Experimental Study on Solar Heat Battery using Phase Change Materials for Parabolic Dish Collectors

Energy consumption has increased with the population increase, and fossil fuel dependency has risen and causing pollutions. Solar energy is suitable to provide society's thermo-electric needs. Thermal energy storage-based concentrated solar receivers are aimed at store heat energy and transportable to the applications. A cavity receiver with two-phase change materials (PCM) is experimentally investigated using a parabolic dish collector to act as the solar heat battery. The selected PCMs are MgCl2.6H2O and KNO3-NaNO3. PCMs are chosen and placed as per the temperature zones of the receiver. The outdoor test was conducted to determine the conical receiver's storage performance using cascaded PCMs. The complete melting of PCM attains at an average receiver surface temperature of 230°C. The complete melting of the PCM in the receiver took around 30 minutes at average radiation around 700 W/m2, and heat stored is approximately 5000 kJ. The estimated number of cavity receivers to be charged on a sunny day is about 10-15 according to the present design and selected PCMs, for later use.


Introduction
Despite using fossil fuels for our thermal needs, an alternative measure is necessary to produce sustainable energy. Solar energy is renewable and available to provide thermal and electrical power using efficient energy conversion systems. Solar radiation is available annually for about 280 days in India. Solar photovoltaic power is effectively stored in Li-ion batteries (Reddy et al., 2013, Reddy et al., 2015. But the major drawbacks of solar thermal systems are intermittent solar radiation and solar energy after sunshine hours. Hence, thermal energy storage (TES) plays a pivotal role in managing such radiation discontinuity. The concentrated solar collectors are helpful to attain higher temperatures for steam production and power generation, while flat plate collectors serve low to medium temperature applications Senthil, 2020a, 2020b).
The parabolic dish solar collector (PDSC) is used for medium to high-temperature applications. Solar energy application requires efficient TES to store heat during peak radiation for later uses. The phase change materials (PCM) contribute to effective thermal management because of their high energy density (Punniakodi and Senthil, 2021). The selective PCMs for PDSC are nitrates (LiNO3, KNO3, NaNO3), hydrated salts, sugar alcohols, and eutectics, depending on applications (Jibin et al., 2012;Reddy et al., 2012).
Several researchers investigated concentrated collectors with a much focus on the poly-generation of producing heating, cooling, electricity, and green fuels (Abid et al., 2020;Aghaziarati et al., 2021;Yilmaz et al., 2020;Ghorbani et al., 2020;Shaikh et al., 2021). Such systems produce high overall conversion efficiency with integrated heat storage. The collector's optical aspects, flux distribution, and mirror coatings are essential to attain the maximum output Rostami et al., 2021;Roosendaal et al., 2020;Senthil and Nishanth, 2017). The photo-thermal efficiency improved 8% for every 0.1 increase of absorptivity.
In PDSC, the cavity-type solar receivers capture the maximum heat flux from the reflector (Bopche et al., 2020;Kasaeian et al. 2021;López et al., 2020;Santoso et al. 2020). A low-cost PDSC of 12.6 m 2 aperture is analyzed for heating applications (Sahu et al., 2021). Cavity receivers produce more heat output than flat receivers by conserving heat flux (Yanping et al., 2020). The fluid inlet temperature is significant for receiver's heat absorption (Senthil et al. 2017a). PCM-based concentrated solar receivers enhance the overall performance by minimizing heat losses (Senthil et al., 2017b(Senthil et al., , 2017cKalidasan et al., 2020;Schöniger et al., 2020;Senthil, 2020). PCM-based solar thermal systems provide promising benefits to match the thermal requirements.
The cavity receiver's smaller end is observed with a higher temperature than the larger end as per the literature. A single PCM in the solar receiver was investigated to improve the productivity of solar energy systems (Yilbasa and. Thus, the earlier research works are carried out with a single PCM in the solar receivers. Employment of two PCMs as per the temperature zones of a receiver is not investigated earlier. Thus, the objective of the present experimental study is to investigate the cavity receivers using cascaded PCMs to capture the varying heat flux. This current work is one of a new kind as per the authors' knowledge.
The enhancement of charging of cascaded PCMs-based solar receiver using is investigated experimentally here. A cavity receiver with two cascaded PCM is aimed to act as a solar heat battery. Outdoor tests were conducted on the PCM-cavity receiver with PDSC, and significant results are reported here. Section 1 derives the need of the study. Section 2 discusses materials and methodology. Section 3 provides a discussion on the results. Section 4 draws significant conclusions.

Materials and Methods
The experimental setup, experimental procedures, performance calculations, uncertainty analysis, are discussed in this section.

Experimental work
A PDSC of 16 m 2 aperture area acts as a concentrator. The detailed specifications of PDSC and conical receiver are provided in Table 1. During the actual testing, the receiver is kept at the focus of PDSC. The receiver is fixed, while the dish tracks the sun. There is no provision for the fluid flow through the solar heat battery as per the present design. The receiver is bifurcated with two parts to hold five kilograms of KNO3:NaNO3 at the base to capture high heat flux, and five kilograms of MgCl2.6H2O filled near the aperture. PCMs are arranged based on the receiver's heat distribution. Fins from the receiving wall protruded to PCM to improve the heat transfer. Figure 1 shows the layout of the proposed receiver. The broader side is filled with MgCl2.6H2O and a smaller conical annular region filled with KNO3:NaNO3. The base side PCM is filled, and the cover plate is welded. The opening side, MgCl2.6H2O, is filled, and the annular surface is welded with the proper vessel. The receiver periphery is covered with appropriate insulation. Properties of PCMs (M/s, Thermo Fisher Scientific India Pvt. Ltd, India) are given in Table 2.

Experimental procedure
The outdoor experiments were conducted on cavity receivers using PDSC. Figure 2 shows a schematic layout of the experimental setup. The tests were conducted in March 2018. Solar radiation, ambient, and receiver temperatures were measured. The outdoor tests started at 8.00 AM; once the receiver reached the optimum temperature of 230°C, the receiver removed from the focus and poured cold water at a temperature around 10°C to cool faster to make the receiver ready for the subsequent trial. Five tests are conducted each day at 8.00 AM, 10.00 AM, 12.00 PM, 2.00 PM, and 4.00 PM.   An infrared thermal imaging camera (Make: Fluke Ti400) is used to measure the average receiver surface temperature during outdoor testing. The thermal imaging camera is used to measure the average surface temperature through its inbuilt function. Every minute, the temperature is observed to track the average receiver surface temperature reaching about 230°C. The experiment is stopped when attaining this temperature. This receiver temperature ensures the complete melting of both PCMs. More than 230°C also increases the total energy stored, but the PCM thermal stability is also accounted. The maximum safe working limit of PCM could be followed to avoid material loss and damage. Operating beyond a temperature of 230°C, other high-temperature PCMs can be selected. Metal PCMs are suitable for more than 500°C.
The receiver was tested for five consecutive days to monitor repeatability and reliability. The readings of the three days are illustrated in Fig. 4. The average surface receiver temperature reaches about 230°C to ensure both the PCMs melted completely in each experiment. Similar input conditions are considered for comparing the solar receivers. The outdoor testing of the cavity receiver with the cascaded PCMs repeated to ensure uniformity and repeatability. Two handles on the receiver periphery are provided to keep on the focus and transport it easily.

Performance calculations
The performance of the PDSC-solar receiver is determined based on energy stored and energy recovered. The total heat stored in PCM is given by Eq. (1), where Ti, Tf, and Tm are the initial, final, and melting temperatures of PCM, mpcm is the mass of PCM, is the latent heat of PCM, dT is the temperature difference, Cpcm is the PCM specific heat.
The overall thermal efficiency is given by Eq. (2) h 67 = ' 8 9 :; <= >; <? @ where is mass of water (kg), E is the specific heat of water (kJ/kg K), Ib is solar beam radiation (W/m 2 ), Ac is the PDSC aperture area (m 2 ), F, , F4 are water's initial and final temperature (K). The storage effectiveness is defined as the ratio of actual heat stored (Q) to the maximum possible heat stored in the solar receiver.

Uncertainty analysis
An uncertainty analysis is done (Kline and McClintock, 1953;Moffat, 1988). The overall uncertainty of the experiment is determined by Eq. (3), X is the measured quantity, Y is the overall uncertainty of the experiment (%), ΔX is uncertainty in the measured quantity (%), ΔY is uncertainty in the derived quantity (%). The uncertainty is determined based on the measurement of the mass of PCM (±0.01 kg), solar radiation using pyranometer (±5 W/m 2 ), temperature measurement (±0.1°C) and wind speed using anemometer (±0.1 m/s) using Eq. (4) and (5) The overall uncertainty of energy efficiency and storage effectiveness is determined by about ± 4.5% and 3.5%, respectively. Thus, the uncertainty is well within the acceptable range of measurements.

Results and Discussion
In this section, PCM charging, the quantity of heat stored, and the prospective applications are discussed. Figure 3 shows the thermal image of temperature distribution on the cavity surface of the PCM-integrated solar receiver. The average temperature of receiver surface is determined using the in-built average function of the camera. The PCM temperature is measured using thermocouples. The heat transfer takes place from the receiver surface to PCM. The amount of heat stored is determined based on the average temperature of the PCMs.  Three days-testing observations are shown in Figures  4.(a)-(c). The surface temperature distribution is observed using a thermal imaging camera, and the solar radiation is measured by a weather station at the test site-higher the beam solar radiation, the charging time of PCM decreases and vice versa. The average beam solar radiation during the experimental duration over the day and the charging time of PCM is compared. The higher the radiation intensity, the charging time of PCM decreases marginally.

PCM charging
The conduction heat transfer dominates while melting of PCM in the both the sections of the receiver. Due to the small thickness of PCM of about 25 mm, the conduction heat transfer occurs in the PCM and the increase in temperature of PCM till the melting point and then, the melting of PCM occurs at near isothermal temperature. Natural convection in the molten PCM improves the heat transfer inside the PCM and leads to increase in temperature of PCM at the liquid state. The charging time of PCM in the solar receiver with parabolic dish collector is on par with the literature (Yilbasa andKhalil Anwar, 2016, Khalil Anwar et al., 2016;Senthil, 2020;Senthil and Cheralathan, 2019). Figure 5 shows the average beam radiation and the corresponding average time for the receiver reaching 230°C on the considered three trial days to ensure the complete PCM melting. The time is taken to melt the PCM and reach an average temperature of about 230°C is around 26-28 minutes. The slight increase or decrease in the charging time is due to average solar beam radiation during testing. The charging time of 30-35 min has been observed in the forenoon (8.00 AM) and afternoon (4.00 AM) because of low solar beam radiation. The low ambient temperature and high wind speed result in increased heat losses from the receiver with a longer charging time.

Heat stored
The heat stored in the solar heat battery is up to 5000 kJ in about 20-30 minutes at average solar radiation of around 700 W/m 2 . Figure 6 shows the heat stored in the different components of the solar receiver in terms of sensible heat of receiver material, sensible and latent heat of PCM. Thus, from 8.00 AM to 4 PM, there is a potential of charging about several solar heat batteries with a total heat quantity of about 75,000 kJ at a concentration ratio of about 100. The heat storage effectiveness is determined based on the actual and maximum allowable temerpature of PCM in the receiver is about 0.65. The maximum allowable temperature of PCM is the critical temperature or degradation temperature of PCM. During the real-time study, the temperature is maintained at a lower than the degradation temperature to avoid the PCM mass loss. The storage effectiveness increases with the increase in PCM temerpature. Due to cascaded PCM arrangements, the operating temperatures are limited to the thermal stability of PCM. The energy efficiency of solar receiver with cascaded PCM is about 65%, based on the temperature attained by the fluid and PCM temperatures. Each heat battery can evaporate two kg of water with effective insulation of the receiver. Thus, there is a possibility of producing about 20-30 kg of steam using about 10-15 solar heat batteries using a PDSC. The heat battery must be kept inside a well-insulated container for effective later use. The thermal output of solar heat batteries using PCM is observed on par with the literature (Peiro et al., 2015;Khalil Anwar et al., 2016;Senthil & Cheralathan, 2019). The applications vary from domestic to industrial based on the PCM selection for the type of concentrated solar collectors.
The temperature drop of single PCM usage is faster than the multiple PCMs due to the cascaded principle. Thus, storing heat at a higher temperature is prone to heat loss to the ambient, whereas keeping the heat at two different temperatures minimizes the heat loss to the surroundings. Therefore, the use of two PCMs is helpful to capture the heat of the respective temperature zone of the solar receiver and minimize the thermal degradation to the ambient conditions.

Potential applications
Fabricated cavity receiver with PCMs is investigated to enhance receiver's heat storage. The novelty of the present design is the transportation feasibility of solar heat batteries to the application sites since the receiver mass is 21 kg. The PCM mass could be decreased or increased based on the material selection and the indented applications. Later, such receivers are taken back to the PDSC site to recharge. The selective PCM could be changed for the applications. The current selection of PCMs is suitable for hot water and cooking. For the cooking, one more vessel to be fabricated to fit into the cavity with the required provisions for personal safety. The suitable applications are hot water, cooking, spaceheating, thermochemical reactions, fuel production, frying, drying, water-splitting, vapor generation, autoclave, and medical sterilization, etc. (Ghazouani et al. 2019;El Mghari et al. 2020;Alqahtani et al. 2020;Touili et al. 2020). In addition, these heat batteries could be beneficial to hydrogen and oxygen splitting through thermochemical routes (Ghorbani et al. 2020;Subramaniam and Senthil, 2021). Further, the current pandemic situations, such as renewable energy-based heat batteries, could benefit the health community.
The challenges of solar heat batteries are the automated charging of PCM-receivers at the PDSC site, handling, transportation, insulation, and relevant safety measures. The PCM-receivers must be monitored and provided with the required tracking control for the safe operation against PCM's overheating during charging.

Conclusions
Fabrication of a cascaded receiver with PCMs is investigated to enhance the receiver's heat storage. The designed receiver stores around 5000 kJ in 30 minutes of solar concentration using PDSC at average solar radiation of about 700 W/m 2 . High absorption of cavity surface coating and reflectivity of PDSC resulted from faster PCM charging. While discharging heat, a single hightemperature PCM's exergy destruction is higher than the multiple PCMs. The solar heat battery benefits domestic hot water and cooking by direct contact or with the help of highly conductive metal-based vessels. Thus, more than two PCMs may be preferable to improve the receiver's performance. The PCMs and enhancements are essential to improve commercial deployment of such solar heat batteries to meet small to large-scale energy requirements. The PCM charging is studied, but the discharging is not analyzed in the current study. Further studies are required to design a suitable heat exchange mechanism like finned vessels with thermal conductivity enhanced vessels to augment heat exchange from the cascaded PCM to practical applications.