The sun is a virtually infinite reservoir of renewable energy. As seen from the graph below, solar energy is by far the largest energy source available. The size of the solar energy reservoir is considerably larger than all other energy sources combined, including both renewable and non-renewable sources. It has been estimated that if only 2% of the solar energy hitting the earth's surface each day was captured, all of humanity's current energy needs would be met and even exceeded.
[caption id="" align="alignright" width="478"] The Size of Various Energy Reservoirs - Source: Perez et al.[/caption]
Given the vast amount of energy that can be sourced from the sun, many recent research and development efforts in renewable energy have focused on improving the ability to harvest, store and distribute solar energy. New installations of solar power continue to grow at an explosive rate as the price of harnessing solar power continues to drop. In 2013, the global solar power generating capacity increased by 26%, from 31 Gigawatts (GW) in 2012 to 39 GW in 2013. In the United States, solar power currently accounts of 1.13% of power generation capacity. Furthermore, the US Department of Energy (DOE) expects to add 1,510 MW of solar power capacity from finishing current solar projects by the end of 2015. The importance of solar power in the future of the energy industry prompts a closer look at 4 ways to utilize solar power.
1. Photovoltaic Modules
Photovoltaic (PV) modules, often referred to solar panels, are the most ubiquitous solar power harvesting technology. The ubiquitous nature of PV modules has prompted many to view the performance of PV modules as the main indicator of the overarching solar industry. Moreover, PV modules have been applied for residential and commercial power generation in buildings, such as Walmart's recent solar installations, as well as for utility-scale solar power plants, with the largest PV generation facility being the 290 MW Agua Caliente plant in Yuma County, Arizona.
PV modules generate electricity by energizing electrons inside their material system. PV modules are made from semiconductor materials, which have the ability to absorb sunlight and create electric energy. When sunlight is absorbed in a semiconductor, the optical energy of the light is transferred to the electrons inside the material. The energized electrons can then be extracted from the semiconductor into an electrical circuit, yielding electricity.
A major reason why semiconductor materials are used for PV modules is their ability to separate electrons into high and low energy states, which are separated by an energy gap that is inherent to the material. When the sunlight is absorbed in the semiconductor, the absorbed energy promotes a low-energy electron to a high-energy electron, which then flows into the external circuit. Positively charged holes are also generated in this process and maintain charge balance in the overall system.
[caption id="" align="alignleft" width="400"] Solar Panel Diagram - Source: ETAP[/caption]
As the amount of absorbed sunlight increases, so does the number of high-energy electrons, which then creates a flowing electric current. The energy difference between low-energy electrons and high-electrons, i.e. the size of the energy gap, determines the voltage output by the PV module.
2. Solar Thermal
Solar thermal, also know concentrating solar power (CSP), is a lesser known method to extract solar power. Unlike PV modules, solar thermal is not applied commonly for residential or commercial building power generation. Solar thermal, however, has a tremendous potential to produce solar power on a utility scale.
The 377MW Ivanpah solar plant in California's Mojave Desert is the world's largest solar energy facility. The $2.2 billion Bechtel built Ivanpah plant was a joint project of NRG, Google, and BrightSource Energy, and its 377 MW of solar power can power 140,000 American homes. In spite of its tremendous potential for utility scale application, solar thermal has recently faced increased skepticism due to its land-intensive nature, as well as environmental concerns relating to avian mortality.
[caption id="" align="alignright" width="515"] Schematic of the Ivanpah Solar Thermal Plant - Source: Greentech.co[/caption]
The energy harvesting process of CSP is inherently different from solar photovoltaics. In CSP, sunlight is directed using mirrors, and then concentrated at a single point. The concentrated optical energy is than used to boil water and generate high-energy steam. The high-energy steam then turns a turbine generator to produce electricity. Steam powered generators are a very established technology to generate electricity that is used by virtually all traditional power plants, including coal, natural gas and nuclear power plants. The inherent different difference between solar thermal and traditional power plants is that CSP uses renewable energy input to generate steam.
3. Biofuels (Natural Photosynthesis)
A major challenges of solar photovoltaics and solar thermal energy generation is their inability to store energy effectively. This creates a demand-response issue, as the generated electricity needs to be consumed instantly.
Plants, however, have the ability to store solar energy using photosynthesis. In the process of photosynthesis, plants take in Co2 and water to produce oxygen and glucose, a form of sugar. Photosynthesis has been extensively studied and the abundance of plants on the earth's surface speaks for the scalability of natural photosynthesis.
The products of the photosynthetic process of certain plants can then be further refined to create chemical fuels that have sufficient energy density for practical applications. This process is already being applied in making ethanol fuels from corn feedstock.
Given the controversy of using corn to generate fuels, researchers are investigating additional crops that can be used for biofuel generation. Their aim is to find biofuel crops that grow quickly and efficiently, have a high energy density, and mitigate the food versus fuel conflict.
[caption id="" align="alignleft" width="300"] A Schematic of the Biofuel Cycle - Source: Ars Technica[/caption]
In spite of the immense scale of biofuels, one of the main drawback of natural photosynthesis remains its inherently low efficiency. The energy conversion efficiency of natural photosynthesis is less than 0.5%, which is a major reason why many biofuel crops require significant refining steps to be converted into usable biofuels.
4. Artificial Photosynthesis
Artificial photosynthesis aims to address the inherent flaw of natural photosynthesis by engineering a chemical scheme that converts sunlight into usable chemical fuels. Artificial photosynthesis intends to mimic the process of natural photosynthesis and to increase the energy conversion efficiency by replacing the biological agents in the natural scheme with solid state materials that drive chemical reactions to produce the desired fuels.
The principal of artificial photosynthesis is based on electrochemistry. Many naturally occurring chemical reactions generate electricity with a distinct voltage. This phenomenon was the scientific basis used to engineer some of the world's first battery systems. Like many phenomena in the chemical world, electrochemical reactions are reversible. This means that if one applies electricity with a certain voltage to a chemical system, one can initiate a chemical reaction that does not occur naturally.
A common example of this concept is the production of aluminum: In its natural state, aluminum is found in various compounds, including aluminum oxide, which is more chemically stable than pure aluminum in nature. In this case, the natural electrochemical reaction is for oxygen to react with the pure aluminum to form aluminum oxide. The reverse reaction can be initiated by flowing electricity through the aluminum oxide to create pure aluminum. This process for making aluminum is known as the Hall-Heroult process and is still a common industrial practice today.
[caption id="" align="alignright" width="459"] Schematic of the Hall-Heroult Process where electricity is applied through a graphite anode - Source: Prenhall[/caption]
In the case of artificial photosynthesis, the energy input used to generate the required electricity is solar energy. The optical solar energy is converted into electricity inside the material system using the same scientific principles as PV modules. Unlike PV modules, the electricity is not extracted into an external circuit but used to initiate chemical reactions at the surface of the material. The chemical reactions can be accelerated with certain catalyst materials, which increase the energy conversion of the overall system when engineered effectively.
The premier DOE hub for artificial photosynthesis research is the Joint Center for Artificial Photosynthesis (JCAP), which is headquartered at the California Institute of Technology. Lawrence Berkeley National Laboratory is home of JCAP North, the second major location of the research hub. Since its inception in 2004, JCAP has designed and created various systems that have the ability to generate hydrogen via water splitting. The solar generated hydrogen is a usable chemical fuel that can be used for a variety of applications. Current artificial photosynthesis systems fabricated by National Renewable Energy Laboratory (NREL) display a solar hydrogen conversion efficiency of 12.4 %. The theoretical limit of solar-to-hydrogen generation has been estimated to be ~ 30%.
JCAP ambitions, however, go far beyond water splitting. JCAP researchers are currently developing systems to convert CO2 and water into alcohols, such as methanol or ethanol, which are already being used as chemical fuels. A material system that uses solar energy to convert CO2 and water into chemical fuels with reasonable efficiency would represent true mastery of the chemistry underlying the photosynthetic process, and could have substantial implications for the energy industry.
Humanity's ability to harness solar power in an effective and economic manner has improved dramatically in the last decade. While solar photovoltaics, solar thermal and biofuels are already being used on industrial scales, the next generation of disruptive solar technologies may be based on artificial photosynthesis technology. Most likely, the future solar energy will include a combination of the 4 methods to meet different types of needs as well as innovative technologies that have not been discovered yet.
Cross-posted from BERC Blog, published online by the Berkeley Energy & Resources Exchange, a network of UC Berkeley scholars and industry professionals.