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The use of solar energy in post-harvest technology

O.P. Singhal and G.E. Thierstein. Farming Systems Research Programme, International Crops Research Institute for the Semi-arid Tropics, Patancheru. India

Household cooking


The present crisis of fossil fuel shortages for producing thermal energy is creating increasing emphasis on the use of solar energy. Solar energy is freely available and is abundant in India for most of the year It is non-polluting and inexhaustible. This paper describes different solar energy systems developed in India with their technical and economical feasibility criteria for the post-harvest technology of cereal crops. These include drying, storage, processing, and household cooking in order to save costly fuels now used for these operations.

Post-harvest technology is a multidisciplinary field and includes various treatments and operations carried out on harvested crops for the purpose of preservation or enhancement of quality for marketing and consumption. In these operations, a considerable amount of produce is lost that needs to be minimized. The factors responsible for these losses include inefficient harvesting, threshing, handling, drying, processing, and cooking.

Solar equipment costs are generally high, relative to fossil fuel equipment costs, but operating costs are much lower. The variability of solar energy also limits its usefulness. However, as fossil fuel supplies dwindle and costs continue to rise, solar energy should become economically feasible for many applications in agriculture.


With the introduction of high-yielding varieties, fertilizer and better land and water management practices, crop yields have increased substantially (from 52 mill. t. in 1951/52 to 130 mill. t. in 1979/80) in India. However, the benefits of increased crop production are lost if management is lacking to handle this increased production with minimum grain losses during the post-harvest period.

Post-harvest technology is a multidisciplinary field and includes various treatments and operations carried out on harvested crops for the purpose of preservation or enhancement of quality for marketing and consumption. The major operations are threshing, drying, storage, processing, packaging, transport, and cooking. In these operations, a considerable amount of produce (about 10 to 15 per cent for cereal grains and 30 to 40 per cent for pulses) is lost, which needs minimization (Ojha 1976). The factors responsible for the above losses of food grains include inefficient harvesting and threshing, handling, drying methods, poor processing techniques, and inadequate cooking methods.

Solar energy can be used in many of the above post-harvest operations to save costly fossil fuel now in use. Some of these are crop conditioning, hot water production, and steam generation for crop processing and household cooking. Solar-powered cold storage is also technically feasible (Pandya et. al. 1980).

Solar energy is free, non-polluting, and inexhaustible. Based on the data available with the Indian Meteorological Department, the average solar radiation for India is about 420 cal/cm2-day ranging from 323 cal/cm2-day at Gulmarg to 511 cal/cm2-day at Jodhpur. The total energy received in India is estimated to be 60 X 10(16) kwh/year (Bhide 1975).

Although solar energy is free, the equipment required to collect and use it is not. Solar energy utilization devices and processes in general follow one of three pathways; (a) utilizing heat from the sun (thermal devices), (b) converting solar energy directly into electrical energy (photovoltaic), and (c) using photosynthetic and biological processes (biological). Major operations in post-harvest technology use heat from the sun at low temperatures (40 C-90 C). These devices have been developed to a stage at which they can be used on a large scale immediately.

Most of the devices depend upon collection of heat by either a flat plate collector or through concentrating systems and utilize thermal energy to heat a working substance (air, water or some other fluid) to do a desired function. The majority of these thermal devices use flat plate collectors that are simple and stationary. Concentrating systems, although capable of giving higher temperatures, are costly and complicated as they have to track the sun. A well-designed flat plate collector system is capable of attaining temperatures of 80-90 C at 70 per cent collection efficiency.

The development of suitable solar energy systems is a potential means of utilizing solar energy most effectively if based on least-cost criteria designs. Solar equipment costs, relative to fossil fuel equipment costs, have slowed solar development. But as fossil fuel supplies dwindle and fuel costs continue to rise, solar energy collection should be economically feasible for many applications in agriculture. Solar energy can reduce our dependence on fossil fuel, but at the present level of technology its availability is too variable and too limited to completely replace fossil fuel.

This paper describes different solar energy systems developed in India with their technical and economical feasibility criteria for the post-harvest technology of cereal crops. These include drying, storage, crop conditioning, processing, and household cooking.


All the cereal crops give higher yields (10-15 per cent) if harvested at optimum moisture content, which is slightly higher than the usual harvest moisture content (Ojha 1976). This reduces shattering losses in the field and transport losses from the field to the threshing yard. It also enables the farmers to plant a second crop after the rainy season crop with the available moisture in the soil (ICRISAT 1981). This early harvest needs adequate drying facilities to reduce grain losses as a result of rotting, discoloration, sprouting of grains, pilferage, bird attack, etc.

The most common method of drying cereal crops is to spread the material on the ground or on a raised platform (sometimes on a metal road) in thin layers to expose it to the sun's radiation. Although this method has the advantages of low cost, it suffers from various drawbacks. One has to depend entirely upon the weather conditions so that in rainy or damp weather, drying is unduly delayed causing spoilage of the material. Even in fair weather, drying is non-uniform, because the top layer dries faster than the bottom one. Considerable losses accrue to the produce due to birds and spillage, and contamination occurs with dirt, dust, and surrounding foreign materials.

The drying of cereal grains on small farms can be done either by natural drying or convective drying using heated or unheated air to remove the moisture from the produce. The use of a natural ventilation system is practical for maize cobs harvested at about 20 to 25 per cent moisture content and stored temporarily in cribs (Boshoff 1979). Drying with unheated air is done under favourable weather conditions using heat from the atmospheric air.

Mechanical drying with heated air is of quite recent origin in India and is used on a very limited scale because of the relatively high initial cost (Ramarao 1974). The system consists of an air-heating system (electrical heater, furnace or burner), a fan, and the drying chamber with appropriate airducts. This method offers the advantage of quick, timely, and uniform drying of grains. A mechanical dryer occupies much less space and is suitable to match large-capacity processing plants (Ramarao 1974).

A solar dryer uses a solar energy collector for heating the air. The drying system has separate areas for collection of solar energy and for drying products. The heat is transferred from the air heater to the drying chamber either by convection or forced circulation of air. A flat plate solar collector can be used to dry agricultural produce efficiently and economically during sunny as well as overcast periods (Singhal 1979).

Several types of solar dryers with natural convection as well as with forced convection have been developed and reported in the literature. A solar dryer with natural convection is slower in performance than the forced convection type and thus has more limited capacity (Singhal and Gupta 1975). However, it is much cheaper to operate.

A 0.5-ton low-cost solar dryer has been developed at the Asian Institute of Technology, Bangkok (Exell and Kornsakoo 1979), which is capable of drying paddy in batches, every few days. A similar dryer using metallic construction has been developed at the Energy Center of the Indian Institute of Technology, Delhi. The major drawback of these devices has been that the entire harvest cannot be dried at once and wet paddy waiting to be dried may spoil.

Small portable batch driers have also been developed in India using forced convection. A trolley-type batch dryer has been designed at Punjab Agricultural University, Ludhiana (Pandya et al. 1980).

A 1-ton-per-day-capacity solar dryer has been designed, fabricated, and installed at the Food Corporation of India Modern Rice Mill at Manlur, Chidambaram (Tamilnadu), to dry parboiled paddy. The collector for air heating on the roof is approximately 58 m2 (8.9 m x 6.6 m). The air flow rate is about 50 m3/min with an average recorded temperature of 15 C above ambient. Hot air is delivered by an air blower into the drying unit. A 5-ton per day capacity drier based on the above design was installed at the Central Farm Corporation, Ludhiana, Punjab, to dry paddy harvested during the rainy season.

At ICRISAT a solar crop dryer using a single glass-covered flat plate solar air heater was designed during 1977/78 in co-operation with the Mechanical Engineering Department, Punjab Agricultural University, Ludhiana, Punjab. The dryer (fig. 4) was designed to dry one ton of grain from a moisture content of 24 to 14 per cent in one day with an overall drying efficiency of 30 to 40 per cent for a solar radiation intensity of 500 cal/cm2-day. The dryer consists of a solar air heater of 50 m2 collector area (10 m x 5 m), a 2.2 kw electrical motor-operated blower, a 1-ton capacity drying bin and ducts to guide the air from the heater to the drying bin. The collector air-heating passage was designed for 1 kg of air per per min. The entire solar air heater was mounted facing south and inclined at an angle of about 22 to the horizontal. Due to mechanical problems, only limited tests could be performed on this dryer from November 1978 to June 1979.

The solar dryer was evaluated for its performance on clear days in November and December 1978. Sorghum grain at about 20 per cent moisture content was dried in the month of December 1978. Also sorghum and chick-pea (1 ton each) at moisture contents of 29 and 20 per cent respectively were dried in the months of February and March 1979 to a moisture content of 12 per cent. A maximum rise in temperature of 21 C above ambient was attained in the month of February; other months gave a rise of 18 to 20 C above ambient temperature at the designed air flow rate between 12 noon and 3 p.m. on days of bright sunshine. The results of sorghum and chick-pea drying are presented in figures 5 (see FIG. 5. Drying of Sorghum Grain Using Solar Energy. Date = 1-2 March 1979; variety = sorghum CSH-6; ambient temperature = 28 + 2 C; hot-air temperature = 40 5 C, overnight tempering = ) and 6 (see FIG. 6. Drying of Chick-pea Grain Using Solar Energy. Date = 22 February 1979; ambient air temperature = 33 1 C; hot-air temperature = 50 5 C). The chick-pea was dried from 29 to 12 per cent moisture content during a total drying period of 10 hours (two days) with overnight tempering. The solar air heater efficiencies varied from 51 to 62 per cent depending upon the time of year. The highest efficiency was observed-during December to March, although the outlet air temperature was low during these months compared to hot months like May and June with lowest thermal efficiencies.

The economics of solar drying, compared with the most common methods of drying such as sun drying and mechanical drying using different air heating systems, is presented in table 1. The systems are compared for drying 1 ton of grain from 24 to 12 per cent moisture content in a day. in situations where the systems are used for either 30 days or 150 days in a year on a farm and an agro-industry respectively.

It is clear from table 1 that sun drying has a low cost of operation as well as low capital investment and hence it is popular, but it does suffer from such inherent drawbacks as spillage of grain from unexpected rains, contamination with dirt and dust, etc. The mechanical dryer using solar energy-heated air comes next with regards to initial investment as well as cost of operation.

TABLE 1 Economics of Grain Drying Using Different Methods


Cost of operation for:


Initial cost

30 days

150 days




Open floor sun drying in one day




Mechanical drying using artifically heated air


Direct oil fired burner  



Coal fired furnace with air heat exchanger  



Husk fired furnace with air heat exchanger  



Husk fired furnace with steam heat exchanger  



Mechanical drying using solar energy





Rice is the staple food in India. More than 50 per cent of the paddy produced is parboiled. The basic requirements of parboiling are soaking, steaming, and drying of paddy (Ali Nawab 1974). A modern parboiling process developed at the Indian Institute of Technology, Kharagpur. utilizes hot water for soaking at 70 to 80 C for 3.5 hours, thus eliminating the steaming process, after which the soaked paddy is dried. Parboiling is done for more than 200 days in a year. It is estimated that with the use of solar energy. a saving of Rs 30,000 (US$3,300) per year can be made for a plant of 1-ton-per-day capacity (Singhal 1979).

A pilot plant of about 100 kg/day capacity for parboiling and drying was designed and tested to determine the feasibility of solar energy utilization (Singhal and Gupta 1974). The parboiling unit consists of a parabolic cylindrical concentrator 3.6 m long and 1.86 m wide, with a focal length of 0.46 m and four well-insulated brick masonary parboiling tanks. The water heated to the required temperature by the concentrator was directed to the parboiling tanks filled with raw paddy. The mixture was allowed to soak for 3 to 4 hours.

The drying system was designed so that the roof of a farmer's house or small rice mill shed can be converted into an air heater. A flat plate air heater of 10.8 m2 surface area was used to heat the atmospheric air. The soaked parboiled paddy was dried at an air temperature of 60 5 C with an air flow rate of 15.25 m3/min.

TABLE 2. Performance of the Paddy Parboiling Systema

Date Time required to parboilb Parboiling
From To
(fur) (fur) (fur) (%) (%) (%)
23 April
10.30 14.00 3.30 35.5 41.4 85.6
13.00 17.00 4.00 32.1 41.4 77.7
25 April 10.00 13.15 3.15 37.2 41.4 89.9
25 April 13.00 17.00 4.00 32.0 41.4 77.3
13.00 16.30 3.30 35.6 41.4 86.1

a. Water temperature: 75 + 2C, number of tanks: 4 (2 at a time)
Insulation thickness: 8 cm; capacity of each tank: 8 kg.
Type of insulation: rice husk; size of concentrator: 3.6 m x 1.8 m.
Reflection material: aluminium sheet; time of investigation: April 1976.
b. Excluding the time required to heat the water by the concentrator.
Source: Singhal 1979.

TABLE 3. Drying Performance of Solar Energy Operated Drying Unit

Time of day Solar
Drying air
Rate of
(h) (cal/cm2 min) (C) (C) (%) (%) (%)
21 May 1976
12.00 1.40 39.5 56.5 54.36 - -
13.00 1.34 41.0 59.0 43.38 11.00 97.50
14.00 1.28 42.0 60.5 33.25 9.60 82.60
15.00 1.16 43.0 600 27.40 640 64.40
16.00 0.98 42.0 59.0 22.30 5.10 52.40
Overnight tempering, 22 May 1976
9.00 1.18 35.5 55.0 23.45 - -
10.00 1.24 36.5 56.0 18.15 5.30 47.60
11.00 1.25 38.0 58.0 16.50 1.65 14.40
Average efficiency (%)   67.40    
Average air-heater efficiency (%)   65.66    
Overall system efficiency (%)   44.25    

Source: Singhal 1979.

The parabolic cylindrical concentrator was able to heat the water to between 70 to 80 C during the collection period, giving a rise of 50 to 55 C above ambient. The efficiency of heat collection was about 55 per cent. The performance of the parboiling system is shown in table 2.

The performance of the solar air heater in drying parboiled paddy is given in table 3. The average drying rate was 5 per cent moisture loss per hour for a 100 kg grain sample, and the total yield and head yield were 75 per cent and 69 per cent for JR-8 variety paddy. The total drying time was 8 3/4 hours. An average drying efficiency of 67 per cent was attained for the system.

The mechanical strength (crushing hardness), milling index (brokers percentage), optimum cooking time and swelling index for cooked rice of the parboiled rice thus produced were intermediate between raw and hot soaked rice as shown in table 4.

TABLE 4 Quality Data of Rice Obtained by Different Parboiling Methods

Method of parboiling Crushing hardness Brokens Optimal cooking time Swelling index




Raw 6.10 22.50 25 00 3.71
Solar energy parboiled 6.60 9.80 27 00 3.75
Hot water soaked 7.40 7.52 31 00 3.81
Steam parboiled 8.60 7 00 41 00 3.85
Raw 5.90 34.25 22.00 3.51
Solar energy parboiled 6.40 10.05 23 00 3.70
Hot water soaked 7.20 10.05 23 00 3.70
Steam parboiled 8.20 7.20 45 00 3.85

Source: Singhal 1979.

A proposed layout of a parboiling plant for a mini-rice mill of 2 ton/day capacity, based on the above investigations, is shown in figure 9 (see FIG. 9. Proposed Layout of Paddy Parboiling System (capacity 2 tonnes/day). 1. Aluminium foil reflector (2 m x 4 m), 2. receiver pipe - 8 cm dia. (black-coated aluminium). 3. 25 mm dia. wheel valve, 4. 25 mm G.I. pipeline well insulated, 5. parboiling tank, 6. thermostat valve, 7. water tank (Singhal 1979)) and figure10 (see FIG. 10. Solar-energy-operated Mechanical Dryer for Parboiled Paddy (capacity: 2 tonnes per day)). The economics of such a system, in comparison with the conventional method has been worked out (Singhal 1979) and is presented in table 5.

TABLE 5. Economics of Solar Energy Operated Parboiling Plant With Conventional Methods (2 ton/day)(a)

(A) Initial investment and area required  
Conventional method Rs 35,000
Solar energy operated Rs 32,000
The effective reflecting surface area for solar energy operated system is  
75 m2 per ton of paddy.  
Mechanical drying  
Mechanical dryer Rs 40,000
Solar energy system Rs 32,000
Collector area of 100 m2/ton of paddy at 14 per cent moisture content is required assuming the drying unit efficiency as 40 per cent.  
(B) Cost of operation  
Parboiling cost Rs/ton
Conventional method of parboiling 34
With coal and fuel 34
With husk as fuel 24
Solar energy 19
Mechanical drying (using artificially heated air)  
Steam heat exchanger with coal fired boiler 31
Steam heat exchanger with husk fired boiler 18
Air heat exchanger with husk fired furnace 19
Direct oil fired burner 25
Solar energy operated mechanical drying 14

a. The cost estimates were made in 1979. Source: Singhal 1979.


Most storage losses of cereal and pulses are caused by insect damage, mould and heating due to excess moisture. Often loss occurs because of moisture accumulation in grain, even though it may be dry when in storage (Shrivastava 1975). It is important that the product is properly stored after it is dried. Stored grain is usually given a chemical treatment to reduce insect infestation for effective storage. Insect infestation can also be prevented by exposing grain to heat treatment or placing it in cold storage. A solar heat treatment machine for destroying initial infestation in maize and other grains has been developed at the University of Udaipur, Rajasthan (Pandya et al. 1980). Technically it is feasible to have a solar-powered cold storage plant to maintain a temperature of 4 to 5 C. but this system has a high initial cost. A 0.5-ton capacity solar powered cold storage unit has been developed at the Indian Institute of Technology, Bombay (Sukhatma et al. 1979).

Household cooking

About 150 million tons of firewood are burnt yearly for cooking in the rural areas of India by the poor sections of society (Garg 1978). The ovens and chulas (fireplaces) used are quite crude and inefficient. Due to scarcity of firewood, the rural population is switching to kerosene as the domestic cooking fuel. This tendency needs to be curbed to conserve foreign exchange reserves, which is why solar cookers or ovens are being developed in this country.

India is a pioneer in the development of solar-cookers. The first cooker was produced in India in 1950 and became commercially available in the 1960s but it did not become popular at that time due to the cheap availability of cooking fuels. However, demand is being felt now. The cookers are becoming popular in Gujarat and Maharashtra states. Two models of solar cookers have been developed. The conical shaped sun-basket, made from fibre glass-reinforced plastic (FRP), serves as solar cooker and baking oven by concentrating the sun's rays at a point where a temperature of 110 C can be achieved within a period of 5 to 10 minutes on clear sunny days. The initial cost of this basket is about Rs 600. The reflector needs to be rotated to track the sun.

The hot box type solar oven works as an air-tight box with double glass covers, in which one or more cooking containers can be placed. It requires more time than the sun basket to cook food but it is able to maintain the temperature for a longer duration, making it useful during intermittent sunshine. The initial cost of this oven is about Rs 500. The major problems being experienced in the adoption of these cooking devices are:

  1. It is difficult to produce a solar cooker at competitive prices for indoor kitchen use. No housewife prefers to cook in the sun.
  2. About 70 per cent of the rural population cook their meals in the early morning before going to work and in the night after returning home. which is not possible by the use of solar cookers. It will be necessary, therefore, either to change cooking habits or to provide low-cost heat storage.
  3. Solar cookers and ovens require more time to cook meals than do conventional methods, and sometimes it becomes difficult to cook even at noon when radiation is very low. This variation is not acceptable to most families.


The use of solar energy is technically feasible in major post-harvest operations affecting cereal crops. Although the total solar energy available in India is immense, the cost of capturing this in usable form is not small. At the present level of technology, the capital cost is high. There is still scope for modifying solar devices to make them acceptable to the farmers and small-scale agro-industries.


Ali, Nawab. 1974. "Studies on Some Engineering Aspects of Parboiling of Paddy." Unpublished Ph.D. thesis, Indian Institute of Technology, Kharagpur.

Bhide, V.G. 1975. "Solar Energy Utilization in India and Abroad." Proceedings, Seminar on Industrial Applications of Solar Energy National Productivity Council, 4-6 June. Madras.

Boshoff, W.H. 1979. "On-farm Maize Drying and Storing Systems for the Humid Tropics FAO/DANIDA, African Rural Storage Centre, International Institute for Tropical Agriculture, Ibadan, Nigeria FAO Technical Report, Rome.

Exell, R.H.B., and Sommai Kornsakoo. 1979 "Solar Rice Dryer." Sun World, 3(3): 75.

Garg, H.P. 1978. The State of Solar Energy Research for Rural Development. Bhagirath, 25 April, 1 -1.

ICRISAT (International Crops Research Institute for the Semi-Arid Tropics). 1981. Farm Power and Equipment Report of Work, 1978/80. ICRISAT, Patancheru. Andhar Pradesh.

Ojha, T.P. 1976. Inaugural Address. Institute on Prevention of Grain Loss by Post-harvest Technology. Indian Institute of Technology, Kharagpur.

Pandya, A.C., Nawab Ali, and Al Alam. 1980. Post-harvest Technology in India." Central Institute of Agricultural Engineering Technical Bulletin, CIAE/80/11, Bhopal.

Ramarao, V.V. 1974. "Studies on Mechanical Drying of Parboiled Paddy." Unpublished Ph.D. Thesis, Agricultural Engineering Department, Indian Institute of Technology, Kharagpur.

Singhal, O.P. 1979. "Utilization of Solar Energy for Parboiling and Drying of Paddy." Unpublished Ph.D. Thesis, Indian Institute of Technology, Kharagpur.

Singhal, O.P., and C.P. Gupta. 1974. "Design of Paddy Parboiling System Using Solar Energy." Proceedings of Conference on the Utilization of Solar Energy, 22-24 Dec. 1974, Allahabad.

___. 1975. "Solar Drying. " Proceedings, Seminar on the Industrial Applications of Solar Energy National Productivity Council, 4 6 June, Madras.

Shrivastava, P.K. 1975. Bulletin of Grain Technology, 9(3): 177-185.

Sukhatma, S.P., S.G. Khandilikar, G. K. Sharma, and S. Arun Kumar. 1979. " Performance Studies of a Flat Plate Collector Along With Booster Mirrors and Associated Hot Water Thermal Storage Tank." Proceedings, National Solar Energy Convention. Indian Institute of Technology, Bombay.

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