## Lesson 18. Performance evaluation of solar cooking

Solar Cookers

Solar cookers are used to cook food using solar radiation as input energy.  Solar radiation is being collected as direct radiation and/or reflected radiation/concentrated radiation.  Using absorbers these radiations are converted into thermal energy.  This heat energy is used to cook food which is kept in the vessels inside the solar cooker.

Energy required for cooking

The energy required for cooking a specified food varies with the properties of ingredients such as water, vegetables etc.  The method cooking such as the temperature required for cooking, minimum temperature to be maintained during cooking, duration of temperature maintenance etc.  are to be considered at the time of cooking.

Thermal energy required for reaching the cooking temperature can be calculated from the following formula.

Q = m  cp  dt, kJ

Where,

Q = Thermal energy requirement for cooking, kJ

m = mass of material taken for cooking, kg

cp = specific energy for the selected material, kJ kg-1 °C-1

dt  = difference between initial and final temperature, °C

The specific heat energy of various materials are given below.

Example

Calculation of energy requirement for cooking 1 kg of rice.  Consider cooking of rice in Aluminium vessel weighing about 0.5 kg.  Water used for cooking rice is 2 kg.  The required heat energy is the sum of thermal energy required for heating rice, water and also the vessel material.  The initial ambient temperature is 30°C and the required cooking temperature is 100°C.

Total thermal energy required for cooking rice  = (m1cp1 + m2cp2 + mcp3) x dt

Where suffix 1 for rice, 2 for water and 3 for vessel material

Hence the total energy is calculated as

=             {(1 x 3.8) + (2 x 4.18) + (0.5 x 0.9)} x (100-30)

=             882.7 kJ

To maintain the same temperature for through cooking, let us assume 30% of energy is required through out the period of cooking.

Total heat energy requirement                 =             882.7 x 1.3

=             1147.51 kJ

Simple payback period

Consider the cooking capacity of solar cooker is for three persons at a time.  Consider three meal per person and cooking energy requirement for one person as 620 kCal per day.  The energy.

Energy required for cooking                        =             3 x 620                  =             1860 kCal/day

Assume about 50% of cooking can be met with solar cookers and hence the energy requirement for cooking

=             1860 x 0.5            =             930 kCal/day

Annual cooking of about 300 days may be considered.