Review of solar dryers with latent heat storage systems for agricultural products

Abstract

Drying of agricultural food products is one of the most attractive and cost-effective application of solar energy as it becomes a potentially viable substitute for fuel–wood in much of the developing world. The intermittent nature of the solar energy, which is the main source of energy in solar drying, is indeed one of the major shortcomings of the solar drying system can be alleviated by storing excess energy during the peak time and use it in off sun hours or when the energy availability is inadequate. Developing efficient and inexpensive energy storage devices in solar dryers is as important as developing new sources of energy and reduce the time between energy supply and energy demand, thereby playing a vital role in energy conservation. It improves the energy systems by smoothening the output and thus increasing the reliability. Therefore, in this paper, an attempt has been taken to summarize the investigation of the solar drying system incorporating with phase change materials (PCMs) for drying agricultural food products.

Introduction

Drying is an important post handling process of agricultural produce which can extend shelf life, improve quality, minimize losses during storage and lower transportation costs since most of the water are taken out from the product during this process [1]. Drying under controlled conditions of temperature and humidity helps the agricultural products to dry reasonably rapidly to safe moisture content and to ensure superior quality of the product [2]. Controlled drying is practiced mostly in industrial drying processes which uses large quantities of fossil fuels. The potential of using solar energy in the agricultural sector has increased due to fluctuation in the price of fossil fuel, environmental concerns and expected depletion of conventional fossil fuels. Solar assisted drying system is one of the most attractive and promising applications of solar energy systems in tropical and subtropical countries. Traditional open sun drying practiced on a large scale in the rural areas of the developing countries suffers from high product losses due to inadequate drying, fungal growth, encroachment of insects, birds and rodents, etc. Properly designed solar dryers may provide a much-needed appropriate alternative for drying of some of the agricultural products in developing countries [3], [4], [5], [6], [7]. The intermittent nature of the solar energy, which is the main source of energy in solar drying, is indeed one of the major shortcomings of the solar drying system can be alleviated by storing excess energy during the peak time and use it in off sun hours or when the energy availability is inadequate [8].

There are numerous technologies for storing solar energy in various forms including mechanical, electrical and thermal energy [9]. Thermal energy can be stored in well-insulated fluids or solids as a change in internal energy of a material as sensible heat, latent heat and thermo-chemical or combination of these. An overview of major technique of storage of solar thermal energy is shown in Fig. 1 [10]. In sensible heat storage (SHS), thermal energy is stored by raising the temperature of a solid or liquid, utilizing the heat capacity and change in temperature of the material during the process of charging and discharging. The amount of heat stored depends on the specific heat of the medium, the temperature change and the amount of storage material. Generally water appears to be the best SHS materials available because it is inexpensive and has a high specific heat. However molten salts, oils and liquid metals, etc. are used above 100 °C. Rock bed type storage materials are used for air heating applications. Latent heat storage (LHS) is the heat absorption or release when a storage material undergoes a change of phase from solid to liquid or liquid to gas or vice versa at more or less constant temperature. Thermo-chemical systems rely on the energy absorbed and released in breaking and reforming molecular bonds in a completely reversible chemical reaction. In this case, the heat stored depends on the amount of storage material, the endothermic heat of reaction, and the extent of conversion [11]. Amongst above thermal heat storage techniques, latent heat thermal energy storage is particularly attractive due to its ability to provide high-energy storage density per unit mass and per unit volume in a more or less isothermal process, i.e. store heat at constant temperature corresponding to the phase transition temperature of phase change material (PCM).

The disadvantages of sensible heat storage systems posses the following: (i) low heat storage capacity per unit volume of the storage medium and (ii) non-isothermal behavior during heat storage (charging) and heat release (discharging) processes. On the other hand, LHS, with solid–liquid phase change, has received considerable attention in solar systems due to the follow advantages:

• It involves PCMs that have high latent heat storage capacity.

• The PCMs melt and solidify at a nearly constant temperature.

• A small volume is required for a latent heat storage system, thereby the heat losses from the system maintains in a reasonable level during the charging and discharging of heat.

A large number of solid–liquid PCMs have been investigated for heating and cooling applications [12], [13], [14], [15], [16], [17], [18], [19], [20]. The PCM to be used in the design of any thermal storage systems should pass desirable thermophysical, kinetics and chemical properties which are given in Table 1 [12], [20]. The ideal phase change material to be used for latent heat storage system must meet following requirements: high sensitive heat capacity and heat of fusion; stable composition; high density and heat conductivity; chemically inert; non-toxic and in spite of these advantages, the main hurdles in its dissemination are reluctance to acceptance as it is a novel technology, intermittent nature of sunshine, limited space availability in urban areas, higher initial costs and convenience issues. The growing urban lifestyle also warrants faster drying which is not possible in normal solar dryers [21].

Sharma et al. [22] studied the changes in the melting point, latent heat of fusion and specific heat of PCMs such as stearic acid, acetamide and paraffin wax, both laboratory-grade and commercial-grade, after a repeated number of melting/freezing cycles. Stearic acid melted over a range of temperatures but was thermally stable. Acetamide and paraffin wax showed reasonably good stability throughout 300 melting/freezing cycles and could be considered as promising PCMs. Acetamide absorbed moisture from surrounding, however. As mentioned by Abhat [12], paraffins qualify as energy storage materials due to their availability in a large temperature range and high heat of fusion. Furthermore, paraffins are known to freeze without any supercooling. A major drawback of paraffins is the low thermal conductivity. This problem is addressed through an increase of the surface area of heat transfer between the heat transfer fluid (HTF) and the PCM. The development of a latent heat storage system involves an understanding of heat exchanger and thermal storage material. Therefore efforts have been focused on the development of the heat exchanger configurations such as shell-and-tube, double pipe, plate or spherical shells and also on phase change materials. The use of finned tubes as well as metal fiber and metal matrix, for example, resulted in an increase of 1- to 5-fold of the effective thermal conductivity of the PCM and hence the rate of heat transfer [23]. Esen et al. [24] studied an energy storage and release in shell-and-tube heat exchanger units. The results indicated that a shorter energy storage and release time upon charging and discharging the PCM existed on the shell side. Choi and Kim [25] investigated some approaches to augment heat transfer within an LHS by the use of finned tubes and concluded that finned tubes increased the effective thermal conductivity of the phase change material due to the high thermal conductivity of the metal fins which are similar to those reported by Lacroix [26]. Wadekar [27] found that the charge and discharge times were reduced considerably in the system of plate heat exchanger. However, the use of plate heat exchangers remains to be found only on a limited scale.

Several Researchers investigated the heat transfer characteristics of PCMs in an LHS during melting and solidification [28], [29], [30], [31], [32], [33]. Sari and Kaygusuz [34] studied the phase transition time, the phase change temperature and the propagation of the solid–liquid interface in both radial and axial directions as well as the effect of the heat flow rate on the phase change stability of steric acid, which was used as the phase change energy storage material. They found that melting and solidification occurred from an upper and lower point in the axial direction, respectively. In the radial direction melting came about from a point closer to the HTF to a point far away from it, while solidification was observed to be in the opposite direction. On the contrary, Sukhatme [35] and Ettouney et al. [23] reported that during the discharge period PCM first solidified at the heat transfer surface. Furthermore, it was indicated that the temperature of HTF affected the charge and discharge times while the effect of the flow rate of HTF in the laminar flow range did not have any effect on heat transfer in both periods. Similar results were reported by Yanadori and Masuda [36] and Sari and Kaygusuz [34]. However, Ettouney et al. [23] found that the effect of natural convection was negligible in melting process for the case of downward HTF flow and also during a solidification process.

Recently, the incorporation of PCM in solar dryers has grown interest to the researcher. Heat storage system using PCM review article are available for any one application except solar dryers for drying of agricultural food products. Therefore, in this paper, an attempt has been taken to summarize the investigation of the solar drying system having latent heat storage with PCMs. This review will help to find the design, development of suitable heat storage unit using PCM for solar specially for agricultural food products.