Solar-assisted smokehouse for the drying of natural rubber on small-scale Indonesian farms

Natural rubber in Indonesia is mainly produced by smallholder farmers and—being the highest foreign currency generating agricultural commodity—is of great importance for the Indonesian economy. Nevertheless, due to the lack of appropriate dryers, more than 80% of the total production has to be sold as low-grade Standard Indonesian Rubber (SIR) for a relatively low price.In order to improve the product quality, a solar-heated rubber sheet dryer was developed. It consists of a flat-plate solar air heater connected to a drying chamber. Part-recirculation of the exhaust air leads to the desired drying air temperature of 45–60°C and results in a significant reduction of the drying time. To enable drying independent of weather conditions, a biomass furnace was incorporated underneath the drying chamber.The tests have shown that during the rainy season, 320 kg of sheets can be uniformly dried to the required moisture content of 0.5% within 5 days. Compared with conventional smokehouses, the firewood consumption could be considerably reduced, from 1.0–1.5 kg per kg dry rubber to 0.3 kg when producing Ribbed Smoked Sheets (RSS). A further reduction can be expected by optimization of the heating intervals. During favourable weather conditions Air Dried Sheets (ADS) can be produced within 6–7 days without supplemented heat. The quality of the RSS and ADS fulfills the international standards.The simple design of the solar-assisted smokehouse allows local production and—after further improvements—seems to be an economical alternative for farmer groups or nucleus estates.