The word Photovoltaic (PV) is composed of two terms: Photo – Photon which means “light” and Voltaic from “Volt” which is the unit used to measure electric potential at a given point.
Photovoltaic systems use cells to convert sunlight into electricity. PV cells can be made from different so-called semiconductor materials. Today, silicon is the most widely used material, but other, usually compound (made from two or more elements) semiconductors is also used. They are silent and non-polluting, utilise a source of energy that renews itself, and require no special training.
The photovoltaic solar energy system converts sunlight directly into electric power to Solar Powerrun lighting or electric appliances. A photovoltaic system requires only daylight to generate electricity.
The solar thermal energy system generates and produces heat. This energy can be used to heat water or air in buildings or in many other applications.
Both use the irradiance of the sun even if the technology is quite different.
A photovoltaic (PV) system is a system which uses solar cells to convert light into electricity.
Solar Power A PV system consists of multiple components, including cells, mechanical and electrical connections and mountings and means of regulating and/or modifying the electrical output. Due to the low voltage of an individual solar cell (typically ca. 0.5V), several cells are combined into photovoltaic modules,which are in turn connected together into an array.
PV systems can be used for homes, offices, public buildings or remote sites where grid connection is either unavailable or too expensive. PV systems can be mounted on roofs or on building facades or operate as a stand-alone system. The innovative PV array technology and mounting systems means that PV can be retrofitted on existing roofs or easily incorporated as part of the building envelope at construction stage. Modern PV technology has advanced rapidly and PV is no longer restricted to square and flat panel arrays but can be curved, flexible and shaped to the building design.
“Grid connected” means that the system is connected to the electricity grid. Connection to the local electricity network allows any excess power produced to feed the electricity grid and to sell it to the utility. Such a PV system is designed to meet all or a portion of the daily energy needs. Typical on-grid applications are roof top systems on private houses.
The figure shows how electricity generated by solar cells in roof-mounted PV modules is transformed by an inverter into AC power suitable for export to the grid network. The householder/generator then has two choices: either to sell all the output to the local power utility (if a feed-in tariff is available) or to use the solar electricity to meet demand in the house itself, and then sell any surplus to the utility.
“Off-grid systems” have no connection to an electricity grid. Off-grid systems are contributing to rural electrification in many developing countries. PV is also used for many industrial applications where grid connection is not possible e.g. telecommunications, especially to link remote rural areas to the rest of the country.
Elements of a grid-connected PV system are: PV modules – converting sunlight into electric power, an inverter to convert direct current into alternating current, sub-construction consisting out of the mounting system, cabling and components used for electrical protection, and a meter to record the quantity of electric power fed into the grid.
Off-grid (stand-alone) systems use charge controllers instead of inverters and have a storage battery for supplying the electric energy when there is no sunlight e.g. during night hours.
When sunlight strikes a photo voltaic cell, direct current [DC] is generated. By putting an electric load across the cell, this current can be utilized. An inverter is an electrical device which converts direct current [DC] to alternating current [AC].
Solar cells produce direct current. Most of the electrical devices we commonly use however, expect a standard AC power supply. An inverter takes the DC from the solar cells and creates a useable form of AC.
An inverter is moreover necessary to connect a PV system to the grid.
Solar electric systems use PV technology to convert sunlight into electricity during daylight hours. In a grid-connected PV system, PV modules pass DC electricity through an inverter to convert it into AC power. If the PV system AC power is greater than the owner’s needs, the inverter sends the surplus to the utility grid for use by others. It allows sending excess solar electricity back to the utility company.
If a home or office requires more electricity than can be provided by the PV system, the balance is provided through the grid connection. The utility provides AC power to the owner at night and during times when the owner’s requirements exceed the capability of the PV system.
In many countries, the utility company purchases all PV electricity generated at a higher rate (feed-in-tariff) than the tariff applied for consumed electricity. In this case, a dedicated metering exists for “PV generation” and a second metering for “power taken from the grid”, applying each different tariffs.
They put a legal obligation on utility companies to buy electricity from renewable energy producers at a premium rate, usually over a guaranteed period, making the installation of renewable energy systems a worthwhile and secure investment for the producer. The extra cost is shared among all energy users, thereby reducing it to a barely noticeable level.
FITs have been empirically proven to generate the fastest, lowest-cost deployment of renewable energy, and with this as a priority for climate protection and security of energy supply, not to mention job creation and competitiveness, FITs are the best vehicle for delivering these benefits.
The FIT system means that the pay-back time for PV is no longer several decades but several years instead.
A PV system needs daylight to work but not direct sunlight. Indeed, if a PV module is exposed to an artificial light, it will also produce electricity.
The light of the sun consists both of direct light and indirect or diffuse light (which is the light that has been scattered by dust and water particles in the atmosphere). Photo voltaic cells not only use the direct component of the light, but also produce electricity when the sky is overcast. It is a common misconception that PV only operates in direct sunshine and is therefore not suitable for use in temperate climates. This is not correct: photo voltaic make use of diffuse solar radiation as well as direct sunlight.
When sunlight strikes a photo voltaic cell, direct current [DC] is generated. By putting an electric load across the cell, this current can be utilized. The amount of useful electricity generated by a PV module is proportional to the intensity of light energy, which falls onto the conversion area. The greater the available solar resource, the higher the electricity generation potential.
However, as the electrical output of a PV module is dependent on the intensity of the light to which it is exposed, it is certain that a PV module exposed to the sun at midday by clear sky, will produce maximum of its output electricity. You can thus indeed say that PV modules will tend to generate more electricity on bright days than when skies are overcast. Nevertheless, photo voltaic systems do not need to be in direct sunlight to work, so even on overcast days a PV module will be generating some electricity.
The electricity production of a PV system depends on external (environmental conditions) and internal (technology, layout of the system) parameters.
The efficiency of the PV module depends on:
The size of the PV system and its technology
The orientation of the PV module towards the sun. The optimal orientation for locations above the Ecuador is the south.
The tilt angle or inclination of the roof. For European countries, the average optima inclination is 30-35 degrees
The irradiance value on site
The climate zone.
Shadows on the modules (also if they appear only at certain times of day) reduce significantly the gain of the whole system and should be avoided.
The map below represents the yearly sum of global irradiation on a horizontal (inclined) surface. Alternatively the maps represent solar electricity [kWh] generated by a 1kWp system per year with horizontal (or inclined) modules.
Grid parity means that, for consumers, photo voltaic electricity will be cheaper than the retail electricity price.
In the light of decreasing solar electricity generation costs and increasing price for conventional electricity, solar power systems will equally become increasingly economic during the next few years. During the next 5-10 years, solar electricity will become cheaper (depending on location and peak hours) for private households than retail electricity.
A considerable advantage of solar electricity is that it is mainly produced around midday when conventional electricity is particularly expensive. Solar electricity largely replaces expensive peak-load electricity at preferential customer prices, which is why it would be wrong to compare it with cheap base-load electricity.
The degradation of the PV modules varies from the type of PV modules installed. The loss of power production within the lifetime of 20 to 25 years is estimated to 10 to 20% for crystalline PV modules.
The CO2 savings of a solar roof will depend on many factors, including:
- The energy source the solar production is replacing (coal, gas, hydro-electric, nuclear…)
- The quantity of energy produced by the solar roof (depending on the roof’s location, orientation, inclination and shading)
- The quantity of electricity needed to manufacture the photo voltaic system (modules, inverter, cables, etc.)
- The “energy habits” of the solar roof owner.
How much CO2 will a solar roof save If your electricity comes from a coal fired power station, each kWh you use will release around 1.000 g of equivalent carbon (various greenhouse gases converted into “equivalent carbon units” for comparison). However, if your original electricity comes from a hydro-electric power station, it is producing much less carbon equivalent emissions (less than 10g).
A very important factor is the design of the system. If a system is wrongly designed (e.g. modules facing the south and 90degree inclination) the electricity output will be very low and therefore the system will not replace much conventional electricity.
So clearly the amount of CO2 you will be saving is very dependent on the source of the energy replaced. Next to CO2 savings, each m² of solar module installed will produce clean and sustainable home-made electricity.
Definitely! With efficient solar power systems, this is sufficient to generate a considerable volume of electricity and heat from solar power.
Hence it is worthwhile producing solar energy not least because this makes us less dependent on energy imports but also because:
- The fuel is free
- It produces no noise, harmful emissions or polluting gases
- PV systems are very safe and highly reliable
- It brings electricity to remote rural areas
- The energy pay-back time of a module is constantly decreasing
- It creates thousands of jobs
- It contributes to improving the security of Australia’s energy supply.
The estimated lifetime of a PV module is 30 years. Furthermore, the modules’ performance is very high providing over 80% of the initial power after 25 years which makes photo voltaic a very reliable technology in the long term.
Most manufacturers in general propose performance guarantees on the modules after 20 years of 80% of the initial output power. On the electronic components and accessories (inverts), the guarantee usually does not exceed 10 years.
But this doesn’t mean that PV systems don’t produce energy after 20/25 years. Most PV systems installed more than 25 years ago, still produce energy today!
If a PV module has a defect or no longer produces electricity or, under identical conditions, produces much less electricity than before, it is generally covered by the manufacturers’ performance guarantee against a drop in efficiency of more than 20%.
Most manufacturers indeed propose performance guarantees on modules of 20 and 25 years for 80% of the initial output power. On the electronic components and accessories (inverts), the guarantee usually does not exceed 5 to 10 years.
In the light of decreasing solar electricity generation costs and increasing costs for conventional electricity (due to oil and gas prices), solar power systems will equally become increasingly economic during the next few years.
A considerable advantage of solar electricity is that it is mainly produced during the day when the demand is high and therefore electricity is particularly expensive. Another important characteristic is that PV is normally produced at the same site than demand; therefore, it is not necessary high investment on extending the electricity infrastructure.
In the long term, solar energy will be much cheaper than conventional energy. However, solar energy is already well on the way: whereas the costs for conventionally generated energy have constantly increased in recent years and – faced with finite resources – will continue to increase by a considerable extent, increasing mass production has enabled the cost of solar energy to drop by an average of 10% per year.