Distribution panel in a modern Italian hotel room

How much energy does it cost to save energy? In this 4-star hotel each room is equipped with a distribution panel including an RCD (very good for electrical and fire safety) and a transformer 2*12V, rated no less than 24VA. Obviously the transformer supplies the door chime, some electronic control gear e. g. for the air conditioner, the electro-magnetic lock and the auto-shut-off: When the key card is removed the room lighting is switched off automatically in 2 groups with different time lags. This requires relays which also need to be powered, although 0.5W for each should suffice. The air condition control is also satisfied with 12V*40mA?0.5W. Still, the transformer is rather warm. This means that it is either highly loaded or over-dimensioned and made with a very poor quality magnetic steel so that it has excessive no-load losses. It is to be assumed that the high transformer power rating is only needed for fractions of a second to release the door lock when inserting the key card. For the rest of the day the bigger transformer causes higher losses; cheap design making losses 3 times as high as necessary, causing electricity costs as high as the price savings once every year.

The answer to this would have been a central 12V or 24V DC supply via remote power feeding, i. e. power through the data line. The short peak demand of the door magnet should then have been boosted by an electrolytic capacitor in order to avoid failure of the opening mechanism due to voltage drop along the long (relatively thin) data line. Not only would the energy efficiency of one (or a few) big feeding station(s) have been a great deal better but also the installation costs would have been lower than for the individual transformers, even though a filter might have become necessary to avoid signal absorption by the electrolytic capacitor. A small reactor connected in series with the electrolytic capacitor, with a reactance great enough to block the HF data signal but small enough not to hamper the power for the magnet, would have done the job and could have been accommodated in each room where now the transformer is or even in smaller piece numbers at somewhat greater intervals.

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