RU2814344C2 - Device for recovery of thermal energy from ground and utilization of heat of phase transitions - Google Patents

Abstract

FIELD: construction.

SUBSTANCE: invention can be used to utilize the heat of the soil in order to improve the energy and environmental efficiency of heat and cold supply to buildings and structures for various purposes. The method for increasing the efficiency of extracting thermal energy from the soil includes a sealed heat exchanger, organizing the circulation of a coolant through it and extracting from the soil and/or discharging low-potential thermal energy into the soil. The ground heat exchanger is located inside the thermal well. So, a special space (layer) is formed between the soil and the heat exchanger, in which the water vapor coming from the soil condenses and the water is subsequently frozen, which provides an additional influx of thermal energy due to the use of the latent heat of phase transitions: first, water vapor into water, then water into ice. By adjusting the thickness of the layer inside the thermal well, in the space between the soil and the soil heat exchanger, the optimal thickness of the ice layer is established, which ensures the maximum inflow of thermal energy from the soil to the heat carrier circulating inside the heat exchanger.

EFFECT: increased efficiency of extracting thermal energy from the soil by optimizing the parameters of the thermal well.

1 cl, 2 dwg

Description

The invention relates to the field of construction, namely to the design of energy-saving decentralized heat and cold supply systems for buildings that make it possible to effectively use non-traditional renewable energy sources, in this case, the heat of the soil mass.

There is a known method of using the heat-accumulation properties of soil (RF patent No. 2351850, F24D 11/02, F28D 1/00, 2007), which includes installing in the soil mass a system for collecting low-potential thermal energy of the soil of the surface layers of the Earth, consisting of thermal wells, discharging recycled material into the soil thermal energy of ventilation emissions of a building, its accumulation in the ground - daily and seasonal, and the selection of thermal energy or cold from the ground for the purposes of heat and cold supply. The disadvantage of this device is its low energy efficiency, especially in the summer. In addition, this method does not allow to fully utilize the heat of phase transitions of water contained in the soil.

The closest to the proposed method is the method of using the telloaccumulation properties of soil (RF patent No. 2416761, F24D 11/02, F28D 5/00 2009), including the installation of sealed heat exchangers in the soil, the organization of coolant circulation through them and extraction from the soil, or/ and discharge of low-grade thermal energy into the ground (prototype). This method of using the heat-accumulating properties of soil requires additional water consumption and electrical energy costs for pumping water into satellite pipelines. In addition, this method does not allow for maximum thermal energy removal per linear meter of a ground heat exchanger. The disadvantages of the proposed technical solution also include the lack of information about the source of water for moistening the soil.

The proposed invention solves the technical problem of increasing the energy efficiency of extracting thermal energy from the soil and maximizing the use of its heat-accumulating properties.

The stated technical problem is solved by changing the geometric parameters of the thermal well, which allows increasing the flow of thermal energy from the ground to the coolant circulating inside the ground heat exchanger.

In thermodynamics, the effect of the so-called critical diameter of insulation is known (see, for example, Theoretical Foundations of Heating and Refrigeration Engineering. Ch. P. Heat Transfer. Textbook. Edited by Prof. E.I. Guigo. L.: Leningrad Publishing House. University, 1976. - 224 p.). The critical insulation diameter is the insulation diameter at which the heat loss of the insulated pipeline is maximum. For isolated pipelines, this effect is a negative factor in the operation of pipelines.

However, during the operation of ground heat exchangers, an unexpected effect is discovered, which consists in the fact that ice freezing on the heat exchanger pipelines at a certain thickness ensures maximum heat flow from the soil mass surrounding the thermal well to the coolant inside the ground heat exchanger.

Ice is a poor heat insulator; however, despite this, when frozen around a ground heat exchanger, it has some resistance to heat transfer the numerical value of which can be calculated using the formula:

where λ _L is the thermal conductivity of ice, W/(m⋅K);

- outer diameter of a thermal well with a freezing layer of ice, m;

- thermal well diameter, m;

α _H - coefficient of heat transfer from the outer surface to the surrounding air, W/(m ² ⋅K).

Analysis of formula (1) shows that as the outer diameter of the ice around the ground heat exchanger increases the first term on the right side of formula (1) increases, the second decreases. Accordingly, at a certain thickness of the ice layer, a condition is formed under which the heat transfer resistance curves of the outer surface and the thermal resistance of the ice layer intersect. On the dependence of the total heat transfer resistance on ice thickness shown in Fig. 1, the curve near the point of intersection of these curves will have a pronounced minimum, corresponding to the so-called critical insulation diameter. On the dependence of the linear heat flux density on thickness, on the contrary, at a given critical insulation thickness a maximum will be observed, i.e. the flow of thermal energy through the surface of the ground heat exchanger with ice build-up at a given critical ice thickness will be greater than without ice frozen on the surface of the ground heat exchanger.

The value of the critical diameter for a specific diameter of the ground heat exchanger can be accurately calculated by taking the derivative of By set it equal to zero and calculate the value

Thus, to increase the heat flow from the ground to the coolant circulating inside the ground heat exchanger, it is necessary to provide a given thickness of the ice layer corresponding to the critical diameter for the selected type of insulation, which in the proposed technical solution is ice freezing around the ground heat exchanger. To do this, a ground heat exchanger is installed inside a thermal well of a given diameter. Thus, a special cavity is artificially created around the ground heat exchanger, the diameter of which corresponds to the critical diameter of the insulation, which allows for maximum heat flow density from the ground surrounding the thermal well to the coolant.

A method for increasing the energy efficiency of extracting thermal energy from the soil includes installing a thermal well, sealed heat exchangers in the soil, organizing the circulation of coolant through them and extracting it from the soil, or/and dumping low-grade thermal energy into the soil and, due to the temperature regime of the coolant, provides an increase in the flow of heat from the soil to coolant circulating inside the ground heat exchanger, and the involvement of latent heat of phase transitions of water in the area of action of the ground heat exchanger into the heat exchange process.

The proposed device allows us to solve the stated technical problem, because the temperature regime of the coolant ensures maximum heat flow density from the soil to the coolant and more efficient involvement in the heat exchange process of latent heat of phase transitions of moisture contained in the soil mass in the vicinity of the soil heat exchanger.

The essence of the proposed method is illustrated by the diagram shown in Fig. 2. Ground heat exchanger 1, with coolant 2 circulating inside it, is located inside a thermal well 3, the diameter of which corresponds to the critical diameter of ice insulation, which ensures maximum heat flow from the soil in its natural state 4 to coolant 2. Thermal well 3 is filled with water and moist air , moistened soil, or other material with known thermal properties (hereinafter referred to as the medium) 5, which makes it possible to regulate the properties of the medium inside the thermal well 3 and thereby ensure effective regulation of the heat flow density from the soil 4 to the coolant 2. The temperature regime of the coolant 2 ensures involvement in the heat exchange process latent heat of phase transitions of water vapor and water inside thermal well 3, located between soil 4 and soil heat exchanger 1.

The principle of operation of the proposed method is as follows: the temperature regime of the coolant provides for at least a one-time decrease below 0°C in the temperature of the coolant at the entrance to the ground heat exchanger 1, condensation of water vapor in the medium 5, filling the free space inside the thermal well 3, and a subsequent phase transition water into ice when extracting heat from the soil and at least a single reverse transition of the coolant temperature through 0°C with the involvement in the heat exchange process of the latent heat of freezing and thawing of pore moisture contained in the medium zone 5 located inside the thermal well 3, and the natural soil 4 located around thermal well 3. The soil in its original natural state has very heterogeneous characteristics, which can change unpredictably and uncontrollably over time (for example, as a result of a decrease in groundwater levels). The creation of an intermediary between the soil 4 and the ground heat exchanger 1 (medium 5 with specified properties) makes it possible to increase the accuracy of the settings of the heat flow parameters from the soil 4 to the coolant 1 (when extracting thermal energy from the soil in the cold season) and from the coolant 1 to the soil 4 (when discharge of thermal energy into the ground during the warm period of the year) and make better regulation of the parameters of the coolant by changing the properties of the medium 5 inside thermal well 3. Selecting the diameter of the thermal well that corresponds to the critical diameter of the insulation for the layer of ice freezing on the heat exchanger allows increasing the heat flow from the ground to the coolant. Supercooled water or another coolant with a negative temperature can be used as coolant 2 of the ground heat exchanger 1, for example in thermal wells. The environment 5 inside the thermal well 3, if necessary, can be additionally moistened.

The proposed technical solution makes it possible to increase the energy efficiency of the prototype, since the freezing of ice inside a thermal well, the diameter of which corresponds to the critical diameter of the insulation (in this case, ice), ensures maximum heat flow from the ground to the coolant and does not require energy to supply water to the environment located inside a thermal well, has higher reliability, allows you to protect the soil from coolant leaks into the ground in the event of a violation of the tightness of the shell of the soil heat exchanger, and to carry out more effective qualitative regulation of the properties of the medium inside the soil heat exchanger.

Claims (1)

A method for increasing the efficiency of extracting thermal energy from the ground, including installing a thermal well, sealed heat exchangers in the ground, organizing coolant circulation through them and extracting low-potential thermal energy from the soil and/or discharging low-grade thermal energy into the ground, characterized in that before installation in the ground the diameter of the thermal well is calculated in this way , so that its value, for the given characteristics of the soil and the soil heat exchanger, corresponds to the critical diameter of the layer freezing inside the thermal well, which ensures an increase in the flow of thermal energy through the surface of the soil heat exchanger.