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Optimisation of energy consumption of a solar-electric dryer during hot air drying of tomato slices
| Content Provider | Semantic Scholar |
|---|---|
| Author | Nwakuba, Nnaemeka Reginald |
| Copyright Year | 2019 |
| Abstract | High-energy demand of convective crop dryers has prompted study on optimization of dryer energy consumption for optimal and cost effective drying operation. This paper presents response surface optimization of energy consumption of a solar-electric dryer during hot air drying of tomato slices. Drying experiments were conducted with 1kg batch of tomato samples using a 3 Central Composite Design (CCD) of Design Expert 7.0 Statistical Package. Three levels of air velocity (1.0, 1.5 and 2.0ms), slice thickness (10, 15 and 20mm) and air temperature (50, 60 and 70C) were used to investigate their effects on energy consumption. A quadratic model was obtained with a high coefficient of determination (R) of 0.9825. The model was validated using the statistical analysis of the experimental parameters and normal probability plot of the energy consumption residuals. Results obtained indicate that the process parameters had significant quadratic effects (p < 0.05) on the energy consumption. The energy consumption varied between 5.42kWh and 99.78kWh; whereas the specific energy consumption varied between 5.53kWhkg and 150.61kWhkg. The desirability index method was applied in predicting the ideal energy consumption and drying conditions for tomato slices in a solar-electric dryer. At optimum drying conditions of 1.94ms air velocity, 10.36mm slice thickness and 68.4C drying air temperature, the corresponding energy consumption was 5.68kWh for maximum desirability index of 0.989. Thermal utilization efficiency (TUE) of the sliced tomato samples ranged between 15 ≤ TUE ≤ 58%. The maximum TUE value was obtained at 70C air temperature, 1.0ms air velocity and 10mm slice thickness treatment combination, whereas the minimum TUE was obtained at 50C air temperature, 2.0ms air velocity and 20mm slice thickness. Recommendation and prospect for further improvement of the dryer system were stated. Ac ce pt ed p ap er Introduction Tomato (Lycopersicon esculentum) is a perishable and seasonal fruit vegetable grown and widely eaten in Nigeria and across the globe for its good health benefits such as reduction of cholesterol, improvement of vision, maintenance of gut, lowering of hypertension, alleviation of diabetes, protection of the skin, prevention of urinary tract infections and gallstones. It is characterized by being in good quality, rich in minerals, vitamins, organic acids, high moisture (usually above 85% wet basis), crude fibre, high lycopene, ascorbic acid and flavonoids (Abano et al., 2014). In Nigeria, tomato yields about 20 – 50 tonnes ha in every harvesting season. Eke (2013) reported that 20 – 60% of tomato produced in Nigeria rot away annually. These losses give rise to short supply and high prices during the off-season. This necessitates the need for efficient and adequate preservation techniques to increase its shelf life. Owing to its seasonal and perishable characteristics, drying becomes a good preservation alternative in order to increase its availability. Its drying is facilitated by slicing and spreading out the product to increase their surface area to hot convective air using a reliable heat source and increasing the airflow around the product. In recent times, dried tomatoes have become a highly attractive product for both domestic and industrial purposes which resulted in increased product demand. This is because the lycopene content, characteristic red colour, non-enzymatic browning and vitamin A (ascorbic acid) content are considered as the most vital quality criteria (Abano et al., 2014). Considerable amount of energy is consumed by convective dryers to dry most freshly harvested agricultural products to safe moisture level as a result of their relatively high moisture content (70 to 95% wet basis) at harvest which requires long drying time, low thermal conductivity during the falling rate drying period which inhibits convective heat transfer to the inner sections of the product structure, relatively low energy efficiency of dryers, and high latent heat of water evaporation(Nwakuba et al., 2016, Motevali et al., Ac ce pt ed p ap er 2014). This high energy consumption, however, has significant impact on the dried product quality such as its nutritional values, shrinkage and other organoleptic properties (Darvishi et al., 2013). Dryer energy consumption is a vital technical information applied for optimal and cost effective design and operation of drying systems as well as adequate meeting of safe storage conditions of crops (Nwakuba et al., 2016). Energy consumption has been identified to vary with crop type, moisture content at harvest, final desired moisture content, specific heat capacity of crop, latent heat of vaporization of water, intended use, gross mass, size, shape, and biological characteristics (such as surface texture, crop porosity, nutritional content), drying times, production capacity, drying air temperatures as well as operating pressure and dryer efficiency (Billiris et al., 2011). Considerable energy savings in drying application can be achieved through partial or full replacement of conventional fuels by renewable energy sources. Extensive research regarding energy consumption of crop dryers has been prompted by the considerable energy consumption in the drying industry, as well as concerns for cost of drying agricultural products, its impact on the food supply chain, and environmental effects like increased prevalent ambient air temperature, increase in greenhouse gas, air pollution, etc. (Koyuncu et al., 2007; Nwakuba et al., 2016). Other reasons prompting the study of energy consumption include: estimating the quantity of fossil fuel saved when using solar energy and the quantity of CO2 emitted into the atmosphere (Tripathy and Kumar, 2009); estimation of the optimum quantity of drying air temperature, air flow, and drying time most appropriate for a particular crop so as to avoid undermining the functional and sensory properties of the product; applicability in the design of appropriate cost and energy effective drying system which would require minimal quantity for any crop type; and for simulation of drying systems. The use of solar energy as a practical power source for crop drying has been stimulated in recent times due to shortages of oil and natural gas fuels and increase in the cost and Ac ce pt ed p ap er depletion of fossil fuels (Nwajinka and Onuegbu, 2014). This power source has been harnessed for heating, cooling, drying, irrigation, pumping, and other numerous thermal processes in food industries (Itodo et al., 2002). Over 90% of agricultural products are sundried in Nigeria and in most African countries (Arinze et al., 1990). Unfortunately, much of this commonly available, renewable and affordable energy from the sun is wasted due to lack of adequate technology to harness it. The daily and seasonal fluctuations in solar radiation as well as its frequent absorption by rain and persistent cloud cover in most parts of the country and the world at large have hampered the optimal use of the Sun's energy for crop drying operation and therefore necessitate the additional use of energy source that permits drying operation during low irradiation and night periods. Researchers Ferreira et al., 2007; Sarsavadia, 2007; Nwakuba et al., 2017) have incorporated electricity as a viable auxiliary source of energy in solar drying systems due to its non-polluting characteristics, ease of usage and high heat density. The increased emphasis on rural development in Nigeria which undoubtedly will necessitate increase in energy demand in the rural sector for drying and other agricultural processes, makes the use of solar-electric dryers cost effective and environmentally friendly. Reducing the energy consumption in these systems irrespective of the crop to be dried would grossly improve the dryer economy. Since the sliced tomato price and quality are functions of energy consumption during drying, it is essential to select optimal drying variables that would yield minimal energy and carbon footprint on the natural environment while keeping the nutritional quality of the sliced tomato unabated and intact with minimal deterioration. The objectives of this study is to investigate the effects of the drying variables (air velocity, sample thickness, and drying air temperature) on the total and specific energy consumption of a solar-electric dryer and to optimize the energy consumption of tomato slices during hot air drying. Ac ce pt ed p ap er Materials and methods Sample preparation A local variety of fresh tomato samples (Gboko Spp.) were procured from a fruit market in Owerri, Imo State, Nigeria. The samples were selected based on uniform colour, and carefully sorted to remove damaged or septic ones, and classified according to relative size, washed and sliced in three layer thicknesses (10, 15 and 20 mm) using a sharp stainless steel knife and a vernier caliper (accuracy 0.05mm) with the direction of cutting perpendicular to the vertical axis of the tomato samples. The initial mass of the sliced samples was measured by a digital weighing balance (of accuracy 0.01 g; Camry instruments, China) and the samples placed on drying racks in such a way that the drying air flows axially into the sample matrix (for faster drying) in thin layers. The mean initial moisture content of 19.57 kg water/dry weight of the samples was determined gravimetrically measured by drying 20g of representative sliced samples at 105C for 24 hours (Koyuncu et al., 2007; Darvishi et al., 2013). Drying system and experimental procedure The solar-electric dryer (Figure 1) was switched on and the required air flow and temperature were selected using a 4 x 4 matrix keypad panel on the control unit. The nucleus of the crop dryer is the arduino microprocessor which controls the overall operation of the system and automates temperature and humidity control, air flow, sample weight loss, and energy consumption through the use of weight |
| Starting Page | 150 |
| Ending Page | 158 |
| Page Count | 9 |
| File Format | PDF HTM / HTML |
| DOI | 10.4081/jae.2018.876 |
| Volume Number | 50 |
| Alternate Webpage(s) | https://agroengineering.it/index.php/jae/article/download/876/756 |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
