**4. Applications of solar driers**

The drying process has been experimentally studied and analyzed to simulate and design a drier. As drying is a process of removing moisture to a safe level, the equilibrium moisture content is defined as the moisture content in equilibrium with the relative humidity of the environment. The equilibrium moisture content is divided into, static and dynamic. While the static is used for food storage process, dynamic is used for drying process. The drying process is experimentally obtained and presented as moisture content on x-axis and rate of drying on y-axis. A deep bed of food grains is assumed to be composed of thin layers normal to the hot air flow direction. The equations for thin layer were written initially, using empirical, theoretical and semitheoretical equations. The conditions of the grain and air, change with position and time during drying of a deep bed of grains. Logarithmic and partial differential equation models to simulate the deep bed dry modeling are dealt in detail (Murthy, 2009).

A computer program in C++ language is developed for modeling of deep bed drying systems and considers eight different configurations of flow of hot air over absorber plates of solar collectors. The usual parameters such as heat removal factor, overall loss coefficient, top loss coefficient, etc., can be determined. The model prompts for basic data (Murthy, 2009) such as amount of grain to be dried, initial moisture content, number of thin layers and weather data.

In a different direction, the first and second law of thermodynamics (Torres-Reyes et al., 2002) have been used to develop the design methods for a particular application. Semi-empirical formulae are developed to calculate the rise in air temperature as it passes through the heater. NTU (number of transfer units) has been defined analogous to the heat exchangers, as a part of design. Using entropy balance the maximum temperature reached by solar collector is written and then Entropy Generation Number is developed to find the entropy generated during thermal conversion of solar energy. Finally, the drying temperature is established as a function of the maximum limit of temperature the material might support.

In all the types of driers stated above, the hot air enters the drying chamber and leaves to the atmosphere. But the hot air can be recirculated to save the energy (McDoom et al., 1999). The drying of coconut and cocoa in a scaled down drier of a large scale drier is considered in which the recirculation of hot air yields 31 and 29% of energy saving, respectively. The recirculation of exhaust/hot air is also applied to hay driers. Lack of uniform drying and inability to accurately predict drying times are some of the existing problems. A new drier is developed which uses forced heated-air circulation through hay stacks. A drying rate difference of 7% is observed due to recirculation of hot air. By recirculating all of the exhaust air, the previous driers either increased drying time or proved to be uneconomical. So only 30% of the hot air is recirculated in the present case. The favorable conditions to recirculate

A drier called FASD (Foldable Agro Solar Dryer) is developed which is a foldable type that can be stored and transported as desired. The performance of the drier is tested to find that the inner temperature is about 8 oC higher than ambient and humidity is lesser by 6% inside. Out of all types, the well known heat pump (Murthy, 2009) principle has been used to dry

The drying process has been experimentally studied and analyzed to simulate and design a drier. As drying is a process of removing moisture to a safe level, the equilibrium moisture content is defined as the moisture content in equilibrium with the relative humidity of the environment. The equilibrium moisture content is divided into, static and dynamic. While the static is used for food storage process, dynamic is used for drying process. The drying process is experimentally obtained and presented as moisture content on x-axis and rate of drying on y-axis. A deep bed of food grains is assumed to be composed of thin layers normal to the hot air flow direction. The equations for thin layer were written initially, using empirical, theoretical and semitheoretical equations. The conditions of the grain and air, change with position and time during drying of a deep bed of grains. Logarithmic and partial differential equation models to simulate the deep bed dry modeling are dealt in

A computer program in C++ language is developed for modeling of deep bed drying systems and considers eight different configurations of flow of hot air over absorber plates of solar collectors. The usual parameters such as heat removal factor, overall loss coefficient, top loss coefficient, etc., can be determined. The model prompts for basic data (Murthy, 2009) such as amount of grain to be dried, initial moisture content, number of thin layers

In a different direction, the first and second law of thermodynamics (Torres-Reyes et al., 2002) have been used to develop the design methods for a particular application. Semi-empirical formulae are developed to calculate the rise in air temperature as it passes through the heater. NTU (number of transfer units) has been defined analogous to the heat exchangers, as a part of design. Using entropy balance the maximum temperature reached by solar collector is written and then Entropy Generation Number is developed to find the entropy generated during thermal conversion of solar energy. Finally, the drying temperature is established as a function

of the maximum limit of temperature the material might support.

the products and this has been found to be excellent alternative to the solar drying.

the exhaust air are presented (Murthy, 2009).

**4. Applications of solar driers** 

detail (Murthy, 2009).

and weather data.

The drying chamber of a drier consists of meshes on which product is spread for drying. Also, the drying chamber (Youcef-Ali et al., 2004) is a wooden cabinet. Hence, the heat loss to the side walls of the drying chamber is considered. As the hot air passes through the mesh, in forced convection driers, turbulence is created. A solar drier without either heat storage or air recycling is considered with a solar collector containing offset plate fins. Experiments are conducted to calculate heat losses (through Nusselt number).

In the above models, the variation of incoming solar radiation is not taken into account. For modeling purpose, a constant artificial flux is adopted to study the drying phenomenon (Hachemi, et al., 1998). A drier with three beds of wool is considered with a solar collector. The drying process in the three zones of the bed is theoretically analyzed. The solar collector is equipped with a flat plate absorber and offset plate fins absorber plate. Under constant incident fluxes, at the same mass flow rate of air, the drying rate and time has been studied to find that offset plate fins collector is better.

The known facts that, the inlet temperature of the air is variable (because of variable incoming solar radiation) and the products shrink as drying process continues are taken into consideration for modeling (Ratti and Mujumdar, 1997). A most common cabinet type drier is considered for the study. A moving co-ordinate is defined to take into account of the shrinkage effects. The experimental data from previous workers is considered for validation of the mathematical model. The carrot cubes are used as product to test the model. It is proposed that the estimation of solar irradiance on the drier is essential to predict the response of the drier (Garg and Kumar, 1998). Considering a semi-cylindrical solar tunnel drier, the irradiance is calculated by taking the geometric quantities, relative motion of sun and optical properties into account.

The change of main variables such as moisture content along the drying tunnel is considered unlike in previous works where uniform distribution is assumed (Condori and Saravia, 2003). This is a study of tunnel green house drier which is continuous type. The conditions for improvement of efficiency are evaluated. A linear relationship between the tunnel output temperature and incident solar radiation is obtained. The drier production is presented by a performance parameter which is defined as the ratio between the energy actually used in the evaporation and the total available energy for the drying process. A non-dimensional variable is also defined, which has all the meteorological information. It is found that, the average moisture content value of the tunnel can be considered to be constant (Murthy, 2009).

The construction and working of solar tunnel drier is explained in detail. Three fans run by a solar module are used to create forced convection. The drying procedure and the instrumentation are also described. The major advantage of solar tunnel drier is that the regulation of the drying temperature is possible. During high insolation periods, more energy is received by the collector, which tends to increase the drying temperature and is compensated by the increase of the air flow rate. The variation of voltage with respect to radiation in a given day and variation of radiation with respect to time of the day are presented. The comparative curves using the tunnel dryer and natural sun drying are presented to show that, the tunnel drying time is less(Murthy, 2009). A substantial increase in the average sugar content is observed. The economics of the drier is worked out to show that, the pay back period is 3 years.

The solar tunnel drier is modified to develop a green house tunnel drier whose working principle and construction is explained in detail. Some additional features of the tunnel drier

Solar-Energy Drying Systems 19

Bena, B., Fuller, R.J., 2002. Natural convection solar dryer with biomass back-up heater.

Condori, M., Echazu, R., Saravia, L., 2001. Solar drying of sweet pepper and garlic using the

Condori, M., Saravia, L., 2003. Analytical model for the performance of the tunnel-type

Ekechukwu, O.V., Norton, B., 1999. Review of solar-energy drying systems II: an overview

Garg, H.P., Kumar, R., 1998. Studies on semi-cylindrical solar tunnel dryers: estimation of

Goyal, R.K., Tiwari, G.N., 1999. Performance of a reverse flat plate absorber cabinet dryer: a new concept. *Energy Conversion & Management*, Vol.40(4), pp. 385–392. Hachemi, A., Abed, B., Asnoun, A., 1998. Theoretical and experimental study of solar dryer.

Hallak, H., Hilal, J., Hilal, F., Rahhal, R., 1996. The staircase solar dryer: design and

Janjai, S., Tung, P., 2005. Performance of a solar dryer using hot air from roof-integrated

Koyuncu, T., 2006. Performance of various design of solar air heaters for drying

Koyuncu, T., 2006. An investigation on the performance improvement of green house type

McDoom, I.A., Ramsaroop, R., Saunders, R., Tang Kai, A., 1999. Optimization of solar

Mumba, J., 1995. Development of a photovoltaic powered forced circulation grain dryer for

Murthy, R. 2009. A review of new technologies, models and experimental investigations of solar driers. *Renewable and Sustinable Energy Revews*, Vol.13, pp. 835-844. Othman, M.Y.H, Sopian, K., Yatim, B., Daud, W.RW., 2006. Development of advanced solar

Prasad, J., Vijay, V.K., 2005. Experimental studies on drying of Zingiber officinale, Curcuma

Ratti, C., Mujumdar, A.S., 1997. Solar drying of foods: modeling and numerical simulation.

Sarsilmaz, C., Yildiz, C., Pehlivan, D., 2000. Drying of apricots in a rotary column cylindrical dryer (RCCD) supported with solar energy. *Renewable Energy, Vol.*21, pp. 117–127. Singh, S., Singh, P.P., Dhaliwal, S.S,. 2004. Multi-shelf portable solar dryer. *Renewable Energy,* 

Shanmugam, V., Natarajan, E., 2006. Experimental investigation of forced convection and desiccant integrated solar dryer. *Renewable Energy*, Vol.31, pp. 1239–1251. Sharma, A., Chen, C. R., Vu Lan, N., 2009. Solar- energy drying systems:A review. *Renewable* 

longa and Tinospora cordifolia in solar-biomass hybrid drier. Renewable Energy,

solar collectors for drying herbs and spices. *Renewable Energy*, Vol.30, pp. 2085-

of solar drying technology. *Energy Conversion & Management*, Vol.40(6), pp. 615-655.

tunnel greenhouse drier. *Renewable Energy*, Vol.22, pp. 447–460.

greenhouse drier. *Renewable Energy*, Vol.28, pp. 467–485.

solar irradiance. *Renewable Energy*, Vol.13, pp. 393–400.

characteristics. *Renewable Energy*, Vol.7, pp. 177-183.

applications. *Renewable Energy*, Vol.31, pp. 1073–1088.

drying. *Renewable Energy*, Vol.16, pp. 749–752.

Vol.30, pp. 2097–2109.

*Vol.*29, pp. 753–765.

*Solar Energy, Vol.*60, pp. 151–157.

agricultural dryers. *Renewable Energy*, Vol.31, pp. 1055–1071.

use in the tropics. *Renewable Energy*, Vol. 6(7), pp.855–862.

*and Sustinable Energy Revews,* Vol.13, pp. 1185-1210.

assisted drying systems. *Renewable Energy, Vol.*31, pp. 703–709.

*Solar Energy*, Vol.72, pp. 75–83.

*Renewable Energy*, Vol.13, pp. 439–451.

2095.

are high lighted such as improvement in the drier efficiency, lowering of the labor cost and ease in installing a conventional heater as an auxiliary heating system for continuous production (Condori et al., 2001). The drier is considered as a solar collector, and its instantaneous efficiency is measured. Products were dried in various configurations, i.e., cut in various ways. The plots of time in a given day vs. moisture content are plotted. The working principle of auxiliary heating system is also presented.

Through out the literature, decrease in drying time has been the main concern. Further, the natural convection type drier is not preferred as low buoyancy forces may cause reverse effect leading to the spoilage of the product. In order to resolve these two issues, an integral type natural convection drier coupled with a biomass stove is developed (Prasad and Vijay, 2005). The constructional details and operation of the drier are presented in detail. Drying time was lowest for solar-biomass method. The uniformity of drying was questionable as there was significant variation in moisture content when samples were tested from trays at top, middle and bottom. Even within a tray, when temperature, relative humidity and velocity of air were measured, variations were observedThe drying efficiency of the drier was evaluated and it is noted that, type of product and its final moisture content level influences the drying efficiency. The final moisture in a product generally requires more energy to extract than the initial moisture and the preparation of the products prior to drying such as slicing, boiling affects the drying efficiency. These factors make it difficult to make comparisons with the drying efficiencies of other solar driers reported in the literature.

#### **5. Conclusions**

This chapter is focused on the available solar dryer's systems and new technologies. The dependence of the drying on the characteristics of product remains still as a problem, for comparison of drying efficiencies of various driers. Author presented a comprehensive review of the various designs, details of construction and operational principles of the wide variety of practically realized designs of solar-energy drying systems. Two broad groups of solar-energy dryers can be identified, viz., passive or natural-circulation solar-energy dryers and active or forced-convection solar-energy dryers (often called hybrid solar dryers). Three sub-groups of these, which differ mainly on their structural arrangement, can also be identified, viz integral or direct mode solar dryers, distributed or indirect-modes. This classification illustrates clearly how these solar dryer designs can be grouped systematically according to either their operating temperature ranges, heating sources and heating modes, operational modes or structural modes. Though properly designed forced-convection (active) solar dryers are agreed generally to be more effective and more controllable than the natural-circulation (passive) types. This chapter also presents some easy-to-fabricate and easy-to-operate dryers that can be suitably employed at small-scale factories. Such low-cost drying technologies can be readily introduced in rural areas to reduce spoilage, improve product quality and overall processing hygiene.

#### **6. References**

Bal, L. M., Satya, S., Naik, S.N., Solar dryer with thermal energy storage systems for drying agricultural food products: A review. *Renewable and Sustinable Energy Revews,*  Vol.14(8), pp. 2298-2314.

are high lighted such as improvement in the drier efficiency, lowering of the labor cost and ease in installing a conventional heater as an auxiliary heating system for continuous production (Condori et al., 2001). The drier is considered as a solar collector, and its instantaneous efficiency is measured. Products were dried in various configurations, i.e., cut in various ways. The plots of time in a given day vs. moisture content are plotted. The

Through out the literature, decrease in drying time has been the main concern. Further, the natural convection type drier is not preferred as low buoyancy forces may cause reverse effect leading to the spoilage of the product. In order to resolve these two issues, an integral type natural convection drier coupled with a biomass stove is developed (Prasad and Vijay, 2005). The constructional details and operation of the drier are presented in detail. Drying time was lowest for solar-biomass method. The uniformity of drying was questionable as there was significant variation in moisture content when samples were tested from trays at top, middle and bottom. Even within a tray, when temperature, relative humidity and velocity of air were measured, variations were observedThe drying efficiency of the drier was evaluated and it is noted that, type of product and its final moisture content level influences the drying efficiency. The final moisture in a product generally requires more energy to extract than the initial moisture and the preparation of the products prior to drying such as slicing, boiling affects the drying efficiency. These factors make it difficult to make comparisons with the drying

This chapter is focused on the available solar dryer's systems and new technologies. The dependence of the drying on the characteristics of product remains still as a problem, for comparison of drying efficiencies of various driers. Author presented a comprehensive review of the various designs, details of construction and operational principles of the wide variety of practically realized designs of solar-energy drying systems. Two broad groups of solar-energy dryers can be identified, viz., passive or natural-circulation solar-energy dryers and active or forced-convection solar-energy dryers (often called hybrid solar dryers). Three sub-groups of these, which differ mainly on their structural arrangement, can also be identified, viz integral or direct mode solar dryers, distributed or indirect-modes. This classification illustrates clearly how these solar dryer designs can be grouped systematically according to either their operating temperature ranges, heating sources and heating modes, operational modes or structural modes. Though properly designed forced-convection (active) solar dryers are agreed generally to be more effective and more controllable than the natural-circulation (passive) types. This chapter also presents some easy-to-fabricate and easy-to-operate dryers that can be suitably employed at small-scale factories. Such low-cost drying technologies can be readily introduced in rural areas to reduce spoilage, improve

Bal, L. M., Satya, S., Naik, S.N., Solar dryer with thermal energy storage systems for drying

agricultural food products: A review. *Renewable and Sustinable Energy Revews,* 

working principle of auxiliary heating system is also presented.

efficiencies of other solar driers reported in the literature.

product quality and overall processing hygiene.

Vol.14(8), pp. 2298-2314.

**5. Conclusions** 

**6. References** 


**2** 

Feyza Akarslan

*Turkey* 

*Süleyman Demirel University, Isparta* 

**Photovoltaic Systems and Applications** 

*Department of Textile Engineering, Engineering and Architectural Faculty,* 

Improvements in quality of life and rapid industrialization in many countries are increasing energy demand significantly, and the potential future gap between energy supply and demand is predicted to be large. Interest in sustainable development and growth has also grown in recent years, motivating the development of environmental benign energy technologies. Research on applications of solar energy technologies have as a consequence expanded rapidly, exploiting the abundant, free and environmentally characteristics of solar energy. However, widespread acceptance of solar energy technology depends on its competitiveness, considering factors such as efficiency, cost-effectiveness, reliability and

Renewable energy sources can be defined as "energy obtained from the continuous or repetitive currents on energy recurring in the natural environment" or as "energy flows which are replenished at the same rate as they are used". All the earth's renewable energy sources are generated from solar radiation, which can be converted directly or indirectly to energy using various technologies. This radiation is perceived as white light since it spans over a wide spectrum of wavelengths, from the short-wave infrared to ultraviolet. Such radiation plays a major role in generating electricity either producing high temperature heat to power an engine mechanical energy which in turn drives an electrical generator or by

(PV) effect. It is well known that PV is the simplest technology to design and install, however it is still one of the most expensive renewable technologies. But its advantage will always lie in the fact it is environmentally friendly and a non-pollutant low maintenance

Some solar thermal systems, such as solar water heaters, air heaters, dryers and distillation devices, have advance notably in decades in terms of efficiency and reliability. Efficiencies of these devices typically range from about 40% to 60% for low- and medium-temperature applications (Thirugnanasambandam et al., 2010). Also, the direct conversion of solar energy to electricity has advanced markedly over the last two decades, leading to significantly reduced prices of photovoltaic modules, and applications have increased especially due to the availability of incentives in many parts of the world (Branker and Pearce, 2010). However, the efficiency of mono crystalline silicon based module is still

**1. Introduction** 

availability (Kumar and Rosen, 2011).

energy source (Chaar et. al., 2011).

directly converting it to electricity by means of the photovoltaic

