Solar Energy

The Sun

Solar energy exists in two forms: photovoltaic, which converts sunlight into electricity, and solar thermal, which harnesses approximately 90% of solar radiation for water and air heating. Solar energy is a product of nuclear fusion occurring within the sun’s core. In this process, four hydrogen atoms fuse to create one helium atom, generating tremendous heat and releasing photons. The sun continuously undergoes this fusion reaction, converting 700 million tons of hydrogen into helium every second. It takes roughly 100,000 years for a photon to travel from the core to the sun’s radiant surface. Although the sun emits about 63 MW/m2 of energy, only 1,367 W/m2 reaches Earth’s atmosphere. Roughly half of this energy is absorbed, with gamma rays, X-rays, and UV rays being converted into heat by the atmosphere. The remaining energy is either reflected back into space, absorbed by clouds, or reaches the Earth’s surface. As the primary source of life on Earth, solar energy holds immense potential for addressing global energy needs.

Solar Panels

There are two main types of solar panels: solar photovoltaics and solar water collectors. Solar photovoltaics derive their name from the Greek words “phōs,” meaning light, and “voltaic,” meaning electric, indicating that such panels generate electricity using light. Solar thermal energy produces heat rather than electricity. This technology uses a collector equipped with an array of pipes through which circulating water is heated by the sun.

To ensure the optimal performance of solar technology, it is important to first consider factors such as building orientation, shading, and materials based on the climate. One option to consider is the use of cool roofs, which have a high solar reflectance or albedo. Additionally, these roofs can reflect sunlight onto the bottom of a solar thermal water heater system, which consists of a box made of metal or black piping within a panel. By doing so, it can provide up to 60% – 70% of the hot water used in a home. The most efficient systems, as per the SRCC OG300 protocol, can achieve a solar fraction of 90%. However, using PV offset for water heating can result in a solar fraction of 100%.

Solar panels are composed of modules, which are made up of solar cells connected in series or parallel. Connecting the cells in series yields a higher voltage, whereas connecting them in parallel results in a higher current. Solar cells are mostly made of silicon, one of the most abundant minerals on Earth. Silicon comprises 14 electrons distributed across three electron shells: the first shell has two electrons, the second has eight, and the outer shell is half full with four.

Solar cells are devices that convert solar radiation into electricity. They are typically made of silicon, which can absorb photons of light and use the energy to eject an electron from its outer shell. However, pure silicon has limited electrical conductivity, which can be improved by doping it with boron or phosphorus at parts per million concentrations. P-type, or Boron-doped, silicon has a positive charge and a hole, while N-type, or phosphorus-doped, silicon has a negative charge and an extra electron. When light is absorbed by the semiconductor material, valence electrons gain energy and transition from the valence band to the conduction band, creating electron-hole pairs that generate an electric current. However, the holes in the valence band tend to quickly disappear due to recombination with electrons, limiting the efficiency of the solar cell.

A P-N junction consists of a thin layer of N-type silicon sandwiched on top of a thicker layer of P-type silicon. This arrangement generates a built-in electric field at the interface of the two materials, which separates the charges of electron-hole pairs created when photons of light are absorbed by the semiconductor material. By creating a depletion zone between the N-type and P-type layers, recombination is reduced, allowing for the generation of electricity. The depletion zone acts as a potential barrier that inhibits the free migration of electrons, leading to a buildup of electrons in the N-type layer and a deficiency of electrons in the P-type layer. This results in a static charge within the depletion zone that generates an electric field.

As the system approaches equilibrium, it becomes increasingly difficult for electrons in the N-type silicon layer to cross over to the holes in the P-type silicon layer. This creates an electric field that separates the two layers and generates the cell voltage. When an external circuit connects the two layers, current flows through it from the N-type silicon layer to the P-type silicon layer, with electrons doing the work that powers the PV cells. In contrast to P-type solar cells, which utilize a silicon wafer doped with boron as a base, N-type solar cells are becoming more popular due to their higher efficiency and immunity to light-induced degradation (LID).4

Solar cells can vary in their materials, design, and manufacturing methods. The most commonly used semiconductors for solar cells are silicon (monocrystalline), Cadmium Telluride (CdTe), Copper Indium Gallium Selenide (CIGS), Perovskite, Gallium Arsenide (GaAs). Other less common semiconductors for solar cells include amorphous or polycrystalline silicon, cadmium sulfide, organic photovoltaics (OPVs), and others. To establish electrical contacts and enable the flow of electricity within a solar cell, metal grids or meshes are situated on the top layer, while metal bases are positioned on the bottom layer.

Efficient energy conversion in a solar cell relies on the ability of the top layers to transmit photons. However, silicon has high reflectivity, making it necessary to apply a coating that minimizes reflection and maximizes the absorption of sunlight. A glass cover is typically placed over the solar panel to provide protection from the environment and prevent heat buildup that can negatively impact cell performance. It is crucial to choose a glass cover with high transmissivity to ensure that the maximum amount of sunlight reaches the solar cells and increases the panel’s overall efficiency.

Solar cells are limited by several factors, one of which is the maximum wavelength at which photons can create an electron-hole pair. For silicon solar cells, this maximum wavelength is 1.1 µm. The actual maximum theoretical efficiency for a silicon solar cell is around 29%, according to the Shockley-Queisser limit. However, this efficiency assumes that only one electron-hole pair is generated per absorbed photon. In practice, multiple electron-hole pairs can be generated per absorbed photon, resulting in a higher potential efficiency. Commercially available cell modules with efficiencies between 15% to 20% and a life span of 25 to 30 years are widely available. Researchers have achieved higher efficiencies through the development of experimental single-crystalline silicon cells, with some reaching up to 25%. Additionally, multiple-junction cells have been constructed with efficiencies exceeding 30%.

Off-grid Solar Power

When living off the grid with solar power, what are the options available when sunlight is not sufficient? Two commonly used choices are backup generators or battery storage. The lifespan of lead-acid batteries usually ranges from 5 to 15 years, while lithium-ion batteries can last up to 20 years or more. PV solar systems, on the other hand, generally have a lifespan of 25 to 30 years. To maximize battery usage, a deep cycle battery can be utilized, which allows stored energy to be discharged over longer periods such as at night or during extended power outages. Alternatively, if it’s possible, connecting to the local utility grid can provide electricity when solar panels are unable to meet energy demands.

Solar cells can sometimes generate more electricity than is needed, resulting in a net-positive situation where excess energy can be sold back to local power companies. However, not all locations have the option to sell back energy since some power companies do not offer this service. When connecting a PV solar system to the utility grid, special safety considerations are necessary to avoid hazardous situations, such as “islanding.” Islanding occurs when solar panels continue to provide power to downed power lines or lines undergoing maintenance, which can cause electrocution and fatal injuries. To address this issue, the National Electrical Code (NEC) added a new feature called module-level rapid shutdown in 2017, which enables solar panel systems to automatically shut down in case of a power outage. Starting January 1, 2019, certain state jurisdictions require that all conductors within an array’s 1-ft boundary must be reduced to 80 V or less within 30 seconds of rapid shutdown initiation.

Environmental Benefits

Despite some environmental impacts during the manufacturing, transportation, and construction of solar systems, solar energy remains one of the cleanest forms of energy available with no harmful air pollution, toxic waste, mining, or drilling causing severe environmental impacts. Solar energy is an environmentally friendly, abundant, and flexible alternative to other energy sources. It is flexible because it can be used in various applications and deployed in different forms, such as rooftop solar panels, ground-mounted solar farms, and portable solar chargers. Additionally, solar energy can be integrated with other energy sources to provide a more reliable and resilient energy supply. Furthermore, advancements in energy storage technology have made it possible to store solar energy for use during periods of low sunlight, increasing the flexibility of solar energy systems.

Emerging Innovations

Compared to coal, crude oil (which is refined into petrol), and natural gas, which require complex supply chains and multiple processing steps to deliver electricity to the grid, solar energy produces direct current in a single step. However, in order to fully realize the potential of solar energy, we need to fundamentally rethink the way we supply and distribute energy through the grid. The traditional electricity grid was designed for centralized power generation from fossil fuels and nuclear energy, making it poorly suited to the decentralized, intermittent nature of solar power. To ensure the seamless integration of large amounts of solar energy into the grid, substantial upgrades to the infrastructure’s design and operation, as well as the implementation of energy storage systems, and the development of new technologies, policies, and regulations, will be necessary.

Flexible solar cells are becoming more prevalent, these cells can be integrated into clothing and accessories, allowing for direct charging of personal appliances. Another innovative application of solar cells is in roof shingles. The installation of conventional solar panels can be challenging and overwhelming for many homeowners. To make rooftop solar panels more accessible, Dow Chemical has developed thin-film PV solar panels that are the size and shape of asphalt shingles. These solar shingles can be nailed down, just like traditional shingles, without the need for elaborate racking systems that penetrate the roof. There are several companies that sell solar shingles, including Tesla with their Solar Roof, CertainTeed with their Apollo II system, and RGS Energy with their Powerhouse shingles. Other companies such as GAF Energy and Sunflare also offer solar shingle systems.

To overcome the high cost of batteries that store excess energy generated by solar panels, Professor Donald Sadoway, a materials chemist at the Massachusetts Institute of Technology, has designed a novel all-liquid metal battery. This battery operates on the same principles as conventional batteries, but it utilizes liquid metal for electrodes and molten salt as an electrolyte, which enables it to absorb electrical currents that are ten times higher than current high-end batteries. This project, which was unveiled in late 2009, has received significant support from the Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E), which awarded a $6.9 million grant in the first round of funding, making it one of the agency’s most significant initial investments.

Spectrolab, a subsidiary of Boeing, is developing multi-junction cells that can achieve an efficiency of over 40%. Spectrolab scientists believe that concentrator cells could achieve efficiencies of 45% to 50% by using more than three junction cells. The multi-junction cells divide the broad solar spectrum into three subcell band gaps, each capturing different wavelengths of light to convert into electricity with higher efficiency. This results in an overall theoretical efficiency of the solar cell up to 58%.

Meanwhile, the DARPA Very High Efficiency Solar Cell (VHESC) program has achieved laboratory efficiencies of up to 54% and production efficiencies of 50%. The cost of installing photovoltaic cells to power an average home can vary depending on various factors such as the size of the system, installation costs, and local incentives or subsidies, but the cost of a typical solar panel system to power an average home can range from $10,000 to $30,000, with an estimated payback period of 8 years. Building a cleanroom manufacturing facility to produce these cells can be very expensive. The cost can range from $100M to $1B, depending on the size and complexity of the facility.

References

  1. Duffie, John A. & William A. Beckman. “Solar Engineering of Thermal Processes”. John Wiley & Sons, Inc., 2006.
  2. Toothman, Jessika & Aldous, Scott. “How Solar Cells Work”.
  3. Gilpin, Lindsey. “8 crazy new solar research breakthroughs”. Innovation.
  4. Pickerel, Kelly. “The difference between n-type and p-type solar cells”. Solar Power World. 2018.
  5. “Appreciating Solar Energy as We Power Toward Net-Zero”. PCL Construction. Mar 6 2023.