تجزیه و تحلیل هزینه چرخه عمر برای سیستم های تهویه مطبوع با حجم ثابت هوا و حجم متغیر هوا

This study presents a life-cycle cost analysis using detailed load profiles and initial and operating costs to evaluate the economic feasibilities of constant-air-volume (CAV) and variable-air-volume (VAV) air-conditioning systems. The present-worth cost method for life-cycle cost analysis is applied to a sample building located in Adana, Turkey which can be conditioned with CAV or VAV systems. In the analysis, two different uses of the building (as a school or as an office center), two different operating scenarios for air-conditioning system (scenario 1 and scenario 2) and two different economic measures (developed and developing economy) are considered. It is found, for all the cases considered, that although initial cost of the VAV system is higher than that of the CAV system, the present-worth cost of the VAV system is lower than that of the CAV system at the end of the lifetime due to lower fan-operating costs.

Selecting the most suitable and economic air-conditioning system among the available many alternatives is one of the important problems that engineers usually face. An air-conditioning system that saves operating costs usually requires a higher initial investment. In this case, engineers should decide whether it is worth paying the extra first cost for a system that has lower operating cost [1]. Air-conditioning systems can be categorized according to the transfer of heating and cooling energy between central plants and conditioned building-spaces. There are four basic system categories: all-air systems, air- and water-systems, all-water systems and packaged unitary equipment systems. All-air systems have been widely used in air-conditioning system applications. Air movement is one of the biggest areas of energy use in these systems. Two main air-distribution systems associated with all-air air-conditioning systems are constant-air-volume and variable-air-volume systems. Different types of these two approaches are available, such as single-duct, dual-duct, reheat and multi-zone systems. CAV systems have been used since the introduction of air-conditioning, while VAV systems have been utilized since the 1960s. Energy saving is one of primary reasons that VAV systems are very popular design choices today for some commercial buildings and many industrial applications. With these systems, the volume of the air delivered is reduced whenever operating loads are less than design loads [1] and [2]. The purpose of this study is to compare CAV and VAV systems considering initial and operating costs together. For this purpose, a sample building located in Adana, which can be air conditioned with a CAV or VAV system, was selected. Two different uses of the sample building (as a school or as an office center) and two different operating scenarios for the air-conditioning system were considered. The operating time of the building and the air-conditioning system is between 8:00 and 17:00 h for scenario 1 and between 8:00 and 24:00 h for scenario 2. Life-cycle cost (LCC) analysis was performed using detailed load-profiles, and initial and operating costs to evaluate the economic feasibility of CAV and VAV systems. The present worth cost (PWC) method for LCC [3], [4], [5], [6] and [7] was used to evaluate total costs. Two different sets of economic measures (interest rate and inflation rate) were used in the LCC analysis, one for a developed economy (ecoset 1) and one for a developing economy such as Turkish economy (ecoset 2). The current exchange rate is 1$ ≅ 1.5 New Turkish Lira (YTL).

In this study, constant-air-volume and variable-air-volume air-conditioning systems were compared calculating initial and operating costs for a sample building located in Adana, Turkey. For comparison, life-cycle cost analysis was used with the present-worth cost method and a comparison was made for eight different cases. It was found that the present-worth cost of the VAV system is always lower than that of the CAV system at the end of the lifetime for all the cases considered. If the number of operating hours of the building is longer (scenario 2), the extra investment of the VAV system with respect to the CAV system pays itself back after approximately 4 years in all the cases considered and the VAV system is a very attractive choice for air-conditioning. However, the VAV system is not an economic alternative with shorter operating hours (scenario 1). In this case, the payback period of the VAV system with respect to the CAV system is always higher than 10 years.