Borrowing the Sun's Energy: How do Solar Panels Work?
The Solar Panel
A typical commercial photovoltaic (PV) solar panel is made up of cells, each of which is a diode. Characterized by their ability to control current so that it travels in only one direction, diodes are semiconductor devices with a “built-in” electric field. These features are key in allowing it to generate current without any applied electric fields. When photons (light) are incident on the diode, a current is generated.
Silicon solar cells, the most common in power generation applications today, have an efficiency ranging from about 15-20%. This is the percentage of incident light which they can effectively convert into electrical energy.
Generated solar power is used in many ways today. Centralized utility-scale solar farms generate power for the electrical grid. Some grid-tied homes may generate electricity through rooftop solar installations and sell the excess through the utility grid (these are called "net-zero" houses if they sell the same amount of energy they consume annually). The QSEC is considered an off-grid house, and uses the electricity it generates for its own needs.
Saving the Sun
As we cannot always control when and how much solar power is being generated, we often want to store it for later use. This is something we do at the QSEC using a battery bank, so that we can use the energy when needed. The decreasing cost of battery technology is one of the driving forces in making solar power more viable on a utility-scale today. The ability to store the energy, when the amount being generated does not match the demand from customers, increases the flexibility of solar power and makes it a more attractive generation option!
The charging of the batteries at the QSEC is facilitated by a Maximum Power Point Tracking (MPPT) charge controller. You can think of the charge controller as the brain of the charging system, because on their own the battery bank and the PV solar panels can't communicate their charging preferences. The controller adjusts the voltage of the module so that it’s always operating as close to its maximum power point as possible. This is the point at which the voltage and current generates the most power, and therefore the most possible Watts.
Direct vs. Alternating Current
There’s still a bit more to the electrical system than this. The electricity drawn from the panels and the battery bank is DC (direct current), but most homes use AC (alternating current), and consequently, most appliances and electronics we use are designed to accept AC power. Currently, the QSEC has some specialized DC equipment installed, such as lights in the electrical room and a refrigerator. Otherwise, an inverter can be used to convert DC to the 120 V and 240 V AC used in homes. The QSEC has a small 300 W inverter installed right now, but this summer we will be installing a 4000 W inverter to power many of the planned projects such as the boiler, and a pump.
Advancements in the Field
Solar technology continues to advance and make solar more competitive with other electricity generating methods. Check out this chart from the United States' NREL (National Renewable Energy Laboratory) to see progress over time in researching the efficiency of different types of solar cells. Note, there are many applications of solar cells and many of these, such as Multi-Junction cells, are not used for terrestrial utility power generation. In fact, solar energy is an ideal power-generation method for space probes and satellites. For utility-scale use, Perovskite and Organic solar cells are two interesting types to keep an eye on.
Other Technologies Contributing to Solar
Apart from increasing the semiconductor efficiency, the total power yield of solar systems is constantly being increased by advances in peripheral technologies. Depending on the conditions of the installation, some methods can be more cost effective than others. Solar tracking allows solar panels to track the sun and move to minimize the incidence angle of light. Increasing the efficiency of MPPT (Maximum Power Point Tracking) controllers or power electronics, such as inverters, is another important process. Even automated panel cleaning systems are being developed to ensure that light is not blocked by dirt, which is especially important in regions prone to dust storms.
One system which QSDT is working towards implementing is a combined PV and solar thermal system. A solar thermal system uses the heat generated by solar radiation to heat a home using water. When used in tandem with PV solar, the PV modules can be cooled by the water which then also heats the home. Ironically, PV cells lose a lot of efficiency as they become hotter, making this sort of system very useful. You can check our live data to see just how hot the QSEC's solar panels get in the middle of a sunny day. This means regulating the temperature would increase the overall efficiency of the system!
Solar PV power is an exciting field, and it has seen significant developments in just the past decade. We hope this article gave you a good starting point for your own research into solar energy! The Queen's Solar Design Team is always looking to experiment with ways to make our electrical system more efficient and put new technologies to the test.
Written by Chris Pennington, Engineering Physics '20.