TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Components: The resonance cavity (7) has an annular shape and is arranged downhole around production tubing (2) in a gas production well. The gas transportation conduit is a gas transport pipeline at a remote location, e.g. underwater and/or underground or in an area without electric power supply facilities. The permeable body (9) comprises a series of stacked plates which are spaced at predetermined spacing from each other. The stacked plates are formed in the annular resonance cavity by coiling a strip around the inner wall of annular resonance tube, and arranging a series of spacers between the adjacent layers of coiled strip. The thermoelectric device comprises a thermocouple connected between a hot spot and cold spot of permeable body, or between hot spot or cold spot of permeable body and a component of which the temperature is unaffected by the standing acoustic wave. The thermocouple forms part of a Peltier element. An array of acoustic signal transducers is arranged in or adjacent to the gas transportation conduit. The transducers detect characteristics of an acoustic wave emitted from the inlet of the acoustic resonance cavity. The transducers comprise microphones (13) converting the acoustic signal into an electric signal, fiber optical signal, or other signal which is transmitted to a flow monitoring assembly. The flow monitoring assembly converts phase differences and/or other characteristics of acoustic signals into an indication of the gas flow velocity in the gas transportation conduit. The microphones are powered by the electricity generated by the thermoelectric cavity and transform the acoustic signal into a pulsed digital acoustical, optical, electrical, or other signal. The signal transducers may include at least one rechargeable battery charged by the thermoelectric device. The flow monitoring system is connected to a flow control assembly which adjusts the gas flow rate in the transportation conduit in response to deviation of the monitored gas velocity from a reference value.