Conservation of energy estimated by second law analysis of a power-consuming process

A refrigeration system removing heat from a cold storage is analyzed to determine exergetic losses of the thermodynamic cycle and the power needed for the flows of the refrigerant and the media transferring heat at a specified temperature. How much the overall power consumption of the system can be decreased by lowering the condensing temperature, by either increasing the external heat transfer at the condenser or by lowering the inlet temperature of the heat exchanging media, is investigated. The latter depends on the relative humidity of the air. The performance of air, water and evaporative-cooled condensers are evaluated as a function of relative humidity of the ambient air. It is shown that the evaporative condenser operates at the lowest condensing temperature and, therefore, the least power consumption of the total system is achieved. Wetting the condenser with water requires only 1% of the overall power consumption but reduces the consumption by 30% as compared with the air-cooled condenser. Precooling the air by a water spray before it enters an air-cooled condenser is of benefit only at relative humidities of 65% or less. At other state conditions of the air, a higher power consumption will result. Lowering the temperature of the surroundings lowers Prev and the second law efficiency must be properly defined so that, for cases of lowest power consumption, highest values of the efficiency will be obtained. Means for the design of least power-consuming air-conditioners are briefly stated, as well as the advantages of evaporative condensers for fog-free operation.