SOLOSOL - Saving the planet one property at a time
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 – that’s about 1018 Kilowatt hours (kWh).
ELECTRICITY FROM THE SUN
Electricity can be generated from solar energy in two ways. The first is to capture heat from the sun and use this to power a conventional turbine or generator. The other is to use the photovoltaic effect, which converts light directly into electricity using materials called semiconductors.
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.
Types of PV cells
By far the most common material for solar cells is crystalline silicon and these cells can be divided
into a number of categories:
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Monocrystalline wafers are the most efficient of the PV technologies in good light conditions. However, since they are cut from cylindrical ingots the cells are normally pseudo-square and cannot completely cover a module without a substantial waste of space. This makes them more expensive than other technologies. |
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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. |
These technologies utilise wafer-based manufacturing techniques. In other words, in each of the above approaches, wafers are processed into solar cells and then soldered together to form a module. |
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Thin film 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 in low light conditions and very sturdy. |
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Hybrid cells are a combination of monocrystalline and thin-film technologies, this has high peak |
Usually, solar cells are electrically connected, and combined into “modules”, or solar panels.
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.
There are four main different types of solar PV. The table below gives an indication of how they compare to each other.
Type of Solar PV |
‘Thin Film’ |
Polycrstaline |
Monocrystalline |
‘Hybrid’* |
Cell Efficiency at STC** |
8-12% |
14-15% |
16-17% |
18-19% |
Module Efficiency |
5-7% |
12-14% |
13-15% |
16-17% |
Area needed per |
Kaneka module: |
Sharp modules: |
Sharp modules: |
Sanyo modules: |
Area needed per kWp (for BIPV) |
Solar metal roofing: 23.5m² Glass-Glass laminate: |
Glass-Glass laminate: 10m² - 30m² (dependent on cell |
C21e tile: 7.8m² Sunslate: |
n/a |
Annual energy |
800 kWp/kWp |
750 kWp/kWp |
800kWp/kWp (C21e) |
900kWh/kWp |
Annual energy |
50-52kWh/m² |
100kWh/m² |
107kWh/m² |
139-150kWh/m² |
Annual CO2 savings per kWp |
344kg/kWp |
323kg/kWp |
323kg/kWp |
387kg/kWp |
Annual CO2 savings per m² |
22kg/m² |
40kg/m² |
46kg/m² |
60-65kg/m² |
* ‘Hybrid’ PV combines both monocrystalline and thin-film silicon to produce cells with the best features of both technologies
** Standard Test Conditions are: 25 °C, light intensity of 1000W/m2, air mass = 1.5
*** kWp = kilowatt ‘peak’. Solar PV products and arrays are rated by the power they generate at STC
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 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.
Hybrid System
A solar system can be combined with another source of power - a biomass generator, a wind turbine or diesel generator - to ensure a consistent supply of electricity. A hybrid system can be grid connected or stand alone.
Photovoltaic/Concentrator Hybrid 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.
