Solar cells are made using semiconductors such as silicon. Semiconductors are useful materials because they can be doped with impurities to change their electrical properties, forming either ‘positive’ (p-type) or ‘negative’ (n-type) material. A solar cell consists of a layer of p-type and a layer of n-type semiconductor sandwiched together to form a pn junction. This pn junction induces a fixed electric field. When the device is exposed to light, consisting of photons, some of the incident photons are absorbed and temporarily liberate electrons from their covalent bonds in the crystal lattice. This can only occur if the energy of the incident photon is greater than the amount of energy required to break the covalent bond, known as the bandgap energy. Each liberated electron leaves behind a hole (a space once occupied by an electron) which acts as a positive charge. In an ordinary semiconductor material, these electron-hole pairs recombine after a short time. In a solar cell, however, the electric field formed by the pn junction attracts electrons to the n side and holes to the p side. This charge separation induces a voltage across the device. When the two sides of the pn junction are connected together via an external circuit, current is able to flow, thus producing electrical energy.
The efficiency of solar cells decrease when temperature increases. During construction, encapsulation materials should be thin as possible for better thermal conductance. The primary method of cooling the cells is airflow over the panels while the car is moving. When the vehicle is stationary the team is allowed to spray it with water, cooling it by evaporation. It is good to note that if the array has no load on it, for example it is not connected to the battery pack, it should be removed from the sun if possible. If the energy has no place to go it will turn into heat causing the cells to expand and possibly damaging the array.
Each sub-array is connected in series, which means that current is the same through each module. Human error during array construction can cause some modules to perform better than others. After testing the modules we group them into similar power outputs. We then split the list into the number of sub-arrays we have giving us a high powered, medium powered, and low powered sub-array. Within the sub-array we make sure to place the best performing modules in spots with the least incident sunlight and our worst modules in spots with the most. Since the sub-array is only as efficient as the weakest link this helps insure we get maximum power.
Maximum Power Point TrackersEdit
An MPPT is a DC to DC converter that optimizes the match between the PV solar array and the battery pack. Essentially it converts the lower DC solar array voltage up to the higher DC voltage needed to charge the batteries.
Maximum Power Point Tracking is electronic (usually digital) tracking. The controller looks at the solar panel output and compares that to the battery voltage. The controller then figures out what the optimal power is that the solar panel can put out to charge the battery. It then takes that and converts it to determine the best voltage to get the maximum current into the battery pack.
Hotspots & Bypass DiodesEditBecause each sub-array is connected in series, if a spot is shaded you have a large amount of power trying to force it's way through the uncooperative spot. If not alleviated, this power is dissipated as heat and may cause an array fire. Hotspot testing helps identify small defects in a solar cell that may cause problems when shaded.
A way to limit the risk impact of hotspots is to use bypass diodes. A bypass diode is placed in parallel with a string of cells; if that string cannot output the current that the rest of the array demands it will be bypassed through the diode.