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Solar Explained

Introduction

The Sun is the original source of almost all the energy used on the Earth. It provides the energy that drives our weather systems and so the energy sources of wind, water and waves are in fact a form of solar energy. Trees and other plant life are sustained by using sunlight to create stored chemical energy through the process of photosynthesis and it is this energy that is released when planet material is burned. Since this energy from the sun is in continuous supply these energy sources can be constantly replenished and hence are categorised as renewables.

Fossil fuels, which the world is at present so dependant on for energy, are deposits formed from the dead vegetation of millions of years ago (an indirect form of solar energy). There are two major issues with fossil fuels that make them an unsustainable energy source. Firstly, being finite, they are not renewable (except over geological timescales) and so will eventually run out. Secondly, when burned they release large volumes of CO2 into the atmosphere. The increase in the atmospheric concentrations of CO2 is the main cause of global warming, which is destabilising the planet’s climate. The Royal Commission on Environmental Pollution’s Report ‘Energy – The Changing Climate’ indicated that the UK needs to reduce emissions of CO2 by 60% by the year 2050 and by 80% by the year 2100. A sustainable energy system must ultimately be based upon renewable energy sources, all of which are ultimately based upon solar radiation. Thankfully, the Earth receives a staggering amount of energy from the sun, as much energy falls on the planet each hour as the total human population uses in a whole year.

Photovoltaic Cells

The word photovoltaic is a marriage of the words ‘photo’, which means light, and ‘voltaic’, which refers to the production of electricity. Photovoltaic technology generates electricity from light. Electricity is the existence (either static or flowing) of negatively charged particles called electrons. Certain materials, called semiconductors, can be adapted to release electrons when they are exposed to light. One of the most common of these materials is silicon (an element found in, amongst other things, sand), which is the main material in 98% of solar PV cells made today. All PV cells have at least two layers of such semiconductors: one that is positively charged and one that is negatively charged. When light shines on the semiconductor, the electric field across the junction between these two layers causes electricity to flow - the greater the intensity of the light, the greater the flow of electricity.

Although the photovoltaic effect was known to the Victorians, it was not until humanity launched into the space race that the unique qualities of solar PV as a power source began to be fully explored. Following this kick-start the technology has raced along a path to commercialisation and the cost of PV generated electricity has plummeted as manufacturing costs have decreased and cell efficiencies have improved.

The following short film explains how photovoltaics work:

Types of PV cells

By far the most common material for solar cells is crystalline silicon and these cells can be divided into four main categories:

  • Monocrystalline

    		Wafers are very thin cells cut from cylindrical ingots. Because the ingots are round the square cells are normally cut with the corners missing, meaning the cells cannot completely cover a module without wasting a bit of space.

  • Poly or multi crystalline

    		Cells are made from cast ingots - large crucibles of molten silicon carefully cooled and solidified. These cells are cheaper than single crystal cells and used to be less efficient but steady developments in PV technology are now delivering comparible performance. They can easily be formed into square shapes that cover a greater percentage of a panel than monocrystalline cells. Both mono and poly crystalline wafers are processed into solar cells and then soldered together to form a solar PV module.

  • Thin film

    		These approaches, in contrast, are module-based. The entire module substrate is coated with the desired layers and a laser scribe is then used to delineate individual cells. Thin film PV is efficient inlow light conditions and very sturdy.

  • Hybrid

    		Cells are a combination of monocrystalline and thin-film technologies, this has high peak output coupled with excellent performance in poor light conditions. Usually, Solar panels have a sheet of glass on the front, and a resin encapsulation behind to keep the semiconductor wafers safe from the elements (rain, hail, etc). Solar cells are usually connected in series in modules, so that their voltages add together.

Types of PV system

  • Grid Connected
    The most popular type of solar PV system for homes and businesses. The solar system is connected to the local electricity network allowing any excess solar electricity produced to be sold to the network. Electricity is taken back from the network outside daylight hours. An inverter is used to convert the DC power produced by the solar system to AC power needed to run normal electrical equipment.
  • Grid Support
    The solar system is connected to the local electricity network and a back-up battery. Any excess solar electricity produced after the battery has been charged is then sold to the network. Ideal for use in areas of unreliable power supply.
  • Off-Grid
    Completely independent of the grid, the solar system is directly connected to a DC battery which stores the electricity generated and acts as the main power supply. An inverter can be used to provide AC power, enabling the use of normal appliances without mains power.
  • Photovoltaic/Concentrator Systems
    In order to save on solar cell cost by maximizing the utilization of expensive high-efficiency (37%) cells, one solution is to use a lens or mirrors to concentrate the solar rays onto a small area.

How photovoltaics work

The Carbon Cycle

The carbon cycle is the cycle by which carbon is exchanged between the geosphere, hydrosphere and atmosphere of the Earth. Human activities, particularly burning fossil fuels and deforestation, are increasing carbon dioxide concentrations in the atmosphere. Because of this, atmospheric carbon dioxide concentrations are higher today than they have been over the last half-million years or longer and the world is starting to warm up.

Carbon dioxide (CO2) is the main source of carbon in the Earth's atmosphere. Although it is a very small part of the overall atmosphere (approximately 0.04% and rising), it plays an important role in supporting life. Other gases containing carbon in the atmosphere are methane and chlorofluorocarbons (which are entirely artificial). These are all greenhouse gases whose concentration in the atmosphere has been increasing in recent decades, contributing to global warming.

Carbon is absorbed from the atmosphere in several ways:

  • When the sun is shining, plants perform photosynthesis to convert carbon dioxide into carbohydrates, releasing oxygen in the process. This process is most prolific in relatively new forests when trees are growing fast.
  • At the surface of the oceans towards the poles, seawater becomes cooler and CO2 is absorbed.
  • Organisms form tissues containing carbon, like carbonate shells, or other hard body parts, resulting in a downward flow of carbon.

Carbon is released back into the atmosphere in many different ways:

  • Through respiration, performed by plants and animals.
  • Through the decay of animal and plant matter.
  • Through the combustion of organic material. Burning fossil fuels such as coal, petrol, oil, and natural gas releases carbon that has been stored in the geosphere for millions of years. This is a main reason for rising atmospheric carbon dioxide levels
  • Through the production of cement. Lime, from which cement is made, is produced by heating limestone, which produces a substantial amount of carbon dioxide.
  • At the surface of the oceans where the water is warmer, dissolved carbon dioxide is released back into the atmosphere.

Solar photovoltaics require energy for manufacture which, (unless it comes from renewable sources!) also produces carbon dioxide. However, the amount of energy used, or carbon dioxide produced during manufacture is normally 'paid-back' (balanced out) within one to four years. Since photovoltaic panels last for roughly 50 years, using solar energy significantly helps reduce carbon dioxide emissions into the Earth's atmosphere.

  

The carbon cycle: Black numbers indicate amounts of carbon stored in the reservoirs, in billions of tons ('GtC' stands for GigaTons of Carbon). Purple numbers indicate the annual movement of carbon between reservoirs. The sediments, as defined in this diagram, do not include the ~70 million GtC of carbonate rock and organic matter in sedimentary rocks.