I have a 20ft deep cave in the basement. It was once used as a place to keep things cool. There is 54 degree air coming out, even in the winter. I would like to know what type of heat exchanger must I put into this cave in order to harness the heat coming from it. The cave is 10 ft long 20 ft deep and 4 ft wide surrounded by huge limestone on each side. I have searched for air to air heat exchangers but they seem to be used exclusively for ventilation purposes. Can anyone help?

Regards,