Due to growing cooling demands as well as emerging global warming and climate change issues , cooling systems should be more efficiently utilized. Boiling is an effective heat transfer mechanism, which has a critical ro le in many cooling systems. Surface modification is considered as the major approach for boiling heat transfer enhancement. In this study, we developed a microbial bio -coating surface modification technique for phase change cooling applications. Thermoacid ophilic Sulfolobus solfataricus coating was implemented using a facile dip coating method on different metallic and non -metallic surfaces. Controlled by drying conditions, the coating exhibited rough and porous morphologies. When tested in a boiling heat t ransfer setup , bio-coated surfaces offered enhancements up to 76.3% in Critical Heat Flux ( CHF ). Next, a 2 miniature evaporator was coated and tested for real -world air -conditioning applications, and coefficient of performance (COP) enhancements up to 11% cl early revealed the potential of bio - coated surfaces for energy saving purpose and reduction in greenhouse gasses. Furthermore, coated evaporators reduced the exergy destruction rate up to 8%. This study not only offers a new type of coating morphology, but the applicability of the proposed bio -coating is also proven in a miniature air conditioning system. Keywords: Bio-coating, Heat transfer enhancement, Environmentally friendly coatings, Electronics cooling, Air conditioning system, Energy and exergy efficiency Graphical abstract: 3 Nomenclature Subscripts and abbreviations Symbols AC Air conditioning system A Heated area [m2] AFM Atomic Force Microscopy Cp Heat Capacity [J/K] BHT Boiling heat transfer h Heat transfer coefficient [W /cm2K] COP Coefficient of performance h Enthalpy [kJ/kg] CHF Critical heat flux I Current [A] cond Condenser K Thermal conductivity [K/W] dest Destruction P Pressure [KPa] ev Evaporator R Thermal resistance [m2K/W] EPS Extracellular polymeric substan ce S Entropy [kJ/kgK] HTC Heat transfer coefficient T Temperature [K] in Inlet T0 Ambient temperature [K] ID Inner diameter Ts Surface temperature [K] ONB Onset nucleate boiling Tsat Saturation temperature [K] OD 600 Optical density at wavelength of 600 nm V Voltage [V] OD Outer diameter x Vapor quality out Outlet ΔT Wall superheat [K] PBS Phosphate -buffered saline W Consumed work by the system [W] PPI Pore number per inch 𝑄𝑐 Heat by the heat source [W] pre Preheater 𝑞″ Net heat flux [W/cm2] SEM Scanning electron microscopy 𝑚̇ Mass flow rate [kg/s] WCA Water contact angle method 𝐸̇ Exergy [W] 1. Introduction Global warming, as a direct consequence of the increase in greenhouse gas emission, has remained the main worldwide concern during the last decades. Heat loss is the predominant form of the energy lost in energy systems/processes. One of the major sources of energy losses is linked with the constant utilization of air conditioning, refrigeration, and heat pipe systems. According to the International Energy Agency (IEA), the cooling deman ds in the air conditioning market is predicted to be tripled by 2050 [1]. Thus, many attempts have been made to enhance the energy 4 efficiency of these systems. Due to its ability to remove large quantities of heat, boiling phenomenon is used as a cooling method in air conditioning and refrigeration systems [2],[3]. Despite its superiorities compared to the conventional methods, boiling has a limit called critical heat flux (CHF), where the vapor columns create a vapor blanket film on the surface, which drastically decre ases the heat transfer rate from the surface. Figure 1 shows the typical boiling regimes . At first, natural convection corresponding to single phase flow happens. The boiling process starts with the inception of bubbles from the nucleation sites on the hea ted surface (cavities and surface impurities). Here, the required temperature for the onset of nucleate boiling (ONB) depends on the surface morphology. As the surface temperature increases, nucleate boiling regime starts, a region characterized by more ac tivated nucleation sites and discrete departed bubbles. Further surface temperature increment results in more generated bubbles leaving the surface in the forms of vapor columns and larger bubbles. The end of the nucleate boiling region is described by excessive generated vapor columns and blankets, which block the coolant path to the surface, thereby leading to boiling crisis or critical heat flux (CHF). Figure 1. Boiling regimes, boiling heat transfer (BHT) and critical heat flux (CHF) enhancements in an ideal flow boiling curve. 5 As indicated in Figure 1, boiling heat transfer enhancement can be achieved by i) reducing ONB temperature, ii) improving heat transfer coefficient (HTC), and iii) increasing CHF [4]. Many surface modification techniques [5], [6] have been proposed and investigated to achieve these goals. These techniques include implementation of nanowire arrays [7], nanorods [8], graphene coating [9], polymer coating [10],[11], virus coating [12], biphilic surfaces [13], porous surfaces [14], laser -engineered surfaces [15], and plasma spray coating [16]. Based on the application and the range of the applied heat fluxes, these three aspects should be considered in the design o f thermal -fluids systems to maximize the heat removal from the surface in nucleate boiling and film boiling regimes. As an example for surface modification, Ma et al. [17] investigated surface area enhancement in flow boiling via micro -pin-fins. The surface revealed the effect of fin height, fluid velocity and liquid sub -cooling on heat transfer. Sa daghiani et al. [18] explored the effect of copper based micro - and nano -structured surfaces on flow boiling. Alam et al. [19] studied the effect of surface roughness on flow boiling. These studies showed that the roughness elements about the size of 1 and 1.6 μm considerably affected the nucleation site density and boiling inception. Roughness elements can majorly be introduced via mechanical modification such as sanding, etching, and deposition [6]. In the study c onducted by Jung et al. [20], the effec ts of treated surfaces by anodization in two acids as the electrolyte (deposition) and with submicron -roughness on subcooled flow boiling of FC -72 were investigated at ambient pressure. Their observations marked an increase in HTC and CHF. Sisman et al. [21] applied roughness with the size of 1 to 4µm on aluminum sheets with mech anical sanding technique and reported that smaller roughness could create more active nucleation sites at the same heat fluxes. In addition to roughness, tuning the porosity is one of the widely used modification techniques in boiling heat transfer. Porous foams generate a high surface area -to-volume ratio. 6 Rainey et al. [22] investigated the effect of micro -porosity using the ABM coating method (Aluminum particles, Devcon Brushable Ceramic epoxy, and Methyl -Ethyl -Keytone) on subcooled flow boiling of FC -72. Compared to non-coated surfaces, boiling inception took place at lower wall superheats and consequently increased heat transfer coefficients. However, the development of microporous coating using Al 2O3 particles with size of less than 10μm resulted in 25% CHF enhancement [23]. Further investigation on R -134a flow boiling in tubes with a metallic porous surface exhibited increased heat transfer c oefficient (up to 200%), as the cell size of the metal foam was changed from 20 PPI (pore number per inch) to 40 PPI [24]. In the study conducted by Pranoto and Leong [25], high thermal conductivity graphite foams with average pore diameter of 350 and 650 µm were used in flow boiling experiments. As a result, mor e nucleation sites on the porous surface could be observed, which raised the heat transfer coefficient by 2.5 times. As can be seen from the abovementioned studies, the available enhancement methods require expensive infrastructure, have a complex process flow for their fabrication/preparation, and most importantly , cause environmental issues due to released hazardous materials as by -products. On the other hand, according to the latest market report published by fortune business insights, the global paint s and coatings market size in 2019 was 154.6 billion USD and is expected to reach an estimated value of 249.7 billion USD in 2027 [26]. This huge budg et is mostly based on the demand for new generation coatings and should be wisely managed in order to meet global energy needs. To address these issues, we used Sulfolobus solfataricus in this study to modify the surfaces by providing organic porous as wel l as roughened surfaces. Previously, this technology was formulated by our team [27]. Here, we propose the next generation bio -coating, which is highly durable, environmentally friendly, and cheap and has a practical coating method, for the implementation to thermal -fluids systems. This study not only offers new types of coating 7 morphologies, but also the applicability of the proposed bio -coating is proven with its implementation to a miniature air conditioning system. 2. Methods and materials 2.1. Sulfolobus solfataricus culture Archaeal cells have a unicellular struct ure with circular chromosome. They resemble to eukaryotic cells due to their metabolisms involving DNA replication and transcription. However, they remarkably exhibit the same structure and morphology and other macromolecule metabolisms as of prokaryotes [28]. Nevertheless, archaeal domains harbor distinctive complex traits at molecular level that enable them to endure extreme physiological conditions such as low or high temperatures (−2 °C to 15 °C or 60 °C to 122 °C), high salinity (2 M to 5 M NaCl), and low or high pH (<4 or >9) [29],[30],[31]. Sulfolobus solfataricus is a hyperthermophilic sulfur metabolizing archaeon, which belongs to Crenarchaeota phylum. This s pecies grows best at temperatures around 80° C and highly acidic pH (2 -4). Sulfolobus solfataricus essentially metabolizes sulfur as a main cellular metabolism. Sulfolobus solfataricus cells generally have a spherical shape processing flagellums, which gen erate frequent lobes [32]. Highly thermostable properties of this species make it a strong candidate for biotechnological applications regarding heat transfer enhancement. The initial batch of Sulfolobus solfataricus was purchased from American Type Cultu re Collection (ATCC® 35092™), and the growth medium was prepared according to the ATCC® Medium 1304 recipe. The medium composition is provided in Table S1. Since Sulfolobus solfataricus prefers anaerobic growth, the initial cultivation rate was relatively slow during the preparation. In order to reach maximum growth rate, the culture volume was stepwise increased. Conical flask containing 30mL of pre -warmed medium was first inoculated using aliquot of the cell vial. Then, the cell growth was monitored using UV Spectrophotometer at OD 600 for 72 - 96h. 8 The culture volume was gradually increased up to 100 mL, 250 mL, and 500 mL by adding pre - warmed medium. To generate the mass production of the archaeon in a sustainable manner, the growth medium was supplied wi th 2 g/L D -sucrose, and the cells were grown at constant temperature and PH of 84°C and 3.9, respectively. When the OD 600 of culture reached to 1, 250 ml of the culture was harvested after cooling down on ice and further harvested using centrifugation at 4500 ×g for 15 min. The remaining culture was added with the fresh medium up to 500 ml and was kept for further incubation. Then , the supernatant was discarded, and the pellet was stored at −80 °C until use. In order to prepare bio -coated surfaces, the pe llet was washed once with 250ml of phosphate buffer (20% (v/v) in H 20) at room temperature and then re -suspended with 250ml of fresh phosphate buffer. Finally, Poly -L-lysine (0.1 % (w/v) in H 2O) was added into the suspension with a ratio of 1:5 to increase the adhesion of the biomaterial to the desired surface. Then, the coating mixture was applied on the silicon wafer with thickness of 500µm and interior side of an evaporator made of aluminum. Subsequently, the coated samples were incubated at room tempera ture as well as at 60°C to initiate the evaporation process. Figure S1 summarizes the culture, coating and drying processes. More information about the coating process of the aluminum tube is provided in the Supporting Information (Figure S2). 2.2. Surface char acterization Once coated surfaces were completely dried, they were processed for the surface characterization. To avoid disparity, which might generate as a result of surface charge formation of biomaterial, and to develop homogeneous surface for high res olution image processing, a thin layer (3 -4 nm) of gold -palladium alloy was employed using SEM sputter coater. A scanning electron microscope (SEM Leo Supra 35VP, Oberkochen, Germany) was used to capture the microscale images of the coated samples. The Sec ondary Electrons (SE) mode was used for the 9 imaging due to its high spatial resolution. The images were taken using the SE mode at low voltage (3KV and 10KV) in 0 and 45 degree titled visions to obtain both full images of the surface area and the cross sec tional area. The wettability of the bio -coated surfaces was measured using the water contact angle (WCA) method [33]. A 5μL of the water drop was applied on the five various spots of each sample, and their relevant average contact angle was determined. Surface characterization using AFM (Atomic Force M icroscopy) indicated that the average thickness in porous and roughened surfaces was 1.2 µm. The drying time of the samples incubated at room temperature was 30 hours, whereas the drying time of the specimens incubated in the oven at 60°C was 1.5 hours. Figure 2a shows the Sulfolobus Solfataricus cells fixed on the substrate surface (using 2.5% glutaraldehyde dissolved in phosphate -buffered saline (PBS) and kept at the room temperature for drying ). The resulting electron microscopy image of the highly dilu ted single cell sample that lacks fixation solution is shown in Figure 2b. This sample was dried under the freeze -drying condition for 2 hours. This figure shows that the morphology of the cell remained intact, while the interior fluid material was complet ely dried as a result of the ice sublimation and formed the porous structure. Figure 2c and Figure 2d demonstrate the dried bio -coating (concentrated cells with poly - l-lysine) on silicon substrates under different drying conditions. In addition to silico n and glass surfaces, the bio -coatings were prepared and characterized on metallic surfaces including copper, aluminum and stainless -steel (Figure S3). Similar to silicon and glass substrates, metallic surfaces also have rough and porous morphologies under dip coating conditions. 10 Figure 2. a) Sulfolobus Solfataricus fixed cells on the substrate. b) Single cell of Sulfolobus Solfataricus in a highly diluted sample and the porous structure inside the cell. 2D and 3D SEM images, AFM results and contact an gles of the coated samples under two different drying conditions: c) Porous surface achieved at 60℃ heat incubation, d) Roughened surface achieved after drying at room temperature. 3. Results and discussion 3.1. New surface morphology for phase change systems The bio -coating surface is inspired by microbial biofilm formation. Generally, the biofilm formation is considered as a significant survival mechanism in microbial life, which protects the microorganism from austere conditions such as nutrient scarcity or un favorable temperature. Similar to bacterial cells, archaeal cells form biofilm, where cells initiate the self -immobilization and build up the stacked layers to adapt to the temporary mutual lifestyle. In the biofilm process, cells inevitably produce multi layer microbial community, which leads to irreversible attachment, subsequent development of micro colonies and a further process for the biofilm maturation. In 11 response to various stress states, death signals induce autolysis of some microbes to provide nutrition for colony survival known as “die for community” [34]. The proposed bio -coating process employs Sulfolobus Solfataricus to form a biofilm on the surface using poly -l-lysine to act as an extracellular polymeric substance (EPS). The structure of the bio - coated layer developed under the room temperature displays roughness mainly due to smooth autolysis of the surf ace microbial biofilm. However, the porous shape structures form due to the rapid release of the gasses such as H 2S, CO 2 and SO 2, which consequently generate a distinct morphological structure on the biofilm (Figure 3). These porous structures identified a s micro and nano structures reflect the trace of the released gasses and demolished cells. In addition, the rapid evaporation withdraws more dissolved gases to the bio -layer/air interface and aligns the micro - organisms in dispersed fashion and thus establi shes the porous structure. On the other hand, evaporation at room temperature corresponding to the steady state form of evaporation allows micro -organisms to slowly descend on the substrate and forms the roughened coating. Most of the Sulfolobus species di splay aerobic growth. However, Sulfolobus Solfataricus has a higher growth rate at low oxygen concentrations [35]. Aerobic microorganisms grow in the presence of the oxygen when the O is the final electron acceptor and water and CO 2 are the final products. Additionally, Sulfolobus species are known to be chemosynthesis microorganisms due to their ability in oxidizing sulfur to metabolize various carbon sources and consequently direct the energy for carbon fixation. Our carbon source to grow Sulfolobus Solfataricus is D-sucrose. Sucrose is a disaccharide consisting of glucose and fructose. Aerobic metabolism generates CO 2 as a result of the glucose oxidation. Basically, they gain energy from growing on sulfur or various organic compounds. Sulfu r metabolism pathways in archaea are part of the food chain survival strategy, when there is a lack of energy source, sunlight, nearby hydrothermal vents, with abundant 12 hydrogen sulfide and carbon dioxide. The SO 42- is the prerequisite in sulfur metabolis m pathway of Sulfolobus Solfataricus, which produces H 2S gas through three sequential reactions [36] as indicated in Figure 3. Moreover, these microorganisms could metabolize organic sulfur an d emit H2S under aerobic conditions [37]. It was reported that in some studies the rate of oxidation is determined by the biogeochemical characteristics of the sediment in sulfur -oxidizing microorganisms [38],[39]. According to these studies, the rate of the sulfur oxidation in such microbes is more on the sediment surface, where enormous amount of the oxidants is available as a result of high rate sulfur reduction [40]. Most of the sulfide oxidizing microorganisms grown in lab cultures are mostly derived from surface sediments [41]. Temperature discrepancy between two samples is the main reason to generate various morphologies defined as roughness and porous coatings. This temperature difference has two main effects on the evaporation rate and resulting cell sedimentation: i) Surface evaporation occurs as a result of vapor transport at the bio -film/air interface. The vapor transport is highly dependent on the diffusion coefficient of the dissolved gas, where the diffusion coefficient exponentially increases w ith the temperature [42]. For example, the diffusion coefficient of air is almost five times higher at 60 ℃ compared to the room temperature. ii) The second reason is derive d from the differences in solubility of the generated gases within the biomaterial at different temperatures . At higher temperatures, the solubility of the generated gases such as H 2S, SO 2, CO 2 and air decreases exponentially and results in major difference in gas release from the bio -film (Table 1). This rapid release of the gasses at 60 ℃ contributes to the alteration in sedimentation profile and also the bio - film rupture. Table 1. Solubil ity of the gases in water (g gas per kg water) [43] Gases in the biomaterial At 20 ℃ At 60 ℃ Percentage of reduction CO 2 1.7 0.6 64.7% 13 H2S 3.85 1.5 61% SO 2 115 45 60.8% Air 0.024 0.013 45.8% Figure 3 . Generation of CO 2, H 2S and SO 2 as a result of Sulfolobus sulfactorius biological metabolism [36]. Figure depicting evaporation of bio -layer under two different conditions (rapid and smooth). The rapid evaporation results in formation of the porous structure, whereas smooth evaporation causes roughened bio -film on the substrate. The schematics represent two various bio - coated morphological surfaces and their function once boiling occurs. 3.2. Testing and analysis method The bio -coated (porous - and rough -coated) surfaces were tested in a flow boiling experimental setup to investigate the effect of surface properties on boiling heat transfer and two - phase flow behavior. More information about the experimental setup and procedure can be found in Supporting Information (Figure S4). A channel with dimensions of 50×15×0.5 mm3 and equipped with pressure and temperature sensors was used to perform flow boiling experiments. 14 The experiments on non-coated and bio -coated surfaces were performed under the at mospheric pressure at the mass flux of ~100 kg/m2s (corresponds to average mass flux of miniature air - conditioning systems [44]). The wall superheat s (ΔT sat) – defined as the temperature difference between the surface (T s) and saturation temperature (T sat) – were obtained for the applied heat input s. The comparisons of the heat transfer performance between non-coated and coated surfaces are presented in Figure 4. It should be noted that each test was repeated for three times to ensure the repeatability of the experiments . The local heat transfer coefficient is calculated as : ℎ=1 𝐴𝑄 ∆𝑇𝑠𝑎𝑡 (1) In equation 1 , 𝑄 is the net heat input (W), ∆𝑇𝑠𝑎𝑡 is the wall superheat (K), and 𝐴 is the surface area (m2). Four temperature measurements were used to calculate the average temperature of the surface in the aluminum block ( at 4 mm beneath the substrates). The average of the measurements was used as the surface average temperature. 𝑇 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 =𝑇1+𝑇2+𝑇3+𝑇4 4 (2) The net heat flux , heat loss, and surface temperatures are calculated using equa tions ( 3) to ( 5): 𝑞″=𝐼𝑉−𝑄𝐿𝑜𝑠𝑠 𝐴 (3) 𝑄𝐿𝑜𝑠𝑠 =𝐼𝑉−𝑚̇𝑐𝑝∆𝑇= 𝐻𝑒𝑎𝑡 𝑙𝑜𝑠𝑠 𝑝𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 × 𝐼𝑉 (4) 𝑇𝑠𝑢𝑟𝑓𝑎𝑐𝑒 =𝑇𝑎𝑣𝑒𝑟𝑎𝑔𝑒 −𝑞″(𝐿𝐴𝑙 𝐾𝐴𝐿+𝐿𝑆𝑖𝑙𝑖𝑐𝑜𝑛 𝐾𝑆𝑖𝑙𝑖𝑐𝑜𝑛+𝑅𝑝𝑎𝑠𝑡𝑒 ) (5) where I , V, and A are the applied current (A), voltage (V), and the heated area (m2), respectively . Heat loss is the difference between input power and the amount of heating needed at a specified mass flow rate at a constant ∆𝑇. The perc entage of the heat loss in the boiling setup was calculated less than 8%. LAl is the vertical length between surface temperature measurement location and 15 substrate, and LSilicon is thickness of the silicon wafer, which is 500 µm. K is the thermal conductiv ity, and 𝑅𝑝𝑎𝑠𝑡𝑒 =6×10−6 𝑚2𝑘 𝑊 is the thermal resistance of the thermal paste used to reduce the thermal resistance between two surfaces . 3.3. Heat transfer performance Figure 4a clearly reveals that the porous -coated surface has higher CHF values comp ared to other surfaces. Furthermore, higher CHF values were obtained at lower wall superheat, which suggests lower surface temperatures for the porous -coated samples. The main reason for enhancements is the interconnected pore networks, which enhance the l iquid wetting and vapor venting at higher heat fluxes (shown in Figure 4b). Accordingly, enhancements of 11%, 20.7%, 74.5%, 94.1% and 76.3% are achieved at 10 (W/cm2), 20 (W/cm2), 40 (W/cm2), 60 (W/cm2), and 80 (W/cm2) heat fluxes, respectively. The obtai ned results for rough -coated surfaces also prove enhancements in HTC and CHF. The roughened surface provides abundant nucleation site densities, and the performance of this surface appears to be better at lower heat fluxes, while its function gradually det eriorates at higher heat fluxes. Enhancements of 22%, 35.6%, 25.7%, 15.7% and 10.7% are obtained at 10 (W/cm2), 20 (W/cm2), 40 (W/cm2), 60 (W/cm2), and 80 (W/cm2) heat fluxes, respectively. As can be seen, compared to other samples, rough surfaces have a b etter cooling performance at low wall superheats (ΔT sat<15K). At this range of wall superheats, the rough -coating offers smaller bubbles compared to the porous -coated surface, which is a result of abundant nucleation sites provided by this type of coating. The visualization studies at the applied heat flux of 20 W/cm2 indicate that the bio-coated samples resulted in the formation of more bubbles and heat transfer enhancement (Figure 4b). 16 Figure 4. The effect of roughened and porous bio -coated surfaces on boiling heat transfer at the mass flux of 100 kg/m2s. a) Local heat transfer coefficient profile and enhancements for nucleate boiling regime and near CHF, b) Wall superheat -heat flux profile and boiling images of the non- coated and bio -coated surfaces at nucleate boiling regime for the heat flux of 20 W/cm2 Visualization studies were performed on the bio -coated and non-coated surfaces. The visualization results on the rough -coated clearly indicate the generation of tiny bubbles (marked in yellow in Figure 5a). This is due to presence of abundant nucleation sites on the rough -coated surfaces. However, on the non-coated surfaces, many slugs (large vapor bubbles) can be observed. These slugs prevent the surface rewetting path and results in a high surface tem perature beneath the large vapor bubbles. On the other hand, on the rough -coated surface, these tiny bubbles move towards the surface, thereby rewetting the surface and increasing the cooling efficiency of the surface (marked in red in Figure 5a) [45]. On the porous -coated surfaces, as a result of the presence of many i nterconnected channels (as shown in SEM images in Figure 2c), transportation of coolant within the bio -coating takes place during the boiling process, especially at higher heat fluxes. These interconnected channels act as liquid supply paths when large vap or blankets form at high temperatures. As a result of this effect, the flow morphology (distribution of vapor phase inside the two -phase flow) changes dramatically. 17 As can be seen in Figure 5b, compared to the non-coated surface, the porous -coated surface houses a more distributed vapor phase, which is more evident at high heat fluxes (q≥40 W/cm2). a) b) Figure 5. Images captured boiling experiments. a) Rough -coated surface at constant heat flux, b) porous -coated at different heat fluxes. Here, liqui d and vapor phases are shown in black and white colors, respectively. 3.4. System performance analysis Based on the obtained promising results from the boiling experiments, a miniature air conditioning system was assembled to examine the performance of the bio -coated evaporators. First and second l aws of thermodynamic are used to analyze the performance of the system. In the energy analysis, when the first law of thermodynamic s is considered , all types of energy are taken into account and the quality of energy types is neglected. For instance, it is not possible to show the change in the quality of thermal energy from the hot ambient air (heat source) to the evaporator surface (cold source). On the other hand, the exergy analysis involving the second law of thermodynamics leads to a better understanding about the energy transfer process. Basically, the exergy of the system represents the maximum amount of the work when the system com es back to its equilibrium with the environment. This is especially important for cooling applications, where the system will have a waiting period for the next cooling cycle. 18 3.4.1 Energy analysis, first law analysis Figure 6a shows the schematic of the lab-made closed -loop system. Detailed information about the experimental setup is provided in Supporting Information (Figure S5). FC -72 was used as the working fluid in this system . The evaporator was made of an aluminum tube with the inner diameter (ID) of 6mm, the outer diameter (OD) of 8mm, and the length of 200 cm. The experiments were performed at four different chamber temperatures. Inside the chamber, an electrical heater fan (2000W) connected to a PID (Proportional Integral Derivative) controller keeps the chamber temperature at a desired constant value. While the electrical he ater heats up the chamber, the coolant passing through the evaporator cools down the hot air. A power -meter device was used to obtain the power consumptions. The power consumption of the electrical heater in the chamber was measured for cases of non-coated surface and bio -coated evaporators. For the air conditioning system, the energy efficiencies of the bio -coated evaporators were investigated by comparing the power consumption of the electrical heaters for bio -coated and non- coated surface evaporators. E ach test lasted for an hour, and the average of power consumption was eventually calculated for each sample. The experiments were performed at four different chamber temperatures of 56.5 ℃, 58.5 ℃, 61℃ and 66 ℃ temperatures (corresponding to 0.5K, 2.5K, 5K and 10K wall superheats, respectively). While the chamber temperature was raised by the heater fan, the evaporator reduced its temperature by absorbing the chamber heat. The PID system dea ctivated and activated the heater when the temperature of the chamber was higher and lower than the regulated temperature, respectively Figure 6b shows the obtained results for the bio -coated and non-coated surface evaporators under experimental conditions . The obtained results indicate that the modified evaporators have higher cooling capacities compared to the non-coated surface specimen. Here, the cooling power is the amount of absorbed heat to keep the chamber at the desired constant temperatures. The w all 19 superheat in the miniature evaporator is defined as the temperature difference between the chamber temperature and the temperature of the fluid before entering the evaporator. On the other hand, the pressure drops for different components of the setup were obtained and compared. Accordingly, the obtained pressure drops for bio -coated evaporators are almost the same as for the non-coated surface evaporator (only ∼3% higher). Coefficient of Performance (COP) was used to evaluate the performance of the system. COP was calculated as: 𝐶𝑂𝑃 𝑐𝑜𝑜𝑙𝑖𝑛𝑔 =𝑄𝑐 𝑊 (6) In Equation 6, 𝑄𝑐 is the heat removed from the heat source (electrical fan) and 𝑊 is the consumed work by the system. The consumed work is the sum of pumping power, preheater power and chiller power. Related information about the energy consumption of the system components is given in Supporting Information. Compared to the Non-coated evapor ator, the porous -coated evaporator results in enhancements of 0.41%, 1.15%, 6.33% and 11.11%, while the rough -coated evaporator leads to enhancements of 1.66%, 4.38%, 4.66% and 4.44% in cooling COP for chamber at air temperatures of 56.5 ℃, 58.5℃, 61℃ and 66 ℃, respectively. The obtained results imply a considerable reduction in energy consumption for thermal -fluids applications. 20 Figure 6. a) Schematic of the lab -made miniature air conditioning system . b) Obtained cooling power for bio -coated and non-coated surface evaporators at different wall superheats and the SEM images of the bio -coated aluminum surfaces. c) Obtained enhancements in COP for the rough and porous coatings at different chamber temperatures According to the obtained re sults, it can be concluded that the proposed coating has the potential to be used in different applications. Rough -coated refrigerator shows higher enhancement at low wall superheats (ΔT<2.5K) compared to porous -coating, which agrees with the obtained results from fundamental studies. This kind of coating is suitable for small -scale refrigerators with low cooling capacities. On the other hand, porous coating offering a superior COP enhancement at medium and high wall superheats is suitable for larger refrig erators with high cooling capacities. 21 3.5. Exergy analysis, second law In this section , the second law analysis is presented to give more insight into the energy recovery (exergy availability) for non-coated and coated evaporators. The following expression can be written based on the general exergy bal ance [46]: 𝐸̇𝑑𝑒𝑠𝑡 =𝐸̇𝑖𝑛−𝐸̇𝑜𝑢𝑡=𝑚̇(𝑒𝑖𝑛−𝑒𝑜𝑢𝑡) (7) where 𝐸̇𝑖𝑛 and 𝐸̇𝑜𝑢𝑡 are the total exergy (transferred by heat, work and mass ) at the inlet and outlet, respectively , and 𝐸̇𝑑𝑒𝑠𝑡 is the rate of exergy destruct ion. For the ambient temperature of 𝑇0 and assuming that the change in kinetic and potential energies is negligible, the exergy is given as: 𝑒=(ℎ−ℎ0)−𝑇0(𝑆−𝑆0) (8) The component -wise analysis for the air conditioning system including evap orator, condenser, preheater and pump are as follows:  Evaporator : 𝐸̇𝑑𝑒𝑠𝑡 ,𝑒𝑣=(𝐸̇1−𝐸̇2)+𝑄̇𝑐(1−𝑇0 𝑇𝐿)=𝑚̇(ℎ1−𝑇0𝑆1)−𝑚̇(ℎ2−𝑇0𝑆2)+𝑄̇𝐿(1−𝑇0 𝑇𝐿) (9)  Condenser : 𝐸̇𝑑𝑒𝑠𝑡 ,𝑐𝑜𝑛𝑑 =(𝐸̇2−𝐸̇3)+(𝐸̇5−𝐸̇6) =𝑚̇(ℎ2−𝑇0𝑆2)−𝑚̇(ℎ3−𝑇0𝑆3)+𝑚̇(ℎ5−𝑇0𝑆5)−𝑚̇(ℎ6−𝑇0𝑆6) (10)  Pump: 𝑊̇𝑝𝑢𝑚𝑝 =𝑚̇(ℎ3−ℎ4) (11) 𝐸̇𝑑𝑒𝑠𝑡 ,𝑝𝑢𝑚𝑝 =(𝐸̇3−𝐸̇4)+𝑊̇𝑝𝑢𝑚𝑝 =𝑚̇(𝑇0(𝑆4−𝑆3)) (12)  Preheater: 𝐸̇𝑑𝑒𝑠𝑡 ,𝑝𝑟𝑒=(𝐸̇1−𝐸̇4)=𝑚̇(ℎ1−𝑇0𝑆1)−𝑚̇(ℎ4−𝑇0𝑆4) (13)  Total: 22 𝐸̇𝑑𝑒𝑠𝑡 ,𝑡𝑜𝑡=𝐸̇𝑑𝑒𝑠𝑡 ,𝑒𝑣+𝐸̇𝑑𝑒𝑠𝑡 ,𝑐𝑜𝑛𝑑 +𝐸̇𝑑𝑒𝑠𝑡 ,𝑝𝑢𝑚𝑝 +𝐸̇𝑑𝑒𝑠𝑡,𝑝𝑟𝑒 (14) Exergy destruction permits the evaluation of exergy destroyed per component. The quality of the vapor was obtained using a transparent channel at the outlet of the evaporator , and the images was post-processed using I mage J software. FC-72, with given properties in Table 2, was used as the working fluid in the closed -loop system. Table 2. FC-72 properties Saturation temperature (K) Liquid density (kg/m3) Vapor density (kg/m3) Liquid enthalpy (kJ/kg) Vapor enthalpy (kJ/kg) Liquid Entropy (KJ/kg.K) Vapor Entropy (KJ/kg.K) 296 1724.8 3.88 58.48 163.78 0.218 0.563 329 1601.55 13.07 94.76 189.81 0.334 0.616 330 1597.8 13.49 95.88 190.82 0.338 0.619 The f irst law analysis highlights that the rough - and porous -coated evaporators work more efficiently at low and high temperatures, respectively . Thus, the exergy analysis was performed on rough - and porous -coated surface at wall superheats of 0.5 K and 5 K, respectively . Vapor qualities, cooling powers, COPs and evaporator exergy destruction rates are tabulated in Table 3. Table 3. Conditions of the exergy analysis Wall superheat (K) Evaporator type Vapor Quality( x) Cooling power (W) Coefficient of Performance (enhancement) Evaporator Exergy destruction (W) (enhancement) 0.5 Non-coated 0 1195 3.428 21.53 Rough coated 0.07 1220 3.485 (1.6%) 19.91 (7.5%) 5 Non-coated 0.38 1505 4.285 14.15 Porous coated 0.47 1595 4.557 (6.6%) 12.98 (8.2%) Figure 7 a and 7b illustrate the pressure -enthalpy diagrams for the low and high wall superheat experiment s, respectively . Under the experimental conditions it can be observed that the rough - 23 coated evaporator enhances the exergy destruction ratio up to 7.5% compared to the non -coated case (as shown in Figure 7c). On the other hand, the porous -coated evaporator demonstrates a 8.2% decrease in the exergy destruction ratio compared to the non-coated case (Figure7d) . The second Law analysis is an additional approach for the analysis of heat exchanging devices and allows to evaluate the energy flow between the evaporator surface and the coolant , combining heat transfer and friction effects . Exergy destruction in flow boiling strongly depends on the distribution of vapor and liquid phases inside the evaporator . Since the bio-coating thicknesses are negligible compared to the tube diameter , the friction al pressure drop in two -phase flows can be considered as the main source of exergy destruction. The large r exergy losses between evaporator inlet and outlet in the non -coated evaporator are due to i) higher wall superheats required for the onset of nucleate boiling , ii) non-uniform surface temperature due to lower number of active nucleation sites , iii) larger frictional pressure drop due to the presence of larger vapor bubbles [47],[48],[49]. Generated larger bubbles (Figure 5) on the non-coated evaporator results in larger bubble slip velocity and higher deformability, which all eventually increase the frictional pressure drop of the boiling flow inside the 2 -m long evaporator [50]. By improving the phase change process , the coated specimens generate more vapors inside the evaporator. Therefore, the inlet vapor quality of the condenser is higher for the coated evaporator compared to the non-coated one . As a res ult, the exergy destruction rate for the condenser is higher for the cycles that utilize enhanced evaporators. 24 Figure 7. a,b) Thermodynamic cycle of two phase air conditioning system with rough - and porous- and non -coated evaporators. Here , the rough -coated evaporator is tested and compared to the non-coated evaporator at wall superheat of 0.5 K and porous -coated evaporator was tested and compared to the non-coated evaporator at wall superheat of 5 K. c,d) Percentage of exergy destruction in different components of the lab -made air conditioning system with the mentioned evaporators Air conditioning systems - most important energy systems - are one of the largest energy consumers in residential and commercial buildings. In the United States alone 400 terawatt - hours of electricity are consumed annually for air conditioning purposes [51]. Setting higher efficiency standards for cooling devices is one of the approaches, which can dramatically reduce the need for new power plants, cut gas emissions, and reduce the costs. Our proposed bio -coating to be implemented to air conditioning systems will not only significantly reduce the green -house gas emission but it will also assist in saving billions of dol lars. 4. Conclusion 25 A new class of bio -based surface modification technique for thermal -fluid applications is presented and validated using a heat resistant microorganism; Sulfolobus solfataricus . The proposed bio -coating was cultured and optimized to provi de organic roughness and porous coating on the desired metallic and non -metallic substrates. Fundamental studies were performed to investigate the enhancement in boiling heat transfer. Afterwards, the coating was successfully adapted and tested in miniatur e air-conditioning systems for performance assessment. Significant enhancements in performances (based on both first and second laws of thermodynamics) were achieved. Unlike most of the available surface modification techniques, which require expensive microfabrication devices (physical modifications) and have damaging impact on the environment (chemical modifications), the proposed technique is economical, scalable, biocompatible and eco - friendly. Supporting Information Coating apparatus, boiling and air conditioning experimental setups, data reduction and uncertainty analysis are provided in the Supporting Information file. Author Contributions S.N. and A.K.S. designed the experimental setups and conducted the experiments. V.O.K. and G.G prepared and opt imized the bio -mixture. S.Ç. characterized the samples. G.G, Ö.K and A.K. provided guidance and comments. A.K.S and A.K. supervised the project. All the authors contributed to the preparation of the manuscript. Declaration of Interests The authors declare no competing interests. Acknowledgements This study was supported by TUBITAK (The Scientific and Technological Research Council of Turkey) Support Program for Scientific and Technological Research Project Grant No. 26 119C026. The equipment and characterizat ion supports are provided by the Sabanci University Nanotechnology Research and Applications Center (SUNUM). The authors also thank Mohammad Jafarpour and Kosar Hassannezhad for their assistance in capturing Scanning Electron Microscopy (SEM) images.