Long-distance Heat Transport Research Articles

Thermo-economic comparison of CO2 and water as a heat carrier for long-distance heat transport from geothermal sources: A Bavarian case study

Deep geothermal energy has tremendous potential for decarbonizing the heating sector. However, one common obstacle can be the mismatch between geologically attractive regions in the countryside and urban areas with a high heat demand density, which are therefore attractive for district heating systems. In the last years, an increasing number of regions consider the transport of geothermal heat into urban clusters. One example of such a region is the South German Molasse Basin in Upper Bavaria. However, such heat transport pipelines come along with massive upfront investment costs due to the required large pipe diameter and insulation thickness. While the classic concept foresees the use of water as a heat carrier in such long-distance heat transportation pipelines, CO2 can be an attractive alternative. This study investigates the thermo-economic performance of CO2 as a heat transport carrier for a potential long-distance heat transmission pipeline with a length of 20 km, which could connect a planned geothermal project in the South of Munich with the existing district heating network of Munich. The results of the base case scenario demonstrate that for both heat carrier options water and CO2 rather low LCOH for the transport of the heat can be achieved.

Heat transfer characteristics and operational visualization of two-phase loop thermosyphon

The advantages of high-efficiency and pumpless long-distance heat transport make the two-phase loop thermosyphon (TPLT) a promising candidate for various energy- and thermal-related applications. In this work, the simultaneous temperature/pressure measurement and high-speed visualization observation were conducted to compare and evaluate the heat transfer performance and two-phase flow characteristics of a vertical-positioned TPLT having an inner diameter of 10 mm. R141b was used as the working fluid at volumetric filling ratios of 35–50 %. The TPLT functioned well and had high heat transfer capacity at relatively high filling ratios (⩾ 40 %), while it failed to operate at a low filling ratio of 35 % under the power inputs greater than 250 W due to the local dry-out. A maximum effective thermal conductivity of 2.0 × 105 W/(m·K) was achieved at the optimal filling ratio of 40 %. Further, a flow regime map was developed to identify the combination effect of filling ratio and heat power on the flow pattern evaluation and two-phase flow instability. The geyser boiling usually manifests itself as an alternation of two or even three flow patterns, and tends to occur at the combination conditions of relatively low filling ratios (35 % and 40 %) and low heat powers or a moderate filling ratio (50 %) and relatively high heat powers. Additionally, the white mist phenomena caused by the flash vaporization was observed in the riser at relatively higher power inputs. The liquid-vapor mixture flow in the riser would partially degrade the TPLT performance due to the inhibition of vapor condensation in the condenser region. This study provides new insight into the operation characteristics of TPLT and gives guidelines for proper design and applications.

Contrasting eddy-driven transport in baroclinically unstable eastward currents and subtropical return flows

We identify and explore the fundamental differences in the dynamics of mesoscale vortices in eastward background (EB) parts in mid-latitude ocean gyres and in westward background (WB) return flows. In contrast to eddy behavior in EB flow, a systematic meridional drift of eddies in WB flow results in poleward expulsion of cold-core cyclones and equatorward expulsion of warm-core anticyclones from the unstable zone with a negative potential vorticity gradient (PVG). Consequently, heat can be transferred further by upper ocean vortices intrinsically coupled with deep opposite sign partners. Such structures can drift through the stable zone with positive PVG in both layers. This mechanism of lateral transfer is not captured by local models of homogeneous turbulence. The crossflow drift is related to the coupling of the upper vortices with opposite sign deep eddies shifted eastward. The abyssal vortices can be viewed as lee Rossby waves induced by their upper-layer partners and described analytically in the vicinity of latitude of marginal stability. Here, we show how such self-amplifying hetons, emerging in homogeneous turbulence, saturate when they approach locally stable regions of inhomogeneous currents. The presented results indicate that subtropical regions with return WB flows in the upper layer favor long-distance heat transport by spatially coherent eddies in accordance with observations and motivate the development of non-local parameterizations of eddy fluxes.

Observation of heat transport mediated by the propagation distance of surface phonon-polaritons over hundreds of micrometers

Efficient heat dissipation in micro/nano electronics requires long-distance propagation of heat carriers operated above room temperature. However, thermal phonons—the primary heat carriers in dielectric nanomaterials—dissipate the thermal energy after just a few hundred nanometers. Theory predicts that the mean free path of surface phonon-polaritons (SPhPs) can be hundreds of micrometers, which may improve the overall dissipation of heat in nanomaterials. In this work, we experimentally demonstrate such long-distance heat transport by SPhPs. Using the 3ω technique, we measure the in-plane thermal conductivity of SiN nanomembranes for different heater-sensor distances, membrane thicknesses, and temperatures. We find that thin nanomembranes support heat transport by SPhPs, as evidenced by an increase in the thermal conductivity with temperature. Remarkably, the thermal conductivity measured 200 μm away from the heater is consistently higher than that measured 100 μm closer. This result suggests that heat conduction by SPhPs is quasi-ballistically over at least hundreds of micrometers. Our findings pave the way for coherent heat manipulations above room temperature over macroscopic distances, which impacts the applications in thermal management and polaritonics.

Techno-economic optimization of large-scale deep geothermal district heating systems with long-distance heat transport

Geothermal energy can play an important role in decarbonizing the heating sector, however, a limiting factor is that heat demand in urban areas does not usually coincide spatially with geological settings favorable to the extraction of geothermal energy. Long-distance heat transport could enable the direct use of geothermal resources even in areas with low or no geothermal potential. This paper proposes the cost-optimal coordinated deployment of geothermal heating plants together with heat transport and distribution networks to simultaneously supply geothermal heat to multiple urban areas. To this end, a holistic approach comprising the mapping of geothermal potential for direct-use, the estimation of district heating potential, and a two-step optimization model to calculate cost-optimal large-scale geothermal district heating systems, is presented and applied to the Free State of Bavaria in Germany. As a result, heat supply costs can be reduced by 15% if fewer geothermal wells are drilled in more geologically favorable areas at greater distances from heat sinks. Calculated levelized costs of heat without local distribution networks of 33–39 €/MWh show that geothermal energy could transition from a local to a regional use if utilized in in scenarios with high full-load hours. The proposed methodology can be adapted to develop expansion strategies for deep geothermal energy in other similar regions worldwide.

Long-distance heat transport based on temperature-dependent magnetization of magnetic fluids

Long-distance heat transport based on temperature-dependent magnetization of magnetic fluids

Heat transfer of temperature-sensitive magnetic fluids around single heating pipe

Temperature-sensitive magnetic fluid (TSMF) is a magnetic nanoparticle suspension with strong temperature-dependent magnetization even at room temperature. TSMF is a refrigerant that enables high heat transport capability and pumpless long-distance heat transport. To enhance the heat transport capacity of the magnetically-driven heat transport device using TSMF, it is effective to use a heating body with a very large heat exchange surface such as a heat sink or a porous medium. In the present study, the thermal flow of TSMF around a single heating pipe under a magnetic field was investigated. Visualization of the temperature field by infrared thermography showed that the application of the magnetic field dramatically developed the thermal boundary layer and improved heat transfer. It was clarified by numerical analysis that this dramatic variation in the thermal boundary layer was associated with several vortexes generated by magnetic force in the vicinity of the heating pipe.

A study on the fluid refrigerant charge in a two-phase mechanically pumped loop system using R134a and R1234yf

Heat management by means of two-phase mechanically pumped loop (TMPL) possesses promising advantages such as long-distance heat transport, high heat density and good temperature control. Knowledge of the fluid charge has great importance for TMPL system efficiency. The present work is a theoretical/experimental study on the refrigerant charge in TMPL system using R134a and R1234yf refrigerants. The mass of refrigerant in the TMPL system was measured for several cooling capacities. The effects of vapor quality in the evaporator (varying from 0.1 to 1), mass flux (300 kg s−1 m−2 and 400 kg s−1 m−2), and saturation temperature (25 °C and 30 °C) on refrigerant charge in the TMPL were experimentally investigated. About 70 experimental points were analyzed and compared with a theoretical model employing eight different prediction tools for the determination of the void fraction. The best results were obtained by Zivi and Hughmark correlations with mean absolute errors of 12% and 13%, respectively.

Theoretical Realizability of Dream-Pipe-Like Oscillating/Pulsating Heat Pipe

The state of the art of thermally self-excited oscillatory heat pipe technology is briefly mentioned to emphasize that there exists no oscillating/pulsating heat pipe (OHP/PHP) suited to long-distance heat transport. Responding to such conditions, this study actively proposes a newly devised conceptually novel type of OHP/PHP. In that heat pipe, the adiabatic section works as it were the dream pipe invented by Kurzweg. This striking quality of the proposed new-style OHP/PHP produces high possibilities of long-distance heat transport. To support such optimistic views, an originally planned mathematical model is introduced for feasibility studies. Hydraulic considerations have first been done to understand what conditions are required for sustaining bubble-train flows in a capillary tube of interest. Theoretical analysis has then been made to solve the momentum and energy equations governing the flow velocity and temperature fields in the adiabatic section. The obtained analytical solutions are arranged to give algebraic expressions of the effective thermal diffusivity, the performance index combined with the tidal displacement, and the required electric power. Computed results of those three are displayed in the figures to demonstrate the realizability of that novel OHP.

Energy transport in cooling device by magnetic fluid

Temperature sensitive magnetic fluid has a great potential with high performance heat transport ability as well as long distance energy (heat) transporting. In the present study experimental set-up was newly designed and constructed in order to measure basic heat transport characteristics under various magnetic field conditions. Angular dependence for the device (heat transfer section) was also taken into consideration for a sake of practical applications. The energy transfer characteristic (heat transport capability) in the magnetically-driven heat transport (cooling) device using the binary TSMF was fully investigated with the set-up. The obtained results indicate that boiling of the organic mixture (before the magnetic fluid itself reaching boiling point) effectively enhances the heat transfer as well as boosting the flow to circulate in the closed loop by itself. A long-distance heat transport of 5m is experimentally confirmed, transferring the thermal energy of 35.8W, even when the device (circulation loop) is horizontally placed. The highlighted results reveal that the proposed cooling device is innovative in a sense of transporting substantial amount of thermal energy (heat) as well as a long distance heat transport. The development of the magnetically-driven heat transport device has a great potential to be replaced for the conventional heat pipe in application of thermal engineering.

Study on a loop heat pipe for a long-distance heat transport under anti-gravity condition

This paper reports the test results and evaluation of gravity effects on long-distance loop heat pipe (LLHP) with 10m distances for heat transport. First, the LLHP was designed based on the one-dimensional steady-state model, fabricated and tested. Test results showed the LLHP could transport heat up to 340W for 10m and the thermal resistance between the evaporator and the condenser was 0.12K/W under horizontal condition. Next, the LLHP was tested in top-heat mode. The maximum heat transports were 310, 270, 220W and the thermal resistances were 0.15, 0.17, 0.22K/W under 30, 60, 100cm anti-gravity condition respectively. The heat transfer efficiency of the LLHP was discussed in detail. The evaluation results showed 71.4% of the 340W heat load was dissipated at the condenser under the horizontal condition. On the other hand, 62.8, 61.8, 57.1% of the maximum heat load was dissipated at the condenser under 30, 60, 100cm anti-gravity condition respectively. The calculation model was in good agreement with the experimental results by considering the existence of a vapor pocket between the evaporator case and the vapor-liquid interface. This analysis indicated the vapor blanket generates easily and becomes thicker as anti-gravity effect increases.

Small Loop Heat Pipe with Plastic Wick for Electronics Cooling

In this paper, fabrication and test results of a small loop heat pipe (LHP) for electric cooling over a distance of 1100 mm are discussed. Poly(tetrafluoroethylene) porous material with 1.2 µm pore radius was used as a wick material for a primary pump of the loop to satisfy in both high thermal performance and low cost. High-grade ethanol was selected as a working fluid inside the loop. The small LHP with an outer diameter of 12 mm and length of 77 mm of an evaporator was made of stainless steel. The vapor and liquid transport lines are 1.8 mm in inner diameter and 1100 mm in length respectively in order to demonstrate the long-distance heat transport in a small tube, against the frictional resistance in the loop. The start-up process was monitored with an infrared camera, and quick start-up and its stable operation after the start-up were visually demonstrated. The loop could operate up to 50 W heat load with the operating temperature of around 70 °C in the evaporator. The thermal resistance between the evaporator and the condenser of the loop was 0.58 K/W at 50 W.

Possibility of long-distance heat transport in weightlessness using supercritical fluids

Heat transport over large distances is classically performed with gravity or capillarity driven heat pipes. We investigate here whether the "piston effect," a thermalization process that is very efficient in weightlessness in compressible fluids, could also be used to perform long-distance heat transfer. Experiments are performed in a modeling heat pipe (16.5 mm long, 3 mm inner diameter closed cylinder), with nearly adiabatic polymethylmethacrylate walls and two copper base plates. The cell is filled with H2 near its gas-liquid critical point (critical temperature: 33 K). Weightlessness is achieved by submitting the fluid to a magnetic force that compensates gravity. Initially the fluid is isothermal. Then heat is sent to one of the bases with an electrical resistance. The instantaneous amount of heat transported by the fluid is measured at the other end. The data are analyzed and compared with a two-dimensional numerical simulation that allows an extrapolation to be made to other fluids (e.g., CO2, with critical temperature of 300 K). The major result is concerned with the existence of a very fast response at early times that is only limited by the thermal properties of the cell materials. The yield in terms of ratio, injected or transported heat power, does not exceed 10-30% and is limited by the heat capacity of the pipe. These results are valid in a large temperature domain around the critical temperature.

Long-distance heat transport system using a hydrogen compressor

A heat transport system using hydrogen absorbing alloys offers several advantages, such as enabling the use of more compact long-distance transport pipes, maintaining a constant level of efficiency regardless of transport distance, and enabling rapid transport. On the other hand, the system supplies heat to a load through a heat pump with a 20 K difference. The system therefore cannot be used for heat loads that require a higher temperature difference. Then, in order to increase heat load applications, we have proposed a hybrid heat transport system that uses a hydrogen compressor. We also developed a system structure and achieved a heat pump with a 40 K difference by using a 0.2-kW experimental system. Furthermore, we focused on using the same structure on the heat source side for supplying various heat loads, and have proposed a basic concept for a wide-area heat utilization system that transports heat to various loads from centralized heat sources.