Heat exchangers are the heart of the energy recovery system which is frequently used to recover waste energy either for industrial or building applications. Heat exchangers are also known as energy recovery ventilators. They transfer energy between the air exhausted from building and the outdoor supply air to reduce the energy consumption associated with the conditioning of ventilation air. The use of energy recovery ventilators reduces the heating energy expenditure appreciably.26 The temperature difference between indoor and outdoor air affects heat recovery efficiency. The bigger the temperature difference is the higher the efficiency.

In the past, heat exchangers were originally made of metal to transfer sensible heat and polymers have been used only as the coating layer to prevent corrosion. But with the demand of using more sustainable and renewable materials and the needs of transferring latent heat, their surface area has been replaced with polymers and their composites. Yliaya gives a detailed review of polymeric membrane-based heat exchangers with a vast description of the effect of chemistry and chemical composition on their performance.27 The main benefit of the polymeric membrane- based heat exchangers is that they can be used to recover energy in the hot-humid area in which they are placed to be integrated with the air conditioning system for energy recovery applications, energy based on heat and mass transfer mechanisms.28 The development of these heat exchangers consists of porous fibers impregnated with a polymer-based solution that are able to capture moisture from the hot and humid air stream. In this system, heat is transferred and moisture is absorbed by the membrane surfaces of the heat exchanger, and hence, the temperature is decreased as much as possible which often depends on its ability and performance factors before entering the air conditioning system. The humid and hotter air is blown from inside, carrying the heat and moisture, and drying up the module as it is blown out to the environment. Therefore, the air conditioning system workload is reduced and so does the energy consumption.29


Iii building structures, the crack formations in concrete structures are common because of its moderately lower tensile strength and action of different load and nonload factors. There are different reasons behind this cracking; some of them are plastic shrinkage, thermal stresses, drying shrinkage, rebar corrosion, external loading, and or coupled effect of multiple factors. Cracks can be manually repaired but there are several issues related to this repair operation, for example, impact on the environment, accessibility, and price. Self-healing is a promising solution to reduce manual intervention.

Self-healing materials are substances that are artificial or synthetically generated with the built-in capacity to involuntarily repair injure to themselves lacking any exterior analysis or human participation. The design of self-healing material gained attracted attention in various buildings and infrastructure applications.30 Though, the majority of this application is incorporating an expansive and nonrenewable element in the structural form like concrete which starts to expand and fill voids and cracks when triggered by carbonation or moisture ingress.31 Zulfiqar et al. reported the fabrication of superhydrophobic surfaces on building materials, which has the added quality to reinstate its properties after rigorous scratch. These superhydrophobic surfaces were constructed from hydrophobic silica nanoparticles and commercially obtainable spray adhesive on three commercially obtainable construction materials, that is, bricks, marble, and glass. These superhydrophobic surfaces were capable to maintain the impact of sand particles traveling at a speed of 11.26 km/h, and also restore their superhydrophobic character by simple acetone treatment upon getting rigorous damages by knife scratches.32

The bio-based self-healing materials have gamed numerous attention in recent years. Sustainable mechanism of self-healing by microbial-induced precipitation of calcium carbonate is studied recently to close and repair cracks. This self-healing of cracks by microbes involves precipitation of calcium carbonate by the direct action of bacteria species including Bacillus subtilis on calcium compounds like calcium lactate or by the disintegration of urea by ureolytic bacteria like Bacillus sphaericus. Calcium carbonate precipitation using microbes is well-suited with concrete and the procedure of formation is environmentally friendly. Bacillus sphaericus is identified as safe to human beings. Gupta et al. give a detailed review of the evaluation of crack healing by bacteria and the effectiveness of factors affecting bacterial self-healing.33 The self-healing materials for building and infrastructure applications give significant benefits, as they would allow conquering the problems related to internal damage analysis and restoration.


Green or biocomposite materials include biopolymers and natural fibers from renewable resources, which reduce the elimination of nonrenewable waste, raw material usage, and decrease greenhouse gas productions. The construction industry uses a huge quantity of materials, the majority of which are originated from nonrenewable resources or resources that necessitate significant time to be renewed. Hence, biocomposite materials are introduced as building materials with a reduced impact on environmental and human health. The biocomposites are used in various construction applications. The use of natural fibers as building materials is an advantage to attain a sustainable construction. Yan and Chouw described the potential of sustainable construction with natural fibers by empathizing the usage of a composite column containing flax fiber reinforced polymer and coir fiber reinforced concrete (CFRC).34 Another study reports the flexural properties of plain concrete and CFRC beams which are externally strengthened by flax fabric reinforced epoxy polymer composites.35 CoDyre et al. reported the effect of foam core density on the behavior of sandwich panels with novel biocomposite unidirectional flax fibers-reinforced polymer skins, with a comparison to panels of conventional glass-FRP skins.36 The improvement effect of alkali treatment on microstructure and mechanical properties of coir fiber reinforced-polymer composites and reinforced-cementitious composites as building materials are also reported.37


Biopolymers are substances naturally produced by living organisms and are hence considered to be eco-friendly and sustainable. Among the biopolymers, the chitosan and cellulose have particular importance due to their numerous applications, large availability, and easiness for modification. They have a large number of environmental applications. These biopolymers have high coagulating and flocculating power and these properties are veiy helpful in wastewater clarification, and it reduces the dependability on synthetic polyelectrolytes. Biopolymer- based hydrogels and nanocomposite films act as efficient biosorbents for eliminating a large number of organic and inorganic pollutants from wastewater. Particularly, it can adsorb heavy metal and dye. It also has many environmental applications like antidesertification, natural biosealants for preventing concrete leaks, and proton conducting membranes in electrochemical devices.

The biopolymers are also used for the enhancement of soil behavior.38 Biopolymers have long been identified as viable soil conditioners because they stabilize soil surface structure and pore continuity. Acid-hydrolyzed cellulose microfibrils can be used for soil stabilization. The productivity of the soils can be unproved by using a low concentration of biopolymers from plant fibers to augment water holding capacity improves the physical properties of soils by binding soils particles together reducing the losses of water by evaporation and deep percolation.39 Soil erosion decreases water quality and agricultural productivity through the loss of valuable “topsoil” and runoff of agricultural chemicals, hi a study, a series of biopolymers added to irrigation water were tested to reduce erosion-induce soil losses. Different polymer additives in irrigation water were tested for their efficacy in reducing soil losses during irrigation. Starch xanthate, cellulose xanthate, and acid hydrolyzed cellulose microfibrils, all appear promising for reducing soil runoff40

Hataf et al. reported the potential of clay soil stabilization by a biocompatible chitosan solution that is synthesized from shrimp shell waste. The chitosan solution is used in different concentrations to evaluate its potentials on the mechanical properties of clay soil at different curing times and conditions. The results showed that the incorporation of chitosan has the potential to increase the interparticle interaction of the soil particles which leads to improved mechanical properties.41 In another study, protein-based biopolymers (casein and sodium caseinate salt) were used for soil strengthening.42 The thermo-gelatin polymers were also used for soil strengthening.43


Plant fibers are a renewable resource and are an important group of reinforcing materials. It attained heavy attention in sustainable technology because of its plentiful occurrence and ease of access. Also, the composite materials of plant fibers are environment friendly, lightweight, and with high particular properties. Plant fibers-reinforced composites are made up of plant fibers and biodegr adable polymer, as a matrix. In recent times, the plant fibers-reinforced composites replaced conventional fibers-reinforced composites, particularly glass fibers-reinforced composites. It is predicted that by 2020 fibers derived from bio-based resources will represent up to 28% of the whole market of reinforcement materials.44 Owing to their peculiar properties like high mechanical strength, exceptional biocompatibility, they have gained extra interest and developing fields in materials technology. Plant fibers-reinforced composites have different industrial applications like internal parts of automotive and building structures and are used as a filler material. It is also widely used in the packaging, construction, transportation, and aviation industry.45


Epoxies are one of the most commonly used engineering thermosets because of their excellent properties like high rigidity, easy to process, greater tensile strength, good chemical as well as thermal resistance, outstanding electrical strength, exceptional solvent resistance, and compositional versatility. Though, these epoxy thermosets have some serious drawbacks like low toughness, high brittleness, and comparatively high cost.46 In order to overcome these limitations, branch generating moiety has been introduced to the epoxy matrix. The presence of large numbers of reactive end functional groups in hyperbranched epoxy resins gives strength and toughness to the epoxy thermosets.47

In order to develop sustainable materials, renewable bio-based feedstocks like cardanol, tannin, lignin, glucose, vegetable oil, etc are used for the modification of epoxy resins. It gives biodegradability to the thermosets.48 Vegetable oils have attained extensive interest because of its benefits like nontoxicity, renewability, effortless modification, biodegradability, and environment-friendly nature. Among the various types of vegetable oils, castor oil has been extensively used in industries due to its easy availability in large quantities and exceptional chemical composition.49 De et al. prepared biodegradable hyperbranched epoxy from the polyester polyol of the monoglyceride of castor oil which exhibited outstanding adhesive strength, chemical resistance, good impact strength, scratch hardness, and biodegradability.50 Das and Karak reported the synthesis of epoxy resin using Mesua ferera seed oil to give flexibility to the prepared epoxy thennoset.51


Inventions of versatile chemical resources from renewable and sustainable materials have attained significant attraction in recent years. In this regard, the production of lactic acid from bacteria is seen as an option to cope with environmental problems and is cost-effective. Lactic acid (2-hydroxy propionic acid or 2-hydroxypropanoic acid) is an important organic acid with molecular formula CH3CHOHCOOH. It is a chiral molecule, which exists as enantiomers L- and D-lactic acid. The lactic acid can be synthesized by using chemical synthesis and by microbial fermentation. By chemical synthesis, a racemic mixture of D- and L-lactic acid is obtained. But in the microbial fermentation process, an optically pure L(+)- or D(-)-lactic acid is obtained.32 Also, this method has many advantages compared to chemical synthesis. Various inexpensive materials like molasses and other residues from agriculture and agro-industry have been used as substrates for lactic acid fermentation. Moreover, the efficiency of microorganisms for lactic acid synthesis can be enhanced by gene modification.53-54 The lactic acid is extensively used in the pharmaceutical, cosmetic, food, and chemical industries. Lactic acid is regarded as one of the most important hydroxycarboxylic acids because of its versatile applications as a flavoring, inhibitor of bacteria, and acidulant. Lactic acid can be easily converted into potentially useful chemicals such as various acids, esters, and biosolvents since it contains both carboxyl and hydroxyl groups.55 Lactic acid is also used as a feedstock monomer for the production of biodegradable poly-L-lactic acid, a superior substitute for synthetic polymers derived from petroleum resources.56-57 hi the sono- chemical synthesis of pyrrole derivatives, lactic acid acts as a bio-based green solvent. Andreev et al. stated that lactic acid fermentation helped to decrease the number of pathogens and to reduce the nutrient loss and hence increasing the agricultural value of plants.5S Thus, the demand for lactic acid has been increasing significantly because of its various promising applications.


The energy consumption of a building is strongly dependent on the characteristics of its envelope. The thermal performance of external walls is one of the important factors to increase the energy efficiency of the construction sector and to decrease greenhouse gas emissions. Thermal insulation is one of the best ways to reduce energy consumption. Thermal insulation systems and materials intend at reducing the transmission of heat flow. Building insulation is usually done with materials obtained from petrochemicals like polystyrene or from natural sources processed with high energy consumptions. These materials cause considerable harmful effects on the environment mostly due to the consumption of nonrenewable materials and fossil energy and troubles in reusing or recycling the products at the end of their lives. Hence, the researchers developed thermal and acoustic insulating materials using natural or recycled materials.

Aditya et al. reported the current progress on the building thermal insulation and also analyzed the life-cycle analysis and potential emissions reduction by using appropriate insulation materials.59 A review of the important commercialized insulation materials is given by Schiavoni and coworkers.60 The agro wastes like cereal straw, hemp, and olive waste were also used for the preparation of building insulation materials.61

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