Adsorption chiller
An adsorption is a sorption - cooling machine , which operates with a solid sorbent. It is mainly used as a cold store.
Adsorption and desorption
The accumulation on a solid is referred to as adsorption in process engineering and the desorption accordingly as dissolution from a solid. In an adsorption, the refrigerant is selected such that with the adsorption or desorption of a change of state is accompanied.
Since the adsorption of the refrigerant involves condensation, it is favored by low temperature and high pressure, reduces the volume of the refrigerant and releases energy in the form of heat. Desorption contains the opposite of condensation (evaporation) and thus has the opposite effect. If the pressure remains constant, this means that the temperature always corresponds to the boiling point of the refrigerant, provided that refrigerant is still present in both states of aggregation. This is the basic requirement for a refrigeration machine and a heat storage system.
Since the coolant should adhere to the sorbent, substances that are very fine-pored and consequently have a very large internal surface are particularly suitable. The following substances meet this requirement:
construction
The adsorption chiller consists of an adsorber or desorber and an evaporator or condenser, which are housed in a common container or connected to one another by a pipe. The advantage of sorption is that the process takes place solely through the temperature control of the sorbent. Therefore, the container can be hermetically sealed and gas-tight. When using z. B. water as a refrigerant, the adsorption chiller always works in the negative pressure range. The structure of the apparatus is relatively simple because only one apparatus is required for the ad- or desorber and evaporator or condenser.
functionality
The main difference to a general refrigeration machine is that the sorbent cannot be circulated in solid form. Therefore, the ad- and desorber as well as the evaporator or condenser must each be the same material in the same container, but fulfill their respective function in a container not simultaneously, but one after the other. As a result, the adsorption refrigeration machine is a discontinuously operating refrigeration machine, in which one phase is accompanied by adsorption and evaporation, while in the other phase, desorption and condensation. Because of this time separation of the phases, this process is also suitable for cold storage .
The refrigerant in the evaporator or condenser is heated, evaporated and thereby cools the same and thus dissipates the supplied heat. The evaporated refrigerant is now adsorbed by the sorbent and releases the heat that is extracted there. As the refrigerant collects there, it must be transported back.
All that is required to transport the refrigerant is appropriate temperature control of the sorbent. If heat is supplied to this, desorption takes place, ie the refrigerant leaves the sorbent in a liquid or gaseous state and then condenses in the evaporator or condenser, which functions as a condenser by removing the heat of condensation. The temperature in the sorbent and thus the pressure are then reduced again.
Alternatively, the output can also be adjusted by regulating the refrigerant flow if the ad- / desorber and evaporator / condenser are housed in separate containers connected to a pipe , in which a gas flow control valve is located in this pipe . The desorbed adsorbent then "sucks" in the amount of refrigerant set on the control valve until it is saturated , thereby removing the heat of evaporation from the evaporator . In this form, the device can also be used as a cold store , e.g. B. transportable, act, which is rechargeable by the sorbent is desorbed at a charging station if necessary.
The main disadvantage is that the "dead heat" is shifted back and forth due to the switching of the apparatus. Therefore, the apparatus construction must be carried out carefully with minimal heat capacities. It has also been proposed to design the adsorption refrigeration machine in several stages, which significantly improves the process efficiency, but also increases the outlay on equipment.
Use in developing countries
Since adsorption and desorption do not necessarily require electrical energy, adsorption chillers can be used in underdeveloped areas without a power grid to cool perishable food, drugs or vaccines . To do this, the sorbent container is connected to the evaporator with water as the coolant, if necessary, and the system is placed under negative pressure with a hand pump.
When the container is saturated with coolant, it can be regenerated by heating it with a solar cooker . In the meantime, additional sorbent containers can be connected one after the other to maintain cooling.
With several containers regenerated when exposed to sunlight, night and cloudy periods can also be bridged.