Experimental evaluation of the performance of solar ...

Journal of Mechanical Science and Technology 28 (11) (2014) 4789~4795 www.springerlink.com/content/1738-494x

DOI 10.1007/s12206-014-1046-x

Experimental evaluation of the performance of solar receivers for compressed air† Ha Neol Kim1, Hyun Jin Lee1,*, Sang Nam Lee1, Jong Kyu Kim1, Kwan Kyo Chai1, Hwan Ki Yoon1, Yong Heack Kang1 and Hyun Seok Cho2 1 Solar Thermal Laboratory, Korea Institute of Energy Research, Daejeon, 305-343, Korea Department of Chemistry and Chemical Engineering, Niigata University, Niigata 950-2181, Japan

2

(Manuscript Received January 15, 2014; Revised June 10, 2014; Accepted July 14, 2014) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Abstract A challenging issue that arises in achieving a combined cycle with concentrated solar power technology is the development of a solar receiver for compressed air. A solar receiver transfers heat from concentrated solar radiation to compressed air so that the air temperature is sufficiently increased to drive an air turbine. Using a simple modular extension, we have developed three solar receivers for compressed air based on the concept of the tubular receiver. Conventional tubular receivers are generally applied in liquid fluids, such as water or molten salt. Cavities and extended surfaces were designed and incorporated into the developed tubular receivers to improve the heat transfer to air. The receivers were also subjected to performance evaluation tests, which were conducted in the solar furnace of the Korea Institute of Energy Research, with ambient air compressed at 5 bar and expressed in terms of outlet temperature and receiver efficiency. Test results indicated the thermal characteristics of the three solar receivers for their proper use and facilitated the comparison of their performances. The results also provided a guideline to construct and simulate a solar receiver system composed of a series of receiver modules. The problems identified in this study can help improve the solar receiver system. Keywords: Cavity; Compressed air; Concentrated solar power (CSP); Extended surface; Solar receiver ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

1. Introduction Concentrated solar power (CSP) technology collects solar radiation energy, heats up a working fluid to drive an engine or turbine, and then produces electricity. Instead of immediately producing electricity from the thermal energy collected during the day, CSP technology reserves part of the energy in thermal storage, such that electricity can be produced after sunset. Thermal storage enables CSP to generate electricity under the control of power suppliers; this feature, also called dispatchability, is the most valuable feature of CSP and is what differentiates it from other renewable energies, such as solar and wind power [1-3]. As such, many believe that the expansion of renewable energies will lead to an increased demand for CSP. The most commercialized CSP plants are currently based on the Rankine cycle, which uses steam produced by the parabolic trough concentrator at a relatively low-concentration ratio. Tower-type CSP plants with a tower and a heliostat field for high concentration have recently gained ground, but the Rankine cycle continues to be used in their power generation *

Corresponding author. Tel.: +82 42 860 3464, Fax.: +82 42 860 3538 E-mail address: [email protected] † Recommended by Associate Editor Jae Dong Chung © KSME & Springer 2014

unit [3]. The combined cycle is the most efficient traditional method of generating power through fossil fuels; implementing the Brayton cycle technology, which employs solar radiation, can result in a breakthrough improvement in the efficiency of CSP technology or reduce the consumption of fossil fuels [4-8]. One of the most challenging issues related to implementing the solar combined cycle is developing a solar receiver that heats compressed air to at least 800°C [7]. Given the low heat transfer performance of air, multiple receivers should be deployed in a serial connection and with multi-stage configuration to obtain high-temperature air [7, 9]. The tubular receiver, which is widely used for water or molten salt, can also be utilized in relatively low-temperature applications. However, this type of receiver tends to perform with low efficiency and result in durability issues in high-temperature applications. As such, volumetric receivers with porous ceramic absorption materials have been actively studied to enhance their efficiency and endurance under highly concentrated heat flux [10]. Unlike tubular receivers, quartz glass is essential in heating compressed air with a volumetric receiver. However, additional efforts are required to cool the glass, prevent its breakage, and seal the compressed air under high-temperature conditions. Consequently, studies are continuously conducted on

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both tubular and volumetric receivers to overcome the weaknesses while retaining the advantages of each [10-13]. In the present study, three types of solar receivers under the concept of the tubular receiver are proposed to enhance the heat transfer performance of the said receiver. The receivers are designed based on a modular structure to account for scalability. Cavity shapes are applied at the front side of the receiver to prevent high losses of radiation, and extended surfaces are added to the flow path to increase the heat transfer area. We also establish performance evaluation equipment for the receivers using compressed air. The experimental results for the different types of receivers are compared, and the scope of the applicablity of the tubular receiver and other related issues is discussed. Future works for further performance improvements are also presented.

Fig. 1. Overview of a tower-type solar power plant and solar receivers.

2. Design of compressed air receivers Depending on the mechanism of the heat exchange between the working fluid and absorber, conventional solar receivers in CSP can generally be classified into either the tubular or volumetric type [1]. In the tubular receiver, as the outer surface of the tube is exposed to concentrated solar flux, the working fluid to be heated flows inside the tube. That is, the absorber separates the surface that absorbs solar radiation energy and the surface that heats the working fluid. As an easily accessible approach, the tubular receiver made of metal is commonly used in conjunction with a working fluid that consists of water or molten salt under a low heat flux. Although the tubular receiver easily handles compressed air, designing a compact modular type and selecting absorber materials under high heat flux are difficult. By contrast, the volumetric receiver utilizes volumetric absorption in metallic or ceramic porous absorbers. Volumetric absorption enables concentrated solar flux to penetrate absorbers at a certain depth while the porous absorbers provide a large contact area. Therefore, this type of receiver is suitable in heating low-heat-transfer working fluids, such as air at a high concentration. When used with compressed air, a volumetric receiver requires the installation of quartz glass, which is essential to isolate the compressed air from the external environment while allowing light to remain incident on the porous absorber. Therefore, additional cooling and sealing devices are required to prevent the contamination and breakage of the quartz glass and the leakage of the compressed air at high temperature. Based on a modular structure that accounts for scalability in large-scale tower-type CSP plants, three types of tubular receivers were designed in this study mainly to improve efficiency, as shown in Fig. 1. Cavity shapes were applied to prevent radiation heat losses at high concentrations. By imitating a volumetric receiver with porous material to ensure a large heat transfer area, fin or metal foam was used to widen the contact area with the air. As shown on the right side of Fig. 2, the first and second receivers were designed, such that a cavity

Fig. 2. Photo and diagram of the rectangular receiver (top left) and the circular receiver (bottom left).

was formed at the center, whose wall consists of tubes that allow the compressed air to circulate internally [14]. The flow paths were separated into the outer and inner paths. Cold compressed air enters from the rear side of the receiver and moves along the outer path toward the front part. By locating the cold air outside the receiver, heat loss to the environment can be reduced while preheating takes place. At the front side of the receiver, the direction of the air flow is shifted toward the inner flow path, and the air is heated through the concentrated solar flux. It then exits through the outlet located at the rear side of the receiver. By installing fins at constant intervals on the cavity wall along the inner flow paths, the heat transfer area is enlarged, and the convective heat transfer is improved through the formation of a turbulence flow. Fig. 2 shows the difference between the first and second receivers: the former has a rectangular shape that facilitates the simple stacking of multiple receivers, whereas the latter has a circular shape that maintains good strength under high-temperature conditions. The third receiver utilizes a porous material made of metal foam installed inside the conventional tubular receiver, as shown in Fig. 3 [15, 16]. A partition wall is installed in the middle of the tube, and the cavity is attached to the front side, such that it is exposed to solar radiation energy. The metal foam is inserted into the tube, which is separated by a partition wall at the opposite side of the cavity. Cold air flows into the tube and is separated by a partition wall from the rear side of the receiver. It then flows through five inlet pipes that are symmetrically arranged to regulate the flow along the axial


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Table 1. Properties of the metal foam. Property

Value

Cell size (μm)

1200 ± 120

Area density (g/m2)

1190 to 1563

Melting point (℃)

1320 to 1350 -6

Coefficient of expansion (10 /K)

15

Thermal conductivity (W/mK)

0.21

Fig. 4. Diagram of the configuration of the performance evaluation equipment of the receivers.

Fig. 3. Photo and diagram of the metal foam receiver.

direction. The metal foam has holes in it to connect the inlet pipes close to the partition wall, such that the cold air bypasses the metal foam and directly strikes the partition wall. As a result, the front side of the receiver under the highest temperature can be effectively cooled as the air temperature increases. After it strikes the partition wall, the air passes through the metal foam, moves toward the rear side of the receiver, and exits through a single outlet pipe. The evaluation of the performance of the receivers was conducted in the solar furnace of the Korea Institute of Energy Research (KIER), the specifications of which are described below. The aforementioned receivers were fabricated according to the constraints of the KIER solar furnace, such as the concentration area, solar heat flux and power, and the experimental unit space. All three types of receivers were made of stainless steel. The front part of the first receiver is square, with the dimensions of 170 mm x 170 mm and an aperture cavity ratio of 50%. The front part of the second receiver is circular, with an outer diameter of 165 mm and a cavity aperture ratio of 38%. Even though the largest possible front area was pursued for the two receivers, the area focused on by the test facility restricted such attempt. The cavity aperture ratio of the first receiver was maximized to the extent permitted by the fabrication constraints. That of the second receiver was reduced for ease of fabrication and for comparison with the first receiver. The lengths of the first and second receivers were the same, both at about 200 mm. The optical simulation in the previous study [14] showed that receiver lengths that are approximately two times larger than the radius of the front area are enough for the cavity effect. For the third receiver, a circular tube with a diameter of 114 mm, length of 250 mm, and front cavity approximately 40 mm long was used. Sizes comparable to that of the first and second receivers were pursued. However, the metal foam used in the experiment was a commercial rather than a customizable product and thereby largely

determined the sizes of the third receiver. The metal foam had a diameter of 100 mm and a length of 100 mm and was composed of Ni, Cr, Fe, and Al. These elements are commonly used in diesel particulate filters; their major properties are summarized in Table 1. Figs. 2 and 3 show photographs of the three fabricated receivers.

3. Configuration of the experimental equipment As in the earlier work, the performance of the receivers was evaluated through the evaluation system installed in the KIER solar furnace [14]. The system consisted of a plane reflector installed outside a building, which changed the direction of sunlight while tracking the sun, and a parabolic reflector installed inside the building, which concentrated the parallel incident light on a focal point. For the performance evaluation, the receiver was installed at the focal point of the parabolic reflector. By installing a shutter array on the wall of the building between the plane and parabolic reflectors, the amount of heat flux concentrated at the receiver could be adjusted. The reflectivity of the plane and parabolic reflectors, which were coated with aluminum and silver, were 0.84 and 0.94, respectively. The concentration area of the solar furnace had a diameter of approximately 150 mm, maximum input power of 40 kW, and maximum concentration ratio of about 5,000 suns in the design condition [17]. Fig. 4 shows a diagram of the configuration of the performance evaluation equipment of the receivers. Roomtemperature air was compressed through a dynamic compressor with a capacity of 500 L. By adjusting the discharge valves, the air at an average of 4 bar (gauge) of pressure was transferred through the pipe and toward the receiver installed at the focal point. Between the compressor and the inlet of the receiver, a pressure gauge, a k-type thermocouple, and a Coriolis-type mass flow meter were installed to measure the air pressure, temperature, and flow rates of the air at the receiver inlet, respectively. The air transferred to the receiver was heated to a high temperature through a highly concentrat-


ed heat flux. Thermocouples were also installed at the inlet, outlet, and inside each receiver to measure the temperature at the inlet and outlet of the receiver and the temperature distribution inside it, respectively. The air heated by the receiver exited through the exhaust line into an area outside the laboratory. Gauges were installed between the receiver and external exhaust lines to monitor the pressure and temperature. The compressor used here was not capable of adjusting the air flow rate. Instead, the valves mounted outside the external exhaust line were opened, and the receiver lowered to approximately 3 bar (gauge), thereby adjusting the pressure and flow rate inside the receiver. The pressure difference applied before and after the receiver did not mean that the pressure drop was due to the receiver. The numerical results of Lim et al. [16] help assess the order of the magnitude of the pressure drop. The direct normal insolation (DNI) was measured with a pyrheliometer. Reaching a temperature high enough to drive a gas turbine with one receiver module is difficult; thus, several receiver modules must be connected in a series. Given the limitations of the concentration area in the KIER furnace, connecting two or more receivers in a series for heating and for evaluating performance is difficult. Therefore, an evaluation experiment was conducted in this study by supplying pre-heated air, which was further heated afterward, such that the equipment could act as a high-temperature receiver in a series coupling. For this purpose, a heat exchanger was installed at the outlet of the receiver, and the incoming air from the compressor to the receiver inlet was preheated with the heated air from the outlet of the receiver. That is, in the non-preheating experiment, the pre-heater was separated, whereas in the preheating experiment, the pre-heater was connected to the pipe at the front end of the receiver inlet.

4. Experiment results and discussions The flow rate and the input power were the major control variables used under a variety of conditions to evaluate the performance of the receivers. The flow rate was adjusted through the valves mounted on both ends of the system inlet/outlet pipes. At the same time, the pressure was varied inversely. For the experiment, the pressure was maintained at an average of 4 bar, and the variation range at around 0.1 bar. When the DNI changed rapidly, the input power was varied subordinately. Therefore, the experiment was conducted when the DNI variation resulting from weather conditions was low, and the desired input power was set by opening and closing the shutter array. In the performance evaluation, the results were represented mainly based on the outlet temperature of the compressed air and efficiency of the receiver. The air temperature can easily be measured through the thermocouples. The efficiency of the receiver is defined as the ratio of the power incident on the receiver opening to the power obtained by the air. The power incident on the receiver opening, Qi , can

Table 2. Conditions for the performance evaluation of the receivers. Receiver

Rectangular

Circular

Metal Foam

DNI (W/m2)

442 to 694

754 to 878

768 to 851

Mass flow rate (10-2 kg/s)

0.61 to 0.62

1.4 to 1.8

1.6 to 2.4

Input power (kW)

4.2 to 9.2

2.9 to 14.0

1.7 to 7.3

POM (kJ/kg)

692 to 1468

162 to 841

88 to 381

Inlet pressure (bar)

5.02 to 5.09

4.89 to 5.05

4.88 to 5.01

800

Outlet Temperature (oC)

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600 400 200 0

0

200

400

Rectangular Circular Metal foam 600 800 1000 1200 1400 POM (kJ/kg)

Fig. 5. Outlet temperature of air when air at room temperature is supplied.

be measured by means of the flux mapping technique [17, 18]. The power obtained by the air originates from the enthalpy difference between the inlet and outlet of the receiver. As a result, the efficiency of the receiver is expressed as h = m& (houtlet - hinlet ) / Qi . The ratio of the mass flow of air to the amount of heat input in the receiver, or the power-toreceiver over the air mass flow rate, was defined as POM = Qi / m& to compare the performance of the receivers under various conditions. The experimental results were plotted with POM as an independent variable, as shown in the figures below. Under plant operation conditions, the steady state must be maintained to drive a turbine, and the receiver must be protected whenever the solar insolation changes. One operational strategy is to control the flow rate of the heat transfer fluid, such that POM is stabilized. Using POM facilitates the effective comparison of various receivers operating in different environments [16, 19]. All of the data were measured at intervals of 10 s. For a performance evaluation in a steady-state condition, the experiment was conducted when the DNI variation is low. For all the variables of the DNI, flow rate, and pressure, a steady state was considered obtained when the coefficient of variation (the standard deviation divided by the mean) was smaller than 5%. Under steady-state conditions, a single representative value was obtained by averaging successive data for at least 5 min. All the results presented below are based on these representative values. Table 2 summarizes the weather conditions and operating ranges used in the performance evaluation of each tubular receiver. Fig. 5 shows the outlet temperature of the air according to POM when air compressed at 4 bar (gauge) at room tempera-

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100

Efficiency (%)

80 70 60 50 40 30 20

800

800

600

600

400

400

200 0

0

200

400

600 800 1000 1200 1400 POM (kJ/kg)

0

200

400

Circular Outlet Metal foam Outlet 200 Circular Inlet Metal foam Inlet 0 600 800

Inlet Temperature (oC)

Rectangular Circular Metal foam

90

Outlet Temperature (oC)


POM (kJ/kg)

Fig. 6. Receiver efficiency when air at room temperature is supplied.

Fig. 7. Inlet and outlet temperatures of the air when preheated air is supplied.

ture was supplied. The rectangular receiver showed the highest outlet temperature, which reached up to 600°C at a POM of 1470 kJ/kg, while the circular receiver showed the maximum outlet temperature of 500°C at a POM of 840 kJ/kg. By contrast, the maximum outlet temperature of the metal foam receiver was only 232°C at a POM of 381 kJ/kg. The rectangular and circular receivers were derived from essentially the same concept, but their shapes and sizes differed. Although an accurate comparison was difficult to conduct because of the lack of data in the POM range of 640 kJ/kg to 910 kJ/kg, the rectangular receiver showed an outlet temperature that was higher than or at least comparable with that of the circular receiver. Given that the receiver with a rectangular cavity was experimentally assessed at a generally low flow rate range, the results were obtained at a high POM range. As described in Sec. 3, the pressure difference was applied before and after the receiver to drive the air flow. The pressure difference did not mean that the pressure drop was due to the receiver; however, in the experiments, the flow rate in the rectangular receiver was smaller than that in the circular receiver despite similar pressure differences (refer to Table 1). This difference may be related to the flow instability issue in the volumetric solar receiver [20]. The high flow resistance in the rectangular receiver prevented the increase of flow rate and consequently resulted in a high POM range. In the case of the metal foam receiver, which contains a porous medium that increases the heat transfer rate, the temperature did not increase significantly when the input power was increased. Consequently, its performance evaluation was conducted at a low POM range. The trend of the temperature increase in the metal foam receiver with POM tended to be smaller than of the circular receiver. Fig. 6 shows the receiver efficiency that corresponds to the data in Fig. 5. In general, as POM increases, the external heat loss increases, and the efficiency tends to decrease. The efficiency of the rectangular receiver was 40% when the outlet temperature of the air reached 600°C. Even at a POM of 973 kJ/kg, the efficiency did not exceed 50%. By contrast, the efficiency of the circular receiver decreased from 68% to 56% at a POM range of 163 kJ/kg to 842 kJ/kg. Comparing the

outlet temperature in Fig. 5 with the efficiency level shown in Fig. 6 leads to the conclusion that the circular receiver outperforms the rectangular receiver. The rectangular receiver with a large aperture ratio was expected to perform better than the receiver with a circular cavity because of the effects of the cavity, but the performance evaluation revealed the opposite. The assumption was that the high flow resistance in the rectangular receiver at large flow rates resulted in poor efficiency at low POM values. Although further studies are needed to identify the physical reasons behind this phenomenon, the flow control issue can certainly be critical at high-temperature conditions, under which the receiver may be damaged. The circular receiver can still be improved by increasing its aperture ratio until it reaches that of the rectangular receiver. The efficiency of the metal foam receiver drastically decreased as POM increased even at a low POM range. Thus, the effect of increasing heat transfer by the porous medium was not properly realized. This finding is explained below. Fig. 7 shows the inlet and outlet temperatures of the receiver when preheated air is supplied to the receiver. The strength of the rectangular receiver diminished as the temperature increased. Therefore, experiments using preheated air were conducted only on the circular and metal foam receivers. Given that the average temperature of the receiver was relatively higher than its counterpart, the experiment was conducted at a relatively low POM range, as shown in Figs. 5 and 6. In the case of the circular receiver, the input air at 597°C was heated to 832°C at the outlet and at a POM of 545 kJ/kg. In the case of the metal foam receivers, the input air at 442°C was heated to 627°C at the outlet and at a POM of 454 kJ/kg. A temperature exceeding 800°C was needed to drive an air turbine, and the circular receiver met this condition only marginally [7]. Therefore, the three receivers should be coupled in a series to drive the air turbines, and in such cases, the volumetric receiver is suitable for use as the final-stage receiver. Fig. 8 shows the receiver efficiency that corresponds to the results shown in Fig. 7. In the absence of preheating, the receiver efficiency tended to be inversely proportional to POM. The efficiency of the circular receiver decreased from 62% to 49% when the POM increased from 206 kJ/kg to 545 kJ/kg.

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100 90

Efficiency (%)

80 70 60 50 40 Circular Metal foam

30 20

0

200

400 POM (kJ/kg)

600

800

Fig. 8. Receiver efficiency when preheated air is supplied.

The efficiency of the metal foam receiver decreased from 53% to 45% when the POM increased from 99 kJ/kg to 454 kJ/kg. Compared with the results in Fig. 6, the efficiency of the circular receiver decreased by about 10% at a POM of approximately 550 kJ/kg. By contrast, the efficiency of the metal foam receiver decreased by about 10% at around a POM of 380 kJ/kg. The European SOLGATE project demonstrated that the outlet temperature should exceed 800°C, at which point the receiver efficiency is approximately 70% [7]. Thus, the efficiency levels of the receivers developed in this study are too low. To improve their efficiency, a secondary concentrator, as adopted in the receivers of the SOLGATE project, was installed at the front side of the receiver. The secondary concentrator needs a cooling device and precise fabrication to reduce inherent losses [5]. Although a secondary concentrator increases the cost of the system, it is beneficial from the standpoint of efficiency because it maximizes the cavity effect of the receiver and therefore minimizes radiation losses. Moreover, the high reflectance of the stainless steel can be offset by applying a selective coating [21]. The selective coatings currently used at high temperatures are expensive or inapplicable, but at least a high-absorptivity coating should be applied to enhance the performance [3]. The metal foam receiver showed the lowest efficiency. The coefficient of expansion of a stainless steel tube is higher than that of the porous metal foam used here. At a high temperature, stainless steel expands more than the metal foam. Therefore, the contact area between the tube and metal foam is reduced, resulting in an increase in contact resistance. Accordingly, the metal foam is unable to sufficiently cool the tube, and effective heat transfer through the air cannot occur. During the experiment, the stainless steel exposed at the front side of the metal foam receiver could not withstand the highly concentrated solar radiation. As a result, the front side was oxidized, and part of this component was ultimately damaged. The results of this study are the preliminary results for the verification of the concept of the proposed receiver. Related follow-up activities are currently being conducted: modeling studies to understand the flow and heat transfer, designing improvements to upgrade the durability of the material at high

temperatures, and designing a feasible secondary concentrator. New receivers will eventually be created based on the circular and metal foam receivers, while the rectangular receiver will be eliminated because of the difficulties encountered in its flow control. In addition, a multi-channel absorption material was developed for the use of the volumetric receiver at high temperatures [22]. Thus, when the fabrication and evaluation of the volumetric receiver for compressed air are completed, the performance evaluation of a multi-stage coupled receiver system can be conducted.

5. Conclusion In this study, three types of solar receivers for compressed air were proposed. The receivers were based on a modular structure to account for scalability for large-scale applications. The receivers featured cavity shapes and extended surfaces to ensure their improved performance. Rectangular, circular, and metal foam receivers were fabricated according to the proposed design. Performance evaluation equipment was also configured, and the proper procedures established in a solar radiation concentration facility. The performance evaluation of the fabricated receivers was conducted with 5 bar of compressed air. The conclusions of this study are as follows: (1) Among the three receivers, the circular receiver exhibited the best performance. It increased the temperature of air from room temperature to 500°C at 56% efficiency and the preheated air at 597°C to 832°C at 49% efficiency. (2) When room-temperature air was heated, the rectangular receiver yielded the highest temperature. However, it showed a slightly worse performance than the circular receiver because of high flow resistance. Thus, it was vulnerable to highly concentrated solar radiation. (3) The contact resistance in the metal foam receiver increased because of a mismatch between the coefficient of expansion of the porous metal foam and that of the stainless steel tube. Consequently, even with preheated air, the maximum increase in the temperature remained below 454°C, and the efficiency was less than 50%. (4) The circular receiver can heat air up to 800°C. Given the need to increase the temperature further, this receiver is suitable for use as a mid-stage receiver. Nevertheless, for practical applications, its efficiency must be improved by installing a secondary concentrator at the front side and by coating the exposed area with a high-absorption material.

Acknowledgment This work was conducted under the framework of the Research and Development Program of the KIER (B2-2415-02 and B4-2444-01).

Nomenclature-----------------------------------------------------------------------η

: Efficiency


m& h Qi CSP DNI POM

: Mass flow rate : Enthalpy : Input power : Concentrated solar power : Direct normal insolation : Power to receiver over air mass flow rate

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Ha Neol Kim is a master’s student in Mechanical Engineering at Inha University (Korea). He received his B.S. degree from Korea National University of Transportation in 2013. He is currently a research assistant in KIER. His research focuses on solar receivers, chemical reactors, and the thermal storage of molten salt. Hyun Jin Lee holds a major in mechanical engineering, specifically radiative properties and heat transfer. He received his masteral and doctoral degrees from Seoul National University and Georgia Institute of Technology, respectively. He joined the Korea Institute of Energy Research in 2009 and has since been studying solar thermal energy.