The industrial revolution triggered an era of fast paced development; from crude mechanical machinery to the age of industries powered by coal and steam. Later, the trend inclined to a more complex form of development and that started when electricity is discovered. Now you might notice that the core of our series focuses more on power harnessing and energy transformations. The reason is to have a better grasp of the process and it's relevance. In today's post we will be discussing about solar energy. But to have good understanding and avoid confusions, especially on some terms. We will first tackle a refresher course about electricity, power and energy.
Electrical Energy: The Middleman
Power plants whether it's solar, hydro or wind have something in common and that is the final or output energy is in the form of electrical energy. I know most of us, if not all, has an idea as to why it is converted into this form. Electrical energy is the most convenient and 'flexible' type of energy to use. It is like a link, a middleman between energy transformations. The power plants harness raw energy and transform it to electrical energy which would later on be regulated in substations before being supplied to households and industries. Of course, there are still many sub-processes between these transformations but it's not necessary at this point as we only wanted to show how electrical energy became a link between these transformations.
The general summary of energy transformations from source(powerplants) to load(households+other industries) is like this:
raw energy(solar, hydro, wind etc) ---> electrical energy(plant output)---> usable energy(light from bulbs, heat from stove etc.).
Electricity vs. Electrical Energy
There is a misconception so widespread that it's even published in some educational materials and this is the misconception that electricity and electrical energy are the same. So why is this a misconception? Are they really different? They are very different as electricity works like a medium of transporting electrical energy. In a simple closed circuit for example with only a battery source and a light bulb load, electricity flows through the loop. You can refer to it on the figure as the red arrow. The flow of electricity is like the loop, it's following a cycle, while the flow of electrical energy is direct. The chemical energy from the battery is converted to electrical energy which flows through the wire, this would later be converted to heat and light energy when it reached the bulb. This is not a cycle as light energy will not go back to the battery unlike electricity which flows through the loop.[1]
Summary of Energy Transformations for the Example:
Chemical Energy(battery) ----> Electrical Energy(flows through the wire) ---> Heat/Light Energy(bulb)
The Law of Conservation of Energy
The law of conservation of energy simply states that energy cannot be created nor destroyed but it can transformed to other forms. In our example the last transformation(light energy) is not the end as the energy continuously transforms to it's other forms. When light strikes an object, energy is absorb in the form of heat which would later 'scatter' through the environment. It is only scattered but not destroyed. And if the battery from the start released a 100 J of energy then after some time 100 J of energy will scatter, nothing more nothing less. This is the law of conservation of energy.
Energy vs. Power
Energy and power are two physical quantities that are very much related with each other but are actually different. There are many cases in which these terms are interchangeably used like buying a generator for example or even simply inquiring information in the electric bill. Most of the time I overheard my neighbors comparing their bills with each other. LOL. Don't be mistaken. I'm not a busybody or something like that. I just often overhear these things. Haha XD
Anyway, let's move on to our topic; what is the difference between energy and power? Energy is commonly defined as the ability to do work. Pushing and pulling while covering a certain distance is work and it obviously needs a certain amount of energy to accomplish. The question is how much? The answer is actually very related to the law of conservation of energy. If 100 y-unit of work is to be done then we need 100 y-unit of energy to accomplish this work. Actually, the work and energy principle given by the formula: Wnet = 0.5mv^2 (final) - 0.5mv^2 (initial) is derived from the law of conservation of energy itself. Do not be overwhelmed by this formula as it just states that the work done is equal to the change in kinetic energy of an object. Based on this we can also conclude that energy required and the work done on an object have the same unit.
We have many different units used in measuring energy. These are Joules, BTU's, calories, Newton-meters etc. But the focus of our discussion is strongly related to electrical energy so the unit of energy we are really interested in is watt-hour(Wh). Yes you read it right the unit for electrical energy is watt-hour, the one you can see in your electric bill(kWh). As consumers, are we paying for electricity, power or energy? In a sense, we are paying for all of these but the one reflected on our bill is the kWh values, so it is more appropriate to say that we are paying for the energy. 1 watt-hour is one watt of electrical power maintained for one hour. This definition gives a hint about the relationship between energy and power. The critical difference between these two physical quantities is time.
Like we said earlier energy answers the question, 'how much?'. This means that energy focuses on quantity or volume. How much energy is delivered in an industry for a period of one year? Power on the other hand answers the question, 'how fast is energy consumed or generated?'. Power is the rate as how fast energy is consumed/generated in a system/load or in other words power is energy per unit time. [2]
The formula for electric power is given by the equation, P = i x v, where P is power, i is current and v is voltage. We can further expand this by substituting q/t to i resulting to P = q v / t where q is charge and t is time. Based on this formula we can also interpret that electric power is the product of charge and voltage over a period of time.
Take a look at the first gif image on our left which describes power consumption on load. When power is consumed like in this case the electrical component(load in this case) acts as a passive device. Passive devices consumed electrical power and later converts them to other energy forms. Some examples of these devices/components are loads like appliances and lighting.
On the otherhand active devices like electric generators and batteries generates power. These devices convert electrical energy from other types like mechanical(generators/turbines) and chemical(batteries) sources. You might notice the flow of charges on the images. This flow is based on the passive sign convention. Power either consumed(flows out) or generated(in) indicating if it's work done the system or work exerted into the system. In passive devices(first gif) the convention is from positive to negative indicating the work done by the charges on that device while in active devices this flow is reversed to negative to positive indicating that the charges are moved by an exterior force or work is exerted on the charges.[3]
So you might wonder why power is an essential quantity. Given that we already have the energy to utilize, why do we need to put emphasis on power? It is very important because of the usage, appliances for example, have different power ratings or requirements. Even if we have enough energy to supply, if the power or the rate doesn't meet these requirements then it will be very inefficient for both the producer and the consumer.
That recap is quite long so I hope you're still with me.I hope this refresher course will help in clearing some misconceptions in the future. There is still solar to discuss today while wind, geothermal and biomass in the near future.
What is Solar Energy?
Most of us have an idea about the term 'solar'. Solar panel, solar-powered and solar eclipse are only few of the many words that have the term solar attached to them. We know that when we talk about solar it has something to do with sun and it's quite funny to think that it's actually the meaning of the word. Solar is used to describe anything that is related to the sun. It originated from the Latin words 'sol' which means the sun and later 'solaris' which means 'of the sun'. Obviously, solar energy is the energy derived from the sun.[4]
The sun is a very powerful source of energy. The scale of energy it can provide to our planet is overwhelming large and this is one of the main reasons why it is a very desirable energy source. Studies have shown that the sun can provide us energy several times larger than our annual energy consumption. According to wikipedia and it's sources,
The Earth receives 174 petawatts (PW) of incoming solar radiation (insolation) at the upper atmosphere. Approximately 30% is reflected back to space while the rest is absorbed by clouds, oceans and land masses. The spectrum of solar light at the Earth's surface is mostly spread across the visible and near-infrared ranges with a small part in the near-ultraviolet. Most of the world's population live in areas with insolation levels of 150–300 watts/m², or 3.5–7.0 kWh/m² per day.
What do these numbers indicate? It only shows the immense potential of this renewable energy. Based on the figure out of the 174 petawatts of energy only 89 petawatts are absorbed by the land and oceans. How big is 89 petawatts? If you can remember our discussion on hydropower we listed there the capacity of the largest dams in the planet.
The Three Gorges Dam tops the list with a maximum power capacity of 22,500MW. If we compare these figures, 89 petawatts is several millions times greater than 22,500MW. Of course, it's quite impossible to harness all of this free energy but even a tiny fraction of this figure is significant enough to create a change.
'Most of the world's live in areas with insolation levels of 150–300 watts/m², or 3.5–7.0 kWh/m² per day.' What the heck does this mean? Just take a 60-watt light bulb lit for an hour as an example and perform some basic arithmetic. That's 60Wh of energy per hour. Now divide the insolation values by 60Wh and we'll get around 58-117. This means that we can lit 58-117 60W- bulbs for an hour in an area of 1 sqm. And if you have a land area of 100 sqm. then the energy you harness can potentially power up 5800-11700 60W-bulbs for an hour. How cool is that?
We now have an idea about the potential of solar energy. Actually, there are many technologies developed just to harness this free energy source like: solar water systems for water heating, HVAC systems for heating cooling and ventilation, solar cookers for cooking, heating and pasteurization and solar powered desalination unit for water treatment. But like usual we will focus our discussion on the mainstream technologies that are strongly related to power generation which are photovoltaic systems and concentrated solar powered systems.
Power Generation and Energy Conversions
There are many methods of harnessing solar energy and generating power out of it, but all of these methods/technologies are commonly based on combinations of two mainstream technologies which are photovoltaics and concentrated solar power.
Photovoltaics
Photovoltaic systems are the most popular way of harnessing solar energy. Photovoltaics(PV) directly converts light energy(photons) to electrical energy. PV's are usually semiconducting materials that exhibit the photovoltaic effect.
The term itself has some unique origin, it is derived from the greek word 'phos'(light) and the other half from the inventor of the battery, Alessandro Volta. Edmond Becquerel, first observed this phenomenon in 1839 and published the results on a scientific journal, Les Comptes Rendus de l'Académie des Sciences. According to his observations, an electric current is produced when sunlight hits two plates of gold or platinum immersed in acidic, alkaline or neutral solutions. Of course, the credit of discovering the photovoltaic effect was given to him.[4]
One downside of using a PV system(solar cells) is it's conversion efficiency. The most efficient cells are only 40% efficient while the global average is 17%. You might think that it is very low but do you know that the first solar cell has a conversion efficiency of a measly 1%. In 1884, Charles Fritts coated selenium with a thin layer of gold, the first solar cell in history. But this invention is not patronized as an alternative energy source at that time, reasons are obviously because of it's low efficiency and second is the cost of producing such technology. Thankfully, selenium p/n junction was discovered by Russell Ohl, raising the efficiency to 5% and this also marked the starting point for a more comprehensive research in this technology.
Why is the efficiency so low? Even today the most advance solar cells do not reach 50% efficiency. To understand the reason behind this let us take a closer look on the process that happens inside a solar cell, the photovoltaic effect.
How does a solar cell work?
Semiconductors like silicon are used in solar cells because they can undergo the process of doping. Basically, doping is the introduction of impurities to modulate the electrical properties of the semiconductor.
A typical solar cell is composed of silicon semiconductors, the p-type and the n-type silicon. A dopant like boron is doped in the p-type silicon and because the boron atoms are one atom short to form a bond with the nearby silicon atoms it will create vacancies of electrons or which we referred to as the 'holes'.
On the other hand the n-type silicon is doped with elements that have one more electron in their energy level, usually phosphorus. When a phosphorus atom bonds with a surrounding silicon atom there will be an excess electron. This electron can freely roam the n-type silicon material.
When the p-type and the n-type silicon material are layered together they will meet at some point known as the p-n junction. Now, this p-n junction is very special because there is an observable interaction in a molecular level between the n-type and the p-type silicon material. The nearest electrons of the the n-type silicon will fill the holes of the nearby p-type silicon. This scenario would negatively ionized the p-type side and positively ionized the n-type side nearest to the p-n junction. The area in which this scenario occurs is known as the depletion zone. Due to the positive and negative charges in the depletion zone an electric field is now present. The formation of ions will create an internal electric field in the depletion zone preventing the free electrons from the n-type material(obviously the majority and the remaining ones out the depletion zone) to fill the holes in the p-type material. In a sense, it's a very unique interaction to create a barrier or a borderline between the p-type and the n-type semiconductor.[5]
Now what would happen when it is exposed to the sun? Some photons will be absorbed by the silicon semiconductor. Most of the time the n-type material is exposed to the sun. When the photons are absorbed, some of them have enough energy to free the valence electrons from their bonds. These free electrons will leave 'holes'. The electrons will tend to move to the n-type layer while the holes to the p-type layer. This electron-hole movement will contribute to a current flow when connected to an external circuit.
Take note that the all the free electrons will not necessary flow through the external circuit, some of them will recombine with the holes left by other electrons. These electron-hole re-combinations is one major factor that causes low efficiency in solar cells. Another major reason is the energy level of the photons. The energy level should be just right to free the electrons. Having a lower energy level will not excite the electrons while having a higher energy level will waste some energy in the process.
Typically, solar cells can only produce 0.5 V but these cells can be connected together in modules and panels to create an additive voltage. The voltage is in DC so it still need to go through an inverter circuit to transform to it's AC equivalent.
Applications
It is undeniable that PV systems have become the most convenient way of harnessing solar energy. The bulk of the demand comes from grid systems which we're using use by households and small to medium sized industries. There is also a small market offgrid PV's. Another application is for space explorations. Actually, the use of photovolataics was popularized when Vanguard I used the earlier version of this technology as a supplementary power source. Unlike in the earth where there are many sources for energy, the options are very limited in space making the power systems for spacecrafts very costly. Although the PV's are also quite inefficient, when things are factored out it is still an excellent supplementary source for spacecrafts.
Summary Energy Conversions( in a typical household, grid)
Light energy(photons)---> Electrical Energy(DC)--->Electrical Energy(AC)
Concentrated Solar Power
Concentrated Solar Power(CSP's) or Concentrated Thermal is quite different from PV's. Unlike the direct conversion we usually see in photovoltaics, there is an intermediate process when it comes to CSP's. It is somehow akin to how we use our magnifying glass to lit a fire in camping trips. CSP's use mirrors and lenses with tracking systems to focus or collect a large area of sunlight onto a smaller area. [6] Usually, the small area is connected to a turbine-generator pair. When heat is absorbed it will make the turbine spin(mechanical energy). The turbine and generator is connected by a shaft. In the generator side a rotor is attached to this shaft. So how is it converted to electrical energy? Let us visit a clip from our previous post.
The answer lies in Faraday's Law of Electromagnetic Induction. Rotors are made up of wire loops stacked around magnetic steel laminations when it is fed by a direct current from a source an electromagnet is formed. The rotation causes the magnetic field to move cutting the flux constantly as the electromagnet passed through the conductors mounted in the stator. This process produces current and develop voltage at the output terminals of the generator.
There are four types of CSP's and some of the main differences lies in their mirrors or lenses. The most developed of them all is the parabolic trough system which uses polished metal mirrors lined in a series of parabolic plates. Incoming sunlight is gathered and concentrated by these plates along the focal line, where the energy will be transformed to heat. Other types of CSP's are the dish stirling, fresnel reflector and solar power tower.
Energy Conversions in typical CSP's
Light energy(Photons)--->Heat Energy(Collected Heat)---->Mechanical Energy(Turbine)--->Electrical Energy(Generator)
Pros and Cons of Harnessing Solar Energy
Pros
1. Abundance of Materials and Resources
The energy source (sunlight) is free and the potential amount we can harness is very very large. The element silicon which is commonly used as semiconductors in solar cells is also very abundant in the earth's crust, comprising 28.2%of element present in the crust.
2. Reliability
What is more reliable to know that the sun is always there. Rivers can ran out of water for some reasons but the assurance of the sun being there is the same assurance of life we have on earth.
3. Flexibility
PV's can are very flexible and can be mounted in structures saving space in urban areas. Both PV and CSP facilities can built and occupy many uninhabited areas near the equator(or anywhere with enough sunlight) and potentially harness unimaginable amount of solar energy.
4. Clean Harnessing Process
CSP's produce very minimal amount of greenhouse gases. It only happened in the heating process when some CO2 is released. PV's on the other hand produce almost zero GHG's in their harnessing process due to direct conversion.
Con's
1. Weather, Particles and the Sun doesn't shine 24/7
Both PV's and CSP's are very much affected by weather conditions like clouds, fog and dust. PV's are less sensitive because they can still harness diffuse sunlight unlike CSP's that demand more concentrated rays. While CSP's are more sensitive to weather changes they usually are built with backup storage that makes them process raw energy even if the sun is down.
2. Cost
Even the resources are abundant it doesn't necessary mean that building the actual technology and the facility is cheap. There have been many improvements in the PV's sector but the CSP's lag behind. Still both PV and CSP technology's costs are not cheap.
3. Ecology
In CSP's, insects usually are attracted to bright light and birds that prey on these insects follow them. The highly concentrated light has enough heat to roast the predator-prey tandem. XD
4. Efficiency
Both PV's and CSP's have low efficiency. Most research in PV and CSP technologies are actually to improve their low efficiencies. These researches focuses more on avoiding the re-combination of the electron-hole pair, absorbing the perfect energy photons etc.
Final Thoughts
Now, that's quite long. I hope you are still reading this . Haha. In my humble opinion, I think solar energy although the way we acquire it is somewhat inefficient, has the most potential in energy sources. We can even put solar farms in wastelands and let them float in polluted waters. But seriously, I believe that somewhere in the future solar energy will be a standard source for many nations in our planet. There is a already a PV hype now and it will still continue in the coming years. What do you think?
Related Readings:
Let's Talk About Renewable Energy #1: Hydropower
Text Credits
[1]-[2]-[3]-[4]-[5]-[6]
http://amasci.com/miscon/energ1.html
https://cleantechnica.com/2015/02/02/power-vs-energy-explanation
https://en.wikipedia.org/wiki/Electric_power
https://www.etymonline.com/word/solar
https://en.wikipedia.org/wiki/Solar_energy
https://en.wikipedia.org/wiki/Photovoltaic_system
https://en.wikipedia.org/wiki/Photovoltaics
https://education.jlab.org/glossary/abund_ele.html
https://en.wikipedia.org/wiki/Concentrated_solar_power
https://www.acs.org/content/acs/en/education/resources/highschool/chemmatters/past-issues/archive-2013-2014/how-a-solar-cell-works.html
Image Credits
Most are from public domain with CC0 and CC ASA attributions.
CC ASA is attributed to Frank van Mierlo, feministo and user A1
Some images are my masterpiece and you are free to use them. XD