STAND ALONE TRACKING SYSTEM WITH SMALL PV MODULE

5 Watt-PV module is utilized for tracking solar oven concentrator system with 2.6 kWTH capacity and 250 Kg weight. The tracking system follows the Sun autonomously in altitude and azimuth using only 5 Watt-peak PV solar module as a tracking energy source. The tracking system is driven by means of two 12 DCV motors of 36 W each, and fed by electrolytic condenser with 78,000 pFd capacity charged properly by PV module. The PV based tracking system has two circuits in H Bridge configuration using N — and P-channel power MOSFET transistors. This electronic circuit commands DC motor rotation way, as a function of the optical sensors for altitude and azimuth position.

The proposed system must be designed based upon local technology and adopted to the needs. Simple design concept is one of the issues in this tracking system to reduce different troubles during its lifetime. The tracking system consists of electrolytic condenser storage, instead of conventional battery and its charge controller configuration. A couple of electrolytic condensers satisfy the total system energy needs. FIG.16 shows "H” configured basic electronic circuit for feeding two DC motors of 36W-each, one for solar altitude and the other for azimuth movements.

The design and construction of effective 2.6kWTH stand-alone solar concentrator oven tracking system was developed using 5 Watts-peak PV module. The objective is focused for simple and robust electronic tracking system for Mexican rural area application.

The generated power at PV module is coupled for charging electrolytic condenser. The maximum module voltage is 16V, and when the electrolytic condenser achieves 15V, the electronic circuit compare, and switches for discharging maximum of about 8.8 Joule of energy as (1/2 CV2), where C is the capacitance and voltage V provided by the module, the plot is shown in FIG. 17.

The energy delivered by the capacitors is conducted towards the selected DC motor according to the optical sensor decision. The DC motor has low internal resistance of ~2Q and considering PV module as a constant current source with about 340mA, the capacitor’s charging time lasts about 2 to 3.5 seconds depending of its charge state. The
electrolytic capacitor charging process for feeding low-resistance DC motor load is illustrated in FIG.17. The figure shows I-V and Voltage-Time curves for PV-module and capacitor charging operation with storage time.

Superposition of the Voltage-time axes indicates the energy charging process in the capacitor and its transference to the DC motor, using MPPES concept.

The energy stored in electrolytic condenser is discharged towards the DC motor by using MPPES (Maximum Power Point Energy Storage) concept. This is a DC/DC converter similar to MPPT (Maximum Power Point Tracking), feeding the load using maximum PV module power point. In the case of MPPES, the energy is temporally stored and discharged, repeating this cycle.

Respect azimuth and altitude mechanical traction, are driven by two independent pulleys connected to each DC motors through v-belts. The PV Module location on the solar oven is shown in FIG. 18.

This tracking configuration has some advantage, which prevents mechanical damage when is compared with the conventional “mechanical-gear” system. This tracking system driven by pulley and v-belts has great flexibility in movements but maintaining precise position.

The energy discharging process on DC motors for azimuth or altitude tracking is given by equation (a):

FIG 18 PV Module location at the top of the main structure of the solar oven

-V^exp-2tdt

Rmotor T

0

From theoretical calculation for voltage-current discharge cycle:

v(t) = Vm exp(-t/t) and i(t) = Vm / Rl exp(-t/x)

Solving equation (a) and using t = 0.156 sec, the stored energy is equal to the consumed energy. If the sun position displaces about 1.5° every 6 minutes, it is enough time for charging DC motor supply energy [4].

CONCLUSIONS

Most of the solar concentrator cooking systems does not posses an autonomous tracking system. We have demonstrated how 5 Watt-peak PV Module can track 2.6KwTH solar concentrator cooking system by means of electrolytic condenser storage system using two DC motors. This cooking system avoids deforestation, one of the mayor rural problems. The PV-based stand-alone tracking system has big energy factor-merit of about 520 times, due to the reduced electrical energy consumption for obtaining high thermal energy. This is thought as the first time that MPPES (Maximum Power Point Energy Storage) concept is used for a stand-alone Sun Tracker system[7]. In addition to the last facts, the Redundancy provided by the use of two detection elements increases the Sun Tracker’s efficiency.

Solar oven reliability is now in their evaluation stage and the total cost is about US$2,500 (by March 2004). The following paragraph describes obtained important fact.

From 60% to 65 % global thermal efficiency (output power available over the incident solar power) has been obtained. The prices of the produced energy is estimated to be US ф 3.0 per kWh as equivalent to electric energy and US$ 1.3 W-peak for an installed total system considering 5.2 peak-hour [2] locally available direct solar radiation resource. This is based on the 30-year estimated system lifetime.

They pay back time is estimated in 3.5 years considering energy price at US ф 15 per kWh (June 2003). This solar oven contributes reducing 2.87 Ton/year of firewood combustion, which means 5.32 Ton/year of CO2 emission to the atmosphere [5, 6].

The cost due the use of the oven is around US ф 30.2 per day, during the 30 years lifetime system, this cost represents US ф 3.8 per individual a day considering 8 people per solar oven.

As a protection issue to the Environment, the solar oven implies big benefits. This prototype can be promoted as a green bonus for CO2, that UNEP (United Nations Environment Program), the GEF (Global Environmental Facility) and the World Bank Institute provide as a result of the Kyoto Protocol to reduce greenhouse effect.