Alternative Cooling Processes 1. Classification of Refrigeration Cycles. Agenda. Hans Jonsson

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1 Alternative Cooling Processes 1 Hans Jonsson Classification of Refrigeration Cycles Mechanically Driven Cycles - Vapor Compression Cycle Heat Driven Processes - Carré Process - Platen-Munters Process - Ejector Cycle Adsorption Processes Expansion Cycles - Joule Cycle - Stirling Cycle - Hilsch Tube (Ranque Vortex Tube) Thermoelectric Processes - Peltier Process Agenda Classification of Refrigerating Cycles Heat Driven Processes - Carré Process - Platen-Munters Process - Ejector Cycle Adsorption Cooling 1

2 Heat Driven Processes 1 Driven mainly, or entirely, by heat Most heat driven processes are absorption refrigeration processes Absorption means that the refrigerant is absorbed by another media, and transported as a liquid solution during a part of the cycle. An absorption process hence consists of a media pair. One refrigerant and one absorbator. Heat Driven Processes 2 Common media pairs (refrigerant + absorbator): - Ammonia + Water (Refrigeration) - Water + Lithium Bromide (Air Conditioning) Refrigerant Carré Processes 1 Weak solution Strong solution 2

3 Carré Processes 2 Condenser, Expansion Device, and Evaporator identical to vapor compression cycle. Compressor of vapor compression cycle replaced by thermal compressor. Thermal compressor: Absorber vessel, pump, generator, regulation valve, heat exchanger. Mainly heat driven. Pressure increase done in liquid phase => Considerably less mechanical energy required! Carré Processes 3 COP? For heat driven processes, COP H is defined Cooling capacity Refrigerant mass flow Latent heat of evaporation COP S H Q& = Q& R 2 H Generator heat input m& = m& H2O S R r ( r + L) m& = m& + m& m& > m& Hence COP H < 1 Solution mass flow Heat of mixing S R Carré Processes 4 3

4 Carré Processes 5 Multi-stage systems improve COP H Carré Processes 6 Carré Processes 7 If there is a heat source available then cooling can be obtained at low cost. Many sustainability aspects involved here Access to process waste heat, combustion gases etc means that you recover heat losses and generate a refrigerating effect instead. Solar driven cooling! Cooling demand increases with solar load With these systems, cooling capacity increses with solar load! Robust technology! 4

5 COP H vs COP 2-1 Is the performance of an absorption process necessarily worse than a vapor compression cycle? COP H vs COP 2-2 The mechanical energy (electricity) for the vapor compression cycle has to be produced! COP H vs COP 2-3 Assume a thermal efficiency for a steam power plant, η t = 0.35 and a COP 2 = 2.4 for a vapor compression cycle. Then Q& 2 Q& 2 COP2 E& COPH,VC = = COP2 = = = Q& H E& Q& H E& COP2 E& = ηt = = = COP2 ηt = Q& H E& ηt = =

6 Carré Processes 8 The Mosquito Coast Novel by Paul Theroux Movie by Peter Weir, 1986 Starring: Harrison Ford, Helen Mirren, River Phoenix An eccentric and dogmatic inventor sells his house and takes his family to Central America to build an ice factory in the middle of the jungle. Conflicts with his family, a local preacher and with nature are only small obstacles to his obsession. Platen-Munters Processes 1 Originated from a MSc Thesis work here at KTH in Absorption process (using three different media) The process has a constant total pressure in all parts of the system. Different evaporation and condensation pressures are achieved by introducing an inert gas into the system. This will make the partial pressure of the refrigerant change in different parts of the system. Completely heat driven! No moving parts - absolutely silent! Platen-Munters Processes 1 For which applications would this system be suitable? Camping equipment (no electricity needed) Mini bars in hotel rooms (absolutely silent) COP H = The company Dometic (formerly Electrolux) announced in June 2006 that the have manufactured their 10 millionth refrigerator in their plant in Motala, Sweden. Manufacturing started

7 Platen-Munters Processes 2 Platen-Munters Processes 3 Hydrogen circuit Water circuit Ammonia is gathered in bowls where evaporation takes place Platen-Munters Evaporator Refrigerant inlet Hydrogen + Ammonia outlet Hydrogen inlet Evaporation of Ammonia due to difference in partial pressure of Ammonia in the bowl and in the hydrogen atmosphere surrounding it (compare diffusion in humid air). Flow downwards is due to buoyancy effects (natural circulation). 7

8 Platen-Munters Absorbator Water inlet Water is gathered in bowls where Ammonia is absorbed Hydrogen outlet Absorption of Ammonia in the water due to difference in Ammonia concentration between Ammonia-water solution in the bowl and the Hydrogen-Ammonia atmosphere surrounding it. Water + ammonia outlet Hydrogen + Ammonia inlet Flow upwards is due to buoyancy effects (natural circulation). Platen-Munters Thermosyphon Pump Head of gasliquid column, H GL Head of liquid column, H L Heater Platen-Munters Thermosyphon Pump The heads have to be equal for the pump to work, hence: ρ L g H L = ρ GL g H GL By adding heat in the thermosyphon pump, some of the liquid is evaporated in such way that: ρ GL = ρ L H H L GL The use of a thermosyphon pump in the system was the invention of Baltsar von Platen and Carl Munters! 8

9 Ammonia liquid Platen-Munters Processes Condenser Hydrogen 19 Hydrogen free of Ammonia Absorber Ammonia vapour and Hydrogen 7 16 Strong solution Low temperature Evaporator Low temperature Pre-cooler 15 High temperature Evaporator High temperature Pre-cooler Pressure equilization line Weak solution Ammonia vapour Rectifier Vapour-liquid separator Lift-tube Heat supplied Heat Exchanger to Boiler Ejector Cycle 1 Ejector cycle Condenser, Expansion Device, and Evaporator identical to vapor compression cycle. Compressor of vapor compression cycle replaced by ejector. Ejector: Acceleration of high pressure vapor causes a low pressure which creates a suction to remove refrigerant vapor from evaporator. Pressure increase done by compression shock followed by a diffusor. Mainly heat driven. Pressure increase done in liquid phase => Considerably less mechanical energy required! Heating of the generator could be done by waste heat, solar heating etc. 9

10 Ejector Cycle 1 Sorption Cooling 10