Annual thermal performance analysis and optimization of earth-air heat exchanger by using a new 1D-2D hybrid numerical model

Abstract

Thermal performance potential analysis is necessary when using the earth-air heat exchanger (EAHE) as a passive way to reduce ventilation energy consumption or achieve thermal comfort in buildings. This work aimed to develop a new simulating model, investigate parametric influence, and build an optimizing criterion. This paper built a 1D-2D hybrid transient numerical model, integrating a number of 2D explicit diffusion models in the earth domain and two 1D implicit convection-diffusion models in the air region. A novel splitting discrete scheme was built on the earth-air coupled interface to separate computations in the two domains while considering their interactions. EAHE thermal performance was analyzed in terms of the outlet temperature/humidity and annual cooling/heating capacity. This paper originally suggested the outlet temperature or cooling/heating capacity limit in an entire year as the criterion for optimizing EAHE's length and depth in any climate.36 one-year-duration simulations of single-pipe EAHE, 27 one-year-duration simulations and a ten-year-duration simulation of multi-pipe EAHE were conducted to analyze influences of the velocity, diameter, length, depth, and pipe-to-pipe space. Results show that this new model could be an effective tool and the novel optimizing criterion is feasible. Both the single-pipe and multi-pipe EAHEs have the same limit of outlet temperature/humidity and it depends on the pipe depth only given a specific climate. The optimal/effective depth is 6 m. The optimal/effective pipe length is independent of pipe depth and increases with air velocity and diameter. Multi-pipe EAHE can be treated as single-pipe EAHE given the pipe-to-pipe space is greater than 3 m.