Desiccant dehumidifiers use a desiccant material, typically Zeolite, which absorbs water vapour in much the same way as silica gel (a packet of which you would often find in a pair of new shoes).
A fan draws air into the dehumidifier and passes it through a section of a slowly rotating wheel which holds the desiccant material. The material draws the moisture out and dries the air which then exits the unit. The moisture is then extracted from the desiccant; this is done by heating a different portion of the wheel not being used to dry the air. The water then evaporates from the desiccant, is passed over a cold plate where it condenses, runs off and is collected in a resevoir or a permanent feed.
Desiccant dehumidifiers work very well at lower temperatures - much better than compressor dehumidifiers - which makes them ideal for use in boats; caravans; motorhomes; garages; cellars; basements; holiday homes and any other unheated or low temperature environments.
Due to the fact that desiccant dehumidifiers are unaffected by temperature they are much more effective at extracting mositure in any real world scenario than a compressor driven unit.
It is important to note that desiccant units tend to be rated in test conditions at a temperature of 20 degrees centigrade and a relative humidity of 60% which is in contrast to compressor driven units that are all rated in test conditions at a temperature of 30 degrees centigrade and a relative humidity of 80%.
It is important to note than in more realistic working environments the actual sensible performance of a compressor driven dehumidifier will be much lower than the nominal rated performance under test conditions as, unlike a desiccant dehumidifier, their performance is affected dramatically by changes in temperature until ultimately in temperatures below around 12 degrees centigrade a basic compressor driven unit will essentially stop extracting moisture.
For example, in a more real world scenario of 27 degrees centigrade temperature and 60% RH (relative humidiity), as opposed to the rated test condition of 30 degrees and 80% RH, a compressor driven unit with a rated capacity of 10 litres per day will in reality tend to only draw 5.6 litres; and a unit rated at 16 litres will draw 8 litres.
As the temperature drops further then the performance of a compressor driven unit will tail off significantly: even at 21 degrees centigrade temperature and 60% RH (which is a more realistic domestic scenario) the unit rated at 10 litres per day will in reality only draw around 3 litres and the unit rated at 16 litres will only draw around 5.5 litres.
This is in stark contrast to the desiccant dehumidifier that will continue to extract its maximum rated capacity irrespective of the temperature.
Ultimately in cooler temperatures below around 12 degrees centigrade basic compressor dehumidifiers will stop working as the evaporator plates are likely ice up and they will stop extracting moisture. In contrast desiccant units tend to be rated in a test condition of 20 degrees centigrade and 60% RH and are unaffected by temperature until it reaches zero degrees centigrade.
Therefore, although the power consumption per hour of a desiccant unit may be slightly higher than a compressor driven unit, in reality it will be much cheaper to run as it will be running for a much shorter period of time to achieve the same net result as the compressor unit as it will extract the mositure much more quickly.
The other major advantage of a desiccant unit is that it will lower the RH to 40% if required which is below the rusting point of metal. This makes them ideal for protecting cars, bikes, guns, machinery and other equipment made of metal from rust.
Desiccant units will always be smaller, lighter and more compact than an 'equivalent' compressor driven unit which makes them much easier for storing.