Sustainable Energy Progress via Integration of Thermal Energy Storage and Other Performance Enhancement Strategies in FPCs: A Synergistic Review

Abstract

Flat plate collectors (FPCs) are the leading solar thermal technology for low-medium range temperature applications. However, their expansion in developing countries is still lacking because of their poor thermal performance. Improving the thermal performance of flat plate collectors (FPCs) is a crucial concern addressed in this review This study comprehensively discussed the performance improvement methods of FPCs, such as design modification, reflectors, working fluid, and energy storage materials, by covering current issues and future recommendations. Design factors such as coating and glass cover thickness, thickness of absorber plate and material, air gap between the glass cover and absorber plate, and riser spacing, along with insulation materials, are examined for their impact on FPC performance. Absorber design changes with selective coatings for improving the heat transmission rate between the working fluid and absorber are critical for enhancing collectors’ thermal output. The nanofluids utilization improved FPC’s thermal performance in terms of energetic and exergetic outcomes in the 20–30% range. Moreover, adding a heat storage unit extends the operating hours and thermal output fluctuations of FPCs. Research suggests that employing turbulators and nanofluids as heat transfer fluids are particularly effective for enhancing heat transfer in FPCs. This comprehensive review serves as a critical tool for evaluating and comparing various heat transfer augmentation techniques, aiding in the selection of the most suitable option.

1. Introduction

Energy is vital for the existence of present and future generations. As per the global energy outlook, a significant amount of energy is used in water/air heating for industries, hospitals, residential buildings, etc. Fossil fuels, i.e., oil, coal, and natural gas, are widely used to meet these global energy demands. High consumption of fossil fuels due to an exponential increase in energy demand is very concerning, as these valuable resources take millions of years to form [1]. Therefore, the concept of energy management/renewable energy utilization is the need of the hour to achieve a sustainable energy future. Moreover, the utilization of ‘renewable energy’ resources has emerged as a top-notch alternative. The renewable energy industry is flourishing, resulting in reduced expenses and the provision of eco-friendly energy sources, affording us the benefit of decreased carbon emissions. Solar energy has the capacity to produce both electricity (via photovoltaic systems) and heat (through solar thermal collectors). Nearly 70% of the total incident solar radiation is transformed into heat, making thermal collectors increasingly popular technologies for sustainable development in applications, such as air/water heating for residential buildings and industrial applications [2]. Solar collectors’ use depends upon various factors, including but not limited to energy demand, desired temperature range, and economic feasibility. Among all solar thermal collectors, flat plate collectors (FPCs) are mostly utilized for low- to medium-temperature (50–100 °C) air/water heating applications in buildings and industries. FPCs are simple in design and construction and have long durability, little maintenance, and economical pricing. The thermal output of the FPCs is a prime variable in the design of economical solar air/water heaters [3].

FPCs suffer from low thermal output issues due to poor heat transmission between the absorber surface and heat transfer fluid. Active and passive approaches can improve the low heat transmission rate. Generally, passive methods increase heat transmission from absorber tubes to HTF. Various researchers have used polymer absorbers, mini/microchannels for fluid flow, various absorber designs, energy storage materials, and nanofluids as working fluids to increase the thermal output of FPCs [4]. The heat transmission rate increased using devices such as twisted tapes, wire coil inserts, vortex generators, and various flow inserts. Different studies utilized reflectors to increase the solar energy flux, which will help improve the thermal output of FPCs [5]. Numerous research studies have used various kinds of heat transfer fluids (mono and hybrid nanofluids), such as aluminum oxides, Al₂O₃, CuO-water, MgO-water, etc., to increase the efficiency of FPCs. Recently, integrating energy storage materials with flat plate collectors has been a primary way to increase their thermal output by storing the heat energy in the late evening hours. It prompted the researchers to propose various performance enhancement methods with FPCs (see Figure 1).

FPCs can be categorized as one/two pass solar air/water heaters depending on the number of flow passes [5]. The recycle pass type air/water heater was also studied with varying airflow percentages in the lower and upper channels [2]. Recycling hot working fluid into incoming air promotes proper air mixing and facilitates convective heat transfer. The FPC is a simplified thermal system featuring a low heat transfer rate, convective heat flow coefficient, low heat conductivity, and rapid heat loss [6]. As we know, natural convection has a lower heat transfer coefficient than forced convection. Air velocity reduces heat transfer resistance, increasing convective heat transfer [7]. Low thermal conductivity and conductive heat transfer reduce convective heat transfer. So, numerous solutions were proposed to solve the demerits of the poor thermal performance of FPCs. Most are expanding the heat transmission surface and generating turbulence in the flowing fluid, using a high heat conductivity material, reducing heat loss, gaining the maximum heat, ensuring proper insulation against heat, and storing the heat for a long time [8]. The study has explored various shapes of the extended surface, including rectangle, conical, inclined, triangular, trapezoidal, and elliptical-formed fins [9]. The augmentation approaches, however, improve heat transfer. A blockage in flow causes a greater pressure decrease. Therefore, greater pumping power is required for forced flow, rendering the system economically unviable [10]. The high-pressure drop can be improved by decreasing the geometrical factors of various roughness. The researchers also examined the morphologies of ribs by altering their positioning on the surface, including staggered, in-line, and regular configurations [11,12]. With increased friction, the convective heat transfer and pressure drop increased due to the presence of the turbulators. The absorber’s surface roughness design minimizes friction losses and maximizes the friction factor and Nusselt number [13]. Different researchers used baffles, varied-shaped obstacles, and a corrugated structure while considering the interaction and flow period with a high-temperature water absorber across the surface. Numerous studies have shown that obstacles and perforated baffles improve the heat transmission rate as related to the plain design [14]. The author replaced standard FPC’s glass-covered riser tubes with a plastic-covered serpentine-shaped collector tube. As a result, the input temperature of the collector was 37.7 °C at 0.75 L/min. The output temperature of the collector ranged from 52 to 61 °C, averaging 58 °C. The collector’s output ranges from 45% to 67%, averaging 58% [15]. Integrating thermal energy storage (TES) increases the output of FPC by increasing the temperature range of the exit working fluid. The phase change materials (PCM) offer the benefit of storing extra heat during charging and releasing heat during late evening/off-sunshine hours [16]. The irreversibility-based exergy analysis of airflow FPC was presented by [17]. As per the results, 84.97% of irreversibility is associated with the absorber plate of the collector. Raising the flow rate to 0.0056 kg/s reduces absorber plate irreversibility to 579.20 W and raises working fluid irreversibility to 158.83 W. The max. exergy output was 33.21% at 0.0056 kg/s and 30 °C. The number of absorber tubes boosts energetic and exergetic efficiency by 34.61 and 92.94%. Suraparaju at al. [18] analyzed the performance of a single-slope solar still using ball marbles (BMSS) and compared it to a conventional solar still (CSS) in Karaikal, India, in October 2020, under various weather conditions. BMSS exhibited higher evaporation rates and productivity due to the sensible heat stored by the ball marbles. On sunny and cloudy days, BMSS improved potable water yield by 21.23% and 22.86%, respectively, with a maximum cumulative productivity of 2950 mL/m².day and 2150 mL/m².day. In economic terms, BMSS had a shorter payback period (5.7 months) and lower cost per liter of potable water (8% less) than CSS. Raja Sekhar et al. [19] explored heat transfer augmentation in a copper tube, simulating a riser tube in a FPC for solar thermal applications. They utilized Al₂O₃/water nanofluids and twisted tapes. The setup included a chiller, collecting and storage tanks, and heating via Nichrome heaters. The study tested various nanoparticle concentrations (0.02%, 0.1%, and 0.5%). Results indicated increased heat transfer and friction coefficients with nanofluids compared to water, particularly a notable 8–12% enhancement in heat transfer coefficients. However, higher particle concentrations led to increased friction coefficients, while increased Reynolds numbers reduced friction coefficients and enhanced Nusselt numbers. Babar et al. [20] explored the impact of PCM-assisted solar drying on green chili using FPC. The experiments were conducted at NIFTEM Kundli, India, with an FPSC solar dryer setup. When PCM was used, the drying chamber experienced a noteworthy temperature rise of 10–14 °C over an extended period compared to experiments without PCM. The maximum temperature reached with PCM was 56.51 °C, employing paraffin as the PCM.

Novelty and Research Methodology of the Study

This review adopts a systematic approach to consolidate current studies investigating the incorporation of phase change materials (PCMs) and other performance augmentation techniques in FPCs, focusing on energy, heat transfer, and economic perspectives. The Scopus database and relevant keywords show a consistent growth in publications related to the use of PCMs in FPCs since 2015. The systematic literature review process is detailed, emphasizing the importance of minimizing bias to generate more reliable data for guiding research in this area. The research methodology adopted for conducting this review study is presented in Figure 2.

Over the past decade, several reviews have examined the thermal performance of FPCs using different performance enhancement methods such as the use of reflectors [21], phase change materials, and nanofluids [22]. Surprisingly, there is no comprehensive analysis of FPCs using PCMs and other performance enhancement techniques regarding the design and development of FPCs for air/water heating are lacking. The current review study provides an overview of recent advancements, practical methods, the need for flat plate collectors, and the challenges associated with these collectors This study also discusses the improvements in the efficiency of FPCs by modifying the absorber design and coatings.

2. Flat Plate Collector: Construction and Working

FPC is a unit that uses solar energy to produce heat output. The heat is used to produce hot air/water in buildings for heating, washing, and bathing. Mostly FPCs are utilized for low-medium temperature heating applications. FPC is an arrangement of an insulated metal box, an absorber plate, and working fluid flow tubes (see Figure 3). Glass or plastic glazing is used to diminish heat transfer losses. In these systems, the working fluid’s (air/water) temperature rises up to 80 °C. It comprises a copper/aluminum made plate. The plate is painted black or artificially formed (porous/corrugated) to improve the collector’s efficiency because the black color absorbs maximum solar radiation. Furthermore, two pipes are placed in the FPC; one is located at the top and the other at the bottom in a horizontal direction, called headers. Many shorter pipes are placed vertically in FPC, known as risers. The working fluid flows via risers and headers to gain heat energy from the absorber surface.

The absorber plate of the collector is designed to absorb heat from the sun and is exposed to direct sunlight. A part of the solar energy that strikes the absorbing plate is turned into heat. As a result, the solar collector becomes hotter. When a fluid is passed through the collector, the fluid’s temperature will increase due to the transfer of heat from the absorbing surface to the working fluid. The residual heat, however, is still lost to the surrounding area. The lower portion is mostly insulated to reduce heat loss. FPC comprises the absorber plate, insulation, glazing cover, tubes, etc. This sub-section deals with the material properties required to alter solar power into heat energy.

• Absorber Plate

The primary role of an absorber is to collect solar radiation, prevent heat loss from the upward and downward sides, and later transfer the remaining heat to the working fluid. The selection of an absorber material depends on many factors, such as its cost and accessibility, corrosion resistance, thermal conductivity, and durability. Copper is a widely used material due to its high thermal conductivity.

• Glazing Materials

In FPCs, glazing refers to the transparent cover placed over the absorber to trap the maximum solar energy while protecting the absorber from outside elements. The glazing is typically made of a transparent material such as glass or plastic sheet, and it is planned to allow the sunlight to pass through while minimizing the amount of heat that escapes from the collector. The glazing serves important function in FPCs, protecting the absorber plate and other parts of the collector from environmental conditions. The glazing material should possess high transmissivity and low absorptivity and reflectivity for the maximum transmission of solar radiation. The most common glazing materials are tempered glass, low-iron glass, and plastic materials such as polycarbonate or acrylic. The selection of glazing material depends on various factors such as specific applications, operating conditions, and cost considerations.

• Riser and Header Tubes

Fins or riser tubes are generally required to move the fluid from the input to the outlet. For the arrangement of a riser tube, factors such as the number, spacing, and thermal conductivity of the bond between the absorber sheet and tube, as well as dimensions of inlet and outflow and intake pressure, should be considered. The following factors affecting the tubing material and its geometry are operating temperature and pressure range, corrosion resistance, thermal expansion coefficient, thermal conductivity, and economic viability. It is essential to consider these factors while selecting the material for the riser and heater of flat plate collectors. It has been found from the published works that with the increase in many tubes, spacing between tubes decreases, which in turn improves the heat removal factor. The frequently used materials for risers and heaters are copper, aluminum, stainless steel, and brass.

• Collector Insulation

Losses due to conduction and convection are common in collectors, and FPCs are more prone. The insulation used should withstand high temperatures, as it should not expand/contract between 30 and 200 °C. It should be fire-resilient and must not attract moisture. The collector output decreases with a rise in the heat conductivity of the insulating material. Because heat conductivity is poor for rock and glass wool, they are the preferred choices for insulation.

• Collector Support

Designers always advise using the same quality of material for collector housing and support to avoid corrosion. Furthermore, a mild steel channel or square pipe is used to hold the weight of the water storage tank and collector.

• Collector Housing

Generally, it protects the insulation and absorber plate and reduces heat losses. Materials such as aluminum, galvanized steel, fiberglass, etc. are used.

2.1. Classification of Flat Plate Collectors

Flat plate collectors are significant for solar air/water heaters. The FPC is the least expensive and easiest to use among different thermal collectors. FPCs are mainly classified based on the working fluids, i.e., air heating and water heating. Figure 4 presents a detailed classification of the FPCs used for different applications.

2.2. Merits, Demerits, and Applications

This sub-section discusses the merits and demerits of the FPCs used for different air/water heating applications. FPCs have several benefits from a manufacturing point of view, such as low cost, longer reliability, and durability. They can also achieve a high performance of up to 75% in the summer season in the presence of high solar radiation. It is the most preferred device for solar thermal applications requiring thermal energy at a temperature lower than 100 °C. FPCs are easy to install and maintenance-free. One of their biggest merits is that there is no need for solar tracking mechanisms. Despite several merits, FPCs have demerits significantly affecting profitability and mechanical feasibility, mostly in cold weather conditions. At low insolation and ambient temperatures, thermal efficiency is low—about 40%. Inadequate thermal insulation can relatively cause heat loss, which, again, is a drawback for FPCs. The heat transfer circulation loop’s high hydraulic resistance is another disadvantage of FPCs [23].

The broader applications of FPCs are presented in Figure 5. In FPCs, solar energy after absorption is transferred to an HTF (water/air/another substance) to meet the daily energy needs of an industrial process. Textile, rubber, paper, petroleum refineries, culinary, hospitals, chemical, and pharmaceutical industries all rely on solar thermal technology for one-third of their energy needs. Solar heat can be used in food for water/air heating, pasteurization, cleaning, cooking, and drying. So, by using FPCs, the consumption of fossil fuels can be reduced and save our environment from global warming.

3. Thermodynamic Analysis of Flat Plate Collector

The FPC’s energy balance under steady-state conditions depicts how well the solar power is being transformed into valuable heat by determining optical and thermal losses. The thermal network and schematic of an FPC with glazing, which includes convection, conduction, and radiation as the main heat transfer phenomenon, are presented in Figure 6.

The heat loss through the collector can be computed with the help of the given equation [24]

$$Q_{loss}=\frac{T_P-T_o}{R}=U_L{A}_c\left(T_P-T_O\right)$$

(1)

U_L represents the total heat transfer coefficient and the sum of the top, bottom, and side loss coefficients. Mathematically, it can be written as

$$U_L=U_t+U_b+U_e$$

(2)

Furthermore, the top loss coefficient can be calculated as [25]

$$U_t=\left\{\frac{\frac{1}{N}}{\frac{C_b}{T_P}\left[\frac{T_P-T_O}{N+f}\right]}0.33+\frac{1}{h_a}\right\}+\left\{\frac{\sigma \left(T_P+T_O\right)+\left(T_P{}^2+T_O{}^2\right)}{{\epsilon }_P+0.5N\left(1-{\epsilon }_P\right)+\frac{2N+f-1}{{\epsilon }_g}-N}\right\}$$

(3)

where, ${\epsilon }_P$—emissivity of the absorber and ${\epsilon }_g$—emissivity of glass, $\beta$—expansion coefficient, C is conduction bond, f—friction factor, and h_a—convective heat coefficient is further calculated by using the below-mentioned relations

$$\left\{\begin{array}{c}C=365.9\times \left(1+0.0001298{\beta }^2-0.00883\beta \right)\\ f=\left(1+0.091N\right)+\left(1-0.0005h_a{}^2+0.04h_a\right)\\ h_a=3.8V+5.7\end{array}\right\}$$

The bottom loss coefficient is computed with the help of the following relation

$$U_b=\frac{k_b}{X_b}$$

(4)

The side loss coefficient will be calculated using the below-given expression

$$U_e=U_b\left(\frac{A_e}{A_c}\right)$$

(5)

After that, the collector fin efficiency can be computed as

$$F=\frac{tanh\left[\frac{m\left(W-D\right)}{2}\right]}{\left[\frac{m\left(W-D\right)}{2}\right]}$$

(6)

where, W—width of the bond, D—diameter of the tube, and m is a module that can be obtained using the below expression

$$m=\sqrt{\frac{U_L}{k\delta }}$$

Mathematically, the efficiency correction factor for FPC can be calculated as [26]

$$F^{\prime }=\frac{\frac{1}{U_L}}{w\left[\frac{1}{U_L\left[D+\left(W-D\right)F\right]}+\frac{1}{C_b}+\frac{1}{\pi D{h}_{fi}}\right]}$$

(7)

Furthermore, the heat removal factor of the FPC can be computed as

$$F_R=\frac{\dot{m}{C}_p}{A_c}\left[1-exp\left(\frac{U_L{F}^{\prime }{A}_c}{\dot{m}{C}_p}\right)\right]$$

(8)

Here, $F^{″}$ is the ratio of F_R and F′, which can be found by the following given formula

$$F^{″}=\frac{\dot{m}{C}_p}{A_c{U}_L{F}^{\prime }}\left[1-exp\left(\frac{U_L{F}^{\prime }{A}_c}{\dot{m}{C}_p}\right)\right]$$

(9)

Furthermore, the absorber temperature can be found by using the equation

$$T_P=T_{fi}+\frac{Q_u}{A_c{F}_R{U}_L}\left(1-F_R\right)$$

(10)

where F_R is the heat removal factor, T_fi—is the initial temperature of flowing fluid. Now, the useful amount of heat gain can be written as

$$Q_u=A_c{F}_R\left[I_t\left(\propto \tau \right)-U_L\left(T_P-T_O\right)\right]$$

(11)

The efficiency of the collector can be calculated as [27]

$${\eta }_c=\frac{Q_u}{I_t{A}_c}$$

(12)

Theoretically, the thermal performance the FPC can be calculated as

$${\eta }_c=F_R\left[\left(\alpha \tau \right)-\frac{U_L\left(T_{fi}-T_O\right)}{I_t}\right]$$

(13)

So, finally, the thermal output of the FPC can be written as

$${\eta }_c=\frac{\dot{m}{C}_p\left(T_P-T_O\right)}{I_t{A}_c}$$

(14)

4. Performance Enhancement Techniques for FPC

Generally, flat plate collectors (FPCs) suffer from low thermal performance due to higher heat losses and insufficient solar energy in non-tropical regions. Previously, various research studies have been completed to enhance the thermal output of FPCs. This section of the current literature review comprehensively discussed the critical findings of several (experimental as well as numerical) studies performed by researchers by altering/adding various factors that affect the thermal output of FPCs.

4.1. Utilization of Energy Storage Materials

FPCs are unable to operate in dark/rainy conditions due to the fluctuating nature of solar energy. Additionally, heat loss from different locations (top, bottom, and side) has the potential to significantly alter a solar collector’s effectiveness. Few studies have used energy storage materials to overcome this issue and increase the thermal performance of FPCs. The excess heat energy available at day time can be stored in two ways: sensible heat storage and latent heat storage. The latent heat storage materials have better energy storage ability and are frequently used in solar thermal applications. Incorporated as latent heat storage (LHS) materials in flat plate solar collectors, phase change materials (PCMs) play a pivotal role. These PCMs have the capacity to absorb or release significant quantities of “latent” heat during state transitions, such as the shift from solid to liquid. During this process, the temperature of the PCM remains constant, ensuring a consistent thermal performance [28].

The ability of the phase change material (PCM) to efficiently capture and release substantial quantities of heat in a regulated manner can be harnessed for energy storage or for maintaining temperature stability within a specific range. This application enhances the thermal effectiveness of end-user products. Figure 7 displays the three major classifications of PCM that may be made based on their sources and properties: organic, inorganic, and eutectic. The selection of PCMs for a flat plate solar collector depends upon the following criteria outlined in Figure 8. By considering the above-outlined selection criteria, researchers can opt for the appropriate PCM for FPC to optimize its performance and efficient energy storage.

Thermophysical, kinetic, and chemical properties, as well as cost and environmental impacts, all play an important role in the selection of PCMs. Paraffin waxes, salt hydrates, and fatty acids are the three most common PCMs, which have been mainly used in solar thermal applications until now. Recently, a few researchers used some other PCMs and their mixtures for thermal energy storage applications in solar collectors. The most popular PCMs and their physical properties are shown in

Table 1. Thermophysical properties of various PCMs used in flat plate solar collectors [29,30].

Type of PCM	Name of PCM	Latent Heat (kJ/kg)	Melting Temperature (°C)	Thermal Conductivity (W/m·K)
Organic PCMs	Paraffin waxes	150–250	0–90	0.20
	Palmitic acid	222	61	0.21
	PEG6000	127.3	60	0.29
	Lauric acid	212	44	0.22
	Acetic acid	192	17	0.26
	Capric acid	139.8	29.60	-
	Stearic acid	244.21	67	0.28
	Myristic acid	228	56	0.22
	Eicosane	247	36.3	-
	Hexadecane	236	18	0.24
Inorganic PCMs	Sodium sulphate	252	32.4	-
	KF·4H2O	231	18.5	-
	Salt hydrate	259	38	1.46
	LiNO3·2H2O	296	30	-
	FeBr3·6H2O	105	21	-
Eutectics PCMs	Capric + Stearic acid	24.7	178.6	-
	Capric + Palmitic	22.1	220	-
	Methyl stearate + Cetyl-stearate	22.3	180	-
	Triethylolethane + urea	29.8	218	-
	Methly-stearate + Cetylpalmitate	28.2	189	-

.

An FPC modified to hold micro-encapsulated PCM under its absorber plate was evaluated by Carmona et al. [31]. The experimental arrangement, along with the simplified model, collected data over a period of 20 days, showcasing two phase change materials (PCMs) with disparate melting points of 45 °C and 60 °C. With a maximum inaccuracy of 4.62%, the model evaluated parameters that were difficult to generate experimentally or expensive to generate. A study by Ronnelid and Dalenback [32] presented a method for enhancing the effectiveness of FPCs by adding inert gas between the absorber and glass instead of simply air, decreasing the heat transfer, humidity, and dust rate. Gases used for this have to fulfill certain requirements, such as sealing off compartments for things such as gases, so that variations in pressure and gas filling volume may be managed. They prepared a mathematical model due to conduction, convection, and radiation and performed the calculations in MATLAB. According to the findings, a thinner collector performs better than a traditional one. Inert gases such as krypton, xenon, and argon proved much better for increasing performance. Vengadesan et al. [33] used serpentine copper tubes to increase surface area and flow time. Airflow inside the collector was increased by baffles. Multifunctional heat storage tubes are attached to the absorber. Solar radiation and water and air flow rates mainly affect the collector’s energy and exergy performances.

The collector with heat storage exhibits a higher peak energy and exergy efficiency of 88.8% and 3.5%, respectively, compared to the system without heat storage, under conditions of 0.025 kg/s water flow and 0.0132 kg/s air mass flow. Heat storage-based collectors have 4.2% lower heat loss coefficients at high flow rates. The hybrid collector has shorter payback periods and better environmental and economic benefits than the standard collector. The test setup of the study is shown in Figure 9. The authors in [34] increased the output of FPC by using an evacuated tube. They conducted a finite element analysis, finding the results using proper sustainable elements, mechanical designs, and long-lasting enclosures. Bellos et al. [35] performed a study of a TES system with solar assistance. The analysis was carried out using a designed mathematical software. It was determined that a temperature of 150 °C is ideal for storage, while a temperature of 75 °C is ideal for the heat pump’s evaporator. In this instance, the organic Rankine cycle output was 18.45%, the heat pump coefficient was 3.704, the electricity recovery ratio was 68.48%, and the collecting area was 150 m². The payback period for the solar field investment in the optimal global case was determined to be 7.8 years. Mukherjee et al. [36] combined an FPC with a thermochemical energy storage system (TCES) and assessed it using a model mainly for space heating in Pune, India. Here, the ambient conditions and the time-varying sun flux (DNI) were utilized for charging and discharging activities. It was discovered that a 16.66% rise in reactive packed bed energy density results in a time-averaged value reduction of 63.97%, during the melting and solidification phases, and 48.06%. The time-averaged value was found to grow by 6.20 and 6.35 times, respectively, during the melting and solidification phases for a reactive packed bed, with a four-fold increase in aspect ratio. Fan et al. [37] introduce a scalable system using fins and MWCNTs to enhance heat transfer and light absorption, improving solar water heating. Their experiments show that this system achieves a water temperature of 79.0 °C under stationary conditions, significantly outperforming other composites. Numerical simulations demonstrate efficient heat transfer and energy storage, maintaining water temperature above 40.0 °C for 51 min, with a remarkable thermal efficiency of 89.2%, surpassing typical solar collectors. Alptekin and Ezan [38] investigated the charging efficiency of a latent heat TES system driven by solar power. The heat transfer phenomena in the latent heat storage system were simulated using an internal 1-D transient mathematical model. The aforementioned literature’s findings further validate the packed bed latent heat TES tank’s mathematical model. A favorable impact on system indicators was also found when the capsule diameter was reduced, and the mass flow rate of the HTF was increased. To prevent collectors from overheating and to deliver power under more favorable temperature circumstances, Wang et al. [39] examined the efficacy of FPC-based SWH combined with PCM. Cascading PCMs with decreasing melting points below PCMs with increasing melting points results in a temperature drop from 19 to 10 °C for a discharge time of 6.4 h. The use of PCM enhanced collector efficiency by 24%. Using fluid with additional zinc oxide nanoparticles as an SHS medium in a storage tank [40] predicted a percentage increase in storage efficiency. Incorporating a nanofluid volume fraction of just 0.1% increased energy storage size by 7.78% and improved daily output by 6.59%. Younes et al. [41] investigated solar still discs of various forms, diameters, reflectors, and thermal energy storage units. As per the results, the productivity of SS was improved by 68%, 86%, and 106% above ordinary stainless steel for flat, corrugated, and finned disc stainless steel (FDSS). Using TESU, the FDSS was 149% more productive than the standard SS. FDSS productivity increased due to external reflectors. FDSS productivity with TESU and reflectors was 184% higher than SS. The cost per liter of freshwater produced by basic SS and FDSS-TESU reflectors is projected to be 0.024 and 0.014 USD, respectively. Dissa et al. [42] conducted research on a solar collector employing hybrid porous composites and non-porous composites as adsorbents. They designed and tested a SAC using a composite adsorbent created by combining a corrugated iron sheet (non-porous) with an aluminum mesh (porous) adsorbent (see Figure 10). The results revealed that the total thermal output reached 61% under different conditions. The study demonstrated that the unsteady mode model can accurately predict collector output.

Chen and Peng [43] numerically examined the impact of a paraffin-filled aluminum foam porous structure as PCM in FPC. Their model included metal foams and paraffin in solid and liquid states, applying a two-temperature model along with Darcy’s law for momentum conservation. The FPC’s sides and bottom were considered adiabatic. Solar energy absorbed during the day was stored in paraffin and transferred to water through tubes at night. The low thermal conductivity of paraffin raised absorber plate temperature, improved heat transfer, paraffin melting rate, and reduced heat loss coefficient through the use of aluminum foams.

Table 2. Recent studies on thermal output improvement methods used in flat plate solar collectors.

Researcher	Setup Location	Working Fluid	Performance Methods	Key Outputs
Absuka et al. [44]	-	Air	Use of PCM as TES	The range of thermal output reported was between 6.05 and 39.99%. And using cherry stone/powder as the PCM allowed the device to yield heat for 5 h after sunset. The collector containing cherry powder improved by 18.7% more than the standard one.
Saedodin et al. [45]	Karaj, Iran	Water	Porous metal foam as energy storage	A porous layer thickness of more than 0.8 provides the best thermal performance. The results revealed that porous medium increased thermal output and Nusselt number by 18.5% and 82%.
Sakhaei and Vaipour et al. [46]	Semnan, Iran	Water	Use of corrugated absorber and PCM	Helically corrugated heat collecting tubes and PCM affect flat plate collector (FPC) thermal performance. Helically corrugated heat collecting tubes reduce FPC heat losses by 39.8%. Discharge hot water in the evening. PCM boosted FPC efficiency from 41.5% to 48.9%.
Badiei et al. [47]	Shiraz, Iran	Water	Use of PCM with fins	The PCM system has lower output temperatures in the morning, thus hot water can be provided for longer in the evening when discharging. Summer PCM collector output increased from 33% to 46%. The use of fins enhanced the storage capacity of PCM. The finned system dissipates more heat into the environment during afternoon discharge, reducing its efficiency.
Sajawal et al. [48]	Taxila, Pakistan	Air	Use of PCM (RT44HC) with fins	Three configurations of a test setup with and without PCM were tested for maximum thermal output. The efficiency analysis shows that the third configuration is better for efficiency. PCM increased its average thermal efficiency by almost 15%.
Al-Kayiem and Lin [49]	Universiti Teknologi PETRONAS (Malaysia)	Water	Paraffin and paraffin with nanoparticle	Flat plate collectors (FPSC) had the maximum efficiency at a 10-degree tilt angle.

depicts the recent study outcomes performed on FPCs with the integration of energy storage materials.

4.2. Modifications in Collector/Absorber Design

Researchers have spent the last few decades examining the performance of various designs and configurations based on solar collectors. Here, the authors discussed the recent design concepts of FPCs. Manjunath et al. [50] utilized an absorber plate with dimple pockets to compare their effects on the surface configuration of FPCs. These two absorber plates were heated, and a computational fluid dynamic (CFD) analysis was conducted with a solar radiation variation from 600 to 1000 W/m² (see Figure 11A). From the results, it was observed that the dimple-based collector shows a 5 °C rise in the average surface temperature than the normal FPC. Furthermore, an improvement of 5.5 °C in the average outlet water temperature was observed. Kundu [51] conducted a comparative study on various absorber profiles, such as rectangular or trapezoidal, with a little modification in their thickness. They extensively studied their performance and optimization, shown in Figure 11B.

The findings showed that a trapezoidal shape was effective for maintaining a constant plate volume. The better performance and ease of manufacture make these profiles superior to others. Pandya and Behura [52] performed a study to assess the impact of both tilt angle and dust particles on the thermal output of FPCs. The study reveals that a rise in tilt angle from 15° to 25° resulted from an increase in the average thermal output from 27% to 30%. Additionally, the growth of dust on the glass surface led to a decrease in thermal output, from 30% to 20%, for a tilt angle of 25%. Kim et al. [53] analyzed the output of both existing FPC-based solar water heaters and a newly proposed one that makes use of a transparent polyurethane tube. Normal water, red, violet, and black colors were used in the new study. As per the results, the thermal output of an FPC-based SWH with black color is increased by 5% more than conventional collectors. To mitigate heat loss from the absorber, Garcia et al. [54] investigated the effectiveness of convective barriers inserted between the glass and the absorber of the FPC. Three convective barriers were chosen as the best options. Depending on the solar collector layout, the optimal number of convective barriers can shift. Utilizing three convective barriers at regular intervals decreased total heat loss by 5.25%. It is suggested to use two covers for an uncoated absorber plate, whereas a single cover is adequate for a coated absorber. Alomar et al. [55] used a v-corrugated absorber plate to improve the performance of an FPC-based SAH. They compared the thermal outputs of the altered and jet-plate-blown corrugated collectors. Model 1 is the modified system, and Model 2 represents the conventional system (see Figure 12).

As per the results, Model 1’s thermal efficiency is 11.5%, 14.5%, 12.3%, and 13.2% higher than Model 2’s at 0.009, 0.018, 0.028, and 0.037 kg/s. Model 1 and Model 2’s optimal thermal performances are 82.3 and 69.1% at 0.037 kg/s, respectively. Model 2 always has a lower output temperature than Model 1. Hellstrom et al. [56] analyzed the behavior of honeycombs and films made of Teflon, checking the glazing for its anti-reflection characteristics, changes in the absorber’s ability to reemit the previously absorbed heat (thermal emittance), and their combination. They investigated the optical characteristics that affected the annual output of FPC in the Swedish environment. The outcomes revealed a combined increase in absorptance to 0.97 and a fall in emittance to 0.05, improving the collector’s performance by 6.7% annually. With the addition of Teflon film at 50 °C, annual performance is enhanced by 5.6%, 12% by Teflon and honeycomb, 6.5% by anti-reflection treatment, and a combination of these gives a rise of 24.6% at 50 °C. Balaji et al. [57] carried out an experiment on absorber tubes using thermal performance enhancers—a copper tube and a copper rod. They used two FPCs, one with an enhancer and one without, and placed them in the sunlight at a slope angle of 13° to ensure the SWHs worked. After a comparison of the design with tubes and the traditional absorber design collector, the thermal enhancer with the rod was found to have better efficiency for every Reynolds number.

Amori and Jabouri [58] fabricated two different FPC-based SWHs and compared their performances. One collector had a conventional absorber, and the other had a new accelerated absorber design. The storage tanks used here were of two types depending on their construction: one had two concentric cylinders, and the other had tubes in a helical shape inside. The results revealed that absorbed heat performs thermally at a 60% level during noon. The instantaneous efficiency for the accelerated absorbed flat plate was 31.5%, and the conventional one was 16.5%. Vengadesan and Senthil [59] investigated the impact of bifunctional rectangular longitudinal flow inserts, which are tilted at an angle of 30° inside the absorber tube, at various mass flow rates. The findings indicate that the highest instantaneous output was 72.93% at 0.025 kg/s. It indicated a 23.6% increase compared to the standard collector output. The modified collector attained its highest instantaneous efficiency at a temperature of 56.7 °C and with the implementation of the suggested flow inserts. Moss et al. [60] compared different absorber types for evacuated FPCs. They identified an absorber that could provide support for the glass cover, is easy to fabricate, and at the same time has excellent efficiency compared to the traditional serpentine tube absorbers. Keeping the above conditions in view, a flooded panel absorber seemed the apt choice, and a hydroforming technique was used to fabricate it from stainless steel. CFD analysis showed that the flow distribution can be optimized by taking the plate at the half and diagonal corners. More than 3% higher efficiency is achieved by using this. Kizildag et al. [61] developed FPC prototypes at a demonstration site. Thermal testing for efficiency, incidence angle modifier, and overheating was carried out before the final installation. The results show that in the winter and spring, a collector with a 40 mm TIM layer can collect energy at rates approximately 2.5 and 1.4 times higher than those of conventional collectors, respectively. Porous insertions were used by Anirudh and Dhinakaran [62] to examine the thermal output improvement in the FPC. These porous blocks were specifically positioned to utilize direct absorption. The sizes of the two porous blocks, specifically their height and length, as well as the absorptivity of the porous medium, are the main factors being taken into account. The findings supported the use of this design for FPC and improved heat transfer. Additionally, the working fluid absorbs more heat due to better thermal contact. Ammar et al. [63] completed a study to improve the output of FPCs. They demonstrated a collector with transparent insulating material (TIM) and conducted a numerical analysis for various slats with varying heights, numbers, materials, tilt angles of the slats, and tilt angles of the collector at various degrees, namely 0°, 30°, 45°, and 90°, and determined their exergy, energy, and thermo-hydraulic efficiency. The outcomes showed that there were minimum thermal losses, due to which thermal efficiency was improved. The 45° tilt angle of the collector will yield the 81% highest thermal performance. Nejlaoui et al. [64] developed and performed the six-sigma robust multi-objective optimization (SSRMO), which takes uncertainty in the design parameters (UDP) into consideration. Three factors—cost, efficiency, and UDP sensitivity—of FPCs are simultaneously taken into account by SSRMO. According to the findings, efficiency and total cost were sensitive by 27% and 31%, respectively. The SSRMO is less sensitive to UDP but offers the same performance.

Channa et al. [65] introduced a substitute that may be utilized in place of accessible absorber material and has low-cost negligible emission. Pebbles are the opted material due to their higher capacity to absorb heat and are widely utilized in solar energy storage systems. They performed various trials with coated and uncoated pebbles and reported their impact on the performances. The results showed that coated pebble absorbers could be a less expensive, less-polluting alternative to conventional, expensive, polluting FPC absorbers. Kansara et al. [66] focused on the cost-effectiveness and feasibility of air-based working fluid Linear Fresnel Reflector (LFR) plants, with ambient air serving as the HTF. They examined a unique FPSC incorporating fins and porous media. Following the CFD analysis, experiments were performed using a solar simulator. CFD analysis was used to provide predictions about how well a flat-plate collector would function when loaded with a porous medium. The investigation shows that porous media transmits heat more effectively than fins and other designs. A collector with porous metal performed better than an empty one by 16.17% for the rise in air temperature. Yehualashet et al. [67] experimented and used a numerical method to examine the newly designed corrugated plate solar surface to check its thermal contact. As per the results, the outlet temperature increased and decreased the ∆T between the plate and fluid via increasing bond conductance due to sinusoidal corrugation. The experimental and numerical results were reported to improve collector efficiency. The outcomes from the experiments and numerical investigation were in good agreement regarding the collector outlet temperature and thermal output.

Table 3. Summary of recent studies on design modifications of flat plate solar collectors.

Researcher	Setup Location	Working Fluid	Performance Methods	Key Outputs
Fan et al. [68]	-	Water	V-corrugated absorber with mini channels	The new collector has an optical/thermal efficiency of around 84.9%/69.4% daily, while the old one only manages 69.1% and 58.6%. The average exergy efficiencies of the novel and traditional collectors are 3.8% and 3.3% at 10–90 g/s. The V-corrugated design absorber improves collector optical and thermal efficiency, reduces pressure drop and power consumption of the pump, and has a negligible effect on exergy output.
Sivakumar et al. [69]	Coimbatore, India	Air	CuO nanoparticle coated surface for absorption	A solar absorber plate of black-painted aluminum and a modified plate of aluminum with a black paint covering with 2 vol.% of CuO to improve the rate of heat transfer. An absorber painted with black paint containing 0.04 vol% of CuO nanoparticles increased collection efficiency by 4%. Also, drying time was cut by 6% in comparison to the conventional dryer. The black paint with a nanoparticle-coated collector had a maximum collector temperature.
Nazari et al. [70]	Urmia, Iran	Water	Coated of CuO on absorber plate	The performance of FPC-SWH with CuO NSs-coated absorber plates was superior to black and plain copper absorber plates. Nanoparticle-coated SWHs were 18.8% and 35.8% more efficient than those with black and flat absorber panels. Nanostructure coating on the absorber boosted solar radiation absorption and outlet temperature, improving collector exergy efficiency by 11.2% more than matte black paint. SWHs with CuO coating decreased their PBP by 0.6 years and saved money and electricity.
Selikhov et al. [71]	-	Air	Polymer material coating	Polymer collector efficiency is 95%, more than conventional collectors with metallic absorption components.
Alkhafaji et al. [72]	Baghdad—Iraq	Water	Addition of fins	(Model B) has a 16.7% higher water temperature inside the tank and a 16.2% higher operating fluid temperature at the outflow than the standard model. The improved collector (model B) has 20.8% higher thermal efficiency than the previous model. This work’s transient model (model B) improves passive systems’ FPSC behavior.
Sharma et al. [73]	India	Water	Circular & trapezoidal corrugated absorber plate	For circular and trapezoidal corrugated surfaces, the thermal output improves by 8.74% and 12.85%. The exergy output improves by 16.88, 23.31%, and the coefficient of heat loss reduces by 7.49 and 8.72%.

shows the outcomes of recent studies conducted to increase the thermal output of FPSCs using different design modifications in absorber plates.

4.3. Use of Reflectors in FPCs

Recently, various researchers used different types of reflectors with flat plate collectors to effectively utilize solar radiation in many applications. Reflectors are optical components that use low-cost and suitable technology to focus light and increase the amount of solar energy. Generally, flat plate, linear Fresnel, CPC, and V-groove secondary reflectors, as well as other reflector designs, have been used to improve the FPC energy input. As per the literature, we know that the ∆T between the collector’s input and outlet kg/h directly correlates with the collector’s efficiency. Keeping this in mind, a reflector is introduced in the collector to focus global solar radiation, eventually increasing ∆T between the inlet and outlet water/air flow. Bollentin and Wilk formed a model and utilized it to calculate how much solar energy came into contact with the FPC in the reflector. It was reported that the use of reflectors caused a high solar radiation density flux [74]. In this study, reflectors in front of the FPC that increase solar radiation were used by the researchers. An increase of 10% was recorded for the yearly irradiation compared to the conventional collector [75].

El-Assal et al. [76] studied how side reflectors affect FPC effectiveness. For the fruitful tilt angles for the reflector and collector, they used TRYNSYS software. The angle of the left-side reflector is 38° in winter and 68° in summer; according to the outcomes of studies and computer models, the right-side reflector angle is 43° in summer and 74.5° in winter. The output of the FPC improved from 46 to 53%. Rajashekaraiah et al. [77] conducted an experiment on an FPC integrated with a trapezoidal reflector. A trapezoidal reflector enhances the output directly and indirectly as it can change its orientation. Thermal enhancement (TE) increased by 13%. Singhal et al. [78] used two reflectors made out of two materials, one from aluminum foil and the other from glass, and studied them to determine their impact on the thermal enhancement of the FPC. They modified the already-present conventional collector, replaced the flat glass cover with a trapezoidal one, and placed aluminum foil reflectors on the sides of the trapezoidal glass. This new modification increased the thermal energy by 29%. Rachedi et al. [79] used a cost-effective reflector to improve the flat plate thermal collector’s output of thermal energy. This was accomplished by tilting four reflectors near a fixed and bi-axially tracked thermal source on the flat plate. The results demonstrated that the completely tracked cases’ innovative tilt angles of the reflectors, which were set at 67° at any time of year, were effective. Contrarily, they changed from month to month in the fixed situation—in both the two-axis solar tracking system and the fixed system without shadowing, they changed by 111.28%. Additionally, it was shown that the daily average benefit ratios of the thermal energy generated by the two-axis system and the incoming solar energy were equivalent to 4.25 and 2.48, respectively. The improvements were 1.52 and 1.33 times greater compared to the fixed case, respectively. Bhowmik et al. [80] made use of a rectangular reflector in their paper to enhance the thermal performance (see Figure 13). The efficiency obtained from it is similar to that obtained without the reflector. The reflector is made of either glass or mercury, two in number, and installed on the collector’s sides.

The reflector was permitted to adjust its angle during the day. The reflector is kept in the best possible position so that maximum radiation can be achieved throughout the day. The left reflector was set to a 30° angle when the sun was 30° south of the collector, and the right reflector to a 60° angle. The maximum amount of reflected light can hit the collector at this tilt angle, increasing the solar collector’s thermal output. Ramesh et al. [81] used a reflector and coated absorber surface to analyze their effects on thermal efficiency. This work employed copper solar absorber panels with nickel-cobalt and black-chrome coatings (BC and Ni-Co, respectively). The striking of solar radiation on the absorber was enhanced by using the reflectors as well. The test data from the setup are collected using the “Response Surface Methodology” and a “Box Behnken design”. The designed experiment’s collected data are examined using an analysis of variance (ANOVA). The thermal output of black-chrome solar FPC is improved by 89.3% as a result of the coating on the absorber and reflectors compared to the Ni-Co panel.

Baccoli et al. [82] created an optimization model for an integrated system comprising a flat solar collector and a flat bottom reflector. Finding the right reflector design to achieve the anticipated maximum system efficiency requires solving such systems’ simulation and optimization problems. The level of shadow and radiation on the collector is described with a thorough formulation that can accommodate finite-length geometry with changeable dimensions. A novel optimization approach that takes into consideration both energetic and economic factors is implemented into the model. The study’s findings are based on solar data from Italy, but they may be simply extrapolated to any place in the world. Murugan et al. [83] made use of corrugated booster reflectors (CBR) to increase the output of centrally finned twist (CFT)-inserted solar collectors and explored them experimentally. The outcomes were compared to both the basic solar collector (SC) with CBR and the flat booster reflector (FBR)-equipped plain tube SC. The standard formulae have been used to validate the test results of the basic CBR-SC and have shown that the disparity of friction factor and Nusselt number is 10.32% and 9.14%, respectively. The CBR has an effective reflector area that is 1.6% larger than the FBR. Because of this, the CBR increased the Nusselt number by about 8.25% over the FBR. The CFT with the lowest twist ratio showed 11.08% more thermal efficiency than the plain tube CBR-SC. The anticipated value’s differences from the test’s value are within 11.02% for the Nu and 10.88% for the f (friction factor). Momeni et al. [84] used parabolic-type reflectors instead of flat-on Fresnel concentrators. This new system’s local concentration ratio distribution has been identified using the ray-trace statistical approach. Compared to a linear Fresnel reflector (LFR), the outcomes have verified that using parabolic reflectors with low curvature significantly affects the absorber pipe diameter. A parabolic system’s absorber tube diameter is smaller than a linear system’s absorber tube diameter, which results in a low rate of heat loss from the pipe surface, according to a performance comparison of both reflectors with the same parameters, i.e., reflector width, absorber pipe height, and reflector number. When comparing both reflectors with the same radiation and absorber pipe data, it became clear that the parabolic reflector could use fewer, wider reflectors, resulting in a simpler control system.

Table 4. Outcomes of recently published studies performed on FPCs using different reflectors.

Researcher	Setup Location	Working Fluid	Performance Improvement Methods	Key Outputs
Ramesh et al. [81]	Tamilnadu, India	Water	Reflector with absorber coating	The authors used both a black chrome and a nickel-cobalt coating for a copper solar absorber panel with a reflector to increase the amount of sunlight striking the absorber. The black–chrome coated collector’s thermal efficiency is 89.3% higher than the Ni-Co panel’s due to a coating on the absorber and the use of reflectors.
Nilolic and Lukic [85]	Kragujevac, Serbia	Working fluid (water/air)	Flat plate reflective surface	Test and theoretical results suggest that at double-exposure, FPSC can outperform a conventional one. The relative thermal power differential of these collectors is 41.79–66.44%.
Pandya and Behura [52]	Jaipur, India	Water	V-trough for reflection	It has been found that the average thermal output of SWH improved from 27% to 30% when the tilt angle went from 15° to 25°. However, when the tilt angle stayed at 25°, dust deposited on the glass decreased the average thermal output of SWH from 30% to 20%.
Rachedi et al. [86]	Southern Algeria	Water	V-trough concentrators	The inclination angle of reflectors changed from one month to the next. Top reflectors’ tilts peak in June and fall to their lowest in December. Daily received energy on collector surface with reflectors is increased by 28.05% than the standard collector.
Housseyn Karoua [87]	-	Air	Linear Fresnel reflector	The outcomes reveal that the air outlet temperature can increase to approximately 150 °C. The developed system works effectively and does not require additional space compared to the FPC.

presents the summarized outcomes of recent studies conducted to increase the thermal outputs of FPCs via the use of various types of reflectors.

4.4. Impact of Nanofluids as the Working Fluid

Nanotechnology is the prime solution for heat transfer applications. The nanofluids are effective HTF when used in thermal collectors. Nanofluid utilization in FPCs will be the most popular way to improve their thermal performance. Nanofluids are a mixture of highly stabilized nanoparticles (1–100 nm) and base fluid. The viscosity of nanofluids mainly affects their heat transfer mechanism. It is the function of the nanoparticle’s shape and size. So, there are three methods mainly used for the production of nanoparticles. Shape and size are important parameters. Nanofluids are prepared by two methods: one is the direct mixing of nano-powders in base fluids. Another method uses powder nanoparticles that have been produced for dispersion in two processes, similar to the previous method. Manufacturers would pick the second approach since nanoparticles are produced on a huge scale [88]. The salient characteristics of nanofluids are shown in Figure 14, which are required for the maximum utilization of nanofluids in solar thermal applications.

At present, various types of nanofluids are available on the market and are used in various industries for solar collectors, space crafts, refrigerating rooms, etc. (See Figure 15).

The viscosity is significantly influenced by factors associated with temperature conditions. For the selection of a specific nanofluid, particle size and temperature must be given the most consideration [89]. Various studies demonstrated the usability and significance of mono and hybrid nanofluids in solar energy collecting systems employing FPCs by increasing the rate of heat transmission and the effect of different parameters on the thermal output of collectors.

Various researchers have used different nanofluids to increase the thermal output of FPCs. Verma et al. [90] explored performance enhancements in FPCs using water/MgO nanofluid (see Figure 16). They performed experiments employing nanofluids with different concentrations and flow rates. The analysis relied on the first and second laws of energy balance. The results indicated a 9.34% increase in thermal output with a 0.75% particle concentration and a 32.23% improvement in exergy efficiency. The Bejan number approached unity (0.97). These improvements surpassed the reduction in pumping power losses, indicating the nanofluid’s positive impact on collector performance.

Choudhary et al. [91] found that employing magnesium oxide/ethylene glycol-DI water with a special kind of surfactant, etyltrimethyl ammonium bromide, boosted efficiency by 16.7%. With a 0.2% vol. percentage, the thermal output of the collector peaked at 69.1%. When the vol. fraction increased above 0.2%, stability was compromised. According to theoretical and experimental findings compiled by Mondragon et al. [92], deposition on the inner side of the tube is caused by a lower concentration ratio of nanoparticles, resulting in a fall in efficiency to 41.5%. Nanoparticle deposition on the tube’s inner side wall is bad news for FPC-based SWH performance. The heat transfer was not satisfactory when the nanofluid viscosity was compared to conventional fluids. Specific heat capacity and conductivity are examples of qualities that play a major role. Alim et al. [93] worked on various parameters such as pressure drop, exergy destruction, and rate of entropy generation of flat plate collectors. They used various nanoparticles, such as Al₂O₃, CuO, SiO_2, and TiO₂, and diffused them into liquid. As a result, heat transfer is improved by 22.15%, while entropy formation is reduced by 4.34% when using CuO. They concluded that CuO-based nanofluids could enhance heat carrier features without the additional cost. The same author utilized Cu-H₂O nanofluid as the working fluid. They conducted tests with particle diameters of 25 nm and 50 nm between the hours of 9:00 and 16:00, with 14.0 l/h as the mass flow rate. The obtained thermal output increased up to 23.83%. The efficiency was decreased with an increase in particle size. The heat gained and temperature by water, respectively, increased by 24.52% and 12.24%. Nasrin et al. [94] used various nanofluids for their analysis. They used four nanofluids: Ag-H₂O, Cu-H₂O, CuO-H₂O, and Al₂O₃-H₂O. For the density of nanofluid, they used the Boussinesq model. They were evaluated based on how they behaved to temperature and velocity distributions, heat transfer coefficients, and mean speeds. Ag nanoparticles showed the highest rate of heat transfer. Yousefi et al. [95] conducted a study on MWCNT-H₂O and showed that increasing some amount of this nanofluid increases efficiency. Hordy et al. [96] performed a study on nanofluid optical characteristics and stability. Different nanofluids, such as water, propylene glycol, ethylene glycol, and Therminol VP-1, were made from the base fluid. Glycol-based nanofluids can remain for a long time, while water-based nanofluids show a gradual change. On the other hand, Therminol VP-1-based nanofluid shows a high rate of deterioration. The ability of these MWCNT-based nanofluids to absorb solar energy throughout a broad spectrum, which can result in around 100% of solar radiation absorption even at low concentrations, was demonstrated in an optical investigation. Goudarzi et al. [97] observed the effects on efficiency by varying the PH values of nanofluids (CuO-H₂O and Al₂O₃-H₂O). This consisted of a two-step process involving mixing nanoparticles in the DI water and then using ultrasonic vibrations to form a homogenous mixture. The results revealed that an increase in the difference in PH values between nanofluids increases the output of the collector. (Shende and Sundara 2015) studied nanofluids and their implications in a direct absorption solar collector (DASC). Highly conductive materials were used to form the nanofluids, particularly the nanostructures of carbon such as CNTs and graphene. Base fluids of de-ionized water and ethylene glycol were used. Due to their black color, fluids can absorb radiation from the far infrared to the far ultraviolet. The heat conductivity of fluids was also increased due to a rise in temperature and concentration. Bamisile et al. [98] made use of nanofluids as a better replacement in thermal systems. This research presents a comprehensive parametric comparison of five mono/hybrid nanofluid applications in solar thermal collectors with those of other fluids. Four regular fluids (water, salt (7NaNO₃·40NaNO₂·53KNO₂), Therminol VP1, and Dowtherm (Q) are compared to five nanofluids with various nano-additives, including Al₂O₃, SiO₂, CuO, Al₂O₃-Fe, and Al₂O₃-ZnO. The nanofluids performed better when used in FPCs and parabolic trough collectors. The useful energy output and thermal efficiency of the HTFs with higher temperature output were lower. Therefore, some specific temperature applications are better suited to base (normal) fluids in flat plate and parabolic trough collectors.

Xu et al. [99] analyzed the effects of nanofluids on the thermal output of the FPC. The results are unclear and occasionally inconsistent. Consequently, this study develops a simple method of forecasting the thermal output of FPCs based on nanofluids. Machine learning models are further employed to establish a connection between the converted thermal output and the stored energy, energy loss, decreased temperature, tilt angle, and nanoparticle size. The outcomes show that the LS-SVR (least-squares support vector regression) is more accurate than other correlations in calculating the FPSC’s thermal output. Chavez et al. [100] used nanofluids in thermal applications, including solar energy production, heat exchangers, and the automotive industry. The use of this technology to improve solar collector heat transfer is one of its potential uses. However, there are significant encounters with the use of nanofluids in thermal collectors. The nanofluids application for large solar collector fields is demonstrated by stability, environmental considerations, and the necessity to create acceptable large-scale production methods. They examined nanofluids in thermal collectors, namely parabolic-trough, at temperatures ranging from 100 to 300 °C in their review of the literature. Sundar et al. [101] evaluated the collector output, heat transmission, and environmental benefits of (water-nano diamond) nanofluid that was permitted to move across the FPC. The stability and thermophysical characteristics were also investigated at particle concentrations of 0.2%, 0.4%, 0.6%, 0.8%, and 1.0%. Thermal conductivity and viscosity are improved by 22.86% and 79.16%, when related to water values at 60 °C. The efficiency of water and 1.0 vol % nanofluids in the collector has been improved by 39.62% to 53% and 74%, respectively. The collection area was decreased to 28.66% by utilizing 1.0 vol. % of nanofluid in contrast to water. The nano fluid’s 1.0 vol. % has a lower embodied energy of 1039.51 MJ than the water-filled collector, which has an embodied energy of 1451.4 MJ. A total of 249.98 kg of CO₂ emissions were measured with 1.0 vol. % of nanofluid in the FPC. A current study by Ajeena et al. [102] presents a thorough summary and examination of recent developments in solar technology, specifically in FPCs using nanofluids. Nanofluids are innovative working fluids that are more efficient than other fluids. The nanofluids are utilized in FPC as heat transfer mediums because of their ability to improve collectors’ thermal performance, and as a result, effective applications can be made for both domestic and commercial use. Mixing the surfactant to nanofluid, proper pH selection, and efficient particle size are all factors that improve collector efficiency. Saffarian et al. [103] focused on the addition of nanofluid and its flow direction in FPC is altered to see the effect on the heat transfer coefficient (HTC). U-shaped, wavy, and spiral pipes with similar pipe lengths are replicated on FPC to achieve it. Nanofluids of Al₂O₃/water and CuO/water are employed in vol. fractions of 1% and 4%, respectively. The outcomes revealed that employing wavy and spiral pipes can greatly raise the Nusselt number and HTC. Additionally noted is that wavy pipes have the greatest pressure drop values; whenever nanofluid is used in place of water, the HTC rises. Mixing nanoparticles to water significantly increased heat conductivity, resulting in a fall in the Nusselt number in all instances save for the CuO 4% case. According to the results, the heat transfer coefficient can rise to 78.25% when wavy pipes and a 4% volume fraction of CuO/water nanofluid are used. Akram et al. [104] tested clove-treated graphene nanoplatelet nanofluids on FPSCs. A zeta potential test showed graphene nanoplatelets–water nanofluid stability for 45 days. The outcomes revealed that the solar collector thermal output of the collector increases with mass concentration and flow rates and falls with decreasing temperature. The solar collector’s best thermal performance is 78% at 0.1 mass% and flow rate 0.0260 kg s⁻¹ m⁻², 18.2% greater than water. Elshazly et al. [105] investigated nanofluids, including MWCNT, Al₂O₃, and the hybrid MWCNT/Al₂O₃ (50:50%), and showed that 0.05 wt% MWCNT resulted in the max. efficiency, with a flow rate of 3.5 L/s, which is around 20% higher than Al₂O₃ under similar conditions; however, it was shown that using hybrid MWCNT/Al₂O₃ offers an improvement in thermal output by 26%, 29%, and 18% for 1.5, 2.5, and 3.3 L/m, and acclaiming the replacement of 50% of the MWCNTs by more economical and eco-friendly Al₂O₃.

Table 5. Summary of recent studies outcomes performed on flat plate solar collectors by using various working fluids.

Researcher	Setup Location	Working Fluid	Performance Methods	Key Outputs
Noghrehabadi et al. [106]	Ahvaz, Iran	Water + nanoparticles	Use of nanofluid as HTF	A square FPC was tested with a 1% SiO2/water nanofluid without surfactants. The nanofluid improved thermal output and temperature performance over pure water.
Saied et al. [107]	-	Water + nanoparticles	Use of hybrid nanofluid (MWCNT+Fe3O4/Water)	A large increase in HTC (26.3%) with a small reduction in pressure drop due (18.9%) due to friction factor has been achieved. Further, the performance parameters were compared using machine learning algorithms and the results are in good agreement.
Mirzaei et al. [108]	Rafsanjan, Iran	Nanofluid + water	Al2O3 as heat transfer fluid	The nano additives of average particle size of 20 nm were used at 0.1% vol. fraction The optimal fluid flow rate was determined to be 2 L/min. The highest efficiency improvement recorded was 23.6% Additionally, the water storage temperature in tank increased by 8.4% more than typical systems.
Verma et al. [109]	Mathura India	Water + hybrid nanoparticles	Use of hybrid nanofluids	Energy efficiency was improved by 18.05% using the hybrid CuO/MWCNTs system, and 4.77% using the CuO/water system. The thermal output increased by 20.52% more than water and 16.28% compared to MgO/water hybrids.
Ashour et al. [110]	Egypt	Water	Use of nanofluids (ZnO and CuO)	A 3-D CFD model predicts outlet and absorber surface temperatures at 1-h intervals throughout the day and their impact on the output of the collector. The best results come from using H2O-CuO nanofluid, which has an average efficiency of 81.64% at a flow rate of 0.0125 kg/s and a volume fraction that is 0.15% higher than water fluid, which is 60.21%.
Tong et al. [111]	Gwangju, Korea	Water + nanoparticles	Use of nanofluid as HTF	Using Al2O3 nanofluid in the FPSC, the efficiency was 21.9% greater than water. Working fluid (water) generated the maximum entropy, while 1.0 vol%-Al2O3 nanofluid generated the least. Using 1.0 vol%-Al2O3 and 0.5 vol%-CuO nanofluids instead of water increased the collector exergy output by 56.9% and 49.6% to water.
Akram et al. [112]	Kuala Lumpur, Malaysia	Water + nanoparticles	Use of different nanofluids	Carbon and metal oxide nanofluids are tested for thermal performance at different heat flux intensities, flow rates, and wt% of (0.025–0.2%). The improvement in the thermal output of the FPC produced at a flow rate of 1.6 kg/min of different nanofluids was 17.45% > 13.05% > 12.36% compared to water.
Mausam et al. [113]	Mathura, India	Cu+MWCNT+DI water	Use of hybrid nanofluids	GRA analysis was utilized to validate and optimize the thermal output of the collector. The thermal output of SHEs improves by 60.05% at 1.5 lpm than 0.5 lpm. The annual reduction in greenhouse gas emissions was 21.42 kg.

presents the findings of recent studies conducted on FPCs using different mono and hybrid nanofluids.

4.5. Use of Other Methods

Numerous researchers have used self-cleaning and jet impingement cooling methods to increase the thermal output of FPCs. The accumulation of dust presents a significant challenge for flat plate collectors, particularly in regions such as MENA with high dust intensity. Various factors such as humidity, adhesion force, rainfall, panel glass type, wind speed, and gravity have a notable impact on the amount of dust accumulated on solar panels [114]. Various cleaning methods are employed to mitigate dust accumulation and ice formation on PV module surfaces. These include manual and automatic cleaning, as well as passive surface treatments. Passive treatments notably decrease dust buildup on glass surfaces, enhancing power generation efficiency. Self-cleaning coatings with unique textures can even enhance light capture. Selecting suitable cleaning methods involves technical and economic considerations. While manual and mechanical cleaning in cold weather can damage solar cell cover glass, transitioning the glass surface from hydrophilic to superhydrophobic passively weakens dust adhesion and ice formation. This holds significance in promoting the PV industry’s high-quality growth towards carbon neutrality [115].

Various studies have been conducted on jet impingement cooling and glass self-cleaning methods to improve the thermal performance of FPCs. Zhao et al. [116] developed a superhydrophobic coating by mixing alcohol and silica sol with ammonia as a catalyst sprayed onto a 220 °C glass surface. After annealing to remove alcohol nanoparticles, a porous coating was found. To achieve super hydrophobicity, PDMS was used. Adding NaCl and high-temperature calcination enhanced the coating’s mechanical strength. Surface modification increased the water contact angle (WCA) to 162.2°, while maintaining a high light transmittance of 86%. Deng et al. [117] utilized candle soot as a template. They applied chemical vapor deposition to envelop the carbon particles with a nano-silicon oxide shell, then subjected the material to high-temperature annealing. This process resulted in a textured surface with nanopores. Following chemical modification, the material attained superhydrophobic properties, ensuring both high light transmittance and long-lasting durability. Ishak et al. [118] introduced a reversed circular flow jet impingement (RCFJI) approach to enhance bifacial photovoltaic thermal (PVT) collector performance. Indoor experiments using a solar simulator assessed the energy, exergy, and economic outputs of the RCFJI bifacial PVT collector. The highest photovoltaic efficiency was 11.38% at 500 W/m² solar irradiance, and the highest thermal output was 61.4% at 900 W/m², both at a 0.14 kg/s mass flow rate. Lower solar irradiance is economically preferable, while higher irradiance benefits energy and exergy performance. The optimal flow rate was determined as 0.06 kg/s.

5. Discussion and Summary of Results

Generally, the ASHRAE and ANSI standards were used by different authors to test the thermal output of FPCs under varying conditions. The outcomes drawn from various performance enhancement methods used in FPCs are discussed in this section and compared with each other. The primary method for increasing the output of FPCs is optimizing the design and shape at minimum cost. Changes can be made in the design of the absorber surface and tubes of the collector to increase the thermal output of collectors. Alami and Aokal [119] used a mechanical position evaluation to determine if a grapheme-covered FPC absorber plate improved heat absorption performance. Ammonia was used to apply the copper oxide coating at first, followed by three applications of the graphene coating. With a thermal efficiency of 69.4%, the copper absorber’s surface coating made it more effective than the standard collector. The coating absorber of the FPC-based SWH with CuO nanostructures enhanced their performance in comparison to black and plain copper absorber plate on solar water heaters. Nanoparticle-coated water heaters outperformed their black and flat absorber plate counterparts by 18.8% and 35.8%, respectively. Based on the exergy results, the absorber with the nanostructures coating was shown to be 11.2% more efficient in converting solar radiation into usable heat than the absorber coated in matte black. Compared to conventional SWHs with a standard cuprous absorber, those with nanostructured coatings saved more money and electricity while decreasing the payback period by 0.6 years. El-Mahallawi et al. [120] used nano-graphite dispersion on a black painting for solar pool heating. In total, 1.5–2.5% nano-graphite particles improve optical absorptance and circulating water temperature in a polymer-based black covering. Sarasar et al. [121] studied the outcome of vortex-type generator inserts (VGI) and nanofluid (TiO₂/water) on the thermal output and rate of heat transfer of an FPC. The heat dissipation rate was reduced by 28.26% at a flow rate of 3 L/min. The use of nanofluid and VGI were combined to find the best volume flow rate for the least heat dissipation and max. optical efficiency. The optimum vol. flow rate boosted effectiveness by 14.5% and lowered heat dissipation by 42.46%.

So, it can be recommended that using nanomaterials in working fluids instead of standard fluids to obtain increased thermal output is one of the latest approaches to improving the output of FPCs. The heat transfer improvement methods that involve the use of nanofluids utilizing various turbulators and integrating TES are the most expensive. Additionally, these approaches result in a greater pressure drop, requiring greater pumping power. As a result, the investigators concluded that surface treatment of the absorber is an efficient way to boost the thermal output of collectors. As per the results of a study using vortex corrugated absorbers and nanofluids, the daily average thermal output of novel and basic collectors was 84.9%/69.4% and 69.1%/58.6%. V-corrugated absorbers enhance collector optical, thermal, pressure drop, and pump power use but slightly affect exergy efficiency. The V-corrugated absorber’s performance enhancement potential is considerable, and the collector’s design and production can be improved. Additionally, FPCs that use PCMs have become increasingly common. The use of PCM, which controls the temperature in the vicinity of the melting point, ensures that there will be a longer time for the delivery of hot water. In addition, it speeds up both the melting and freezing processes. The thermal output of such a system can be increased. The FPC systems with PCM have better thermal efficiency. PCMs extend the collector’s irradiation advantage. It can increase by 3.5 h daily. It regulates the water outflow temperature up to 10 °C higher than without PCM. PCM can boost thermal efficiency by 40%. Most investigations used paraffin. Most studies place PCM beneath HTF tubes at a 2–5 cm thickness for optimal operation. Many recent studies discuss their production, thermophysical characteristics, and ability to improve heat transport. The installation of fins will boost the heat exchange area and sometimes incorporate metallic particles into the PCM. The results show a considerable increase in average thermal performance due to the use of coated absorber plates/tubes, reflectors, nanofluids, and PCMs. By employing glass cooling via jet impingement and self-cleaning techniques, the thermal output of FPCs can be significantly improved, leading to higher energy efficiency and increased longevity. From the results, it was observed that injecting a smooth liquid into a superhydrophobic surface yields a liquid-infused porous surface with exceptional anti-icing capabilities. This prevents droplets from infiltrating and forming condensation within micro-nanostructures. However, it is important to acknowledge that ensuring mechanical stability remains a substantial challenge for both superhydrophobic and liquid-infused porous coatings. Constructing an appropriate rough and porous structure is essential to addressing this challenge.

6. Current Challenges Associated with FPCs

In this section, the challenges associated with performance improvement methods applied in FPCs are discussed comprehensively. To the best of the author’s knowledge, as of September 2022, there were several challenges associated with flat plate solar collectors (FPCs). These challenges may still be relevant, but it is essential to note that the solar energy field evolves rapidly. Here are some key challenges:

• Temperature fluctuations: FPCs can experience overheating in high solar radiation and freezing in cold climates. Managing temperature fluctuations without complex thermal management systems is a challenge.
• Dust and soiling: The accumulation of dust, dirt, or bird droppings on the collector’s surface reduces its efficiency. Cleaning can be labor-intensive, particularly in remote installations.
• Space requirements: FPCs require a significant amount of space, making them less suitable for densely populated urban areas with limited rooftop or ground space.
• Cost: The initial cost of purchasing and installing FPCs can be high, deterring some potential users. Reducing the cost to make solar energy more accessible is an ongoing challenge.
• Durability and longevity: FPC components are exposed to weather conditions and UV radiation, potentially leading to degradation over time. Ensuring long-term durability and reliability is a challenge.
• Heat loss: Heat losses from the glazing and frame of the collector. Improving insulation and reducing thermal losses is an ongoing challenge.
• Environmental impact: The environmental impact of producing FPC components, particularly the glazing materials, should be considered. Reducing the environmental footprint of manufacturing is a challenge.

Even though collector shape, riser tube, and absorber significantly impact the functions of collectors, more research could be conducted to find cheaper ways to make highly efficient solar collectors. Although selective coatings on absorbers and glass coverings have been studied, more research on the “life cycle assessment” of improved absorbers and coating materials is needed. Researchers are familiar with the working of FPCs with different reflectors. However, to find the optimum solution, the effectiveness of different ways to install reflectors must be compared and analyzed in terms of cost, complexity, and operation. The problem of how to use solar trackers with FPCs has not been solved yet because the economics have not been looked at in-depth, and there are not many estimates of how much energy trackers use over time. In addition, the greater viscosity of hybrid nanofluid is a main issue that must be resolved if the thermal output is to be enhanced. At the moment, it is not cost-effective to use nanofluids on a large scale in a commercial setting. So, more research needs to be carried out on hybrid nanofluids. Most efforts are focused on enhancing heat transport phenomena in PCMs. The use of highly-conductive nanoparticles has improved performance and is shaping future research. Superhydrophobic coatings face limitations due to their unique surface structures and demanding application criteria. These challenges pose a significant barrier to transitioning from nature-inspired concepts to practical industrial implementation.

7. Conclusions and Future Recommendations

7.1. Conclusions

Several strategies have been employed to enhance the overall thermal performance of the FPC. One straightforward and efficient method is optimizing the collector’s design to achieve peak efficiency by mitigating heat loss and pressure drops. Additionally, the convective HTC between the riser tube and air/water significantly influences the collector’s thermal output. Notably, the collector’s performance tends to improve, especially when active/passive techniques are utilized, requiring minimal expenditure. Several important conclusions should be highlighted regarding how the collector’s performance can be enhanced through different strategies.

• Energy storage materials (PCMs) can improve efficiency and prolong collector operational flexibility by eliminating overheating and frost issues in adverse conditions. So, PCMs and composite PCMs can increase the thermal output of FPCs.
• Absorber plate shape is a crucial design factor in improving FPC efficiency; generally, corrugated absorber plates outperform conventional absorbers due to augmented friction and turbulence.
• V-trough collector performance is increased by 18–19% due to the optimized combination of absorber type and tube inserts compared to a flat absorber collector. Regarding cost-effectiveness, a low flow rate is preferable for FPC-based air/water heaters of the serpentine type because the payback period is short.
• The use of different shapes, such as twisted tapes, turbulators, and fins, can improve the heat transfer rate (due to proper mixing of fluid) and thermal output of FPC-based solar air heaters.
• Nanoparticles in nanofluids boost the heat transmission rate due to their high conductivity and random fluid flow. By decreasing nanofluid viscosity, increased shear rate decreases the boundary layer at heat transfer surfaces, improving the heat transmission rate that will lead to improved thermal output of collectors.
• Hybrid nanofluids are effective heat transfer fluids that improve both the energy and exergy efficiency of FPCs compared to water-based mono nonfluids.
• Glass surfaces that possess superhydrophobic characteristics effectively minimize the likelihood of dust adherence, showcasing remarkable self-cleaning capabilities. Practical trials in specific regions are anticipated to introduce an enhanced approach for mitigating dust accumulation in photovoltaic systems.

7.2. Future Recommendations

This comprehensive literature survey explores diverse strategies employed by researchers to increase the performance of FPCs. Yet opportunities remain for advancing the thermal performance of FPCs. The collector’s design, a crucial factor, could benefit from optimization using machine learning and other algorithms for improved outcomes. Furthermore, future investigations should focus on nano-enhanced approaches to enhance heat transfer, thereby achieving superior thermal output from collectors. In the future, there is a need for the continued advancement of superhydrophobic glass technologies, particularly those incorporating photocatalytic, self-repairing, and thermally insulating attributes.