1. Introduction

 Recent years, the crisis of energy is growing seriously and also the contamination of ecology have been reverberated as nternational problems(Lee, 2000). The social concern with energy crisis has been growing for the last several years. To realize effective air conditioning system and to solve the energy problem, the desiccant cooling system and its adoption idea are in the spotlight of new concept of air conditioning system. This study is to introduce the new concept of air conditioning system driven by photo voltaic energy as operating power and oyster shell used for desiccant from fishery wastes.

 In warm climates, cooling is important for space conditioning of most buildings and the cooling load in refrigerator or air  conditioning system increases during the daytime. So the efforts of reducing the cooling load have been performed at many places and it is regarded as necessary key point to who design the system of cooling.

 The adsorption operations exploit the ability of certain solids preferentially to concentrate specific substances from solution onto their surfaces. In this manner, the components of either gaseous or liquid solutions can be separated from each other. In the field of gaseous separations, adsorption is used to dehumidifying air and other gases, to remove objectionable odors and impurities from industrial gases such as carbon dioxide, to recover valuable solvent vapors form dilute mixtures with air and other gases, and to fractionate mixtures of hydrocarbon gases containing such substances as methane, ethylene, ethane, propylene, and propane.

 Some studies(Cooer and Alley, 1994; Benitez, 1993; Buonicore and Davis, 1992; Nevers, 1995; Noll et al., 1992; Dubinin, 1995; Polanyi, 1932) on desiccant are concentrated in finding and developing the raw material of adsorbent.

 The goal of this study is to clarify the possibility of oyster shell as a desiccant in a air conditioning system. Especially, the peltier element and photo voltaic cells were used for remove adsorption heat generation of oyster shell in the process of adsorption.

2. Experiment of Adsorption Amount of Adsorbent with Adsorption Mechanism

 Fig. 1 show the apparent(Fig. 1(a), (b)) and microscopic photo(Fig. 1(c)) of oyster shell. Generally known, oyster shell has many cavities on its surface and these are widening the surface area. The table 1 shows the typical specific surface areas of adsorbents for usually used in industrial fields.

Fig. 1. Apparent and microscopic photos of oyster shell.

Table 1. Typical specific surface areas of adsorbents

 The specific surface area of activated carbon indicates the most high value than others in Table 1.
Although the value of oyster shell is very low. It can be derived from nature without any other efforts and costs.

 Under the constant temperature condition, the process of adsorption on porous material can be illustrated as Fig. 2.

Fig. 2. Mechanism of adsorption.

 It is mainly divided into three steps as follows(Kondo, et al, 2003).
▶ Step 1 : Molecule diffusion process into the thin layer of fluid(Called as fluid film) which is attached on the adsorbent.
▶ Step 2 : According to developing of diffusion, the surface diffusion process which attached the vapour or gas along the
pores. It is called as mixed diffusion because there exist two diffusion of pore diffusion and surface diffusion.
▶ Step 3 : Adsorption process in the pore adsorption sites.

 As mentioned at introduction, to investigate the probability of oyster shell as a desiccant, some desiccants were used for examining the adsorption amount of vapour on their surfaces. Fig. 3 is the schematic of experiment.

Fig. 3. Experimental schematic for measuring adsorption

 The apparatus is mainly consisted of constant temperature and humidity chamber(Samheung instrument, SH-FDO 150), electronic balance(OHAUS, adventurer, ± 2mg) and test section used for setting the desiccants.

In order to investigate the effect of cooling of oyster shell, peltier element and photo voltaic cells were used to remove adsorption heat generation in the oyster shell.

The driving energy of peltier element is derived from solar. Fig. 4 is the result of inducing current for cooling effect. From Fig. 4, it is cleared that the cooling effect exists in this experiment and this effect is generally known phenomena among some references(Cooer and Alley, 1994; Benitez, 1993).

Fig. 4. Non-dimensional adsorption amount(Comparison of adsorbent species).

 The increase in electric current induced into peltier element is effectively release the heat generation of adsorption. Consequently, the non-dimensional adsorption amount is increasing according to increase in electric current.

 Fig. 4 indicates the results of adsorption experiments of silica-gel, activated charcoal, hi-dry(Japan made; commercially used), and oyster shell. From Fig. 4, it is clearly known that the highest value of non-dimensional adsorption amount exists at activated charcoal among any other materials.

 This result would be explained that the specific surface area indicated in table 1 of activated charcoal is the highest value so the value of adsorption amount would increase with this proportion.

 Fig. 5 is the results of induce current for cooling effect. From Fig. 5, it is clear that the cooling effect exists in this experiment and this effect is generally known phenomena among some references(Ono and Suzuki, 2005).

Fig. 5. Non-dimensional adsorption amount of oyster shell(Effect of induced current; so called cooling effect).

 The increase in electric current induced into peltier elements is effectively release the heat generation of adsorption. Consequently, the non-dimensional adsorption amount would increase with increase in electric current.

 However, in the case of 0.8 [Amp] of induced current, the desorption process has been existed at the end region of experiment.

 The reason of this phenomenon can be explained from the characteristic of peltier effect. The heating side of this experimental apparatus is composed with rectangular fins to release the heat generation of peltier element. According to the increase of induced current into peltier elements, the heat release fins can not release the adsorption heat sufficiently. Consequently, the heat of hot side of peltier is transferred into the cold side so the desorption process is occurred.

 The typical types of adsorption isotherm lines are well known as Fig. 6(Kondo, et al, 2003). To compare with Fig.6, the acquired results were re-plotted as Fig. 7 and Fig. 8.

Fig. 6. Typical types of adsorption isotherm lines(Kondo, et al., 2003).

Fig. 7. Non-dimensional adsorption amount of adsorbent.

Fig. 8. Effect of induced current into peltier elements.

 From Fig. 7, oyster shell and other materials indicate the similar tendency with type III of Fig. 6. The type III is physical adsorption characteristic which is suitable for multi-layered molecular adsorption. And it is generally considered unfavourable for adsorption.

 On the other hand, the data of adsorption amounts in Fig. 8 are increased with the increasing of induced current into peltier elements.

 This means that the effect of adsorption heat release enhances the efficiency of adsorption. Consequently, the non-dimensional adsorption amount gradually approaches to the type II of Fig. 6 with increasing electric current. The type II is considered favourable for adsorption.

3. Experimental Apparatus and Method for Dehumidifying Air Conditioning Chamber

 Fig. 9 shows the schematic of the experimental apparatus and the details of peltier cooling duct. The experimental apparatus mainly consisted of a test section of dehumidifying air conditioning chamber and photo voltaic cells for supplying electricity.

Fig. 9. Schematic diagram of experimental apparatus for dehumidifying air conditioning chamber.

 In the chamber, the flow rate of circulating air is controlled by DC fans(12 V, 0.1 A, 3 EA) and the temperature of air is controlled by four peltier elements. Two of them are for latent heat releasing and others are for sensible heat releasing.

 The T-type thermocouples were used for measuring the temperature of air and Anemometer was used for measuring the velocity of circulating air.

 To check the variation of humidity in the chamber, the hygrometer was installed at the top of chamber and all of data were gathered every second.

4. Experimental Results and Discussions

 Fig. 10 and 11 show the variations of saturation pressure and absolute humidity in chamber. In these graphs, the values of saturation pressure and absolute humidity decrease with increase in the mass of oyster shell. The increasing of mass of adsorbent means the increasing in surface area for adsorption so the amount of vapor in chamber is decreased with increasing in mass of oyster shell. And the adsorption heat generation is effectively released by thermoelectric device so the capability of adsorption amount of oyster shell would be increased as shown in Fig. 5 Consequently, the saturation pressure of vapor is kept to the lower range. However, in the case of 0.8 [Amp] of induced current, desorption process has been existed at the end region of experiment. The reason of this phenomenon can be explained from the characteristic of peltier effect. According to the increase of induced current into peltier element the heat release fins can not release the adsorption heat sufficiently. Therefore, the heat of hot side of peltier is transferred into the cold side so desorption process is occurred.

Fig. 10. Variation of saturation pressure.

Fig. 11. Variation of absolute humidity.

 Fig. 12 is the variation of room air temperature. As mentioned above, the peltier elements are working to cool the circulating air so the room air temperature is controlled. And also the oyster shell captures the moist of air so the latent heat load for this system reduces. For these reasons, the room air temperature decreases with increase in the mass of adsorbent(Oyster shell).

Fig. 12. Variation of room air temperature.

 Using the results of Fig. 12, the enthalpy of moist air is calculated as followed equation. This equation is usually used for calculating the enthalpy of moist air. And Fig. 13 shows the calculated result of circulating room air enthalpy.

Fig. 13. Enthalpy variation of moist air.

 Where, Cpair = Specific heat of dry air [J/(kg·K)]
            Cpvap = Specific heat of vapour [J/(kg·K)]
            Te = Ambient temperature [K]
            x = Absolute humidity [g/g_DA]
            γ = Latent heat of evaporation [J/kg]

5. Conclusions

 The probability of oyster shell desiccant cooling system driven by renewable energy of photo-voltaic effect was experimentally examined. This study has mainly performed in two parts of experiments. The one is for examine the adsorption characteristics of oyster shell and the other one is for adoption to the dehumidifying air-conditioning system. From this study, it is clarified that the oyster shell has sufficient probability for using as desiccant in an air-conditioning system. Moreover, the heat releasing device would be attached in the system, the system based with oyster shell can be operated with high efficiency. The acquired main conclusions are summarized as follows.

 1) Oyster shell has similar surface feature with other adsorption materials. This means that it can be used as desiccant material in a air-conditioning system.
2) As general characteristic behavior of adsorbent, all species of adsorbent used in this experiment showed that the non-dimensional adsorption amount have tendencies of increasing at higher saturation pressure of vapor.
3) Increase in electric current induced into peltier elements is effectively release the heat generation of adsorption. Consequently, the non-dimensional adsorption amount would increase with increase in electric current.
4) The non-dimensional adsorption amount of oyster shell gradually approaches to the type II(Typical adsorption isotherm line; favorable for adsorption) with increasing electric current.
5) It was found that the enthalpy of room air was kept lower range by the increase in mass of oyster shell.

Acknowledgement

This research was financially supported by the Ministry of Education, Science Technology(MEST) and National Research Foundation of Korea(NRF)(No. 2010-0022234).