Fig. 25.1 shows a simple box-type solar cooker. The insulated box has a matt black metal base plate. A glass cover sits on top of the box. A reflective alum...

Marking Scheme

• (a) [2 marks] Matt black is a good absorber of radiation / absorbs the most solar radiation [1]; this allows the base plate to absorb the maximum possible amount of energy from sunlight [1]
• (b) [2 marks] Glass is transparent to the short-wavelength visible light from the Sun, which passes through to the base [1]; the warm base emits longer-wavelength infrared radiation, which is absorbed by the glass and cannot escape, trapping thermal energy inside [1]
• (c) [1 mark] The reflective panel directs additional sunlight through the glass cover onto the base, increasing the total solar energy entering the cooker [1]
• (d) [1 mark] To reduce thermal energy loss by conduction through the walls to the cooler surroundings [1]
• (e) [1 mark] On an overcast day, clouds block and scatter much of the direct sunlight, so much less solar energy reaches the cooker and the temperature inside does not rise high enough to cook food [1]
• (f) [1 mark] It uses no fuel, so there is no cost for energy and no smoke or harmful emissions [1]

Explanation

(a) The matt black surface has the highest absorptivity of any surface type. When solar radiation (a mixture of visible light and infrared) strikes the base plate, the matt black finish absorbs nearly all of it, converting the radiant energy into thermal energy that heats the plate. A shiny or light-coloured surface would reflect a large fraction of the incoming radiation, reducing the energy available for cooking. The matt finish is important because a glossy black surface, while still dark, reflects more light at certain angles than a matt surface does.

(b) This is the greenhouse effect applied to cooking. The Sun emits radiation with a peak in the visible spectrum (short wavelengths, around 500 nm). Glass is transparent to these wavelengths, so the sunlight passes through the glass cover and reaches the black base plate. The base absorbs this energy and heats up to 130 °C. At this temperature, the base re-emits radiation, but because it is much cooler than the Sun, the emitted radiation has much longer wavelengths (peaking in the infrared, around 8000-10000 nm). Glass is largely opaque to these longer wavelengths, so it absorbs the outgoing infrared radiation instead of transmitting it. The energy is effectively trapped inside the box, raising the temperature well above what would be achieved without the glass.

(c) The reflective aluminium panel acts as a mirror, redirecting sunlight that would otherwise miss the cooker. By angling the panel, the user can bounce additional solar radiation through the glass cover and onto the base plate. This effectively increases the amount of solar energy entering the cooker beyond what the glass opening alone would collect. The more reflective the panel, the more additional energy is directed into the cooker.

(d) The insulated walls reduce the rate at which thermal energy escapes from the hot interior (130 °C) to the cooler surroundings (32 °C). The temperature difference of 98 °C would drive rapid conduction through uninsulated walls. The insulation (which contains trapped air pockets) has a very low thermal conductivity, creating a high thermal resistance. This keeps the interior hot enough to cook food by ensuring that most of the solar energy absorbed by the base stays inside the box.

(e) Solar cookers rely on direct sunlight to function. On an overcast day, the cloud cover scatters and absorbs a large proportion of the incoming solar radiation before it reaches the ground. The diffuse light that does reach the cooker is much less intense than direct sunlight and arrives from many directions, so it cannot be effectively focused by the reflective panel. The reduced energy input means the temperature inside the cooker does not rise high enough to cook food.

(f) A solar cooker uses free solar energy and produces no smoke, soot, or harmful emissions during operation. A wood-burning stove requires fuel (wood or charcoal), which costs money, contributes to deforestation, and produces smoke and particulate matter that cause respiratory health problems, especially when used indoors. The solar cooker also produces no carbon dioxide from combustion, making it more environmentally friendly.