Solar energy will transform electricity production in the next 10-20 years. Learn how in this engineering PDH course.

In this 2-hour engineering PDH course, we will answer the following questions in detail.

2. How does a photovoltaic solar cell produce electricity?

3. What different types of cells exist today? How efficient are they?

4. How much does solar power cost?

5. How does the cost of solar power plants compare to other electricity-producing technologies?

6. What are the disadvantages of solar energy?

Here's a summary of some key points.

How much energy do we receive from the sun?

The Earth receives 174 petajoules of energy from the sun every second. Over a year, enough solar energy falls on the Earth's surface to replace all of today's global energy production. Its potential surpasses by far the combined energy output of coal, oil, natural gas, hydro, nuclear, and biofuels. Photovoltaic systems allow us to tap into this abundant resource by converting solar energy into electricity. In 2018, solar PV systems produced 2.4% of the global electricity supply, generating 505 GW worldwide and growing.

Solar PV World Electricity Production by Country

 Source: RENEWABLES 2019 GLOBAL STATUS REPORT 

According to Eicke Weber, Director of Germany's Fraunhofer Institute for Solar Energy Systems,

Between 2030 and 2050, we will see 10-30% of global energy demand covered by solar PV (photovoltaics). Right now, we are in an embryonic state compared to where we are going in a few decades. 

In this course, we will provide an overview of the use of solar energy for electricity production, specifically with solar photovoltaic technologies (also known as solar PV).

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How does a photovoltaic solar cell produce electricity?

The solar cell is made of a semiconductor, usually silicon that has been "doped" with an agent that has the propensity to either gain or lose electrons. Examples of doping agents are phosphorus and boron.

Phosphorous-doped silicon can lose electrons and is known as an N-type semiconductor. Since an N-type semiconductor wants to lose electrons, it acquires a positive charge.

On the other hand, boron-doped silicon has a shortage of electrons, or an abundance of "holes," and is known as a P-type semiconductor. Since P-type silicon wants to lose holes and gain electrons, it acquires a negative charge.

Joining these two types of silicon forms a PN junction, the foundation of the solar cell.

The PN Junction of a Solar Cell

Source: Physics Stack exchange 

An internal electric field (E) forms at the junction. The force of the electric prevents the free charges found in the P-type and N-type silicon from recombining.

When a photon hits the solar cell, it gives its energy to an electron in the crystal lattice. This energy excites an electron from the P-type silicon and "kicks" it into the conduction band, where it is free to move around within the semiconductor. The difference in energy between the valence band and the conduction band is known as the bandgap.

When the electron leaves the P-type silicon, it leaves its place empty, creating a "hole." An electron from neighbouring atom moves into the "hole," leaving another hole behind.

In this way, holes propagate throughout the crystal lattice. As this happens, electricity begins to flow in the circuit. This phenomenon is known as the photovoltaic effect.

Ideally, a photon with an energy equal to that of the bandgap hits the solar cell and excites an electron from the valence band to the conduction band.

Note that different materials have different bandgap energies, as shown in this table.

Bandgap Energies of Various Semiconductors 

Most solar panels today are made of silicon solar cells. However, most photons hitting the Earth have greater energies than the bandgap of silicon. The solar cell still absorbs these high-energy photons, but their "extra energy" is converted into heat rather than electricity. When this happens, the efficiency of the solar cell decreases.

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What different types of cells exist today? 

How efficient are solar cells at converting solar energy into electricity?

Researchers are continually working to improve the efficiency of solar cells. Each new wave of solar cells gives rise to a new generation of photovoltaic devices.

Today, there exist three generations. They are,

1. Crystalline Silicon (Monocrystalline and polycrystalline solar cells)
2. Thin film
3. Emerging PV

Commercially available mono and multi-crystalline silicon solar cells have efficiencies of around 18-23%. 

Second-generation solar cells, or thin-film solar cells, include amorphous, cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) solar cells. In the lab, both CIGS and CdTe have achieved cell efficiencies of 18% while the efficiency of amorphous-Si is only 13%.

The third generation of solar cells includes several thin-film technologies often described as "emerging photovoltaics." Most third-generation solar cells have yet to be commercialized and are still in the research and development phase. Today, the efficiency of this type of solar cell is about 10%.

Solar Cell Efficiencies

Source: IRENA Letting in the Light 

An example of emerging photovoltaics is the Perovskite solar cell. It is based on low-cost materials and shows much promise in terms of efficiency and cost.

Perovskite solar cell efficiency under laboratory conditions has improved dramatically, increasing from 14% to 22% in recent years.

Here is a picture of a perovskite solar module printed by inkjet on a flexible substrate. An external LED shows that the module can generate power under scattered light conditions.

Perovskite Solar Module

Source: Saule Technologies 

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How much does solar power cost?

When analyzing the cost of any utility, investors look at the "Levelized Cost of Energy" (LCOE). The LCOE is the ratio between the lifetime cost of the energy utility and its energy production. In short, the LCOE is the price the project must earn per megawatt-hour to break even.

This figure shows the range of LCOE for PV projects from 2010 to 2017, taking into account variations in the costs and sizes of individual projects. Between 2010 and 2017, the global weighted average LCOE fell 72%, from 0.36 USD/kWh to 0.10 USD/kWh (360 USD /MWh to 100 USD/MWh).

Cost of Solar PV Has Dropped 72% between 2010 - 2017

Source: IRENA Renewable Power Generation Costs 

The projected reduction in installation costs could result in an average 59% decrease in LCOE of commercial-scale PV projects between 2015 and 2025. By 2025, the global weighted average LCOE could be as low as 0.03 USD/kWh (30 USD/MWh).

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How does the cost of solar power plants compare to other electricity-producing technologies?

According to IRENA's 2018 report, "Renewable Power Generation Costs in 2017", solar PV utility-scale plant costs are comparable with other renewable sources of energy as well as fossil fuel-fired plants.

This figure shows the global weighted average LCOE of each technology, excluding any subsidies.

Average LCOE of Solar PV is Comparable to Fossil-Fuel Fired Power Plants

Source: IRENA Renewable Power Generation Costs

Notice that in 2017, the global average LCOE of solar was well within the range of fossil fuel-fired power plants for many regions around the world, as shown in this figure. It shows the regional average LCOE of each technology and excludes any subsidies. In Asia, Oceania, North America, and Europe, the regional average LCOE falls within the range of fossil-fuel-fired power plants based on 2017 data. The LCOE of solar ranges from 9.5 cents US per kWh in Asia to a high of 17 cents US per kWh in Africa and Eurasia.

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What are the disadvantages of solar energy?

Different electricity sources also have different environmental costs. Today, the world is focused on carbon emissions and greenhouse gases (GHGs), often presented in tonnes of CO2 equivalent. Solar power plants generate few  emissions, whereas other types of generation can produce substantially more.

However, reliability is a significant concern, as system operators are mandated to ensure supply meets demand at all times. Intermittent sources of power, such as wind and solar, must be backed up by other sources of generation. Which brings us to the topic of an upcoming course - energy storage. But first, we invite you to preview the first video of this engineering PDH course below.

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