Convective Sun Dryer from PMB to farmers
Paddy harvesting Sri Lanka is now done mostly by mechanical harvesters. This is done before the grains are well dried to reduce shattering losses. Moisture content of machine harvested paddy is typically about 22 per cent. Drying of this wet paddy to 14 per cent before storage is a basic requirement but farmers face difficulty drying due to shortage of facilities, 
Drying facilities are best provided at Paddy Marketing Board (PMB) stores rather than at farmer households because it allows use by many farmers. It reduces handling, dry paddy can be sold to PMB and it also increases PMB’s procurement.
PMB has provided concrete drying yards at some stores for farmers to dry moist paddy within the store premises. The 1,000 m2 drying yard recently constructed at the Weerawila store which cost of over Rs. 5 million, can dry about 12,000 kg of wet paddy on a sunny day from 22 per cent to 14 per cent. High labour requirement for unloading, spreading, mixing and bagging is a common issue with sun drying yards.
Sri Lanka receives about 6 kWh of heat energy per square meter per day. Solar energy falling on 1000 m2 Weerawila drying yard, amounting to 6,000 kWh is sufficient to dry about 100,000 kg of wet paddy. However, it dries only 12,000 kg of paddy in practice. The drying efficiency is about 12 per cent and this low efficiency is due to several reasons.
Paddy is spread on the drying yard in a layer about 25 mm thickness. Paddy grains on the top receive thermal energy from the sun and get heated, but the heat does not pass to grains deep in the layer because paddy is a poor thermal conductor. The top layer reaches about 60 C during mid-day while the bottom remains little above ambient. The top layer, by virtue of its high temperature, loses about 50 per cent of the heat to the ambient by radiation and convection.
This phenomenon limits the thermal efficiency of sun drying process to about 30 per cent. Mixing the paddy layers several times during the day helps improve the efficiency but it still remains low. Further, it is labour intensive and adds to the cost. High temperature experienced by the top layer can induce cracks and reduce the milling yield, another disadvantage of conventional sun drying.
Sun dryer
The convective sun dryer was developed to increase the efficiency of sun drying process and to produce a higher quality product. The dryer is a trough with a porous deck placed on a plenum connected to a suction fan.
Paddy is placed on the porous deck exposed to direct sun. The fan draws air through the paddy. Heat received by the top layer of paddy heats the air and then dries lower layers of paddy. Top layer does not reach high temperature hence losses by radiation is minimised. Convective losses are prevented entirely.
Ambient air has a high relative humidity in the morning but air gets warmer and drier by mid-day. Warm dry air has a high drying potential. The convective sun dryer uses this potential effectively for drying paddy. The contribution to drying duty by ambient air is usually higher than the contribution by direct solar energy falling on the dryer.
A prototype convective sun dryer was constructed at Halabawa PMB store in Anuradhapura District in March 2020. This unit was designed to hold 2 tonnes of paddy, harvest of 1 acre field and also a typical tractor load. This load was estimated to dry in about six hours on a sunny day. The direct cost of construction of the dryer was about Rs. 90,000. A concrete drying yard for the same duty costs over Rs. 600,000 and has about 15 times the footprint.
In operation, the labour cost is much lower compared to sun drying. There is no need for intermittent mixing. Spreading and collection are much easier. The cost of electricity is only Rs. 100 per 2 tonne batch.
Trials conducted at Halabawa verified the design predictions. More important than performance was the end user response. The farmers who provided paddy and used the dryer were happy with the outcome and foresaw wide acceptance by the farming community.
The top layer of paddy in the dryer does not get heated like in open sun drying. Therefore, it produces superior quality paddy that will yield a higher milling outturn. The dryer is inexpensive, simple to construct and operate. The dryer is sized to suit the average farmer. Wide application of this dryer has the potential to benefit the local paddy-rice economy. It can also be used dry other product like maize and soybeans.
Dryers available to farmers
Seven dryers are made available to farmers in seven regions of the PMB during the present Yala harvest. The response from the farmers will be used to decide on the wider use of this technology. They can get details from PMB officials in the region it the use of these dryers.
One issue that arose during initial use of this dryer was the suggestion to have a roof cover for using this dryer in rainy weather. This is not recommended because with high humidity of air and lack of sunshine, drying will not be effective on rainy days. Rainy weather is a drawback in all forms of sun drying. A tarpaulin is kept close-by to use as an emergency rain cover during sporadic rains
There are two distinct drying problems faced by paddy farmers. First is drying of moderately wet mechanically harvested wet paddy on sunny days. This issue is addressed by the convective sun dryer. The second is drying wet paddy during rainy days. Large number of mechanical dryers available in rice mills that are not fully utilised can be used to address this issue. A programme to implement this will be launched in this season on a limited scale.
Another issue was the interest by farmers to build these simple dryers in their homesteads. This involves cutting down trees to provide open area for placing the dryers, that will used only for a few days of the year. A much better alternative is to build multiple dryers that can be used by many farmers at PMB stores where large open areas are available.
The convective sun dryer was initially developed by the author for a different wet material and was used successfully on a large scale. It received the Sri Lanka Patent 9094 in 1983 which has long expired. The construction of these seven dryers did not involve government funds.
(The writer can be reached at jatalm@live.com)