Multi-bandgap Solar Energy Conversion via Combination of Microalgal Photosynthesis and Spectrally Selective Photovoltaic Cell
Microalgal photosynthesis is a promising solar energy conversion process to produce high concentration biomass, which can be utilized in the various fields including bioenergy, food resources, and medicine. In this research, we study the optical design rule for microalgal cultivation systems, to efficiently utilize the solar energy and improve the photosynthesis efficiency. First, an organic luminescent dye of 3,6-Bis(4′-(diphenylamino)-1,1′-biphenyl-4-yl)-2,5-dihexyl-2,5-dihydropyrrolo3,4-c pyrrole.1,4-dione (D1) was coated on a photobioreactor (PBR) for microalgal cultivation. Unlike previous reports, there was no enhancement in the biomass productivities under artificial solar illuminations of 0.2 and 0.6 sun. We analyze the limitations and future design principles of the PBRs using photoluminescence under strong illumination. Second, as a multiple-bandgaps-scheme to maximize the conversion efficiency of solar energy, we propose a dual-energy generator that combines microalgal cultivation with spectrally selective photovoltaic cells (PVs). In the proposed system, the blue and green photons, of which high energy is not efficiently utilized in photosynthesis, are absorbed by a large-bandgap PV, generating electricity with a high open-circuit voltage (Voc) in reward for narrowing the absorption spectrum. Then, the unabsorbed red photons are guided into PBR and utilized for photosynthesis with high efficiency. Under an illumination of 7.2 kWh m −2 d −1. we experimentally verified that our dual-energy generator with C60-based PV can simultaneously produce 20.3 g m −2 d −1 of biomass and 220 Wh m −2 d −1 of electricity by utilizing multiple bandgaps in a single system.
In addition to the electricity generated from solar and wind power, biofuel is an attractive renewable energy source, especially for systems such as transportation, which require liquid forms of energy. The aquatic microalgal biomass is considered to be one of the best-suited feedstocks for this purpose, and it does not require arable land area 1,2,3. over, a wide range of the potential applications of the microalgal biomass (food, medicine, agriculture, etc.) makes it more promising in the view of marketability. However, to become economically viable for mass production, the fundamental issue of limited biomass yield, although one-order higher than terrestrial plants, must be resolved.
As a process to convert sunlight into a utilizable form, photosynthesis has a similarity to the photovoltaic devices (PVs); however, photosynthesis for terrestrial or microalgal biomass production suffers from the limited power conversion efficiency (PCE), approximately one order lower than that of PVs 4. restricting the productivity per area. In chlorophyll, which is to be considered a semiconductor with a Band-gap (Eg) of 1.78 eV, infrared (IR) photons below the Band-gap are not absorbed (53%) and the corresponding maximum electron flux is 1.2 × 10 21 m −2 s −1 under 1 sun, as shown in Fig. 1a. In this case, the maximum power conversion efficiency (PCE) reaches only 27%, and the excess energy of the absorbed visible photons is lost as heat. This PCE is lower than the maximum achievable value of 34% at the optimal Band-gap of 1.34 eV, called the Shockley-Queisser limit 5,6. over, in the photosynthesis process depicted in Fig. 1b, through photosystems (PSs) I and II, 48 photons are consumed to produce one molecule of glucose (C6H12O6) with a chemical energy 29.8 eV. The corresponding maximum photosynthesis efficiency (PE) becomes [(1.2 × 10 21 m −2 s −1 ) × (29.8 eV/48)/(1000 W m −2 )] ~ 12%. In reality, as not all the absorbed photons enter the PSs, it is known that at least 57 photons 7,8,9 are consumed per glucose molecule, and the maximum PE becomes ~10% (assuming no loss during the conversion from glucose into real biomass consisting of diverse molecules). While diverse single-junction PVs, such as GaAs (Eg ~ 1.4 eV), crystalline silicone (c-Si, Eg ~ 1.1 eV), and perovskite (Eg ~ 1.5 eV).based PVs, have achieved high PCEs of more than 70% of their theoretical limits, as shown in Fig. 1a 10. the PEs of outdoor microalgae cultivation under sunlight are reported to be only ~4% for photobioreactor (PBR) systems 11,12,13,14,15 and 3- to 5-fold lower for open pond systems 3 in warm locations, far below their theoretical limit, implying significant opportunity for further technical improvement. It should be noted that some of the previous reports 13,14 chose a different definition of PE and did not count IR photons as inputs.
Results and Discussion
Spectral conversion: bioreactor adopting luminescent materials
While most previous reports on the spectral characteristics of microalgal photosynthesis have been based on light emitting diode (LED)-based environments, the quantity and quality of LED were significantly different from the sun and the results may not be applicable to outdoor conditions. For this reason, adopting the appropriate illumination, which mimics the sun in a controllable manner, is crucial to study the optical behavior of an aquatic photosynthesis system in outdoor conditions. Accordingly, we implemented an artificial AM 1.5 G light source, integrating optical filters with a white metal-halide lamp. As shown in Fig. 2a, despite the sharp fluctuations of the measured spectrum of our light source, the integrated proportions of visible photons (400–500 nm, 500–600 nm, and 600–700 nm) roughly matched the AM 1.5 G spectrum, with an error range of 5–15% 9. over, periodic boundary conditions (PBCs) were applied to simulate large-scale cultivation using smaller bioreactors with limited illumination area. A reflecting metal (aluminum foil or stainless-steel walls) on the sides can prevent energy efflux from inside to outside and energy influx from outside to inside, as depicted in Fig. 2b. In the optical view, the system is identical to the large-scale reactor, where the influx and efflux of light can be compensated. The illumination area was confined by the aperture to strictly define the input energy and prevent overestimation.
Under simulated solar illumination, we examined the change of the algal growth rate by adopting a spectral conversion material. For this experiment, we chose an organic dye, 3,6-Bis(4′-(diphenylamino)-1,1′-biphenyl-4-yl)-2,5-dihexyl-2,5-dihydropyrrolo3,4-c pyrrole.1,4-dione (D1) 37. as a spectral conversion material, which absorbs photons with wavelengths shorter than 550 nm (i.e. blue and green light) (peak at 495 nm) and emits photons with a photoluminescence (PL) peak at 618 nm (i.e. red light), as shown in Fig. 2c. Such spectral characteristics of optical absorption and luminescence are well-matched with the optical properties of microalgae. While the typical red luminescent dyes show a limited quantum yield of 10–20% in the film state, mainly due to the aggregation and π–π stacking interactions of the molecules in the solid state, D1 was designed to minimize such quenching loss in the film using the phenomenon of aggregation induced emission (AIE) 38,39. The measured quantum yield of D1 was 24.7% in the film state on a quartz plate 40. and such outstanding optical properties make D1 an ideal candidate for our proposed system.
Figures 2d and S1 show the results of the microalgae (Chlorella sp.) cultivation with and without D1. The light intensity was set to be 0.2 or 0.6 sun with continuous illumination, corresponding to 400 and 1200 μmol m −2 s −1 of visible photons, or 4.8 and 14.4 kWh m −2 d −1 of energy. D1 was coated on the polycarbonate cover of the bioreactors with a volume of 500 ml and illumination area of 56 cm 2. The results of the duplicate reactors were compared with another duplicate reference reactors without dye coatings, as depicted in the inset of Fig. 2d. From their average values, we found no evidence of improved biomass production from the experiments using spectral converting materials. While the average growth rates were already high for the reference reactors (0.41 and 0.49 g L −1 d −1 ), with the aid of a high-density photon supply (0.2 and 0.6 sun, respectively), the coating of D1 failed to add productivity and yielded even lower growth rates (0.39 and 0.47 g L −1 d −1. respectively).
Our results contradict previous research 11,27,28,29,30,31,32,33,34 that reported enhancements in the biomass productivity by modifying the quality of incident illumination. We speculate that these discrepancies may be due to our different illumination conditions, that is, simulating sunlight. First, organic dyes tend to degrade easily by photooxidation under strong illumination 41. We observed that the color of D1 after the cultivation was not as dark as the initial state, suggesting degradation. Second, even with identical systems, the light intensity can produce different results. Typical LED light sources have a photon flux far below 0.1 sun and spectra that poorly match chlorophyll absorption spectra. So, the additional illumination with converted red photons may help to enhance the photosynthesis rate. On the other hand, the artificial sunlight we used was as strong as outdoor illumination and contained a sufficient number of red photons to saturate the photosynthesis. Hence, the photon supply may not be a limiting factor for photosynthesis. In previous research 11. while up to a 40% enhancement of photosynthesis efficiency was achieved with spectral conversion under 0.05 sun, no enhancement was observed in the same system under 0.15–0.2 sun.
Materials and Methods
Synthesis and characterization of D1
Refer to SI and literature 37,55.
Microalgae cultivation with spectral conversion material
Chlorella sp. was used for the luminescence experiments. The seed culture was inoculated into a photobioreactor containing 500 ml of BG-11 medium with an illumination area of 56 cm 2 with initial dry cell weight (DCW) of 0.15–0.25 g L −1. It was then cultivated under the following conditions: shaking at 110 rpm, temperature of 32 °C, 0.2 or 0.6 sun of light intensity, and 2% (v/v) of CO2 supplementation with 0.4 vvm. The cultivations were prepared in duplicate. The biomass concentration was monitored by measuring the optical density (OD = −log10 Transmission) and dry cell weight. In particular, the OD was measured at 680 nm with a UV-Vis spectrophotometer (DR 5000, HACH), and the dry cell weight was obtained by Standard Methods 56.
PV fabrication and characterization
The current density (J)-voltage (V) curve was obtained from the full device (area ~0.15 cm 2 ) of glass/indium tin oxide (ITO, 75 nm)/MoO3 (10 nm)/tetraphenyldibenzoperiflanthene (DBP, OSM, Korea):C60 (99.9%, OSM, Korea) (1:9, 50 nm)/C60 (10 nm)/bathocuproine (BCP, 99.9%, Lumtec, Taiwan, 7 nm)/Ag (150 nm). The device was fabricated by sequentially depositing MoO3, DBP:C60, C60, BCP, and Ag layers onto a pre-cleaned ITO substrate in a vacuum chamber (J-V and EQE characteristics of this device were measured using a K201 LAB55 (McScience, Korea) solar simulator and a K3100 IQX (McScience, Korea), respectively. The PCE was obtained at the point of maximum power (Pmax = MAX(−J × V)) in the J–V curve, where Pmax was 30 W m −2 at V = 0.70 V under illumination of 1 sun. The photocurrent density (Jph) of PV can be obtained by integrating EQE with the equation ∫ q EQE(λ) ΦAM1.5G(λ) dλ, where q is the elementary charge of 1.6 × 10 −19 C and ΦAM1.5G is the flux of solar illumination (# m −2 μm −1 s −1 ), equal to AM 1.5 G spectrum over the photon energy (hc/λ, where λ is the wavelength, h is the Planck constant of 6.62 × 10 −34 m 2 kg s −1. and c is the speed of light = 3.00 × 10 8 m s −1 ). Both the Jph and short-circuit current density (Jsc = JV=0) are shown to be the same (~6 mA cm −2 ) validating the confidence of our measurement.
Microalgae cultivation with spectrally selective photovoltaic cell
For the dual-energy generator, Chlorella vulgaris from the University of Texas (UTEX-265) was cultivated under CO2 aeration (5% v/v and 1 vvm) with a fixed volume of 500 ml, pH of ~7, and illumination area of 45 cm 2 in the water bath (27 °C). The reactors containing BG-11 medium were continuously shaken at 95 rpm. The biomass concentration was determined from the measured OD at a wavelength of 680 nm. The biomass concentration was calibrated by measuring the DCW, which was shown to be 0.52 × OD g L −1 (R 2 = 0.9991 for linear fitting). Deionized (DI) water was added at a rate of 16 ml d −1 to compensate for the evaporation of water and maintain the total volume. A heat value of 4.2 kcal g −1 of the biomass, which was measured using a calorimeter (Parr-1261, Parr Instrument), was used for calculating the PE.
Photosynthesis modeling
To model the photosynthesis that occurred in the dual-energy generator, a recently reported custom-made simulation 9. based on ray-tracing 54,57,58,59,60. was used to obtain the optical absorption profile inside the system. Subsequently, the photosynthesis profile was calculated using the model 49,50,51
where #Ph.(λ, x, z) is the absorption rate of photons per volume at a given position (x, z) and wavelength (λ), and Aaction(λ) is an action spectrum indicating the ratio of the photons used for photosynthesis to the total absorption, assumed to be the same as the value for Chlorella pyrenoidosa 16,17 reported previously 9. A biomass concentration (Cvolume) of 1.4 g L −1 was used for the calculation based on a typical concentration in the middle of the growth phase in the experiments. The maximum photosynthesis rate per biomass (Rmax) was the only fitting parameter, and we selected a value of 0.30 W g −1. yielding an overall biomass productivity of ~20 g m −2 d −1 under 7.2 kWh m 2 d −1. best fit to our experimental values 9. #Ph.(λ, x, z) was obtained by the 2D ray-optical simulation 9,54 with a measured light extinction coefficient of α = 2.1 cm −1. The effects of photoinhibition and change of lipid accumulation as a spectral response were not accounted for this model.
Algae powered solar panels
Billions of years before the invention of solar panels, algae was already harnessing the sun’s energy. Because algae has been optimized for light absorption through its evolution, the key to more efficient solar panels may be unlocked by working with algae.
That’s what a team of researchers from Yale University, Princeton University, Lincoln University and NASA banked on, and now the team has found a way to use algae to enhance organic solar panels.
The research was led by André Taylor. associate professor of chemical and environmental engineering at Yale, who runs the university’s Transformative Materials and Devices Laboratory.
Cheaper Alternative
When sunlight strikes an organic solar cell, electrons in the organic “active” layers pick up energy and begin moving that energy through the core of the solar panel.
The “active layer” materials currently used in solar cells are expensive and very rare. Diatom, on the other hand, is cheap and can be found almost anywhere. So figuring out how to harness diatom can help bring down the cost of organic solar panels.
To this end, the researchers worked with fossilized diatoms, which is also known as diatomaceous earth — a cheap, naturally recurring substance derived from dead algae.
Algae Nanostructures are Key
A common problem that occurs when manufacturing organic solar cells is that the active layers of organic material need to be thin, which both reduces their efficiency and can make it expensive to accomplish.
To solve this problem, the researchers developed a method for grinding the diatoms up into smaller bits, as the diatoms were initially too large to be placed in the active layer of the panel. They found that the electrical output levels stayed constant after their grinding method, even as the amount of material was reduced.
“We saw work by Jeremiah Toster et. al in Nanoscale where they implemented diatoms into dye sensitized solar cells and observed an enhanced power conversion efficiency,” said Lyndsey McMillon-Brown. a doctoral student in Taylor’s lab and co-lead author of the paper. “This sparked our interest as we were curious to see if diatoms could be successfully implemented into organic solar cells for an enhanced performance as well.”
What’s Next?
“We hope that this work will shed more light on the opportunity to utilize biomimicry or bio-inspired designs to solve engineering problems,” said McMillon-Brown. “Nature has developed many solutions that can be used to address many of our engineering problems – we just have to learn how to adapt and apply them.”
The researchers are keen on improving their technology and on finding a better-fitting strain of algae that they could use for higher performance.
“Moving forward we’re interested in optimizing the integration of diatomaceous earth into organic solar cells,” said McMillon-Brown. “In the future, we’d like to take care to select a species of diatoms that fit well into a high-performing polymer device.”
For more about this research, please see the paper published in Organic Electronics.
In addition to Taylor and McMillon-Brown, the study’s authors are Marina Mariano (co-lead author), YunHui L.Lin, Jinyang Li. Sara M. Hashmi. Andrey Semichaevsky and Barry P. Rand.
Microalgae and Their Intervening Role in Buildings’ Design
Microalgae are photosynthetic microorganisms which convert water and CO2 into organic compounds and oxygen using light energy. As the oldest resident of our planet, they have an important role in building our atmosphere. Due to their fast growth rate and high content of nutritional and bioactive compounds, they can be cultivated for various applications in bioenergy, cosmetics, pharmaceutical, agricultural, and food industries. Various factors including cultivation systems, the type of algal species, environmental conditions, and algae–bacteria interaction can affect the biomass productivity of microalgae and the biochemical composition. The algae theoretical efficiency of solar energy conversion to biomass is 9%, which is at least three times higher than the amount related to C4 plants. Microalgae are known as carbon mitigators due to their high capacity of CO2 sequestration by uptaking 1.8 kg of CO2 per 1 kg of biomass. The decarbonization and oxygenation (more than 75% of the oxygen needed is produced by microalgae) through photosynthesis could be a cost-effective and sustainable way to address global environmental issues [30,31,32,33].
Microalgae have a promising role in the bioremediation of anthropogenic pollutions to air, soil, and water. Their environmental benefits consist of atmospheric decarbonation through photosynthetic carbon capture, wastewater treatment through taking nutrients and organic wastes from the wastewater, and soil decontamination through biosorption and bioaccumulation of heavy metals. On the other hand, the use of agricultural land with a limited area capacity and a limited capacity of fuel obtained from agricultural products which form the basis of fuel products, makes algae more productive among other types of biomasses [34,35,36].
There are different systems for microalgae cultivation, including an open cultivation system, a closed cultivation system, or a hybrid of both. Open cultivation systems including different configurations of open ponds have the advantages of low initial and operational cost and low operational energy, but they need larger ground areas for light availability and are more susceptible to contamination, water evaporation, and unfavorable weather conditions. On the other hand, the closed cultivation systems, including different configurations of PBRs, have overcome these limitations by minimizing the required space and better controlling the growing conditions. However, they incur more costs and energy consumption for installation, operation, and maintenance. It is reported that PBRs yield 13 times more productivity compared with open raceways. The feasibility study of a microalgae-integrated system is vital for understanding the key cultivation technologies and environmental growth parameters such as nutrients, pH, temperature, light intensity, salinity, and carbon concentration, and also production conditions such as aeration, mixing, dilution rate, and harvesting frequency [37,38,39].
Recently microalgae-integrated buildings have been considered by architects and designers. The multifunctionality of microalgae, such as phycoremediation and the production of biomass as feedstock, offers advantages for integrating microalgae system into urban green buildings [40,41]. Microalgae can grow in different aquatic habitats and tolerate a wide range of environmental conditions. They become a part of building materials or service systems in a microalgae-integrated building. Due to their Rapid growth (a key factor of their superiority), microalgae can reach high densities and cover the façade in a short time. They also can be cultivated all year round [25]. By remediating wastewater coupled with capturing CO2, generating O2, and producing renewable energy potentials, they benefit the building and the occupants. They also reduce the building energy consumption due to providing effective shading in summer, solar heating in winter, and year-round daylighting penetration. These multi-functionalities and multiple benefits have caused various approaches to be introduced by architects, designers, and engineers for integrating algal systems in different scales of the built environment [42]. Evaluating different types of building envelope technologies in terms of two factors, namely, energy efficiency and compliance with the building, shows that algae bioreactor façades are in the first place in terms of energy efficiency. Biological energy generated in this system results in the reduction of environmental pollution as well as high efficiency. On the other hand, in terms of compatibility with architecture, the highest score goes to the algae bioreactor façade, showing that the algae bioreactor façade, among building-interactive technologies, should be considered a promising example [43].
Façade-Integrated Algae
The building-integrated microalgae cultivation system is an innovative technology for high-performance adaptive architecture that offers multiple benefits, including sequestration of CO2 and production of O2 to reduce the air pollution, conversion of solar radiation into heat and biomass, providing shading through changes in algal density, and creating sound insulation. It can also make a dynamic exterior vision due to the color changes of the algal culture [44]. Algae façade-integrated buildings as living designs are the result of applying biomimicry in architecture design and planning; they their required energy and water from their location and are adapted to their environment and climate. They also do not pollute the environment. The responsiveness of the building’s skin to natural factors including wind, rain, and sunlight, and vital functions including breathing, carbon capture, and water consumptions, should be considered in the biodesign of such buildings [45,46]. In algae façade-integrated buildings, the lifesaving sources of CO2 and nutrients for microalgae growth are obtained from occupants and building operations. A simulation study of PBR façade-integrated building showed a 13% reduction in CO2 level compared to a standard building with 200 occupants [27,47]. The high concentration of CO2 produced by occupants’ respiration and nutrients from domestic wastewater increases the biomass productivity. This symbiotic relation causes CO2 fixation, wastewater treatment, and biomass production for various uses [42].
The microalgae façade is a patent-pending system under development by XTU Architects for many years. There are some conceptual designs or built examples of a microalgae system in the area of architecture interventions. Table 1 summarizes some of these microalgae application in architecture interventions. Green Loop Tower (2011) is a project of retrofitting old building enclosures with green technologies. It is a proposed building intervention of the Marina City Tower where the parking deck and roof top are enclosed with microalgae systems for the purpose of CO2 reduction, wastewater treatment, bioenergy production, and net zero energy building (ZEB) [42,48]. Process Zero (2011) is a proposed office retrofitting, speculated for the General Services Administration in Los Angeles, where the building is enclosed with microalgae reactors with different densities depending on open view provision and solar availability [42,49]. Algae BRA (2011) is another concept proposal, speculated for a fashion company housing offices and commercial spaces. It is a proposed office building installed with external and internal tubular PBRs, presenting thermal regulation, passive cooling, decarbonation, biomass production, and flexible spatial organization [42,50]. The FSMA Tower (2011) is a project researching the integration of biological systems and skyscrapers. It is a speculative skyscraper enclosed with PBRs dispersed across the vertical surface, supporting social interaction and environmental benefits [42,51]. Algae Therapeia (2011) is a proposed research complex near the coastline, enclosed with PBRs as an external environmental skin to filter light, heat, sound, and air. It is a dome-shaped building, focusing on seawater and algae for medical, nutritional, and industrial usages [42,52]. UrbanLab (2012) is a speculative RD office building enclosed with plastic PBRs with the aim of developing the microalgae technology for biofuel production along with wastewater treatment. The project will be implemented with the participation of Ennesys, a French-based startup, and Origin Oil, a company dedicated to transforming algae into biofuels in La Defense, France. Using approximately 10,000 m 2 of bioreactor panels, it is expected that the building will be capable of reducing water usage by 80% along with an energy saving of 80% [42]. BIQ (Bio-Intelligent Quotien) house (2013) is a real-world application of flat PBRs installed on a residential building with the purpose of energy saving, carbon sequestration, and biomass production. The building is enclosed by microalgae glass panels on two sides. The solar energy stored in the panel and the grown algal biomass supply the renewable energy required for building operation. The installation sequesters approximately 16 kg of CO2 per day with a biomass production equivalent to 30 kWh/m 2 year and heat production of 150 kWh/m 2 year [13,42,53]. The CSTB Prototype (2014) is a technology demonstration project including bioreactor curtain walls installed on an office building in France and focusing on carbon sequestration and air quality improvement. This project is the first real-world application since microalgae façade experiments in 2009 experimenting with different configurations and density effects in daylight penetration [42,54]. In Vivo (2016) is another project with the purpose of attracting social attention and openness toward a sustainable city. It consists of three buildings; each has a unique façade integrated with different biological systems and functions. One grows microalgae for medical research with potential solar energy revival for heat supply and domestic hot water. In Vivo is a design competition-winning project and should increase the visual appeal of a building for advertising its environmental, economic, and social benefits, leading to increased acceptance for general use [42,55]. French Dream Tower (2018) is a speculative mixed-use project, combating glass towers’ environmental problems related to quick energy transmission and high energy consumption. It consists of towers enclosed with flat bioreactors for regulating solar energy and thermal insulation, collecting rainwater, and filtering outdoor air [42,56]. Algae Tower (2021) is an office tower in Melbourne, Australia, whose façade elements can be adjusted to the optimal sun angle to maximize shading and biomass production [45,57]. Microalgae Ivy (2021) is a patent-pending project for low-performing Windows retrofitting. Its full-scale prototype was installed at the School of Architecture at the University of North Carolina (UNC), Charlotte. It consists of a network of interlocking bioreactors, providing the possibility of cultivating different strains for different uses and aesthetics. The prototype demonstration filled with five strains ( Chlorella. Chlorococcum. Haematococcus. Scenedesmus. and Spirulina ) for energy efficiency, biofuel production, and indoor air quality enhancement was able to sequester CO2 produced by three occupants and output 500 g of biomass per day [42].
Algae-Powered Buildings: Energy Efficiency and Environmental Performance
One of the building energy efficiency indicators is energy use intensity (EUI), which explains the level of building energy performance and is determined by dividing total annual energy use by building. Comparing different buildings across energy efficiency is conducted according to this index. A lower EUI indicates lower usage of energy or higher building efficiency. An average primary EUI is around 120 kBtu/ft 2 /year and 200 kBtu/ft 2 /year for residential and commercial stocks, respectively [42]. Space heating and cooling, lighting, water heating, and ventilation consume more than half of the building’s energy usage. Major energy loss has been related to poor building envelope construction and inefficient HVAC systems. Other factors affecting energy consumption include the geometry of the building, energy characteristics of opaque walls and Windows, WWR, and microclimate control such as shading, trees, and landscape. Indoor air quality is also affected by building enclosure and some other factors such as off-gassing interior materials, molds/bacteria due to leaks, or lack of ventilation. Energy management in general requires more efficient use of energy, water, and air quality protection, and wastes and pollution control. Energy interventions play an important role in reducing pollutant emissions and energy bills. The energy cost savings due to energy efficiency and on-site energy production can improve living affordability. Integrating climate-responsive design strategies with energy-efficient active systems and renewable energy generation typically increases upfront cost. However, it can lead to faster economic payoff with operational energy savings [42].
The Paris Agreement aims for GHG reduction in the world. Public acceptance to improve building energy efficiency is intensified by greater awareness of climate emergency and economic returns. However, to deal with the climate crisis, both mandatory and voluntary implementations are required. New York City obligates carbon neutrality by 2050 and demands improvement of buildings energy efficiency up to 23% above 2012 levels by 2030. It is targeted to reach 40% GHG reduction by 2025 and 50% by 2030 [42]. At the voluntary level, over 65,000 Passive House (PH) (a voluntary standard for energy efficiency in a building which reduces the building’s ecological footprint) buildings are certified around the world, starting in Germany in 1990s. The performance requirements are 15 kWh/m 2 of each heating and cooling demand FOCUS with maximum 60 kWh/m 2 /year of renewable primary energy demand (heating, hot water, and domestic electricity use) [42,60,61]. To meet the energy requirements, there are strategies such as high insulative building enclosures, energy-efficient Windows, thermal breaks, and air tightness, which are related to high-performance building enclosures, and ventilation heat recovery, which is related to energy-efficient HVAC systems.
Buildings supplying their required energy (heat and electricity) from microalgae (Figure 1) can serve as an alternative building system. The mechanism of the process is as follows: first, water containing nutrients is being filled in the façade PBRs, where daylight and CO2 are converted to algal biomass through photosynthesis; secondly, the biomass and heat generated by the façade element are transferred through a closed loop system to the plant room, where both forms of energy are exchanged by a separator and a heat exchanger, respectively. For the supply of hot water and heating the building, a hot water pump is used to adjust the temperature levels of the generated heat [62,63].
Microalgae enclosures buildings not only generate clean energy but also play a role in GHG mitigation and can be considered as a carbon-neutral power source of energy. Alongside the positive environmental effects, they also have financial profitability due to the reduction of energy and operating costs and taxes which consequently cause lower life cycle costs and increased rental costs without decreasing occupancy [45,64]. These systems are also of interest in the field of net zero energy because of their effectiveness in improving building energy efficiency, renewable power generation, and good air quality. Resulting in better temperature control, PBR façade-integrated buildings can reduce energy consumption by more than 33% in terms of fuel consumption and 10% in terms of electricity consumption [25,29,54]. There is a micro-community integrated with microalgae systems which restores wastes from buildings and converts them into operational valuable resources; they can achieve off-grid power and water independency along with polluted air decarbonation and wastewater treatment [42,65,66]. In 2013, an algae-powered building was implemented in Hamburg, Germany. Since then, there have not been any implemented real-world applications other than small-scale experiments for testing feasibility [42].
Real-World Examples of ABT
There are a few real-world applications of algae buildings, summarized in Table 4. The world’s first PBR façade project is the BIQ building, which is a part of the International Building Exhibition in Hamburg. BIQ consists of a penthouse plus 15 apartments located on four floors. The integrated PBR system are installed on the southwest and southeast faces of the building, consists of 129 flat panel glass bioreactors with dimensions of 2.5 × 0.7 × 0.08 m, with capacity of 150 kWh/m 2 and 30 kWh/m 2 of thermal energy and bioenergy production, respectively. The transformation efficiencies of the thermal energy and bioenergy are determined to be 38% (compared with a typical solar thermal source which is 60–65%) and 10% (compared with a conventional photovoltaic (PV) system which is 12–15%), respectively. The produced biomass is harvested in an energy management center where the generated heat is recovered by a heat exchanger to be reintroduced to the system or stored in an underground aquifer. Methane is generated by conversion of approximately 80% of the harvested biomass in an outdoor plant, and is returned to the building for heat and electricity generation [11,13,85,86]. According to Arup, the implemented system on the Hamburg building has high efficiency for growing the algal culture and requires minimal maintenance [45,53,59,87,88]. Currently, the overall energy needs of the building are reduced by 50%, and 100% is expected to be achieved if solar panels are used to power the pumps and heat exchangers [62].
The first curtain wall PBR prototypes were constructed by a French consortium in the University of Nantes, which shares the PBR expenses via an efficient building integration through a symbiosis of thermal energy, light quality, and air quality [54]. The CSTB prototype includes a 200 m 2 PBR curtain wall located at CSTB (Scientific and Technical Centre for Building) site in Champs-sur-Marne, a town slightly east of Paris, France. Built on microalgae façade experience since 2009, this project became the first technology demonstration installed in a real-world application, testing different configurations and density effects on daylight penetration. The project capitalizes on high growth rate and superior carbon sequestration, in which 1 m 3 of microalgae absorbs the same amount of carbon dioxide as 80–100 trees. The operation and monitoring system helps the year-round algae growth, and such technological demonstrations help raise awareness of its possibility for benefiting human and built environments [42,54].
SYMBIO2 is another project in France, implemented in Nantes. It integrates a 300 m 2 biofaçade in a waste processing plant for simultaneous microalgae production (0.7–1 ton/year) and partial treatment of flue gas (CO2 biofixation: 1–1.8 ton/year). It is proposed that the economic and technical feasibility of this new approach for the production of microalgae be demonstrated by this project. The final interest of this concept will be the result of mutual benefits achieved between the buildings and the needs of microalgae; thus, optimization of the symbiosis is critical here [36,89].
Urban Morphogenesis Lab—UCL and Synthetic Landscapes Lab—University of Innsbruck, in collaboration with ecoLogicStudio, a London-based architecture and urban design studio, presented PhotoSynthetica in Dublin during the Climate Innovation Summit, 2018. PhotoSynthetica is a photosynthetic building cladding system which removes CO2 and pollutants from the atmosphere and produces a valuable food resource in the form of algae, using the algal power. It shows how biotechnology integration with our cities helps to achieve carbon neutrality. Conceived as an “urban curtain”, the system captures approximately one kilogram of CO2 per day, equivalent to that of 20 large trees. The installation on the Irish Revenue and Custom building in Dublin contains 16 custom-made bioplastic containers (2 × 7 m), each of which functions as a PBR. The modules are designed digitally to utilize daylight for feeding the algal cultures and release luminescent shades at night which is very scenic. CO2 molecules and air pollutants in the inlet air introduced at the bottom of the biofaçade are captured and stored by the algae and grow into reusable biomass while air bubbles naturally raise through the watery medium within the bioplastic PBRs. The harvested biomass can be employed for the production of bioplastic raw material that constitutes the main building material of the PBRs. To culminate the process, freshly photosynthesized oxygen is released at the top of each façade unit into the urban microclimate. In order to hold the carbon for as long as possible, the PBRs are designed in the serpentine scheme so that the algae can process it. As in other ecoLogicStudio projects, the curtain is a form of biomimetic, a design that copies structures and processes from nature [90,91].
Ingenious ‘control panel’ in algae provides blueprint for super-efficient future solar cells
A single ingenious protein complex makes it possible for algae and cyanobacteria to use and store solar energy more efficiently than any other organism on earth. Scientists at the universities of Utrecht and Birmingham have unravelled the mechanism, which could serve as source of inspiration for super-efficient photovoltaic cells. They published their results in the respected scientific journal CellChem.
Like plants, algae store the sun’s energy in biomass via photosynthesis, but while plants only store an average of 12 percent of the energy, algae can store up to 98 percent. “That enormous degree of efficiency makes algae ideal for energy storage and conversion”, explains Sem Tamara, PhD Candidate at Utrecht University.
Highly complex light harvesting system
Tamara conducts research into the molecular structure that facilitates the efficient photosynthesis process in algae. A single algae has many protrusions on its surface, called antennae, which form vital components of its light harvesting system. “It’s a highly complex system. Each protrusion is made up of stacks of tiny disks. Inside each disk, there is a ‘gamma’ building block that passes the light efficiently into the system.”
Tamara used mass spectrometry (MS) to discover that there may be up to 20 different types of gamma building blocks. “MS allows you to determine the weight of molecules. Each specific molecule has its own weight. The number of peaks in our mass spectrum then displays the number of different forms of a specific type of molecule.” So far, Tamara has accurately defined four different gamma building blocks. “And some of them can convert the light better than others.”
The system is more complicated than a Swiss watch. This is the product of three billion years of evolution, and engineers could learn a lot from it.
Efficient through diversity
The wide diversity of molecules that let light through does not mean that one form of the light harvesting system is more efficient than another, however. According to Professor of Mass Spectrometry Albert Heck, Tamara’s PhD supervisor: “I think that the diversity of gamma building blocks is what makes the system work optimally under all circumstances. It can constantly adapt, so it is much more refined than we earlier thought.”
Heck hopes that today’s solar panels, which have a yield of 20 percent at most, may eventually be improved with help from the same system that algae use. “The ingenious control panel that algae use to convert sunlight into usable energy is more complicated than a Swiss watch. This is the product of three billion years of evolution, and engineers could learn a lot from it. A primal organism that gives us the blueprint for the ultimate super-efficient solar cells.”
Solar Biopanels
With his firm Greenfluidics. Mexican biotechnician Adán Ramirez Sánchez has made solar panels powered by algae instead of minerals mined from the earth.
Algae has the capability to generate clean energy, create biomass for fertiliser, convert CO2 into oxygen and can even be used in space. The intelligent Solar Biopanels are the only multipurpose system in the world using microalgae and nanotechnology, which absorb CO2 and convert it into both electricity and oxygen.
The intriguing triangular green panels measure a metre across and provide an ‘avant-garde’ design to buildings while purifying the air simultaneously.
The innovative biopanels are still very new. they are currently in the testing phase of the technology for green buildings and hope to expand their use for future space colonies in the future.
To learn more about this exciting new technology, head to the Greenfluidics website.
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