Published on EME 812: Utility Solar Power and Concentration (https://www.e-education.psu.edu/eme812)

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Lesson 1: Solar Energy Conversion and Utility Solar Power

Overview

Overview

Welcome to the first lesson of the EME 812. In this lesson, we will overview the main types and principles of solar energy conversion to usable outputs, such as electricity, heat, and fuel. There are quite a few technologies that help to do that. Some of those technologies are quite old and well-known, and some are still subject to current research. We will read a couple of recent review papers to learn about those technologies and their impact. Also, we will spend some time reviewing the concept of efficiency, which is a key metric of any process of energy conversion. Also, at the end of this lesson, I will ask you to refresh your knowledge of units and main terms used in the solar energy studies.

Learning Objectives

By the end of this lesson, you should be able to:

  1. explain the principles of solar energy conversion;
  2. list the examples and parameters of solar systems across the scale;
  3. calculate efficiency of a solar system, based on system performance information.

Readings

Journal article: Crabtree, G.W. and Lewis, N.S., Solar Energy Conversion, Physics Today, 60(3), 37 (2007) [1].

Journal article: Hernandez, R.R. et al., Environmental Impacts of Utility Scale Solar Energy, Renewable and Sustainable Energy Reviews, 29, 766 (2014) [2].

L. Radovic, Efficiency of Energy Conversion [3]

J. R. Brownson, EME 810 Solar Resource Assessment and Economics. 2.2. Basic Solar Jargon for Energy and Power [4]

1.1 Solar Energy Conversion - Overview

1.1 Solar Energy Conversion - Overview

The energy that is naturally available from the solar source is quite enormous. The sun delivers 1.2 x 105 TW of radiative power onto the Earth, the amount that surpasses any other energy resource by capacity and availability. That would convert to 3.78 x 1012 TJ of energy per year. For comparison, according to Crabtree and Lewis (2007), all recoverable Earth's oil reserves (~3 trillion barrels) account for 1.7 x 1010 TJ of energy. Thus, the sun supplies this amount of energy to the Earth in only ~1.6 days!

A few more stats: 

According to reviews of University of Oxford [5], the current global energy utilization is close to 1.6 x 105 TWh per year (i.e. 5.76 x 108 TJ/year). If we again compare this amount to the global solar energy flux, the Sun is able to cover this demand in only 1 hour and 20 min! It is sort of mind blowing.. 

However, 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?

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 reaching the Earth:
    • 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.
Credit: Victor Blacus via Electromagnetic Spectrum from Wikimedia Commons [6]

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). [1]

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 1

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

Check Your Understanding - Essay Question 2

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

Check Your Understanding - Essay Question 3

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

Check Your Understanding - Essay Question 4

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

Check Your Understanding - Essay Question 5

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

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.

1.2 Efficiency of Conversion

1.2 Efficiency of Conversion

Efficiency is a very important metric in energy conversion. It is most commonly used for evaluating and comparing various methods and devices in terms of technical performance, which is, in turn, related to cost of the technology.

Efficiency is the term we often hear and use in daily life and it comes up frequently in various system analyses, cost estimates, and decision making studies. So, we should spend some time on refreshing our basic understanding of the efficiency as a universal metric of conversion systems.

Reading Assignment

So, first of all, please refer to this Efficiency of Energy Conversion book chapter [3], and refresh your basic knowledge of the efficiency definition and use. This text uses a number of simple efficiency calculation examples describing fossil fuel systems. I encourage you to learn from them, and further we will see how the same approaches may apply to solar energy systems and facilities.

Based on this reading, can you answer the following questions?

Check Your Understanding - Essay Question 6

If an electric motor consumes 150 W of electrical power to produce 120 W of mechanical power, what is the efficiency of this device?

Check Your Understanding - Essay Question 7

How would you determine the energy conversion efficiency of a power plant that consists of three conversion sub-systems with efficiencies η1, η2, and η3, respectively?

Check Your Understanding - Essay Question 8

A light bulb converts electric energy to light and heat. Can you estimate efficiency of a 40 W light bulb emitting 950 lumens of light energy (assume 1 lumen equivalent to 0.001496 W of power)?

We see that efficiency of conversion η is an important metric of system performance. When applied to solar energy conversion systems, efficiency of solar energy conversion (η) is defined as the ratio of the useful output power (delivered by the conversion device) to the incident power (of the solar radiation):

η= P out P in ×100%

What can we learn from efficiency analysis?

  • What fraction of available energy is lost in the conversion
  • How one device is compared to another
  • What is the performance limit

Photovoltaic cells are compared by nominal efficiency measured at standard conditions [ASTM G173 guide]:

  • Air temperature 25°C
  • Irradiance of 1000 W/m2 (clear sky)
  • Air mass (AM) of 1.5G
  • Panel oriented perpendicular to the light beam

When the exterior conditions are kept constant, measured efficiency is solely a device characteristic. If we have solar irradiance measured and device performance data collected, we can tell at what efficiency the system operates.

[Solar Conversion Efficiency Cheat Sheet [7]].

Example

Generally, to estimate the efficiency of solar energy conversion, you would need:

  1. solar irradiance data, and
  2. performance data

Consider the example below, which shows estimation of the standard efficiency of a PV module.

solar panel & graph: current on y & voltage x axis Starts @ 5.6 amps & steadily declines till 45V then declines fast down 2 0 amps @ 52V

Figure 1.2. The connection of efficiency with performance. A solar module of total cell area 2 m2 produces a voltage of 45V and a current of 5A at the peak power.
Credit: Mark Fedkin

Standard solar input (irradiance) at the module surface: S = 1000 W/m2

Identifying power input and output:  Pin = S =1000 W/m2   

Identifying power output from the PV cell:  Pout = E × I / Area = 112.5 W/m2  (Note: from physics, power is equal voltage times current)

Then, for efficiency, we can write: η = Pout / Pin = 112.5 / 1000 ×100% = 11.25%

Conclusion: only 11.25% of energy flowing to the panel is converted to electricity.

The reason that energy conversion systems have less than 100% efficiency is that there are losses. The origin of those losses can be a complex issue, which could be better understood based on the physics and design of a particular conversion device – PV cell, concentrator, or thermal collector. We will get back to those considerations when talking about specific conversion technologies in detail in respective lessons of this course.

There is an important distinction between the total power (measured in W) and power density or flux (measured in W/m2). When we talk about the performance of a particular solar energy conversion device (for example, a solar cell), power density characterizes the "quality" of the energy conversion - how much power is generated by each square meter of the cell area. That may depend on various properties of the cell material, design, and physical principles behind the conversion process. In contrast, the total power reflects the overall output - the "quantity" of usable energy generated by the whole device per unit of time. In applications of solar energy (say, if we want to power a building), we always look at the total wattage of the system and ways to maximize that total "quantity" of energy supply.

For example, imagine a solar module. At a particular moment of operation, the output power of the device can be expressed as

Pout (total) = Pin(total) (η/100%) = S A (η/100%)

  • η = efficiency (%)
  • S = sunlight power density (irradiance) at the cell surface (W/m2)
  • A = total cell area (m2)

Logically, to increase the total output from that module, we need to either increase the efficiency or increase the total input power.

The avenue of raising cell efficiency leads us to the physics of the conversion process, materials properties, and cell design. The main research and development question here is how to make a better working cell.

The avenue of increasing the total input power lead us to three issues: (i) concentration of light, (ii) sun tracking, and (iii) system scale-up. Concentrating the ambient incident light would indeed increase the amount of energy supplied to the module per unit of time via increasing the S parameter in the above equation. Tracking - i.e., the orientation of the solar panel perpendicular to the sunlight beam - is another way to maximize the amount of absorbable radiation and also contributes to increasing the S parameter. Finally, increasing the size of the module by adding more cells to the system, increasing cell area, or multiplying modules (scale-up) would increase the total active area of conversion (A).

The technology scale-up is the way to match the solar power to commercial applications and consumers. The utility-scale solar power, which is the primary focus of this course, is discussed in the next section.

1.3 Utility Scale Power

1.3 Utility Scale Power

There are two main solar technologies that are being considered for large scale power generation: (1) Photovoltacs (PV) and (2) Concentrating Solar Power (CSP). Another type - concentrating photovoltaic (CPV) is currently not a major player, but there are a few large facilities that use CPV technology. PV and CSP are pricipally different in the type of energy conversion and type of solar resource they rely on. We are going to review the basics of those technologies and their current state in energy market in this lesson before considering more technical details further on.

Photovoltaics (PV)

Reading Assignment

So, what do we mean by the Utility-Scale Solar Power?

Please read the introduction on the website of the Solar Energy Industries Association (SEIA) [8] and watch the video below to get the basic idea about utility-scale photovoltaic systems.

Click for a transcript

PRESENTER: All right, we all know that the sun's energy creates heat and light. But it can also be converted to make electricity and lots of it. One technology is called solar photovoltaics, or PV for short. You've probably seen PV panels around for years. But recent advancements have greatly improved their efficiency and electrical output. Enough energy from the sun hits the Earth every hour to power the planet for an entire year.

Here's how it works. You see, sunlight is made up of tiny packets of energy called photons. These photons radiate out from the sun. And about 93 million miles later, they collide with a semiconductor on a solar panel here on Earth. It all happens at the speed of light. Take a closer look, and you can see the panel is made up of several individual cells, each with a positive and a negative layer-- which create an electric field. It works something like a battery.

So the photons strike the cell, and their energy frees some electrons in the semiconductor material. The electrons create an electric current, which is harnessed by wires connected to the positive and negative sides of the cell. The electricity created is multiplied by the number of cells in each panel and the number of panels in each solar array. Combined, a solar array can make a lot of electricity for your home or business. This rooftop solar array powers this home. And the array on top of this warehouse creates enough electricity for about 1,000 homes.

OK, there are some obvious advantages to Solar PV technology. It produces clean energy. It has no emissions, no moving parts. It doesn't make any noise, and it doesn't need water or fossil fuels to produce power. And it can be located right where the power is needed, in the middle of nowhere, or it can be tied into the power grid. Solar PV is growing fast. And it can play a big role in America's clean energy economy-- anywhere the sun shines.

Understanding the limitations in efficiency of solar energy conversion and taking into account the demands of centralized power generation, the technology scale-up is one of the important issues being developed by the government agencies in order to build sustainable energy future.

Obviously, there is a strong push for large-scale systems from the government and industry. But, along with the promise, the scale-up process brings new challenges to the energy conversion system design. Some of those challenges are:

  • lower than desired efficiency (theoretical limits suggest it can be much higher);
  • high up-front cost of materials and equipment;
  • energy storage (electricity or heat);
  • power distribution and transmission.

All these issues deserve more attention and will be covered in more detail in further lessons of this course. In this lesson, we are not yet digging into any technical details of the considered technologies but, rather, taking a plunge into the context.

The following materials will give you an idea of the current state of utlility scale solar market in the US.

Reading Assignment

Industry Report: U.S. SOLAR MARKET INSIGHT [9], 2018 year in review, Executive summary, SEIA, Wood Mackenzie Power and Renewables, Published March 2019. (You can access the executive summary of the Report in this Lesson).

The SEIA 2018 Market Report provides a general outlook of the role of PV solar technology at the scale of national energy development. In the year of 2018, additional 10.6 GW (DC) of PV capacity was installed, again showing continuing penetration of solar into the national energy market. Residential solar market grew by 7% in 2018, at the same time, from the reported data, we can observe the tug-of-war situation between photovoltaic and natural gas utility plants, which resulted in some contraction of utility scale solar share, which might also be due to uncertainties associated with solar tarifs.

In more detail, photovoltaic technologies will be studied in Lessons 4-6.

Cencentrating Solar Power (CSP)

The other prominent technology developed on the utility scale in the US and worldwide is Concentrating Solar Power (CSP). While CSP is currently outpaced by PV on the global and domestic market, this technology may be advantageous in the areas with high annual insolation.

Watch this 2-min video to overview the utility-scale Concentrating Solar Power (CSP) systems:

Click for a transcript

Ok Take the natural heat from the sun, reflect it against a mirror, focus all of that heat on one area, send it through a power system, and you've got a renewable way of making electricity.

It's called concentrating solar power, or CSP. Now, there are many types of CSP technologies. Towers, dishes, linear mirrors, and troughs. Have a look at this parabolic trough system. Parabolic troughs are large mirrors shaped like a giant "U." These troughs are connected together in long lines and will track the sun throughout the day. When the sun's heat is reflected off the mirror, the curved shape sends most of that reflected heat onto a receiver. The receiver tube is filled with a fluid. It could be oil, molten salt, something that holds the heat well. Basically, this super-hot liquid heats water in this thing called a heat exchanger, and the water turns to steam. Now, the steam is sent off to a turbine, and from there, it's business as usual inside a power plant. A steam turbine spins a generator and the generator makes electricity. Once the fluid transfers its heat, it's recycled and used over and over. And the steam is also cooled, condensed, and recycled again and again.

One big advantage of these trough systems is that the heated fluid can be stored and used later to keep making electricity when the sun isn't shining. Sunny skies and hot temperatures make the southwest U.S. an ideal place for these kinds of power plants. Many concentrated solar power plants could be built within the next several years. And a single plant can generate 250 megawatts or more, which is enough to power about 90,000 homes. That's a lot of electricity to meet America's power needs

Reading Assignment

Web article: 2018: The Year Sees Explosive Expansion of Concentrated Solar Power Capacity Globally [10], HELI SCSP, Accessed: April 2019.

While PV system significantly outpaced CSP in growth over the past decade, there is still a significant economic potential for converting solar thermal energy into power in a number of locations around the globe.  

I hope these materials give you a clear idea what kind of systems will be the subject for learning in this course. The following self-check questions allow you to iterate the basics once more before we move ahead.

Check Your Understanding - Essay Question 9

List the key technologies that have been involved in utility scale solar power generation:

Check Your Understanding - Essay Question 10

How is the utility scale power different from the distributed generation power?

Check Your Understanding - Essay Question 11

What were the top-3 ranked states for installing PV solar energy systems?

1.4 Environmental Impact of Utility Scale Solar Power

1.4 Environmental Impact of Utility Scale Solar Power

Utility-scale solar power systems are on the rise worldwide - the tendency being fostered by advances in technology, new energy policies, and markets. Because of this growth, there has been an increased interest among stakeholders to understand the environmental impact of such systems. In spite of the often idealistic public perception of solar technology as "green" panacea, an objective examination reveals both positive and negative impacts. Careful impact assessment of developing technologies is important in order to steer towards the optimal solutions that would take into account economic, environmental, and social sides of the trend.

Please read the following review article, which nicely covers the multiple effects of utility solar power.

Reading Assignment

Journal review article: Hernandez, R.R. et al., Environmental Impacts of Utility-Scale Solar Energy [2], Renewable and Sustainable Energy Reviews, 29, 766 (2014).

This article will be the background of the Lesson 1 forum discussion, and you will get a few questions on this material in the reading quiz (see the Summary and Activities page of this lesson for more details).

1.5 Refresher on Units and Terminology

1.5 Refresher on Units and Terminology

At the conclusion of this lesson, I want to refer you to some resources on basic energy units, conversions, and terminology that specifically applies to solar energy systems.

Please check this unit from the EME 810 Course (Solar Resource Assessment and Economics) by Prof. Jeffrey Brownson that explains the basic jargon and units used to measure solar energy and power. Even if you have taken EME 810 as a prerequisite, I encourage you to review the page Basic Solar Jargon for Energy and Power [4] as a refresher. Also, it would be useful to look through the original technical paper by Beckman et al. (1978), too, and use it the future if any notation questions arise. The main purpose of this material is for everyone to be on the same page when analyzing the solar energy conversion technologies further in this course.

Supplemental Materials:

These presentations provide additional explanations and illustrations to the concepts of energy conversion and efficiency. These resources are optional, but can be helpful if you need to refresh the basics.  

[11] [12]

  

Summary and Activities

Summary and Activities

Readings and activities within this lesson give you a general perspective of this course and set the context without yet addressing the specific science of the main solar energy conversion technologies. Here, we try to figure out what aspects and what impacts would be important when the systems are scaled up to the utility level. One of the key metrics for technology assessment is conversion efficiency; so, we started to explore that concept to some extent.

Lesson 1 Assignments
Type Description / Instructions
Reading Complete all necessary reading assigned in this lesson.
Discussion  Discussion Forum "Environmental Impact of Solar Power" 
  • Read the review article Hernandez, R.R. et al., Environmental Impacts of Utility Scale Solar Energy, Renewable and Sustainable Energy Reviews, 29, 766 (2014) in section 1.4 of this lesson.
  • Choose one beneficial and one adverse environmental impact of the utility scale solar energy development that you believe are most important. Do you think the pros outweigh the cons? Express your opinion in ~200-word post on the Lesson #1 Discussion Forum.
  • Comment on at least two other posts (choose those not yet commented upon). Respond to all comments to your post.
Reading Quiz Complete the Lesson 1 Quiz.

References for Lesson 1

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

Duffie, J.A. and Beckman, W.A., Solar Engineering of Thermal Processes, John Wiley & Sons 2013.


Source URL: https://www.e-education.psu.edu/eme812/node/3

Links
[1] https://physicstoday.scitation.org/doi/10.1063/1.2718755
[2] https://www.e-education.psu.edu/eme812/sites/www.e-education.psu.edu.eme812/files/1-s2.0-S1364032113005819-main.pdf
[3] http://www.ems.psu.edu/~radovic/Chapter4.pdf
[4] https://www.e-education.psu.edu/eme810/node/483
[5] https://ourworldindata.org/energy#all-charts-preview
[6] http://commons.wikimedia.org/wiki/File:Electromagnetic-Spectrum.svg
[7] https://www.e-education.psu.edu/eme812/sites/www.e-education.psu.edu.eme812/files/Slide_Efficiency_of_conversion.pdf
[8] http://www.seia.org/policy/power-plant-development/utility-scale-solar-power
[9] https://www.seia.org/research-resources/solar-market-insight-report-2018-year-review
[10] http://helioscsp.com/2018-the-year-sees-explosive-expansion-of-concentrated-solar-power-capacity-globally/
[11] https://www.e-education.psu.edu/eme812/sites/www.e-education.psu.edu.eme812/files/Energy%20and%20Power%20Basics_0.pdf
[12] https://www.e-education.psu.edu/eme812/sites/www.e-education.psu.edu.eme812/files/Energy%20Efficiency%20Basics%20SU20.pdf