Abstract: A heat exchanger installed under the ground or water includes an outer pipe installed vertically or installed inclinedly under the ground or water; and a heating heat-transfer pipe inserted into the outer pipe, and evaporating refrigerant injected thereto. A heat medium is sealed in the outer pipe. A heat exchange is performed by using a change in phase of the heat medium and a heat medium guiding section for guiding the heat medium condensed on an outer surface of the heating heat-transfer pipe to an inner wall surface of the outer pipe by using surface tension of the heat medium or by reducing an angle of contact on the inner surface is formed in an upper section of the outer pipe.

Abstract: A heat exchanger for exchanging heat under the ground or water includes an outer pipe (51) to be installed under the ground or water; and a cooling heat-transfer pipe (52) which is inserted into the outer pipe (51), and which dissipates heat from refrigerant injected thereto. A heat medium is sealed in the outer pipe (51). The outer pipe (51) and the cooling heat-transfer pipe (52) are arranged so that the liquid heat medium is held between an inner wall surface of the outer pipe (51) and an outer wall surface of the cooling heat-transfer pipe (52).

Abstract: A formaldehyde detector (10) includes a detection reagent that produces a color by reaction with formaldehyde. The detector (10) further includes a detection element (30) including: a base material (31) constituting an optical waveguide through which light propagates while repeating total reflection; and a mesoporous layer (32) that is formed on at least part of a light-reflecting surface constituting a surface of the base material (31) and carries the detection reagent. The detector (10) further includes: a light emitter for emitting light to the base material (31); and a light receiver for receiving light emitted from the base material (31). In addition, the detector (10) includes a signal processing part.