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Basics of Harnessing Solar Energy

  • Solar energy is harnessed via two general technologies: solar thermal and photovoltaics

  • Solar thermal technologies convert sunlight directly into heat

  • Photovoltaics convert sunlight into electricity by liberating electrons within a special type of material called a semiconductor

  • Challenges for solar technologies include cost, efficiency, durability, and material/resource use

  • Intermittency of solar irradiation provides challenges for grid incorporation, and highlights the need for storage technologies

Solar energy is by far the most abundant source of energy on earth, with 173,000 TWh (terawatt hours) of energy from the sun striking the earth every hour. However, this energy is spread out over the earth’s surface unevenly over space and over time. How do we humans harness this energy and convert it into a useful form for us?


There are two general ways in which humans harness energy from the sun: solar thermal and photovoltaics. This module will briefly describe each of these technologies, then provide some basic level considerations, comparisons, and potential future outlooks.

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Solar overview

Solar Thermal Energy

In solar thermal technologies, solar energy is converted into heat, which then can either be used for commercial or household heating and cooling (solar heating and cooling, SHC). For example, a very simple solar thermal system might heat water for use in a shower. This thermal technology can be deployed at industrial scale to boil water into steam to turn a turbine and generate electricity (concentrating solar power, CSP).

A simple solar water heater runs water through pipes to heat the water on a sunny day. The warmed water is then stored in an insulated tank until use.

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Rooftop Solar Water Heaters: Source Alt Energy Mag

In a CSP plant, sunlight is reflected by an array of mirrors to a central location. This could be either a pipe running through a series of curved mirrors, or a tower at the center of a circular arrangement of mirrors. The concentrated sunlight heats a liquid, solid particles, or gaseous materials. In a power tower, this fluid can reach 1000°F. The hot fluid heats water into steam, which turns a turbine to generate electricity or the heat could be used for industrial processes.

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Concentrating Solar Power: Figure modified and annotated from the US Department of Energy: Solar Energy Technologies Office

Photovoltaics

Solar photovoltaics (PV) convert sunlight directly into electricity by taking advantage of special properties of materials called semiconductors. When sunlight hits the semiconductor, electrons are liberated and can freely move around randomly through the material. In order to turn this into usable electricity, the electrons must move in a common direction. This is accomplished in solar cells by stacking two types of semiconductors together, described in more detail below.

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Electron movement: In solar photovoltaics, solar energy in the form of photons prompts electrons to move. Electrons in a semiconductor material move around once a photon with enough energy to excite that type of material's electrons (from the sun, for example) is absorbed. The most common type of semiconductor material used in solar photovoltaics is silicon.

Electrical flow: To create a solar cell, typically two different kinds of semiconductors are stacked on top of each other. One kind of semiconductor has extra electrons and the other has extra “holes” (lack of electrons). In silicon PVs, these different materials are made by adding small amounts of other elements into the silicon: phosphorus is often added for extra electrons (negative), and boron is often added for extra “holes” (positive). 

 

The difference between the two types of semiconductors causes the mobile electrons to flow in a certain direction, creating electric current. Metal contacts (such as silver) accept these mobile electrons from the semiconductor, and the electrons are directed through a wire to consumer use.

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One solar cell only generates a small amount of electricity, so solar cells are linked together to form modules and arrays, which cumulatively helps generate more electricity.

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Figure copied from from pveducation.com

General considerations for developing solar technologies

For electricity generation, CSP and PV offer two very different approaches that come with their own advantages and challenges. Some of the biggest considerations are cost, efficiency, and material/resource use.

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Cost
Costs for PV solar energy decreased by about 80% between 2010-2020. Solar electricity is currently cheaper than most fossil fuel alternatives. Three main factors contributed to this rapid decline: technological advances, manufacturing scale, and policy incentives. Technological advances mostly include improvements in the manufacture of the polycrystalline silicon, as well as the manufacture and design of the solar cells themselves. 90% of these falling costs resulted from economies of scale as the solar sector has grown, and government driven initiatives such as the Sunshot Challenge. At this point, PV electricity is cheaper than CSP.

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Efficiency
A continuing challenge for solar energy conversion is efficiency. The maximum efficiency for a silicon solar cell is 33%. Technological advances look towards other materials, such as perovskites, or new cell compositions, such as double-sided cells or tandem cells. These approaches might absorb and convert more solar radiation, and better engineering may help maximize efficiency and prevent energy loss. 

 

A CSP plant uses thermal energy from the sun, which can actually be stored for short periods of time before being converted to electricity or used directly as heat for industrial processes.

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Materials and resource use
At the moment, PV technology relies on silicon. Silicon is the second most abundant element in the earth’s crust and is a major component of sand. But in order to work as a semiconductor, both for solar panels as well as computer chips, the silicon must be exceptionally pure and in crystalline form. Silicon solar panels are relatively long lasting, with an estimated life of 30-35 years, so solar panels are only beginning to enter the waste stream.

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For CSP plants, one of the largest resource uses is water to clean off the mirrors. As the mirrors become covered with dirt and dust, their capacity to reflect the sun becomes limited. This is an important consideration, since many CSP plants are located in deserts that already face water shortages.

Grid incorporation and storage

The biggest challenge with solar energy is its intermittency. The sun doesn’t shine at night, and clouds diminish solar radiation. Solar radiation also varies by season due to the different angle that it hits during winter and summer months. Since electricity must be consumed when it is generated, intermittency means that solar energy either needs to be stored for use during periods of low generation (for example, night-time), or it needs to be paired with other sources of electricity in the complex power grid. Electricity demand also can be shifted to take advantage of inexpensive solar energy during daylight hours. One advantage of CSP is that the fluid used can store solar energy (in some plants up to 17 hours), allowing for electricity generation a few hours after the sun goes away. Incorporating solar energy into the grid is a balancing act among energy sources for supply, demand, and storage.

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Source. California Independent System Operator (CAISO)

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Questions for deeper thinking

  • What are some of the current barriers to widespread solar implementation?

  • While solar is an increasingly attractive energy source, what are broader factors that need to be considered with incorporating solar energy?

  • What are some advantages/disadvantages for CSP vs. PV? 

  • An attractive aspect of solar energy is its scalability - solar installations can range from household roofs to utility scale. What are considerations for the different scales of solar?

Sources and further reading

Solar basics

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Costs of solar

Page last updated: September 6, 2022​

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