"recharged" to drive off the humidity, typically by heating it. The desiccant can be mounted on a belt or other means of transporting it during a cycle of operation. Dehumidifiers which work according to the absorption principle are especially suited for high humidity levels at low temperatures. They are often used in various sectors in industry so that humidity levels below 35% can be achieved. Because of the lack of compressor parts, desiccant dehumidifiers are often lighter and quieter than compressor dehumidifiers. Desiccant dehumidifiers can also operate at lower temperatures than compressor dehumidifiers as the unit does not rely on cooled coils for which the moisture condensing efficiency decreases at lower temperatures. The reason for the limited acceptance of desiccant dehumidification can be attributed to initial installation costs, the fact that operational benefits are not fully understood, lack of technology awareness, and company priorities, which are not focused on benefits of new technology. Membrane dehumidification Several approaches can remove water vapor by flowing air past a membrane that allows vapor to enter. Dehumidification with membranes can allow for water vapor removal without condensation; this avoids the energy required with the enthalpy of vaporization, offering high efficiency for well-designed systems. Such dehumidification can be done passively with a reject air stream; see Energy recovery ventilation. Active systems can use pressure gradients or electrocatalytic approaches. Selective membrane dehumidification Selective membranes use materials that block other ambient gases besides water vapor. Water vapor will then diffuse through these membranes under a concentration difference. Such a difference in concentration (partial pressure) can be caused by vacuum pumping, or simply passing by an airstream with a lower concentration of water. The most efficient configurations save energy by using two membranes that isolate a vacuum pump from ambient air. This dramatically reduces the pressure across the vacuum pump, saving energy. While such systems are often termed "Isothermal Membrane Dehumidification," recent research has show that these systems can be made more efficient by combining them with heat exchange. Such integration can improve performance by improving the vapor compression cycle's COP (by operating between closer temperatures), and enhancing air mixing near the membrane. Selective membranes can be made by immobilizing a liquid that that can absorb water (or another solute) within a membranes, dubbed "supported liquid membranes". Typically, there are two layer types; a highly porous membrane that contains the absorbing liquid, and a trapping layer that prevents