Withering of tea leaves using heat. A pipe heat exchanger utilising low-temperature geothermal energy

The volume of Indonesian tea exports to the European Union (EU) decreased by 43% in 2014 because of the EU setting a maximum residue limit of anthraquinone (AQ) for tea as 0.02 mg/kg. The content of AQ in tea leaves increases during the process of the withering and drying, especially when there is incomplete combustion of firewood, the energy source for withering and drying of tea leaves. This study aims to develop and test a new concept for the direct use of low-temperature geothermal energy with a heat pipe heat exchanger (HPHE) for the withering of tea leaves as a solution for energy sources free from AQ. The geothermal fluid simulators used water heated by a 9000W heater and flowed by a pump. The HPHE consisted of 42 heat pipes and 181 fins. The heat pipe was 700mm long with an outer diameter of 10mm; the fluid in the heat pipe worked using water with a filling ratio of 50%. Each fin was made of aluminum with a thickness of 0.105mm and a size of 76mm × 345mm. The results show that the effectiveness of the HPHE varied from 66% to 79.59%. For 100g of fresh tea leaves, the heating energy produced ranges from 15.21W to 45.07W, meaning it can wither tea leaves from 80% to 54% of the wet base (w.b.) in a time varying from 11h 56m to only 49.6 minutes. The Page mathematical model is the best model to represent the behaviour of the tea leaves with this HPHE system. The use of an HPHE is a new concept for the direct use of low-temperature geothermal energy for the withering of tea leaves, which is acceptable and feasible to use as renewable energy in the tea-making industry and as a substitute for fossil fuels.