Heat Exchange Area Research Articles

Exergoeconomic Optimization of Polymeric Heat Exchangers for Geothermal Direct Applications

The highest economic costs of a geothermal plant are basically related to well drilling and heat exchanger maintenance cost due to the chemical aggressiveness of geothermal fluid. The possibility to reduce these costs represents an opportunity to push toward geothermal plants development. Such challenges are even more important in the sites with a low-medium temperature geothermal fluids (90–120 °C) availability, where the use of these fluids for direct thermal uses can be very advantageous. For this reason, in this study, a direct geothermal heating system for a building will be investigated by considering a plastic plate heat exchanger. The choice of a polymeric heat exchanger for this application is upheld by its lower purchase cost and its higher fouling resistance than the common metal heat exchangers, overcoming the economic issues related to conventional geothermal plant. Thus, the plastic plate heat exchanger was, firstly, geometrical and thermodynamical modeled and, after, exergoeconomic optimized. In particular, an exergoeconomic analysis was assessed on the heat exchanger system by using a MATLAB and REFPROP environment, that allows for determination of the exergoeconomic costs of the geothermal fluid extraction, the heat exchanger, and the heating production. A sensitivity analysis was performed to evaluate the effect of main design variable (number of plates/channels) and thermodynamic variable (inlet temperature of geothermal fluid) on yearly exergoeconomic product cost. Then, the proposed methodology was applied to a case study in South of Italy, where a low-medium enthalpy geothermal potential exists. The plate-heat exchanger was used to meet the space heating requests of a single building by the exploitation of low-medium temperature geothermal fluids availability in the selected area. The results show that the inlet temperature of geothermal fluid influences the exergoeconomic cost more than the geometrical parameter. The variation of the exergoeconomic cost of heat exchanger with the inlet geothermal fluid temperature is higher than the change of the exergoeconomic costs associated to wells drilling and pumping with respect to the same variable. This is due the fact that, in the selected zone of South of Italy, it is possible to find geothermal fluid in the temperature range of 90–120 °C, at shallow depth. The product exergoeconomic cost is the lowest when the temperature is higher than 105 °C; thus, the smallest heat exchange area is required. The exergoeconomic optimization determines an optimum solution with a total product cost of 922 €/y for a temperature of geothermal fluid equal to 117 °C and with a number of plates equal to 15.

Thermo-Economic Comparison Between Organic Rankine Cycle and Binary-Flashing Cycle for Geothermal Energy

Geothermal energy is a characteristic of widely distributed, high capacity factor, high reliability, and lower environmental impact potential values. And it will play an important role in achieving the goal of carbon neutral carbon peak. Nonetheless, geothermal energy presents its own particular challenges, i.e., the high investment cost and long payback period. The binary flashing cycle (BFC) system is proved to be a promising power generation technology due to the efficient and full utilization of a low-grade heat source. While the economic performance still needs further evaluation, in the present study, the thermo-economic comparison between organic Rankine cycle (ORC) and the BFC for geothermal energy has been investigated. R245fa has been chosen as the working fluid. Considering the thermodynamic and economic performance simutaneously, several evaluation indicators were selected including thermal efficiency, exergy efficiency, net power output per ton geothermal water, heat exchanger area, and heat recovery efficiency, and the system modeling and comparison were presented. The simulation results reveal that the BFC system obtains 32% more net power output than the ORC system under the working conditions investigated. The heat recovery efficiency of the BFC is 1.96 times as much as that of the ORC, which indicates that the BFC can realize the full utilization of low-grade energy. And more heat exchanger areas are required in the BFC system. What is more, the preliminary discussion of the economic feasibility of BFC system applied in the FengShun geothermal power plant is presented. The payback period of the BFC is just 6.0 years under the generation pressure of 600 kPa. It is indicated that the BFC system has obvious economic benefits, especially in a nonflowing geothermal well.

Numerical study on the in-situ pyrolysis process of steeply dipping oil shale deposits by injecting superheated water steam: A case study on Jimsar oil shale in Xinjiang, China

Taking the oil shale deposits in the Jimsar area of Xinjiang, China as an example, this paper proposes an in-situ pyrolysis method using superheated steam suitable for steeply dipping oil shale. This method involves drilling parallel to the bedding and fracturing fractures perpendicular to bedding direction. High-temperature fluid is injected from the injection well to develop a larger heat exchange area through fracturing fractures and oil shale deposits. In this study, a thermal-hydraulic-mechanical coupling mathematical model considering oil shale anisotropy was established to study the in-situ conversion process. The results show that the new proposed method (hydraulic fracture number = 3) has the same heating efficiency as the traditional well layout method (drilling well perpendicular to beddings, fracture number = 3), but has the advantage of preventing shear damage to the wellbore and reducing the drilling cost; For the optimization of hydraulic fracture number and length, when the hydraulic fracture number is increased to 4 (fracture spacing = 20 m), the heating efficiency no longer significantly increases. When the fracture length increased, the heating efficiency does not significantly change. The results of this paper can serve as a reference for the study of in-situ pyrolysis of steeply dipping oil shale deposits.

Automated method of calculation of parameters for non-traditional heating technologies and conditioning of buildings

Purpose. Develop an automated method for calculating parameters for heat pump systems for heating, air conditioning and hot water supply, designed for use in domestic conditions with non-standard heat transfer flows. Methodology. Mathematical modeling of thermodynamic processes occurring in heating, air conditioning and hot water supply systems. Findings. The automated method of calculating the parameters of non-traditional technology, which uses standard heat pump equipment of the water heating system for the cooling mode of the air in the warm period of the year, and the discharge of heat dissipated into the ground, was substantiated and developed; and for the needs of hot water supply heat pump air-liquid, acting as a high-speed water heater. The estimation of technologies of the thermal energy utilization in buildings developed earlier by authors is executed. The first technology involved the use of a heat pump and heat accumulator scheme in the cold season, and halved the consumption of conventional fuel compared to a gas column for hot water at the same facility. The second technology involved heat recovery with the help of a heat accumulator in a complex system of air conditioning and hot water supply in the warm period, which saves from 74 to 82% of conventional fuel compared to the scheme with boiler and air conditioner without heat accumulator. Critical conclusions were drawn about the need to use additional dimensional equipment for these technologies and the excess amount of hot water received. Possibilities of realization of such scheme were analyzed. Analytically substantiated recommendations on the design (ribbing of heat exchange surfaces) of heating devices and parameters of their operating modes in the cold and warm periods of the year were given. The condition of invariance of heat exchange areas of heating devices and basic water consumption in the heating system was fulfilled. The need to regulate the air conditioning regime by changing the water flow in the system to maintain a constant indoor air temperature with fluctuations in outdoor air temperature was substantiated. Originality. For the first time, attention is paid to the study of non-traditional methods of using heat pump heating for heating, air conditioning and hot water supply of residential premises. The automated method for determining rational parameters for these technologies was developed. Practical value. The automated method of forming the control dependence of the mass flow of water in the system on the outside air temperature on the condition of constancy of the set comfortable indoor air temperature was developed. The use of air-liquid heat pump for hot water supply in the warm period was analyzed, a high energy conversion factor was noted (14 ... 22). The savings of conventional fuel from the application of the considered technology from 13% to 18% in comparison with the technology using a heat accumulator were substantiated.

Optimal Heat Exchanger Area Distribution and Low-Temperature Heat Sink Temperature for Power Optimization of an Endoreversible Space Carnot Cycle.

Using finite-time thermodynamics, a model of an endoreversible Carnot cycle for a space power plant is established in this paper. The expressions of the cycle power output and thermal efficiency are derived. Using numerical calculations and taking the cycle power output as the optimization objective, the surface area distributions of three heat exchangers are optimized, and the maximum power output is obtained when the total heat transfer area of the three heat exchangers of the whole plant is fixed. Furthermore, the double-maximum power output is obtained by optimizing the temperature of a low-temperature heat sink. Finally, the influences of fixed plant parameters on the maximum power output performance are analyzed. The results show that there is an optimal temperature of the low-temperature heat sink and a couple of optimal area distributions that allow one to obtain the double-maximum power output. The results obtained have some guidelines for the design and optimization of actual space power plants.

Dynamic Response Characteristics Analysis and Energy, Exergy, and Economic (3E) Evaluation of Dual Loop Organic Rankine Cycle (DORC) for CNG Engine Waste Heat Recovery

Frequent fluctuations of CNG engine operating conditions make the waste heat source have uncertain, nonlinear, and strong coupling characteristics. These characteristics are not conducive to the efficient recovery of the DORC system. The systematic evaluation of the CNG engine waste heat source and the comprehensive performance of the DORC system is conducive to the efficient use of waste heat. Based on the theory of internal combustion (IC) engine thermal balance, this paper analyzes the dynamic characteristics of compressed natural gas (CNG) engine waste heat energy under full operating conditions. Then, based on the operating characteristics of the dual loop organic Rankine cycle (DORC) system, thermodynamic models, heat transfer models, and economic models are constructed. The dynamic response characteristics analysis and energy, exergy, and economic (3E) evaluation of the DORC system under full operating conditions are carried out. The results show that the maximum values of net power output, heat exchange area, and the minimum values of EPC (electricity production cost) and PBT (payback time) are all obtained under rated condition, which are 174.03 kW, 25.86 kW, 37.54 kW, 24.76 m2, 0.15 $/kW·h and 3.46 years. Therefore, the rated condition is a relatively ideal design operating point for the DORC system. The research in this paper not only provides a reliable reference for the comprehensive analysis and evaluation of the performance of the DORC system, but also provides useful guidance for the selection of appropriate DORC system design operating points.

Constructal thermodynamic optimization for a novel Kalina-organic Rankine combined cycle to utilize waste heat

Kalina cycle and organic Rankine cycle have different optimal heat source temperatures. To realize cascade utilization of low-temperature waste heat, a model of novel Kalina-organic Rankine combined cycle is established by using a dual-pressure Kalina cycle as top cycle and a dual-pressure organic Rankine cycle as bottom cycle, and constructal thermodynamic optimization is carried out by uniting constructal theory and finite-time thermodynamics. With the total turbine volume and total heat exchanger area being fixed, the optimal tube external diameters of the evaporators in the dual-pressure Kalina cycle and dual-pressure organic Rankine cycle are obtained by maximizing the net power output of the Kalina-organic Rankine combined cycle system. The results prove that the net power outputs of the Kalina-organic Rankine combined cycle system after four progressive optimizations are increased by 2.62%, 5.41%, 15.05% and 16.17%, respectively, compared to the initial one. In the quartic constructal thermodynamic optimization, the optimal tube external diameters of the high- and low-temperature evaporators in the DPKC are 0.024m and 0.023m, respectively, and the those in the DPORC are 0.020m and 0.024m, respectively. The results obtained will improve the performance of the KORCC and promote the efficient utilization of waste heat.

Investigation of heat energy storage of RT26 organic materials in circular and elliptical heat exchangers in melting and solidification process

Energy storage, particularly thermal energy, is one of the most important issues in responding to human energy needs. In this numerical work, by using the latent heat of the organic paraffin RT26 and its melting and freezing points, an attempt has been made to study thermal and heat energy storage. In this theory, by analyzing different circular and elliptical shapes of heat exchangers and internal tubes, an attempt has been made to increase the thermal efficiency and deal with the slow rate of completing the process, which is mainly based on buoyancy. In this work, by employing MWCNT and GO, it has been tried to increase the thermal conductivity of organic material. The results also show that increasing the eccentricity of the condenser in the melting process causes a thermal equilibrium to be created in heat exchanger area in such a way that by increasing the eccentricity from 0.02 to 0.04, and 0.06, the percentage of heat efficiency of heat exchanger will increase by 27%, 49% and 58%, respectively. It was also found that by reducing the cold wall temperature from 280 K to 270 K, the solidification process is accelerated so that the time required to complete the process is reduced by 17%.

Heat Exchangers in Carnot Batteries: Condensation and Evaporation in a Reversible Device

The combined heat pump–organic Rankine cycle is a thermal–electrical storage concept which allows the reversible use of components in both operation modes (loading and unloading the storage). This saves in terms of investment costs but also creates challenges during design and operation. A heat exchanger is an expensive component destined to be used for the reversible purposes of a heat pump condenser and an organic Rankine cycle evaporator. In this study, the operation of such an apparatus was evaluated based on an analytical model, experimental data and thermal imaging. This study shows that the model can predict the filling of the apparatus distinguished by liquid, vapour and the two-phase region. The thermal imaging supports the model and gives the location of the regions. Connecting both methods, a valid statement about the current condition of the heat exchanger is possible. Due to very small pinch points, the apparatus is not efficiently used in the investigated modes. Extending the pinch to 2 K can already save up to 46.1% of the heat exchange area. The quality of the heat transfer in the evaporator (q˙ORC = 10.9 kW/m2) is clearly higher than in the condenser (q˙HP = 6.1 kW/m2).

Numerical simulation of a phase change material in a spherical capsule with a hollow fin

The global objective of this paper is to study the incorporation of phase change materials (PCM) into passive solar construction and other applications for the storage and stability of thermal energy. The PCM used is the n-octadecane and Gallium confined in a spherical capsule with normal and hollow fins. CFD numerical simulations have been carried out to increase the heat exchange area and to compare the effect of the regular fin with hollow one at different angles. The temperature changes inside the spherical capsule on the horizontal and vertical lines for a deep understanding of the PCM behavior are also realized. The results show that the melting time is reduced 14 times in the case of PCM with hollow fin compared to a configuration without fins.

Evaluation of pressure drop effect on COP of single-stage vapor compression refrigeration cycles

This paper addresses a theoretical investigation about the effect of pressure drop along the heat exchangers on the coefficient of performance, heat transfer area and compressor capacity of a single-stage vapor compression refrigeration system. This analysis is performed based on a model of the complete system with one-dimensional heat exchangers, compressor and expansion device modelled as concentrated parameters. The fluid thermodynamic state along the heat exchangers is evaluated based on the energy and momentum balance considering predictive methods available in the open literature for heat transfer coefficient, pressure drop and void fraction. The compressor is modelled based on single isentropic efficiencies and the expansion device is assumed as adiabatic. Uniform wall temperatures are imposed in both heat exchangers to mimic heat removal from a refrigerated space and rejection to external ambient. Commercially available tube diameters were considered for the channel dimensions. Based on the analysis, it can be concluded that the pressure drop presents significant impact at the efficiency and sizing of the main components in a refrigeration system depending on the operational conditions. COP reductions of more than 15% for R600a and R134a were observed, as well as up to 29.2% increase of the heat exchanger area for the condenser. Correlations were proposed to prompt estimative of the impact of pressure drop on the COP and heat exchangers area.

Stochastic Bi-Objective Optimization for Closed Wet Cooling Tower Systems Based on a Simplified Analytical Model

A fast and accurate representation of heat and mass transfer processes of closed-wet cooling tower (CWCT) systems for optimal design and optimization is still challenging, especially considering the impact of uncertain weather conditions. This study develops a systematic approach for the stochastic bi-objective optimization of CWCT systems based on a simplified analytical model. The analytical model is established using an effectiveness-Number of Transfer Units method, which is used as a surrogate model of the CWCT systems, and is embedded in the stochastic non-linear programming model for seeking the optimums of two competing objectives. A tailored decomposition strategy is proposed to convert the original problem into multiple bi-objective optimization sub-problems by sequentially discretizing the design and uncertain spaces. Each bi-objective optimization problem is solved using an augmented e -constraint method, resulting in a set of Pareto solutions. An efficient decision-making method, namely Technique for Order Preference by Similarity to Ideal Situation, is applied for selecting the preferred solution out of the Pareto solutions obtained. From an illustrative case study, we conclude that the stochastic approach outperforms the deterministic approach since it allows for a certain design margin of heat exchanger area to reduce the impact of the variability in weather.

Analyzes of thermal characteristics of a hydrothermal coaxial closed-loop geothermal system in a horizontal well

The effective exploitation of deep hydrothermal resource is an emphasis in the global geothermal industry. A horizontal-well coaxial closed-loop geothermal system (CCGS) is presented to realize its efficient development by significantly increasing the heat exchange area based on a long horizontal well. However, few researchers focus on the complex convection process in the hydrothermal reservoir, making it difficult to effectively evaluate the thermal characteristics of a geothermal system. Therefore, a 3D transient flow and heat transfer model considering the forced and natural convection in the reservoir is established to achieve a comprehensive CCGS analyzes. Then, the flow and temperature fields in the formation with forced and natural convection are compared. The thermal characteristics of the horizontal-well CCGS with and without convection are discussed. Also, the influences of key factors on the heat production of the horizontal-well CCGS are investigated. The differences between the horizontal-well and vertical-well CCGSs in exploiting the hydrothermal resource are analyzed. The results show that the fluid convection in the reservoir, particularly the forced convection, can greatly enhance heat transfer by affecting heat convection. The temperature field in the reservoir may return to the original state before the next heating season when a high convection velocity occurs, which is beneficial to realize 100% sustainable heat production. The horizontal length, permeability, and reverse circulation should be the primary considerations in improving heat extraction. Compared with the limited performance of vertical-well CCGS, the horizontal-well CCGS can fully extract the heat stored in the hydrothermal reservoir via a long horizontal well. The findings provide a vital reference for the CCGS scientific study and engineering application.

Relaxation of the requirements on loop height and heat transfer area of a passive heat removal system in integral SMR using a self-powered booster

In this study, compactness improvement capacity of a self-powered efficiency booster is analyzed for natural circulation systems used in integral small modular reactors (SMR). Based on thermoelectric principle, the booster converts heat energy into electricity directly, which is used to produce additional driving force for natural circulation flow and heat transfer. Thus, flow and heat transfer are enhanced and such systems do not rely on gravity solely. Consequently, requirement on loop height and heat transfer area can be relaxed. Numerical simulations and experimental tests have been carried out. The results indicate that, using the booster, loop height is no longer a determining factor for natural circulation systems, and lower height can be used for the same performance. Furthermore, if the desirable heat transfer rate was kept, the heat exchanger area can be reduced by a factor of 25%–78%. With these results, compactness improvement capacity of the booster is proved.

Improving thermal energy recovery systems using welded plate heat exchangers

The aim of this paper is to show the use of welded plate heat exchanger (WPHE) technology in energy recovery systems to reduce the number of units and the potential benefits in revamping projects. The implementation of multifluid units allows for the reduction in the number of units in heat exchanger networks. A thermohydraulic model is developed to determine the unit dimensions that fulfil the required heat duty within the pressure drop restrictions. The approach to multifluid cases is demonstrated on a crude oil preheat train that contains a total of 12 heat exchangers. Various options for the simplification of the network structure are analysed but in principle, with the possibility of implementing multifluid structures, two options are analysed, the reduction to seven units and the reduction to three units. The total heat exchanger area using conventional shell and tube exchangers for the original structure is 9350.96 m2, compared to 3598 m2 required by the proposed multi-stream arrangement. An additional revamping projection consisting of the direct replacing of the original exchangers with WPHE technology using the same installed area, shows a potential energy reduction of 25% and a payback of 1.6 years.

HEAT EXCHANGER DESIGN OPTIMISATION

In this paper, an optimized model of Shell and Tube Parallel Flow Heat Exchanger of water and oil type is proposed using C++ programming language. Shell and tube heat exchangers are of special importance in boilers, oil coolers, condensers, pre-heaters etc. They are also used in high pressure operations, refrigeration and air conditioning industry and process applications. In this paper, a number of practical cases of shell and tube heat exchangers are taken and the analysis of thermal and constructional design of every case is done. The optimised design is obtained by minimising the pressure drop by maximising the heat exchanger area to reduce pumping and running cost. Also considering that large sizes lead to increased capital cost, heat exchanger area is also optimised to overcome this problem. Effect of design parameters on pressure drop and heat exchanger area is also explained.

Mathematical and experimental evaluation of a mini-channel PV/T and thermal panel in summer mode

In this paper, a mini-channel PV/T and mini-channel thermal panel hot water system is presented. The thermal panels in this system use mini-channel tube as the heat exchanger, which has a small hydraulic diameter and large heat exchanger area, and this special structure can improve the heat transfer coefficient at the same flow rate than the conventional type. The performance of this system for generating hot water and electricity in summer has been tested, and a simulation model of this operating mode has been developed. Based on a typical day’s weather data, the simulation model is verified, and the experimental and simulated results agree with each other very well. The results reveal that the experimental and simulated electrical efficiencies of PV/T panels are 11.5% and 12.6%, respectively. The experimental and simulated thermal efficiencies of thermal collectors are 46.8% and 48.0%, respectively. The experimental and simulated final water temperatures in the tank are 59.3 °C and 60.9 °C, respectively. Based on these results, an error analysis is carried out. The experimental and simulation results of the system in summer provide a fundamental data and method for predicting the annual performance of the system in the future.

Experimental Verification of CFD Simulation When Evaluating the Operative Temperature and Mean Radiation Temperature for Radiator Heating and Floor Heating

The assessment of heating systems is not only interested in the efficiency of the heating system itself, but also in the quality of the environment that the heating system creates. The quality of the environment and the heat-humidity microclimate is closely related to thermal comfort. A suitable environment has a positive effect, for example, on the efficiency of work at the workplace. The range of temperatures, humidity and operating temperatures in workplaces is often also legally prescribed in such a way that there is no thermal discomfort for users in the heated space. In terms of savings, it is therefore best to use heating systems that can create a comfortable environment with the lowest possible energy costs. During their development, variations are possible with temperature gradients, the size of the heat exchange area, or the ratio of the radiant and convective components of heat transfer. When developing such systems, it is appropriate to consider CFD simulations. The comparison of the results of CFD simulation and experimental measurement is also in the following article, where the comparison of the operating temperature and the mean radiation temperature of two different heating systems in the model office is monitored.

Costs vs. Flexibility of Process Heat Recovery Solutions Considering Short-Term Process Variability and Uncertain Long-Term Development

To significantly decrease fossil carbon emissions from oil refineries, a combination of climate mitigation options will be necessary, with potential options including energy efficiency, carbon capture and storage/utilization, biomass integration and electrification. Since existing refinery processes as well as many of the potential new processes are characterized by large heating demands, but also offer large opportunities for process excess heat recovery, heat integration plays a major role for energy efficient refinery operation after the implementation of such measures. Consequently, the process heat recovery systems should not only be able to handle current operating conditions, but also allow for flexibility towards possible future developments. Evaluation of the flexibility of process heat recovery measures with both these perspectives enables a more accurate screening and selection of alternative process design options. This paper proposes a new approach for assessing the trade-off between total annual cost and potential operating flexibility for the heat exchanger network in short-as well as in long-term perspectives. The flexibility assessment is based on the evaluation of a flexibility ratio (similar to the conventional flexibility index) to determine the range in which operating conditions may vary while at the same time achieving feasible operation. The method is further based on identification of critical operating points to achieve pre-defined flexibility targets. This is followed by optimization of design properties (i.e., heat exchanger areas) such that feasible operation is ensured in the critical operating points and costs are minimized for representative operating conditions. The procedure is repeated for a range of different flexibility targets, resulting in a curve that shows the costs as a function of desired flexibility ratio. The approach is illustrated by an example representing a heat exchanger network retrofit at a large oil refinery. Finally, the paper illustrates a way to evaluate the cost penalty if the retrofit is optimized for one operating point but then operated under changed conditions. Consequently, the presented approach provides knowledge about cost and flexibility towards short-term variations considering also changes in operating conditions due to long-term development.

Techno-Economic Optimization of an Integrated Biomass Waste Gasifier–Solid Oxide Fuel Cell Plant

With a growing energy demand in a carbon-constrained society, fuels cells powered by renewable fuels, and specifically solid waste, are seen as interesting contributors to the energy portfolio. The alternative energy industry needs to reduce costs, enhance efficiency, and demonstrate durability and reliability to be economically feasible and attractive. This paper addresses biomass waste gasification in distributed energy systems, using a solid oxide fuel cell (SOFC) to produce electricity and heat. The potential and optimal plant efficiency and layout (i.e., anode off-gas (AOG) recirculation point via small-scale turbomachinery and heat exchanger network) are analyzed through a multi-stage approach that includes scenario evaluation and multi-objective optimization via a hybrid optimization strategy with heuristics and mathematical programming. The results in this paper summarize the most convenient operating conditions and provide an optimized heat exchanger network (HEN). The AOG recirculation toward the gasifier combustor is the preferred option; the electrical and thermal efficiencies can separately go up to 49 and 47%, respectively. The combined total efficiency ranges between 76 and 82%, and the area of heat exchange, which corresponds to an amount of heat exchanged between 91 and 117 kW, is within 6–14 m2.