Design, Simulation, Construction and Performance Comparison of Mixed-mode Solar Crop Dryers With and Without Thermal Storage

Design, Simulation, Construction and Performance Comparison of Mixed-mode Solar Crop Dryers With and Without Thermal Storage

Chapter One

Aim and Objectives

The aim of the work is todesign, simulate,construct and carry out the performance comparison of mixed-mode solar crop dryers with and without thermal storage material integrated with the absorber for yam drying under the meteorological conditions of Zaria.

The specific objectives are to:

Carry outdesign analysis to determine the dimensions of the solar crop dryingsystems;
Simulate the performance of the drying systems with and without thermal storage material under varying meteorological conditions ofZaria;
Construct solar crop dryers with and without thermal storage materials; and
Evaluate experimentally the performance of the solar dryers

CHAPTER TWO

LITERATURE REVIEW

General Introduction on Yam

Yam is an economically useful plant belonging to the genus Dioscorea or the Tubers/rhizomes of the plants. Yams are cultivated for the consumption of their starchy tubers in Africa, Asia, Latin America and Oceania. The global yam production was almost 48.7 million tonnes per annum with 97% of this coming from sub-Saharan Africa. West and Central Africa accounted for 94%, Nigeria emerging as the largest producer with 34million tonnes, Cote d‟voire 5 million tonnes and Ghana 3.9 million tonnes (Amma, 2014). There are many widespread varieties of yam species, the most economic important species grown include: Dioscorearotundata (white yam), D.cayenensis (yellow yam), D.alata (water yam),D.esculenta (Chinese yam), D.bulbifera (aerial yam) and D.dumentorum (trifoliate yam) (Amma, 2014). Yam is a food security crop and a source of industrial starch in some sub-Saharan African countries. Besides, Yam plays a vital role in traditional culture, rituals and religion as well as a source of income of the African people. The utilization of yam in Nigeria has not been fully achieved due to insufficient processing technology obtainable in other developing countries. Its high moisture content poses a great challenge for its storage (Liberty et al.,2013).

Solar Dryers and Drying Principle

Solar crop drying utilizes the sun‟s energy either directly, indirectly or in the mixed- mode. Air flow can be generated by either natural or forced convection. In natural convection dryers the driving force for airflow is the buoyancy while the forced convection type uses a fan to aid the air flow (Bukola and Ayoola, 2008). The drying process can be categorized into two; constant rate and falling rate drying periods. In constant rate drying period evaporations occur at pelt surface of the material being dried. In this condition, the rate of drying is controlled by the rate of evaporation from the surface of the material being dried.As the drying proceeds, a point is reached at which the rate of transfer of moisture within the material to the surface is less than the rate of evaporation from the surface. At this point the falling rate drying period starts. In falling rate drying involves two processes; movement of moisture within the material to the surface and removal of the moisture from the surface (Abdullahi, 1995). During this process, there is a possibility of the product being dried to form dry surface layers which are impervious to subsequent moisture transfer if the drying rate is very rapid (Drew, 2011). To avoid this effect, the heat transfers and evaporation rates must be closely controlled to guarantee optimum drying rates (Rajkumar, 2007). Quick drying is enhanced by high temperature, high wind speed, and low relative humidity (Bukolaand Ayoola, 2008).

Classification of Drying Systems

Drying equipment may be classified in several ways. The two most useful classifications are based on Hiiet al.,(2012):

The method of transferring heat to the wet solids
The handling characteristics and physical properties of the wet

A classification chart of drying on the basis of heat transfer is shown in figure 2.1,

Three distinct sub-classes of either the active or passive solar drying systems can be identified namely(Hiiet al., 2012):

Direct type solar dryers
Indirect type solar dryers
Mixed-mode/Hybrid solar dryers

Types of Solar Dryer

On the basis of the mode of drying, e.g. direct or indirect, solar dryers may be classified as passive and active ones(Hiiet al., 2012):

CHAPTER THREE

MATERIALS AND METHOD

Description of the Two Solar Crop Dryers

The solar dryers consist of the following components.

i.) Solar Collector: The collector which is essentially the air heater is an insulated box with a transparent glass cover. The collector was separated from the drying chamber by a duct, one side was opened to let the atmospheric air into the collector and the other end was connected to the air-duct. The connecting- air-duct was then connected to the drying chamber. During sunlight exposure, the absorber heats both the air and the thermal storage material. The storage material emits the stored energy during low sunshine hour and cloud cover.

ii.) Absorber: This was made up of black painted galvanized iron sheet metal, which absorbs the solar radiation. The black paint enhances the absorptivity of the galvanized sheet. A good absorber is needed to raise the temperature of the collector. Sean (2013) describes a good absorber as one that absorbs almost all the incoming radiation, lose a small portion to the surroundings and efficiently transfer the absorbed solar energy to the passing air within the collector.

iii.) Thermal Storage Materials: The storage material collects heat during sunlight exposure (high insolation) and dissipates during low sunshine, cloud cover and rains. A thermal storage (gravels) was integrated with one of the collectors and the other has only a plain absorber.

iv.) Drying Cabinet: It houses the wire mesh trays in which the yam slices were spread on. At the side of the cabinet, chimneys were provided to facilitate and control the convective flow of air through the dryer. The two opposite sides of the drying chamber were glazed to collect additional solar radiation. This further increases the convective air flow and additional heating necessary for drying. Access door to the drying chamber was also provided. The interior of the drying cabinet was painted black to enhance elevated temperature within the dryer.

v.) Tray: The black painted trays were constructed from strong wire mesh that can be easily slotted and removed for maintenance purpose. The trays were designed for easier cleaning, loading and offloading of the yam slices.

vi.) Support: The supporting structure bears the load of the drying cabinet and the air heater. It was constructed from mild steel material.A sketch of the front view of the mixed-mode solar crop cabinet dryer is shown in figure 3.1.

CHAPTER FOUR

RESULTS AND DISCUSSION

Simulated and Experimental Results for the Solar Dryers

Reading of solar insolation I(W/m²), Ambient temperature Ta (°C), Collector air temperature Tc (°C), Temperature of air into different Trays in the drying chamber (°C), and moisture loss were measured during the experiments. The experimental data obtained were tabulated in Tables B.1, B.2,B.3,B.4,B.5,B.6,B.7,B.8,B.9,B.10,B.11, and B.12, as shown in appendix B. Table B.13,B.14, and B.15 are the simulated results also shown in appendix B.Figure 4.1,4.2, 4.3, 4.4, 4.5, 4.6,4.7,4.8,4.9, 4.10,4.11,4.12,4.13,4.14, 4.15, and 4.16were drawn from the results in Tables B.1, B.2, B.3, B.4, B.5,B.6,B.7,B.8,B.9,B.10,B.11,B.12,B.13,B.14 and B.15.

CHAPTER FIVE

SUMMARY, CONCLUSION AND RECOMMENDATIONS

Summary

The solar dryers were designed, simulated, constructed, and tested at Ahmadu Bello University, Zaria using the meteorological conditions of Zaria, Nigeria.

The dryer was designed to dry 7 kg of yam slices. The dimensions of the dryer are: Collector length = 0.65m, collector area = 0.277 m², Height of the drying chamber, =0.9,total chimney height, = 2.7 m, Length of the drying chamber, =

1.64 m and Width of the drying chamber, W= 0.43 m. However, a section of 0.5 m of the dryer along the dryer length with chimney height of 0.7 m each were fabricated and tested with 2 kg of yam slices in each of the drying chambers.

The dryers‟ conditions were simulated using TRANSYS 16.0 and MATLAB R2013a Softwares. Typical Meteorological Year (TMY) solar data of Zaria obtained fromwww.solaranalytical.com was processed to obtain the monthly average daily solar resources of Zaria using the solar radiation and weather data processor TYPE 109 component of TRANSYS 16 software. Programmes were written using MATLAB software to generate the weight loss of the yam slices in the drying chamber. The solar dryers were tested one with and the other without thermal storage materials at the Mechanical Engineering Workshop ABU Zaria, Kaduna State, Nigeria from 18^th to 20^th of June 2016. The hourly percentage moisture loss (wet basis), drying rates, system efficiencies and collector‟s efficiencies were all calculated from the results of the testing.The dryer with the storage materials gives better drying efficiency.

Conclusion

The performances of the solar collectors were simulated under varying meteorological conditions of Zaria. TRANSYS 16.0 software was used to simulate the collectors‟ with and without thermal storage outlet air Since a drying model is not available in the TRNSYS library, a MATLAB code was written to computes the weight loss of the yam slices in the drying chamber. The computed NSE values and its corresponding RMSE between the modeled collector air temperature and weight loss of yam slices and the observed collector air temperature and weight loss of yam slices with and without thermal storage for 19^th and 20^th of June, 2016 confirms that the model formulation using TRANSYS 16 and Matlab Softwares used for the performance simulation of the system is valid and realistic owning to the good quality of fit between the experimental and the simulated results.
The performances of the drying systems were evaluated using the percentage moisture loss, drying rate, collector efficiency and drying efficiency. The average drying rates, collector efficiency, and drying efficiency of the drying system with and without thermal storageare 82× 10⁻⁵ kg/s and 2.55× 10⁻⁵ kg/s, 78.25 % and 42.20 %, 29.15 % and 25.35% respectively. Maximum collector outlet air temperature of 66°C and 52°C at 872W/m² for collector with and without thermal storage under no load conditions were obtained. Temperatures of 48°C and 42°C at 818 W/m²and 43°C and 41°C at 755 W/m²for second and third days of drying for collector with and without thermal storage under load test were measured. The collector with the storage materials shows increase in both the collector and drying efficiencies. During the experiment, the results in table B.11 shows the drying rates do not vary much with the coordinate positions of the trays. This is because the inlet drying air temperature goes to the individual trays without picking moisture from lower bed first to the upper bed. The air ducting provided serves this purpose.

Recommendations

The developed dryers can help in mitigating the problem ofnon-uniform drying. Therefore, Government should encourage local farmers on the use of the improved solar dryers.

The dryer performance can be improved by using flexible air distributing duct to reduce the pressure drop across the duct, this will increase the drying
The dryer can be tested using different thickness of yam slices to get the optimum value under the meteorological conditions of
The performance of the dryer can be improved by optimizing the design.

REFERENCES

Aasa, S.A, Ajayi O.O and Omotosho O.A (2012). Design Optimization of Hot Air Dryer for Yam Flour Chunk. Asian Journal of Scientific Research, 5(3): 143-152
Abdullahi, A. F., (1995).Development and Performance Evaluation of an On-Farm Forced Convection Solar Dryer for Tomato Drying. (Unpublished) M.Sc. Thesis Department of Agricultural Engineering, Ahmadu Bello University, Zaria. Nigeria.
Abhishek, S. and Varun, G., (2013). Solar Air Heaters with Thermal Storages. Chinese Journal of Engineering,2013 (11).
Adzimah, K.S, and Seckley, E., (2009). Improvement on the Design of a Solar Cabinet Drier. American Journal of Engineering and Applied Science, 2(1): 217-288
Ajao, K. R., and Adedeji, A.A.,(2008). Assessing the Drying Rates of Some Crops in Solar Dryer. Journal of Research Information in Civil Engineering, 5(1):1-12
Alamu, O.J., Nwaokocha, C.N and Adumola, O.(2010). Design and Construction of a Domestic Passive Solar Food Dryer. Leornado Journal of Sciences,9(16):71-82
Amma, A. S. (2014). Effect of Different Drying Methods on the Functional and Physicochemical Properties of the Flour of Selected Yam Cultivators in Ghana. (Unpublished) M.Sc. Thesis Department of Food Science and Technology. Kwame Nkrumah University of Science and Technology, Kumasi Ghana.

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