After that empty test, a mutton carcass containing couscous has been introduced in the oven and the following parameters measurements were carried out:

— inside temperature,

— absorber temperature,

— meat weight and temperature

— ambient temperature, wind velocity, solar radiation (global radiation+ beam radiation reflected onto the oven)

3.3 Computation of the efficiency in load

To compute the efficiency of the oven let us consider the following equations:

Pi =Pu +Ps +Pi

i) Estimation of Pi:

The incident power is given by the solar radiation falling directly on the glass and the reflecting power from the reflectors.

P = T(g, +pGt )

ii) Estimation of Pu

Pu =^Mmeat*Cmeat*(iTmeat—’Tamb )

Pbot, om ={Tp — Tamb k, + ^ / Ik,

The sides lost are given by the conduction in the glass wool and the wood board

Usides —1/{ei/ki+ei/ki )

iii) Estimation of the stored heat Ps

It is constituted by the heat of the air in the oven and the heat in the stones

Ps =Ms, ones*Cstones*(Tstone^Tamb }±Mair*Cair*(Tair-Tamb )

iv) Estimation of the heat lost p

P —Ptop +Pbottom + Psides Ptop ~Ut*(Tp — Tamb )

Value of Ut for flat-plate collectors is given by Duffie and Beckman in Solar Engineering Thermal Processes.

The efficiency is given:

Eff ~Pu / P

The calculated efficiency of the oven is given by the following curve.

3.4 Discussion

The Test with the loaded oven has been run during a typical day of Sahelian region: fairly sunny, important diffuse radiation (~ 20%), a little windy, ambient temperature varying from 20-22 C° in the morning to 40-42 C°, at the beginning of the afternoon.

As we can see, the efficiency is very low, particularly during the early hours, when the inertia of the system has to be overcome. In fact it is not very satisfactory as a whole. An other prototype, with a new design in order to improve the performances has been constructed and is being experimented. For example, by reducing the height of the oven, convection losses are reduced (see curve Fig.7)

и

c

a)

=5

re

Ф

Q.

E

ф

Hours

—•—(Tp-Tc) New —■—(Tp-Tc) old

Fig.7