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MODELING AND

OPTIMIZATION OF

RENEWABLE ENERGY

SYSTEMS

Edited by Arzu Şencan Şahin

MODELING AND

OPTIMIZATION OF

RENEWABLE ENERGY

SYSTEMS

Edited by Arzu Şencan Şahin

Contents

Preface IX

Chapter 1 Solar-Energy Drying Systems 1 Feyza Akarslan

Chapter 2 Photovoltaic Systems and Applications 21 Feyza Akarslan

Chapter 3 A New Adaptive Method for Distribution System Protection Considering Distributed Generation Units Using Simulated Annealing Method 53 Hamidreza Akhondi and Mostafa Saifali

Chapter 4 Exergoeconomic Analysis and Optimization of Solar Thermal Power Plants 65 Ali Baghernejad and Mahmood Yaghoubi

Chapter 5 Optimization of Renewable Energy Systems: The Case of Desalination 89 Karim Bourouni

Chapter 6 Heat Transfer Modeling of the Ground Heat Exchangers for the Ground-Coupled Heat Pump Systems 117 Yi Man, Ping Cui and Zhaohong Fang

Chapter 7 Promoting and Improving Renewable Energy Projects Through Local Capacity Development 147 Rafael Escobar, David Vilar, Enrique Velo, Laia Ferrer-Martí and Bruno Domenech

Chapter 8 Utilization of Permanent Grassland for Biogas Production 171 Pavel Fuksa, Josef Hakl, Zuzana Hrevušová, Jaromír Šantrůček, Ilona Gerndtová and Jan Habart

Preface

Energy needs are continuously increasing and the demand for electrical power continues to grow rapidly. The world energy market has to date depended almost entirely on nonrenewable, but low cost, fossil fuels.

Renewable energy is the inevitable choice for sustainable economic growth, for the harmonious coexistence of human and environment as well as for the sustainable

development. As we learn how to economically harness the renewable energy sources, they will get cheaper and cheaper while fossil fuels get more and more expensive. A wind, solar or geothermal power plant may be more expensive to build now than a fossil power plant, but the future cost of fuel will be zero. In addition, the effects of the pollution fossil fuels produce become more and more destructive. The cost of controlling these pollutants is growing every day.

Arzu Şencan Şahin Süleyman Demirel University, Technology Faculty, Energy System Engineering, Isparta, Turkey

Solar-Energy Drying Systems

Feyza Akarslan

Department of Textile Engineering, Engineering and Architectural Faculty,

Süleyman Demirel Univercity, Isparta

Turkey

1. Introduction

Energy is important for the existence and development of humankind and is a key issue in international politics, the economy, military preparedness, and diplomacy. To reduce the impact of conventional energy sources on the environment, much attention should be paid to the development of new energy and renewable energy resources. Solar energy, which is environment friendly, is renewable and can serve as a sustainable energy source. Hence, it will certainly become an important part of the future energy structure with the increasingly drying up of the terrestrial fossil fuel. However, the lower energy density and seasonal doing with geographical dependence are the major challenges in identifying suitable applications using solar energy as the heat source. Consequently, exploring high efficiency solar energy concentration technology is necessary and realistic (Xie et al., 2011).

Solar energy is free, environmentally clean, and therefore is recognized as one of the most promising alternative energy recourses options. In near future, the large-scale introduction of solar energy systems, directly converting solar radiation into heat, can be looked forward. However, solar energy is intermittent by its nature; there is no sun at night. Its total available value is seasonal and is dependent on the meteorological conditions of the location. Unreliability is the biggest retarding factor for extensive solar energy utilization. Of course, reliability of solar energy can be increased by storing its portion when it is in excess of the load and using the stored energy whenever needed. (Bal et al., 2010).

Solar drying is a potential decentralized thermal application of solar energy particularly in developing countries (Sharma et al., 2009). However, so far, there has been very little field penetration of solar drying technology. In the initial phase of dissemination, identification of suitable areas for using solar dryers would be extremely helpful towards their market penetration.

Solar drying is often differentiated from “sun drying” by the use of equipment to collect the sun’s radiation in order to harness the radiative energy for drying applications. Sun drying is a common farming and agricultural process in many countries, particularly where the outdoor temperature reaches 30 °C or higher. In many parts of South East Asia, spice s and herbs are routinely dried. However, weather conditions often preclude the use of sun drying

2 Modeling and Optimization of Renewable Energy Systems

because of spoilage due to rehydration during unexpected rainy days. Furthermore, any direct exposure to the sun during high temperature days might cause case hardening, where a hard shell develops on the outside of the agricultural products, trapping moisture inside. Therefore, the employment of solar dryer taps on the freely available sun energy while ensuring good product quality via judicious control of the radiative heat. Solar energy has been used throughout the world to dry products. Such is the diversity of solar dryers that commonly solar-dried products include grains, fruits, meat, vegetables and fish. A typical solar dryer improves upon the traditional open-air sun system in five important ways (Sharma et al., 2009):

 It is faster. Matetrials can be dried in a shorter period of time. Solar dryers enhance drying times in two ways. Firstly, the translucent, or transparent, glazing over the collection area traps heat inside the dryer, raising the temperature of the air. Secondly, the flexibility of enlarging the solar collection area allows for greater collection of the sun’s energy.  It is more efficient. Since materials can be dried more quickly, less will be lost to spoilage immediately after harvest. This is especially true of products that require immediate drying such as freshly harvested grain with high moisture content. In this way, a larger percentage of product will be available for human consumption. Also, less of the harvest will be lost to marauding animals and insects since the products are in safely enclosed compartments.It is hygienic. Since materials are dried in a controlled environment, they are less likely to be contaminated by pests, and can be stored with less likelihood of the growth of toxic fungi.It is healthier. Drying materials at optimum temperatures and in a shorter amount of time enables them to retain more of their nutritional value such as vitamin C. An added bonus is that products will look better, which enhances their marketability and hence provides better financial returns for the farmers.It is cheap. Using freely available solar energy instead of conventional fuels to dry products, or using a cheap supplementary supply of solar heat, so reducing conventional fuel demand can result in significant cost savings.

2. Classification of drying systems

All drying systems can be classifed primarily according to their operating temperature ranges into two main groups of high temperature dryers and low temperature dryers. However, dryers are more commonly classifed broadly according to their heating sources into fossil fuel dryers (more commonly known as conventional dryers) and solar-energy dryers. Strictly, all practically-realised designs of high temperature dryers are fossil fuel powered, while the low temperature dryers are either fossil fuel or solar-energy based systems (Ekechukwu and Norton, 1999).

2.1 High temperature dryers

High temperature dryers are necessary when very fast drying is desired. They are usually employed when the products require a short exposure to the drying air. Their operating temperatures are such that, if the drying air remains in contact with the product until equilibrium moisture content is reached, serious over drying will occur. Thus, the products are only dried to the required moisture contents and later cooled. High temperature dryers

4 Modeling and Optimization of Renewable Energy Systems

The three modes of drying are: (i) open sun, (ii) direct and (iii) indirect in the presence of solar energy. The working principle of these modes mainly depends upon the method of solar-energy collection and its conversion to useful thermal energy.

3.1 Open sun drying (OSD)

Fig. 1 shows the working principle of open sun drying by using solar energy. The short wavelength solar energy falls on the uneven product surface. A part of this energy is reflected back and the remaining part is absorbed by the surface. The absorbed radiation is converted into thermal energy and the temperature of product stars increasing. This result in long wavelength radiation loss from the surface of product to ambient air through moist air. In addition to long wavelength radiation loss there is convective heat loss too due to the blowing wind through moist air over the material surface. Evaporation of moisture takes place in the form of evaporative losses and so the material is dried. Further a part of absorbed thermal energy is conducted into the interior of the product. This causes a rise in temperature and formation of water vapor inside the material and then diffuses towards the surface of the and finally losses thermal energy in the and then diffuses towards the surface of the and finally losses the thermal energy in the form of evaporation. In the initial stages, the moisture removal is rapid since the excess moisture on the surface of the product presents a wet surface to the drying air. Subsequently, drying depends upon the rate at which the moisture within the product moves to the surface by a diffusion process depending upon the type of the product (Sodha, 1985).

Fig. 1. Working principle of open sun drying.

In open sun drying, there is a considerable loss due to various reasons such as rodents, birds, insects and micro-organisms. The unexpected rain or storm further worsens the situation. Further, over drying, insufficient drying, contamination by foreign material like dust dirt, insects, and micro-organism as well discolouring by UV radiation are characteristic for open sun drying. In general, open sun drying does not fulfill the international quality standards and therefore it cannot be sold in the international market (Sharma et al., 2009).

Solar-Energy Drying Systems 5

With the awareness of inadequacies involved in open sun drying, a more scientific method of solar-energy utilization for drying has emerged termed as controlled drying or solar drying.

The main features of typical designs of the direct an of indirect types solar -energy dryers are illustrated in Table 1.

Table 1. Typical solar energy dryer designs (Ekechukwu and Norton, 1999).

3.2 Direct type solar drying (DSD)

Direct solar drying is also called natural convection cabinet dryer. Direct solar dryers use only the natural movement of heated air. A part of incidence solar radiation on the glass cover is reflected back to atmosphere and remaining is transmitted inside cabin dryer. Further, a part of transmitted radiation is reflected back from the surface of the product. The remaining part is absorbed by the surface of the material. Due to the absorption of solar radiation, product temperature increase and the material starts emitting long wavelength radiation which is not allowed to escape to atmosphere due to presence of glass cover unlike open sun drying. Thus the temperature above the product inside chamber becomes higher. The glass cover server one more purpose of reducing direct convective losses to the ambient which further become beneficial for rise in product and chamber temperature respectively (Sharma et al., 2009). However, convective and evaporative losses ocur insidethe chamber from the heated material. The moisture is takenaway by the air entering into the chamber from below and escaping through another opening provide at the top as shown in Fig. 2. A direct solar dryer is one in which the material is directly exposed to the sun’s rays. This dryer comprises of a drying chamber that is covered by a transparent cover made of glass or plastic. The drying chamber is usually a shallow, insulated box with air-holes in it to allow air to enter and exit the box. The product samples are placed on a perforated tray that allows the air to flow through it and the material. Fig. 2 shows a schematic of a simple direct dryer (Murthy, 2009). Solar radiation passes through the transparent cover and is converted to low-grade heat when it strikes an

Solar-Energy Drying Systems 7

3.3 Indirect type solar drying (ISD)

The is not directly exposed to solar radiation to minimize discolouration and cracking on the surface of the. Goyal and Tiwari (1999) have proposed and analyzed reverse absorber cabinet dryer (RACD). The schematic view of RACD is shown in Fig. 4. The drying chamber is used for keeping the in wire mesh tray. A downward facing absorber is fixed below the drying chamber at a sufficient distance from the bottom of the drying chamber. A cylindrical reflector is placed under the absorber fitted with the glass cover on its aperture to minimize convective heat losses from the absorber. The absorber can be selectively coated. The inclination of the glass cover is taken as 45 o^ from horizontal to receive maximum radiation. The area of absorber and glass cover are taken equal to the area of bottom of drying chamber. Solar radiation after passing through the glass cover is reflected by cylindrical reflector toward a absorber. After absorber, a part of this is lost to ambient through a glass cover and remaining is transferred to the flowing air above it by convection. The flowing air is thus heated and passes through the placed in the drying chamber. The is heated and moisture is removed through a vent provided at the top of drying chamber (Sharma et al., 2009).

Fig. 4. Reverse absorber cabinet drier.

Fig. 5 describes another principle of indirect solar drying which is generally known as conventional dryer. In this case, a separate unit termed as solar air heater is used for solar- energy collection for heating of entering air into this unit. The air heater is connected to a separate drying chamber where the product is kept. The heated air is allowed to flow through wet material. Here, the heat from moisture evaporation is provided by convective heat transfer between the hot air and the wet material. The drying is basically by the difference in moisture concentration between the drying air and the air in the vicinity of product surface. A better control over drying is achieved in indirect type of solar drying systems and the product obtained is good quality.

8 Modeling and Optimization of Renewable Energy Systems

Fig. 5. İndirect solar drier ( Forced convection solar drier)

There are several types of driers developed to serve the various purposes of drying products as per local need and available technology. The best potential and popular ones are natural convection cabinet type, forced convection indirect type and green house type. Apart from the above three, as seen from the literature, ‘‘Solar tunnel drier’’ is also found to be popular. These conventional types are shown in Figs 6-7.

Fig. 6. Green house type solar drier.

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Table 2. Comparisons of natural-circulation solar-energy dryers

Fig. 8. Multiple-shelf portable solar drier.

A staircase type dryer (Hallak et al., 1996) is developed which is in the shape of a metal staircase with its base and sides covered with doublewalled galvanized metal sheets with a cavity filled with nondegradable thermal insulation (see Fig.9). The upper surface is covered with transparent polycarbon sheet to allow the sun’s rays to pass through and be trapped. The upper polycarbon glazed surface is divided into three equal parts which can swing open, to provide access to the three compartment inside the dryer. The base of the dryer has four entry

Solar-Energy Drying Systems 11

points. The partition walls between the compartments also have four port holes for easy air flow. Air moves by natural convection as it enters through the bottom and leaves from the top.

Fig. 9. Staircase solar drier.

Another system called rotary column cylindrical dryer (Sarsilmaz et al., 2000) is developed which contains essentially three parts—air blow region (fan), air heater region (solar collector) and drying region (rotary chamber) (see Fig. 10). A fan with variable speed of air flow rate is connected to the solar collector using a tent fabric. The connection to the dryer or rotary chamber was again through another tent fabric. The dryer is manufactured from wooden plates at the top and bottom and thin ply wood plates at the sides to make cylindrical shape. A rectangular slot is opened on side wall where it faces the solar air heater for the passage of hot air via tent fabric. On the opposite side of this wall a door is provided for loading and unloading of the products. A column is constructed at the center of the rotary chamber to mount the products and the column rotates due to a 12 V dc motor and a pulley and belt system.

Fig. 10. Rotary column cylindrical drier.