| The heating capacity of the air source heat pump decrease sharply as the outdoor temperature falls in winter; the cooling capacity also decrease as the outdoor temperature raise in summer. And the traditional tunnel ventilation system supplied air of poor smell influencing indoor air quality. In order to overcome the problem of air source heat pump and take full advantage of the heat storage of the soil, this paper proposes a new air source heat pump system in which the tunnel is used to temper the ambient air, which is then used for heat exchange with a heat pump.Tunnel air temperature, at a depth of about 10 m or more, stays fairly constant throughout the year, and is approximately equal to the average annual ambient air temperature. In this heat pump system, coefficient of performance of the heat pump is always high because the temperature of underground tunnel air in summer is lower than that of atmosphere air, whereas that of underground tunnel air in winter is higher than that of atmosphere air. This system not only has obvious energy saving effect, but also makes full re-use of abundant underground tunnel resources in our country. From protecting the atmospheric environment and renewable energy utilization point of view, the study of air source heat pump based on the underground tunnel air is of great significance to low the energy consumption of building and protect the environment.Firstly, energy source of soil was discussed. The change law of soil temperature and the effect which has an impact on soil temperature caused by thermo-physical parameters are analyzed through the subject. The mathematic model of soil temperature is established. Based on the mathematic model, we can get the formula of soil temperature when the ground temperature varies harmonically. According to this formula, the characteristics of soil temperature are discussed. The attenuation and delay action of soil temperature offer the air source heat pump system which uses the air of underground air tunnel as the heat source precondition for its practical application.Secondly, the heat exchanging process that outdoor air flow through underground tunnel analyzed. On the basis of the characteristics of heat transmitting process in continuous ventilating tunnel, the unsteady coupled heat transfer between air and the tunnel walls are discussed. A mathematical model of unstable coupled heat transmitting process under changeable conditions is established using whole-field method. Subsequently, numerical simulations about the model of unsteady-state conjugated heat transfer with three-dimensional between airflow and tunnel is carried out with CFD software. By comparing measured tunnel data in Jinan to the numerical results, we can find that the numerical results of the mathematical model are in agreement with the measured ones, and its computational accuracy can satisfy the demand of practical engineer.According to the calculation results under different working conditions, we find that length of the tunnel, the buried depth of tunnel and velocity of air in the tunnel, inlet air temperature, equivalent diameter of the tunnel, and material of the tunnel all have big effects on the heat transfer process in tunnel. In this paper, concept "tunnel heating efficiency" is introduced. The amount of heat-exchanged between the air and the tunnel is influenced by the parameters like:length of the tunnel, velocity of air in the tunnel, the buried depth of tunnel and equivalent diameter of the tunnel. How these factors influence the tunnel heating efficiency are also analyzed. Based all the reaserches, the optimal scope of design about various parameters are illustrated. These advices could provide academic guidance to the system engineer designer.At last, experimental research of this system performance in winter is also carried out in laboratory located in Weifang. The calculated results show that air source heat pump based on the underground tunnel air, as compared to the heat pump without using the tunnel air as heat source, can not only raise heat capacity but also increase the heat pump coefficient of performance especially in the lowest temperature. In addition, air source heat pump based on the underground tunnel air can run stablely and reliablely without frosting in winter by theoretical analysis. The study of air source heat pump based on the underground tunnel air provides instructive and meaningful guidance to the optimization and efficient operation of system.This subject is a sub subject of scientific research project in the Ministry of Construction of China, which is "The air source heat pump based on the underground tunnel air" (Number05-K3-13).
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