A split liquid desiccant air conditioning system for treating an air stream flowing into a space in a building, said split liquid desiccant air-conditioning system being switchable between operating in a warm weather operation mode and a cold weather operation mode, the split liquid desiccant air conditioning system comprising: a conditioner located inside the building, said conditioner including a plurality of structures arranged in a substantially vertical orientation, each structure having at least one surface across which a liquid desiccant can flow, each structure also including a passage through which a heat transfer fluid can flow, wherein the air stream to be treated flows between the structures such that the liquid desiccant dehumidifies and cools the air stream in the warm weather operation mode and humidifies and heats the air stream in the cold weather operation mode, the conditioner further comprising a sheet of material positioned proximate to the at least one surface of each structure between the liquid desiccant and the air stream, said sheet of material permitting transfer of water vapor between the liquid desiccant and the air stream; a regenerator located outside the building connected to the conditioner by liquid desiccant pipes for exchanging liquid desiccant with the conditioner, said regenerator including a plurality of structures arranged in a substantially vertical orientation, each structure having at least one surface across which the liquid desiccant can flow, each structure also including a passage through which a heat transfer fluid can flow, said regenerator causing the liquid desiccant to desorb water in the warm weather operation mode and to absorb water in the cold weather operation mode to or from an air stream flowing through the regenerator; a reversible heat pump located outside the building coupled to the conditioner and to the regenerator by heat transfer fluid pipes, wherein the heat pump pumps heat from the heat transfer fluid flowing in the conditioner to the heat transfer fluid flowing in the regenerator in the warm weather operation mode, and wherein the heat pump pumps heat from the heat transfer fluid flowing in the regenerator to the heat transfer fluid flowing in the conditioner in the cold weather operation mode; an apparatus for moving the air stream through the conditioner; an apparatus for circulating the liquid desiccant through the conditioner and regenerator; and an apparatus for circulating heat transfer fluid through the conditioner and the reversible heat pump; and an apparatus for circulating heat transfer fluid through the regenerator and the reversible heat pump. a conditioner located inside the building, said conditioner including a plurality of structures arranged in a substantially vertical orientation, each structure having at least one surface across which a liquid desiccant can flow, each structure also including a passage through which a heat transfer fluid can flow, wherein the air stream to be treated flows between the structures such that the liquid desiccant dehumidifies and cools the air stream in the warm weather operation mode and humidifies and heats the air stream in the cold weather operation mode, the conditioner further comprising a sheet of material positioned proximate to the at least one surface of each structure between the liquid desiccant and the air stream, said sheet of material permitting transfer of water vapor between the liquid desiccant and the air stream; a regenerator located outside the building connected to the conditioner by liquid desiccant pipes for exchanging liquid desiccant with the conditioner, said regenerator including a plurality of structures arranged in a substantially vertical orientation, each structure having at least one surface across which the liquid desiccant can flow, each structure also including a passage through which a heat transfer fluid can flow, said regenerator causing the liquid desiccant to desorb water in the warm weather operation mode and to absorb water in the cold weather operation mode to or from an air stream flowing through the regenerator; a reversible heat pump located outside the building coupled to the conditioner and to the regenerator by heat transfer fluid pipes, wherein the heat pump pumps heat from the heat transfer fluid flowing in the conditioner to the heat transfer fluid flowing in the regenerator in the warm weather operation mode, and wherein the heat pump pumps heat from the heat transfer fluid flowing in the regenerator to the heat transfer fluid flowing in the conditioner in the cold weather operation mode; an apparatus for moving the air stream through the conditioner; an apparatus for circulating the liquid desiccant through the conditioner and regenerator; and an apparatus for circulating heat transfer fluid through the conditioner and the reversible heat pump; and an apparatus for circulating heat transfer fluid through the regenerator and the reversible heat pump.