In our experimental research, a commercial solar collector, SH 1500G, was used, glazed to reduce the losses by radiation and convection, and thermally insulated to avoid heat dissipation. This type of collector is a double pass with baffles to create flow turbulence and to increase the absorber surface; the same technology is also used in [12,13]. The casing and the absorber material are extruded aluminum. The collector has, on the upper side, a 14 W photovoltaic panel to deliver energy to the small, variable speed axial fan placed on the upper back side, to hasten the exit of warm air. The air collector’s surface is 1.9 m², and 0.5 m² for the photovoltaic panel. The air enters at the bottom of the panel and exits through the upper side via ducts with a diameter of 127 mm. The unit can be controlled from a thermostat. The operation of a solar collector can be done with optimal control of parameters in order to maximize the energy, and ensure thermal comfort, by stopping the system from overheating.

Romania’s geographical location, taking, as reference, the city of Bucharest, located at 44°25' North latitude and 25°6' East longitude, was taken into account in establishing the incident solar global radiation, at an angle of 50° for the winter months, and 35° for the summer months. Firstly, a proprietary software to establish the radiation on a tilted surface, was used to set I_g, based on time and atmospheric temperature, T_a. Temperature and radiation are variable over time, and they were obtained from measured values every 10 min for January and July. Values are shown in Figure 1a and Figure 2a, and they were compared with the values determined by the application JRC PVGIS [14], pictured in Figure 1b and Figure 2b. I’_g and T’_a are interpolated values with sixth degree polynomials. I’_g from Figure 1a,b have similar values in January. I’_g from Figure 2a,b are between 800 and 1100 W/m² in July, for the time intervals of 10–15.

The SH 1500G solar collector, see Figure 3a, was tested in the Polytechnic University of Bucharest, in the Hydraulic Laboratory. The experimental stand is mobile, and has the ability to modify the inclination angle of the panel, as shown in the chart in Figure 3b.

We measured the ambient temperature—inlet air temperature, the outlet temperature, the solar radiation, outlet air velocity, and the time intervals. For the heating process, the collector was exposed to the sun, for the cooling process the mobile stand was moved to the shade, with the fan continuously functioning.

The solar radiation was measured with a Voltcraft PL 110SM portable pyranometer (Conrad Electronic, Berlin, Germany), with accuracy ±10 W/m², the temperature was measured with a laser thermometer Fluke 576 (Fluke Corporation, Everett, WA, USA) (accuracy ±0.5 °C) and the air speed leaving the collector was measured with an anemometer (accuracy ±3%).

In order to estimate the heat flux delivered by the solar air collector the following equation was applied: (1) Q = c p ⋅ m ⋅ Δ T = c p ⋅ m ⋅ ( T fout − T fin ) where c_p = 1008 kJ/kgK.

In Equation (1), m is the mass flow rate in (kg/s), given by Equation (2): (2) m = ρ ⋅ V ˙ = ρ ⋅ π ⋅ d 2 4 ⋅ v The efficiency of the solar collector was calculated with the following formula: (3) η = Q I ′ g ⋅ A c

The atmospheric temperature, equal with the inlet air temperature, the outlet temperature, solar radiation, outlet air velocity, and the time intervals were measured. The heating characteristics of the collector, the air mass flow rate, the heat flux, the variation of the temperature, and, finally, the efficiency of conversion of solar energy into thermal energy, were calculated. For the cooling process the steps were the same, moving the collector to a shaded area, with the fan in operation.

The results were centralized in Figure 4 and Figure 5: heat fluxes and outlet temperatures in the heating process (a), and in the cooling process (b), in July, and the same features on a day in January, Figure 5a,b. In addition, relationships between the solar global irradiation, the variation of temperatures, and outlet air velocity, were established, shown in Figure 6.

In the heating process, a stable functioning was found, representing the optimal operating point of the solar air collector, after about 50 min in summer, and 70 min in winter. In terms of cooling, it was done over a longer time in summer and in a shorter time in winter.

Figure 6 illustrates the increasing of the velocity of warm air, to the exit, from the collector, simultaneous with the increase of solar radiation, and the variation of air temperature, simultaneous with the decrease of the density of air.

The air velocity at the collector outlet could not be measured for solar radiation below 200 W/m² as the solar powered fan did not operate. From the mathematical point of view, a linear variation between I_g and v, respectively, ΔT and v, of the form Equations (4) and (5), setting intersection with axes, can be found: (4) I g = 343.5 ⋅ v (5) Δ T = 13.057 ⋅ v

Figure 7 illustrates the efficiency of the solar collector versus solar radiation I_g, and performance variation over time, to achieve the optimal operating point. From interpolation, Equation (6) is obtained, which can quickly determine the efficiency of the air collector when solar radiation is known: (6) η = − 9 E − 8 I g 3 + 0.0001 I g 2 − 0.0078 I g

Similar results regarding efficiency were found by Morson [10]. The results are better than those presented by Visagavel [15], with 10%–12% due to the type of collector used in present experiments—double pass with baffles in galvanized aluminum. In Visagavel’s experiments [15], a galvanized iron and single pass collector with a maximal efficiency of about 38% was used. In our experiments the collector eficiency is over 50% at the optimal operating point.

In summer, to evacuate the warm and polluted air from industrial or residential buildings, three computational models can be used to calculate natural ventilation: first, taking into account the internal thermal pressure, second, with the influence of external air currents, and, third, taking account of the heat transfer inside of solar collectors, located on the roof of the building.

Use of the solar air collectors is an efficient solution to induce supplementary natural ventilation. In this case, the mathematical model estimates the airflow rate through the tilted solar collector with a heat-absorbing material.

The main parameters are: the absorbing material and the glass surface temperature, the inlet and outlet air temperatures, the atmospheric temperature, the mass flow rate, the collector’s surface, and the openings surfaces.

The mathematical model consists in thermal balance equations at the passing of the solar radiation through the glass cover, heat transfer to the air between the glass cover and the absorbing material, and through the channels of the absorbing material, in the following assumptions: laminar flow in the air channel; one-dimensional processes; the air inlet temperature is equal to the atmospheric temperature; and heat losses are neglected.

Using Equations (4)–(6), for different values of solar radiation, the air velocity, the variation of temperature delivered by the collector, and its efficiency, can be obtained.

The equation for the energetic balance is: (7) Q = η ⋅ I g ⋅ A c

With heat flux: (8) Q = A 0 ⋅ v ⋅ ρ ⋅ ( T f o u t − T f i n ) ⋅ c p where A₀ is area of the outlet section (m²).

Area of the solar collector is calculated, equaling Equations (7) and (8), and results: (9) A c = A 0 ⋅ v ⋅ ρ ⋅ ( T f o u t − T f i n ) ⋅ c p η ⋅ I g = l ⋅ L

By the increasing of ΔT, the air’s density decreases and results a new mass flow rate, from Equation (2) and a new heat flux, from Equation (1). With this heat flux, a new iteration is performed to obtain/to verify the final A_c (Equation (7)).

As can be seen in the previous paragraph, for different global solar radiations I_g, air velocities v and conversion efficiency values are different. For a family residence of about 200 m³ living space, the surface of the solar air collector results are as listed in Table 1.

Optimization strategies in solar collector operation involve the limitation of the exhaust air velocity to about 2.5 m/s. Limitation is imposed by the axial fan that works with photovoltaic energy. The graph in Figure 8 shows the variation of the useful area for solar collectors, used in improving the ventilation of the building, depending on solar radiation. The temperature variation ΔT = T_fout − T_fin in three cases, 10 °C, 15 °C, and 20 °C, was considered.

The numerical results are summarized in Figure 8. As can be seen from the figure, for an average solar radiation of 500 W/m², ventilation of a house, of 200 m³, can be provided with about a 4 m² collector area, which means two solar panels of the studied model. For radiation of 700 W/m² or better, a single air collector is sufficient.

In Table 2, the same analysis is made for an office building, with about a 1000 m³ workspace, and results:

As can be seen in Figure 9, for radiation of 500 W/m², to achieve a difference of temperature of 10 °C, a surface of 12 m²is required, which means six solar collectors. For a variation of temperature of 20 °C, 26 m² are required, which means 13 solar air collectors, to improve natural ventilation.

By introducing the concept of air change per hour, ACH, as in Visagavel [15], given by Equation (9): (10) A C H = V ˙ ⋅ 3600 V r with V_r (m³) the volume of the room/ventilated space, values for ACH were found, as in Table 3, compared to the values indicated by Mathur [20], Bassioiny [21] and Visagavel [15].