Kathryn Cook
Kathryn Domalgalski
Solar Energy and Other Renewables
Human use of energy is natural and necessary; it is fundamental to life.[1] Energy is defined as an indirectly observed quantity or the capacity of a system to do work.[2] Energy comes in many different forms including electricity, fossil fuels, wind energy, and many more. However, solar energy represents the world’s most abundant permanent source of energy. Solar energy is radiant energy from solar rays that hit the earth. The sun emits radiation from its surface with an energy distribution at 6000 K.[3] The solar radiation is received at the rate of 19.3 kcal/min/m2 or 1.363 kilowatts (kW)/m2, which is equivalent to 6000 times the total energy budget for the United States.3 This means that the sun and other stars are indirectly responsible for all of our energy.[4]Although aprodigious amount of radiant energy is wasted everyday, the technology needed to harness this unlimited source is just beginning to develop. The most common solar energy system is the photovoltaic (PV) systems, which directly convert sunlight into electricity through the use of solar panels (Scheme 1).3 Concentrating solar power (CSP) is positioned to become an additional source of renewable electricity generation. Concentrated solar power systems use mirrors or lenses to concentrate a large area of sunlight onto a small area.[5] Electrical power is produced when the concentrated light is converted to heat, which drives a steam turbine connected to an electrical power generator (Scheme 1).5 The conventional solar hot water heater system is a well-established technology but has recently been combined with the emerging technologies of nanofluids and liquid nanoparticle suspensions. These heaters work by absorbing solar energy on a copper plate, coupled to a thermal carrying fluid that removes heat from the plate and heats the water.[6] However, the nanofluid-based system uses absorbing metal or carbon nanoparticles dispersed in a thermal fluid. The nanofluids directly absorb the incident radiation to produce water vapor without the requirement of heating the fluid (Scheme 1).[7] Microalgae has also emerged as amethod for mass production of transportation fuels due to their fast growth rate, habitat preferences, and oil yields. Algae efficiently stores excess solar energy as fatty acid triacylglycerides (TAGs) in lipid bodies, which can be converted to their derivatives via transesterification.[8]Transesterification is the acid-catalyzed method of converting lipids within algal biomass to the high-energy, lightweight fuel, biodiesel (Scheme 1). In the future, hydrogen may begin to replace some and/or reduce the use of fossil fuels. This is because hydrogen can be produced cost effectively from renewable energy sources, such as water. Photocatalysis and photosensitization are the photo-electrochemical processes that produce hydrogen directly from sunlight, thus converting solar energy into chemical energy.1 Hydrogen can be stored and transported for use in heating, generating electricity, industrial processes, and transportation (Scheme 1). Solar energy has gained interest for its potential to provide power for a variety of processes without relying on fossil fuels. The number one advantage for using solar power is that its generation does not produce emissions. Solar energy reduces approximately 90 percent of air pollutants when compared to conventional fossil fuel technologies. Another advantage of solar energy is the independence it can afford, allowing people anywhere to harness it and/or gain access to it.[9] Solar energy has allowed the transition from an energy system based on fossil fuels to an energy system based on more efficient, less expensive, cleaner, and renewable energy sources.9
Scheme 1. Solar Panel Systems (A), Concentrating Solar Power System (B), Solar Hot Water Heater System (C), Biomass Fuels from Algae (D), PhotocatalyticHydrogen Production (E).
The basic principle behind the solar cell, aka photovoltaic cell (PV cell), is that sunlight is being harnessed and directly converted into electricity.4 Sunlight knocks the electrons loose within the solar cell, causing them to move towards the surface, creating an electron imbalance.4When joined by a connector, a current of electricity occurs between the negativelyand positively charged sides.4There are four main types of these solar cells: semiconductor solar cells, sensitized inorganic solar cells, organic dye-sensitized solar cells, and organic polymer solar cells. In the semiconductor solar cells, the semiconductor absorbs protons when exposed to solar radiation because excited electrons move from valence band and to conduction band, leaving a positive hole (Scheme 2).3This hole is keptempty by creating an intrinsic voltage in the solar cell material, which separates the PV charge carriers. An example of the material used in a semiconductor solar cell is silicon crystals doped with arsenic.3 Sensitized inorganic solar cells take a more molecular approach, which has been the focus of inorganic photochemistry. In this type of solar cell, inorganic compounds sensitize wide band gap semiconductors to visible light, showing charge-transfer excited states (Scheme 2).[10] Single crystal and polycrystalline films of tin oxide or titanium dioxide in photoelectrochemical cells are examples of this type of solar cell.10Organic dye-sensitizedsolar cells most mimic the natural environment to create electricity comparatively. Just as in photosynthesis, the dye in the solar cell absorbs the light from the sun and uses it to excite electrons.[11] Different, however, are the materials used in the solar cell. For example, a film of interconnected nanometer-size titanium dioxide particles are used as the electron accepter while iodide and triiodide are used as the electron donor and oxidation product (Scheme 2).11A common dye used is a ruthenium-based dye, N719 (Scheme 3). The organic polymer solar cell, though not efficient for commercial use, uses a transparent ITO positive electrode and low work function negative electrode to surround a layer of conjugated polymer donors and fullerene derivative acceptors (Scheme 2).[12] For this solar cell to function a narrow band gap is needed with broad absorption, a low-lying HOMO, and high hole mobility. A material, Poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothia-diazole-4,7-diyl-2,5-thiophenediyl](PCDTBT), is a type of polycarbazole conjugated polymer that is an example of a polymer used in these solar cells; along with Poly(3-hexylthiophene-2,5-diyl), known as P3HT (Scheme 3). These polymers have been shown to give the highest yields in energyin recent experiments. All four of these types of solar cells give hope to researchers for a sustainable future for our generations.
Scheme 2. Semiconductor Solar Cell (A), Sensitized Inorganic Dye Solar Cell (B), Sensitized Organic Dye Solar Cell (C), Polymer Solar Cell (D).
The junctions in these four different types of solar cells can either be homojunction or hetero-junction. A homojunction is the region between positive holes (p-layer) and negative holes (n-layer) of a single material.[13] Usually formed by two semiconductors, a heterojunction is the region of electrical contact between twodifferent types of materials.13Usually, this type of junction is used when the solar cell is made from thin-film materials.
Scheme 3. One Chemical Structure of an Organic Dye used in Gratzel Cells (A), Two Chemical Structures of Polymers used in Solar Cells: PCDTBT (B) and N719 (C).
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