تجزیه و تحلیل عملکرد سیستم مناسب تبرید چرخه هوا جهت حمل و نقل جاده ای

The performance of an air-cycle refrigeration unit for road transport, which had been previously reported, was analysed in detail and compared with the original design model and an equivalent Thermo King SL200 vapour-cycle refrigeration unit. Poor heat exchanger performance was found to be the major contributor to low coefficient of performance values. Using state-of-the-art, but achievable performance levels for turbomachinery and heat exchangers, the performance of an optimised air-cycle refrigeration unit for the same application was predicted. The power requirement of the optimised air-cycle unit was 7% greater than the equivalent vapour-cycle unit at full-load operation. However, at part-load operation the air-cycle unit was estimated to absorb 35% less power than the vapour-cycle unit. The analysis demonstrated that the air-cycle system could potentially match the overall fuel consumption of the vapour-cycle transport refrigeration unit, while delivering the benefit of a completely refrigerant free system.

The concept of air-cycle refrigeration was identified in the early 1800s and the first commercial air-cycle machine appears to have been in service in 1844. A succinct historical account of developments in the field of air-cycle refrigeration is provided by Bhatti [1]. The reciprocating compression and expansion machinery used for early air-cycle machines rendered the systems inefficient and they were replaced by CO2 vapour compression systems prior to the development of chlorofluorocarbon refrigerants. However, awareness of the environmental risks associated with using HCFC and HFC refrigerant fluids has spurred interest in alterative, natural refrigerant fluids that can deliver safe and sustainable refrigeration in the future. Today's highly efficient turbomachinery, which was not available to early air-cycle systems, has enhanced the performance of the air-cycle. A previous paper by Spence et al. [2] has reported the design, construction and testing of an air-cycle refrigeration unit for road transport. The programme was supported by Enterprise Ireland and Thermo King (Ireland). The unit constructed was a demonstrator plant that was directly comparable to Thermo King's SL200 trailer refrigeration unit. The demonstrator incorporated commercially available components that were not optimised for the air-cycle system and consequently the system would not be capable of achieving the optimum performance. Testing of the demonstrator unit on Thermo King's calorimeter test facility confirmed that the original objective had been met, which was to demonstrate that an air-cycle system could fit within the existing restrictive physical envelope of the SL200 unit and develop an equivalent level of cooling power to the existing vapour-cycle unit. The measured performance of the air-cycle demonstrator is summarised in Table 1. For comparison; the standard SL200 vapour-cycle unit delivered 7.2 kW of cooling duty at −20 °C and 12 kW at 0 °C. As previously reported, the fuel consumption of the air-cycle demonstrator was much greater than that of the SL200 vapour-cycle unit. At full load operation, the air-cycle fuel consumption was over three times greater than the vapour-cycle unit, although at part load operation the fuel consumption penalty reduced from over 200% to around 80%. Table 1. Measured performance for the air-cycle demonstrator plant Full-load, −20 °C Part-load, −20 °C Full-load, 0 °C Cooling capacity (W) 7800 3400 9500 Ambient temperature (°C) 29.3 29.9 30.6 Trailer temperature (°C) −20.0 −20.0 0.0 Discharge air temperature (°C) −46.4 −35.8 −29.4 Engine speed (rpm) 2210 1760 2210 Table options Since the air-cycle demonstrator had been constructed using modified commercially available turbomachinery and compromised heat exchanger configurations, achieving good energy efficiency was never an expectation. This paper reports detailed measurements taken throughout the air-cycle demonstrator system and identifies the potential performance improvements necessary for the air-cycle system to compete on energy efficiency terms with the standard vapour-cycle unit.

The performance of an air-cycle refrigeration unit for road transport, which had been previously reported, was analysed in detail and compared with the original design model. Overall, the turbomachinery was found to satisfy the original design requirements, but the heat exchangers proved to be a major performance handicap. The heat exchanger effectiveness values were several percentage points lower than anticipated and the heat exchanger pressure losses were excessive. As a consequence, the air-cycle demonstrator unit absorbed around 25% more power than the design value and more than twice as much power as the equivalent vapour-cycle unit. Using state-of-the-art, but achievable performance levels for turbomachinery and heat exchangers, the performance of an optimised air-cycle refrigeration unit was predicted. The power requirement of the optimised air-cycle unit was 7% greater than the equivalent vapour-cycle unit at full-load operation. However, at part-load operation the air-cycle unit was estimated to absorb 35% less power than the vapour-cycle unit. The analysis demonstrated that the air-cycle system could potentially match the overall fuel consumption of the vapour-cycle transport refrigeration unit, while delivering the benefit of a completely refrigerant free system. Bringing the optimised air-cycle to fruition would require the introduction of several new technologies into the transport refrigeration sector including air bearings, plate-fin heat exchangers, turbomachinery and high speed electric drives. The amount of development work and the associated investment to bring a range of air-cycle transport refrigeration units to market would be considerable, and would be unlikely in the absence of legislation to encourage such refrigerant free systems.