Imagine placing a black frying pan in the Mojave Desert during the daytime. The pan gradually gets hot to the touch until its temperature exceeds 90°C (195°F). You toss an egg into the pan, and in minutes it's fully cooked. You may not realize it, but this simple experiment is an illustration of solar energy harvesting.
Solar energy harvesting is most commonly associated with the solar panels you see sitting on residential rooftops. However, the commercialized adoption of solar energy harvesting spans a variety of applications that provide astounding amounts of energy to the world. Let's look at five innovative solar energy harvesting technologies.
1) Photovoltaic solar panels
Photovoltaic (PV) solar panels use the sun's power to create a flow of electricity. This is the most widely adopted method of harvesting solar energy today. These panels, which range in size from a few square centimeters to a few square meters, are constructed from many PV cells arranged in an intricate matrix. Intuitively, the larger the surface area available for sunlight to penetrate the PV cells, the more solar energy that gets harvested.
Each PV solar cell is generally made up of a compound semiconductor wafer structure, which can either be a monocrystalline or polycrystalline structure. The structure's two thin semiconductor wafers, one P-type and one N-type, are each grown separately. The two wafers are placed on top of each other, and the natural reaction that occurs between the two semiconductor types creates a depletion zone that reaches an equilibrium point, without generating any electricity. Due to the PV cell, when light photons pass through and connect with the semiconductor wafers, their interaction releases enough energy to create an equilibrium disruption in the depletion region. That action subsequently creates a brief flow of electricity. However, because of the constant presence of light, this interaction occurs continuously and can produce massive amounts of electrical energy.
The power produced by a single photon interaction replicates across the entire surface of the PV cell. It's compounded into a whole panel of solar cells and then into a vast PV panel array. This minor interaction in the depletion zone can be repeated and multiplied, resulting in a significant amount of electricity. PV solar arrays, however, produce DC power. To be integrated with modern power transmission technology, such as the outlets in your home, this DC energy must be converted to AC power using an inverter. There are a variety of proprietary iterations of this fundamental technology that seek to optimize the efficiency of each PV cell on a molecular level, the assembly of the panel, and the panel's ability to be integrated into a larger solar array.
2) Thermal energy harvesting: Energy of electromagnetic radiation
The sun produces a broad spectrum of radiation of many different wavelengths, including infrared. This spectrum efficiently transfers thermal energy to bodies that can absorb it. Elements that can effectively absorb this thermal electromagnetic energy are referred to as 'black bodies,' as the color black absorbs all wavelengths of radiation that are visible to the human eye. An ideal black body can correctly absorb, and emit, all wavelengths of the electromagnetic radiation spectrum.
Electromagnetic radiation has long been used for heating in many passive heating systems, such as the egg cooking example, in Roman bathhouses and Ancient Egyptian homes, and modern solutions such as thermal solar panels and thermosiphons. These thermal solar energy harvesting strategies rely heavily on black body radiation physics and their ability to absorb and transfer electromagnetic radiation. On a residential level, thermal energy is gathered most often for use in water heating systems. However, these solutions are less suitable for energy generation on an industrial scale.
3) Solar water heaters
A great example of a thermal solar energy harvesting application that's commonly implemented in sunny climates around the globe is a solar water heater. The simplest version of a solar water heater system uses a pump to circulate cool water through a black body panel. This visually resembles a PV solar panel, where the black surface efficiently absorbs thermal energy, which is then cooled by the circulated water, thereby heating the water. The water is continually circulated through this loop, creating warm water throughout the solar activity. Some systems can forgo a pump system by utilizing the buoyancy created by the heated water. This warmer water 'floats' and the colder water sinks, producing low amounts of flow in the system, creating a thermosiphon. These systems require the storage tank to be above the solar absorption source, as shown here.
4) Vacuum Tube Solar Water Heater
More advanced and efficient solar water heating systems utilize vacuum tubes and self-contained heat pipes to transfer thermal energy to a secondary tank. The vacuum tube ensures that radiant energy can enter the system, but all energy that gets turned into thermal energy is contained in the tube. The heat pipe absorbs this energy and subsequently transfers it to the large water tank. These systems are significantly more efficient at heating water during cold months, as minimal amounts of thermal energy escape the vacuum tube, allowing nearly all radiant energy to be converted into thermal energy.
5) Molten salt solar power
Relatively recent breakthroughs in molten salt systems are pushing the boundaries of power generation using solar energy. However, much like the previously discussed solar-powered water heating systems, molten salt power plants utilize electromagnetic radiation to melt salt. This molten salt then gets transferred to a heat exchanger, which heats water into steam that is then driven through a steam turbine to generate electricity. Molten salt power plants, such as the Ivanpah Solar Plant, rely on an extensive network of heliostat mirrors to redirect sunlight to a single point, most often referred to as a power tower or central tower. This tower collects the energy from all surrounding heliostats, which is enough power to melt the salt at nearly 1500°F. This molten salt is then stored in insulated tanks, allowing for the energy to be used even when the sun is no longer shining.
The future of harvesting solar energy
Solar energy harvesting technology is increasingly utilized as an alternative to electricity generated by fossil fuel. While various methods of solar energy harvesting exist, they all fundamentally use the sun to perform work in a specifically desired way, something we traditionally rely on electricity to do. Increases in efficiencies and process optimizations will continue to unveil the productivity of solar harvesting efforts into the future and may eliminate the need for fossil fuel use altogether.