Electrolux Refrigration

• Electrolux developed in 1925 a household absorption refrigerator (marketed in USA by Servel), which had no need of compressor, based on a 1923 patent by Swedish students C. Munters and B. von Platen. The Electrolux system of fully heat-powered absorption refrigeration is shown in fig it has no moving parts and slightly different vaporiser and condenser pressures, allowing for natural-thermal-convection pumping (thermo-siphon). Einstein and Szilard patented in 1928 a similar pump-free absorption refrigerator (using ammonia, water and butane), but the difficulties of dealing with ternary mixtures (and freon panacea at that time) relegated those pump-free refrigerators to a marginal place in the market.
• The absorption refrigeration effect can easily be achieved in a simple intermittent device (named Iceball), which basically consists on two thick spherical steel vessels (to withstand a few MPa) connected at the top through a pipe and holding a two-phase ammonia/water mixture (nearly half and half). An ammonia separator device is needed for effective operation, but we do not consider it in this conceptual mode of operation. First the device is ‘charged” by heating for some time one sphere (e.g. with a burner) while the other is immersed in room water; in that way, the liquid remaining in the hot vessel gets weak on ammonia, whereas some ammonia-concentrated solution condenses on the room-temperature side (nearly pure ammonia when using the separator. The device is then made to work to produce cold by just cooling the weak liquid in room water, what lowers its pressure and sucks vapours from the other sphere that gets cold due to vaporisation (this sphere can be put inside an ice chest, or some ice-tray built directly on it). With a valve in the connecting pipe, the charged-state can be kept for later use (a pressure jump builds up).

• The energy efficiency (coefficient of performance, COP) of heat-driven refrigeration machines which extracts Q_cold at T_cold by expending Q_hot at T_hot, in the presence of an environment at T_amb, is often defined in terms of energy extracted divided by energy consumed (the Carnot efficiency can be derived by combining a heat engine using Q_hot coupled to a mechanical refrigerator pumping Q_cold, both working against T_amb):

• what makes difficult the comparison with vapour-compression machines. The use of exergy efficiencies would remediate that situation, not only in refrigeration systems but in heat pumps and heat engines, but this is uncommon. Energy efficiency of heat-driven refrigeration is much smaller than work-driven refrigeration.
  
  
  Notice finally that some systems covered under Evaporative cooling (below), and other sorption and chemically reactive systems (not covered here), are very close to absorption refrigeration machines (the refrigerant is adsorbed by a solid desiccant or by a solid reactant). High endothermic processes like adsorption of ammonia in some halide salts, may be used for freezers (e.g. BaCl₂(s)+8NH₃(g)=BaCl₂(NH₃)₈(s) has been demonstrated to produce cooling down to −30 ºC; afterwards, the halide is regenerated at some 100 ºC with solar energy or waste heat).