Low Heat Load Research Articles

Dynamic characteristics of an integrated cooling system comprising vapor compression and thermosyphon loop for electronics cooling

An integrated cooling system comprising passive cooling-thermosyphon loop and active cooling-vapor compression was proposed for the cooling of electronics, especially avionics, under a wide range of heat loads and ambient temperatures. A prototype of the integrated system with the weight of 2.8 kg was developed by microchannel heat exchangers and a micro rotary compressor. The dynamic characteristics of the thermosyphon loop mode and vapor compression mode, and the switching mode were investigated experimentally, with second as time scale. Under the premise that the average wall temperature does not exceed 50 °C, the thermosyphon loop mode can handle a heat load of 600 W with COP of 35.7, and the vapor compression mode can handle a heat load of about 1000 W with COP of 3.1, at the ambient temperature of 20 °C. The developed integrated cooling system can effectively handle the sharp change in heat load (two heat loads of 400 W and 800 W like square wave) and ambient temperature (from 10 °C to 32 °C) by switching working modes. It automatically switched to TL mode at low heat load and ambient temperature for energy conservation, and switched to VC mode at high heat load and ambient temperature for safety.

A theoretical study of the effects of different heating loads on the exergy performance of water-based and air-based space heating systems in buildings

The present study investigated theoretically the exergy performance of floor heating, radiators, and air heating under three different space heating loads of 10, 30 and 50 W/m2. The effects of different supply and return water temperatures were studied for the radiators, and the effects of different supply air temperatures were studied for the air heating system. All systems were assumed to be connected to a boiler. The floor heating system was further analyzed assuming an air-to-water heat pump, and a ground-source heat pump.Floor heating was the optimal heating system due to its low exergy demand. The separation of thermal environmental conditioning and ventilation was an efficient solution. The results prove thermodynamically that renewables (i.e., ground source heat in the present study) and low temperature heating systems (i.e., floor heating in the present study) are a resource- and exergy-efficient combination proven by their exergy efficiencies up to 10.5%. The critical COP concept was validated (2.57 in this study).The power use of auxiliary components might seem negligible in terms of energy; however, it is critical in terms of exergy as it affects the exergy performance drastically. The relative importance of auxiliary power becomes more critical at low space heating loads.

Simulation and experimental validation of pulsating heat pipes

• Numerical model improvements for the thermal performance prediction of pulsating heat pipes. • Model validation with new experimental data for vertical and horizontal orientation. • Flow behavior analysis on the simulated results. The one-dimensional transient numerical code of JJ Cooling Innovation has been significantly improved to simulate the thermal/hydraulic performance of pulsating heat pipes (PHPs). Using a mechanistic approach, the liquid slug and vapor plug flows and their interaction with the surrounding wall are modeled. The heat transfer is predicted using the Three-Zone Model of flow boiling in microchannels, which includes liquid film evaporation and liquid film condensation of vapor plugs and convection of liquid slugs. Discretizing the entire PHP serpentine into one continuous grid, the code solves all variables based on local conditions. The code allows the user to define the working fluid, the channel’s shape, material and wall roughness, the serpentine’s pitch, the geometrical parameters of evaporator, adiabatic region and condenser zones, and the operating parameters. The boundary conditions in the evaporator zone can either be constant heat flux or constant temperature, while on the condenser side, the coolant temperature is constant as well as the heat transfer coefficient. The code works without any fitting factor and predicts all local variables in the 1D grid as a function of time, including the liquid film thickness and the onset of boiling for the formation of new vapor plugs within the evaporating and adiabatic zones, if local nucleation conditions are reached. Growth, collapse and coalescence of generated bubbles are taken into account. The film thermal resistance is corrected to a radial conduction model. The implementation of the initial liquid film thickness laid behind a liquid slug is updated to include the effect of the bubble velocity and acceleration as well as the length of the leading slug. The nucleation of new vapor plug has also been updated. The onset of nucleation is only dependent on the local thermodynamic state. If the threshold is reached, a new method is used to simulate micro-bubbles formation and coalescence to a channel-size vapor plug. These modifications greatly improve the code accuracy and stability allowing simulations with high pressure fluid rather than ethanol only. Furthermore, in the on-going EU project Pulsating Heat Pipes for Hybrid Propulsion Systems (PHP2), a new PHP test bench has been designed and built at Provides Metalmeccanica. A copper tube PHP with a 19-turn of 2.0 mm internal diameter and 12 mm tube pitch was tested over a wide range of imposed heat loads. A microchannel water-cooling plate was used at the condenser side with an inlet temperature of 20 °C. Then, tests were done for vertical and horizontal orientations with R1233zd(E) as the working fluid. The code, which was previously validated for ethanol PHP test data from KAIST, is now successfully validated for the present experimental data points. Accurate results were obtained from low heat loads up to 250 W for a set of 49 data points, predicting 97% within an error bandwidth of 30%.

Beam-induced surface modifications as a critical source of heat loads in the Large Hadron Collider

Beam-induced heat loads on the cryogenic regions of the Large Hadron Collider (LHC) exhibit a wide and unexpected dispersion along the accelerator, with potential impact on the performance of its High-Luminosity upgrade. Studies related the heat load source to the avalanche multiplication of electrons at the surface of the beam vacuum chamber, a phenomenon known as electron could build-up. Here, we demonstrate that the topmost copper surface of beam pipes extracted from a low heat load region of the LHC consists of native Cu2O, while the pipe surface from a high heat load region had been oxidized to CuO during LHC operation and maintenance cycles. Experiments show that this process increases the secondary electron yield and inhibits efficient surface conditioning, thus enhancing the electron cloud intensity during LHC operation. This study relates the abnormal LHC heat loads to beam-induced surface modifications of its beam pipes, enabling the development of curative solutions to overcome this critical limitation.

Heat transfer investigation of a flat-plate oscillating heat pipe with tandem dual channels under nonuniform heating

To provide cooling of electronics under nonuniform heating from multiple heat sources, a novel tandem dual-channel flat-plate oscillating heat pipe (OHP) is tailored and experimentally studied with a “center-multiple-heat-source nonuniform heating and two-end air cooling” layout under different inclination angles, heat loads and nonuniform heating levels. Flow pattern diagrams together with thermal resistance contour maps are organized for two tandem mirror-symmetric halves of the flat-plate OHP, and the thermal performance differences under nonuniform and uniform heating are compared. It is indicated that, compared with uniform heating, nonuniform heating produces better thermohydrodynamic performance at a low heat load with quasi-steady “stop-oscillation” fluid motion in the flat-plate OHP, while it results in poorer thermohydrodynamic performance when the heat load is large enough to maintain sustainable fluid flow in the flat-plate OHP. Under nonuniform heating conditions, concentrating more heat to overcome the negative effects rather than to amplify the positive effects on the fluid flow and heat transfer in OHPs is more beneficial to the overall thermal performance. For the 540 tested conditions, the flat-plate OHP has a much smaller overall thermal resistance than a pure 6063 aluminum alloy plate with the same configuration (approximately 15.3% on average) along a heat transfer distance of 101.8 mm. In addition, it possesses good adaptability to gravity, with coefficients of variation of 27.6% in the overall thermal resistance for inclination angles from 0° to 90°. Accordingly, this flat-plate OHP has good potential for high-efficiency and good-adaptability cooling of electronics under nonuniform heating by multiple heat sources.

Investigation of temperature oscillations in a novel loop heat pipe with a vapor-driven jet injector

A novel loop heat pipe with a vapor-driven jet injector (LHPI) shows excellent heat transfer performance. However, high-amplitude temperature oscillations are observed at low heat loads. In this study, a LHPI with transparent compensation chamber is designed and tested to reveal the mechanism of temperature oscillations. The relations between the internal flow and temperature oscillations are investigated by visualization study, and the results show that high-amplitude temperature oscillations are caused by the intermittent liquid supply of the compensation chamber. Meanwhile, the temperature oscillation characteristics of the LHPI under a variety of operating conditions are also studied. Five operation modes of the LHPI are classified as start-up failure mode, high-frequency/low-amplitude oscillating mode, low-frequency/high-amplitude oscillating mode, overshoot mode and smooth mode. Furthermore, a low filling ratio can eliminate the low-frequency/high-amplitude oscillating mode and is therefore conducive to the stable operation of the LHPI, but it elevates the operating temperature under high heat loads. In addition, increasing the sink temperature can also suppress temperature oscillations.

Examining a Non-Mixing Concept for Hydronic Heating Systems: A Simulation Study

While buffer tanks are common to address the short-cycling concern of hydronic heating systems, they involve water mixing, where the boiler’s hot water is mixed with the system’s return water. This water mixing condition brings concerns of the performance of the boilers and the in-space heating equipment (e.g., radiant floors and terminal units) due to the reduced temperature differences (ΔT) through these devices. Then, a non-mixing concept is introduced by putting a movable separation plate in a buffer tank, which can store supply hot water and return cold water separately. To examine this non-mixing concept, a simulation study is conducted by developing two hydronic system models with mixing and non-mixing concepts. After simulating these models under continuous heating loads, it is observed that the non-mixing model can achieve better system efficiency (e.g., 86.7% versus 82.0% for low heating loads) due to better boiler’s efficiency and less system cycles. In addition, it is noted that the non-mixing concept allows better control of supply water temperature, which can better support the outdoor reset control of boilers.

Operating Performance of External Non-Involute Spur˝and Helical Gears: A Review

In practical use, most gears have an involute shape of tooth flanks. However, external involute gears have some drawbacks, such as unfavourable kinematic conditions at the beginning and end of meshing, a limited minimum number of teeth, and the highly loaded convex-convex (i.e., non-conformal) contact. Researchers have developed and analysed various non-involute forms of tooth flanks, but they have not been widely accepted. The main reasons are higher manufacturing costs and sensitivity to manufacturing and assembly errors. Analyses of non-involute forms of teeth are mostly theoretical (analytical and numerical), while there is a lack of experimental confirmations of theoretical assumptions. This paper reviews external non-involute shapes, their operating characteristics and possibilities of use compared to involute gears. Established criteria, such as Hertzian pressure, oil film thickness, bending stress at the root of the tooth, contact temperature, and gear noise, were used for assessment. The results of analytical studies and experimental research on S-gears are presented in more detail. S-gears have a higher surface durability and a lower heat load when compared to involute gears. The usability of non-involute gears is increasing with the development of new technologies and materials. However, the advantages of non-involute shapes are not so significant that they could easily displace involute gears, which are cheaper to manufacture.

Techno-economic evaluation of electricity price-driven heat production of a river water heat pump in a German district heating system

Large scale heat pumps (HP) are an important technology that will link district heating (DH) systems to the electricity sector in future smart energy systems. This paper examines the feasibility of the integration of a river water HP at a combined heat and power plant in Germany. It is part of a more extensive study about the transformation of a DH system in an urban district towards a 4th generation DH system. The focus is on operational characteristics and economic efficiency of electricity price-driven heat production. A novel method for estimating the coefficient of performance (COP) of two-stage ammonia HPs based on the difference between sink and source temperature is presented. The HP achieves a seasonal COP in the range of 3.4 to 3.7. The 15-year simulation with the software energyPRO shows that electricity-price driven operation is especially relevant for lower heat loads during the non-heating season. The correlation between volatility of electricity market price change and flexible operation is analysed. Finally, the levelized cost of heat for four designs with heat outputs of the HP from 4.7 MWth to 6.1 MWth and increasing storage sizes are compared. The results indicate that electricity costs are reduced in more flexible systems, but cost parity to the minimum dimensioning is not yet reached with the underlying economic framework conditions. However, the parameters that benefit the economic efficiency of more flexible systems are discussed.

Modelling the Spatial Distribution of ASF-Positive Wild Boar Carcasses in South Korea Using 2019-2020 National Surveillance Data.

Simple SummarySince African swine fever (ASF) virus in wild boar populations can spill over to domestic pigs, it is crucial to understand the disease determinants in the wild compartment. However, the imperfect detection sensitivity of wild boar surveillance jeopardizes our ability to understand ASF spatial distribution. In this study, we used national surveillance data of ASF in wild boars collected in the Republic of Korea from 2019–2020 to model the spatial distribution of ASF-positive carcasses for two successive study periods associated with different surveillance intensity. The model allowed us to identify disease risk factors in the Republic of Korea, determine the spatial distribution of the risk of ASF, and estimate the sensitivity of surveillance. The outputs of this study are relevant to policy makers for developing and improving risk-based surveillance programs for ASF in wild boars.In September 2019, African swine fever (ASF) was reported in South Korea for the first time. Since then, more than 651 ASF cases in wild boars and 14 farm outbreaks have been notified in the country. Despite the efforts to eradicate ASF among wild boar populations, the number of reported ASF-positive wild boar carcasses have increased recently. The purpose of this study was to characterize the spatial distribution of ASF-positive wild boar carcasses to identify the risk factors associated with the presence and number of ASF-positive wild boar carcasses in the affected areas. Because surveillance efforts have substantially increased in early 2020, we divided the study into two periods (2 October 2019 to 19 January 2020, and 19 January to 28 April 2020) based on the number of reported cases and aggregated the number of reported ASF-positive carcasses into a regular grid of hexagons of 3-km diameter. To account for imperfect detection of positive carcasses, we adjusted spatial zero-inflated Poisson regression models to the number of ASF-positive wild boar carcasses per hexagon. During the first study period, proximity to North Korea was identified as the major risk factor for the presence of African swine fever virus. In addition, there were more positive carcasses reported in affected hexagons with high habitat suitability for wild boars, low heat load index (HLI), and high human density. During the second study period, proximity to an ASF-positive carcass reported during the first period was the only significant risk factor for the presence of ASF-positive carcasses. Additionally, low HLI and elevation were associated with an increased number of ASF-positive carcasses reported in the affected hexagons. Although the proportion of ASF-affected hexagons increased from 0.06 (95% credible interval (CrI): 0.05–0.07) to 0.09 (95% CrI: 0.08–0.10), the probability of reporting at least one positive carcass in ASF-affected hexagons increased from 0.49 (95% CrI: 0.41–0.57) to 0.73 (95% CrI: 0.66–0.81) between the two study periods. These results can be used to further advance risk-based surveillance strategies in the Republic of Korea.

Steady-state modelling of dual-evaporator loop heat pipe

To promote the thermal management of loop heat pipe in aerospace and electronics, a steady-state heat transfer model of the asymmetric dual-evaporator loop heat pipe (DE-LHP) is established, in which the mass flow distribution at the junction of the liquid line is modified. The control of operating temperature between two compensation chambers (CCs), heat sharing characteristics and influence of heat sink and ambient temperature on operating temperature are analyzed. The control of operating temperature is influenced by the CCs position and heat load distribution between two evaporators. Two domination zones, namely the position domination zone and heat load distribution domination zone, are observed when the heat load of the distal evaporator is larger than that of the proximal evaporator. A formula for determining the influencing range of the position domination zone is also proposed. The operating temperature reaches the minimum when the heat loads applied to the two evaporators are equal. The ambient temperature influences the operating temperature at low system heat load while the heat sink temperature prevails at high heat load. The heat sharing is affected by the total heat load and the loop pressure distribution, and proportional to the flow distribution at the junction of vapor line.

Development of a Miniature Loop Heat Pipe for High Heat Flux Devices and Evaluation of Its Heat Transport Performance

This paper describes the design, fabrication, and the heat transfer performance of a miniature loop heat pipe (mLHP) aiming for cooling a small and high heat flux devices. For the design process, a one-dimensional numerical model was developed. Based on the numerical model, an mLHP was fabricated. The mLHP comprises an evaporator (15×16×t3 mm3), a condenser (12×12.5×t2 mm3) and transport lines (O.D.=1.6 mm, L=100 mm). The mLHP uses Shirasu porous glass (SPG) as a wick and is mainly made of stainless steel. The heat transport performance was investigated in horizontal attitude, anti-gravity (top heat mode) and gravity-assisted (bottom heat mode) with working fluid of water. When the condenser was dissipated by natural air convection, the mLHP performed 5-W (20-W/cm2) heat transport at 170-℃ heat source temperature in horizontal attitude. The thermal resistance was 7.9℃/W. In the bottom heat mode, the lowest thermal resistance of 2.6℃/W was obtained due to the gravity assist. Additionally, the mLHP was tested under the condition that the evaporator side was kept horizontal but the condenser side was bent upwards by 90 degree around the middle of transport lines. Consequently, in low heat load condition the bent mLHP showed the same performance with the bottom heat mode, but shifted to the horizontal mode as the heat load increased. Thus, the heat transport performance was comprehensively investigated for considering various uses.

Impact of cold plasma on the techno-functional and sensory properties of whey dairy beverage added with xylooligosaccharide

This study aimed to evaluate the impact of cold plasma (0, 5, 10, or 15 min) on the techno-functional and sensory properties of whey dairy beverages added with xylooligosaccharide (XOS, 1.5% p/v). Untreated and pasteurized whey beverages were also evaluated. The products were evaluated for physicochemical characteristics, bioactive compounds, XOS stability, rheological properties, and sensory characteristics. Cold plasma and pasteurized products presented lower color intensity (L*=87.4–87.9, a*=-0.24- −0.60, b*=2.41–5.19), reduced consistency (K = 4.31–42.21 mPa.sn and N = 0.57–0.95), and similar apparent viscosity, XOS chemical stability, and sensory characteristics compared with the untreated product. However, the cold plasma-treated beverages presented lower heat load indicators (hydroxymethylfurfural [HMF] values of 1.91–2.10 µmol/L and whey protein nitrogen index [WPNI] of 6.09–6.66 µmol/L) and a higher concentration of bioactive compounds (antioxidant activity [5.31–9.30%], and inhibition of ACE [14.17–22.53%], α-amylase [18.52–25.67%] and α-glucosidase [22.50–27.50%] activities) than the pasteurized product, being the effects more pronounced for the higher exposure times. Overall, cold plasma has important advantages for the processing of whey beverages added with XOS.

Examination of heat load at different zones of a Nimonic alloy combustor liner of different configurations by finite element method

In modern ages, the usage of gas turbine systems in aviation and power sectors has been increased tremendously. To achieve the desirable output, the components used in the gas turbine systems are heated to high temperatures. The increase in temperature may increase the heat load on the components which in turn increase the thermal stress and the component may undergo thermal expansion. Due to the exposure of these components to heat regularly, it may also increase the fatigue and creep of the material, thereby the life of the materials used for the fabrication of these components may decrease. As the life of the components decrease, the efficiency of the gas turbine system also decreases. Combustor liner of the gas turbine system undergo high thermal stress because of the combustion of fuel and air inside it. Temperature distribution and the heat load on the combustor is mandatory to make the design of engine reliable. The present work focuses on the design of combustor liner with different configurations for a gas turbine engine used for land-based power generation. The distribution of temperature at the primary. Secondary and dilution zones of the combustor liner for proposed configurations was analysed using ANSYS. The heat load at the zones of the combustor liner was also studied with the help of ANSYS. The results observed in the analysis helps out to study the effects of design variation on temperature distribution and heat load at different zones of the liner. The results also helped in the selection of a best configuration with low heat load at different zones of the liner.

Effects of a shallow SAS divertor on detachment in KSTAR

For long pulse operation of fusion reactors, it is important to reduce sputter-erosion and power loading of the divertor target by means of plasma detachment. It has been reported that the small-angle-slot (SAS) divertor employed by the DIII-D tokamak can initiate detachment at a relatively low upstream plasma density as it can effectively dissipate heat by concentrating neutrals near the target. Motivated by these findings in DIII-D, we investigated the effects of a SAS-like divertor in KSTAR using SOLPS-ITER simulations without drifts. One remarkable feature revealed by our simulation study is that even a very shallow SAS can lead to a considerably lower heat load on the divertor target than the original flat, open divertor of KSTAR. Deuterium neutrals are concentrated along the divertor separatrix line in the shallow SAS, while deuterium density in the open divertor peaks in the far-scrape-off layer. Furthermore it was found that the neutral density and temperature-drop induced by SAS are both fairly incentive to the depth of the slot. The highest heat dissipation was obtained for a SAS depth of 10.3 cm.

Application of biporous wick in flat-plate loop heat pipe with long heat transfer distance

Loop heat pipe, acting as one of the applications of porous media, has been extensively researched for its reliability and robustness. In order to enhance the heat transfer capacity of the loop heat pipe and adapt it to long-distance thermal control system, high-performance capillary wick is urgently required. In this paper, a comprehensive study about the biporous wick fabrication and operating mechanism is introduced. Contrasting to the conventional monoporous wick, the coaction of large and small pores could reduce the flow resistance and sustain sufficient capillary force. The driving force for loop circulation was comprised of capillary force, phase-change driving force, and gravity force. Furthermore, a flat-plate loop heat pipe with long heat transfer distance was constructed to test the performance limit of the biporous wick. Test results under horizontal condition indicated that the loop transferred the maximum heat load of 110 W (heat flux of 6.6 W/cm2) and the minimum thermal resistance was 0.382 °C/W. Besides, the intermittent formation of vapor phase inside compensation chamber would cause the temperature pulsation during low heat load range. While increasing the heat load, the subcooled liquid became sufficient and the pulsation was restrained. By introducing gravity effect at a tilt angle of 4.6°, the working performance was improved for the wick could be quickly infiltrated by the subcooled fluid even under long heat transfer distance. The maximum heat load was 330 W (heat flux of 19.9 W/cm2) and the minimum thermal resistance was 0.161 °C/W at a heat load of 240 W.

Experimental investigation on the start-up characteristics of single loop thermosyphon for motorized spindle bearing-shaft system cooling

Single loop thermosyphon (SLTP) is a promising method for handling the thermal dissipation problem of motorized spindle bearing-shaft system. The SLTP used for motorized spindle shaft cooling is with different structure from that of the existing SLTP. Therefore, it is necessary to conduct comprehensive study for its working mechanism. A visualization and thermal characterization experiment was carried out to research the effect of different heat load (Q) and liquid filling ratio (FR) on the start-up characteristics of the SLTP in this paper. The results show that the heat transfer in the SLTP with high FR at low heat load is intermittent boiling. The working fluid oscillates at the boiling incipience due to the U-shape structure of evaporator section, and temperature of the evaporator wall has an overshoot because of subcooled fluid and “interfacial vaporization heat sink effect”. At high heat load, the SLTP with high FR will translate from natural convection to nucleate boiling when starting, while liquid in the storage pool will be pushed into condenser section by vapor because of the excessive liquid level. However, the SLTP with low FR will not boil at low heat load, it forms a circulation mode of natural convection and condensation. With the increase of heat load, the heat transfer mechanism of the SLTP with low FR develops into nucleate boiling, and the liquid film returning from condenser section will be boiling when flows through evaporator section. In present experiment, the SLTP with 30% FR has the best start-up and flow performance.

Study on the Performance of Multiple Sources and Multiple Uses Heat Pump System in Three Different Cities

Various efforts have been made worldwide to reduce energy use for heating, ventilation, and air-conditioning (HVAC) systems and lower carbon dioxide (CO2) emissions. Research and development are essential to ensuring the efficient use of renewable energy systems. This study proposes a multiple sources and multiple uses heat pump (MMHP) system that can efficiently respond to heating, cooling, and domestic hot water (DHW) loads using multiple natural heat sources. The MMHP system uses ground and air heat as its primary heat sources and solar heat for heat storage operations and ground temperature recovery. For the efficient use of each heat source, it also determines the heat source required for operation by comparing the heat source temperatures in the same time zone. A model for predicting the heat source temperatures, electricity use, and coefficient of performance (COP) was constructed through simulation. To analyze the efficiency of the proposed system by comparing the existing air source heat pump with ground source heat pump systems, a performance analysis was conducted by setting regional and system configurations as case conditions. The results demonstrate that the electricity use of the MMHP system was 13–19% and 1–3% lower than those of air source heat pump (ASHP) and ground source (GSHP) systems, respectively. In addition, the MMHP system was the most favorable in regions with a low heating load.

Impact of Silica Nanofluid Deposition on Thermosyphon Performance

Thermosyphons are among the most efficient passive heat transfer devices known nowadays. Due to heat and mass transfer limitations of working fluids, much effort is invested in developing a new generation of enhanced fluids, such as nanofluids. Previous studies reported improved thermal capacity when water was replaced by nanofluid. The interplay between nanoparticles and the evaporator surface is crucial here. However, the details of the deposition mechanism and its impact on boiling are still not known. This paper focusses on determining how the boiling process in the thermosyphon affects the inner wall of the evaporator and the nanofluid itself, and how it influences the thermal performance. For that purpose, we conducted the experimental study on a thermosyphon filled with silica nanofluid and then analyzed the samples of evaporator surface and silica nanofluid under a scanning electron microscope. Silica deposited into a porous layer during boiling, which enabled capillary wicking. This in turn decreased the overheating of the evaporator wall. Moreover, silica nanofluid increased heat transfer efficiency and transferred more thermal energy at low heat loads. After opening the thermosyphon, the deposited layer dried and split up. Based on two methods of crack pattern analysis, the mean thickness of the layer was estimated.

Long-term thermal performance analysis of deep coaxial borehole heat exchanger based on field test

Geothermal energy is an abundant and clean energy source that is abundant and widely and naturally distributed in the earth. Deep coaxial borehole heat exchanger (DCBHE) can efficiently extract heat from rock-soil for building heating. And DCBHE systems have superior application performance to that of the traditional BHEs. However, the existing related researches are primarily focused on theory and modeling, via which the temperature characteristics and thermal processes cannot be truly reflected, thus field testing is necessary. In this study, the long-term thermal performance of a DCBHE is developed based on field test with a distributed optical fiber temperature sensor (DOFTS). The numerical model is validated by field test data while considering power failure. The effects of the heat load mode, flow rate, heat load, radius ratio and soil thermal conductivity on the fluid and soil are analyzed and discussed. Moreover, the thermal process in the formation is analyzed. It is demonstrated that the results of the numerical simulation fit well with the test data. The temporal and spatial distributions of the fluid temperature and thermal process can be affected by two heat load modes, for which thermal compensation should be applied. A higher flow rate, lower heat load and larger radius ratio can promote the outlet fluid temperature. The thermal equilibrium points in the formation will move downward with the increase of the soil thermal conductivity, and main impact scope can reach 40m. A higher flow rate and larger radius ratio will also obviously strengthen the pressure drop, and the power consumption will increase accordingly. Furthermore, two methods respectively called “Intercept” and “Guide” are proposed to reply the reverse heat conduction in shallow soil for DCBHE. The results can provide a reference for optimum design of DCBHEs for practical engineering.