DESIGN OF A SOLAR/LPG DRYER UNIT

Transcription

1 DESIGN OF A SOLAR/LPG DRYER UNIT For the design of the solar dryer we will concentrate on a low-cost solar dryer that can be built in rural area from almost any kind of available building materials and by locally available workmen. When the sun is not able to heat the dryer the dryer will be powered by LPG. First a literary research on the available solar dryers on the market was done. Different solar dryers were found. They can be organized in direct and indirect solar dryers. The direct solar dryers will make use of the sun to heat the drying chamber directly, whereas an indirect solar dryer will have collector that will be heated by the sun. The hot air from the collector will be transferred to the drying chamber. Then there are also passive and active solar dryers. Active dryers mean that hot air gets transferred by the use of a fan, whereas passive dryers depend on wind speed in the air vents. Then there are mixed mode dryers with combine the direct and indirect methods. All of these different dryers have their own advantages and disadvantages. After these we looked into the specifications and advantages of all the solar dryers and analyzed what the best options for the solar dryer would be. The results and the design for the solar dryer can be seen and read below. The calculations can be found at the end of the report. PRINCIPLE OF A DRYER Firstly the principle of the solar dryer is explained so that further expanse in the report can be followed. In the process of drying relative and absolute humidity are of great importance. Air can take up moisture, but only up to a limit. This limit is the absolute humidity, the maximum, is also dependent on temperature. When air passes over the sheaths it will take up the moisture until it is fully saturated. Because the humidity is dependent on the temperature, the capacity of the air for taking up this moisture will grow with the rising temperature. If air is warmed, the amount of moisture in it remains the same, but the relative humidity falls. Thus the air is therefore enabled to take up the moisture from the sheaths. CONSIDERATIONS With the design of the solar dryer there are some considerations that need to keep in mind to make a working design. These considerations are listed below. The amount of moisture that needs to be removed, approximately 10% The material to be dried is the sheath from the areca palm leaves. The raw sheaths have a moisture content of 14%-17%. (Kalita, et al. 2008) The required moisture content needs to be above 5%, as it will result in cracks in the material. This means that roughly 10% of moisture needs to be removed from the product. The harvesting period, thus temperature and humidity The harvesting period for the sheaths is all year round. As temperature does not really vary over the seasons this can be considered to be around 32 C C all around the year. (Information World Weather and Climate sd). The humidity will change around the year; it will differ from 28% in the dry season till 88% relative humidity, because of the monsoon. It will be a difficult task to dry the leaves during the monsoon time, as then an alternative energy source for drying the sheaths will be needed. The quantity of air needed The quantity of air needed to dry the leaf sheaths will determine the size of the dryer chamber. The quantity of air needed can be calculated by taking the moisture content of the leaf sheaths and the temperature inside the drying chamber. Daily solar radiation and daily sun hours Byse is located in Karnataka, India, at a latitude of North and longitude of, 75 0 East. Solar radiation over the year on horizontal surface in Byse is found to be W/m2 by the National Solar Radiation Database. So the total radiation on a 13 tilted surface is calculated as Wh/m2. It will have a daily sun hour rate of 8 hours per day all year round. The size and weight of the sheaths For the size of the sheaths were taken; maximum length: 1000 mm, maximum width: 350 mm. The maximum weight of the sheaths has been chosen as 1 kg.

2 The scale of the operations, the budget, the use of local materials and skills The design of the solar dryer will be designed to be easily put together by local craftsmen and the use of local craftsmen. The design will be made with certain materials if afterwards the chosen materials are not available there is thought of other material options. The quantity of sheaths to needs to be dried The quantity of sheaths the machine will dry at once, in two days maximum will be around the thirty. So in a month more than 400 leaf sheaths can be dried. RESEARCH Other research made us seen some other important data that may improve the dryer or are important for the features of the dryer. Also a filter needs to be installed to prevent bugs and dust to cause harm to the leaves. A reflective surface on the inside will keep the heat inside. But a reflective surface next to the collector can increase the efficiency of the dryer. The vents need to be closed by night, to prevent or reduce thermo siphoning and rehydration. Aluminum screens can be used as an collector. The sides of the drying chamber need to be insulated to prevent the solar radiation and decrease the heat loss. The leaf sheaths are not favourable to change under the circumstances of sun. This makes a mode of direct solar drying also possible. It would be the most profitable when the leaf are dried standing. This because the part of the sheath that needs to be dried the most will be on the bottom and thus get the most dry air but also because the storage capacity will increase. REQUIREMENTS Efficiency: The solar dryer needs to be more efficient than the normal drying process. For the solar dryer we want to achieve an efficiency of about 50%. Durability: We want to achieve a durability of the solar dryer of 5 years. In this five years regular adjustment and maintainance need to be held. Since the dryer has to be stay outside in the monsoon we need to think about materials can handle the humidity and rain. Sustainability: The solar dryer needs to be as sustainable as possible, also to ensure the sustainability of the plates. Capacity: The capacity needs to be as high as possible in a maintainable storage space. Easy to use: The local villagers need to be able to work with the solar dryer and preform maintainence if needed. Easy to build: The villagers need to be able to build it themselves Easy to maintain: The materials used in the solar dryer need to be locally available so if there is need of new materials the local villagers can get them easily for a low price. Needs to work on LPG when the sun is not there. Airthight: to prevent leakage of heat and thus efficiency of the solar dryer, there needs to make sure the solar dryer is airthight.

3 THE DESIGN The design and is parameters can be seen below. The dryer consist of 3 parts, the collector, the drying chamber and the kiln. Figure 1: Overview solar dryer Figure 2: Side view solar dryer

4 The collector The collector exists of a cover plate an absorber plate and an insulator, these will all be build into a frame of wood. The cover plate is a plate to keep the heat inside, it needs to let the light through thus is must be transparent, but it also needs to trap the heat inside so it must have the right thermal properties. Using a clear solid, like a glass cover, will allow light to enter, but once the light is absorbed and converted to heat, glass cover will trap the heat inside. This makes it possible to reach similar temperatures on cold and windy days as on hot days. So the cover plate can be made of glass, but if glass is not available thin film, fiberglass or a plastic are also options. Glass will be the best option since this has the best properties to get the light in and hold the heath. But glass may be hard to obtain, fiberglass is the next best option since it is strong and durable. The cover plate will let the light through, the absorber will collect this light and transform it into heat. To improve the collector it must be painted black, this will improve the effectiveness of turning light into heat. The collector can be made from any black painted metal sheet. A sheet of galvanised iron is the best option since this is also used in the building of houses and thus may be the easiest to get our hands on. This plate can be put parallel to the cover plate but is can also be perforated and be put under a small angle so the amount of the air that can be heated will grow. This seems like the best option. The black surface paint must be able to withstand repeated and prolonged exposure to high temperatures without appreciable deterioration or out gassing, this needs to be checked when buying the paint. The dimensions of the collector were found to be; length: 2000 mm and width 1000 mm. See calculations. This means that the glass area is also 2 square meters, same for the absorber. The glass sheet is most efficient for a thickness of 4 or 5 mm. The collector will be tilt over an angle of 23, to catch the most of solar radiation. This is determined by 10 + the North latitude. The collecter must be faced north to south to make sure the sun will fall on the collector. Everything must also be isolated to reduce the loss of heat. To insulate it, also the drying chamber, aluminium foil can be used or polystyrene. This will be put on the bottom of the collector and also on the sides of the collector and drying chamber The heated air leaving the collector is immediately driven into the drying chamber, this does not hold the help of a external fan it is a properties of heated air itself. Drying chamber The heated air will want to get to the top of the drying chamber. The design of the solar drying chamber is the most important part, because the air flow depends on the design of chamber. Careful considerations has made that we did not choose a step pattern but just to stack the trays on top of each other. This may slow down the drying process of the leaves on the top but it will leave the best quality. The drying chamber is a rectangle shaped box with the dimensions of; length: 1000 mm, width: 2000 mm and height: 1300 mm. The drying chamber can be opened by two doors that can open by hinges, when the trays are in the drying chamber these doors can be shut with a lock. The hot air will then leave through the chimney. A chimney has been put at the top because air velocity will increase with the height of the chimney. The wind speed in Byse could not be determined, to be sure that there is an air stream through the collector and drying chamber there will be a chimney Calculations about the height of the chimney can not be made. But we expect the height to be 200 mm, this has to be decided by trial and error. The same goes for the air vents at the bottom of the collector. The kiln The kiln can be made or a little stove can be connected to the bottom of the drying chamber. The hot air will then proceed from the stove to the drying chamber by a pipe. When working with a stove connected to the drying chamber it is important to not get the smoke into the drying chamber. This can be done by taking a pipe system at a lower point, beneath the burning of the mass, and connecting this to the dryer chamber. Another pipe can be used as a chimney for the smoke. Calculations for this method has not been found or made yet.

5 PHYSICAL FEATURES The physical features will be about the dryer type, size, shape, collector area, tray area, number of tray and drying capacity. The choices made will also be explained. Type The dryer will be an indirect passive dryer. We chose for this type of dryer because of the advantage that is does not have an effect on the quality of the leaves. Studies also showed that the indirect dryers are also more efficient, just slightly though. We choose not to use an external fan, this because than electricity needs to be used which may be a rare resource in the villages. If the dryer does not work as well without a dryer we can still incorporate this in the design. When the sun is not shining the dryer will be heated with a little LPG or coal powered kiln. This kiln will be then attached to the bottom of the drying chamber via a pipe, so the heated air can get into the drying chamber. Collector area The collector area was found to be 1.9 square meters, see calculations. We have transformed this to 2.0 square meters since it is easier to work with. Dimensions The dimensions and size can be seen in the design. In the calculations the explanation for these can be found. Tray At first it seemed that the most profitable way to put the leaf sheaths upwards in the drying chamber. This will save up room so more leaf sheaths can be dried at once. Since the bottom of the sheaths is the thickest part it will hold the most of moisture. When drying then upwards the heated and thus dry air will pick up the most moisture here, this can be valuable solution.but since it is not done before we will not know if this drying method will be more efficient. When drying the leaf sheaths flat it is also more easy to keep track of the quality and the drying process of the leaf sheaths. When drying them flat it can be beneficial to slightly overlap the leaf sheaths, when done correctly it will not influence the drying. The number of trays to fit into the drying chamber will be four, the space between these racks is found to be 200 mm. With this height between the racks we give the air enough space, but not that much air that it is able to escape or get cold. The frame of the trays will be made from wood and covered with a mesh, this may be a mosquito net or any other kind. The number of trays will be four and the size of the trays will be; length 1000 mm and width Since this may be a bit large it is easiest to divide the drying chamber in half so there will be eight trays of 1000x1000 mm. The trays will stand on a small rail made out of a piece of wood. Drying capacity The drying capacity is somewhere above 30 sheaths in two days. This means that in one month about 400 leaf sheaths can be dried. PRODUCT QUALITIES Appearance Due to the indirect drying system the appearance of the of the leaves will not be damaged. Over drying and rehydration capacity Over drying may happen, but since the drying is mostly for the storing this is not a problem. Also there is a greater chance that the leaf sheaths will get to the humidity equilibrium, a equilibrium with the humidity of the air and the moisture in the sheath. After the drying the leaf sheaths will be cleaned and soaked in water which will make them regain some of the moisture lost. When the sheaths are over dried, their are likely to get moisture back from the water or from the humidity of the storage space. ECONOMICS For the economical features of the dryer, there has been looked at the costs of making the dryer, the costs for drying, payback and requirements needed for space or skills.

6 Cost of dryer including costs of drying The costs of the materials need to be made on site, since we do not know what is available and what will be the price of those materials. The materials chosen are quiet cheap and easy to get, so we do not expect sky high costs. The payback of the costs of the dryer will be done by the selling of the dinnerware. Seen there has not been looked at the business plan for the selling, no confirmation can be made about the payback. Floor space requirements The floor space required is about 3000x3000mm of open air space. When the dryer is placed under the trees, if can not get its full efficiency. Skilled personal requirements The requirements for using the dryer are not high. Once in awhile the dryer needs to be checked on quality, this will also been seen in the quality of the leaf sheaths and the dryer needs to be checked for any unwanted air vents or other leakage of heat. And for the correct drying process the leaf sheaths have to be laid down correctly. If at the installation of the drying chamber the method for correct drying will be shown, the villagers of Byse can easily do it themselves afterwards since it is not that hard or technical. And the villagers themselves will have the best eye to see when the sheaths are dried enough. As safety requirements it will be recommended to wear gloves when taking the trays out of the dryer chamber, as these trays can be heated by the hot air. For the thermal performance features there can be looked at the calculations.

7 CALCULATIONS For the design of the solar dryer first the calculations were done for the solar collector. The drying chamber will have an area proportional to the solar collector. The properties Areca palm Sheath 14%/17% moisture content >5 % moisture to prevent cracks in the material Weight : g Length x width: X cm Drying rate (assumption) Weight moisture content (average) : M i M f m w = m p = 0, M f = 0,702kg m w= moisture weight [kg] m p = Initial mass of the product (20 sheaths) M i = Initial moisture content on wet basis [%] M f = final moisture content on wet basis [%] Ideal drying time: 2 days, 18h m dr = m w t d = 0,702kg 18 = 0,039 kg m dr = Drying rate [kg/h] t d = drying time [h] Mass of air needed for drying M = Drying rate: 0,039 kg/h Rel. humidity 27 % m w m p m w = o With the psychometric chart : Moisture content on dry basis 0, ,702 Humidity ratio initial at 32 C : 0,0085kg water/kg dry air Humidity ratio final(assumption 50 C):0,010 kg water/ kg dry air water = 0,0023 kg dry solids kg M= moisture content on dry basis [kg water/kg dry solids ] Water activity a w= 1 exp[ exp 0, ,5639 ln M = 0,078 a w = ERH ERH = 7,8 100 Mass flow rate of air m = m dr 0,039 = w f w i 0,010 0,0085 = 26 kg a w = water activity [-] ERH = equilibrium relative humidity m = mass flow rate of air [kg/h] w f = humidity ratio final Kg H 20/ kg dry air w i=humidity ratio initial Kg H 20/ kg dry air

8 Temperature: From January to May the average temperature is 32 C Solar radiation 23 tilted surface is calculated as Wh/m2 ->18,473 MJ/m 2 /day Psychometric chart: o Ti = 32 C -> hi = 35 kj/kg o Tf = 50 C ->hf =52 kj/kg Area of the collector A c Iη = E = m f i t d = = 17,238 kj A c = E Iη = 17,238 = 1,87 m2 18,473 0,5 h = enthalpy of moist air [ J/kg] A c = Area of the solar collector [m 2 ] E = total usefull energy received from drying air [KJ] I = the global radiation on the horizontal surface during drying period [ kj/m 2 ] η = collectors efficiency M A = Area of the vent 1 w f w i m w = V = M ART p = 1 0,010 0,0085 0,702 = 468 kg 468 0,291 ( ) = 410,51m 3 101,3 V a = W a t d =/2,3 P = the atmospheric pressure [101,3 KPa] V = the volume of air [m 3 ] M A=the mass of the air [kg] T = the absolute temperature [K] R= the gas constant [0,291 kpa m 3 /kg K] A v= area of the vent [m 2 ] V a= volumetric airflow rate [ V w= wind speed [m/s] V a = W a t d =/2,3 A v = V a V w =/2,3