Analysis of Air Heating in Winter in Underground Heat Exchangers and in Water Bodies During Water Freezing on Submerged Pipes

Abstract

Background. Air heating in winter in the ambient temperatures range below -5 °С is possible both by the heat of the soil and by the heat of the phase transition of water to ice. Heated air reduces energy consumption in ventilation systems and heat pumps such as air–water and air–air during peak loads on the heating system, which reduces the installed capacity of the heat-generating equipment. Objective. The aim of the paper is determination of the influence of air velocity (flow rate) on the desired length of pipes at a constant value of the diameter of the channel with air, as well as determination of thermal resistances and linear heat flux density with a comparison of the processes of soil cooling and water crystallization. Methods. Air with a temperature below -5 °С is passed through elements of the natural environment for use in ventilation systems, heat pumps, and in the buffer zones of buildings during the frost period. The lower the air temperature, the greater the economic and energy efficiency. Results. The calculated analysis of the effect of air velocity (flow rate) on the required length of pipes embedded in the ground and submerged in water subject to a phase transition of water to ice was carried out. It is shown that the required length of the ground heat exchanger is strongly influenced by the mode of operation (continuous operation without interruptions or operation with interruptions). To heat the air from -10 to -3 °С, depending on the air speed in the ground heat exchangers, the needed length of the pipes should be 1.5–2 times longer because the thermal resistance of the soil is greater than that of ice and the ice thickness is lower than that of the cooled soil, due to the high heat value of water crystallization. At high speeds, the linear density of the heat flux reaches 40–60 W/m2. Conclusions. When placing a channel in the form of a pipe in the ground or in water, it is possible to pre-heat the frosty air during peak loads on heating systems, which makes it possible to reduce the installed power of the heat-generating equipment. The technology of water freezing during air heating has significant advantages, especially when operating in annual mode.

Keywords

• Heat flow
• Heat exchanger
• Ventilation systems
• Heat supply systems
• Thermal resistance
• Linear density
• Air heating
• Heat of crystallization
• Heat generating equipment