Rural Applications of Solar Energy. Course: Rural Energy Systems RDL 722 1

Transcription

1 Rural Applications of Solar Energy Course: Rural Energy Systems RDL 722 1

2 Solar Drying

3 Importance of solar drying The world population is more than 7 billion and about million people do not have enough food to eat. There are three methods to solve hunger problem: Increase food production Reduce population growth Reduce loss of food during and after harvesting It has been estimated that world as a whole more than percent food grains and percent vegetables, fruits/fish etc. are lost before it reaches to the consumers. Drying is a traditional method for preserving food. Solar drying is an effective method to preserve food. Solar energy is diffuse in nature and thus suitable for crop drying, locally available and thus saves transportation, solar dryers can be made locally of any size and capacity and solar dryers are economical if cash crops are dried.

4 Advantages in Favour of Crop Drying It permits early harvesting and reduces the field losses of the products, Reduces the risk of field losses caused by wild animals, It permits better planning of harvesting season, It reduces spoilage in storage drastically, It permits the farmer to sell his product at better price during early period of harvesting season, Quality of the product gets enhanced significantly and hence farmer gets more money for his product, and Transportation is easy with dried product.

5 OPEN SUN DRYING It is the simplest method of drying used in most developing countries. The food / crop is spread out in the open under the sun. Advantages (1) No technology involved (2) Very low cost (cost of labour only) Disadvantages (1) Contamination of the product due to dirt and insects. (2) Wastage by birds / mice. (3) Spoilage due to sudden and unpredicted rain. (4) There is no control of temperature over crop drying. (5) Overdrying may cause loss of germination power, nutritional changes, sometimes complete damage.

6 CLASSIFICATION OF SOLAR DRYERS DIRECT TYPE DRYERS : In direct or natural convection type dryers, the agricultural product is placed in shallow layers in a blackened enclosure with a transparent cover. The solar radiations are directly absorbed by the product itself. The food product is heated up and the moisture from the product evaporates and goes out by the natural convection/circulation. INDIRECT TYPE DRYERS : In these dryers the food product is placed in a drying chamber. The air is heated in solar air heaters and then blown through the drying chamber. In some of the designs, dryers receive direct solar radiations and also heated air from solar air heaters. In these dryers manipulation of temperature, humidity and drying rate is possible to some extent. FORCED CIRCULATION TYPE DRYERS : In these dryers, hot air is continuously blown over the food product. The food product itself is loaded or unload continuously or periodically. These kind of dryers are comparatively thermodynamically efficient, faster and can be used for drying large agricultural product. These dryers can be of Cross-flow type, concurrent flow type or counter-flow type.

7 TYPE OF SOLAR DRYERS (c) Forced circulation type solar dryers (a) Direct type solar dryers (b) Indirect type solar dryers

8 Natural Convection or Direct type Solar Dryer These dryers appear to be more attractive for use in developing countries since these do not use fan or blower to be operated by electrical energy. These dryers are low in cost and easy to operate. Some of the problems with these dryers are: slow drying, no control on temperature and humidity, small quantity can be dried, and some products change colour and flavour due to direct exposure to sun. Several direct type dryers are fabricated, tested, and analysed in many countries. The simplest direct type solar dryer is solar cabinet dryer.

9 Solar Cabinet Dryer The solar cabinet dryer in its simple form consists of a wooden (or of any other material) box of certain width and length (length is generally kept as three times its width), insulated at its base and preferably at the sides and covered with a transparent roof. The inside surfaces of the box are coated with black paint and the product to be dried is kept in the trays made of wire mesh bottom. These trays loaded with product are kept through an openable door provided on the rear side of the drier. Ventilation holes are made in the bottom through which fresh outside air is sucked automatically. Holes are also provided on the upper sides of the dryer through which moist warm air escapes. This dryer has given encouraging results and reduced the drying time by one third compared to open sun drying.

10 Details of solar cabinet dryer

11 Photograph of Solar Cabinet Dryer

12 Mixed Mode Type Solar Dryer In the mixed mode type of solar dryers, the solar air heater with or without any electric fan along with a drying bin is used. Such simple mixed mode type solar dryer was developed at AIT Bangkok for drying paddy and therefore named as rice dryer. It consists of a solar air heater made of a frame of bamboo poles and wire covered with 0.15 mm thick transparent PVC sheet. The ground is covered with burnt rice husk which absorbs the solar radiation and heats the air in contact. The hot air in this air heater rises to the drying chamber which either consists of transparent PVC sheets on bamboo frame absorbing directly the solar radiation or a bamboo frame covered from all the four side with some opaque material. The drying material (rice etc.) is kept on the nylon net tray in thin layer through which hot air heated from air heaters enters its bottom and goes up into the chimney.

13 Mixed Mode Type Solar Dryer (contd.) The chimney is a long cylinder made of bamboo frame covered with black PVC to keep the inside air warm. There is a cap at the top of the chimney, leaving some space in between chimney top and cap to allow warm humid air to go out and protecting the product from rain and other foreign materials. The height of the chimney and the hot air inside it creates a pressure difference between its top and bottom thereby creating forced movement of air through the rice bed to the top of the chimney. The drying rate will depend on the depth of the bed, initial moisture content of the material, solar insolation, ambient temperature, and the design of the dryer.

14 Cross section of chimney type solar dryer

15 Photograph of Solar Rice Dryer

16 Auxiliary energy may be supplied either by electric heating or oil or gas burners and used only when solar air heaters or the heat from the thermal storage device is not sufficient to supply necessary energy for drying the product. Forced Circulation Type Solar Dryer As the name implies, such dryers use some kind of one or several electric operated blower/exhaust fan to circulate air between air heater/storage bin/drying chamber. Such dryers are more efficient, faster, reliable, preferred and can be used for drying large quantities of agricultural products. These dryters can be used at low as well as at high temperatures and used for drying large quantities of product. These dryers are of bin type, tunnel type, belt type, column type, or rotary type. Some forced circulation type solar dryers use some kind of thermal storage unit, heat recovery wheel and auxiliary heating arrangement.

17 Forced Circulation Type Solar Dryer (contd.) Several storage systems are proposed but the most preferred one is the rock bed storage system which stores the heat in the form of sensible heat and performs the dual function of storing the heat and that of a heat exchanger. A hybrid solar dryer (solar assisted) was developed at Fresno, California for drying large amount of fruits and vegetables. It consists of several solar air heaters with a total area of 1350 m 2, a thermal storage (rock bed type) of 350m 3 volume, a rotary wheel type heat recovery whell and a tunnel dehydration in which 14 trucks loaded with prepared food move at a rate of 24 hours per truck in one direction and the heated air is sent from the other direction. The system is designed for a fixed air flow rate of 9.5m 3 /s to the dehydrator 24 hours a day. The drying temperature varies from 60 C in the beginning of June to 66 C in August to September. The solar contribution in this hybrid systems is 1582 MJ/hr which is about 60 per cent of the total heat requirement of drying.

18 Photograph of Forced Circulation Type Solar Dryer

19 Leather Dryer with Roof mounted Solar Air Heaters (4 x 167m 2 area) at Ranipet, Chennai

20 Typical grain drying bin

21 SOLAR DESALINATION

22 Solar Desalination Systems Water is one of the most abundant resources on earth, covering three-fourths of the planet's surface. About 97% of the earth's water is salt water in the oceans; 3% of all fresh water is in ground water, lakes and rivers, which supply most of human and animal needs. The only nearly inexhaustible sources of water are the oceans Their main drawback, however, is their high salinity. It would be attractive to tackle the water-shortage problem with desalination of this water.

23 WATER DESALINATION TECHNOLOGY Nature is carrying out the process of water desalination since ages. Oceanic water due to solar heating converts into vapours and pours down as precipitation on earth in the form of fresh water. Water is the most needed substance on the earth for sustenance of life. Due to rapid expansion of population, accelerated industrial growth and enhanced agricultural production, there is ever increasing demand for fresh water. Demand of fresh water (potable water) has increased from litres/person/day to litres/person/day. The ocean covers 71 recent of the earth's surface-140 million square miles with a volume of 330 million cubic miles and has an average salt content of 35,000 ppm. Brackish/saline water is strictly defined as the water with less dissolved salts than sea water but more than 500 ppm.

24 SOLAR DESALINATION TECHNIQUES Potable Water Requirement Sources of Potable Water Less than 550 ppm Domestic, Industries and Agriculture Rivers, Lakes, Ponds, Wells etc. Demand of Potable Water litres / person / day (OLD) litres / person / day (NEW) Underground Saline Water Sea Water 2,000 2,500 ppm 30,000 50,000 ppm

25 WATER DESALINATION TECHNOLOGY Potable water (fresh water) suitable for human consumption should not contain dissolved salts more than 500 ppm. For agricultural purposes, water containing salt content of 1000 ppm is considered as the upper limit. Potable water is required for domestic, agriculture and industries. Some applications in industries like cooling purposes, sea water is feasible despite the corrosion problems while other industries use higher quality water than is acceptable for drinking water. Modern steam power generation plant need water with less than 10 ppm. Potable/fresh water is available from rivers, lakes, ponds, wells, etc. Underground saline/brackish water contains dissolved salts of about 2,000-2,500 ppm.

26 METHODS OF CONVERTING BRACKISH WATER INTO POTABLE WATER DESALINATION: The saline water is evaporated using thermal energy and the resulting steam is collected and condensed as final product. VAPOR COMPRESSION: Here water vapour from boiling water is compressed adiabatically and vapour gets superheated. The superheated vapor is first cooled to saturation temperature and then condensed at constant pressure. This process is derived by mechanical energy. REVERSE OSMOSIS: Here saline water is pushed at high pressure through special membranes allowing water molecules pass selectively and not the dissolved salts. ELECTRODIALYSIS: Here a pair of special membranes, perpendicular to which there is an electric field are used and water is passed through them. Water does not pass through the membranes while dissolved salts pass selectively. In distillation; thermal energy is used while in vapour compression, reverse osmosis, electrodialysis, etc. some mechanical and electrical energy is used.

27 Schematic of basin-type solar still

28 COMPONENTS OF SINGLE EFFECT SOLAR STILL 1. Basin 2. Black Liner 3. Transparent Cover 4. Condensate Channel 5. Sealant 6. Insulation 7. Supply and Delivery System

29 SOLAR STILL OUTPUT DEPENDS ON MANY PARAMETERS 1. Climatic Parameters I. Solar Radiation II. Ambient Temperature III. Wind Speed IV. Outside Humidity V. Sky Conditions 2. Design Parameters I. Single slope or double slope II. Glazing material III. Water depth in Basin IV. Bottom insulation V. Orientation of still VI. Inclination of glazing VII. Spacing between water and glazing VIII. Type of solar still

30 SOLAR STILL OUTPUT DEPENDS ON MANY 3. Operational parameters I. Water Depth II. III. IV. Preheating of Water Colouring of Water Salinity of Water V. Rate of Algae Growth VI. PARAMETERS Contd Input Water supply arrangement (continuously or in batches)

31 Double sloped experimental solar still

32 MULTIBASIN STEPPED SOLAR STILL 1. In this still small basins are put in steps like a stair case and named as multibasin system. 2. The still consists of saline water trays in an airtight box with glass cover, charging "pump, and distillate collecting tray. 3. The projected basin area of the still is m2 and the glass cover is inclined at 45 degree. The depth of water in tray was 5.0 cm. 4. This still has not given higher output because of higher heat loss.

33 Multibasin stepped solar still

34 WICK TYPE SOLAR STILL ADVANTAGES 1. Low thermal capacity resulting in higher output, 2. Receives and absorb more solar radiation because of optimum inclination. PROBLEMS 1. Chocking of pores of wick with salt in due course of time, 2. Deterioration of the wick cloth, 3. Decolouration of wick cloth, 4. Difficult to maintain uniform flow of water.

35 Cross section of titled multiple wick type solar still

36 Regenerative inclined step solar still

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38 Photograph of the multi effect H-D Solar Distillation unit at IIT Delhi

39 Multiple effect diffusion solar still

40 SOLAR COOKING

41 BOX SOLAR COOKER

42 Parabolic Domestic Solar Cooker

43 Principle of working of Scheffler Community Solar Cooker

44 World's Largest Solar Steam Cooking System at Tirupati, Andhra Pradesh Location Installed at the temple town of Tirumala, Andhra Pradesh with nearly 50 percent funding from MNES System Employs automatic tracking solar dish concentrators to convert water In to high pressure steam which Is used for cooking purpose* In the community kitchen. Technical Details Solar dish concentrators (106 Nos) with total reflector area of about 1000m 2. Modular in nature and consists of several units (parallel & series) connected to central pipeline system. Each dish consists of scheffler mirrors with an aperture area of 9.4 sq.m. Generates 4,000 kg of steam per day at 180 C and 10 Kg/cm 2. Cook meals for around 15,000 persons per day. The cooker saves about 1,20,000 litres of diesel per year. The total cost of the system Is about Rs. 110 lakh. Implementing Agency Ministry of Non-Conventional Energy Sources (MNRE).

45 World s Largest Solar Steam Cooking System

46 WORLD S LARGEST SOLAR STEAM COOKING SYSTEM AT TIRUPATI

47 Solar Cookers Solar cookers are used and developed in several countries including India. In India all the three types of solar cookers like box type, reflector type (SK type) and large size (Scheffler type) are in use. Typical box type solar cooker takes hours for cooking of food and is 60x60x17 cm in size. Parabolic domestic solar cooker (SK-14) with reflector diameter 140 cm gives about 200 C temperature and cooks food for 8-10 persons at a time. Scheffler community solar cooker with single or multiple dish (each of about 10 m 2 reflector) is suitable for community use and one such solar cooker for cooking meal for people is in use at tirupati, A.P with total reflector area of 1000 m 2.

48 Thank You