Solar energy is seeing major growth in demand, but also in technological development, reports Neil Eisberg
Solar energy is playing an increasing role in power generation, especially in the UK. Due to the good 2020 Spring weather in the UK, solar energy generation peaked at 9.68GW, beating the previous record of 9.55GW achieved in May 2019 (C&I, 2020, 84, 5, 4).
The global coronavirus pandemic, however, is significantly affecting US solar plant development and will curb the record installation levels expected this year, according to the US Solar Energy Industry Association (SEIA) in March 2020.
The latest forecasts before the crisis predicted annual photovoltaic (PV) installations would soar by 47% in 2020 to almost 20GW, SEIA and consultant Wood Mackenzie said in a joint report.
This represents an upward revision of around 2GW from previous forecasts made by the groups in September 2019. Growth would mainly be driven by the utility-scale market and total annual installations were expected to creep above 20GW in 2021, SEIA said.
As regards the global picture, the most recent report from the International Renewable Energy Agency (IRENA): Global renewable Outlook, notes in its Transforming Energy Scenario (TES) that solar PV, plus wind, will lead the way to meet the 2050 zero carbon emissions target. In the TES, over 60% of all power generation and some 73% of installed capacity will be solar PV and wind, up from the current 10% level of generation. Solar PV is predicted to supply over one third of total 2050 energy demand from a capacity of 8519GW, up from 384GW in 2017, while wind will supply around 25% or 6044GW, up from 514GW.
Efficiency & materials
This increase in demand is matched by increasing developments to improve PV solar cell efficiency and in alternative materials.
Silicon cells generally have efficiencies in the range 18-21%, with a maximum of just under 27%. Scientists at the University of Colorado Boulder, US, have increased the 21% efficiency of a silicon cell by a third by layering a perovskite solar cell on top of the silicon cell to produce what is also known as a tandem or multijunction cell. The solution involves a triple-halide alloy of chlorine, bromine and iodine, which in the correct proportions stabilises and improves the cell’s efficiency.
Halide perovskite cells look promising, but toxicity and stability are key factors in their use. The most efficient, offering over 25% efficiency, contain lead, which presents an environmental challenge, however, replacing lead with less toxic materials is particularly challenging. Tin has been shown to be a possible replacement, however, despite excellent optical characteristics, efficiency is not good and is said to fall off rapidly due to tin reacting with oxygen in the environment.
Researchers from the Helmholtz Zentrum Berlin für Materialien und Energie (HZB) and China’s Institute of Functional Nano & Soft Materials at Soochow University have used tin in a two-dimensional structure containing phenylethylammonium chloride (PEACl), in so-called 2D Ruddlesden-Popper phases. PEACI is added to the perovskite layers and heat treatment applied while the PEACI molecules migrate between the perovskite layers, resulting in vertically-ordered stacks of 2D crystals that prevent the tin cations from oxidising.
Tandem solar cells have also been created using halide perovskite cells with CIGS cells, comprising copper, indium, gallium and selenium, which can both be deposited in micrometer-thick layers to produce a solar cell with a thickness below 5µm. Researchers at HZB connected the CIGS bottom cell directly to the perovskite top cell so there are only two electrical contacts. In addition, an organised monomolecular layer of so-called SAM molecules was added to the CIGS layer to improve the contact with the perovskite. They have also introduced rubidium into the CIGS layer, which ‘has significantly improved the CIGS absorber material,’ according to Christian Kaufmann from PVcomB at HZB. The resulting two terminal tandem cell achieves an efficiency of 24.16%, a new world record for tandem cells. The cell is said to be particularly suitable for space applications due to its proton radiation hardness (Joule, doi: org/10.1016/j.joule.2020.03.006 ).
A six-junction solar cell has been developed at the US National Renewable Energy Laboratory (NREL) that has set a world record for the highest solar conversion efficiency at 47.1%, when measured under concentrated illumination equivalent to 143 suns. A variation of the same cell achieved 39.2% efficiency under one-sun illumination. Each of the six junctions or photoactive layers is designed to capture light from a specific part of the solar spectrum. A total of 140 layers of different materials from groups III and V in the Periodic Table support the performance of these different junctions in the device, which is three times thinner than a human hair. Like the CIGS-perovskite tandem cell, it has possible applications in satellite technology (Nature Energy, doi: 10.1038/s41560-020-0598-5).
Since conventional silicon PV cells only convert a small portion of incident sunlight into electricity, the remainder becomes heat, which degrades the efficiency of the panel. This heat needs to be dissipated without consuming energy, thereby excluding the use of refrigeration or air conditioning. This is a particular issue for panels installed in hot environments such as deserts, which are attractive locations due to the availability of sunlight.
Researchers in the Water Desalination and Reuse Center of Saudi Arabia’s King Abdullah University of Science and Technology (KAUST) have developed a cooling system that overcomes this problem. They have used a polymer that contains the desiccant, calcium chloride, which absorbs the moisture in humid air at night, for example, and swells into a gel, doubling its weight. By also incorporating carbon nanotubes into the polymer, the researchers were able to reverse this absorption releasing the water on heating during the day.
Since the gel is able to self-adhere to surfaces, by coating the underside of solar panels, they discovered that the water released was sufficient to cool the panels by up to 10°C in indoor testing, with greater efficiencies being obtained in outdoor testing, probably due to improved heat and mass transfer.
In addition to cooling solar panels, the researchers believe the approach could be used in other applications. According to KAUST researcher Renyuan Li: ‘The technology could be made as small as several millimetres for electronic devices, hundreds of square metres for a building, or even larger for passive cooling of power plants.’
In the dark
PV power generation relies on sunlight – or does it? Research in the Department of Electrical and Computer Engineering at the University of California Davis in the US has resulted in a PV cell that can generate up to 50W/m2 under ideal conditions at night, some 25% of the level of a normal cell in sunlight (ACS Photonics, doi: 10.1021/acsphotonics.9b00679 ).
Professor Jeremy Munday and graduate student Tristan Deppe are hoping to improve the power output and efficiency of the devices they have developed, which are based on a similar process to conventional PV cells, ‘but in reverse’. These new devices are thermoradiative cells that produce infrared light when pointed at the night sky, as they are hotter than their surroundings, so the current and voltage go in the opposite direction but still generate power. The cells will also operate during the day if sunlight is blocked or they face away from the sun.
And what about PV cells that can operate underwater, for example, powering autonomous submersibles? Certainly, conventional silicon cells will only operate in shallow water due to their narrow band gaps. Water is a good absorber of red and infrared radiation like silicon and amorphous silicon so optimal band gap values are needed for deeper water.
Researchers at New York University, US, have therefore looked for semiconductors with wider band gaps; at 2m depth, the optimum band width would be around 1.8eV, while at 50m, it would be 2.4eV. Organic cells known to function under low-light conditions, as well as alloys of elements from groups III and V in the Periodic Table, would fit this requirement (Joule, doi: 10.1016/j.joule.2020.02.005).
Increase in US solar installations predicted by the SEIA before the Covid-19 crisis
Global solar capacity forecast for 2050 by IRENA
Electricity generated at night by the University of California’s ‘reverse’ solar cell