FREQUENTLY ASKED QUESTIONS – solar Energy


1. What is Photovoltaic solar energy?

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.

2. What difference is there between thermal solar energy and Photovoltaic solar energy?

 The photovoltaic solar energy system converts sunlight directly into electric power to run lighting or electric appliances. A photovoltaic system requires only daylight (non direct sunlight) 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.

3. What is a Photovoltaic system?

A photovoltaic (PV) system is a system which uses solar cells to convert light into electricity.

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.

4. What is a Photovoltaic system composed of?

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.

5.What is an inverter?

When sunlight strikes a photovoltaic 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 usable form of AC.

An inverter is moreover necessary to connect a PV system to the grid.

6. What is net metering?

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.

7. What is the feed-in tariff (FIT) and how does it work?

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. In countries such as Germany and Spain the demand for renewable energy systems has risen dramatically and the installation costs are coming down fast.

FITs can be shaped according to a country’s RE resources, its electricity distribution system and its RE targets. There are many design options to help take account of these variables, including some which make the system more compatible with liberalised energy markets (but carry higher investment risk). The important thing is that each technology is supported if viable.

8. Does Photovoltaic technology need bright sunshine to work properly?

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). Photovoltaic 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: photovoltaic make use of diffuse solar radiation as well as direct sunlight.

When sunlight strikes a photovoltaic cell, direct current [DC] is generated. By putting an electric load across the cell, this current can be utilised. 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, photovoltaic systems do not need to be in direct sunlight to work, so even on overcast days a PV module will be generating some electricity.

9. How much electricity does a Photovoltaic system produce?

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 Asian  countries like India, the average optima inclination is 15-30 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.

10. What does grid parity mean?

Grid parity means that, for consumers, photovoltaic 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.

Grid parity (competitiveness with retail electricity prices) will be reached progressively from 2010 onwards in several European markets. Countries with the highest solar irradiation and higher electricity prices, such as Italy and Spain have the potential to reach grid parity starting in 2010 and 2012, respectively. Grid parity will be reached in India in 2017 and cover progressively most other EU countries up until 2020.

11. Do Photovoltaic modules loose efficiency each year?

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.

12. Can renewable energy sources guarantee a secure energy supply despite their dependence on the weather?

The best way forward to ensure a secure energy supply for the future is an energy mix of renewable energy sources, intelligent load management in combination with energy storage. This will enable renewable energy sources to ensure a secure, climate-friendly and sustainable energy supply.

Solar power is particularly available during periods of peak load demand (midday and in summer) and is excellently complemented by wind power, whose peak values are principally reached in winter. Biomass and hydro power  energy are continually available and balance out any deficits.

13. What is the lifetime of a Photovoltaic system?

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 photovoltaic 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 (inverters), 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!

14. What if there is a problem with the Photovoltaic system?

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 (inverters), the guarantee usually does not exceed 10 years although longer inverter insurances can be arranged.

15. Is solar energy more expensive than conventional energy?

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, like all energy production technologies (coal, gas, nuclear, etc.) in the past and present, need financial support from the government to further develop the technology and thus reduce prices to become competitive

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 more than 10% per year.

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