A review on compressed-air energy use and energy savings

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Abstract

Compressed-air systems account for about 10% of total industrial-energy use for few selected countries as found in literatures. Compressed air is typically one of the most expensive utilities in an industrial facility. This paper describes a comprehensive literature review about compressed air energy use, savings, and payback period of energy efficient strategies. This paper compiles latest literatures in terms of thesis (MS and PhD), journal articles, conference proceedings, web materials, reports, books, handbooks on compressed air energy use, efficiency, energy savings strategies. Computer tools for compressed air analysis have been reviewed and presented in this paper. Various energy-saving measures, such as use of highly efficient motors, VSD, leak prevention, use of outside intake air, reducing pressure drop, recovering waste heat, use of efficient nozzle, and use of variable displacement compressor to save compressed-air energy have been reviewed. Based on review results, it has been found that for an electric motor used in a compressed-air system, a sizeable amount of electric energy and utility bill can be saved using high efficient motors and applying VSDs in matching speed requirements. Also, significant amounts of energy and emission are reducible through various energy-saving strategies. Payback periods for different energy savings measures have been identified and found to be economically viable in most cases.

Introduction

Use of compressed air in industry and in service sectors is common as its production and handling are safe and easy. In most industrial facilities, compressed air is necessary to manufacturing. Compressed-air generation is energy intensive, and for most industrial operations, energy cost fraction of compressed air is significant compared with overall energy costs. Yet, there is a vacuum of reliable information on the energy efficiency of a typical compressed-air system [1], [2], [3], [4], [5], [6].

As a general rule, compressed air should be used only if safety enhancements, significant productivity gains, or labour reduction, will result as it is very expensive (see Fig. 1). Greenough [7] also reported how to select compressed-air system for an industrial facility.

Annual operating costs of air compressors, dryers, and supporting equipment, can account from 70% [9], [10], [11] to 90% [12] of the total electric bill.

Compressed air accounts for as much as 10% of industrial electricity consumption in the European Union [13]. Fig. 2 shows compressed-air energy use in 15 EU countries. Compressed-air systems in China use 9.4% of China's electricity. Compressed air is probably the most expensive form of energy in a plant, because only 19% of its power are usable. In the US, compressed-air systems account for about 10% of total industrial-energy use [14], as in Malaysia [15]. In South Africa, compressed air consumes about 9% of total energy consumption [16], [17]. Table 1 shows the industrial application of compressed-air system.

According to the total life cycle costs (LCC), initial investment and maintenance represents only a small portion of the overall cost of compressed-air equipment, and the power required to operate the compressor is usually 75%, or more, of the annual cost of compressed air, as Fig. 2 shows. Improvement to compressed-air systems can achieve 20–50% energy savings [18]. Over a compressed-air system's lifetime, operating energy is its single greatest cost (see Fig. 3), in many cases exceeding five times the initial equipment cost [19], [20], [21], [22], [23], [24].

Two of the most important factors influencing the cost of compressed air are the type of compressor control and the proper compressor sizing. Oversized compressors and compressors operating in inefficient control modes have the highest unit energy and the highest annual operating costs [25], [26], [27], [28].

Manufacturers are quick to identify energy (and thus money) losses from hot surfaces and to insulate those surfaces but somehow are not alert towards saving compressed air as they view air to be free; the only time air leaks and dirty air filters get any attention is when air and pressure losses interfere with normal operation of the plant. However, paying attention to compressed-air systems and practising simple conservation measures can save considerable energy and cost. The cost of electric power operating an air compressor continuously for a year is usually greater than the initial price of the equipment. From this perspective, any efforts to reduce energy consumption pay for themselves immediately and produce ongoing savings [9].

Although technology changes improve compressed-air efficiency, institutional and behavioral change, which involves government and public-interest facilitators, produce greater effects. Still, many industrial facilities do not take the time to study the costs involved in the generation of what is probably their most expensive plant utility energy source [2]. Small modifications have been proven to result in large savings and short payback periods. Such modifications include reducing leaks, matching supply with demand, reducing pressure setting if low pressure is adequate, using a smaller compressor at full load instead of a large one at part load, reducing average inlet temperature by using outside air, using waste heat from the cooling fluid to heat the facility in winter, using high-efficiency motors, turning off the compressor at night and during lunch break and using an after cooler, all which impact energy savings [26], [29], [30], [31], [32], [33].

In this study, the authors give an overview of energy-saving measures, complete with an analysis on potential savings of energy and cost, and simple payback periods. The authors hope that the information, will be useful to policy makers, researchers, and industrial-energy users. It is expected that the review results presented in this paper will create awareness on the potential energy savings of compressed-air systems for industrial-energy users.

Section snippets

Methodology

This section explains the energy audit, the data needed for energy analysis, in estimating energy savings and emission reductions by high-efficiency motor, variable speed drive, preventing leak, use of intake air temperature, reducing pressure drop, recovering waste heat and use of efficient nozzle.

AIRMaster+ [110]

AIRMaster+, developed by the U.S. Department of Energy (DOE) Industrial Technologies Program (ITP), provides a systematic approach for assessing the supply-side performance of compressed-air systems. Using plant-specific data, the software effectively evaluates supply-side operational costs for various equipment configurations and system profiles. It provides useful estimates of the potential savings to be gained from selected energy efficiency measures and calculates the associated simple

Review results and discussions on compressed-air energy savings, payback periods, and associated emission reductions

Based on the results presented by [15] in Table 9 and by analyzing data, it was determined that 1765, 2703, and 3605 MWh of total energy can be saved by using energy-efficient motors for 50, 75 and 100% motor loading, respectively. Similarly, associated bill savings for the estimated amount of energy savings are US$115,936 US$173,019 and US$230,693, respectively. It also has been found that the payback period for using energy efficient motors ranges from 0.53 to 5.05 years for different

Conclusions

From the review, it has been identified that energy audit is an effective tool that helps to collect data necessary for estimating compressed-air energy use. It also helps to identify where energy waste is taking place so that necessary measures can be implemented. Based on literature review it has been identified that only about 10–20% of total input energy is utilized for useful work in compressed-air system. Major energy lost takes place in the form of waste heat and through the leakage of

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