EME 812
Utility Solar Power and Concentration

1.1 Solar Energy Conversion - Overview

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The energy that is naturally available from the solar source is quite enormous. The sun delivers 1.2 x 105 TW of power onto the Earth, the amount that surpasses any other energy resource by capacity and availability. For comparison, according to Crabtree and Lewis (2007), all recoverable Earth's oil reserves (~3 trillion barrels) account for 1.7 x 1022 Joules of energy. The sun supplies this amount of energy to the Earth in 1.5 days. To be utilized, the solar radiation needs to be converted into other forms of energy, such as electricity or usable heat. The question is: can we effectively do that at the scale of our demands?

According to International Energy Outlook of the U.S. Energy Information Association (EIA), current global demand of energy utilization is ~ 13 TW. This converts to 4.1 x 1020 Joules of energy per year. This demand is expected to grow up to ~20 TW in 2020.

Apparently, the solar resource contains enough energy to cover those demands. However, the critical limitations in solar energy conversion will be the efficiency of existing technologies and their overall capacity to produce power sustainably.

Before considering various types of conversion of solar energy, let us briefly review what solar radiation actually is. Here are a few main things we know from physics:

  • Solar energy is electromagnetic radiation.
  • Main components of solar radiation:
    • Infrared ( 52 – 55% λ > 700 nm)
    • Visible (42-43% 400 < λ < 700 nm)
    • Ultraviolet (3-5% 100 < λ < 400 nm) - see Figure 1.1
  • Solar radiation near the earth surface is essentially in the range λ 290 – 2500 nm.
  • Quantum (unit energy) of electromagnetic radiation - photon (E = hv) - is a often a more convenient term in the mechanism of solar conversion.
Diagram of electromagnetic spectrum. Including visible light within the solar light range
Figure 1.1. Electromagnetic spectrum.

This diagram lists all existing types of electromagnetic radiation across the scale of radiation wave length and frequency. The solar radiation reaching the earth’s surface predominantly consists of infrared (52-55%), visible (42-43%), and ultraviolet (3-5%) radiation, covering the wave length range from ~ 290 nm to 2.5 μm [Duffie, Beckman, 2013]

This mix of various types of electromagnetic radiation allows the sunlight to be converted through a variety physical mechanisms, which are:

  • direct conversion to electricity (through photovoltaic effect);
  • conversion to usable heat (for example, via thermal collectors);
  • conversion to matter / fuel (for example, production of biomass through photosynthesis).

Now, let us look in more detail at the state of the art of these technologies and learn what the main objective and challenges are there.

Read the following article to overview the main types of solar energy conversion, and try to find the answers to the self-check questions below.

Reading Assignment

Journal article: Crabtree, G.W. and Lewis, N.S., Solar Energy Conversion, Physics Today, 60(3), 37 (2007).

This article reviews the multiple possibilities to convert solar radiation into usable forms of energy. It discusses various ideas and recent advances in scientific research directed towards raising the conversion efficiency through better understanding the physicochemical phenomena.

Check Your Understanding - Essay Question

What is energy conversion efficiency? How would you define it in your own words?


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ANSWER:

Efficiency shows what percentage of energy input is converted to usable energy output.

Check Your Understanding - Essay Question

What is Shockley-Quesisser limit, and what is its value?


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ANSWER:

Maximum theoretical efficiency for a single p-n junction solar cell illuminated with unconcentrated light - 31%

Check Your Understanding - Essay Question

What are possible approaches to reach higher efficiency of sunlight to electricity conversion in solar cells?


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ANSWER:

(a) concentrating light, (b) use of stacked cells with different bandgaps to capture greater fraction of light spectrum, (c) use of materials that enable carrier multiplication - multiple electron excitement per single photon, (d) dye sensitization, (e) nanofabrication of solar cells to reduce transport losses

Check Your Understanding - Essay Question

What are possible approaches to reach higher efficiency of sunlight to heat conversion?


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ANSWER:

(a) concentrating light to reach higher temperature, (b) scaling-up conversion device, (c) separating infrared and visible light for combined photovoltaic and thermal conversion, (d) use of thermoelectric materials

Check Your Understanding - Essay Question

What are possible approaches to reach higher efficiency of sunlight to fuel conversion?


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ANSWER:

(a) creating faster growing biomass (e.g. by breeding), (b) re-ordering photosynthesis stages to avoid inefficiencies, (c) designing artificial nano-assemblies to produce fuel from water and CO2

As we perceive from this reading, numerous technologies and areas of research and innovation in solar energy conversion target the overarching objective to raise the device efficiency, thus making it more economically viable for implementation. This is especially true in the light of quite high capital costs for solar energy systems. This challenge is related to both initial materials and manufacturing.

We will talk more about efficiency on the next page of this lesson.