Solar Energy Technology
Few technologies have received so much research and undergone so much improvement in efficiency as solar energy
Solar panels are arrays of semiconductor devices, mostly silicon-based, which convert sunlight into electricity. The first solar panels had an efficiency (percentage of solar energy converted to electricity) of just 6%, and were first used commercially in 1955. The photovoltaic industry was given a stimulus by the space industry, which first used solar cells on satellites as early as 1958 (Vanguard 1). Efficiencies have now reached around 18-20%, and production costs and economies of scale have drastically improved their commercial viability. Solar cells are available in a variety of forms and are used in over a hundred countries in large quantities.
Solar radiation is energy in the form of electromagnetic radiation from infrared (long) to ultraviolet (short) wavelengths. The amount of solar energy that strikes the earth’s surface (solar constant) is about 1,000 watts per square meter under clear skies, depending on the weather, location and topographic orientation. This energy can be used directly through solar thermal collectors, for heating, cooling, water disinfection, etc, or for generating electricity with photovoltaic cells (PV). Large-scale installations that collect the solar energy contained in the sunlight are generally referred to as solar power plants.
Types of solar panels:
Crystalline silicon: more than 90% of worldwide PV production. Since being introduced around 1990, multicrystalline has gradually increased its share of the market compared to monocrystalline silicon, overtaking it around 2005. In 2014, the global market share of crystalline silicon cells was about 90%, with multi-Si making up 55% and mono-Si 35%. According to forecasts, silicon cells will continue to be the dominant photovoltaic technology in the long term and will be the “workhorse” of power generation together with wind power plants.
Thin-film technologies: 10% of the world’s production of PV in 2013. The use of thin-film technology has fallen from its maximum of over 30% in 1995, since it has not been able to match silicon-based performances. As performance improves, the lower overall cost makes it likely to be a competitor in the future.
CIGS: copper, indium, gallium, selenide. CIGS is made by depositing a thin layer of the four metals on a substrate, such as rigid glass or flexible plastic. The higher absorption coefficient of the material permits the cell to be thinner than conventional solar cells. EMPA (Swiss Federal Laboratories for Materials Science and Technology) and ZSW (Zentrum für Sonnenenergie und Wasserstoff Forschung, Deutschland) have both achieved efficiencies in excess of 20% for CIGS. EMPA achieved 20.4% efficiency with a polymer substrate, and ZSW 21.7% with a glass substrate.
CdTe: cadmium telluride. In multi-kilowatt systems, CdTE has lower costs than conventional crystalline silicon PV. Its shorter pay-back time is a major advantage, but the presence of the toxic metal cadmium obliges the owner to recycle the material at the end of life. Tellurium is also a rare metal, so places a limit on this technology’s potential for large-scale applications. CdTe thin film technology accounts for 50% of all the thin film market, but still only 5% of world PV production, despite it being used in some of the world’s largest photovoltaic power stations, such as the Topaz Solar Farm.
a-Si: Amorphous silicon. Known more commonly for its use in LCD displays, this thin-film non-crystalline form of silicon is deposited on flexible or rigid substrates, such as plastic, metal and glass. Its chief advantage is that it does not contain any toxic metals such as lead or cadmium. The amorphous structure indicates that the atoms are not held in a rigid crystalline lattice, affecting its properties, including transparency.
Solar modules can produce electricity from light with frequencies in or close to the visible light EMR band range (400–700 nanometres), and cells designed for a single frequency (colour) are the most efficient. Splitting white light into a number of its composite frequencies and generating electricity from these separately can reach efficiencies of 50%. Another three-junction design has a theoretical limit of 58%, but to date these have proved impractical for commercial applications.
However, more usual cells are pushing to reach 20% efficiency. Solar modules used for domestic applications typically have power densities of up to 175 W/m2.
The semi-conductor photovoltaic cells in solar modules generate electricity by ‘knocking off’ electrons in a wafer of two types of conductors (at so-called ‘n’ and ‘p’ junctions). The flow of electrons which results is DC (direct current), which is the form of electricity used in all batteries, and other low-voltage equipment, such as mobile devices and computers. Whereas a refrigerator will run on AC at 220-240V, a transformer is required to convert the AC current from the grid to DC for devices such as a computer (12-18V).
In order to integrate the electricity generated by a rooftop array of solar modules, it needs to undergo the opposite conversion – DC to AC. This permits it to be fed into the household mains system, or indeed fed back into the external mains grid to be used by other consumers.
A solar inverter is a device which converts solar panel DC current to AC, and adjusts its voltage and frequency (50 cycles per second, or 50 Hz) to match that of the mains. The inverter is also referred to as a critical balance of system (BOS) component. There are three main types of solar inverters:
- Stand-alone: DC charging of battery from DC PV, but batteries may also be charged from an AC supply if available
- Grid-tie: matches DC phase of the solar generated electricity to that of the grid
- Battery backup: charges batteries (DC-DC), but also allows excess energy to be fed to the grid (DC-AC).
MPPT maximum power point tracking is employed by solar inverters to obtain maximum power from a PV array. The MPPT allows the current-voltage (I-V) curve to be monitored in real time to adjust optimal loading in response to changing environmental conditions.
|No. occupants||Power consumption kWh/year||No. solar modules||Generated power kWh/year (average/day)||Roof surface area /m2|
The average consumption per day per person is of the order of 3-4 kWh. A storage battery of around 4-8 kWh can be used in combination with grid backup to match a household’s consumption. By comparison, an electric car with a lithium-ion battery has typically 24 kWh capacity. Electric vehicles in the garage can therefore be used effectively as recipients of excess solar generated power, and as ancillary storage systems to be tapped for domestic use on cloudy days.
Solar Power Plants
A solar power plant refers to a largescale industrial plant for the capturing and conversion of solar energy. There are various types and designs of solar power plants, chiefly among which are solar photovoltaic farms, solarthermal plants, and solar towers.
The Webberville Solar Farm, Texas, has a peak output of 34MW. It consists of 128 thousand solar panels, and covers an area of 150 ha (1.5 km2). It cost $250 million and is expected to produce around 60 GWh per year. Austin Energy purchases the power under a 25-year PPA (Power Purchasing Agreement), and has the ambition of obtaining 35% of its electricity from renewable sources by 2020.
Some solar facts
The Earth receives 122 PW (1.070 EWh per year) of solar energy at its surface. The Sun offers 7,500 times more energy than the entire anthropogenic demand. We would need to tap into just 0.013% of available solar energy (0.44% over land) to replace all other energy sources.
When the solar radiation arrives at the earth’s atmosphere, the average intensity is 1,367 kW / m2 (solar constant). After reflection, absorption, and conversion processes as the sunlight passes through the atmosphere, an average of about 330 W/m2 (24% of the solar constant) is incident at the earth’s surface over 12 hours per day.
Humans consumed 505,000 petajoules primary energy in 2010. 17% of this was for electricity generation. The market value of global energy consumed in 2015 was approximately 9.1 trillion US dollars.