Reverse Brayton Cycle Research Articles

Nonlinear operational optimization of an industrial power-to-heat system with a high temperature heat pump, a thermal energy storage and wind energy

The heat demand for industrial processes is often provided in the form of steam generated by various fossil fueled equipment. In order to reduce CO2 emissions, the heat demand has to be covered by renewable energy sources. Electrified steam generation relies on complex energy systems, that can be operated according to energy availability and cost developments. However, such a multi component industrial energy system poses a challenge in modeling and determining the cost- or emission-optimal operation of the system. This study develops a methodology to model a multi component industrial energy system on the basis of a case study. By optimal system operation, either costs or emissions are minimized in response to fluctuating renewable wind energy and electricity prices.A high temperature heat pump (HTHP), a sensible thermal energy storage (TES) and a wind turbine are combined to create an electrified energy system to supply super-heated steam. During periods of low wind speed, additional grid electricity is purchased to ensure a steady heat supply. The HTHP offers a high operational flexibility and thus, enables the charging and discharging of the TES. A model of the closed reverse Brayton cycle HTHP, which is able to simulate part load behavior, is created in a process simulation software and consolidated in nonlinear surrogate models. The component behavior of a TES is represented by a combination of equations based on heat exchanger relations. Finally, the resulting algebraic nonconvex, nonlinear optimization problem based on the proposed system is solved using the local interior point optimizer (IPOPT) solver equipped with a multi-start approach to determine an optimal operation over a reference week with respect to the current wind power generation, grid emissions and electricity prices.The results of the optimization show, that optimal operating strategies enable a high potential to decarbonize future industries at minimum operational costs or emissions.

Thermodynamic analysis of a dual reverse Brayton cycle (dual-RBC) hydrogen liquefaction process precooled by mixed-refrigerant

In order to reduce hydrogen liquefaction power consumption, a dual reverse Brayton cycle (dual-RBC) process is proposed and analyzed in this paper, which consists of a mixed-refrigerant J-T cycle for precooling, a hydrogen RBC for hydrogen liquefaction, and a helium RBC for liquid hydrogen subcooling. Compared with typical dual-Claude processes, the two-phase low-pressure cycling hydrogen for liquid hydrogen product subcooling is replaced by an independent helium RBC, reducing the temperature difference and exergy loss in subcooling heat exchangers. Consequently, the specific power consumption (SPC) of dual-RBC process is 4.16% lower than dual-Claude process under the same conditions. The mixed-refrigerant J-T cycle (driven by single-stage oil-lubricated compressor, no liquid/two-phase expanders) could reduce precooling power consumption by >45% comparing with liquid nitrogen. The dual-RBC process could reach a SPC of 7.08 kWh kg−1 under typical conditions. While it is more realizable with mature technologies than novel processes.

Thermodynamic investigation of a centrifugal reverse Brayton cycle for refrigeration in air conditioning filed

The reverse Brayton cycle (RBC) is known to exhibit a lower coefficient of performance (COP) when compared to conventional vapor-compression refrigeration systems. To tackle this issue, this study proposes a centrifugal reverse Brayton cycle (CRBC) that features a rotating heat exchanger, which utilizes the conversion mechanism between equal potential energy and pressure energy to enhance system performance. The findings indicate that the COP of the CRBC is roughly inversely proportional to the working fluid density, making CO2 a more suitable option for CRBC systems than conventional working fluids. We propose three different cycles for the CRBC system: the diamond cycle, trapezoid cycle, and rectangle cycle. From both technical and operational perspectives, the trapezoid cycle appears to be the preferred choice for CRBC. When the isentropic efficiency of the CRBC system exceeds 95%, its coefficient of performance (COP) ranges from 5.31 to 5.7, whereas the COP of transcritical carbon dioxide refrigeration cycle (tCO2), air RBC, and CO2 RBC are around 4.3, 1.9, and 0.79, respectively. This suggests that the performance of the CRBC system surpasses that of RBC by a significant margin, while achieving comparable performance to conventional vapor compression refrigeration systems. Moreover, the dimensionless power reflecting the installation capacity requirements for CRBC, tCO2, air RBC and CO2 RBC are around 0.2, 0.35, 5.3 and 1.9, respectively. This indicates that the compressor/expander capacity demand for system CRBC approaches the thermodynamic minimum.

Using of saltwater to increase the output power of a steam injected gas turbine (STIG) by connecting spray freezing desalination to it

Gas turbines are one of the common methods for generating electrical energy. However, reducing the output power by increasing the air temperature is one of the problems of these turbines. Common methods to compensate for the power drop of gas turbines are often based on turbine inlet air cooling (TIAC) technology or injecting steam or water into the turbine as humidified gas turbine (HGT) that causes an increase in its output power. However, these technologies often require high consumption of fresh water which limits their use in dry and low-water areas. Therefore this study has investigated the connection of a spray freezing desalination to a steam-injected gas turbine (STIG) through a reverse Brayton cycle for increasing its output power by consuming saltwater. This spray freezing desalination is a process based on eutectic freeze crystallization (EFC) technology that uses compressed air in a turbo expander to produce chilled air that can crystallize the droplet of sprayed saline water. This desalination technology has the potential of zero liquid discharge (ZLD). Therefore, with the possibility of using a saline water source such as reverse osmosis (RO) brine in this process, in addition to increasing the output power of gas turbines, it will be possible to reduce the environmental problems related to its disposal.

Energy, Exergy, and Economic (3E) Analysis of Boil-Off Gas Re-Liquefaction Systems Using LNG Cold Energy for LNG-Fueled Ships

Liquefied natural gas (LNG)-fueled ships have the effect of reducing most pollutants, which is advantageous for responding to strict regulations. Because boil-off gas (BOG) is generated in the LNG storage tank of an LNG-fueled ship, a BOG re-liquefaction system is required. The representative systems for LNG-fueled ships were proposed by Kwak and Shen, but their exergy efficiencies were only 19.6% and 24.9%, respectively. To improve the system, this paper proposes novel BOG re-liquefaction systems combined with the fuel gas supply system. The systems utilize LNG cold energy in the BOG stream and N2 reverse Brayton cycle, respectively. The proposed systems were simulated using a commercial program and were optimized using a genetic algorithm. The results of energy, exergy, and economic (3E) analyses performed for comprehensive evaluation of the proposed system show that the system in which LNG cold energy is applied to the BOG stream has the best performance. Specific energy consumption, exergy efficiency, and total annual costs of this system were improved by up to 78.6%, 69.2%, and 68.2%, respectively, compared to those of the existing systems. The overwhelmingly superior system is expected to greatly contribute to the improvement of the BOG re-liquefaction system for LNG-fueled ships.

Geothermal assisted hydrogen liquefaction systems integrated with liquid nitrogen precooling; Thermoeconomic comparison of Claude and reverse Brayton cycle for liquid nitrogen supply

Hydrogen liquefaction systems are of paramount importance in achieving a low liquid hydrogen price in developing the hydrogen supply chain. In these systems, reduction of compression work and efficient supply of liquid nitrogen for precooling purposes is essential. In this respect the present research proposes an investigation of two efficient configurations comprised of a Claude cycle for hydrogen liquefaction integrated with geothermal driven organic Rankine cycle to compensate some of the consumed power. In the proposed schemes, either the Claude or reverse Brayton cycles are employed and modeled for liquid nitrogen supply, while in most of previous research a fixed value was considered for compression work to supply the liquid nitrogen, reducing the accuracy of their reported outcomes. A comprehensive thermoeconomic assessment was implemented to determine a better scheme under various operating conditions using a parametric study. The results revealed the vital importance of integrated modeling of the liquid nitrogen supply unit with that of the hydrogen liquefaction system.

Comparison of mean-line design methods on matching characteristics of three coaxial-impellers in an aircraft environmental control system

High-speed turbo-cooler of coaxial expander-compressor is a promising solution for reverse Brayton cycle refrigeration systems to improve compactness, power recovery and efficiency. The performance curves of coaxial impellers are crucial for their matching characteristics as well as the system efficiency under off-design conditions. In this paper, three mean-line methods for the design of high-speed turbo-coolers are evaluated by comparing the matching characteristics of three coaxial-impellers, including the specific speed and characteristic ratio (SS-CR) approach, the power ratio (PR) approach and velocity coefficient (VC) approach. A coupling model is established to predict the off-design performance of turbo-cooler and the system size. The results show that the rotating speed and mass flow rate by all three methods lie within 2% of each other. At the design point, SS-CR approach gives 84.0% expander efficiency compared to 83.0% by PR approach and 80.9% by VC approach, but the PR approach has better performance at speeds below 35 krpm. The system volume is the lowest at the aspect ratio of 1.5 of the heat exchanger designed with all the three methods. However, SS-CR gives 6.0% advantage in system volume 5.6% over VC approach and 3.7% over PR approach. When the cooling capacity is controlled at 2000 W, SS-CR approach gives 79.3% expander efficiency compared to 78.9% by PR approach and 76.4% by VC approach.

The original and impactful exploitation of chemical energy in heat pumps

In light of reducing carbon emissions and the tension on non-renewable energy sources, heat pumps are being investigated as an alternative to fossil-fuel-dependent energy converters in heat demanding applications. On the basis of the impactful results the authors obtained in previous research around the potential of using reactive fluids–instead of inert ones–as novel working fluids in power cycles, the study has been extended to heat pump systems. The thermodynamic analysis of heat pumps operating with reactive gases as working fluids is thus the objective of the present work. More specifically, the analysis is based on the use of instantaneously equilibrated fictive gaseous reactions, with the aim to thoroughly assess the impact of different stoichiometries and thermochemical characteristics of chemical reactions. This studied heat pump is based on the reverse Brayton cycle. Furthermore, the behavior of the fluid in each unit operation of the cycle is investigated and all the results are compared to those of a heat pump utilizing comparable inert fluids; that is to preliminarily quantify the potential gains in performance. For the considered spectrum of reactive fluids, operating conditions, and reaction stoichiometries, the corresponding results show a range of potential reactive fluids that can be utilized in heat pumps and reveal an increase of more than 200% in the system's coefficient of performance compared to inert-fluid heat pumps.

Experimental Assessment of a Reverse Brayton Cycle Based On Automotive Turbochargers and E-Chargers for Cryogenic Applications

Abstract The utilization of automotive engine components for the development of a reverse Brayton cycle is shown in this study. The well-known characteristics of turbochargers and electrical centrifugal compressors commonly used in the automotive industry is a starting point for developing an experimental cycle configuration of a reverse Brayton refrigeration cycle with the ability to achieve up to 115 K after a multi-stage compression with intermediate cooling and a single-stage expansion in a radial turbine using air (R-729) as working fluid. The use of a variable nozzle turbine allows to evaluate the optimum rack position for each operating point, while having control over refrigeration capacity. Studies over the minimum cycle pressure at the inlet of the first stage of compression have been performed to assess the possible benefit of creating vacuum or pressurize the working fluid. A 1 m^3 cooling chamber was attached to the installation to test the ability of cooling different thermal loads working under different operation ways, open or closed cycle, this is, letting the working fluid to interact with the thermal load or cooling it through a heat exchanger. The evaluation of thermodynamic parameters allows to obtain the coefficient of performance (COP) of the installation and the efficiency of each component. The analysis of turbomachinery performance will allow identifying weak points to improve cycle performance, and the coupling model between rotatory machines as well as to adapt the size of the installation to the refrigeration capacity required for different applications.

Thermodynamic and economic analysis of two-stage CO2 Heat Pump with reverse Brayton cycle

At present, the output water temperature of most CO2 heat pumps is below 95°C. Herein, the reverse-Brayton cycle is innovatively applied to the CO2 heat pump to increase its output water temperature. A numerical model of a two-stage reverse-Brayton cycle with refrigerant injection and internal heat exchanger is proposed. The effects of discharge pressure, suction temperature, injection mass ratio, and pressure of refrigerant injection on COP of this system at different heat source temperatures are analyzed. Simulation results show that the highest COP can reach 3.05 with a discharge temperature of 129.1°C. The highest discharge temperature is 157.1°C and the temperature lift is 87.1°C with a COP of 2.85 correspondingly. High temperature hot water or steam is supplied by the proposed cycle with a respectable COP, which will broaden the application scope of CO2 heat pumps in industries. Moreover, relationship between COP and the payback period is revealed under typical conditions to evaluate the benefit of the proposed system.

Analysis of Modified Reverse Brayton Cycle-Based Systems for Cold Generation at Low Temperature Using Exergoeconomics

Abstract High-temperature superconductor (HTS)-based technology is getting increased attention as the most appropriate technology for electric power transmission towards reduction in CO2 emissions and augmentation of the energy supply. However, this cryogenic technology demands highly efficient and economically reliable cooling systems, having a cooling capacity of 2–10 kW at a temperature range of 65–80 K. The refrigeration cost is one of the critical issues for such superconducting technologies to penetrate the marketplace. Thus, cryocoolers with modification based on reverse Brayton cycle (RBC) comprising different turbine arrangements are analyzed to meet the demand of cryogenic technologies. The exergoeconomic analysis which combines the exergy and economic analysis is performed to identify the source of exergy destruction that affects the economy and performance of the systems based on RBC. The optimized RBC configurations using the exergoeconomic tool are evaluated and compared to find a thermodynamically enhanced configuration with reduced refrigeration cost. These configurations are compared using exergy, economic, exergoeconomic, design evaluation parameters, and environmental footprint. The results indicate that the cost of exergy destruction reduces around 30% while optimizing these novel RBC configurations. The saving with successful running for ten years is around 6% of the total investment using parallel expansion with different pressure ratio (PED) RBC configuration, which is one of the best performances with 41.1% exergy efficiency. Based on the study, recommendations based on exergoeconomic parameters are also provided for further improvement in the cycles.

Research on the Design of Auxiliary Generator for Enthalpy Reduction and Steady Speed Scroll Expander

To help the reverse Brayton cycle cool the refrigerant from 100 K to 50 K, an auxiliary generator, with a housed stator, is studied and optimized, and the influences of weights in the cost- function on the results are discussed. The power demand and adiabatic characteristics of reverse Brayton cycle expansion are analyzed, after which the optimization indexes, including output rated power, efficiency, the air gap between rotor and stator, loss, and volume, are decided. The initial model of the auxiliary generator is then constructed and the parameters to be optimized are also determined. Taking the low loss and sinusoidal back-EMF as the evaluation indexes, the single parameter optimizations of the auxiliary generator are carried out. The co-simulation of the generator and its corresponding driving circuit is investigated, with which the power generation efficiency is calculated. The global optimizations of the generator parameters are carried out using a genetic algorithm. A suitable analytic hierarchy process (AHP) model is proposed, with which a three-order judgment matrix is constructed, and the effects of different weight combinations, in the cost-function, on generator performance are compared. The experimental results show that the output back-EMF amplitude is 28.2 V, which is about 10% smaller than the simulation results; the output power of the auxiliary generator under load is about 3.7 W, meeting the rated demand.

Design and analysis of an efficient hydrogen liquefaction process based on helium reverse Brayton cycle integrating with steam methane reforming and liquefied natural gas cold energy utilization

In order to solve the high energy consumption of the hydrogen liquefaction process, the conceptual design of an integrated hydrogen liquefaction system is proposed and performed by integrating the steam methane reforming (SMR) process and the utilization of liquefied natural gas (LNG) cold energy. Before LNG enters the SMR process, its cooling energy is utilized in the pre-cooling section, which can reduce the use of liquid nitrogen (LN2) and improve performance. The inert gas helium is relatively safe as a refrigerant in the cooling and cryogenic section and performs well in small and medium-sized hydrogen liquefaction systems. The proposed design which produces 5 t/d of liquid hydrogen (LH2) is compared to a based case only using LN2 for pre-cooling to verify its effectiveness and feasibility. Comprehensive comparative analysis reveals that the specific energy consumption (SEC) of the proposed process is reduced from 10.78 kWh/kg LH2 to 7.948 kWh/kg LH2, and the COP is improved from 0.1205 to 0.1634. The use of LNG cold energy makes the fuel cost of the proposed system only account for 73.7% of the base case, which makes it perform better in terms of recurring investment. In conclusion, the present method and results can provide a reference for the design of integrated hydrogen production and liquefaction system by reducing energy consumption, taking into account the recovery of the cold energy of LNG.

Concept and evaluation of energy-efficient boil-off gas reliquefiers in LNG carrier ships propelled by dual-fuel engines

Ships carrying liquefied natural gas (LNG) are generally propelled by dual-fuel enginesthat use boil-off gas (BOG) or diesel as fuel. Fuelling the engine with pumped and vaporized LNG is operationally more convenient than compressing BOG. BOG is usually reliquefied using reverse Brayton cycle (RBC) or Claude cycles. In this paper, both these cycles are reconfigured to extract the cold of pumped LNG to reduce power consumption. Steady-state simulations are done on process simulator Aspen HYSYS V8.6®. Capturing the cold from the LNG fuel improves the exergy efficiency of RBC and Claude reliquefiers from 37.8% to 57.5% and 48.5% to 75.6% respectively. The surface area of heat exchangers in the Claude system is one-third that of RBC. Mass flow through compressor and turbine in Claude system is one-fifth and one-ninth of RBC, while volume flow at turbine inlet is only one-fourteenth in the Claude system.The evaluation shows that all equipment needs to be designed with sufficientoperational flexibility to enable the system toextract cold effectively from the LNG fuel supply because it varies widely during the voyage. Claude system with cold extraction from LNG fuel is highly compact and the most efficient reliquefier designed so far.

Energy and exergy analyses of a cryogenic reverse Brayton cycle with waste heat recovery

In a typical reverse Brayton cycle (RBC), a great amount of compression heat is just wasted, for which there are two different temperature levels. The basic organic Rankine cycle (ORC) and the regenerative ORC with improvement were then applied to recover waste heat. Considering that the waste gas after ORC was reused, the total exergy efficiency was defined to evaluate different cycles. A simulation method was conducted to predict cycle performances and the coupling between RBC and ORC was clarified. The feasibility of replacing the after cooler with the evaporator was analysed. Compared with the typical RBC, a relative improvement of 18.2% was achieved for COP in the combined system and the cooling exergy efficiency was increased by 16.8%. The correspondence of the total exergy efficiency and the COP of whole system exited in different cycle layouts. Proper organisation of the RBC with ORC could promote the energy efficiency.

Energy and exergy analyses of a cryogenic reverse Brayton cycle with waste heat recovery

Energy and exergy analyses of a cryogenic reverse Brayton cycle with waste heat recovery

Process optimization, exergy and economic analysis of boil-off gas re-liquefaction processes for LNG carriers

A re-liquefier based on the reverse Brayton cycle is the first choice for the re-liquefaction process owing to its compactness, simplicity, and safety. However, due to the low efficiency of the heat exchangers, the energy consumption of the re-liquefaction process is relatively high. Therefore, a novel cascade nitrogen expansion re-liquefaction process with high heat exchange efficiency is proposed and compared with the parallel expansion process. In the designed process, the condensation and subcooling stages of the boil-off gas (BOG) are performed in different heat exchangers. The performance improvement of the designed process is analyzed using the Aspen HYSYS and the genetic algorithm is used to optimize the processes. The optimization results show that the specific energy consumption (SEC) and exergy loss rate of the designed process are 0.727 kWh/kgLNG and 148.14 kW, which are 11.7% and 16.7% lower than those of the reference process, respectively. Moreover, the exergy efficiency (EXE) of the designed process is 0.352, which is 13.2% higher than that of the reference process. Because of the higher EXE and lower refrigerant demand, the designed process saves 7.2% of the total annual costs consisting of annual capital and operating costs compared with those of the reference process.

Essential design criteria for safe and efficient operation of an LNG boil-off gas reliquefier under deteriorated performance of system components

Reverse Brayton cycle (RBC) working with nitrogen is used to reliquefy the boil-off gas (BOG) generated in LNG storage tanks. The reliquefier consumes relatively low power if the heat exchanger surface area and the operating parameters are optimally chosen. However, these parameters depend on the isentropic efficiencies of the compressors and turbine that show degradation over years of operation. This paper attempts to find a way to maintain efficient and safe operation of the system even with deteriorated efficiencies of compressor and turbine. The analysis shows that while deterioration of compression system can be made up by applications of variable frequency drive and inlet guide vanes keeping the reliquefaction quantity unaltered, the deterioration of turbine efficiency demands that the pressure ratio of the compressor system should be higher. If appropriate flexibility is incorporated right at the design stage, a reliquefier can be re-adjusted to mitigate the system deterioration. If the isentropic efficiency turbine deteriorates from 90% to 75%, the pressure ratio of nitrogen compressor has to be increased from 3.7 to 4.7 to mitigate increased power input to the reliquefier by as much as 310 kW for a ship carrying 266,000 m3 of LNG.

Integrated hydrogen liquefaction processes with LNG production by two-stage helium reverse Brayton cycles taking industrial by-products as feedstock gas

Hydrogen-rich industrial by-products, such as coke oven gas (COG) and synthetic ammonia exhaust tail gas, are often simply utilized as fuel gas, or even not utilized at all in many cases. An innovative and efficient utilization method is proposed and investigated in this paper. Aiming to coproduce liquid hydrogen and liquefied natural gas (LNG), three helium reverse Brayton cycles are constructed, optimized, analyzed and compared. The LN-He-1 process is constructed with reference to conventional industrial hydrogen liquefaction processes, but with different feedstock gas. The LN-He-2 and N–He processes, which are an improvement on the basis of LN-He-1. A combination of distillation and flash is utilized to reduce the separation load. Analysis of load variation, power consumption, exergy, heat transfer and turbomachinery efficiencies are carried out. For different feedstock gas compositions, the specific power consumptions of LN-He-1, LN-He-2 and N–He processes are within 26.45–54.78 kWh/kmol (feedstock gas), 26.32–53.78 kWh/kmol (feedstock gas) and 21.94–50.42 kWh/kmol (feedstock gas), respectively. The exergy efficiencies are between 13.0% and 66.5%.

Exergetic analysis of reverse Brayton cryocooler with different turbine arrangements for HTS power cables

The cooling systems of High-Temperature Superconducting (HTS) power transmission cables are unable to achieve the desired performance as prescribed in the cryogenic road map. Reverse Brayton cycle based Cryocooler (RBC) has been identified as a refrigerator that has the potential to achieve the desired performance for higher cooling load demand. In this article, an exergy analysis of the basic Reverse Brayton Cycle (RBC) and its modifications has been investigated for identification of appropriate cycle configuration that will meet the demand for practical applications of HTS cables. The modifications in the basic cycle are carried out with different arrangements of turbines such as parallel or series expansion that may increase the exergy efficiency of the cooling system with a heat load of 10 kW @ 65 K. The analysis is carried out using Aspen HYSYS V8.6, and helium is used as a working fluid. The effects of important process parameters such as pressure ratio, compressor inlet pressure, and flow diversion on the performance are investigated and are used to optimize these cycles. The best cycle configuration based on RBC is identified and recommended based on the results of the analysis.