temperature of the steam generator was 115.4 °C when using material B. According to Ref. 26, the temperature of water at 115.4 °C corresponded to a saturation vapor pressure of 0.171 MPa, but the pressure of water vapor was lower than this saturation pressure. When the pressure of the generator was 0.123 MPa, the corresponding saturation temperature was 105.5 °C, which was lower than the measured temperature. When the incident irradiation power was 2.3 kW, some of the water was converted into steam at 115.4 °C, while the remainder persisted in liquid phase and was continuously heated. Therefore, we needed to separately consider heat absorption in the liquid and the gaseous phases. The contents of water in the liquid and gas phases could not be directly obtained owing to the experimental conditions considered here. We, thus, provide the distribution of thermal efficiency of the generator by assuming that the water was in the pure liquid and pure gas phases under an irradiation power of 2.3 kW, as shown in Fig. 11. Figure 12 shows variations in the temperature on the surface of quartz glass in the steam generator with material B over time. Its temperature increased with the incident irradiation power, as did the temperature of the porous ceramic material. This led to increasing radiation-induced heat exchange between the porous ceramic material and quartz glass, which in turn increased radiation-induced heat exchange between the latter and the environment to increase convection-induced heat exchange. We know from Eq. (2) that as the incident irradiation power increases, the thermal efficiency of the steam generator decreases because the ratio of loss of heat in it is higher in this case.

The results of the influence of the incident irradiation power on the outlet temperature and the thermal efficiency of the steam generator were consistent with those reported in Ref. 27, the results of which showed that the outlet temperature increased and the thermal efficiency decreased with increasing incident irradiation power.

Figures 13 and 14 show variations in the outlet temperature of the steam generator over time and its average distribution once the water in it had reached a steady state. The generator produced steam with a maximum temperature of 800 °C when the incident irradiation power was 6 kW, water flow at the inlet was 0.92 L/h, and the temperature of water at the inlet was 39 °C. When the incident irradiation power was kept constant, the outlet temperature of steam in the generator decreased as the rate of water flow at the inlet was increased. Moreover, the heat exchange per unit mass of steam decreased as the rate of water flow at the inlet was increased, resulting in a decline in the outlet temperature of steam.

Based on Eq. (2), Fig. 15 shows the thermal efficiency of the steam generator once it had reached a steady state using material A, with a constant incident irradiation power and different rates of water flow at the inlet. It shows that the thermal efficiency of the steam generator increased with the rate of water flow while the incident irradiation power was kept constant. When the incident irradiation power was maintained at 6 kW and the rate of water flow at the inlet was low, steam entered the porous ceramic material, and failed to obtain most of the energy absorbed by it. It then entered the space of heat exchange between the quartz glass and the porous ceramic material through convective heat exchange. It exchanged a small amount of heat with the quartz glass owing to the low rate of flow, and this eventually led to a large temperature gradient between the quartz glass and the porous ceramic material. The radiation-induced heat exchange between them was higher, as was that between the quartz glass and the environment. Therefore, the ratio of loss of heat in each part of the steam generator was higher than the heat exchanged by steam at low flow rates, because of which its thermal efficiency was lower. As the rate of water flow at the inlet was increased, the ratio of loss of heat of each part of the steam generator decreased and its thermal efficiency increased. When the rate of flow rate was high, the temperature of the exported steam decreased. Therefore, under the premise of considering the temperature of the exported steam and the thermal efficiency of the generator, selecting a suitable rate of water flow at the inlet is an important means of ensuring the thermal performance of the steam generator during its operation.

The results of the influence of the rate of water flow at the inlet on the temperature of steam at the outlet and the thermal efficiency of the generator are consistent with those reported in Ref. 27.

Figure 16 shows that under the same incident irradiation power and rate of flow at the inlet of the steam generator, silicon carbide ceramic materials with different porosities generated steam at different temperatures at the outlet. The other parameters of the materials were kept the same as above. The steam generator using material A produced steam at a higher temperature at the outlet than that using material B. This is because the lower the porosity of the material was, the larger was the corresponding specific surface for the convective heat transfer for steam inside it. This led to an increase in the coefficient of bulk heat transfer as well as the amount of heat absorbed by the steam, and in turn increased the temperature at the outlet.

Figure 17 shows that the use of porous silicon carbide ceramics with different porosities led to varying efficiencies of the steam generator while the incident irradiation power and the rate of flow of water at the inlet were kept constant. The steam generator using material A had a higher thermal efficiency than that using material B. This is because the lower the porosity of the material was, the higher was the coefficient of bulk heat transfer for convective heat transfer for the steam inside it. This led to a larger amount of heat absorbed by the steam, and increased the thermal efficiency of the material with lower porosity at the same incident irradiation power. However, the porosity of the porous ceramic material should be set to within a reasonable range by considering such factors as the feasibility of the process and the drop in the pressure of the porous material.

The results of the influence of the porosity of the material on the outlet temperature of steam and the thermal efficiency of the steam generator were consistent with those reported in Ref. 28.

We simulated a steep drop in the input irradiation to the front surface of the steam generator in the experiments by turning off the irradiation lamps of the solar simulator once the generator had reached a steady state. We then investigated its ability to resist such drops in irradiation, such as in case the sun is shaded by clouds, by obtaining the reduction in the temperature of steam at the outlet 255 s after the simulator had been turned off.

The steam generator reached a steady state under different irradiation powers and a constant rate of water flow at the inlet. All irradiation lamps of the solar simulator were then turned off. The resulting reductions in the temperature of the generator 255 s later are shown in Figs. 18–21. They show that the temperature of steam at the outlet of the generator at different incident irradiation powers decreased after the sudden shutdown. As the incident irradiation power was increased, the reduction in temperature following the sudden shutdown of the simulator increased for both the steam generators using materials A and B. The temperature of the porous ceramic material also increased with the incident irradiation power, and the increased convective heat exchange between it and the steam led to an increase in heat absorption by water. This in turn increased the temperature of steam at the outlet while the rate of water flow at the inlet was kept constant. Suddenly turning off the irradiation lamps of the solar simulator, and the associated decrease in solar irradiation, caused a reduction in convective heat exchange between the porous material and the steam, heat absorption by water, and the temperature of steam at the outlet. Therefore, the higher the incident irradiation power was, the higher was the drop in temperature once the simulator had been turned off.

Figure 22 shows that the use of porous silicon carbide ceramic materials with different porosities at the same incident irradiation power and the rate of flow at the inlet led to significant differences in the robustness of the steam generator to fluctuations in irradiance. The value of $θ$ of the steam generator using material A was higher than that using material B after the sudden shutdown of the simulator, the changes in it were minor, and thus the resistance of the generator to interference was higher.

The above result was obtained because the smaller the porosity of the ceramic was, the larger was its solid mass. It, thus, had a higher heat capacity, its temperature changed slowly, and it was more resistant to interference. The status of heat transfer in the evaporation stage of water under the two-phase gas–liquid flow was more complex, and required considering the change in the phase of water when the coefficient of heat transfer increased significantly as well as the effects of changes in the porosity of the ceramic materials on its heat capacity. Therefore, the dimensionless temperature of material A was lower than that of material B.

The authors of this study designed a steam generator with a high capacity for heat transfer by installing a porous ceramic material inside it and using spray cooling technology. Prevalent steam generators produce steam at a temperature that rarely reaches 700–1000 °C. Our proposed steam generator can use concentrated solar energy to produce high-temperature steam (up to 800 °C) and has significant advantages in terms of generating high-temperature steam compared with the generators proposed in Refs. 20 and 21. Moreover, we tested two porous silicon carbide ceramic materials under various experimental conditions to assess the thermal performance of the steam generator and provide guidance on its structural optimization. We investigated the effects of the incident irradiation power, rate of water flow at the inlet, and porosity of the porous ceramic material (ranging from 70% to 85%) on the outlet temperature, thermal efficiency, and robustness of the generator to fluctuations in irradiance. The following conclusions were obtained:

• As the incident irradiation power was increased, the outlet temperature of both steam generators using ceramic materials with porosities of 70% and 85% increased. The lower the porosity was (70%), the higher was the outlet temperature. However, the thermal efficiency of the generator decreased with increasing irradiation power, because of which the steam generator using the ceramic material with a low porosity (70%) had better thermal characteristics. It obtained steam with a higher temperature and yielded a higher thermal efficiency under the same irradiation power compared with the steam generator that used the ceramic with a high porosity.

• The outlet temperature of the steam generator decreased and its thermal efficiency increased as the rate of water flow at the inlet was increased. Therefore, appropriately increasing the rate of water flow at the inlet can improve the thermal efficiency of the generator for a given outlet temperature.

• The lower the porosity of the ceramic material was, the better the resistance of the steam generator to disturbances.

We focused here on the effects of specific parameters on the thermal performance of our proposed high-temperature steam generator, and the results provide a useful reference for its optimization. We were unable to analyze the influence of certain other parameters due to particularities of the experimental conditions. Future research in the area should examine the influence of the thickness and pore density of the porous material on the thermal performance of the steam generator. The structure of the steam generator can be optimized based on experimental data to further improve its efficiency of photothermal conversion.

The present work was supported by the Beijing Natural Science Foundation (No. 3222049) and the National Natural Science Foundation of China (No. 52176209). This research was also funded by International Partnership Program of Chinese Academy of Sciences (No. 182111KYSB20200021). The authors thank the Institute of Electrical Engineering, CAS (No. E155710101) for financial support.

The authors have no conflicts to disclose.

Hongjun Wang: Conceptualization (equal); Data curation (equal); Methodology (equal); Writing – original draft (equal); Writing – review & editing (equal). Qiangqiang Zhang: Conceptualization (equal); Funding acquisition (equal); Investigation (equal); Supervision (equal); Writing – review & editing (equal). Xin Li: Conceptualization (equal); Supervision (equal); Writing – review & editing (equal). Xia Zhang: Investigation (equal); Supervision (equal). Tianzeng Ma: Investigation (equal); Supervision (equal).

The data that support the findings of this study are available within the article.