BU-413: Charging with Solar, Turbine

Folks concerned about the environment gravitate towards using renewable energy. The sun provides peak power of about 1,000 watts per square meter (93W/sq ft) and a solar panel transforms this power into roughly 130W per square meter (12W/sq ft). This energy harvest corresponds to a clear day with the solar panel facing the sun. Surface dust on the solar panels and high heat reduce the overall efficiency.

Generating electricity by sunlight goes back to 1839 when Edmond Becquerel (1820–1891) first discovered the photovoltaic effect. It took another century before researchers understood the process on an atomic level, which works similar to a solid-state device with n-type and p-type silicon bonded together.

Commercial photovoltaic (PV) systems are 10 to 20 percent efficient. Of these, the flexible panels are only in the 10 percent range and the solid panels are about 20 percent efficient. Multi-junction cell technologies are being tested that achieve efficiencies of 40 percent and higher.

Global warming will affect solar panels negatively. A study from the Massachusetts Institute of Technology (MIT) reveals that a one degree Celsius increase in temperature reduces the photovoltaic power output by 0.45%. Like a battery, heat also reduces the lifespan of solar cells.

At 25°C (77°F), a high quality monocrystalline silicon solar panel produces about 0.60V open circuit (OCV). Like batteries, solar cells can be connected in series and parallel to get higher voltages and currents. (See BU-302: Series and Parallel Battery Configurations) The surface temperature in full sunlight will likely rise to 45°C (113°F) and higher, reducing the open circuit voltage to 0.55 V per cell due to lower efficiency. Solar cells become more efficient at low temperatures, but caution is necessary when charging batteries below freezing temperatures. (See BU-410: Charging at High and Low Temperatures) The internal resistance of a solar cell is relatively high: with a commercial cell, the series resistance is typically one ohm per square centimeter (1Ωcm2).

A solar charging system is not complete without a charge controller. The charge controller takes the energy from the solar panels or wind turbine and converts the voltage so it’s suitable for battery charging. The supply voltage for a 12V battery bank is about 16V. This allows charging lead acid to 14.40V (6 x 2.40V/cell) and Li-ion to 12.60 (3 x 4.20V/cell). Note that 2.40V/cell for lead acid and 4.20V/cell for lithium-ion are the full-charge voltage thresholds.

Charge controllers are also available for lithium-ion to charge 10.8V packs (3 cells in series). When acquiring a charge controller, observe the voltage requirements. The standard Li-ion family has a nominal voltage of 3.6V/cell; lithium iron phosphate is 3.20V/cell. Only connect the correct batteries for which the charge controller is designed. Do not connect a lead acid battery to a charge controller designed for Li-ion and vice-versa. This could compromise the safety and longevity of the batteries as the charge algorithms and voltage settings are different.

A lower-cost charge controller only produces an output voltage when sufficient light is available. With a diminishing light source, the charge controller simply turns off and resumes when sufficient levels of light are restored. Most of these devices cannot utilize fringe power present at dawn and dusk and this limits them to applications with ideal lighting conditions.

An advanced charge controller tracks power by measuring the voltage and adjusting the current to get maximum power transfer with prevailing light conditions. This is made possible with maximum power point tracking (MPPT). Figure 1 illustrates the voltage and current source from a solar cell with varying sunlight. Optimal power is available at the voltage knee where the dropping voltage line meets the vertical power line. MPPT determines this point.

MPPT finds the best power point which is at the crossing point of the vertical power line. (V x A = W). The top horizontal line gets the most light. Wind turbines have a lower internal resistance than PV and the MPPT differs.

It should be noted that not all MPPT circuits function equally well. Some are coarse and do not respond immediately to light changes, causing the output to fall or turn off if a shadow falls on the panel. Other systems drop off too early and do not fully utilize low light conditions.

A common MPPT method is perturb and observe (P&O). The circuit increases the voltage by a small amount and measures power. If the power increases by the equal amount, further voltage increases are applied until the optimal setting is reached. P&O achieves good efficiency but it can be sluggish and result in oscillations.

Another method is incremental conductance that computes the maximum power point by comparing current and voltage deltas. This requires more computation but has an improved tracking ability over P&O. Current sweep is a method that observes the current and voltage characteristics of the PV array to calculate the maximum power point.

Solar panels are normally connected in series, each providing about 20V on a sunny day. The controller reads the overall string voltage but if one panel gets shaded, the MPPT loses effectiveness. Advanced systems process each panel or group of panes individually. This allows voltage tracking of shaded panels down to 5V. The negative is higher system costs.

You may ask, “Why can I not simply plug a 12V solar panel directly into my laptop or mobile phone?” This should work in principle but is not recommended. The charge controller transforms the incoming DC voltage from the solar panel or wind turbine to the correct voltage range. In bright sunlight, the voltage of a 12V solar panel can go up to 40V, and this could damage your device.

From 1998 to 2011, the price of commercial photovoltaic (PV) systems dropped by 5–7 percent annually and analysis suggests that the price-drop will continue. It now costs between US$4 and $5 per watt for a typical residential solar installation capable of delivering 5kW. Larger installations cost $3 to $4 per watt with further reductions for megawatt systems.

Maintenance Charger

A maintenance charger is usually powered by a small solar cell that provides a trickle charge to a battery on a sunny day. These devices help prevent sulfation of a lead acid battery when stored. Even a small float charge will keep the battery at full charge.

Choose a maintenance charger that switches to a controlled float charge when the battery is fully charged. A prolonged charge, even at a low current, could overcharge the battery and promote internal corrosion. A float charge that is correctly adjusted only replenishes what the battery loses through self-discharge. (See also BU-403: Charging Lead Acid)