Evaluating the optimum load range for box-type solar cookers

摘要

This paper introduces a new concept of Optimum Load Range (OLR) for solar cookers. OLR gives the load values for which cooker preferably shows good thermal as well as good cooking performance; it may be considered a crucial parameter for solar cookers. This OLR concept is based on the dependence of rate of rise of load temperature on different heat transfer processes between load and cooker interior. This concept illustrates solar cooking in two simple steps. The total time required to complete these steps puts an essential constraint for cooking of any load amount. The maximum value of load (upper limit of OLR) till which cooker shows satisfactory cooking may be determined from this constraint. This constraint requires determination of two OLR parameters which are t_(step Ⅰ) and t_(step Ⅱ). The load for which cooker remain almost 30% efficient, may be referred as lower limit (minimum value) of OLR. For the verification of OLR, experimental studies have been conducted with a solar cooker named SFSC. The OLR parameters along with different thermal performance parameters (TPPs) (second figure of merit (F_2), utilization efficiency (η_u) etc.) suggested by different researches for solar cookers in water load condition have been computed from the measured thermal profiles of different loads (0.8-3.0 kg). From the curve analysis of different TPPs with load, the existence of upper limit of OLR is observed. The values of rate of rise of load temperature at water temperatures 80,85 and 90 "C for different loads also confirm the same. The OLR of SFSC is found to be 1.2-1.6 kg.