A review of the energy recovery and energy pressure of liquid

Due to the rapid development of today's industry, it not only greatly accelerates the consumption of energy, but also causes serious pollution to the environment. Therefore, reducing energy waste and recovering energy has become an important research direction today. This paper introduces the future development trend of renewable energy, discusses the progress of recovery of nonpolluting energy such as marine, bioenergy and thermal energy, and analyzes the efficiency and social benefits of various types of energy recovery. Energy recovery methods for natural gas pipelines are briefly analyzed, and current research advances in energy pressure recovery devices are provided, leading to a summary of methods to improve the efficiency of fluid energy recovery. Finally, it is concluded that although the current energy sources are diversified, the most valued energy sources at present and in the future should be solar energy and ocean, which are infinitely recyclable. This paper will benefit researchers to further understand and study the various aspects of energy recovery.


| INTRODUCTION
The recovery of liquid energy in industrial processes is one of the most important means of saving energy and reducing emissions.Moreover, improving the energy recovery rate not only avoids the waste of energy, but also indirectly reduces environmental pollution.Therefore, how to recover liquid energy in processes such as hydrogen liquefaction, seawater desalination and liquefied natural gas (LNG) with high efficiency has become a hot topic of research in related fields internationally. 1 Currently, one of the main problems facing the hydrogen liquefaction industry is the high specific energy consumption and its high manufacturing costs.In response, Masoud Taghavi has developed a new structure for the production of liquid hydrogen.A sensitivity analysis has shown that the unit has improved both the inlet liquid air mass flow rate and the power provided by the solar panels, resulting in good production efficiency and reduced energy consumption compared with previous liquid hydrogen production structures. 2 Figure 1 indicates that the use of LNG cold energy is also very popular in some Asian countries, such as Japan, where the use of LNG cold energy is divided into two main categories: first, for integration with external plants or cooling systems, such as air separation, liquid CO 2 and dry ice freezers, and second, for the use of the receiving station itself, such as deep cooling for power generation and recondensing of natural gas.In Korea, LNG cold energy is mainly used in food freezing and refrigeration and air separation.To be able to recover the waste cold energy released during the regasification of LNG.
Le et al. 3 used a direct expansion configuration involving different steps of expansion and mass flow rate extraction at intermediate pressure levels in a mathematical model for pressure energy recovery, followed by a direct configuration of an Organic Rankine Cycle (ORC) to recover the remaining cold energy.][6] Figure 2 the system components to achieve the cold energy step can be divided into five parts: the gasoline vapor recovery part, the power generation part, where the azeotropic mixture is used to drive the expander and generate electricity through phase change; the low temperature methanol scrubbing part, where the cold energy is used to remove hydrogen sulfide (H 2 S), carbon dioxide, carbonyl sulfur (COS), and other impurities from the feed gas; the air conditioning part, where a 50% glycol solution is used as the working fluid to transfer the LNG cold energy for residential and office buildings; and the air liquide storage part, where air is used as the working fluid to store the additional cold energy in a storage tank.
Figure 3 illustrates that among other things, ORC can be used not only for cold energy recovery, but also for energy recovery from solar, industrial heat, biomass, geothermal and bottom power cycles.
Memon et al. 7 started using a reverse osmosis (RO) system to be able to recover energy from oil and gas fields and produce fresh water, and tested it against three data models.The tests showed that the RO system with two energy recovery units was more efficient and that the energy consumption of such a system was significantly lower and that the unit was suitable for working in areas where water was scarce. 8,9odification of the energy recovery device (ERD) is also a hot topic of research at present.To achieve rapid interchangeability in the work of ERDs.Sun witnessed the innovative development of a new type of isobaric ERD, called the swashplate-plunger energy recovery device (SP-ERD), based on the structural characteristics and working mode of swashplate axial plunger pumps.After several experimental tests, the results showed that F I G U R E 1 Application of liquefied natural gas cold energy recovery technology in selected Asian countries. 2I G U R E 2 New structure for realizing cascade utilization of cold energy. 5he commercial utilization of the device was very high due to reduced volume mixing and improved volume utilization.10 As less research has been reported on smallscale ERDS in reduced seawater reverse osmosis (SWRO) desalination, a new piston type high-pressure pumpenergy recovery device (HPP-ERD), which simultaneously pressurizes raw seawater and recovers water energy from concentrated brine, was invented by Song et al. 11 and tested on a simulated laboratory platform.The test results show that the HPP-ERD is not a single high-pressure pump and also significantly reduces the payback period and desalination costs of the proposed project.Twin-screw expanders are one of many types of ERDs that recover the pressure energy lost during throttling, but how to adjust the capacity of the expander under fluctuating conditions has recently become a challenge.To this end, Tian Yafen has developed a thermodynamic mathematical model to investigate this challenge. Th simulation results show that the speed has a more pronounced effect on the performance of the twin-screw expander.Therefore, Tian Yafen concluded that speed regulation is the most effective way to adjust the capacity of an expander.12 In recent years, the contradiction between energy shortage and excessive energy consumption in industrial production has become more and more pronounced, and for this reason research into liquid energy recovery has developed in the direction of specialization and diversification.In the past few years, research related to liquid energy recovery at home and abroad has been developing rapidly, with advanced technologies abroad already achieving sufficiently satisfactory recovery efficiency and performance, and domestic research gradually getting on the right track.So it follows that research into liquid energy recovery is extremely important.
The structure of this paper is presented below.Chapter 2 focuses on the recovery of energy from three sources: ocean and bioenergy.Chapter 3 focuses on process optimization, project development and common recovery methods for recovering pressure energy from natural gas pipelines.Chapter 4 concludes the text.

| TYPES OF ENERGY RECOVERY
The continuous development of human population has greatly increased energy consumption, resulting in energy shortage and serious environmental pollution.Therefore, it is necessary to shift from fossil fuels to renewable energy, including wind energy, solar energy, marine energy and bioenergy.Among them, compared with other renewable resources, bioenergy utilization shows many advantages because of easy transportation/ storage and rich availability.However, the research on the recycling of other energy sources is relatively few and the advantages are relatively few. Figure 4 the system composition for realizing the cascade utilization of cold energy can be divided into five parts: gasoline steam recovery part, in which propane is used as the working fluid to transmit cold energy to the oil-gas separator; The power generation part, in which the azeotropic mixture is used to drive the expander and generate power through phase change; The lowtemperature methanol washing part can be used to remove H 2 S, carbon dioxide, COS and other impurities in the feed gas; Air conditioning part, in which 50% glycol solution is used as working fluid to transmit LNG cold energy for residential and office buildings; And a liquefied air energy storage component, wherein air is used as a working fluid to store additional cold energy in a storage tank.
Figure 5 integrated energy recovery systems for natural gas engines: including Supercritical Brayton Cycles, absorption refrigeration cycles, organic flash cycles, refrigeration storage units and effluent purification units.

| Wind energy recovery
In the 20th century, the world's energy consumption increased more than ten times than before.In the 21st century, the world's energy consumption will further increase, and the industrialization and energy demand of developing countries will continue to grow.Therefore, to solve this problem, attention should be paid to the effective use of all available energy.
The Savonius wind turbine with housing is installed on the outlet side of the industrial exhaust fan to collect exhaust energy for power generation.Compared with other wind turbines, it has the following advantages: low noise, self starting, low wind speed rotation, simple structure, low manufacturing cost and easy maintenance.However, these industries must install return fans to keep the working environment at medium temperature.These fans circulate air from inside to outside at a high speed of 6 to 12 m/s.In developing countries, the technology has become more popular as new devices are installed throughout the country.One of them is Kombolcha Textile Co., Ltd.located in Kombolcha, Ethiopia.Kombolcha Textile Co., Ltd. is a large company that consumes 2.5-3.0 mw power.The company purchased electricity from the Ethiopian Electricity, Lighting and Power Authority (EELPA).With the rising cost of electricity per kilowatt hour, the company has restored alternative power sources.Therefore, exploring and developing the method of using non natural energy will be a wise solution.
Some scholars have studied: in kombolcha Textile Co., Ltd., Savonius wind turbine is used to collect unnatural wind energy.The blade is made of mild steel sheet metal because this material has higher maximum applied stress resistance than aluminum.The blade is designed with solid works-2015 software and analyzed with ansys-19 software.A prototype rotor blade was manufactured, tested and inspected. 13arlos Méndez with solar chimney and wind energy as the leading technologies, to study the feasibility of using an integrated system for electricity generation and fresh water, this paper analyzes the system as a whole and examines each subsystem separately to determine the overall efficiency associated with each subsystem.Numerical simulations were carried out in an Engineering Equation Solver (EES).The results show that the implementation of a multisystem approach increases the overall efficiency (using wind farms) to 52.53% during discharge and 52.51% when storing fresh water, respectively, 14  F I G U R E 4 Composition of energy recovery application fields. 13I G U R E 5 Comprehensive energy recovery system of natural gas engine. 14hong et al. 15 proposed a green energy power generation system with two vertical axis wind turbines and a housing component mounted above the cooling tower.The housing consists of guide blades and diffuser plates that increase power by increasing the turbine's rotational speed.The Angle of the guide blade and the inclined Angle of the diffuser are optimized.Through testing on cooling towers, the performance of vertical axis wind turbines improved by 7%-8%, with no significant change in the current consumption of fan motors.Installed in a cooling tower driven by a 7.5 kW fan motor, the system runs for 16 h a day and is expected to recover about 17.5 GWh/year from 3000 cooling towers in the business district.Chong et al. 16 also enhanced the wind power by improving the vertical axis wind turbine, increasing the speed by 7% and reducing the time required for maximum speed) by 41%, while also making the installation of the downstream system simpler.
The wake characteristics are important in the performance of rear floating horizontal axis wind turbines and the layout of floating wind farms.Duan et al. 17 studied the effect of platform surge motion on the wake characteristics of a floating horizontal axis wind turbine.Through the research method of parameter values, it was verified that surge motion has an impact on the wake structure and energy recovery of floating horizontal axis wind turbines.In addition, the period of wind and waves is the main factor affecting the energy recovery of the wake.Due to the short period of wind and waves, the interval between floating wind farms should be longer than that of traditional wind farms.In the case of moderate wind and wave periods, the interval may be slightly shorter than in traditional wind farms.Tabatabaeikia et al. 18 optimized the design of exhaust gas recovery wind turbine by computational fluid dynamics, the design not only optimizes the power generation of the system but also ensures the performance of the cooling tower, and their strategy can be proved feasible through a series of experiments, and the optimized device can increase the power generation by 48.6%.
As can be seen in the above articles, the recycling technology of wind energy, which has developed a lot today, is mostly optimized in a way that is relevant for turbines.

| Energy recovery of small RO seawater desalination unit
Fresh water is the main demand for life and health.The total reserves of water resources in the world are about 1.4 billion cubic kilometers.Of these, 97% are in the ocean and 2% are stored in the form of ice in the polar regions.The percentage of available fresh water is about 1%, which is used for the living needs of plants, animals and human beings.
In recent years, groundwater has been polluted due to industry, agriculture and population growth.Clean water is the most precious resource in the world.Seawater desalination and brackish water desalination are important ways to solve the world drinking water shortage.There are two different ways to obtain fresh water by desalination.The first method is called phase change desalination, or thermal desalination, while the second method is called membrane method.In the process of phase change desalination, the heat used by water conservancy changes from liquid phase to steam, and then returns to the liquid again through condensation.In nature, phase change desalination produces rainwater, which is the main source of fresh water supply on our earth.The thermal desalination process requires a lot of energy.It is estimated that 22 tons are produced per day × 106 cubic meters of water requires about 203 m 3 / year × 106 tons of oil.
Based on previous studies, it can be concluded that the use of RO membrane desalination technology is very important for the production of drinking water at an economic price in remote areas.This study attempts to fill the gap in the use of a simple, economical, easy to control but efficient ERS in remote areas.From the perspective of efficiency and economy, the proposed ERS is combined with RO and compared with previous tests. 19n the experiment, the energy recovery system coupled with the RO device is used, and the output fresh water is tested to confirm the drinkability of gas field water.Establish a mathematical model to study the impact of different parameters on factory productivity and efficiency.Compare the experimental and simulation results with previous studies to check the visibility of the proposed system from an economic point of view.
T A B L E 1 Inputs, outputs, and residue of the multigeneration system. 14 3911

Inputs
The current research proposes an easy controller, which can change the controller to adapt to different operating conditions and can use small RO devices to produce cheap fresh water in remote areas.The system reduces power consumption by reducing the size of the main high-pressure pump.In addition, by increasing the pressure of partially desalted water in the membrane and using the saline wastewater in the membrane, the operation cost is reduced.The proposed system is verified by experiments, and the simulation model is designed and compared with the experimental results to test the performance of the system under different working conditions.In addition, an economic study on the feasibility of the plant is also carried out. 20n addition, some researchers have carried out extensive research on seawater energy recovery.As less research has been reported on small-scale ERDS in SWRO desalination, a new piston-type HPP-ERD that simultaneously pressurizes raw seawater and recovers water energy from concentrated brine was invented by Daiwang et al. 11 and tested on a simulated laboratory platform.The test results show that the HPP-ERD is not a single high-pressure pump and also significantly reduces the payback period and desalination costs of the proposed project.To solve the problem of limiting the working time and exploration depth of the Deep Sea Argo due to energy supply, Xue et al. 21proposed a scheme to use seawater pressure for energy recovery and conservation.The case results show that the input pressure of the hydraulic motor is related to its working time and working frequency.The conclusion verifies the feasibility of the scheme of using seawater pressure for energy recovery and preservation of deep-sea exploration floats, and provides a basis for further application research.Touati et al. 22 designed wastewater in terms of both energy recovery and water exchange to investigate the integration of pressure delayed osmosis (PRO) and SRO.The first swro-pro design ("1RO-2PRO" design) performed lower than the second design ("2RO-2PRO" design) at the recommended RO process recovery rate (40-50%).The study shows that two factors strongly influence the performance of the PRO, namely the capacity of the SWRO and the initial seawater concentration.The process capital cost of adding additional components to the SWRO plant has implications for the economic viability of the combination of the two PRO subsystems.
Jianbo et al. 23 explored the feasibility of recovering Salinity Gradient Energy (SGE) by the red method and investigated the effect of different inlet parameters on the performance of the red chimney.An experimental study using the red chimney to recover SGE from desalinated seawater and seawater verified the feasibility of energy recovery through comparative experiments.The results show that the power density of desalinated seawater (3.0 mol/L)/seawater (0.50 mol/L) is slightly higher than that of seawater/freshwater (0.060 mol/L) and lower than that of desalinated seawater/freshwater.High concentrations, high temperatures and high flow rates can effectively improve the performance of the stack. 24,25leber Marques Lisboa To analyze the impact of membranes on the energy efficiency of heat recovery direct contact membrane distillation (DCMD) systems for desalination, numerical and experimental results from previously reported porous media models are used for validation.The results show that the joint development of membranes and heat recovery systems is the most effective way to improve the competitiveness of membrane distillation compared with other thermal desalination technologies, making membrane distillation a commercially viable option for brackish water and seawater desalination. 26 Note: The permeate inlet temperature is 17.5°C for all cases. 26esult, many experts have studied and analyzed these membrane modules.Nowadays, making seawater desalination and reducing energy consumption is still a challenging problem, so Mohamed Abdelgaied et al. 27 proposed a hybrid HDH-RO system, which consists of five components: photovoltaic panels, vacuum tube SWC, SAC, HDH desalination unit and RO desalination system, and after experimental tests and performance analysis, they found that The water recovery rate of the new hybrid HDD has been greatly improved, with an efficiency of roughly between 48% and 49.8%.A comparison with the previous system also revealed that the new system was more energy efficient.To solve the problem of the ERD affecting the operational stability of the reduced SWRO system due to the formation of switching, Sun Zheng et al. 28 is developing a three-cylinder energy recovery device (TC-ERD) and a matching control strategy with transient overlap function.Several experimental tests were conducted under the prescribed parameter settings, and the final results showed that there was a substantial increase in the high pressure brine flow rate pulsation.
Mansour 29,30 to improve the performance of smallscale RO plants in remote areas.A simulation model was designed and implemented The validity of the proposed ERS was verified by comparing experimental results with model results at different system pressures.The results show that the experimental values do not differ significantly from the simulated values and that the recovery rate has an impact on the energy savings.The ERS saves 80% of the electricity consumption.The total cost of a small-scale RO desalination plant with ERS is significantly reduced.Rami S. El-Emam et al. 31 combined turbines that can be used as energy recovery with desalination plants to study the desalination of seawater with different salinity values.After experimenting with their system, it was known that high pressure pumps and RO modules are extremely important in RO systems based on the efficacy of the system, and the economy of the system they studied was substantially improved.M. Kurihara et al. 30 investigated single-stage seawater at high temperatures and salinity in the Middle East by using PEC-1000 RO membrane modules.In their experiments, they found that this membrane module exhibited excellent performance and durability in SWRO.
Through the above, we found that some innovations in membrane modules are still mainly changes in membrane module materials or new combinations with other machinery and equipment.Among the more important equipment related to desalination are the turbines. 32,33The related literature is described in detail in subsequent sections.

| Bioenergy recovery
Microalgae are widely discussed as a promising biomass raw material for biofuel production.Microalgae are photosynthetic organisms that can fix carbon dioxide and can grow in different habitats and harsh conditions.Many research reports have discussed the advantages of microalgae biomass as raw materials for biodiesel production.However, cultivating microalgae species with high fat yield is still a challenge for the commercialization of microalgae biodiesel technology.For microalgae, many chemical stresses (such as sulfur and nitrogen limitation) have been studied as a simple and cheap method to increase lipid content.On the other hand, the applied stress conditions may inhibit cell growth and significantly reduce volumetric lipid productivity.Although the supply of exogenous organic carbon such as glucose, plant hormones or vitamins may stimulate the growth and lipid content of microalgae, the use of these expensive compounds will increase the production cost of biodiesel.Despite the rapid development in the field of genetic engineering in the past decade, few microalgae species have shown stable expression of transgenic proteins except Volvox catri and rheinhardy Yiyuan, despite the successful integration of transgenic DNA.
Many studies have reported that cold plasma can be effectively used for random mutagenesis of microalgae to enhance some metabolites.However, many developed countries have endless debates and ethical issues about mutant outdoor cultivation.Therefore, the outdoor cultivation of transgenic microalgae needs more rigorous risk assessment and environmental management strategies than the current unmodified wild-type.With strict regulations, options for genetically modified biofuel production and other large-scale industrial applications are limited.In addition, the application of physical stress is a valuable method to improve the lipid productivity of microalgae.For example, the stress of 35°C resulted in excessive accumulation of neutral lipids.Some studies have reported that high-dose cold atmospheric pressure plasma (CAPP) exposure will damage the cell membrane and lead to necrosis, while low-dose will stimulate the separation of cells from cultured cell slices, which may be beneficial to microalgae.However, there is no research on the effect of CAPP as a physical stress on microalgae, rather than its application as a mutagen in the production of biodiesel.In previous screening studies, ordinary C.C. showed the highest biomass productivity and strong adaptability to growth in wastewater, which can be used for economic large-scale planting.Some scholars have studied the effects of CAPP pretreatment on the growth, photosynthetic efficiency, fat accumulation, fatty acid spectrum and biodiesel recovery of common grass.5][36][37][38][39][40][41] Rhee et al. 42 presents an environmentally friendly and improved renewable energy system design.A biorefinery targeting biofuel productivity and incorporating a resource recovery process is designed and a comparative case study is conducted to assess system performance.The results showed that the hydrothermal liquefaction (HTL) had better net operating margin (NOM) and greenhouse-gas (GHG) emission performance compared with the ester exchange, at $8982 and 15567 kg CO 2 equivalent respectively.In conclusion, the HTL-based system showed superior economic and environmental performance.Rios-Del Toro et al. 43 achieves the complete conversion of TB to energy and products by coupling biochemical and thermochemical biorefinery platforms.By applying a central composite design and using two hydrolysates as substrates.The results show that the hydrogen production performance of the detoxification hydrolysate is superior to that of the nondetoxification hydrolysate.Overall, coupling the biochemical and thermochemical platforms of TB allows for the simultaneous refining of gas-liquid products, the extraction of thermal energy and the treatment of bio-waste from the tequila industry.
In order to improve the stability and energy recovery of AD process and avoid the inhibition of oxygen on methane production, two-stage ad of corn straw containing trace salts was used.Some scholars have investigated hydrogen fermentation (the first stage) and methane production (the second stage).The initial pH value and oxygen carrying capacity of hydrogen production by micro oxygen were optimized.The traditional two-stage and one-stage ad (excluding micromanipulation) are also used as a control to reflect the impact of micromanipulation and two-stage method on AD process.In addition, the energy recovery rate of corn straw ad was calculated according to the output of hydrogen and methane.][46][47][48][49][50][51] Cheng et al. 52 investigated the mass and energy balance of high solids AnMBR in FW and SES codigestion and performed an EROI analysis to determine the key factors for improving the net energy balance and whether the bioenergy recovered from the system could be used as a viable alternative to fossil fuels.The results show that high solids AnMBR has the potential to support smart cities in the future in both SES and FW treatments.The relationship between the net energy balance and the carbon to nitrogen ratio is revealed for the first time in this paper, and the sigmoid function developed can predict the energy balance in different regions. 53Yanshan Wang used AD as a biomass pretreatment method in combination with hydrothermal carbonization (HTC) for three typical garden wastes (leaves, branches and grasses) to achieve sustainable resource utilization of garden wastes with high lignocellulosic content.The results show that AD pretreatment can effectively change the surface composition and structural properties of the feedstock, thereby modulating the properties of downstream hydrocarbons.[56]

| Energy recovery of waste heat
The main categories of waste heat recovery are these, which are flue gas waste heat recovery, cooling medium waste heat recovery, waste steam and waste water waste heat recovery, chemical reaction waste heat recovery, high temperature product and slag waste heat recovery, and combustible waste gas and waste waste heat recovery.
In places such as factories or hotels, the cooling fans or cooling towers of the unit coolers used release energy, and this released energy is waste heat pollution. 57,58he industrial waste heat recovery is not only effective in helping enterprises to save energy and create greater benefits, but also to protect the environment and create a green environment.Through the understanding of waste heat recovery in most enterprises, we found that high-quality waste heat is generally recovered for heating, heating water and process make-up heat, while the rest of waste heat is all discharged in the form of cooling or discharged directly.Some enterprises are not mature in waste heat recovery technology or do not invest in waste heat renovation, resulting in a large part of low-grade heat energy all wasted, while other places that need heat have to be heated by boilers. 59Therefore, industrial waste heat heating is one of the most energyefficient and environmentally friendly ways to be promoted.
Several industrial waste heat recovery technologies are summarized in Figure 6.Much of the waste heat energy in the thermal desalination system is not recovered, and combining the energy recovery component with the thermal desalination system can lead to energy savings.The heat exchanger is an important part of waste heat recovery, and the multiflow structure can reduce the heat exchange area of the heat exchanger, thus saving cost and improving efficiency.In addition, the combination of indirect evaporative cooler and heat exchanger can improve the energy transfer efficiency.Organic Rankine cycle technology can recover more heat, and the use of a three-fluid wound tube heat exchanger as a cooler and heat exchanger can further improve energy efficiency.The integration of a two-stage tandem organic Rankine cycle and LNG cooling system can improve energy efficiency.The integration of thermal and chemical energy storage systems can provide good energy recovery for other processes.Two machine learning methods, artificial neural network and nonlinear regression, are built to predict energy consumption to obtain energy reduction solutions.Thermodynamic analysis is also one of the common methods for industrial waste heat recovery to effectively reduce energy use.
Combining energy recovery components with thermal desalination systems can improve performance and reduce costs.Jamil et al. 60 developed a numerical model for cost optimization through the optimization capability of a genetic algorithm and set the heat transfer coefficient, pressure drop, and effluent cost to increase with increasing influent flow.After the calculations, they found that the overall cost of the optimized plant was reduced by 52.7%.Martín et al. 61 developed a hydrothermal model that allows the determination of the unit sizes that satisfy the requirements under the pressure drop constraint.They demonstrated it on a crude oil preheat train containing 12 heat exchangers, from which it was found that the heat exchange area of the heat exchangers was reduced by a multi-flow structure.The results showed that energy recovery could be improved by 25%.To investigate a more efficient application of indirect evaporative cooler (IEC) in heat recovery components, Li et al. 62 numerically studied the thermal performance and energy transfer in three basic types of cross-flow, counter-flow and parallel flow, respectively, and found that irreversible heat transfer in the primary air passage and irreversible mass transfer in the secondary air passage caused 90% of the total energy loss.It is recommended to combine IEC with sensible heat exchanger and energy recovery exchanger to improve the energy transfer efficiency.To solve the problem of high mass flow rate and temperature fluctuations of industrial waste heat, Orumiyehei Aida et al. 63 proposed that a continuous and stable heat load is required for two sectors of the organic Rankine cycle.The first of these sectors is the combination of a waste heat recovery system (WHRS) with an ORC for low temperature gases in an electric arc furnace (EAF), followed by the mixing of the WHRS and ORC with a linear Fresnel collector (LFC).The results show that in the LFC is the device with the highest energy loss.The process of obtaining 2,3-butanediol by high-pressure distillation is both expensive and environmentally unfriendly, and Lee et al. 64 proposed the use of 2-heptanol as an extraction solvent.The feasibility of heat recovery was verified by molecular simulations and experimental measurements using the kneading technique, and the proposed process was 30% less energetic than that of the existing process.
To achieve more effective cascade application of multi-stage waste heat from internal combustion engines (ICE), Yu et al. 65 proposed a two-stage cascade ORC and integrated with LNG cold energy system to harvest the medium and low temperature emission heat from ICEs.By modeling, investigating the parameters and nondominated ranking genetic algorithm to optimize the objective, the results showed that the condenser consumes the most energy and optimizing the condenser can significantly reduce the cost and improve the efficiency.Temiz et al. 66 developed a new solar energy based integrated system for power generation and cooling.Thermal and chemical energy storage systems were integrated into a molten salt cold and hot storage tank and a hydrogen-based energy storage system with heat recovery through Rankine cycle and lithium-bromine absorption refrigeration cycle, and practical application results showed at least 10% improvement in energy efficiency.Khayyam H et al. 67 presented an innovative F I G U R E 6 Industrial waste heat recovery. 58pproach for process design and analysis of a new waste heat recovery system for carbon fiber manufacturing.Two optimization methods, namely artificial neural networks and nonlinear regression, were constructed to predict energy consumption.The results showed that both recovery and electrical energy consumption were optimized by using a recovery system through a heat exchanger.
To improve thermal efficiency and waste heat recovery, this paper presents a new approach to liquid air production using ocean hydrodynamics, waves, wind and solar energy.The expected benefits of grid RES integration with decoupled LAES systems in gridconnected applications include reduced operating costs and emissions, improved efficiency-constrained mode creep, and increased reliability, flexibility, and resilience. 68To reduce fuel costs and pollution, Sanjay et al. proposed a pinch analysis-based energy localization method for evaporative systems (e.g., dryers).In addition, an analysis-based heat recovery technique was developed, mainly for direct heat recovery and hybrid heat recovery in spray dryers.The results show that the drive potential of new and retrofit spray dryers with heat recovery systems can be used appropriately to save energy as well. 69The purpose of this paper is to explore the fact that the use of wall-mounted heat recovery units in existing buildings will provide some energy benefits and minimize energy losses.The annual energy demand of single-family and multi-family buildings was analyzed to discover the possible energy benefits of using such devices.The results show a low risk of exhaust air return and high ventilation efficiency.The wall heat recovery device is a clean, safe and beneficial device. 70he objective of this paper is to investigate how to improve the oxygen recovery and energy efficiency of a reconfigured single column air separation unit, using two operational strategies.The results show that by varying the high pressure air pressure, a 3%-9% reduction in SPC is shown.However, the first operation method has high turbine regulation requirements, which is an area for improvement.The second operation method requires two additional motors to drive the last two stages of the booster compressor, which increases the expense. 71The objective of this paper is to verify that it is feasible to apply closed-loop geothermal technology for commercial applications in deep, high-enthalpy systems.The Duhamel convolution method in the oil and gas industry was applied to model a one-sided closed-loop geothermal energy recovery system.A natural coupling analysis approach is used to deal with the complexity of the system.The results show that the closed-loop geothermal system is very sensitive to the thermal conductivity of the reservoir and that the reservoir's thermal conductivity-controlled outlet temperature level varies with disturbances over time. 72o summarize the above, we found that with the advancement of technology, many experts and scholars in waste heat energy are studying the reduction of waste heat energy loss or the improvement of waste heat energy utilization efficiency through intelligent optimization methods.Also, we believe that intelligent optimization approaches, such as artificial neural network approaches, can be used over other energy recovery uses.

| TYPES OF PRESSURE ENERGY
Pressure energy recycling is one of the common energy saving directions in industries and enterprises.In natural gas engineering, seawater desalination technologies and municipal wastewater treatment industries, there is a certain amount of pressure energy wastage, and the reasonable recycling of this energy can improve energy efficiency and save resources.

| Pressure energy in natural gas engineering
As a clean energy source, natural gas is becoming more and more important in today's energy mix.Natural gas is generally delivered to city gate stations or large users using a high-pressure pipeline network, and is depressurized using pressure regulators according to different user needs.The pipeline network contains a large amount of available pressure energy during the depressurization process, which has gradually developed in large gas supply systems: long distances, large diameters and high pressures.Natural gas pipelines are generally transported at pressures of 8-12 MPa. research results show that if efficient recovery of gas pipeline pressure energy can be carried out in a timely manner, resource conservation can be fully realized, which is of great help in building an environmentally friendly society.Table 3 shows the base composition and thermodynamic properties of natural gas.
Today's more common methods of pressure energy recovery include: pressure energy refrigeration, pressure energy generation, which can be used in the air conditioning industry, liquefaction, rubber crushing and other industries.][77] First of all, the optimization of process equipment can be studied and analyzed for the existing pressure ERDs, and process optimization analysis and equipment improvement treatment can be carried out with specific conditions.In addition, a comparative analysis can be conducted for various types of current processes, and a comprehensive planning operation can be conducted after the comparison, so as to obtain the best pressure energy recovery process, which is of great help to improve the energy utilization rate and is a basic method to promote the completion of efficient use of pressure energy recovery. 78cond, the development of pressure energy utilization projects, combined with the actual situation of the enrichment of pressure energy utilization methods, and timely strengthening of technical and economic aspects of the index analysis.Most of the current pressure energy utilization is used in the refrigeration and power generation industries, and can be used in cold storage, ice making, LNG peaking and other aspects of cold applications.However, it is necessary to pay attention to the actual method of pressure energy recovery related to project specificity.For example, micro turbines can be established near small and medium-sized regulator stations for gas recovery purposes.In the area around large regulating stations, gas peaking power plants can be established in due course so as to ensure that the gas peaking efficacy meets the expected requirements.The fuel systems and burners of the power plants need to be designed with fuel specificity in mind.If the winter power usage is low, more heat supply and less power generation can be handled by the operation, and the residual energy of the regulating station and recovered power can also be put back into the power plant to complete the system energy supply.Actively strengthen the development and application of pressure energy recovery methods, has a significant impact on national economic construction and social development. 79igure 7 illustrates several technical means of natural gas energy conservation.The extraction of natural gas from natural gas hydrates is performed using guest T A B L E 3 The composition and thermodynamic properties of natural gas. 73,74tural gas analysis Value (vol%) | 3917 replacement technology, where higher replacement pressure and temperature facilitate higher replacement degree and natural gas production.Low-frequency voltage heating-assisted pressure reduction can lead to higher gas hydrate decomposition yields and greater energy efficiency.Artificial intelligence enables the system to learn different states of thermal stimulation, which can lead to dynamic optimization of efficiency.
The recovery process of flare gas from natural gas is improved to a new recovery process based on liquid ring compressor, which can improve the recovery efficiency.The recovery of cold energy from natural gas is an integrated energy use system based on the Allam cycle for natural gas pressure reduction stations.When natural gas is used for power generation, a combined system of stand-alone power generation subsystem and liquid air energy storage subsystem.Cold energy from LNG is stored during low periods and released during peak periods.The turbine expander has an important position in the natural gas transmission process.Adjusting its blade angle can increase efficiency, and the combination of dual expander and organic Rankine cycle can also achieve energy saving.The oil-free twin-screw expander is simpler to seal and can make up for the defects of the turbine expander applied to the sampling area at high speed.
Turbine expanders are used in natural gas delivery processes where system sustainability is unstable due to natural gas flow variations.Saryazdi 80 designed a natural gas pressure reduction station system equipped with a radial turbine expander to improve turbine efficiency by adjusting the angle of the nozzle blades.The variable inlet guide vane improves turbine efficiency by up to 60% compared with a fixed inlet guide vane.Additional expanders installed during natural gas transmission recover pressure energy but with low efficiency, Xiong et al. 81 proposed a dual expander and an ORC with a significant daily round-trip efficiency improvement.The results obtained by performing thermodynamic analysis of the differences show that the contribution of the dual expander is much lower than that of the ORC cycle driven by solar energy.Wang et al. 73 proposed an integrated energy utilization system for natural gas pressure reduction stations based on the Allam cycle.By establishing a mathematical model, the effects of turbine inlet temperature, inlet and outlet pressure isentropic efficiency and natural gas expansion stage on the performance of the proposed system were investigated, and the results showed that the energy efficiency was improved.Diao et al. 82 designed the first oilfree twinscrew expander for application in natural gas energy recovery in urban gas stations.This design has simpler seals and easier bearing selection compared with the conventional turbine expander, which can compensate for the shortcomings of the turbine expander applied to the sampling area at high speeds.The system has been shown to be effective in recovering the pressure energy of natural gas through practical applications.Natural gas hydrate can achieve both natural gas production and CO 2 sequestration by guest replacement technology.Mok et al. 83 investigated the effect of replacement pressure and temperature on the replacement of CO 2 and N 2 , and the results showed that the higher degree of replacement at higher pressure and temperature is beneficial to improve the productivity of natural gas in different environments.To improve the contribution of hydrate decomposition to natural gas production, Zhao et al. 84 proposed a low-frequency electric heating-assisted pressure reduction in a five-point well model.By building a numerical simulation model, the simulation results indicated that the hydrate decomposition gas production was improved after implementing electric heating and also obtained a larger energy efficiency ratio.Efficient recovery of flare gas generated in the oil and gas industry is a hot research problem.Asadi et al. 85 proposed a new flare gas recovery process based on a liquid ring compressor, which was simulated by Aspen HYSYS and MATLAB, and the results showed that the absorption efficiency of H2S can be improved by reducing the temperature of the amine or increasing the flow rate of the recovered amine.In addition combining this system with a refinery producing H 2 S can recover 87% of the calorific value of H 2 S. The fluctuation of LNG supply causes instability in power generation.To solve this problem, Wu et al. 86 proposed a combined system containing a stand-alone power generation subsystem and a liquid air energy storage subsystem.The cold energy of LNG is stored in the low period and released in the peak period.A Gaussian distribution model was introduced and the results showed that at least 29% performance optimization was achieved.The combination of pressure reduction and thermal stimulation is one of the hot technologies for natural gas extraction from natural gas hydrates.Yang et al. 87 proposed a combined artificial intelligence-based model for dynamic optimization of energy use efficiency based on learning and evaluation of different thermal stimulation policies.The results show that this method provides a solution for the optimization of dynamic thermal stimulation parameters.

| Pressure energy recovery in seawater desalination
With the world's population growth and economic development, the global shortage of fresh water resources is becoming more and more serious, and countries are continuously strengthening the research and development of seawater desalination technology.At present, the international desalination mainstream technology includes thermal method and membrane method, and each of them contains a variety of technologies.Among them, RO desalination technology has the advantages of small energy consumption, low investment cost and high equipment life, and has become the mainstream technology in the market.The reduction of energy consumption in the process is an important factor for the rapid development of RO technology.The pressure during the operation of RO desalination technology is between 6-8 MPa, and the pressure of the concentrated brine discharged from the membrane module after separating fresh water is still as high as 5-6.5 MPa, which will cause huge waste if it is discharged directly.Research shows that the loss caused by the direct release of concentrated brine pressure energy accounts for about 30%-50% of the total cost of desalination and 75% of the operating cost.The desalination system installed with pressure ERD will greatly reduce the cost and can save nearly 50% of energy consumption.][90] Pressure ERDe has a variety of classification methods, according to the working principle can be divided into centrifugal and positive displacement pressure ERD.Centrifugal type uses high-pressure concentrated brine to push the hydraulic turbine to rotate and drive the pump to pressurize the feeding seawater, realizing the conversion of "pressure energy-mechanical energy-pressure energy," reducing the High-pressure pump power consumption, saving the cost of water treatment; positive displacement type using the incompressibility of the liquid, directly realize the "pressure energy-pressure energy" energy transfer between high-pressure concentrated brine and low-pressure feed seawater, no energy conversion loss in the middle.Positive displacement pressure ERD surpasses centrifugal device in all aspects of performance, and is the mainstream technology in engineering practice in the world.According to the separation of high-pressure and low-pressure fluids, there are two types of recovery devices, namely rotor type pressure ERD and piston type valvecontrolled pressure ERD. 91,92Figure 8 shows a representation of ERDs in each country.
Khaled et al. 22 investigated the integration of delayed osmosis (PRO) with SRO, evaluating two designs: a single-stage PRO and a proposed two-stage PRO, showing that the two-stage PRO had better performance and was influenced by the SWRO seawater concentration.Sun 28 indicated that the pump selection could be optimized to improve this problem, with a total energy recovery efficiency of 97.1%.To address the problem of membrane biofouling in seawater RO plants, Winters Harvey et al. 93 found that bacterial aggregates are dispersed from the rotating ERD.The flow rate of seawater through the rotary energy recovery unit is an important factor in the dispersal of bacterial aggregates to the RO, and reducing the flow rate can lead to a reduction in membrane biofouling formation.Solar energy is an effective energy source for desalination process, so Lai et al. 94  technology with high recovery rate to obtain more fresh water and energy storage by consuming renewable electricity from a disc-shaped solar Stirling engine.Based on the theoretical derivation and performance optimization, the results show that the recovery rate is improved at different pressures.While high energy costs and the environmental impact of salt water treatment are the challenges facing desalination plants today, Benjamin et al. 95 have developed a Python-based model.This model can be used by users to simulate a new extended PRO system.The results show that membrane properties are the most influential parameter in the performance of the PRO system.The system was found to have a high energy productivity and some competitiveness in the market.Abdelgaied Mohamed et al. 96 simulated a new hybrid seawater desalination device, which uses two common high fresh water production technologies: humidification and dehumidification (HDH) and RO.Next, the comprehensive performance of the system is evaluated, five coupling models are deduced, and each model is verified with the previous experimental data.The results show that the system is a low energy consumption fresh water production system.The maximum fresh water output per hour of the new HDH-RO mixed desalination system is between 192 and 200 L, and the water recovery rate is between 48% and 49.8%.In addition, its specific power consumption (SPC) is between 1.22 and 1.24 kWh/m 3 , saving an average of 14.7% to 65% compared with the previous RO desalination system technology.
Wastewater treatment often uses various methods and technical measures to separate or degrade the pollutants contained in wastewater into nontoxic, nonhazardous and stable substances.Common wastewater treatment can be divided into three main categories according to technical principles: they can be divided into physical, chemical and biological methods.Modern wastewater treatment technology, according to the degree of treatment, can be divided into primary, secondary and tertiary treatment. 97,98 Primary treatment, mainly to remove the solid pollutants in suspension in the sewage, most of the physical treatment method can only complete the requirements of primary treatment.After the first level of treatment of sewage, BOD can generally be removed by about 30%, which can not meet the discharge standards.The primary treatment is the pretreatment of the secondary treatment.
Secondary treatment, the main removal of organic pollutants (BOD, COD substances) in the sewage is colloidal and dissolved state, the removal rate of up to 90% or more, so that organic pollutants to meet the emission standards.
Tertiary treatment, further treatment of difficult to degrade organic matter, nitrogen and phosphorus and other soluble inorganic substances that can lead to eutrophication of water bodies.The main methods are biological nitrogen removal and phosphorus removal, coagulation and sedimentation, sand filtration, activated carbon adsorption, ion exchange and electro-osmosis analysis.
The whole process is that the raw sewage through the coarse grate is lifted by the sewage lifting pump, after the grate or sand filter, after which it enters the sand sedimentation tank, and the sewage after sandwater separation enters the primary sedimentation tank, and the above is the primary treatment (i.e., physical treatment), and the effluent of the primary sedimentation tank enters the biological treatment equipment, with activated sludge method and biofilm method, (where the reactors of activated sludge method are aeration tank, oxidation ditch, etc., and biofilm method includes Biological filter, biological turntable, biological contact oxidation and biological fluidized bed), 100 the effluent from the biological treatment equipment enters the secondary sedimentation tank, the effluent from the secondary sedimentation tank is disinfected and discharged or enters the tertiary treatment, the primary treatment ends here for the secondary treatment, the tertiary treatment includes biological de-phosphorization, coagulation and sedimentation, sand filtration, activated carbon adsorption, ion exchange and electrodialysis.The sludge from the secondary sedimentation tank is partly returned to the primary sedimentation tank or biological treatment facility, partly to the sludge thickener, and then to the sludge digester, where the sludge is finally used after dewatering and drying equipment. 101,102gure 9 illustrates the types of energy-efficient technologies for wastewater treatment.Energy-saving technologies in wastewater treatment processes include three main categories: biological communities in wastewater, biofilm and overall process improvement, among which biofilm method and RO technology are more widely used.
3][104] Its advantages are low energy consumption, bioenergy recovery, less microorganisms generated per unit mass of substrate removed, no oxygen required for the whole process, thus not limited by oxygen transfer capacity, and high loading capacity for organic matter.In addition to these advantages, the single-stage pilot-scale submerged anaerobic membrane bioreactor expressed in the paper has higher COD removal and more efficient bioenergy recovery compared with the conventional anaerobic membrane bioreactor.In addition, the overall process improvement can also achieve energy saving and efficiency improvement.A new anaerobic membrane F I G U R E 9 Energy saving technology for wastewater treatment. 103ioreactor-biochar adsorption-RO process can reduce the carbon emissions of the original process system by 12% and the overall energy consumption by 42%, providing a reference for process improvement.The combination of an anaerobic membrane bioreactor and a microbial electrolyzer can lead to more resource recovery from wastewater treatment.In low salinity wastewater RO desalination, improving its permeability and fouling resistance can save 56% of the overall energy consumption, while the membrane area will be reduced by 25%.In addition, energy saving optimization can be investigated in other areas.
In the context of increasing demand for energy and wastewater recovery technologies, Zhang et al. 105 developed a AnMBR-biochar adsorption-RO process to be used for municipal wastewater treatment.The results showed that this process was able to reduce process operating costs and energy consumption by 42% in energy consumption and carbon emissions to approximately 12% compared with the original process.Methanogens occupy an important position in the energy recovery process of wastewater treatment plants.Li et al. 106 investigated the diversity of methanogens by functional gene sequencing.16S rRNA and mcrA gene sequencing results were constant, and unclassified methanogenic bacteria may be necessary to complete the community function, and further studies are beneficial to improve energy recovery in wastewater treatment plants.Municipal wastewater treatment has much unrecovered energy.Kong et al. 107 used a single-stage pilot-scale submerged AnMBR to recover bioenergy from wastewater treatment and achieved over 90% COD removal in 217 days of operation.The bioenergy recovery efficiency calculated by empirical equations was 69.4%.AnMBRs are widely used for wastewater treatment, but their membrane fouling and loss of dissolved methane increase operating costs, so EI Kik Olga et al. 108 combined AnMBRs with microbial electrolytic cells (MECs).The effectiveness of the coupled AnFMBR-MEC system was tested by monitoring the metrics of the wastewater treatment process.The results showed that this system improved the resource recovery of wastewater. 109To address the problem of low leachate treatment efficiency, the feasibility of pretreating a mixture of leachate and municipal wastewater (1:4) in a microbial desalination cell MDC was first investigated by Rahman et al. 99 The desalination efficiency and energy recovery were measured by monitoring the indicators and measuring the desalination efficiency at different applied external resistance (Rext).The results showed higher desalination rates and higher NH3-N removal with greater Rext, while COD removal was lower.To extract distillation of 1,4-dioxane from industrial wastewater more efficiently, Li et al. 110 conducted an energy, external energy, economic and environmental macroscopic analysis of an energyefficient extraction thermal coupling process with thermal integration function.The results showed that the extraction partition wall column with thermally integrated process had the greatest energy savings, with a 28.18% and 24.28% reduction in total energy demand and investment cost, respectively, compared with the conventional process.To save energy and increase freshwater production in wastewater treatment processes, Li et al. 111 demonstrated theoretically and experimentally that high permeability and good fouling resistance in RO desalination of low salinity wastewater can achieve this goal.After modeling simulations and parameter optimization, zwitterions of different molecular weights were synthesized and hollow fiber membranes were fabricated, which saved 56% energy and reduced membrane area by 25% compared with existing membrane bioreactors.

| CONCLUSION
Due to the world's attention to energy shortage and energy waste phenomenon, energy recycling technology is gradually developed and matured.Energy source from traditional nonrenewable energy to renewable energy is the future development trend, the use of wind, solar, ocean energy and bio-energy can not only achieve the purpose of energy recycling, but also achieve the protection of the environment.In natural gas transmission and seawater desalination treatment, there is a large amount of pressure energy waste phenomenon, rational use of this energy for refrigeration and power generation industry to achieve the maximum efficiency of energy utilization.
1. Wind power in has a huge range of applications and is mostly optimized in ways such as improving turbines to increase energy recovery.2. RO technology in seawater desalination has the advantages of low energy consumption and low investment.There is a large pressure energy in the filtered concentrated brine, which is wasted by direct discharge.Energy utilization can be improved by combining RES and RO.A new HDH-RO system has been developed that consumes less energy compared with a stand-alone RO system.When PRO is integrated with SRO, the two-stage PRO has higher performance than the single-stage PRO.Reducing the flow rate and pressure pulsation of high and low pressure seawater, and optimizing the pump size and structure also enhance the pressure energy recovery efficiency.

Microalgae and corn stover have abundant bioenergy.
Microalgae can be used as feedstock for biodiesel production, chemical stress can increase their lipid content, cold plasma can enhance their metabolism, and CAPP has an effect on microalgae metabolic activity to improve net energy recovery.Corn stover is rich in carbohydrates, and its compact structure and complex composition lead to low recovery efficiency.Two-stage AD is used for hydrogen fermentation and methane production, with the former providing favorable conditions for the latter to improve energy recovery efficiency.4. Heat exchanger is an important component of the waste heat recovery process.The use of multi-flow structure can reduce its heat transfer area, and the combination of indirect evaporative cooler and heat exchanger can improve the energy efficiency.The two-stage tandem ORC technology can also improve the energy utilization rate.The combination of thermal desalination system and energy recovery can save energy.The construction of two machine learning methods, artificial neural network and nonlinear regression, has research prospects.5.A large amount of pressure energy exists in the natural gas transportation process.The turbine expander is used for natural gas transportation, and the turbine efficiency can be improved by adjusting the angle of the blades.The combination of double expander and ORC makes the daily round-trip efficiency increase significantly, and the oil-free twin-screw expander is designed for natural gas energy recovery, and its simple sealing has been proved to be effective in recovering the pressure energy of natural gas.6.The main targets for improvement of energy-saving wastewater treatment processes are: biological communities, biofilms and overall process improvement.Anaerobic biofilm method has low energy consumption, single-stage pilot-scale submerged anaerobic membrane bioreactor with higher COD removal rate and higher bioenergy will be efficient.The new anaerobic membrane bioreactor-biochar adsorption-RO process reduces carbon emissions by 12% and overall energy consumption by 42%.High permeability and fouling resistance can save 56% of overall energy consumption and 25% reduction of membrane area.

7
Natural gas energy saving technology.79JI ET AL.
proposed a self-diluting 2-stage RO F I G U R E 8 Classification of energy recovery devices by country and representative products. 91JI ET AL.| 3919

Table 2
shows the comparison of the feed and permeate outlet temperatures of the porous media model with experimental results from the literature.The permeate inlet temperature is 17.5°C for all cases 2.2.2 | Seawater desalination system and energy recovery unit Comparison of the feed and permeate outlet temperatures of the porous media model with experimental results from the literature.
Currently, seawater desalination and energy recovery units cannot be achieved without RO membrane components, which are involved in many studies.As aT A B L E 2 Table 4 summarized the characteristics of raw leachate, MWW, and leachate-MWW mixture.
99aracteristics of raw leachate, MWW and leachate-MWW mixture.99 T A B L E 4