安徽建筑大学
water-source air conditioning systems. Air source air conditioning systems can be classified into two types: chilled water air conditioning systems and air conditioners. Water-source air conditioning systems includes water source heat pump air conditioning systems and so on. For the absence of energy efficiency standards for water source heat pump, it is not discussed in this paper.
In a chilled water air conditioning system, cold water from the water chilling units is flowing through the coil that cools and dehumidifies the room’s air. According to the type of condensers, water chillers are divided into water-cooled, air-cooled and evaporative cooling ones. In terms of the whole chilled water air conditioning systems, if cooling towers and fan coils can be considered as part of the condenser and the evaporator, respectively, the heat flow direction of the chilled water air conditioning system is from indoor to outdoor. Therefore, chilled water air conditioning systems can be equivalently considered as air source air conditioning systems.
Air source air conditioners, as air–air units for short, include room air conditioners, variable speed room air conditioners, unity air conditioners and multi-connected air conditioners (heat pump) unit. They omit the secondary water cycle compared with chilled water air conditioning systems. They transfer the heat from indoor space to outdoor space with one or several main units. In terms of energy transfer, chilled water air conditioning systems and air source air conditioners have a common theoretical foundation.
3. Analysis of thermodynamic for air conditioning systems 3.1. Thermodynamic cycle of air conditioning systems
The energy efficiency for air conditioning systems can comprehensively reflect the manufacturing technology level, and the improvement of the energy efficiency is constrained by the principle of thermodynamics and heat transfer. In theory its limit value is the efficiency of the reversed Carnot cycle. The reversed Carnot cycle consists of two reversible isothermal processes and two isentropic processes. There is no temperature difference between heat source and working fluid. However in the practical refrigeration cycle, the compression and expansion processes are not isothermal processes and there is a limited temperature difference in two heat exchangers. Due to variable temperature heat source such as inlet and outlet water temperature or air temperature, that the Lorenz refrigeration cycle can serve as the ideal cycle for air conditioning systems. Fig. 1 shows the evolution progress from the Lorenz refrigeration cycle to the theoretic vapor compression refrigeration and then to actual compression refrigeration cycle [8].
Cycle A stands for the Lorenz refrigeration cycle. It is determined by the temperatures of inlet and outlet air of actual high and low temperature heat sources, and there is no fraction and disturbance. Besides the compression and expansion processes are isentropic.
Cycle B is the equivalent reversed Carnot cycle to the inverse Lorenz cycle. The high and low temperature heat sources of the equivalent Carnot cycle are equal to the inlet and outlet thermodynamic average temperature of two heat sources of the Lorenz cycle, and the energy efficiency of this cycle is considered as Carnot cycle efficiency.
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安徽建筑大学
Cycle C is the theoretical vapor compression refrigeration cycle. In this cycle the compression and expansion processes are isentropic, and the throttling progress is enthalpy. What is more, evaporation and condensing processes are carried out under isobaric condition. The evaporation and condensing temperature are corresponding with cold and heat temperature heat source. This cycle is useful for the theoretical analysis.
Cycle D is the practical refrigeration cycle vapor compression cycle. This cycle takes into account the actual isentropic efficiency of the compression processes, pressure drops in the evaporation and condensation progresses and heat absorption of the expansion valve. The efficiency of this cycle can be obtained from the practical test.
3.2. General principles of thermodynamic perfectibility
The thermodynamic perfectibility is the efficiency of the Second Law of Thermodynamics, which reflects how much the practical cycle deviates from the ideal cycle. The thermodynamic perfectibility of the cycle must be more than 0 and less than 1, thus it can be applied to analyzing how much the practical cycle deviates from the ideal cycle. The thermodynamic perfectibility of the ideal cycle is 1, but in the practical cycle it cannot be achieved. Putting thermodynamic perfectibility at 0.7 or 0.8 is not cost-effective enough for mass production. However, the thermodynamic perfectibility cannot be too low, and there exists a reasonable range of its value based on present technology. In this paper, the thermodynamic perfectibility is put up, making the compatible analysis of energy efficiency of energy efficiency standards for different air conditioning products.
The energy efficiency ratio (EERc) of the Lorenz cycle is defined as follows:
EERc?TlmThm?Tlm
Here Tlm is the thermodynamic average temperature of T1 and T4 and Thm is the thermodynamic average temperature of T2 and T3.Tlm and Thm are defined, respectively as follows:
Tlm?T1?T4ln(T1T4) T2?T3ln(T2T3)
Thm? The thermodynamic perfectibility under refrigeration conditions of the practical cycle is defined as follows:
?re?EERrEERc
Here EERr is the energy efficiency ratio of the practical cycle, and it can be obtained by testing.
4. Analysis of the thermodynamic perfectibility for air conditioning systems 4.1. Definition of the EERc of the ideal air conditioning systems cycle
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