Solar power – a primer

26 JUN 2019
Opinion articles Tech Trends

The Basics

The energy that the Earth receives from the Sun in just one hour is equal to the total amount of energy consumed by humans in one year. Part of this energy can be converted into electricity by photovoltaic systems. The basic unit of these systems is the light-absorbing solar cell, also called a photovoltaic (PV) cell. Conventional solar cells contain a rigid semiconductor wafer made of crystalline silicon as the PV material, although other PV materials are now in use as well.

Solar cells are usually about four inches square, and can be connected together to form a solar module or solar panel. Solar modules, in turn, can be combined with the necessary wiring, support structures, and other electronic components to create a complete solar installation, able to supply electricity for various purposes, such as direct use in appliances, lightning, battery charging, or to feed the grid.

Solar PV technologies

Solar panels can be classified into various key technologies depending on the type of solar cell material employed. Each technology has specific characteristics that optimise it for different applications in the solar market.

Crystalline silicon. Crystalline silicon, the semiconductor familiar from the electronics industry, is also the most conventional PV material. Higher efficiencies make it favourable for utility-scale applications, but also for residential and commercial rooftop markets. Nowadays, crystalline silicon accounts for more than 90% of the PV market. In the last 10 years, the efficiency of average commercial silicon modules increased from about 12% to 17%. In the laboratory, best performing solar cells are based on silicon heterojunction with an efficiency of 26.6%. Record efficiencies demonstrate the potential for further advances at the production level.

Inorganic thin-films. Inorganic thin-film PV use lightweight, flexible materials like amorphous silicon – which has been in the marketplace for over two decades – cadmium telluride (CdTe) or CIGS. Overall, the flexibility of thin-films makes them a good fit for consumer electronics and building integration. Thin-films can also be used in utility-scale projects in remote areas. The current lab record efficiency for thin-films is 23.3%, and belongs to CIGS solar cells. Unfortunately, most commercially available thin film panels have modest efficiencies of around 15%, which has led to a poor market share (around 5% of the current PV market).

Perovskites. Recently, the discovery of perovskite solar cells has drawn the attention of academia and industry due to the potential for low-cost manufacturing and high conversion efficiencies. Perovskites are essentially a type of inorganic thin-film solar cells, which show favourable properties like transparency and flexibility. Currently, pure perovskite solar cells have a record efficiency of 24.2%, but suffer from poor stability, which means they will struggle to make a dent in the market. Tandem solar cells combining perovskites and silicon constitute a more promising alternative, thanks higher record efficiencies (28%) and greater stability.

Multi-junction and high-concentrating PV (HCPV) systems. Multi-junction (MJ) PV cells use various layers of solar cells in a sandwich-like structure to achieve high power output, with staggering record efficiencies of 46%. Initially developed to power satellites, MJ solar cells are still prohibitively expensive, but are starting to be used in conjunction with concentrated mirror systems at small utility scale for peak-load assistance.

Organic PV (OPV) and Grätzel PV. Organic PVs use thin films of polymers or organic molecules to convert sunlight into electricity, while Grätzel PVs (named after Prof. Michael Grätzel and also known as dye sensitised solar cells-DSSC) employ a cell structure that contains an electrolyte, like a battery. Both offer light weight and flexibility, making them a potential good fit for a range of personal electronics applications. Though OPV and Grätzel cells offer similar advantages to inorganic thin-films, their overall stage of development is much more immature, such that their current efficiencies lag behind all other technologies, with a record of 15.6%. Their contribution to the overall solar market share is negligible, and is expected to remain in the background as perovskite solar cells gain more traction.

Solar installations

Solar cells and panels are rarely used in isolation. They need to be combined with the necessary wiring, support structures, and other “balance of system” components, to create a solar-power system with the desired voltage. Solar-power systems can be used to charge batteries for energy storage, to power motors, and/or to provide electricity for appliances and lighting. They can either be used independently or be connected to a central utility grid. In the latter case, power can usually be shared back and forth between the grid and the site of the solar-power system: When the solar-power system generates excess power, it can send it to the grid, and when it generates insufficient power, the user can pull reserve power from the grid. This arrangement with a local utility is referred to as net metering.

New developments

  • Bifacial solar panels: Bifacial modules offer a simple approach to improve the energy generation of a solar installation. By collecting light from both front and rear sides of the panel, manufacturers claim bifacial modules can provide up to 30% more energy than conventional panels in optimal locations with high reflectivity; depending on the design and layout. Motivated by their simplicity, many tier-one manufacturers like Jinko or Panasonic have started to offer bifacial panels as part of their portfolio. In the coming years, performance data from the first utility-scale installations will determine the real-life efficiency gains of bifacial panels.
  • Perovskite tandems: Perovskite tandems are a type of multi-junction structure where perovskite cells are combined with more mature photovoltaic technologies, such as crystalline silicon, enabling improved utilisation of the solar spectrum. To date, laboratory tandem cells from leading developer Oxford PV hold a record efficiency of 28%, already outperforming the best-in-class silicon heterojunction cells (26.7%). This landmark result sets high expectations for perovskite-based tandem cells to enter the market in the next two years.
    Floating solar installations: Over the past 10 years, floating PV projects have moved from niche deployments to large-scale utility applications. Floating solar systems are already competitive with utility-scale installations when land acquisition costs are high. In the coming years, new deployments and competition from novel start-ups aside from market leader Ciel et Terre will further drive down costs.