How does vapor absorption chiller work?

Answer 1

A chemical heat pump is based on the principles that water molecules bind more efficiently to certain hygroscopic salts or Silica Gel than to other water molecules. As a consequence, when using two separate bowls - one containing water (evaporator), and the other containing hygroscopic salt (reactor) - in a confined space, water will evaporate to the salt that absorbs the water. When the confined space is in a state of vacuum the water transport will be so high that the water will start boiling in order to produce vapour at the same speed as it is absorbed by the salt. Such evaporation requires energy. If the energy is not supplied from outside the system it will be taken from the water itself, which as a consequence gets colder. In essence the evaporation process transports thermal energy (heat) from the water to the salt. The temperature difference increases until a maximum difference (ΔT), at which the salt is no longer able to absorb more water. If the cold evaporator is connected to the indoor heat exchange system we will get air conditioning (refrigeration). Chemical Heat Pumps have historically either been based on a solid or a fluid reactor substance, where only absorption machines based on fluid reactor principles have made any commercial success. The fluid salt solution reactor is efficient in its ability to absorb energy. It has a high charge and discharge power. However, since the fluid solution is efficient in absorbing heat the reactor temperature continuously increases. As a consequence the loading capacity of the system decreases to the point where the ΔT no longer is sufficient to run the process at all. Moreover, the good heat absorption characteristics of the fluid also mean it will not store energy efficiently. As a consequence, fluid reactors are designed for converting thermal energy to cooling in a continuous process, and cannot for practical purposes be powered by a source of energy that fluctuates over time, such as solar energy. The positive aspects of using a solid reactor substance are that it enables a constant loading ΔT between the evaporator and the reactor. In other words, the solid reactor substance does not increase much in temperature as it is charged. Energy is efficiently stored, (in terms of energy per volume unit) in the solid salt but has a very poor charge and discharge power. Because of its poor heat conductivity characteristics there are very few practical applications for absorption machines based of a solid substance. The implications of the 3-phase system are that a constant temperature difference between evaporator and reactor is achieved at that same time as excellent storage capabilities and high charge and discharge power are maintained. All the positive aspects of the solid substance and the liquid substance converge into one technology. In essence the initially fluid salt solution reactor substance is crystallized into a solid state as thermal energy is added to the reactor. Energy is stored efficiently and with practically no losses in the dried salt. The practical implications of this energy storage solution are that a fluctuating source of energy for the first time can be used efficiently to deliver cooling and heating 24/7.