Length Of Heat Exchanger Research Articles

An investigation of recovering the energy of exhaust heat for improving conventional compression ignition to low temperature combustion by adding diesel vapor

Reducing exhaust emissions and, consequently, recovering wasted energy and exergy of an engine are the primary purposes of engine research. In previous studies, improving the combustion efficiency and reducing the exhaust emissions by homogeneous combustion, and the recovery of engine wasted energy (especially exhaust wasted energy) to use in the downstream cycles have been researched. In the present study, the feasibility of recovering exhaust energy to evaporate diesel fuel and the effects of adding diesel vapor on combustion efficiency, exhaust emissions, and overall engine efficiency have been investigated. To achieve these purposes, an experimental investigation on the effects of adding diesel vapor was performed. And, the numerical analysis was used to investigate the feasibility of the diesel evaporation by recovering exhaust energy. The experimental tests were performed at a constant speed (1000 rpm) and three amounts of fuel injection. In each amount of fuel injection, the amount of diesel vapor increased in three steps up to the knock threshold. In the experimental investigation, the share of the indicated work, combustion irreversibilities, heat transfer loss, and exhaust gas from the inlet exergy have been shown. Also, the effects of adding diesel vapor on exhaust emissions (PM, NO, CO and HC) and the cycle variation were investigated. Furthermore, the chemical exergy of each one of the species in the exhaust stream was investigated in detail. In the numerical investigation, the following results were investigated: the available exhaust gas exergy up to the dew point, the share of the exhaust recoverable energy from the diesel evaporation energy, the improvement of the second law efficiency by recovering exhaust gas energy, the reduction of exhaust gas temperature, and the required length of heat exchanger for increasing the diesel temperature from ambient temperature up to the exhaust gas temperature. According to the achieved results, adding diesel vapor reduces combustion irreversibility and heat transfer loss; also, the second law efficiency improves from 30% to 44%. Using a heat exchanger can recover 20–45% of the required evaporation heat of diesel. The reduction of exhaust gas temperature after the heat exchanger is less than 2%. The maximum required heat exchanger length for increasing the diesel temperature from ambient temperature up to the exhaust gas temperature is 50 cm. The addition of diesel vapor decreases NO and soot emissions and, simultaneously, increases CO and HC emissions. The addition of diesel vapor decreases the cycle variation, especially in low-load conditions. According to the chemical exergy of the various species in the exhaust gas mixture, the CO2 and H2O emissions have the most destructive effect on the environment; also, the detrimental environmental impact of CO emission is more than NO until being oxidated by a catalyst.

A new multiscale modeling framework for investigating thermally-induced flow maldistribution in multi-stream plate-fin heat exchangers

Multi-stream Plate-fin heat exchangers (MSPFHEs) are among the most efficient heat exchangers in energy-based industries. They should be carefully designed to have maximum effectiveness and to avoid wasting energy. Flow maldistribution could significantly degrade the performance of heat exchangers. This study presents a novel modeling framework to capture the thermally-induced maldistribution in two-phase MSPFHEs. The developed model is used to show how thermally-induced flow maldistribution affects the performance of the heat exchanger, and some possible modifications are investigated to reduce its consequences. Our case study results show that thermally-induced maldistribution decreases the total heat transfer by 10.8%. Heat leakage helps to induce less flow near the leakage region, so it can be managed to overcome the initial thermally-induced flow. Moreover, thermally-induced maldistribution affects a limited region along the heat exchanger height direction. So, thermal performance deterioration decreases with increasing the number of layers for each stream. Nevertheless, increasing the heat exchanger length increases the degraded region in the heat exchanger. Thus, increasing the core length is not a good way to counteract the thermal performance deterioration caused by thermally-induced maldistribution. It is also found that an optimum layer arrangement could help to eliminate the thermally-induced maldistribution in the heat exchanger.

Design of a 2 MW ZrC/W-based molten-salt-to-sCO2 PCHE for concentrated solar power

To increase the power cycle efficiency and lower the levelized cost of electricity (LCOE) of concentrated solar power (CSP) plants, printed circuit heat exchangers (PCHEs) capable of operating above 700 °C with molten chloride salt and a sCO2-based fluid are needed. In this paper, the design of a high-pressure, high-temperature, 2 MW PCHE comprised of a thermomechanically-robust, zirconium carbide/tungsten (ZrC/W) composite is conducted for CSP plants. The ZrC/W composite is a material with high thermal conductivity, high stiffness, and high failure strength at high temperatures, along with excellent resistance to thermal cycling and thermal shock. In this work, a thermomechanical design analysis was conducted to select appropriate material thicknesses of the ZrC/W plates, and to determine the geometrical dimensions and the thermal performance of the PCHE. The influences of the plate number and heat exchanger length on power density and pressure drop have also been systematically investigated. Economic analyses were conducted to compare the cost of ZrC/W-based PCHEs to those comprised of IN740H (a state-of-the-art, nickel-based superalloy) and 316 stainless steel (316SS). At a sufficiently high plate production rate, the manufacturing cost of ZrC/W-based PCHEs can be significantly lower, while achieving a much higher power density, compared with state-of-the-art, nickel alloy-based and stainless steel-based PCHEs.

Economic Optimization of Rotary Heat Exchangers Using CO2 Pricing Scenarios Based on Validated Fluid Dynamic and Thermodynamic-Based Simulation

Rotary heat exchangers have been widely used in paint shops, combustion power plants, and in heating, ventilation, and air conditioning systems in buildings. For these processes, many types of heat exchangers are available in the market: Tube-shell heat exchangers, plate heat exchangers, and rotary heat exchangers, among others. For the rotary heat exchangers, the problem is that there is no net present value method and lifecycle assessment method-based optimization found in the literature. In this work, we address this issue: An optimization is carried out with help of an empirically validated simulation model, a life-cycle assessment model, an economical assessment, and an optimization algorithm. The objective function of the optimization simultaneously considers economic and environmental aspects by using different CO2 pricing. Different CO2 pricing scenarios lead to different optimization results. The ambient air empty tube velocity va, 2.1 optimum was found at 1.2 m/s, which corresponds to a specific mass flow msp of 5.4 kg/(m2·h). For the wave angle β, the optimum was found in the range between 58° and 60°. For the wave height h* the optimum values were found to be between 2.64 mm and 2.77 mm. Finally, for the rotary heat exchanger length l, the optimum was found to be between 220 mm and 236 mm. The optimization results show that there is still potential for technical improvements in the design and operation of rotary heat exchangers. In general terms, we recommend that the optimized rotary heat exchanger should cause less pressure drop while resulting in similar heat recovery efficiency. This is because the life cycle assessment shows that the use phase for rotary heat exchangers has the biggest impact on greenhouse gases, specifically by saving on Scope 2 emissions.

Effects of compressor frequency and heat exchanger geometry on dynamic performance characteristics of heat pump dryers

A heat pump dryer (HPD) exhibits significant transient behaviors owing to the combined system of the heat pump cycle and closed-loop air cycle. However, a complete transient simulation for HPDs has not been yet developed owing to its complicated equations and lack of experimental data. In this study, a complete transient simulation of an HPD was developed using the finite volume method and was validated with experimental data, presenting validation errors less than 6.1%. Using the developed simulation model, the dynamic behavior and performance of the HPD were analyzed by varying operation modes and heat exchanger length. The operation mode with a higher compressor frequency showed a 16.5% higher moisture extraction rate (MER) due to the decreased drying time, whereas that with a lower frequency showed a 34.8% higher specific moisture extraction rate (SMER) due to the decreased energy consumption. As the heat exchanger length increased from 80% to 120%, the MER increased by 10% and the SMER increased by 14%. Additionally, as the compressor frequency increased, the optimum heat exchanger length for achieving maximum performance increased. In conclusion, the compressor control logic and heat exchanger length should be optimized according to the drying time, power consumption, and initial cost.

Modelling and Parametric Analysis of Wire Finned Coiled Tube Heat Exchanger in a Small J-T Refrigerator

In the present study, numerical analysis of coiled tube heat exchanger used in J-T refrigerator is carried out. A computer code is developed to estimate the length of the heat exchanger by giving mass flow rate, diameter of tube and shell as input parameters. This code is verified against experimental data. Two different configurations are considered in this study viz., heat exchanger with wire fin wound around the inner tube of heat exchanger and without wirefin. Three different refrigerant mixtures are used to evaluate the performance of heat exchangers. The variations of temperature and heat transfer coefficient are brought out as result. Significant reduction in the length of heat exchanger is observed in all the cases. Mixture-1 causes 33% reduction in length of heat exchanger with wirefin. Similarly, for mixture-2 and mixture-3 the length is reduced by 15% and 30%. Additionally, heat transfer coefficient (HTC) values are also estimated for heat exchanger with wirefin and without wirefin. Considerable increase in HTC values is observed in the heat exchanger with wirefin.

Effects of Edge Shapes on Thermal-Fluid Processes in Oscillatory Flows

Thermoacoustic machines, Stirling engines or coolers, and pulse tube coolers are examples of energy systems that operate based on oscillatory flow principles. This class of technology would achieve an improved efficiency from appropriately designed heat exchangers, stacks, regenerators and thermal buffer tubes. In this paper, heat transfer and oscillatory flow behaviour in three identical parallel-plate heat exchangers, one ‘heat source’ positioned between two ‘heat sinks’, are investigated using numerical method. The effect of different plate edge shapes on heat transfer, flow structures and acoustic pressure drop are examined at a selected drive ratio of 0.3 – 2.0%. Flow parameters show a strong dependency on drive ratio and flow direction, especially at low excitation where gas displacements are below or comparable to the heat exchanger length. Cone edge shape minimises the flow complexity better than other shapes with a negligible effect on the heat transfer. The result of this study will benefit the design and development of compact and high-efficiency heat exchangers for the next generation of oscillatory-flow energy and thermal management systems.

Sensitivity analysis of a vertical geothermal heat pump system in a hot dry climate

Ground source heat pumps (GSHPs) are considered to provide very efficient air conditioning systems, and this assists in achieving sustainability goals as well as reducing electricity consumption and carbon emissions. The design of such systems, however, depends on the climatic conditions and the geological characteristics of the site; in practice, the optimum design of the system also depends on the characteristics of the surrounding soil. The majority of the research into GSHPs has been focused on cold regions, thus this work is a very useful reference and important in GSHPs’ design in Saudi Arabia and therefore in other hot and arid regions.For the first time in such a hot/dry climate, a rigorous sensitivity analysis method applying the Ground Loop Design software has been employed to analyse 12 parameters that most affect the behaviour of the system. It is concluded that the most important four design parameters are the: thermal conductivity, soil temperature, building load, and fluid flowrate. On increasing these values they have opposing effects on arid environmental conditions, such as those found in the city of Riyadh in Saudi Arabia.In addition, the near-optimum design of a GSHP system in the city of Riyadh in Saudi Arabia has been investigated. The temperature of the water entering the heat pump is an adjustable factor that determines the performance of a GSHP unit. Because of the coefficient of performance of the heat pump, it can also be measured by the value of the entering water temperature. Thus, the entering water temperature (EWT) was selected to be the performance measure of the system, as all the heat transfer processed between the ground heat exchanger components and the surrounding soil is manifested in the entering water temperature. The value of the EWT was fixed both at the baseline and in the proposed design systems, and these were tested over the lifespan of 20 years. The results showed that the length of the ground heat exchanger and power consumption in the proposed design could be reduced by 15% and 1.12% respectively, compared to the baseline system. These results can be used to simplify the design of buildings in similar arid regions worldwide.

Assessment of the Impact of Temperature on Biofilm Composition with a Laboratory Heat Exchanger Module.

Temperature change over the length of heat exchangers might be an important factor affecting biofouling. This research aimed at assessing the impact of temperature on biofilm accumulation and composition with respect to bacterial community and extracellular polymeric substances. Two identical laboratory-scale plate heat exchanger modules were developed and tested. Tap water supplemented with nutrients was fed to the two modules to enhance biofilm formation. One “reference” module was kept at 20.0 ± 1.4 °C and one “heated” module was operated with a counter-flow hot water stream resulting in a bulk water gradient from 20 to 27 °C. Biofilms were grown during 40 days, sampled, and characterized using 16S rRNA gene amplicon sequencing, EPS extraction, FTIR, protein and polysaccharide quantifications. The experiments were performed in consecutive triplicate. Monitoring of heat transfer resistance in the heated module displayed a replicable biofilm growth profile. The module was shown suitable to study the impact of temperature on biofouling formation. Biofilm analyses revealed: (i) comparable amounts of biofilms and EPS yield in the reference and heated modules, (ii) a significantly different protein to polysaccharide ratio in the EPS of the reference (5.4 ± 1.0%) and heated modules (7.8 ± 2.1%), caused by a relatively lower extracellular sugar production at elevated temperatures, and (iii) a strong shift in bacterial community composition with increasing temperature. The outcomes of the study, therefore, suggest that heat induces a change in biofilm bacterial community members and EPS composition, which should be taken into consideration when investigating heat exchanger biofouling and cleaning strategies. Research potential and optimization of the heat exchanger modules are discussed.

10-year simulation of photovoltaic-thermal road assisted ground source heat pump system for accommodation building heating in expressway service area

The photovoltaic-thermal (PVT)-ground source heat pump (GSHP) system has been proved to have great potential in improving building energy efficiency, but ordinary PVT collector requires a large installation area and limits the popularization of PVT. In this paper, a novel solar energy collector, called photovoltaic-thermal road (PVTR), is proposed to replace the traditional PVT collector, and a novel energy system that is PVTR-GSHP system is proposed. In order to study the system performance, taking a typical expressway service accommodation building in Beijing as an example, a 10-year simulation is carried out to analyze the heating operation characteristics of the proposed system. Moreover, the effects of the PVTR area, the length and layout of ground heat exchanger (GHE) on the system performance are also discussed. Results show that, when the length of GHE is 5000 m, and the number of boreholes is 25 and the borehole depth is 100 m, the minimum area of PVTR satisfying the soil heat balance and meeting the system power consumption are 187.2 m2 and 216 m2, respectively. When the PVTR area is 216 m2, the minimum GHE length satisfying the soil heat balance is 4000 m. Finally, to realize system zero energy consumption and soil heat balance, the best combination of GHE length and PVTR area is 4000 m and 216 m2 under the cases of this paper. And in this case, the average COPHP and COPsys in ten years are 5.59 and 2.31 respectively.

РЕЗУЛЬТАТЫ УПРОЩЕННОГО МАТЕМАТИЧЕСКОГО МОДЕЛИРОВАНИЯ РАСПРЕДЕЛЕНИЯ ТЕМПЕРАТУРЫ ЖИДКОСТИ В ТЕПЛООБМЕННОМ КАНАЛЕ ЭЛЕКТРОНАГРЕВАТЕЛЯ С ВНУТРЕННИМ ИСТОЧНИКОМ ТЕПЛОТЫ

Indirect electrical resistance heating systems are heat-ers with internal heat sources and are widely used in agri-culture for heating gaseous and liquid media. Such sys-tems are characterized by insufficient intensity of heat ex-change processes. This implies a large heat transfer sur-face area and significant geometric size. Earlier, an attempt was made to solve the problem of increasing the efficiency of heat transfer processes and minimizing the geometric size of the heat exchanger. For this purpose, the heat ex-change characteristics were simulated and the geometric dimensions of three heat exchange systems were deter-mined: “pipe with internal heat sources in a dielectric pipe”, “pipe with an internal heat source -a membraneof heated liquid” and “cylindrical coil -heated liquid”. The analysis of these heat exchange systems has shown that the most promising is a coil-type heat exchanger. This system has the best heat transfer characteristics and the most compact size. To confirm the correctness of the applied method of calculating the heat exchange and geometric parameters of the heat exchanger, the simulation of the temperature dis-tribution of the heated liquid in the channel of the coil heat exchanger is implemented in this work. The verification calculations carried out under the formulated assumptions, using the example of a coil heat exchanger, show that the method for determining the heat exchange and geometric parameters of heat exchangers is correct. As a result of the simulation, it has been found that the error in determining the required channel length of the coil heat exchanger, the number of turns and the height of the coil to reach the liq-uid temperature at the outlet of 75°C does not exceed 4%. A similar conclusion can be made regarding the heat ex-changers of the types “pipe with internal heat sources -heated liquid” and “pipe with an internal heat source -a membraneof heated liquid”.

Моделирование охлаждения масла погружного электродвигателя с помощью теплообменника

Submersible motors are part of submersible oil production pumps that convert electrical energy, which is supplied through a cable from VSD, into mechanical energy of pump rotation. Currently, in about 30% of cases, the failure of an electrical submersible pump is due to a failure of the submersible motor. One of the main causes of failures is overheating of the stator winding insulation. Overheating of submersible oil-filled electric motors occurs because more heat is generated inside the motor than is removed through its outer surface. To intensify the heat removal, it is proposed to connect a heat exchanger in series with the motor and to organize the circulation of the oil in a closed loop. Both in the submersible motor and in the heat exchanger, oil flows along the annular gap along the inner surface of the housing, the oil channel is closed through a hole inside the shaft. The aim of the work is to select such a configuration of the annular channel, in which its length would be minimal. Intensification of heat removal by increasing the speed of the coolant is not advisable, because requires the motor to be equipped with a powerful pump for pumping oil, which will become an additional source of heat. Therefore, it was decided to increase the surface area of the annular channel through which heat, through the body of the installation, is removed to the well fluid. A series of calculations was performed for heat exchangers with smooth walls, with fins (perpendicular to the flow direction), and with spiral grooves (which additionally increase the length of the trajectory of oil particles and the time of their thermal contact with the stacks of the heat exchanger body). Computational fluid dynamics calculations showed that heat exchangers made according to the first two design options removed less than half of the heat. According to the third option, the oil was cooled practically to the temperature of the well fluid with a heat exchanger length of about 10% of the submersible motor length.

Optimization of Earth Air Tube Heat Exchanger for Cooling Application Using Taguchi Technique

Earth air tube heat exchanger (EATHE) is one of the passive technologies which utilize the earth stored heat (renewable energy) for heating/cooling the buildings. EATHE releases heat to earth for cooling space in summer, making the earth a heat sink and extracts earth-stored energy for heating space in winter and makes the earth a heat source. This paper optimizes the Length of the ground heat exchanger and overall heat transfer coefficient of earth air heat exchanger using the Taguchi technique for cooling application. For this purpose, we select six factors such as installation depth of Pipe (A), Pipe's inner diameter (B), Thermal conductivity of pipe material (C), Inlet air temperature (D), Outlet air temperature (E), Inlet air velocity (F). All these factors are taken at three levels, and we select an L27 orthogonal array for experimental runs. The ground heat exchanger's Length and the overall heat transfer coefficient were then calculated for each experimental run. In the Taguchi method, we find the signal to noise ratio for an optimal combination of all six factors and ANOVA to find the order of influencing parameters and their percentage contributions for both the objective parameters. According to our results, the best combination for all the six factors for ground heat exchanger length and overall heat transfer coefficient were A1B1C3D1E3F1 and A2B3C2D3E1F3, respectively. The highest and lowest influencing factors for ground heat exchanger length were the pipe's inner diameter and the pipe's installation depth with their contribution factors of 69.12 and 0.32%, respectively. In contrast, the highest and lowest influencing factors for the overall heat transfer coefficient were the pipe's inner diameter and thermal conductivity of pipe material with their contribution factors of 75.97and 0%, respectively. Hence the order of influence of all the six factors for both the objective parameters was BEFDCA.

A printed-circuit heat exchanger consideration by exploiting an Al2O3-water nanofluid: Effect of the nanoparticles interfacial layer on heat transfer

Supercritical carbon dioxide (S-CO2) Brayton cycle is an encouraging power conversion technology pertaining to waste heat recovery applications, because of the high compactness and efficiency it presents. A key technological challenge for the commercialization of this technology is the improvement of the cooling process of these cycles. In this study, an analytical investigation of a printed-circuit heat exchanger (PCHE) used as precooler for S-CO2 Brayton cycles and employing an Al2O3-water nanofluid is presented. In particular, the heat exchanger is modeled as segments in series to investigate the nanofluid impact on the PCHE’s thermal-hydraulic performance. Regarding the nanoparticles consideration, the selected theoretical model for the estimation of the thermal conductivity takes into account the radius of the nanoparticles and the nanolayer thickness which is formed around it. In brief, the maximum used nanoparticle volume fraction of 5% results in an improvement of 75% for the heat transfer coefficient leading, in turn, to a reduction of 1% for the heat exchanger length and a pressure drop increase of 8%. Finally, the increase of nanoparticle radius results in a reduced effective thermal conductivity, while the nanolayer thickness of 2 nm showed an improved heat transfer coefficient by 43% compared to the minimum nanolayer thickness of 0.5 nm.

Technico-economic modelling of ground and air source heat pumps in a hot and dry climate

In a hot and dry country such as Saudi Arabia, air-conditioning systems consume seventy per cent of the electrical energy. In order to reduce this demand, conventional air -conditioning technology should be replaced by more efficient renewable energy systems. These should be compared to the current standard systems which use air source heat pumps (ASHPs). These have a poor performance when the air temperature is high. In Saudi Arabia, this can be as much as 50 °C. The purpose of this work, therefore, is to simulate and evaluate the performance of ground source heat pumps (GSHPs) compared with systems employing (ASHPs). For the first time, both systems were comprehensively modelled and simulated using the Transient System Simulation (TRNSYS). In addition, the Ground Loop Design (GLD) software was used to design the length of the ground loop heat exchanger. In order to assess this configuration, an evaluation of a model of a single story office building, based on the climatic conditions and geological characteristics that occur in the city of Riyadh in Saudi Arabia was investigated. The period of evaluation was twenty years in order to determine the Coefficient of Performance (COP), Energy Efficiency Ratio (EER) and power consumption. The simulation results show that the GSHP system has a high performance when compared to ASHP. The average annual COP and EER were 4.1 and 15.5 for the GSHP compared to 3.8 and 11 for the ASHP respectively, and the GSHP is a feasible alternative to ASHP with an 11 years payback period with an 18% total cost saving over the simulation period and 36% lower annual energy consumption. The TRNSYS model shows that despite the positive results of the modeling, the high rate of the underground thermal imbalance (88%) could lead to a system failure in the long term

Analysis of Heat Exchange Performance of Heat Exchange Tubes of Evaporative Heat Exchanger Based on Fluent

In order to study the influence of the heat exchange tubes length of the evaporative heat exchanger on the heat exchange performance of the organic Rankine cycle technology (ORC) in automobile exhaust heat recovery, four kinds of three-layer tube heat exchange tube models with the length of 500mm, 800mm, 1100mm and 1400mm are designed by DM modeling software, and they are simplified into a quarter calculation domain model. The simulation calculations of the coupling heat exchange of four different lengths of heat exchange tubes are performed by using the Couple algorithm of Fluent fluid simulation software and the κ-ε model. Through the simulation calculation of the heat exchange of the four three-layer heat exchange tubes, it can be seen that the temperature in the three-layer heat exchange tubes changes along the Z axis direction. The heat exchange performance of different lengths of heat exchange tubes can be obtained by analyzing the temperature cloud diagrams of the XY plane at both ends of the heat exchange tubes and the temperature change curve along the Z axis direction. The heat exchange simulation analysis results of the four three-layer heat exchange tubes show that under the set parameter model, only the inlet and outlet boundary temperature is considered, and the three-layer heat exchange tube with a length of 1400mm has better heat exchange performance.

The application of thermoelectric and ejector in a CO2 direct-expansion ground source heat pump; energy and exergy analysis

The thermodynamic analysis of a horizontal CO2 direct-expansion ground source heat pump in transcritical cycle was performed to investigate the influence of using thermoelectric sub-cooler and the ejector on the performance parameters of the cycle. A comparative analysis was conducted between the Base cycle and the cycles with ejector and thermoelectric sub-cooler considering the pressure drops in the gas cooler and the ground heat exchanger. The effects of compressor inlet temperature, soil temperature, thermoelectric sub-cooler length, thermoelectric sub-cooler cooling water mass flow rate, and ground heat exchanger outlet temperature on different performance parameters of the cycles were studied. The results demonstrated that the application of ejector exhibits a more stable operation of the cycle and increasing its COP by 16.5% as compared to the Base cycle considering the same ground heat exchanger length. Also, the length of ground heat exchanger is considerably decreased by using the ejector considering the same COP for the Base and EJR cycles. Besides, thermoelectric sub-cooler decreases the irreversibilities of the cycle by generation more power, thereby COP is boosted by 16% as compared to the Base cycle. Furthermore, the major irreversibility in the cycle with thermoelectric and ejector is occurred in the compressor and followed by ejector, ground heat exchanger, gas cooler, thermoelectric sub-cooler, and the expansion valve, respectively.

Cooling of office buildings in cold climates using direct ground-coupled active chilled beams

This study investigates the use of a direct ground cooling system (DGCS) using active chilled beams for the cooling of office buildings in Sweden. The methodology of the study entails laboratory experiments to develop and validate a simulation model of the cooling system. The sensitivity of the input parameters, such as borehole heat exchanger (BHE) length, internal heat gains and room temperature set point, are studied with respect to BHE outlet fluid temperature and room thermal comfort. The results provide a practical insight into designing DGCSs with regard to borehole outlet fluid temperatures. The results also show that the thermal comfort criteria in the room are met by applying the DGCS even under the most critical design conditions of undisturbed ground temperature and internal heat gains. The sensitivity study quantifies the influence of the room temperature setpoint and internal heat gain intensity on the BHE length. The BHE outlet temperature level is more sensitive in shorter BHEs than in the longer ones, and BHE length and room temperature levels are highly correlated. Thus, the sizing of DGCS can benefit from a control system to allow the room temperature to float within a certain range.

Analysis of influence of the length of ground heat exchangers on the operation characteristics and economy of ground source heat pumps

A three-dimensional finite element dynamic simulation platform of the ground source heat pump system (GSHPS) is established. According to the outlet temperature of ground heat exchangers (GHEs) required by the code in summer and winter, the calculated minimum buried depth of GHEs meeting the requirements is 60 m, when the number of borehole is 9. By using the established platform, the annual operation performance and cost of the GSHPS under different buried pipe depths are studied. The results show that the deeper the buried depth of GHEs is, the better the heat exchange effect of GHEs is. Compared with the GHEs with 60 m buried depth, when the buried depth of GHEs is 65 m, 70 m, 75 m and 80 m, the average coefficient of performance (COP) of the unit increases by 4.1%, 6.3%, 7.7% and 8.2% in cooling period and 1.0%, 1.6%, 1.8% and 1.9% in heating period, respectively. Considering the performance and initial investment of the GHSPS comprehensively, the optimal buried depth of GHEs is 60 m. However, considering the performance the system and the total cost of the system running for 20 years comprehensively, the optimal buried depth of GHEs is 70 m.

Numerical investigation of thermal and hydraulic characteristics of sCO2-water printed circuit heat exchangers with zigzag channels

Since the precooler and the recuperator are the largest components of a supercritical carbon dioxide Brayton cycle, their design can substantially affect the performance and size of the whole system. Although the design of a precooler with zigzag channel geometry as an alternative to straight channels can reduce its size significantly, the applicability of available correlations (e.g. 0.7 < Pr<2.2) for the zigzag channel geometries is limited to the operational range of recuperators only. The current study, therefore, aims to develop correlations and understating of the complex flow and heat transfer characteristics in the zigzag channel printed circuit heat exchangers (PCHEs) operating under precooler conditions (2.2<Pr<13) of supercritical carbon dioxide Brayton cycle(sCO2-BC). Thermal and hydraulic characteristics of the PCHEs are computed numerically for a wide range of Reynolds numbers (5000<Re<7000) and Prandtl number (2.2<Pr<13). Also, a new data reduction method based on segmental averaged values has been proposed to handle adverse variations in the thermophysical properties of CO2 under precooler conditions. To ensure accurate evaluations, steep variations in the thermophysical properties of CO2 are implemented by supplying high-resolution real gas (RGP) property tables. Results suggest that thermal and hydraulic characteristics associated with zigzag channel vary substantially along the length of heat exchanger thus the conventional data reduction methods based on the channel average values cannot be used for true evaluations. Instead, segmental average values are used to develop pressure drop and heat transfer correlations for a broader range of Reynolds number and Prandtl number. The proposed correlations should be useful in the design of compact heat exchanger systems using zigzag channels for a wider range of cooling loads.