Exergy Analysis of Food Drying Processes

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Neslihan Colak, Mustafa T. Balta, Filiz Ifier, Ebru Kuzgunkaya, Arif Hepbasli and Zafer Erbay

15.1 Introduction

It is well known that micro-level system parameters may have some great impact on macro-level energy aspects, the environment, and sustainability. Of course, if one wants to approach these thermodynamically, there are two ways: energy analysis through the first law of thermodynamics and exergy analysis through the second law of thermodynamics. Exergy analysis is an essential tool to expose the impacts of a power generating device on exergy-based sustainability; sustainability is necessary to overcome current ecological, economic, and developmental problems (Dincer and Rosen, 2005). In this regard, some new exergy-based sustainability parameters for a PEM fuel cell have been developed and studied parametrically to investigate how they will help measure the level of environmental impact and sustainable development (Midilli and Dincer, 2009). These types of parameters may also be applied to other energy-related systems.

Drying has been used worldwide for centuries to preserve different food and agricultural products. Nowadays, the drying process is one of the major procedures of food preservation. The basic target of food dehydration is to remove water to a final concentration, which assures microbial spoilage of the product and minimizes chemical and physical changes of the food during storage (Crapiste and Rotstein, 1997). Drying is an energy-intensive operation consuming 9-25% of national energy in the developed countries (Mujumdar, 1995). In many practical applications, drying is a process that requires high-energy input because of the high latent heat of water evaporation and relatively low-energy efficiency of industrial dryers.

For development of sustainable energy, three important technological changes have been required: energy economies on the demand side, efficiency improvements in the energy production, and renewing of fossil fuels by various sources of renewable energy.

I. Dincer et al. (eds.), Global Warming, Green Energy and Technology,

DOI 10.1007/978-1-4419-1017-2_15, © Springer Science+Business Media, LLC 2010

Heat pumps (HPs) are devices for raising the temperature of low grade heat energy to a more useful level using a relatively small amount of high grade energy. Using HPs in convective hot air dryers has been recognized as an ideal area for HP applications (Schmidt et al., 1998). The energy efficiencies of conventional dryers are generally very low, a value of 35% being representative of the upper performance range (Lawton, 1978). Strommen et al. (2002) found that HPDs consume between 60% and 80% less energy than conventional dryers operating at the same temperature. This makes such dryers a feasible option for users who are not satisfied with the comparatively high energy consumption of directly heated dryers (Schmidt et al., 1998).

The most effective way to reduce energy demand is to use energy more efficiently. In this regard, exergy analysis, based on the second law of thermodynamics, successfully identifies the magnitudes and locations of energy degradations, inefficient uses of natural resources, and the pollution of the environment by means of waste energy (Kotas, 1985). If the less energy goes into production and marketing of foods, the less global warming pollution is created.

Exergy analysis evaluates the available energy at different points in a system. In the design of a system, the exergy method can be used to extract useful information to aid the task of choosing the most appropriate component design and operation procedure. This information is much more effective in determining the plant and operation costs, energy conservation, fuel versatility, and pollution levels. Bejan (1982) pointed out that the minimization of lost work in the system would provide the most efficient system. Moreover, Bejan (1988) and Szargut et al. (1988) emphasized that the effect of operating conditions on the system efficiency was much stronger for lost-work analysis than it is for the heat balance analysis. This explanation is required to determine the inefficient processes, equipment, or operating procedures during drying. For evaluating the performance of food drying systems, energy analysis method has been widely used, while the studies on exergy analysis are relatively few in number.

Fresh fruits and vegetables have both important nutritional and economic value. Recently, the market demand for naturally processed fruits and vegetables has undergone an important rise because of increasing health-conscious consumers. In vegetables broccoli is described as a vegetable with a high nutritional value due to its important content of vitamins, antioxidants, and anti-carcinogenic compounds (Nestle, 1998). Broccoli dehydration has not been investigated to a great extent and a few data are available in the literature (Bon et al., 1997; Simal et al., 1998; Mulet et al., 1999; Sanjuan et al., 2001; Mrkic et al., 2007). Annual broccoli production of Turkey increased 90.3% from 2005 to 2006 and reached 16,178 tonnes (Turkish Statistical Institute, 2006). It is estimated that production of this vegetable will rise gradually in Turkey and the world.

In this study, exergy analysis of food drying processes is presented and applied to broccoli drying process in a heat pump-driven conveyor dryer as a case study. Broccoli florets were dried at 45oC, 50oC, or 55oC drying air temperatures and 0.5 m/s, 1 m/s, or 1.5 m/s, drying air velocities. Effects of temperatures and mass flow rates on the exergy losses, exergy efficiencies, and improvement potentials of the drying process were investigated.

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