Several factors should be considered in selecting a proper panel for the cultivation of microalgae: 1) available system types of microalgae cultivation; and 2) the main purpose of the selected panel, which might be employed as a building façade or a double-glazed window. Generally, there are two main types of cultivation system such as open-air ponds and closed PBRs (see [27, 31, 32] for more details). The latter type is of interest herein because such a system could be employed in the building façades and the double glazed windows, which is the ultimate aim of this research. It can be noted that the first two factors mentioned above closely rely upon each other. The attempt made in this study is to fabricate a new regional compatible PBR-type panel that suits Iran cities. The regional compatibility of those panels could be achieved by employing a standard window dimensions industrially produced in Iran, along with using a native microalga in order to facilitate preparing the required cultivation inputs.

The PBR panel examined in the present work is akin to the double glazed window, whereas in the PBR panel, the enclosed space between two glasses is filled with cultivation of microalgae instead of Aragon gas. The standard width and height of a residential building window are ranged from 40 to 90 cm, and from 100 to 180 cm, respectively [33]. The minimum sizes (i.e. 40 cm × 100 cm) of a window are selected for saving in effort and expense of the experiments. Moreover, based on the recommendation provided by Hormozgan Construction Engineering Organization [33], the standard thickness of the space enclosed by two glasses could be considered as 8, 12, 16, 20, and 24 mm in the double glazed window. In this regard, Degen et al. [34] depicted that the PBR panel with thickness of 15 mm would be more productive rather than the other thickness values. Therefore, by comparing those values recommended by Hormozgan Construction Engineering Organization [33] and Degen et al. [34], the culture layer thickness of 16 mm is selected. It is worthy of notice that, by applying such a thickness value in producing the PBR panel, the light zone would be proportional to dark zone. In addition, in designing the PBR panel, the surface area to volume ratio is a paramount parameter because it can optimize the performance of panel by increasing the panel area exposed to light as well as reducing the travel distance of light through the panel [35–37]. In the other words, absorption of light has a considerable effect over an efficacy of the PBR panel performance.

There is a variety of microalgaes which can be used to sequester the CO₂ [38, 39]. Various factors can effectively contribute to better selection of the proper microalgae for the purpose of the current study. The primary question should be addressed here is that, to account for an economical manufacturing the proposed PBR panel, how the general production expenses could be considerably decreased. These expenses consist of costs that should be paid for preparing the panel structure, sufficient provision of the microalge, and nutrients required for feeding the microlage. Due to using the standard panel as a double-glazed window, the first mentioned expenditure may be unalterable. Moreover, the last one (i.e. the nutrients) depends on the type of microalgae would be selected for cultivation. Hence, the type of microalgae should be carefully selected to afford the optimized PBR panel. Consequently, an economical production of the PBR panel would rely on the accessibility and productivity of the microalgae as well as its cost. To obtain the aforementioned objective and fulfill indigenous conditions of the studied region for microalgae cultivation, Chlorella Vulgaris species is selected. It should be noted that this species could meet the requirements for growth in the seawater of Caspian Sea by applying some treatments [40]. Furthermore, the Caspian Sea is not far away from the area of interest. For the sake of convenience and reduction in expenses, the mentioned species is provided by the Algae Biotechnology Center of Persian Gulf, Bushehr, Iran.

As noted earlier, light absorption is a vital parameter in microalgae cultivation. Hence, a crucial need for preparing an optimized condition for light absorption should be fully met. In this regard, the intensity of light over the cultivation medium should be controlled because if this intensity increases, large amounts of microalgae cells will be died, on the contrary, a serious shortage of light could be led to decrease of microalgae photosynthesis and CO₂ absorption as well as productivity of the PBR panel [41, 42].

An aqueous solution with 25 ppt salinity is developed by combining the urban water and the sea-salt. This solution was also used by Emad abadi et al. [43] to cultivate the Chlorella Vulgaris. A working volume of the PBR panel culture medium is five liters. Under sterilized conditions, the working culture medium is prepared by mixing the abovementioned aqueous solution of 4 L and 1 L the stoke solution of Chlorella Vulgaris.

The PBR panel is set up as a window of a building located at a highly air-polluted region of Tehran (i.e. capital of Iran). A schematic representation of the PBR panel used as the double-glazed window is shown in Fig. 1. In addition to those details are drawn in Fig. 1, an air compressor is employed to pump the polluted air uniformly from outdoor environment into the culture site by using an air controller valve and an electronic flowmeter. The pumped air is fed by a bubble blower embedded at the bottom of window. Those bubbles ascend thorough the cultivation media and then the process of photosynthesis is conducted by the Chlorella Vulgaris. During this process, CO₂ is absorbed and oxygen is produced by the noted microalgae. A void space of 30 cm height above the surface level of microalgae cultivation solution is considered to maintain the produced oxygen gas, which is collected by a gas oxygen sucker and then is released into the air. Moreover, there is a possibility of entering a fraction of this oxygen gas into the photobioreactor using a setting device. Therefore, the dissolved oxygen in the cultivation media is collected by the soluble oxygen sucker. Additionally, the release process of the dissolved oxygen is identical to the corresponding process mentioned above for the oxygen gas.

The users of the PBR panel should deal with the issue of sedimentation of biomass in the course of cultivation process. Here, an air injection mechanism from the bottom of PBR panel is employed to produce bubbles, which cause floatation of the microalgae and, in turn, preventing from sedimentation of biomass.

The present section is devoted to the performed tests and their results. The proposed green window was evaluated in two different sites and conditions. The most important difference of those conditions regards how the PBR panel was installed and located. The first set of tests was carried out on a the PBR panel installed as a double glazed window located in a wall opening, whereas in the second one, the noted PBR panel was examined in a laboratory condition in which the panel was placed on the floor because the required wall opening was not available. Moreover, it should be mentioned that the first site was located in Tehran city, while the laboratory implemented as a new site for conducting the second set of tests, was located in Babolsar city. Moreover, there are two main causes for testing at the second site (i.e. the laboratory in Babolsar city) where the current research was followed up: 1) limitation of time and place, to accomplish the objectives of the tests in a certain period of time; 2) limitation in accessing an apparatus to measure the intensity of light passing through the studied panel. However, in comparison with the second site, it must be acknowledged that the first place of tests was in further agreement with the practical situations. In the following subsections, first the results of the tests performed in Tehran will be addressed and then laboratory measurements of Babolsar city will be presented.

Table 1 shows the data measured regarding the input, output and absorption amounts of CO₂ during the six days according to the proposed methodology. Furthermore, at the same time, temperature of input air to the cultivation medium along with evaporation from the aqueous solution are reported in Table 1. Such additional records were used to describe in more detail the conditions under how cultivation was performed during all the time of test.

The first measurements of those noted parameters (i.e. at 10 am of first experiment day) were conducted using the aqueous solution (which had volume of 4 L) in the absence of the stoke solution (which had volume of about 1 L) of Chlorella Vulgaris. The mentioned measurement led to obtaining the input and output CO₂ amounts of 1091 and 962 ppm, respectively (Table 1). This means that the PBR panel absorbed 11.8 percent of the CO₂ without adding any stoke solution of Chlorella Vulgaris to culture medium. In the following stage, 1 L stoke solution of Chlorella Vulgaris was immediately added to the aqueous solution (this means that mixture of the aqueous solution and the stoke one in 4:1 (v/v) ratio) and three hours thereafter (i.e. at 1 pm of first experiment day) the measurements of interest were conducted. In this stage, the CO₂ absorption decreased from 11.8 to 6.1 percent. Perhaps such a reduction would be due to shocking of microalgae arising from their settling down to the new conditions. The data shows that absorption of CO₂ was insignificantly increased to 7.1 percent at 4 pm of first day of experiment.

In the second day of experiment, the CO₂ absorption percentage was notably increased to 24.5 at 10 am. This may be due to matching up the microalgae and their culture medium along with improving efficiency of the system over the time. Table 1 indicates that at the third day, at 1 and 4 pm, the green window absorbed a greater relative amount of CO₂ than its preceding two days. Thus, it could be concluded that the microalgae grew progressively. At the next day of test (i.e. fourth day), relatively similar amounts of CO₂ absorption were obtained for measuring at 10 am and 1 pm, compared to the results of the third day. However, at the 4 pm of fourth day, the CO₂ amount absorbed by the green window (6.5 % absorption) was lower than the corresponding value for third day (CO₂ absorption of 13.8 %). A noteworthy increase of the CO₂ absorption over the last two days of experiment (i.e. the fifth and sixth days) demonstrates a considerable improvement in the efficiency of the green window.

By comparing the amounts of CO₂ which were absorbed by the green window at 4 pm (in the middle of dark period) in each day with the corresponding value for 10 am (end of lighting period), it could be noted that, in the presence of light of the fluorescent lamp, the microalgae contribute more in absorption of CO₂ during the photosynthesis. Such an issue was due to the respiration of the microalgae during the dark period and also evapotranspiration. However, because of using the closed PBR here, the latter could be considered as of no importance. In addition, it can be clearly observed from Fig. 2 that the average percentage of CO₂ absorption per day generally increased from 6.6 to 45.4 % in the course of experiment. Nevertheless, the aforementioned figure depicts that this parameter interestingly decreased at the fourth day of test.

As depicted in Fig. 3, the growth of microalgae results in lowering the transparency of the green window as well as its color over the time. Such a color variation can demonstrate a wide range of color spectrum due to growing of microalgae and increasing its cell density. This issue can be employed to achieve three different applications by using either the green window proposed here or other forms of PBR. These applications include: 1) From the perspective of urban landscape design, the building façades can be designed by organizing the green windows having the different elapsed times from the start of cultivation process in order to construct a color spectrum towards different directions (see Fig. 4) as well as having a dynamic color over the time (regarding Fig. 4, it should be noted that the changes in color of the windows are predicted using the fitted polynomial equation which will be presented in Fig. 6. This is done based on the assumption that there is a relationship between color spectrum and optical density of cultivation medium); 2) Utilizing the green window as an apparatus for producing shade in regions with hot and dry climate (such a shade could decrease the indoor temperature of building); 3) Decrease of the window transparency can lead to limiting the visibility (this issue is compatible with the Iranian culture regarding staying concealed the inhabitants’ privacy from non-mahram men). Moreover, it is worthy to emphasize that there are other applications of PBRs such as an open microalgae PBR as an urban fountain, biofilters over the buildings in improving the air quality, bioinsulator in decreasing the noise, artificial trees in urban spaces, closed circle PBRs for parking canopies, curtain-wall PBRs, and algae biofaçades [31]. Fig. 5 illustrates a microalgae biofaçade that is designed based upon the color spectrum of indoor light which is produced by the stained window in the Iranian houses.

As can be seen in Table 2, the input CO₂ of this stage of experiment was observed as 611 ppm where 11 ppm was absorbed solely by the initial aqueous solution. Moreover, the light intensity passed from the green window was measured as 950 lux. This value demonstrates that the initial aqueous solution could decrease light intensity of 50 lux due to its opacity. In the next step, after adding the 0.5 L stoke solution of Chlorella Vulgaris at the start point of light period, data shows that the culture medium produced CO₂ by 31.1% (i.e. 190 ppm). As earlier described, settling down of microalgae to a new condition would led to their shocking.

Hereafter, the data were recorded every 12 h at the end of light and dark periods. After 12 h (i.e. end point of light period for first day), 27 ppm CO₂ was absorbed by the culture medium (Table 2). Moreover, the temperatures of input and output air were 23.8 and 24.4°C, respectively. This increase of 0.6°C in temperature demonstrates that the photosynthesis of microalgae is an exothermic process. Such an issue would be carefully considered in providing the thermal comfort of buildings. In comparison with data of the prior stage, second measurements recorded at 24 h after adding the stoke solution (end of dark period) indicate that the CO₂ absorption has been approximately doubled (63 ppm). The trend of measurements observed in the present work is in good agreement with the described general phases of algae growth in the literature. (see [44] for more details).

To assay the growth rate of the Chlorella Vulgaris species, the turbidity technique has been utilized in the Babolsar site. In order to find the proper time of balanced cell growth by this method, the standard curve in seven dilutions of main stock (i.e. 1:1, 1:2, 1:5, 1:10, 1:20, 1:50, 1:100 stock solutions) was developed. This curve was calculated based on the optical density in order to gain the cell numbers of each reading. As it is prevalent in the turbidity technique, an optical density of the microalgae cultivation medium was measured by a spectrophotometer. All of the readings were carefully performed on the proper λ_max which was calculated as a suitable wavelength for reading to get an approximate cell numbers. In accordance with the results of this test, the optical density of 0.033 for the wavelength of 540 nm was obtained.

Fig. 6 provides information on the variation of the optical density as a function of cultivation time. In this figure, it can be seen that, the optical density increases slightly until the cultivation time of 72 h. It then rises dramatically by almost 150% from the time of 72 h to about 192 h; thereafter, it declined suddenly by less than 10% until the end of experiment. Hence such a variation indicates the dynamic characteristic of visual transmittance of the green window over the time. Moreover, equation of the fourth-order polynomial curve fitted to the data points is shown in Fig. 6. This equation depicts the optical density variation as a function of cultivation time for results of tests carried out in the laboratory located in Babolsar.

According to the results reported here, this technique may be a proper method to measure the amount of transmitted light through the proposed green double-glazed window. It is worth, however, that the aforementioned amount can be varied through the entire life cycles of microalgae (i.e. the growth of microalgae) in the cultivation medium because these cells can cause absorption and scatter of light. Such a feature in glazing systems can be employed to control the visual transmittance of those systems under the different seasonal conditions. Furthermore, the color of glazing can play an important role in the satisfaction of occupants in buildings with various functions such as residential, official, commercial, and educational buildings, and those for religious purposes. With regards to the capability of the proposed green window in changing both the visual transmittance and color of glazing systems, it should be noted that these characteristics could significantly affect several issues such as working performance, mood, alertness and sleepiness, and visual satisfaction for the occupants [45]. Umdu et al. [46] investigated the ability of the façade-integrated PBRs to act as an effective insulation system. Therefore, the noted green window could also be utilized to improve thermal performance of buildings.