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Heat transfer to and from a reversible thermosiphon placed in porous media
Abstract and Figures
The primary focus of this work is an assessment of heat transfer to and
from a reversible thermosiphon imbedded in porous media. The interest in
this study is the improvement of underground thermal energy storage
(UTES) system performance with an innovative ground coupling using an
array of reversible (pump-assisted) thermosiphons for air conditioning
or space cooling applications. The dominant mechanisms, including the
potential for heat transfer enhancement due to natural convection, of
seasonal storage of "cold" in water-saturated porous media is evaluated
experimentally and numerically. Winter and summer modes of operation are
studied. A set of 6 experiments are reported that describe the heat
transfer in both fine and coarse sand in a 0.32 cubic meter circular
tank, saturated with water, under freezing (due to heat extraction) and
thawing (due to heat injection) conditions, driven by the heat transfer
to or from the vertical thermosiphon in the center of the tank. It was
found that moderate to strong natural convection was induced at Rayleigh
numbers of 30 or higher. Also, near water freezing temperatures
(0°C-10°C), due to higher viscosity of water at lower
temperatures, almost no natural convection was observed. A commercial
heat transfer code, ANSYS FLUENT, was used to simulate both the heating
and cooling conditions, including liquid/solid phase change. The
numerical simulations of heat extraction from different permeability and
temperature water-saturated porous media showed that enhancement to heat
transfer by convection becomes significant only under conditions where
the Rayleigh number is in the range of 100 or above. Those conditions
would be found only for heat storage applications with higher
temperatures of water (thus, its lower viscosity) and large temperature
gradients at the beginning of heat injection (or removal) into (from)
soil. For "cold" storage applications, the contribution of natural
convection to heat transfer in water-saturated soils would be
negligible. Thus, the dominant heat transfer mechanism for air
conditioning applications of UTES can be assumed to be conduction. An
evaluation of the potential for heat transfer enhancement in
air-saturated media is also reported. It was found that natural
convection in soils with high permeability and air saturations near 1
becomes more important as temperatures drop significantly below
freezing.
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... Applied hybrid systems (GCT-HP) solutions for house-heating purposes are adopted in Germany and Austria [27,37,65,66]. A smart heating and cooling system based on the thermosiphon principle was used by [34][35][36]67,68]. GCT technology was also successfully implemented in mega-projects, such as the Alaska pipeline, where over 120,000 thermosiphons were installed for an oil pipeline crossing Alaska. ...
Compared to conventional ground heat exchangers that require a separate pump or other mechanical devices to circulate the heat transfer fluid, ground coupled thermosiphons or naturally circulating ground heat exchangers do not require additional equipment for fluid circulation in the loop. This might lead to a better overall efficiency and much simpler operation. This paper provides a review of the current published literature on the different types of existing ground coupled thermosiphons for use in applications requiring moderate and low temperatures. Effort has been focused on their classification according to type, configurations, major designs, and chronological year of apparition. Important technological findings and characteristics are provided in summary tables. Advances are identified in terms of the latest device developments and innovative concepts of thermosiphon technology used for the heat transfer to and from the soil. Applications are presented in a novel, well-defined classification in which major ground coupled thermosiphon applications are categorized in terms of medium and low temperature technologies. Finally, performance evaluation is meticulously discussed in terms of modeling, simulations, parametric, and experimental studies.
... The thermal conductivity of the salt was measured in a transient process of heating the central heater rod for a short period of time and letting heat to diffuse inside the salt. Measuring the temperature response of the central heater rod after it was turned off (Fig. 3) allowed calculation of the thermal conductivity of the salt as described in Ref. [20] and shown in Fig. 3. Permeability of the salt was measured by passing argon gas through the salt and measuring the pressure difference created at both ends of the reactor as shown by Han et al. [18]. ...
Intermittency of sustainable energy or waste heat availability calls for energy storage systems such as thermal batteries. Thermochemical batteries based on a reversible solid-gas (MgCl2-NH3) reactions and NH3 liquid-gas phase change are of specific interest since the kinetics of absorption are fast and the heat transfer rates for liquid-vapor phase change are high. Thus, a thermochemical battery based on reversible reaction between magnesium chloride and ammonia was studied. Two-dimensional experimental studies were conducted on a reactor in which temperature profiles within the solid matrix and pressure and flow rates of gas were obtained during discharging processes. A numerical model based on heat and mass transfer within the salt and salt-gas reactions was developed to simulate the NH3 absorption processes within the solid matrix, and the results were compared with experimental data to determine dominant heat and mass transfer processes within the salt. It is shown that for high permeability salt beds, the reactor uniformly adsorbs gaseous ammonia until the bed reaches the equilibrium temperature, then adsorbs gas near the cooled boundaries as the reaction front moves inward. In that mode, the heat transfer is the dominant factor in determining reaction rates.
... The thermal conductivity of the salt was measured in a transient process of heating the central heater rod for a short period of time and letting heat to diffuse inside the salt. Measuring the temperature response of the central heater rod after it was turned off (Fig. 3) allowed calculation of the thermal conductivity of the salt as described in Ref. [20] and shown in Fig. 3. Permeability of the salt was measured by passing argon gas through the salt and measuring the pressure difference created at both ends of the reactor as shown by Han et al. [18]. ...
Intermittency of sustainable energy or waste heat availability calls for energy storage systems such as thermal batteries. Thermo-chemical batteries are particularly appealing for energy storage applications due to their high energy densities and ability to store thermal energy as chemical energy for long periods of time without any energy loss. Thermo-chemical batteries based on a reversible solid-gas (MgCl2 - NH3) reactions and NH3 liquid-gas phase change are of specific interest since the kinetics of absorption are fast and the heat transfer rates for liquid — vapor phase change are high. Thus, a thermo-chemical battery based on reversible reaction between magnesium chloride and ammonia was studied.
Experimental studies were conducted on a reactor in which temperature profiles within the solid matrix and pressure and flow rates of gas were obtained during charging processes. A numerical model based on heat and mass transfer within the salt and salt-gas reactions was developed to simulate the absorption processes within the solid matrix and the results were compared with experimental data. The studies were used to determine dominant heat and mass transfer processes within the salt.
It is shown that for high permeability materials, heat transfer is the dominant factor in determining reaction rates. However increasing thermal conductivity might decrease permeability and reduce reaction rates. The effect of constraining mass flow rate on the temperature and reaction propagation is also studied. These results show that optimized heat and mass transfer within the solid-gas reactor will lead to improved performance for heating and air conditioning applications.
Copyright © 2016 by ASME Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature
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A short overview is given of the Dutch and the IEA Solar energy R and D programmes, serving as a sketch of the framework for a number of contributions to this conference.
In climates with hot summers and cold winters, it is thermo-dynamically possible to provide all heating and air-conditioning needs without significant fuel or electrical energy input if adequate thermal energy storage capacity exists. Because all buildings sit on massive volumes of soil, water and rock, capacity is not the problem. The obstacle in meeting that goal is the lack of technology to readily transfer heat to and from soil in a cost effective manner. Enhancement of heat transfer rates coupled with an order of magnitude drop in installation cost over current practices would likely lead to the widespread use of seasonal underground thermal energy storage (UTES). Smart thermosiphon technology may be a path to those technical and economic goals. This technology uses conventional passive thermosiphon technology to transfer energy out of soil, and controlled rate transfer of energy into the soil. In this paper, we describe how smart thermosiphon technology can facilitate ample seasonal energy storage to meet air conditioning and winter heating needs. Simulations of soil freezing within an array of thermosiphons and the use of those frozen soils to provide air conditioning demands will be presented. A comparison of heat transfer from looped tubes inserted in vertical wellbores with the calculated heat transfer from thermosiphons shows that the same total heat transfer rate can be realized using 40% of the borehole length if smart thermosiphon technology is used. Results of the testing of a lab-scale experimental smart thermosiphon demonstrate that uniform temperatures and heat fluxes can be maintained on the inside wall of the thermosiphon pipe, thus proving the potential for dramatic enhancements of heat transfer rates. Winter heating and summer air conditioning modes have been demonstrated. The first pilot-scale installation of smart thermosiphons for seasonal UTES has demonstrated the ability to install the devices using inexpensive direct push techniques.
Closed-loop ground-coupled heat pump systems offer several advantages over conventional HVAC systems. For instance, they collect renewable ground heat or recuperate building heat rejection that accumulated in the ground during the cooling season. Furthermore, because of their relatively high coefficient of performance (COP) in both heating and cooling they are, as noted by the U.S. Department of Energy and the U.S. Environmental Protection Agency, among the most energy-efficient and environment-friendly heating and cooling systems. Finally, they emit less greenhouse gas than conventional HVAC systems given the power generation mix found in most jurisdictions.