Challenges of Making Solar Energy Economical
Edis Osmanbasic posted on December 30, 2019 |


How much energy can the sun provide in an hour? As anyone who’s ever fallen asleep while sunbathing can attest, it’s enough to leave a mark. The average solar power upon the Earth’s surface is 174.7 watts per square meter, according to a paper published by Sandia National Laboratories. Multiply that value by the Earth’s surface area (4πr2, where r is roughly 6378 km) and then again by 60 minutes (that’s 3600 seconds) and we have our answer: in one hour, the sun provides about 3.21 x 1020 joules of energy to Earth. That’s equivalent to 76,841 megatons of TNT. In less than two hours, the sun provides more energy than the entire planet used in all of 2017.

In short, with efficiency improvements, solar power generation technology could have significant potential as an energy resource.

The article discusses the emerging technologies in solar energy sources that could increase the technical and economical effectiveness of this source, thus boosting its popularity. The currently developing technologies have the potential to replace the dominant crystalline silicon (c-Si) technology in the future and significantly increase the efficiency of photovoltaic (PV) cells.

In the past the solar technology has been expensive and relatively inefficient. Technological advances over the last twenty years have significantly increased its efficiency and decreased its costs. This resulted in the rapid growth of solar energy capacity. The solar energy system costs are less than half of what they were 20 years ago. However, solar energy still requires government support and incentives to be financially competitive with commercial energy sources. In addition to this, the efficiency of commonly used c-Si cells is already close to its theoretical maximum, which imposes a need for new technology. According to the IHS Markit global scenarios, only 5% of total generation capacity in the ranking top 10 countries came from solar (Figure 1).

Generation capacity 2019 in GW, ranking top 10 countries (IHS Markit Autonomy scenario, July 2018).
Generation capacity 2019 in GW, ranking top 10 countries (IHS Markit Autonomy scenario, July 2018).


The new upcoming solar technologies promise growth in solar energy usage by decreasing its costs and increasing its efficiency. The most effective way of using solar energy is by distributing solar power generation, such as electricity produced by households with rooftop systems. Individual owners using distributed solar generation will produce electricity for their own use, with excess power production sold to a power company. The distributed solar power generation has numerous benefits such as: it is clean energy; it is cost effective; and it reduces the load on grid generation and reduces the infrastructure needed for transmission and distribution facilities, etc. However, many engineers working in the power generation industry will cringe when reading this, as distributed power generation has challenges in many areas due to the way many of today’s power grids are set up.

Solar PV Cell Design

Solar cells contain light energy absorbing materials and convert it into electrical energy. The cell semiconductor material is defined by the difference between two energy levels: the valence band and the conduction band. The lower-energy valence band contains negatively charged electrons, while the higher-energy conduction band is empty. When the electrons are hit by photons with energy greater than the bandgap, they can absorb enough energy to be excited from the valence band to the conduction band, producing an electron-hole pair. An internal electrochemical potential separates the electron-hole pairs, causing the flow of electrons and holes, which creates an electric current. The internal electrochemical potential is caused by semiconductor doping, where one part of the semiconductor interface is doped with electron donors (n-type doping) and another with electron acceptors (p-type doping) creating a p-n junction.

Basic PV cell design. Image courtesy of Apricus.
Basic PV cell design. Image courtesy of Apricus.

Efficiency

The solar cell efficiency is limited because only one electron can be excited by one photon, regardless of the photon energy. Similar to the wind power plants’ limitations for maximum theoretical efficiency (which according to the Betz's law 16/27 (59.3%)), the solar PV cells also have limited maximum efficiency, known as Shockley–Queisser limit. The maximum solar conversion efficiency of a solar cell with a single p-n junction is approximately 33.7%, achieved at a bandgap of 1.34 eV. This limit (the peak of the graph from figure 3) is experimentally obtained by combining materials with different bandgaps into tandem solar cells. Figure 3 illustrates the dependance of cell efficiency on the bandgap. A high bandgap inhibits photons from causing the PV effect. In the case of a low bandgap, the photon energy is higher than the energy required to excite electrons across the bandgap, and the excess energy will be wasted. The commonly used semiconductors have bandgap values placed near the peak of the efficiency graph (for example silicon (1.1 eV) and CdTe (1.5 eV)).

Solar cell efficiency vs material badgap for a single p-n junction cells. Image courtesy of Sbyrnes321 from Wikipedia
Solar cell efficiency vs material badgap for a single p-n junction cells. Image courtesy of Sbyrnes321 from Wikipedia

The mentioned limitation applies only for a solar cell with a single p-n junction. The maximum efficiency can be increased by using tandem solar cells with multiple layers. Theoretically a tandem solar cell with an infinite number of layers could reach an efficiency of 86.8% while using concentrated sunlight. Multi-junction solar cells are made of different semiconductor materials which form multiple p-n junctions with multiple bandgaps. Different materials absorb different wavelengths of light, and they are optimized for each section of the spectrum.

Emerging Technologies Can Increase the Cost-Effectiveness

In order to impose a new technology on the market, economic and technical requirements must be met. The technology should be more efficient but still price-competitive with the currently available technologies. The new cutting-edge PV technologies that could replace c-Si include: concentrated photovoltaics, quantum-dot cells, perovskite, multi-junction cells, organic photovoltaics, cadmium telluride, copper indium gallium selenide (CIGS), and graphene. The first three technologies mentioned are the most promising ones.

Concentrated Photovoltaics

Concentrated PV (CPV) systems, similar to telescopes, contain lenses and curved mirrors which focus light on multi-junction solar cells. They also include solar tracking technology, providing more efficient sunlight absorbtion. CPV systems with a wide range of magnification ratio can be designed. They are grouped in three classes:


  • low concentration (LCPV), magnification ratio is less than 10X,
  • medium concentration (MCPV), magnification ratio between 10X and 150X;
  • high concentration, magnification ratio above 150X, usually less than 1000X.

According to the National Renewable Energy Laboratory (NREL), CPV has the best PV research-cell efficiency. In 2014, Fraunhofer Institute for Solar Energy Systems successfully developed a multi-junction CPV with 46% efficiency. The maximum efficiency of c-Si ever reached was 27.6% (with concentrated sunlight) and approximately 15% in commercial usage. Considering the cell efficiency, CPV has the potential to be the future technology. High cell efficiency provides a lower unit cost, because it requires less surface area to generate the same watt peak of electricity (Wp - the output power generated by a solar cell under a full solar radiation), thus decreasing the required number of cells.

The maximum potential of the CPV technology can be achieved by using the nonconventional multi-junction solar cells. Multi-junction cells use several different materials arranged in multiple layers (conventional single-junction solar cells are built from one layer of a single type of material).

Although the technology was tested in 1983, it has never achieved mass commercial usage.  The CPV requires expensive components, such as the solar tracking modules that precisely orient the cells directly towards the sun, which ultimately increases the design complexity and the balance of system costs (BoS – including all components of a PV system). This PV system is suitable only in the regions with high direct solar radiation, which limits its potential market. However, its high efficiency is promising for the markets where direct normal irradiance is the highest, such as the Middle East, North Africa, and Australia.

Quantum-dot Photovoltaics

This PV solar cell design uses quantum dots (semiconductor nanocrystals) as the absorbing PV material instead of bulk materials such as silicon, copper indium gallium selenide (CIGS), or cadmium telluride (CdTe). The distinctive feature of this technology are the tunable bandgaps. While the conventional bulk materials have fixed bandgaps, quantum-dot cell bangaps are tunable across a wide range of energy levels by varying the size of the quantum dots. The bandgap can be changed without changing the material or the construction techniques. This property makes the quantum-dot technology attractive for multi-junction solar cells. The quantum dots can be used as one junction in a multi-junction cell, capturing additional solar energy that is usually lost as heat. Lower bandgaps are more suitable to generate electricity from photons with lower energy (and vice versa). Although this technology has been identified as a key future technology for the solar industry, quantum dot solar cells are not yet commercially viable on the mass scale, mostly because of the low efficiency, which can reach only 8%. However the efficiency is not the only parameter determining the cost efficiency. The quantum-dot design can be combined with important existing technologies, thus improving their efficiency. This feature opens the posibility for it to be a technology of the future.

Perovskite

The perovskite solar cell (PSC) type includes a perovskite structured compound (a hybrid organic-inorganic lead or tin halide-based material) as the light-harvesting active layer. The materials have high absorption coefficients, providing ultrathin films (approximately 500 nm) for complete visible solar spectrum absorption. Perovskite materials are cheap and simple to manufacture. When it comes to efficiency, PSC efficiencies have extremely increased - from 3.8% in 2009 to 25.2% in 2019 in a single-junction design source. When combined with silicon-based technology, an efficiency of 28.0% can be reached, higher than the maximum efficiency of single-junction silicon solar cells. Therefore, the perovskite solar technology is the fastest-advancing solar technology. Considering its efficiency, thin, lightweight design of modules, and low manufacturing costs, this cell type is commercially attractive as the future technology. Currently the main challenge for peroskite is its sensitivity to moisture. It degrades quickly and needs to be protected by a watertight seal.

Summary


Although the c-Si is the dominant solar PV technology for the time being, it does not have the potential to remain so forever. The presented emerging technologies have the potential to replace the c-Si in the long term. The most promising emerging technologies are provided in the table below. However, the leading countries should make an effort to develop the emerging PV technologies to their full potential.









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