Experimental Studies on the Influence of Spatial Orientation of a Passive Air Solar Collector on Its Efficiency

Abstract

The solar collector is used to convert solar energy into thermal energy. First, the internal energy of the absorber increases, which is reflected in the increase in its temperature. This energy is transferred to the working fluid in direct contact with the absorber. Depending on the type of fluid, liquid or air solar collectors are distinguished. When the flow of the working medium takes place naturally, without the support of pumps or fans, the solar collector is treated as a passive device. The gravitational movement of air in the inner space of an air solar collector depends on its construction and its spatial orientation in relation to both the source of radiation and the direction of the force of gravity. This paper describes the results of laboratory experimental tests of a prototype passive air solar collector, including: the influence of radiation intensity and the deflection of the solar collector from the vertical on the increase in the temperature of the air flowing through the collector, the mass flow rate of the air and the efficiency of the device. The tests were carried out using an air solar collector with the dimensions 2080 × 1040 × 180 (height × width × thickness) and radiation intensity in the range of I = 0 ÷ 550 W/m². It was found that the vertical arrangement of the collector does not ensure maximum efficiency of the device.

1. Introduction

The demand for energy increases during the development of human society. This applies to both electricity and heat. Fossil fuel resources are shrinking, making them more expensive and less available. Renewable energy sources are an alternative to conventional energy sources. One of the commonly used methods for using thermal energy is photothermal conversion in solar collectors. At the moment, the market of solar collectors is highly developed and there are many types of collectors available. Currently, numerous attempts to increase the efficiency of these devices are described in the world literature. A positive impact on the performance of solar collectors can be achieved by modifying the structure, optimizing the angle of inclination and orientation towards the sides of the world, as well as the use of working media other than water. Francesconi et al. [1] conducted an assessment of the optical efficiency in solar collectors. The aim of the study was to create a more robust and repeatable way to improve the optical efficiency of a solar collector. A transient heating of the not-loaded collector was taken as an indicator to deduce its optical efficiency. No measurement of the fluid temperature rise was conducted. A compound parabolic collector and an evacuated pipe were examined to compare the methodology proposed by the authors with other proposed techniques based on fluid heating. Application of multiple solar selective-absorbing coatings was studied in [2]. The authors proved that the ideal and the multi-section system performed a better thermal efficiency than the conventional system. The heat loss for the multi-section system was reduced by 29.3%, and the thermal efficiency was enhanced by 4.3%. Qiu et al. [3] investigated the efficiency enhancement of a solar trough collector by combining solar and hot mirrors. The experimental data indicated that the new type of the receiver could achieve a higher optical efficiency (78.02%). The modification of the jet impingement with a v-corrugated absorber plate was analyzed by Alomar et al. [4]. The efficiencies of the two modifications of the collector were compared. The study shows that the use of the modified model has a positive effect on the heat exchange efficiency. Bhowmik et al. [5] proposed a novel technology to improve the performance of the solar thermal collectors. A reflector was used to improve the reflectivity of the collector. The reflector was changing its angle with the daytime. The authors obtained an increase in the collector’s efficiency by 10%. An enhancement of the solar trough collector efficiency was presented by [6]. The author aimed at enhancing the amount of reflected solar irradiance onto the receiver tube by optimizing the reflecting mirror profile. The overall efficiency of the trough was enhanced by ~8%. Sheikholeslami et al. [7] investigated numerically a novel turbulator in the solar collector system. Eight cases have been analyzed that considered the variable Re. The authors found that the increase in the Reynolds number results in augmentation in the Nusselt number by about 75.22%, but the friction factor reduces this by 12.43%. A low emissivity thin-film coating was studied to enhance the thermal conversion efficiency of selective solar absorbers in high vacuum flat plate collectors by Caldarelli et al. [8]. Luo et al. [9] analyzed the profits and efficiency enhancement of a cylindrical solar collector by structural modification of a helical tube. A thermal efficiency improvement of the parabolic trough solar collector was studied by Ajbar et al. [10] using different kinds of hybrid nanofluids. Shojaeizadeh et al. [11] conducted research on the thermal efficiency of a ferrofluid-based cylindrical solar collector with a helical pipe receiver under the effect of a magnetic field. Ultrahigh-efficiency solar energy harvesting via a non-concentrating evacuated aerogel flat plate solar collector was presented by Gao et al. [12]. Peng and Sadaghiani [13] used a sunrays trap in a direct-absorption solar collector to enhance the thermal efficiency of the collector. Many studies include the use of the addition of nanofluids to the working fluids of the collector. Ahmadlouydarab et al. [14] conducted a comparison on the research for flat plate solar collectors’ thermal efficiency. Efficiency increase was noted when the nanofluids were used as working media. Farhana et al. [15] analyzed the efficiency enhancement of flat plate solar collectors using crystal nano-cellulose (CNC) nanofluids. Mahbubul et al. [16] investigated a carbon nanotube nanofluid in enhancing the efficiency of an evacuated tube solar collector. The effect of the nanoparticle shape of Al₂O₃/pure water nanofluid on an evacuated U-Tube solar collector efficiency was presented by Kaya et al. [17]. Yan et al. [18] studied the effect of a U-shaped absorber tube on thermal-hydraulic performance and the efficiency of two-fluid parabolic solar collectors containing two-phase hybrid non-Newtonian nanofluids. Norouzi et al. [19] conducted research on the efficiency enhancement of the parabolic trough solar collector using the rotating absorber tube and nanoparticles.

The addition of diamonds into the working liquid was tested by Alklaibi et al. [20]. Multi-resonance plasmonic nanoparticles were also examined for improving the solar collector efficiency [21]. Said et al. [22] used Fe₃O₄/water hybrid nanofluids for thermal efficiency improvement. The water/Al₂O₃ mixture was tested by [23,24]. Paraffin-wax/graphene oxide carbon-based fluids were examined by Abu-Hamdeh et al. [25]. Iron oxide nanofluid addition was analyzed by Choudhary et al. [26]. Dutkowski and Kruzel [27,28] experimentally investigated the influence of microencapsulated phase-change material slurry on the thermal efficiency of flat plate solar collectors. Feng et al. [29] presented a parametric study on the efficiency of a solar evacuated tube collector using phase change materials.

There are many ways to use solar energy for living purposes. The most affordable is the use of air collectors, which are a simple and failure-free design at a low price. Alomar et al. [4] analyzed the efficiency of solar air heater collectors by modifying the jet impingement with a v-corrugated absorber plate. The thermal efficiencies of the modified collector versus the normal v-corrugated collector with a jet plate blown system were compared. Sevik et al. [30] investigated the relative roughness height effect in a solar air collector with convex dimples. Experimental investigation on the heat transfer performance of a solar collector with baffles and semicircular loop fins under varied air mass flow rates was presented by Rani and Tripathy [31]. The authors analyzed the temperature distribution, heat transfer coefficients and thermal efficiency of a solar air heater integrated with baffles and fins in the form of hollow semicircular loops. Chabane and Aouissi [32] investigated the thermal efficiency of a solar air collector with transverse rectangular baffles incline by an angle of 135°. Oztürk and Çiftçi [33] conducted research on upgrading the performance of a solar air collector with flexible aluminum air ducts and graphene nanoplatelet-enhanced absorber coating. Technical and economic performance analysis of large flat plate solar collectors coupled with an air source heat pump heating system was conducted by Li et al. [34]. Lingayat and Chandramohan [35] conducted a numerical investigation on a solar air collector and its practical application in the indirect solar dryer with energy and exergy analysis. Numerical and experimental investigation of a solar air collector with internal swirling flow was shown by He et al. [36]. Reddy et al. [37] provided a performance evaluation of a sand-coated absorber based solar air collector. Dong et al. [38] provided a thermal economic analysis of a double-channel solar air collector coupled with a draught fan. Selimefendigil et al. [39] studied the performance of a greenhouse drying system by using a triple-flow solar air collector with nano-enhanced absorber coating.

The methods described above increase the efficiency of solar energy conversion, but they entail financial outlays. Passive methods allow for heating of the building through a properly made partition (glass), drying of agricultural products, and heating of water directly through the walls of the accumulation tank. Passive collectors do not require an additional power source (e.g., pumps, fans, etc.) to force the flow of the medium, and thus do not incur continuous operating costs. The purchase and installation of the collector is the only expense incurred by the investor. The costs related to the installation design, fees for the control systems, working liquid tanks and the working liquid itself, intermediate heat exchangers, service, maintenance, etc., are avoided. Passive air solar collectors are an excellent choice where direct heating of the room through the windows is sometimes impossible due to location, technical or security reasons. This occurs in rooms with windows on the north side of the building, elevated basements, storage halls or workshops. In addition, passive solar collectors can be used both to force the movement of warm air into a heated room and to cool it, when the installation of the collector supports the intake of cooler air and the discharge of heated air into the environment. However, air solar collectors have enjoyed little interest from investors compared to liquid collectors, due to the lack of universal, reliable data on their energy efficiency in operating conditions. Thus, there is a strong motivation for experimental work, because the amount of reliable data for the technology of passive solar air collectors is relatively small, especially for the passive collectors with dimensions and construction similar to liquid solar collectors.

Since the airflow in a passive solar collector is driven by a density gradient caused by the temperature of the medium, the orientation of the collector relative to the vector of gravity due to gravity and the oppositely directed buoyancy force is very important. The presented work shows the results of experimental research on the construction of a passive air collector with connection stubs, the length of which corresponds to the thickness of the thermally insulated external building partition, typical in climatic conditions corresponding to the areas of Central Europe. The novelty is that the vertical (parallel to the wall of the building) position of the passive air solar collector is not the optimal position. It is possible to intensify convective air movements in an air solar collector only by tilting the collector from the vertical.

The paper is divided into the following sections: introduction in Section 1; description of the experimental procedure, including the test stand, with particular emphasis on the prototype collector structure (Section 2.1) and the measuring equipment used (Section 2.2), the scope of experimental tests (Section 2.3) and the method of converting measurement data (Section 2.4); results of direct measurements (Section 3.1 and Section 3.2) and results obtained from the calculations (Section 3.3 and Section 3.4) with their discussion; summary and conclusions in Section 4.

2. Experimental Methods

Currently, there are no standardized guidelines for testing air solar collectors, therefore, during the construction of the laboratory stand, the guidelines contained in the PN-EN 12976-2:2019-05 standard [40] and the ASHRAE 93-77 standard [41] regarding the testing of liquid solar collectors were used. The schematic diagram of the test stand is shown in Figure 1.

The test stand consisted of the tested prototype flat air solar collector (1) placed on a supporting structure. The supporting structure made it possible to set the collector in a vertical position and tilt it from the vertical in relation to the horizontal axis of symmetry. At the opposite end of the supporting structure there was a radiation source (2), which changed its position along with the collector. In this way, the rays fall perpendicularly on the collector surface, regardless of its position relative to the horizontal. The sources of radiation were three Philips HeLeN Infrared halogen lamps with a maximum power of 2 kW each. The characteristics of the radiation spectrum of the halogen lamps used against the background of the solar radiation characteristics are shown in Figure 2.

Each of the lamps was connected to an electric power regulator, by means of which the intensity of the emitted radiation was set. The distance of the set of radiators from the collector (3 m) and the direction of their operation were selected so as to obtain a uniform distribution of radiation on the front surface of the collector. The radiation intensity was measured at 30 points evenly distributed over the surface in 3 columns and 10 rows. A sufficiently large number of measurement points made it possible to check the uniform distribution of radiation and to determine the average value of radiation intensity on the basis of local values. The radiation intensity on the collector surface was measured using a pyranometer CMP11 (secondary standard) by Kipp & Zonen.

In order to maintain an undisturbed influence of the environment on the gravitational air flow through the collector, it was placed 0.5 m above the floor surface. The test bench was away from the walls and other objects in the laboratory.

In addition to measuring the collector’s radiation intensity, ambient temperature, air temperature in the inlet and outlet ducts and air flow velocity in the inlet section of the duct supplying air to the collector were measured.

2.1. The Construction of a Prototype Flat Air Solar Collector

The collector (Figure 3a) was made of an aluminum housing with external dimensions of 1.04 m × 2.08 m × 0.18 m (width) × (height) × (depth). The air inlet and outlet from the collector took place through pipe ducts with internal diameters d_in = 110 mm for the inlet duct and d_in = 130 mm for the outlet duct, respectively. The length of the ducts was L = 0.5 m. The axes of the inlet and outlet ducts were located in the vertical symmetry plane of the collector, offset by 110 mm from the lower and upper edges of the collector, respectively. The collector housing was insulated with 50 mm thick mineral wool on the rear surface and was 20 mm thick on the side walls. The absorber was made of 0.5 mm thick aluminum sheet covered with black, matte paint. In this way, the absorber surface emissivity coefficient ε = 0.98 was obtained. The collector had Solarglas PV class P1 glazing with a thickness of 3.2 mm (Figure 3b). Solar glass dedicated to solar energy applications has been tested and approved for use in solar collectors by the Fur Solartechnik SPF Institute in Switzerland.

2.2. Measuring Equipment

Measurements of the air temperature in the inlet and outlet sections of the air ducts and the temperature of the air surrounding the collector were carried out with individually made type K thermocouples. Each of the thermocouples was calibrated (in the range of 0 ÷ 80 °C) against a standard glass thermometer with an elementary division of 0.02 °C. It was found that the indication error of the thermocouples did not exceed ±0.2 K. The air temperature in the inlet and outlet ducts of the collector was determined as the average of 3 local values. The average value of the results archived by the recorder (Memograph RSG40 by Endress & Hauser) within 5 min of the measurement was taken as the value of the local temperature.

In order to determine the amount of air flowing through the collector, the measurement of its average velocity in the inlet section of the duct through which air flowed into the solar collector was used. The determination of the average air velocity was based on the measurement of the local velocity at five points of the channel cross-section. A portable hot-wire anemometer TESTO 524 was used for the measurements. The measurement uncertainty of a single measurement was ±0.03 m/s + 5% of the measured value. Hence, the maximum measurement uncertainty of the test results was ±0.085 m/s.

Kipp & Zonen CMP11 pyranometer, the “secondary standard” class, was used to measure the intensity of radiation reaching the collector surface. This is the highest accuracy class of this type of measuring device. The measurement of radiation intensity in the visible and infrared range was made by placing the device directly on the surface of the collector glazing. The measurement was carried out at 30 precisely defined points based on a regular grid (3 columns × 10 rows). The voltage signal from the pyranometer was fed to the recorder (Memograph RSG40). The recorder recorded the electrical signal as an average value in the interval of 3 s. The pyranometer calibration constant given by the device manufacturer was 8.66 µV for every 1 W/m².

2.3. The Scope of Research

The tests were carried out in laboratory conditions, in the range of an average radiation intensity 0–550 W/m² and five different values of the angle ψ of the collector position relative to the horizontal (Figure 1). The collector was tilted back by setting the angle ψ of 45°, 60°, 70°, 80° and 90°. The value of the angle ψ = 90° meant that the collector was in a vertical position.

All measurements of temperature, airflow velocity and radiation intensity reaching the collector were carried out in steady states of the solar air collector operation. The steady state of the collector operation (test results not presented in this paper) required waiting for about 15 min. After this time, the following were recorded for further calculations: the averaged (over 3 s) local value of radiation intensity and the averaged (over 5 min) local value of temperature and airflow velocity. The data stored in the recorder’s memory made it possible to calculate the average value of the radiation intensity reaching the collector surface, the average temperature and air velocity in the collector’s inlet duct and the average temperature of the air leaving the collector’s outlet duct.

2.4. Conversion of Measurement Data

The thermal efficiency of a passive air solar collector is defined as the ratio of the heat flux transferred to the working medium (air) by the heated surface of the absorber to the energy flux delivered to the device in the form of radiation. In general form:

$$\eta =\frac{Q}{I·A}[-]$$

(1)

where:

Q [W] is the thermal power of the collector determined from the following relation:

$$Q=m·c_p·\left(T_{out}-T_{in}\right)$$

(2)

and:

I [W/m²]—measured average radiant energy flux delivered to the collector,

A [m²]—is the collector aperture area.

In the Formula (2) it was assumed that:

T_out [K]—is the average temperature of the air leaving the collector,

T_in [K]—is the average temperature of the air at the entrance to the collector (this temperature was equal to the temperature of the air around the collector),

c_p—is the specific heat of the air—c_p = 1.006 kJ/(kgK),

$\dot{m}$ [kg/s]—is the air mass flow rate through the collector.

Air mass flow rate $\dot{m}$ was determined using the relationship:

$$\dot{m}=\rho ·\stackrel{-}{w}·\frac{\pi d_{in}^2}{4}$$

(3)

where:

ρ [kg/m³]—air density calculated from the Clapeyron equation for air parameters at the entrance to the collector inlet duct,

$\stackrel{-}{w}$ [m/s]—average air flow velocity in the collector inlet duct,

d_in [m]—diameter of the inlet duct collector.

3. Data Reduction

3.1. Air Temperature Increase in the Collector

Figure 4 shows the change in the temperature of the air flowing through the collector. It was noticed that regardless of the degree of inclination of the collector, the increase in the air temperature was at the same level and depended only on the average value of the thermal radiation intensity. The increase in the air temperature in the collector was the greater the value of the average radial intensity reaching the collector. The maximum value of the increase in air temperature reached 35 K with an average radiation intensity of I = 550 W/m².

It can therefore be concluded that the amount of energy reaching the collector and absorbed by the absorber was independent of the spatial orientation of the collector. This was due to the location of the radiators in relation to the air solar collector. Their position in relation to the collector did not change (they deflected together with the collector), so the amount of absorbed energy converted into heat and transferred to the flowing air depending only on the current power with which the radiators worked.

3.2. The Velocity of the Air Flow through the Collector

The graph in Figure 5 shows the results of the velocity measurement in the collector inlet channel. The points represent the average value of the velocity in the inlet section of the duct as a function of the average radiation intensity, for different values of the collector angle. The higher the radiation intensity, the higher the velocity of the air sucked into the collector. The maximum flow velocities were obtained for the inclination angle ψ = 80°. The maximum average velocity at the entrance to the collector inlet duct was $\stackrel{-}{w}$ = 0.95 m/s (at the radiation intensity I = 460 W/m²), which corresponded to the airflow rate of 43 m³/h. Figure 5 shows that the collector’s deviation from the vertical by 10° (ψ = 80°) may have resulted in an increase in the airflow velocity by about 10% compared to the value obtained for a vertically positioned collector. The reason for such air behavior may be the detachment of the liquid stream from the surface of the absorber in the upper part of the collector and its free flow toward the outlet channel. Confirmation of this effect requires flow visualization using CFD methods.

Further inclination of the collector resulted in a significant decrease in the air flow velocity. This may be due to the fact that as the manifold is tilted backwards, both the inlet and outlet ducts began to line up vertically. This was particularly important in the case of the exhaust duct, where the warm air leaving the collector was forced to move downwards, i.e., against the convective buoyancy forces that were the driving force of the movement.

3.3. Collector’s Heat Flux

Figure 6 shows the change of the collector’s thermal power depending on the average radiation intensity and the level of the collector’s position relative to the horizontal. Depending on the collector’s angle, the air flow velocity was reflected in the power of the air collector. Again, the highest values were obtained for the collector set at an angle of 80°. The maximum value of the thermal power was Q = 420 W, already for the average radiation intensity of the order of I = 460 W/m².

As shown in results from earlier analyses, the change of the collector thermal power depending on the inclination angle was a consequence of changes in the airflow velocity at the inlet to the collector. A change in the average air velocity resulted in a change in the airflow rate inside the collector and connection pipes. Airflow was caused only by changes in its local density, which was a function of the temperature. As shown in Figure 4, the change in air temperature did not depend on the angle of inclination of the collector, so there was an assumption that the orientation of the connection stubs relative to the direction of gravity influenced the flow of fluid and, consequently, the thermal power of the collector.

3.4. Collector Efficiency

Figure 7 shows the characteristics of the collector efficiency as a function of the average radiation intensity and the collector inclination. It was noticed that as the average radiation intensity increased, the efficiency of the collector asymptotically tended to a certain maximum value. The highest efficiency levels were obtained for the collector set at an angle of 80° to the horizontal. The efficiency achieved in laboratory conditions for this collector setting was η = 53%. Placing the collector in a vertical position resulted in a deterioration of efficiency by more than 10% compared to the maximum values in the entire radiation intensity range.

As follows from Formula (1), the efficiency of the collector depends on the energy supplied to the collector in the form of radiation (including heat), which in the experimental conditions is not dependent on the spatial orientation of the collector. The next component of the equation was the thermal power Q obtained during energy conversion and transferred to the air. It was this value that depended on the orientation of the collector and resulted in changes in the efficiency of the collector. These changes were caused not by changes in the local density of the fluid, as these were identical regardless of the inclination of the collector, but the mass flow rate of the air, which was a consequence of the path they had to pass, especially in the connection channels of the collector. The deflection of the collector caused these channels to be set vertically, making it difficult for air to flow through the entire structure. It has been shown that the air solar collector of the proposed design achieves the highest efficiency when it is tilted backward by an angle of 10°.

4. Summary and Conclusions

Experimental tests in laboratory conditions were carried out on a prototype structure of a flat air solar collector. It was assumed that the air collector would be installed in a vertical position on the southern façade of the building. The task of the collector was to heat the air in the room directly behind the wall, therefore, the collector was equipped with stub pipes whose length (L = 500 mm) corresponded to the width of the building partition. The tests were carried out in laboratory conditions, assuming no external influences, e.g., wind. The collector worked in conditions of natural convection with the radiation intensity reaching 550 W/m². The collector with dimensions of 2080 mm in height, 1040 mm in width and 180 mm in depth with two connection pipes (d_in = 110 mm—inlet duct and d_in = 130 mm—outlet duct) was set at the angle ψ with the value of 45°, 60°, 70°, 80° and 90° (vertical) to the horizontal. An artificial radiation source placed 3 m from the collector emitted waves perpendicular to the collector surface. As a result of the experimental research, it was found that:

• an increase in the temperature of the air flowing through the collector depended on the radiation intensity (ΔT _max = 35 K for I = 550 W/m²) and did not depend on the collector’s inclination;
• deflection of the collector from the vertical changed the air flow rate through the collector while maintaining the other parameters unchanged;
• tilting the collector backwards by 10° resulted in a 10% increase in the velocity of air flow through the collector. Further deflection of the collector backward caused a decrease in the value of the air flow velocity, regardless of the radiation intensity;
• further tests (including CFD) are necessary to check the reasons for the increase in the air flow velocity through the manifold when it is deviated from the vertical and what effect the length of the connection pipes–inlet ducts has on the value of the optimal angle of the collector deflection.