Long Term Performance of Small Scale Indirect Solar Dryer

Abstract

Strictly speaking, all forms of energy on the earth are derived from the sun. However, the more conventional forms of energy, the fossil fuels received their solar energy input eons ago and possesses the energy in a greatly concentrated form. These highly concentrated solar energy sources are being used as such at a rapid rate that they will be depleted in not-too distant future.

Drying is a part of many conventional food preparations. It also helps to impart special taste and store food items for longer periods. Solar dryer technology is simple and therefore easily adoptable by households and small communities. A solar dryer is an enclosed unit which allows the solar isolation to pass through a glazing and get absorbed. The heated surface, in turn, heats up a draft of air which then flows across/through food items and leads to their drying. Drying is dependent on two fundamental processes: mass and heat transfer. In the indirect type solar dryer, heat has to be first transferred from the heated absorber plate in the collector to the air draft. Heat has to then transfer from the flowing hot air to the moist material in the drying chamber which is then followed by moisture removal to attain the desired moisture level of the product. The effectiveness of drying depends both on the rate of drying and the extent of drying.

Hence, various food materials (aamla, methi, ginger, garlic, onion, potato chips and grapes) were efficiently dried using specially designed indirect solar dryer and its rate of drying and extent of drying were calculated. Also, nutritional values for the dried grapes were analysed.

For mathematically designed dryer, the drying capacity was 12 kg food product per day and area of collector was 3.088m2. Specially designed V-trough were arranged for an enhancement of collectors efficiency which was estimated to be ηc = 51.2%. Aluminium painted with black nickel as a absorber plate was used and thermocole as a insulating material with tempered glass for cover plate.

Performance analysis of solar dryer is carried out for no load condition in open loop as well as in closed loop. The steady state mathematical model based on heat balance concept of solar dryer without load is applied to identify the dimensionless parameter called no-load performance index (NLPI). For load condition in open loop, different food materials are dried in dryer as well as in open sun drying.