Thermal Performance and Cost Assessment Analysis of a Double-Pass V-Trough Solar Air Heater

Abstract

Solar air heating systems offer an effective alternative for reducing greenhouse gas emissions at a profitable cost. This work details the design, construction, and experimental evaluation of a novel double-pass V-trough solar air heater with semicircular receivers, which was built with low-cost materials readily available in the Mexican market. Thermal performance tests were conducted in accordance with the ANSI-ASHRAE 93-2010 standard. The results indicated a peak collector efficiency of 0.4461 and total heat losses of 8.8793 W/(m² °C), with an air mass flow rate of 0.0174 kg/s. The instantaneous thermal efficiency varied between 0.2603 and 0.5633 with different air flow rates and an inlet air temperature close to the ambient temperature. The outlet air temperature reached 70 °C, making it suitable for dehydrating fruits or vegetables at competitive operating costs. Additionally, a second-law analysis was carried out, and the exergy efficiency was between 0.0037 and 0.0407. Finally, a Levelized Cost of Energy analysis was performed, and the result was USD 0.079/kWh, which was 31% lower than that of a conventional electric air heater system.

1. Introduction

Using conventional fuels to provide energy for human activities and the consequent emission of greenhouse gases have caused global climate change. Clean energy, such as solar energy, is a viable technical and economic option for reducing these emissions. Renewable thermal sources can drive the air-heating process for dry food products or space conditioning. The agricultural sector uses about 30% of the global energy consumption, and 3.62% is for drying processes [1,2].

The drying process involves heat and mass transfer, in which the moisture is removed from any material, and it is widely used in the food industry to preserve food by reducing water and microbiological activity. Besides being expensive, conventional dryers require large amounts of fossil fuels that increase greenhouse gases [3]; in this regard, solar air heaters can be coupled to conventional dryers to reduce fuel consumption [4]. Solar energy is a suitable option among renewable energy sources since the technology that involves thermal solar energy is well known, cheap, and available in many regions.

To improve the performance of conventional solar air heaters, the following actions are commonly carried out: reducing thermal losses from the environment by using appropriate insulation; increasing the convection coefficient with vortex generators; or increasing the heat transfer area with novel designs [5]. Since the performance of solar air heaters is affected by various parameters and operation conditions [6], different designs have been proposed and studied: flat plate [7], “V”-plate channel [8], with baffles to increase heat transfer [9], solar concentrating, and single- and multiple-pass air flow [10], among others.

Several research studies have been carried out on evaluating double-pass solar heaters for the drying process. In this regard, Banout et al. [11] evaluated the performance of a double-pass solar dryer with a cabinet-type solar dryer to dry chili. According to the authors, the overall drying efficiency of a double-pass dryer was two times higher than that of a cabinet-type dryer. Hussein et al. [12] analyzed the performance of a double-pass solar air heater with a new solar absorber. According to the authors, the proposed configuration reaches a maximum effective efficiency of 80.9% with an air mass flow rate of 0.03 kg/s, which means that this configuration was 4.8% higher than the conventional double-pass solar air heater with a flat plate solar absorber.

On the other hand, Chávez-Bermúdez et al. [13] reported a mathematical model for a double-pass CPC-type solar air heater by comparing its performance with that of a single-pass CPC-type air heater. The results showed that higher efficiency and temperature gains can be achieved in the double-pass solar heater by providing higher instantaneous performance and outlet air temperature than conventional ones, with improvements of up to 73.1% and 9.4%, respectively. Mesgarpour et al. [14] conducted a numerical simulation to evaluate the effect of different geometrical parameters on a double-pass solar air heater with a helical flow pathfinding increase of at least 16.5% due to optimization.

On the other hand, some works have been focused on the experimental analysis of double-pass solar air collectors. Sharol et al. [15] investigated the influence of a double-pass solar air heater’s thermal energy storage material inside the tube (with a cross-matrix absorber). They reported an improved maximum thermal efficiency of 68.23% and an exergy efficiency of 23%. El-Said et al. [16] carried out an experimental evaluation of a double-pass solar air heater that used a corrugated absorber plate and an integrated external reflector. The maximum average thermal efficiency for the proposed heater was 19.33%. Kumar et al. [17] investigated a curved design of a double-pass counterflow solar air heater with arched baffles. Khatri et al. [18] designed and developed a double-pass, unidirectional-flow solar air heater with an absorber plate for higher thermal performance, in which cylindrical fins were also adapted on the upper surface of the absorber.

V-trough reflectors are used to increase the solar energy collected, mainly in water heating or photovoltaic/thermal applications [19,20,21,22]; for air heating applications, the reports are scarce: Sunilraj and Eswaramoorthy [23,24] proposed and analyzed a V-trough solar air heater with phase change materials for thermal energy storage.

Based on the literature review, new designs of solar air heaters using double-pass and/or solar concentration configurations are promising ways to increase air temperature, which can be used in industrial drying or space heating processes. Therefore, this paper presents the design, construction, and experimental evaluation of the thermal performance of a double-pass V-channel solar air heater. The novelty of this design is the V-trough geometry, which is simple to manufacture and has scarcely been reported in the literature. In addition, V-trough solar collectors typically use flat receivers; however, the developed air heater uses semicircular receivers to couple the fans for the air flow requirements. Additionally, low-cost materials readily available in the Mexican market were used in its construction, whereby a cost–benefit study based on the Levelized Cost of Energy (LCOE) was conducted.

2. Materials and Methods

2.1. Geometry

The geometry of the solar heater is based on a V-trough concentrator, considering a baseline b and a V-trough vertex angle θ. The geometrical concentration ratio for incidence angle φ = θ/2 = 0 and number of reflections is equal to one [25]:

$$C={\displaystyle \frac{a}{b}}=1+2\rho \cos \theta$$

(1)

where ρ is the reflectance of the side wall for solar radiation (ρ = 1 for geometric concentration ratio). The aspect ratio is calculated as follows [25]:

$${\displaystyle \frac{L}{b}}={\displaystyle \frac{\mathit{cos}\theta }{\mathit{sin}\left(\theta /2\right)}}$$

(2)

where L is the lateral wall length, and b is the V-trough bottom width. Figure 1 shows the transversal geometry of the air heater, where the reflector boosters with length L are shown.

Figure 2 shows the transversal geometry of the air heater, where the reflective surfaces, the cover, and the receivers can be observed. These two cylindrical receivers were considered to efficiently distribute the air, adapting them to the exhaust fans available in the Mexican market. Finally, there was an air gap between the receivers and the sides and bottom to avoid thermal losses by conduction.

Additionally, Figure 2 shows the operation of the solar air heater, where the air first circulates and is preheated through the cavity formed by cover, reflectors, and receivers. Then, the air flows inside the receivers, where the air is finally heated. Two exhaust fans drive the air at the outlet of the cavity of the solar air heater. The two outlet ducts can be connected to a collection or drying chamber.

A V-trough vertex angle of θ = 40° and a V-trough bottom width b = 0.22 m were considered in the air dryer design. The diameter of the circular receivers was 0.1016 m and was placed on the collector cavity. With these parameters and from Equations (1) and (2), C = 2.53, L = 0.493 m, and a = 0.557 m. However, the reflector’s upper ends were truncated to a value of L = 0.406 m to use a commercial reflective sheet with a width of 1.22 m. With these dimensions, a new aperture width a = 0.50 m and a new geometric concentration ratio C = 2.27 were calculated.

However, considering that only the two upper semicircles in the receptor will collect solar radiation, the geometric concentrator ratio was calculated as C = a/πD.

Table 1. Geometrical parameters of the V-trough solar concentrator.

Description	Value
V-trough vertex angle (θ)	40°
V-trough bottom width (b)	0.22 m
Aperture width (a)	0.50 m
Lateral wall length (L)	0.41 m
Collector length (LC)	2.44 m
Diameter receiver (R)	0.1016 m
Geometric concentration ratio (C)	1.56
Aperture area (Aa)	1.22 m2
Gross area (Ag)	1.38 m2
Receiver area (Ar)	0.78 m2

shows the geometrical parameters of the final V-trough solar concentrator design.

Finally, the reflective material was a galvanized steel sheet with a reflectance of 0.59 [26], the cover was a transparent alveolar polycarbonate with a transmittance of 0.86 [27], and the receiver coating was matte black paint with an absorptance of 0.95 [28]. Additionally, a selective surface was not used to reduce manufacturing costs since there is no thermal storage and the operating temperature is low.

2.2. Ray Tracing Analysis

Since the lateral reflector length was reduced and the receptor was not a flat plate but two semicircles, a ray-tracing analysis was performed to determine the solar air heater’s radiation distribution and optical performance. The incidence angles of 0° and ±23.45° were used for the simulation with the Soltrace 3.1.0 Software, considering that the collector tilt is the same as the latitude of the site (18.88° N for Giuseppe, Morelos, Mexico). In addition, 3.0 mrad and 3.73 mrad were considered for specular and slope errors, respectively [29].

2.3. Solar Thermal Analysis

The solar collectors’ thermal performance tests included the instantaneous efficiency, the incidence angle modifier, and the time constant. These parameters indicate the efficiency of the captured solar radiation by the collector transferred to the working fluid. The instantaneous efficiency and the time constant are determined at near φ = 0 conditions, while the incidence angle modifier allows determining how this efficiency is affected for off-normal incidence angles. All three parameters were obtained experimentally following the ANSI-ASHRAE 93-2010 standard [30].

The following assumptions were considered to simplify the solar thermal analysis: the solar collector performance was under a steady state, the air thermal capacity was constant, the useful energy gain was a function of the inlet air temperature, and the overall loss coefficient was independent of the temperature and wind speed.

The instantaneous efficiency of the solar collector (η) is defined as the ratio between the useful energy gain and the solar energy intercepted by the gross collector area, determined at normal incidence; it is expressed as follows:

$$\eta ={\displaystyle \frac{\dot{m}{C}_p\left(T_o-T_i\right)}{A_{g}G}}={\eta }_o-{\displaystyle \frac{F_R{U}_L}{C}}{\displaystyle \frac{\left(T_i-T_a\right)}{G}}$$

(3)

where $\dot{m}$ is the mass flow rate of the air, C_p is its thermal capacity, T_o is the outlet air temperature, T_i is the inlet air temperature, A_g is the gross area of the solar collector, G is the global solar irradiance, η_o is the peak collector efficiency (η when T_i is equal to T_a so the efficiency only depends on the optical performance), F_R is the heat removal factor (equivalent to the effectiveness of a conventional heat exchanger; this is the ratio of the heat transfer obtained to the maximum possible heat transfer), U_L is the total heat loss coefficient, and T_a is the ambient temperature.

On the other hand, since the peak collector efficiency term in the instantaneous efficiency equation becomes more complex when the incidence angle increases, the incidence angle modifier allows quantifying the thermal efficiency reduction with the incidence angle increasing between the direct beam and the normal to aperture [31].

The incidence angle modifier (K_τα) is defined as follows:

$$K_{\tau \alpha }\left(\phi \right)={\displaystyle \frac{{\left(\tau \alpha \right)}_e\rho \gamma }{{\left[{\left(\tau \alpha \right)}_e\rho \gamma \right]}_n}}={\displaystyle \frac{{\left[{\eta }_o\right]}_{\phi }}{{\left[{\eta }_o\right]}_n}}$$

(4)

where (τα)_e is the effective transmittance–absorptance product, γ is the fraction of specularly reflected radiation from the reflector intercepted by the receiver, subindex φ represents the incidence angle of interest, and subindex n represents normal incidence condition.

Therefore, the instantaneous efficiency for off-normal incidence angles is determined by the following:

$$\eta =K_{\tau \alpha }{\eta }_o-{\displaystyle \frac{F_R{U}_L}{C}}{\displaystyle \frac{\left(T_i-T_a\right)}{G}}$$

(5)

Finally, the time constant is the time taken for the fluid leaving the collector to reach 63.2% of its final steady-state value as it passes from a condition of no radiation to one of incident radiation, and it is calculated as follows:

$${\displaystyle \frac{T_{st}-T_i}{T_{ss}-T_i}}=0.632$$

(6)

where T_st is the outlet air temperature at the time t, and T_ss is the outlet air temperature when the irradiance is unblocked for the heating time constant and blocked for the cooling time constant.

2.4. Solar Collector Exergy Analysis

An exergy analysis was carried out to quantify the energy quality and the energy flow direction during the entropy generation. The solar collector exergy rate transfer to a working fluid (EX_u) with respect to a reference state (environment) was calculated according to the following [32] definition:

$${EX}_u=\dot{m}{C}_p\left[\left(T_o-T_i\right)-T_{a}ln\left({\displaystyle \frac{T_o}{T_i}}\right)\right]$$

(7)

Assuming that the sun is an infinite thermal source, the exergy rate of thermal emission at apparent sun temperature, T_s, according to [33], is defined as follows:

$${EX}_a=A_{a}G\left[1+{{\displaystyle \frac{1}{3}}\left({\displaystyle \frac{T_a}{T_s}}\right)}^4-{\displaystyle \frac{4T_a}{3T_s}}\right]$$

(8)

where EX_a is the term related to the absorbed solar radiation exergy by the collector. The sun’s apparent temperature is 5762 K [34].

The exergy efficiency of the solar air heater was calculated as the ratio of EX_u and EX_a [35]:

$${\eta }_{ex}={\displaystyle \frac{{EX}_u}{{EX}_a}}$$

(9)

2.5. Measurement Uncertainty Analysis

Since solar collectors’ thermal performance testing reliability depends on the measurement of diverse parameters under specific operational and climatic conditions to apply the steady-state model described previously, it was necessary to determine the measurement uncertainties.

Therefore, uncertainties were determined according to the guidelines of ISO GUM:2008 [36], focusing only on Type A uncertainties. These uncertainties are determined by statistical methods and are related to the stability of the measurements. The methodology established by Mathioulakis et al. [37] was applied to propagate these uncertainties. Finally, a coverage factor k = 2 was used to achieve a 95% level of confidence in the measurements.

2.6. Levelized Cost of Energy

To estimate the technical–economic feasibility of the solar air heater, the Levelized Cost of Energy (LCOE), which is defined as the cost of the unit of energy generated averaged over the useful life of the system [38,39], was used.

$$LCOE={\displaystyle \frac{TC}{\sum _{y=1}^N{AEP}_y{\times \left(1+r\right)}^{-y}}}$$

(10)

where TC is the present value discount of the total cost of energy generation, AEP is the energy generated by the system in a typical year, and r is the discount rate. TC was calculated as follows:

$$TC=I+\sum _{y=1}^N\left(O_y+M_y\right){\left(1+r\right)}^{-y}-SV{\left(1+r\right)}^{-N}$$

(11)

where I is the investment cost of the system, $O_y$ is the yearly operation cost, and $M_y$ is the maintenance cost over a year; additionally, N is the useful life of the system in years, SV is the salvage cost after year N, and finally r—this term is defined as the ratio at which the nominal rate a exceeds the inflation rate i:

$$1+r={\displaystyle \frac{1+a}{1+i}}$$

(12)

The LCOE was calculated for the solar double-pass V-trough air heating system with an auxiliary electrical system and was compared to the LCOE of a conventional electric air heater calculated with the same methodology used for the solar system indicated in Equations (9)–(11). The following considerations were assumed:

• The solar system is composed of a single solar air heater.
• Energy requirements were considered with the peak power of the solar air heater multiplied by operation hours.
• The interest was not included in the investment cost during the system construction.
• The energy generated over a typical year was considered.
• The salvage cost is given after the last year of energy production.

For the solar system with the auxiliary electrical system of 733 W, the respective TCs were added. A solar fraction of F = 0.8 was used, and the SV was considered 10% of the investment cost.

Table 2. Values used in LCOE calculation.

Description	Value
Discount rate (r)	0.1
Electrical tariff	0.0953 USD/kWh
Equipment useful lifetime (N)	15 years
Fan power	44 W
Daily operation time	6 h
Annual operation time	2190 h
Resistor efficiency	0.95
Energy generated in a typical year (AEP)	1606 kWh
Investment in the solar system	USD 388.58
Investment in the electrical system	USD 150.63

shows the values considered for calculating the LCOE of the solar system with the electrical auxiliary and for the LCOE of an electric air heater. The investment costs of the auxiliary and the conventional electrical systems were considered the same.

Associated Costs

Table 3. Associated cost for the solar double-pass V-trough air heater.

Description	Quantity	Cost (USD)
Square steel profile	19.64 m	USD 33.81
Galvanized steel sheet	5.04 m	USD 52.97
Galvanized steel duct	5.52 m	USD 56.70
Transparent polycarbonate sheet	2.1 m2	USD 41.94
Air extractor	2	USD 47.90
Galvanized steel bridles	2	USD 16.90
Galvanized steel elbows	2	USD 33.25
Manufacturing and assembly	1	USD 74.00
Other associated costs	1	USD 31.11
	Total	USD 388.58

shows the associated cost for the solar double-pass V-trough air heater, including manufacturing costs.

The total cost per collector was USD 388.58. However, the cost per unit of area was 305.97 USD/m². The solar double-pass V-trough air heater was built on a welded tubular structure with a square steel profile. The tubular structure supports the reflective and exterior sheets, the top cover, and the two transparent side covers. In addition, the side covers support the circular receivers and air extractors.

3. Experimental Setup

The solar double-pass V-trough air heater was experimentally evaluated. Figure 3 shows the experimental setup diagram, where air extractors were placed out of the cavity, and mixing chambers were used to control the air inlet temperature. Therefore, four thermocouples at the inlet, four at the outlet, and two inside the cavity were placed to measure the air temperature of the collector; additionally, two transducers were installed inside the cavity.

A pyranometer was installed on the collector plane to measure the global solar irradiance, and the air flow rate was measured at the collector inlet and outlet. Finally, the ambient temperature, the wind speed, and the pressure difference between the collector inlet and outlet were measured.

Table 4. Accuracy of the measured variables.

Variable	Instrument	Operation Range	Accuracy
Temperature	Type T thermocouples	−185 to 300 °C	±0.5 °C
Air flow rate	Turbine anemometer	0 to 45 m/s	±3%
Wind speed	Turbine anemometer	0 to 45 m/s	±3%
Pressure difference	Differential pressure gauge	±39.99 kPa	±1%

shows the instruments used and their features. Figure 4 shows the solar collector prototype.

The experimental evaluation was carried out under clear overcast conditions from November 2021 to February 2022 in Jiutepec, Morelos, Mexico (18.88° N, 99.16 W). The ANSI/ASHRAE Standard 93-2010 [31] testing procedures in steady-state conditions were followed to determine the thermal performance of the double-pass V-trough solar air heater collector. Therefore, the collector operation and environmental conditions (air flow rate and inlet temperature, as well as solar irradiance and ambient temperature) remain constant during the test period. The operating conditions were inlet temperature between ambient and 70 °C, mass flow rate of 0.0176 kg/s, wind speed lower than 3 m/s, and solar irradiance above 700 W/m² in the solar collector plane. The solar collector was aligned on an east–west axis.

4. Results

A ray tracing analysis was carried out to estimate the optical performance of the solar air heater for three different incidence angles, considering the variation in solar declination along the year. Figure 5 shows the ray tracing for two cases with incidence angles of (a) φ = 0 and (b) φ = 23.45°. Ray tracing was performed with up to 2500 million generated rays; at this condition, the uncertainty was lower by 0.5%.

Table 5. Thermal power absorbed by both receivers.

Incidence Angle (°)	Thermal Power (W)
	Receiver 1	Receiver 2
0	400.13	399.68
23.45	278.37	214.46
−23.45	214.46	278.37

shows the thermal power absorbed by each receiver for the incidence angle, assuming an irradiance of 1000 W/m² at the aperture area corresponding to 1220 W for the V-trough solar concentrator prototype. Therefore, when the angle of incidence was 23.45°, receiver 1 absorbed 22.82% and receiver 2 absorbed 17.58% of the irradiance; accordingly, the optical efficiency in this incidence condition is 40.4%. In addition, when the incidence angle is normal to the receiver, the optical efficiency rises to 65.6%.

As mentioned before, the air temperatures, air flow rate, relative humidity, pressure difference at the inlet and outlet of the solar air heater, and incident solar irradiance were measured to quantify the thermal and exergy efficiencies. Figure 6 shows the instantaneous thermal efficiency as a function of (T_i − T_a)/G, where the error bars represent the uncertainty in each value. The test conditions were a constant air mass flow rate of 0.0174 kg/s, average irradiance G = 1099 ± 20.5 W/m², and inlet air temperatures of 33.40 ± 0.35 °C, 42.37 ± 0.81 °C, and 72.06 ± 0.58 °C. As observed, the uncertainties for the (T_i − T_a)/G parameter and the instantaneous thermal efficiency were 0.0010 K m²/W to 0.0026 K m²/W and 0.0062 to 0.019, respectively.

For the instantaneous thermal efficiency linear fit, if F_R and U_L are both constant, the slope represents the product of the removal factor F_R by the overall loss coefficient U_L, divided by the concentration ratio C. As can be seen in Figure 6, there is some data scattering; this means both F_R and U_L are not constant. In general, F_R depends moderately on temperature, and U_L is a function of temperature and wind speed. Therefore, the peak collector efficiency (η_o) and total heat losses (F_RU_L)/C were 0.4461 and 8.8793 W/(m² °C), respectively. The total heat losses were close to 9 W/(m² °C) since, to maintain a low cost of the solar double-pass V-trough air heater, a selective coated on the receiver surface was not applied; in addition, the selected polycarbonate cover was partially opaque to infrared radiation.

An interesting piece of data on the graph (Figure 6) is the last one, where an instantaneous thermal efficiency of 2.3 ± 1.88% was obtained. This efficiency corresponds to a high inlet air temperature of 72.06 ± 0.58 °C, allowing a mean increment of only 1.7 °C. Therefore, to guarantee an acceptable thermal efficiency and a temperature increment higher than 25 °C, this solar collector would be used only in applications requiring heating environmental air at temperatures near ambient by 20 °C or less. However, the calculated thermal efficiency was similar to the values reported in the literature for concentrated air solar collector configuration [40].

Compared with the other solar air heaters reported in the literature, the thermal efficiency of a double-pass solar air collector was from 54.7% to 60.3%, evaluated with air flow rates from 0.00864 kg/s to 0.01317 kg/s [41,42]. Another analysis [43] tested the air flow rates from 0.012 kg/s to 0.038 kg/s, and the maximum thermal efficiency was 63.74%. On the other hand, Aldabbagh et al. [44] reported a thermal efficiency of 83.65% with an air flow rate of 0.038 kg/s. Regarding the evacuated tube solar collectors, thermal efficiencies of 25% were reached when the heat transfer fluid was oil [45]; other authors demonstrated that the efficiency increases as the flow rate increases reach 27.15% at 2.81 kg/m [46]. When coupled to a device for drying, the thermal efficiency can be improved to 39.9% [47]. The maximum thermal efficiency value is achieved by integrating thermal storage to 67.5% [48].

Besides the efficiency of the experimental device, another highlight performance feature is the hot air temperature reached, since it is suitable for drying several products such as fish (40 to 50 °C) [49], bananas (40 to 65 °C) [50], grapes (52 to 65 °C) [51], mango (~35 °C) [52], pineapple (34 to 64 °C) [53], chili (34 to 71 °C) [54], and others [55].

The instantaneous thermal efficiency for different mass flow rates was calculated with the air inlet temperature equal to the ambient temperature. It can be observed in Figure 7 that the thermal efficiency increased as the mass flow increased. The minimum efficiency was 0.2603 ± 0.0076 with an air mass flow rate of 0.0098 kg/s, and the maximum value was 0.5633 ± 0.021 with an air mass flow rate of 0.0287 ± 0.0001 kg/s. In addition, the uncertainty for the mass flow rate was between 0.000016 and 0.000138 kg/s, and for the instantaneous thermal efficiency, it was from 0.005 to 0.021.

Figure 8 shows the incidence angle modifier (K_τα), which was calculated following the ANSI-ASHRAE 93-2010 standard. The test was performed with an air mass flow rate of 0.0260 ± 0.0001 kg/s, and the solar irradiance was between 570 ± 7.6 W/m² and 1044 ± 11.6 W/m². Interestingly, the incidence angle modifier’s uncertainty was high, between 0.0656 and 0.1344. However, that could be expected since this modifier was calculated as the ratio between the value obtained at the incidence angle of interest and the peak collector efficiency; therefore, it depended on eight variables. A third-order polynomial equation was obtained, where K_τα = 1.0049 − 0.0012φ − 0.0002φ² − 2.0 × 10⁻¹⁹φ³, valid to calculate the incidence angle modifier from 0° to 65°. According to these results, the V-trough solar air heater is more susceptible to incidence angle variations than the flat-plate collectors. As reported by Summ et al. [56], the median and the minimum of flat-plate collectors’ incidence angle modifiers at incidence angles of 30° are 0.98 and 0.94, respectively. These values represent a loss of up to 6% of the optical efficiency related to the incidence angle. In contrast, the V-trough air collector analysis showed a loss of 20% of optical efficiency at 30° of incidence angle, consistent with the ray tracing analysis.

The time constant of the solar air heater (Figure 9) was calculated under the ANSI-ASHRAE 93-2010 standard, which was 516 s for heating, with an average irradiance of 1071.43 ± 23.8 W/m² and 0.0169 kg/s of air mass flow rate. This time constant is better than other air solar heaters, which require more time to compensate for the thermal inertia of the solar collector [57].

Now then, Figure 10 shows the exergy efficiency of the V-trough solar air heater as a function of (a) outlet and inlet temperatures ratio with a constant air mass flow rate of 0.0174 kg/s, and (b) the air mass flow rate with inlet air temperature close to the ambient temperature. According to Figure 10a, as the temperature ratio increases, the exergy efficiency increases. Figure 10b shows the effect of the air mass flow rate on the exergy efficiency; it can be seen that as the air mass flow increases, the exergy efficiency decreases because the air mass flow increment reduces the outlet air temperature. The maximum exergy efficiency obtained was 0.0407 ± 0.0030, with a mass flow rate of 0.0217 ± 0.0001 kg/s, which could be considered low. However, the exergy efficiencies between 0.0226 ± 0.0007 and 0.0407 ± 0.0030 are like those reported for other solar collectors. Gupta and Kaushik [58] evaluated a solar air heater with different artificial roughness geometries and reported a maximum exergy efficiency of 0.015 to 0.024, depending on the roughness geometry, with a Reynolds number of 4000. For their part, Jafarkazemi and Ahmadifard [59] evaluated flat solar collectors with liquids as work fluids and determined that exergy efficiencies were up to 0.07.

Finally,

Table 6. LCOE results.

Description	Cost (USD)
Solar hybrid V-trough air heater
TC solar	USD 537.81
TC electric auxiliary	USD 426.52
LCOE	USD 0.079/kWh
Conventional electric air heater
TC	USD 1407.01
LCOE	USD 0.115/kWh

shows the results of the LCOE analysis, where the LCOE of the hybrid solar system was USD 0.079/kWh, and the conventional electric air heater system was USD 0.115/kWh, so the hybrid solar system is profitable. This result was lower than the levelized energy cost of a solar heating system for buildings (USD 0.1/kWh), the technology most similar in the literature [60].

5. Conclusions

A double-pass V-channel solar air heater has been developed and experimentally evaluated. The instantaneous thermal efficiency for an air mass flow rate of 0.0174 kg/s was 0.4461, with total heat losses of 8.8793 W/(m² °C). The effect of varying the air flow rate on thermal efficiency was analyzed. The maximum efficiency value was 0.5633 with an air mass flow rate of 0.0287 kg/s, while the minimum efficiency was 0.2603 with an air mass flow rate of 0.0103 kg/s.

Furthermore, a third-order polynomial equation was fitted for the angle of incidence modifier, and a time constant of 516 s was calculated. In addition, a second law of thermodynamics analysis was performed, obtaining a value of the exergy efficiency between 0.0037 and 0.0407.

According to the results, the double-pass V-channel solar air heater increased the air temperature by more than 30 °C for inlet temperatures close to ambient temperature. An air outlet temperature of up to 70 °C was achieved, which can dry most food products such as fish, beef, banana, grapes, mango, pineapple, chili, and others.

The solar air heater evaluated in this work was built with low-cost materials available in the Mexican market and had an approximate cost-effectiveness of 305.97 USD/m². The calculated LCOE of the presented double-pass V-channel solar air heater was USD 0.079/kWh, which is 31.3% lower than the LCOE of a similar electric air heater system.

Finally, compared to other technologies, the developed solar air heater has first- and second-law thermal efficiencies similar to other technologies such as flat-plate solar collectors and vacuum tubes. Despite this, using local materials limits the thermal efficiency of the device, but it could be improved using higher-quality materials. However, the prototype described in this paper is an affordable, easy-to-manufacture, low-maintenance solar alternative that can be integrated into production processes, providing thermal energy sustainably and cleanly.