Solar thermal uses panels that absorb sunlight to heat a liquid or gas inside of tubes. Some systems directly heat the water that can be used in your home. These systems are very efficient but have no way of preventing water from freezing outside during cold months and offer few ways to prevent heat loss when the water is not being used.
Another type of home solar thermal systems heats a gas or some type of antifreeze in tubes and then puts those tubes into close contact with the water pipes in your home so the heat is transferred from the tubes with the heated gas/liquid to the water pipes. This process is known as Heat Exchange. By heating the water you use in your home with sunlight you are reducing your gas or electricity use.
Larger scale solar thermal systems use the same concept of heating a gas or liquid to create electricity. Some states such as California and Nevada have Solar Thermal Farms where acres of land are used to put rows of parabolic mirrors that reflect and focus sunlight onto long tubes with a gas or liquid in them.
This liquid is heated to incredibly high temperatures. For example in systems that heat oil in the tubes, the oil can reach temperatures up to 750oF. Through heat exchange the hot liquid transfers its heat to water in a chamber.
The water reaches a temperature high enough that it turns to steam (212oF) which is used to power steam engines. A steam engine allows steam to quickly pass through fans that turn and generate electricity. The steam is then sent to a storage unit where it condenses and is put back into the system. Heat storage in solar thermal farms can be very effective so energy can be generated at night even when the sun is not shining. The problem solar thermal farms have is their need for new water due to steam escaping the cooling towers. Farms can require around 800 gallons of water per MWh generated. Since solar thermal farms are most often located in deserts finding a source of water is a challenge.
To address this challenge the use of dry cooling is beginning to show up in solar thermal farms. This new way of cooling the plant can reduce the water use up to 90% but it reduces the efficiency and electricity output (Dry Cooling).
Scientists Experiment with Carbon Nanotubes as Potentially Being the Solar Cell of the Future
Within the past year and a half scientists have experimented with a three dimensional solar cell composed of carbon nanotubes as a means to potentially replace doped silicon wafers as the basis for creating energy from sunlight. The carbon nanotubes are composed of graphene, an allotrope of carbon which is one single layer of atoms thick, and is rolled up forming a tube shape as seen below in Figure 1. Approximately the size of a DNA molecule, these cells are smaller, lighter and less mechanically complex in comparison to a doped silicon solar cell.
Figure 1. (to the right) Carbon nanotubes structure for solar cell applications (In a carbon nanotube-based photodiode, electrons (blue) and holes (red) – the positively charged areas where electrons used to be before becoming excited – release their excess energy to efficiently create more electron-hole pairs when light is shined on the device.).
Additionally, the carbon nanotubes have the ability to capture nearly all of the light that strikes them. In conventional flat solar cells, the photovoltaic coatings must be thick enough to capture the photons. The energy of the photons then frees electrons from the doped silicon wafer semiconductor to create electrical current. However inefficiencies result in each mobile electron leaving behind what is known as a “hole” in the atomic matrix of the coating.
The longer it takes electrons to exit the photovoltaic material, the more likely it is that recombination occurs. Recombination is when an electron comes into contact with a hole, eliminating the hole-thus reducing the electrical current in the solar cell. Extra energy is also lost in the form of heat throughout the entire process, and the doped silicon cells require constant external cooling for efficient operation.
Since the three dimensional carbon nanotube cells absorb more of the incoming photons than conventional cells, their coatings can be made thinner, allowing the electrons to exit more quickly. This greatly reduces the likelihood that recombination will take place. The lower likelihood of recombination increases the quantum efficiency, or the rate at which absorbed photons are converted to mobile electrons, of the solar cell.
This increased efficiency is what makes carbon nanotubes so revolutionary. It could potentially have a profound impact on the solar energy industry and offer the renewable solution our society is looking for to eliminate our dependency on fossil fuels.
Tests to validate the physics behind this technology have been conducted at Cornell University and Georgia Tech University. The tests have proven that the high efficiency of these carbon nanotubes is not only factual, but tangible. Individual and strings of carbon nanotubes have been created and tested in laboratories, many of which have proven the potential usefulness of the carbon nanotube as a solar cell.
However, full modules and arrays of carbon nanotubes have not yet been produced and tested. The problem from here lies in making the carbon nanotubes replicable and deployable on a large commercial scale. This involves making the solar cells quicker and cheaper to manufacture, which is likely to take many years (if not decades). Once this can be accomplished, PV modules may reach efficiencies upwards of 90-99%, but until then they will remain at their current efficiency levels of 10%-20%.
Definitions:
Allotrope: One or more forms of an elementary substance.
Photon: Elementary particle and basic unit of light.
Ever wonder how sunlight can be turned into the electricity used in your home?
Layers of a panel
Solar panels are made up of layers. The top layer is a clear glass or plastic which protects the solar cells. Under the protective layer there is another layer of glass that helps guide sunlight into the solar cells by reducing the amount of light that is reflected off the panel. Solar cells are the part of the panel that harvest solar energy (generally blue or black) and are made of crystalline silicon. On the top of the solar cells are a conductive metallic strips serving as a path for excited electrons to travel along. These thin strips form an electric circuit connecting all of cells in the panel. Below the solar cell is a metallic backing that serves as a conductor. The top surface of the solar cell is positively charged while the bottom backing is the negative end.It is important to balance the area the conducting grid takes up on the solar cell so not to lower the efficiency of the cell.
The Solar Cell
Doped crystalline silicon is frequently used in the electronics and solar industries. It acts as a semiconductor (meaning it allows electrons to move more freely than an insulator and less freely than a conductor). This silicon is mined from the earth and then purified and doped until it is cut into the “wafers” used for a solar cell. Wafers can be either monocrystalline (typically rectangular with rounded corners) or polycrystalline (typically rectangular). Monocrystalline is silicon where the crystal lattice is one solid piece and Polycrystalline is made up of many small silicon crystallite lattices. Monocrystalline wafers take longer to produce and typically offer a higher production efficiency compared to surface area, so it is typically more expensive than polycrystalline.
Monocrystalline silicon is made from highly purified silicon that is melted down in a quartz crucible. The silicon is slowly removed from the crucible taking a cylindrical shape. When the silicon cylinder, also referred to as an ingot, is finished being formed it is allowed to cool for several hours so it is easier to handle. Next the ingot is grinded to a desired diameter and cut into wafers. For a more detailed explanation on how monocrystalline wafers are made check out Silicon Wafer Fabrication Process. This involved process is the reason monocrystalline cells are more expensive but also more efficient than polycrystalline cells.
Doping is the process of adding small amounts of impurity elements to a substance. For a solar cell, boron, phosphorus, and Arsenic are typically added to the crystalline silicon to help the silicon increase the conductivity of the material. The addition of the doping elements forms a P-N junction in the cell where boron acts as the P-type (missing an electron) material and Phosphorus or Arsenic act as the N-type material (has an extra electron). The P-N junction causes the flow of electrons to only move in one direction (from P to N). For more information about P-N junctions check out Solar Cells & Panels.
Sunlight shines on the cell and the energy from the sun light is transferred into the electrons in the cell. Electrons gain enough energy to break free from their electron shell in the solar cell making them “freed electrons”. The freed electrons form a current in the solar cell which then travels down the metallic strips on the cell. The process of light causing electrons to break from their shell and move freely is called the photoelectric effect and the way of creating electricity from sunlight is called photovoltaics. For more information on the photoelectric effect check out Photoelectric Effect.
