Efficient Working Fluid Research Articles

Thermo-economic optimization of hybrid solar-biomass driven organic rankine cycle integrated heat pump and PEM electrolyser for combined power, heating, and green hydrogen applications

The energy sector prioritizes innovative clean energy production. This research explores a versatile hybrid solar-biomass trigeneration system, integrating organic Rankine cycle (ORC), a heat pump, and a proton exchange membrane electrolyser. Sustainable and adaptable, it produces electricity, heating, and hydrogen for Indian paper and pulp industries. The study assesses the system's thermodynamic, economic, and environmental feasibility. Initially, parametric analysis and first layer multi-objective optimization (MOO) are performed to select the efficient working fluid and operating temperatures. Benzene outperforms other fluids, achieving an energy utilization ratio (EUR) of 0.27 and exergy efficiency of 75 %. A typical heat requirement of 250 kW for processing 150 kg of paper necessitates collector areas of 3500 m2 and 4500 m2 in Aurangabad and Tiruchirappalli, respectively, and biomass waste of 0.05 kg/s. The combined system performance during day time and night time operation was studied by altering the critical solar radiation, ambient temperature and biomass waste flow rate. The second layer MOO revealed an optimum EUR of 0.25 and exergy efficiency of 9 % at the lowest cost of 33 $/h for biomass waste operation. In comparison to the conventional electricity-powered system, the proposed system would reduce CO2 emissions by 90 kg/h to 270 kg/h.

Thermo-hydraulic characteristics and structural optimization of supercritical CO2-cooled microchannel heat sink

Supercritical carbon dioxide (sCO2) serves as an efficient working fluid in microchannel heat sinks (MCHS), effectively mitigating heat dissipation challenges in high heat flux microelectronic devices. Nonetheless, the issue of flow acceleration affecting micro-scale heat transfer, especially in channels with diameters <0.2 mm, warrants additional investigation. Flow acceleration exacerbates temperature non-uniformity, consequently diminishing heat transfer efficiency. Numerical simulations were conducted to examine the thermo-hydraulic characteristics of sCO2 in microchannels featuring complex structures including ribs and cavities. The channel had a diameter of 0.133 mm, with heat flux to mass flow rate (q/G) ranging from 0.1 to 0.5 kJ/kg. Three-dimensional entropy production rate (Sg) maps and corresponding heat maps were utilized to establish boundary conditions and evaluate heat transfer optimization. Additionally, structural optimizations were performed to rectify temperature non-uniformity in sCO2 within MCHS. The results indicate that the entropy production rate (Sg) of sCO2 initially decreased and then increased with the increase in q/G and dimensionless enthalpy (h*). The optimized MCHS demonstrated enhanced temperature uniformity, with a maximum temperature difference of 1.8 K, notably lower than that of the non-optimized MCHS (5.3 K). This study offers a theoretical basis for the design and implementation of sCO2 in micro-scale MCHS.

Range of application of heat pump systems on various natural refrigerants

GROUNDING: The utilization of heat pumps (variation of vapor compressing machines) for heat transfer between different temperature levels is a promising development trend, considering low cost of heat energy, environmental friendliness of working mediums, economically sufficient solutions and low cost maintenance. Heat pumps are used for heating in building structures where low potential heat sources are wastewater streams, groundwater streams, air, heat of the soil. It is also possible to use low potential heat of heat emissions from industrial plants, that can be cost effectively in terms of production resources usage.&#x0D; AIMS: Selection of the refrigerant and its composition in order to provide operational stability for heat pumps at temperature levels from 48 to 95 ̊C with minimum energy consumption for the most widespread thermodynamical cycles.&#x0D; MATERIALS AND METHODS: Standard thermodynamic cycles of heat pumps were simulated using Aspen HYSYS software. Further optimization and parameter selection were performed using MATLAB module Global Optimization Toolbox.&#x0D; RESULTS: In compliance with demanded design parameters the results of each working medium simulation were obtained. Energy efficient parameters of the common heat pumps thermodynamical cycles were determined.&#x0D; CONCLUSIONS: According to exergy analysis of the thermodynamical cycles it has been determined that mixed refrigerants, pentane and ammonia are the most suitable and energy efficient working fluids for heat pumps.

Binary liquid film condensation from water-ammonia vapors mixture flowing downward along a parallel plate condenser

The ammonia-water film condensation is used as an efficient working fluid in industrial applications such as refrigeration, plate condenser and evaporator, absorber/generator heat exchange, air-conditioning, heat pumps and separation processes. The present work focuses on a numerical investigation of water-ammonia condensation on a falling binary liquid film inside a parallel plate condenser by mixed convection. The parallel plate condenser is composed by two parallel vertical plates. One of the plates is wetted by liquidfilm and cooled by the thermal flux cooling while the other plate is isothermal and dry. Parametric computations were performed to investigate the effects of the inlet parameters of gas, the properties of the binary liquid film as well as the thermal flux cooling on the combined mass and heat transfer and on the efficiency of the parallel plate condenser. The results show that an increase in the inlet vapor of ammonia as well as of vapor water enhances the efficiency of the parallel plate condenser. It is shown also that an enhancement of efficiency of the parallel plate condenser has been recorded when the thermal flux cooling and inlet liquid flow rate is elevated. Whereas the increase of the inlet liquid concentration of ammonia inhibits the efficiency of the parallel plate condenser.

Investigating the performance of heat exchangers in absorption heat pump systems using both numerical and experimental methods

To achieve better heating efficiency and lower CO2 emission, this study has proposed an air source absorption heat pump system with a tube-finned evaporator, a vertical falling film absorber, and a generator. To analyzeboth heat and mass transfer performances and optimize the sizes of both the absorber and the generator, a distributed parameter model and a two-dimensional numerical model have been adopted, both validated. To develop an environmentally efficient working fluid pair for absorption heating for cold climate, a calculation method adopting fugacity and activity models was developed. The defrosting control strategy of this system was developed based on a spatial and temporal frost development model, which determines the characteristics of frost distribution, frost growth, and frost inhomogeneity. To evaluate the functionality of this system, a test rig was constructed, with a heating capacity of 36.88 kW, a coefficient of performance of 1.54 under evaporation temperature and supply water temperature of −9.2 °C and 38.4 °C, respectively. Validation results showed a 1.5 % higher prediction accuracy for the two-dimensional model with correction, comparing to the distributed parameter model. R134a-DMF and R161-DMF were recommended at an ambient temperature of −2 °C. This is because with ambient temperature of −7 °C and supply water temperature of 41 °C, the predicted coefficient of performance was 1.04 and 1.06 for R134a-DMF and R161-DMF, respectively. The frost prediction indicated that at the time of 3,600 s, the thickness of the frost layer in the heavy frost area was 0.94 mm, with a total frost mass of 3,995 g. Compared with the initial stage, the sensible and latent heat transfer rates decreased by 22.1 % and 24.2 %, respectively.

Enhanced heat transfer characteristics of the mini hexagonal tube heat sink using hybrid nanofluids

The thermal performance of different hybrid nanofluids on the mini hexagonal tube heat sink has been investigated experimentally. Al2O3/SiO2-DIW, Al2O3/CuO-DIW, and CuO/SiO2-DIW nanofluids have been used as the working fluids. The aggregate of 0.01 volume fraction of Al2O3, CuO, and SiO2 nanoparticles for hybridizing ratios (80:20) has been considered to prepare deionized water-based hybrid nanofluids. The thermal performance of three different hybrid nanofluids inside the mini hexagonal tube heat sink has been assessed. The varying volumetric flow rates of 15–50 lph have been maintained on the hexagonal tube side and 50 lph on the mini passage side. Herein, the proposed hexagonal-shaped heat sink provides a larger heat contact surface to exchange the heat between the hybrid nanofluids and hot DIW. It is divulged that the increase in the volume flow rate of nanofluids promotes the higher Brownian motion, leading to an increase in convective heat transfer. Amongst three hybrid nanofluids, CuO/SiO2-DIW nanofluids have revealed a slight increase in pumping power while increasing the volume flow rate. From the experimental results, it is shown that the heat transfer coefficient, effectiveness, and Nusselt number have been improved by 54.5%, 42%, and 28.4%, respectively, for Al2O3/CuO-DIW nanofluids. Thus, Al2O3/CuO-DIW nanofluid can be deemed to be an efficient working fluid for the miniature heat sinks to accelerate the heat transfer rates.

Does Particle Size in Nanofluid Synthesis Affect Their Performance as Heat Transfer Fluid in Flat Plate Collectors?—An Energy and Exergy Analysis

A flat-plate collector is one of the most common solar collector systems due to its comparatively lower cost and maintenance. However, the performance of this type of collector is low; thus, research studies in the past decades have focused on improving its efficiency through various plate configurations and efficient working fluids. This study recognizes the research gap regarding the influence of nanoparticle shapes and their effects on improving the heat transfer properties in flat-plate collectors. In this study, fly ash nanofluid at 0.5% (with a range of nanoparticle sizes) was used as a working fluid to investigate the performance of a flat-plate collector. This study analyzed the behavior of the collector system via a range of Reynolds numbers in its laminar regime, between 800 and 2000. The results measured in this study showed that the maximum energy efficiency measured was 73.8%, which was recorded for the fly ash nanofluid at a nanoparticle size of 11.5 nm. At a Reynolds number of 2000, the fly ash nanofluid with a nanoparticle size of 11.5 nm showed a top heat loss coefficient of 4.78 W/m2K, while the top heat loss coefficient of a nanoparticle size (NPS) of 114 nm was 5.17 W/m2K. This study provides a framework for the significance of the nanoparticle size in the synthesis of nanofluids in both mono and hybrid composites and application in solar collector systems.

Computational Analysis of Water/Cu Nano Fluid Dynamic Viscosity using Molecular Dynamic Simulations

To enhance thermal management for micro-electric devices, nanofluids become promising working fluids for many thermo-fluid applications. Thermal characteristics of the working fluids can be improved by nano particles additives dispersed in the working fluid such as Cu-nanoparticles in water. The nanoparticle additives manage to alter thermal and dynamic properties of the working fluids such as the dynamic viscosity which plays an important role in specifying thermal and dynamic behaviour of the working media. In order to understand the effect of modifying the dynamic viscosity of the working media, the effective value of this property must be determined. The molecular dynamic (MD) simulation has been used to estimate the Cu/water nanofluid dynamic viscosity at partial volume fractions of φ=0.0125 % and φ= 0.02478 %, and at working temperatures 293 K, 303 K, 313 K, 323 K and 333 K. The used spherical shape nanoparticles are made up of numbers of 0.3-nm-diameter Cu-atoms. The MD simulation results have been compared to reliable experimental and analytical results. The estimated values of the dynamic viscosity using MD simulations converge very well to the experimental and analytical values of the dynamic viscosity, which reveals the advantages of using MD simulations to determine physical properties of nanofluid working medias and hence to design more efficient working fluids. The RDF shows good results for the SPCE model

Comprehensive thermodynamic and exergoeconomic analyses and multi-objective optimization of a compressed air energy storage hybridized with a parabolic trough solar collectors

This paper designs a novel power plant consisting of a medium-temperature solar field based on parabolic trough solar collectors, an organic Rankine cycle, and a compressed air energy storage unit. The solar field supplies the energy required by the organic Rankine cycle at the charging period, providing the power for the compressors and high-temperature energy storage. Also, for the performance improvement of the system, zeotropic mixtures are employed as the working fluids in the Rankine cycle. The system was assessed from energy, exergy, and exergoeconomic viewpoints. A comprehensive parametric study was performed to examine the effects of some key parameters on the performance of the devised system. And, the MOPSO optimization algorithm is selected to optimize the calculations, wherein the TOPSIS decision-making method identifies the optimal solutions. According to the results, Isopentane/R142b mixture was selected as the efficient zeotropic working fluid of the Rankine cycle. Hence, the optimum energy efficiency and exergy efficiency were computed to be 5.08% and 5.04%, respectively. Besides, the optimum round trip efficiency and exergetic round trip efficiency were obtained by 57.06% and 66.20%, respectively. Moreover, considering electricity cost of 0.36 $.GJ−1, the lowest payback period of 4.39 years, and the highest profitability were obtained.

State-of-the-art review on water-based nanofluids for low temperature solar thermal collector application

In the last decade, nanofluids have set significant milestones as efficient working fluids in the field of solar energy conversion to meet rising energy demand. Research on thermophysical properties, long-term stability, and rheology is progressing to achieve effective practical deployment of nanofluids in renewable solar photo-thermal energy conversion sectors (i.e., solar collectors). Nonetheless, researchers and engineers are having a difficult time coping with nearly infinite culpable variables influencing the output of various types of nanofluids. This paper aims to provide an up-to-date analysis of the developments and challenges of widely used water-based nanofluids, with a focus on formulation methods, main properties (thermophysical, stability, and rheological), and effective implementation in low temperature solar collector systems. Previous experimental and numerical studies on the subject have been compiled and thoroughly scrutinized, providing crucial phenomena, mechanisms, flaws, and responsible parameters for achieving stable and optimized thermal properties that integrate with heat transfer performance. It has been discovered that optimizing the critical factors leads to superior behavior of the nanofluids, which results in improved thermal efficiency of the solar collectors. Finally, emerging concerns are identified, as are potential recommendations to resolve existing problems in the field for future advancement that would mobilize rapid progress and practical engineering use of water based nanofluids on solar collectors. • Water-based nanofluids are studied comprehensively. • Formulation methods and crucial thermo-physical properties are critically assessed. • Stability of the nanofluids is investigated in detail with associated factors. • Application of the nanofluids on low temperature solar collectors is scrutinized. • Existing challenges are disclosed, and future recommendations are made.

Effect of working fluids in a novel geothermal-based integration of organic-flash and power/cooling generation cycles with hydrogen and freshwater production units

One of the essential steps to design energy conversion-based systems is choosing an efficient working fluid under the design goals to access stable products with high efficiency and overcome environmental issues. In this regard, the current paper is motivated to devise and evaluate a novel geothermal-driven multigeneration system under the effect of various working fluids. The proposed system consists of a flash-binary geothermal power plant, an organic flash cycle (OFC), a power/cooling subsystem (an organic Rankine cycle (ORC) and a thermoelectric generator incorporated with a compression refrigeration cycle), and freshwater and hydrogen production units utilizing a humidification-dehumidification desalination unit and a low-temperature electrolyzer. Considering the design potential of the OFC and ORC, four different environmentally-friendly working fluids, i.e., R123 and R600 in the OFC and R1234yf and R1234ze(e) in the ORC are selected and classified in four groups to introduce the best one, under the energy, exergy, and economic (3E analysis) approaches. Also, the whole system is optimized through a genetic algorithm, respecting the optimal solution for the energy efficiency and unit exergy cost of the products. According to the results, R123/R1234ze(e) shows the highest cooling, hydrogen, freshwater production rates, and energy efficiency. Likewise, the maximum power generation and exergy efficiency belong to R600/R1234ze(e). Moreover, R600/R1234yf has the lowest unit exergy cost of products.

Thermal enhancement in hybrid nano-polymer using novel models for hybrid nanoparticles

Novel correlations for thermophysical properties of base fluid (micropolar fluid) and hybrid nanostructures are used to analyze the impact of the inclusion of copper oxide and titanium dioxide thermal enhancement in the polymer with microstructures. The polymers are assumed to be characterized by micropolar rheological equations. The models from governing laws are derived and are simplified by boundary approximations. The boundary conditions are based on no-slip. The finite element method (FEM) is used for numerical solutions. The mesh and convergence analysis are tested. The convergent solutions are further utilized for the simulation process. The wall shear and couple stresses are examined and the heat transfer rate is visualized. This study has predicted thermal optimization due to the simultaneous inclusion of copper oxide and titanium dioxide nanoparticles. The hybrid nanofluid is an efficient working fluid than the pure fluid. However, the presence of hybrid nanoparticles in micropolar fluid makes the viscous region wider. It is found that nano polymers with CuO experience less resistive force due to porous medium as compare to hybrid nanopolymers (CuO−TiO2−polymers). The angular motion slows down as the vortex viscosity parameter is increased. However, this decrease is more noticeable in hybrid nano-polymers than that in nanopolymers.

Process Systems Engineering Evaluation of Prospective Working Fluids for Organic Rankine Cycles Facilitated by Biogas Combustion Flue Gases

The organic Rankine cycle (ORC) has recently emerged as a practical approach for generating electricity from low-to-high-temperature waste industrial streams. Several ORC-based waste heat utilization plants are already operational; however, improving plant cost-effectiveness and competitiveness is challenging. The use of thermally efficient and cost-competitive working fluids (WFs) improves the overall efficiency and economics of ORC systems. This study evaluates ORC systems, facilitated by biogas combustion flue gases, using n-butanol, i-butanol, and methylcyclohexane, as WFs technically and economically, from a process system engineering perspective. Furthermore, the performance of the aforementioned WFs is compared with that of toluene, a well-known WF, and it is concluded that i-butanol and n-butanol are the most competitive alternatives in terms of work output, exergy efficiency, thermal efficiency, total annual cost, and annual profit. Moreover, the i-butanol and n-butanol-based ORC systems yielded 24.4 and 23.4% more power, respectively, than the toluene-based ORC system; in addition, they exhibited competitive thermal (18.4 and 18.3%, respectively) and exergy efficiencies (38 and 37.7%, respectively). Moreover, economically, i-butanol and n-butanol showed the potential of generating 48.7 and 46% more profit than that of toluene. Therefore, this study concludes that i-butanol and n-butanol are promising WFs for high-temperature ORC systems, and their technical and economic performance compares with that of toluene. The findings of this study will lead to energy efficient ORC systems for generating power.

Synthesis of therminol‐graphite nanofluids and photo‐thermal conversion properties

Therminol-based nanofluids with graphite nanopowders (GNs) were proposed as an efficient working fluid in direct-absorption solar collectors (DASCs), especially for elevated-temperature applications. In this work, Therminol VP-1/GN nanofluids (TG nanofluids) were prepared in two steps with surfactant (Triton X-100). In the synthesis procedure, the vortex generator for coating GN with sticky surfactant (μ = 240 mPa⋅s) and the ultra-sonicator for diluting GN/Triton X-100 mixture with Therminol VP-1 are used. We have demonstrated excellent dispersion stability up to 4 weeks and even after heating over 100°C without any surface modification of nanoparticles. The feasibility of using the synthesized TG nanofluid as a working fluid in DASCs was experimentally demonstrated by measuring its absorption coefficient as well as by characterizing photo-thermal conversion performance. The absorption coefficient of the TG nanofluid at 0.004 wt.% of GNs was measured to be 1.12 cm−1, which is more than eight times enhanced from that of bare Therminol VP-1 (ie, 0.13 cm−1) in the wavelengths from 350 to 1600 nm. Finally, the temperature increment of 43°C was achieved after t = 3600 s under the 5-sun condition in the static photo-thermal conversion experiment. Moreover, the low viscosity of TG nanofluids (only 2.2% higher than that of the bare Therminol VP-1) supports the superiority of TG nanofluids to be applied as working fluid in DASCs.

Assessment of a solar-driven cogeneration system for electricity and desalination

The goal of the present paper is the investigation of a solar desalination system with an organic Rankine cycle system for power and freshwater production. This system is an environmentally friendly technology that is able to utilize solar energy properly in a novel cogeneration application. A parabolic trough concentrator with a smooth and corrugated receiver was employed as the heat source of the desalination system. A humidifier–dehumidifier desalination technology was used for producing freshwater. The electricity is produced by an organic Rankine cycle which is fed both by the solar field and by the hot brine. The present analysis is performed by using a detailed numerical model which is validated by experimental literature data. Based on the final results, the corrugated tube has a maximum performance of 66.59%, and it is more efficient than the smooth tube with 63.11%. The average freshwater productions were estimated equal to 13.09 kg hr−1 and 12.71 kg hr−1 for the corrugated and smooth tubes, respectively. The maximum net work production is found at 7.57 kW with R113, while the less efficient working fluid is R134a. It was found that the application of the developed desalination system leads to the production of high amounts of fresh water and a significant reduction of the equivalent CO2 emissions.

Effect of helium xenon as working fluid on centrifugal compressor of power conversion unit of closed Brayton cycle power plant

Closed Brayton cycle (CBC) having single-shaft, centrifugal type compressor is considered as an efficient energy-conversion option associated for gas-cooled reactor (GCR) heat source. In terrestrial power plants and space power systems noble gases are considered as an efficient working fluid for most of the GCR's and CBC engines. The effectiveness of various noble gases as working fluid in closed cycle power plants for the power conversion units is of imperative concern. Although pure helium is relatively difficult to compress nonetheless it is measured as the best coolants for closed Brayton cycle power plants due to its better transport properties. Due to compression properties, its use resulted in the requirement of more mass, bigger size, higher cost and relatively more dynamic problems of rotatory machines in energy conversion system. The mixture of xenon with helium up to a molecular weight of <40 g/mol resulted in an increase of the coefficient of heat transfer as well as the significant increase in the loading of the compressor. Therefore, performance analysis is conducted for a novel design of helium xenon centrifugal compressor. The performance analysis is conducted with different molecular weight mixtures of helium xenon using similarity criteria. It is concluded that the use of helium xenon 40 g/mol is the optimum choice for space applications and 15 g/mol for closed Brayton cycle terrestrial power plants.

Thermo-economic performance and sensitivity analysis of ternary hybrid nanofluids

The thermo-economic performance of the Al2O3-TiO2-Cu/water ternary hybrid nanofluid was investigated experimentally. The shear stress, viscosity, and thermal conductivity were measured at 0.005–1 vol% over temperature range of 20–60 °C. The morphology and stability of nanofluids were characterized by SEM and UV–Vis spectrophotometer analyses. To obtain the optimum nanoparticle mixture ratio, sensitivity analysis of viscosity and thermal conductivity was conducted for 1 vol% mixtures with five different nanoparticle mixture ratios. The results showed that the optimum mixture ratio with the highest thermal conductivity and lowest viscosity was found to be 40:40:20 (Al2O3:TiO2:Cu). Following this, the experimental data were compared with literature data for the corresponding single nanofluids and hybrid nanofluids. Due to different sizes of nanoparticles in ternary hybrid nanofluids, small and densely-packed clusters will result in relatively higher thermal conductivity and lower viscosity compared with single and binary hybrid nanofluids. Economic analysis revealed that Al2O3-TiO2-Cu/water ternary hybrid nanofluids with 0.7 vol% and 1 vol% are the most efficient working fluids in the laminar and turbulent flow regimes, respectively. These results highlight the strong potential of ternary hybrid nanofluids for use in industrial heat transfer applications

Selected water thermal properties from molecular dynamics for engineering purposes

Water is the most commonly used fluid in most heat transfer applications. Estimating water thermal properties in an efficient and reliable way is thus important. We propose here molecular dynamic simulations for this purpose and explore a methodology for calculating water thermal properties from such simulations. This is significant, particularly to compare fluids to design an efficient working fluid for heat exchangers or cooling microelectronic devices. Lammps and Moltemplate models have been used in this study to calculate selected water properties. The atomistic model of water is the extended simple point charge (SPC/E) model. The thermal properties are thermal conductivity, viscosity, density, specific isochoric heat capacity, specific isobaric heat capacity, Prandtl Number, and Volumetric thermal expansion coefficient. These results are compared with experimental published results for water at temperatures 288 K, 300 K, 312 K, and 324 K and pressure 1 atm. The simulated values are sufficiently close to reliable published values, this demonstrates the usefulness of molecular dynamics simulations., which can thus be used as an effective and relatively cheap tool to investigate thermal and dynamic properties of working fluids in thermofluid design.

Thermo-hydraulic performance of nanofluids in enhanced tubes - a review

Technological developments are continuously growing to improve the efficiency of thermal systems on the demand of present era. To get rid of the scientists and engineers from traditional heat transfer intensification methods and to miniaturize thermal transport devices, novel and innovative techniques are obligatory. In this whole drive, the effective tube geometries and efficient working fluids perform vital role in the betterment of heat exchange devices. Geometrical argumentations/surface modifications impose on smooth tubes are called as enhanced tubes. These tubes contribute towards high rate of heat transfer by reducing the formation of boundary layer near the wall surfaces of the tube. Also, nanofluids with superior thermo-physical properties compared to conventional fluids also assist the rate of heat exchange in various heat transfer applications. This article begins with the introduction of enhanced tubes and nanofluids and followed by overview of nanofluid synthesis and different tube geometries. An attempt has been made to summarize last decade research contributions in the field of forced convection heat transfer and flow characteristics (HTFC) with employing the combined effect of enhanced tubes and nanofluids. Additional heat transfer enhancement techniques (swirl flow devices, magnetic field, agitation, rotation and pulsation) brace with present ones, are also discreetly deliberated. Proposed correlations by authors are also represented which shows the good agreement with experimental data. Important findings and outcomes from reviewed studies are compiled in discussion section. A number of suggestions and guidelines are highlighted to intensify the heat transfer rate as well as to mitigate the effects of pressure losses in fluid flow pipes.

Numerical investigations of CO2 and N2 miscible flow as the working fluid in enhanced geothermal systems

Abstract In this study, the heat extraction from geothermal reservoirs with the application of CO2 and N2 miscible flow as the novel working fluids has been investigated based on the discrete fracture network model. The miscible flow with different CO2 and N2 proportions and highly pressure and temperature dependent properties have been integrated into reservoir simulations. The heat production processes for CO2 and N2 miscible flow have been simulated with two different discrete fracture networks, which also take the thermal-hydraulic-mechanical (THM) coupling mechanisms into consideration. Based on simulation results, it is revealed that the heat extraction efficiencies show an irregular trend with the increasing proportions of N2 in the mixture. It is found that the miscible flow with two different N2 proportions (20% and 40%) can be a more efficient working fluid than the one with larger N2 proportions. Based on simulation results, cumulative heat production curves that represent ten CO2 and N2 proportions can be divided into three categories. Evaluations of relevant permeabilities and effective normal stresses at sample points were made, which contributed to the comprehensive analysis of the heat transfer processes. It is also found that there are reasonable ranges for the miscible flow with different CO2 and N2 proportions as working fluids that allows higher heat extraction efficiencies, which are validated by a series of comparisons. Thus, in order to define the humps on the heat extraction efficiency curves, a new concept of the optimized heat extraction efficiency range is proposed and validated. It is proved that fluid properties of the miscible flow with different CO2 and N2 proportions and the reservoir temperature determine the optimized heat extraction efficiency range directly. This study proposes the miscible flow of CO2 and N2 as the working fluid, which provides a new alternative for geothermal energy production.