Capillary mat heat exchangers are increasingly used in transportation energy tunnels owing to their large heat-transfer area and uniform temperature. Thermal energy tunnels, a new type of energy tunnel, differ from transportation energy tunnels because of the heat source inside such tunnels. To examine the feasibility of applying capillary mat heat exchangers in thermal energy tunnels under endothermic conditions, heat transfer performance was experimentally investigated using a 1:1 intermittent operating mode. The results showed that the higher the initial air temperature (T₀) in the tunnel, the greater the heat flux. With the inlet temperature of circulating water (t_in) fixed at 5 ℃, as the temperature difference between T₀ and t_in increases by 10 ℃, the heat flux increases by 45.9%. The heat flux also increases with the increase of circulating water velocity (u); whereas u increases up to 0.1 m/s, the heat transfer rate saturates and approaches 187.22 W/m². The lower the t_in is, the greater the heat flux. When the T₀ is 50 ℃ and the u is 0.075 m/s, for every 1 ℃ increase in the t_in, the heat transfer rate decreases by 2.04%.