A thermally conductive cylinder for being installed in the earth, pool, lake, river or ocean comprising: --- U-type piping: made of two or more U-type tubular bodies for a thermal conductive fluid to pass through, wherein the U-type tubular bodies are radially arranged within a columnar thermal conductive body which is composed of solid state or colloidal object made of thermal conductive material; each of the U-type tubular bodies has a first fluid inlet and outlet terminal and a second fluid inlet and outlet terminal, the segments of piping near the first fluid inlet and outlet terminal are concentrated adjacent to the core of the columnar thermal conductive body, and the piping segments near the second fluid inlet and outlet terminal are radially arranged along the axial direction of the columnar thermal conductive body and near the edge of the columnar thermal conductive body; and --- Thermal insulating device: placed between the piping segments of the U-type tubular bodies near the first fluid inlet and outlet terminal of each of the U-type tubular bodies and the piping segments near the second fluid inlet and outlet terminal of each of the U-type tubular bodies, the piping segments near the first fluid inlet and outlet terminal being enveloped by the thermal insulating material; the thermal insulating device is installed around the U-type piping section which is concentrated towards the axial core of the thermally conductive cylinder; the thermal insulating device substantially reduces the thermal energy loss caused by thermal conduction between adjacent piping segments of the first fluid inlet and outlet terminal and the second fluid inlet and outlet terminal on the same side of the U-type piping. --- U-type piping: made of two or more U-type tubular bodies for a thermal conductive fluid to pass through, wherein the U-type tubular bodies are radially arranged within a columnar thermal conductive body which is composed of solid state or colloidal object made of thermal conductive material; each of the U-type tubular bodies has a first fluid inlet and outlet terminal and a second fluid inlet and outlet terminal, the segments of piping near the first fluid inlet and outlet terminal are concentrated adjacent to the core of the columnar thermal conductive body, and the piping segments near the second fluid inlet and outlet terminal are radially arranged along the axial direction of the columnar thermal conductive body and near the edge of the columnar thermal conductive body; and --- Thermal insulating device: placed between the piping segments of the U-type tubular bodies near the first fluid inlet and outlet terminal of each of the U-type tubular bodies and the piping segments near the second fluid inlet and outlet terminal of each of the U-type tubular bodies, the piping segments near the first fluid inlet and outlet terminal being enveloped by the thermal insulating material; the thermal insulating device is installed around the U-type piping section which is concentrated towards the axial core of the thermally conductive cylinder; the thermal insulating device substantially reduces the thermal energy loss caused by thermal conduction between adjacent piping segments of the first fluid inlet and outlet terminal and the second fluid inlet and outlet terminal on the same side of the U-type piping.