Sustainability

Sustainability and Sustainable Design

Sustainability is a difficult concept to give one definitive definition to. Sustainability was coined and introduced in the Brundtland Report in 1987. The concept of sustainability is split up into three major categories: environmental, social, and economic. Environmental sustainability focuses on avoiding the depletion of nature's resources and maintain an ecological balance. Social sustainability promotes the connection and communication of cultures to create a greater quality of life. Economic sustainability lies on the basis of promoting economic growth while not harming the environment. Each individual category are intertwined which creates the large web of sustainability.

Sustainable design pursues to reduce any previously man made negative impacts to the environment. The minimal goal of sustainable design is to reduce the use of non-renewable resources, reduce waste, and help improve the environment.

Sustainable design principles are to:

• Optimize land potential
• Minimize the use of non-renewable resources
• Use environmentally safe products
• Protect nature's resources, i.e. water
• Optimize operation and maintenance of equipment

Solar cells are a perfect example of sustainable design. Solar cells harness energy from the sun and produce useful electricity that humans can use. Current commercial solar cells, which are small portions of solar panels, are made using silicon. Silicon is a perfect choice for current designs. Silicon is the 2nd most abundant resource in the world and is a main portion of sand. Silicon is also a conductive non-metal which makes it ideal for creating thin films, like solar panels.

Silicon Solar Cells

Solar Cells Design Process

Creating a Silicon Solar Cell follows the process of:

1. Starting with a raw material, which will be sand.
2. The sand is melted in very high temperatures to form cylindrical ingots. During this process Boron is added to give the silicon a positive electrical property.
3. The silicon ingots will then be cut into thin sheets called wafers. The side of the wafer that will be facing the sun will have an anti-reflective coating.
4. In an oven-like chamber, phosphorus is added to one side of the wafer to give the silicon negative electrical properties.
5. Solar Cells are then soldered together to form a solar panel. A thin piece of glass will be placed over the entire solar panel.
6. The panel is put in a tester to determine efficiency and problems of the solar cell.

How Do Solar Cells Work?

The positive Boron and negative Phosphorus portions of the solar cell meet and form a p-n junction. In a small portion of the middle of the junction electrons move from the negative Phosphorus to the positive Boron portion. This creates an electric field between each portion. Photons from the sun knock loose electrons which reach the electric field and shoots the electron out to metal conductor strips and into a wire, which will generate electricity. Electrons will then flow into the N-type Boron portion of the solar cell to be reused.

Constraints of Silicon Solar Cells

Solar cells today are not perfect by any means. They are a beginning to what can be a better, greener future. Today's best solar cells reach 25% efficiency and commercially is closer to 18%. Current estimates put the efficiency limits of silicon solar cells as described above at 31%. This low efficiency is in part to several flaws with the silicon solar cells. Photons do not all come in with the same energy. The silicon solar cells have a bandgap of certain energies that can be utilized. If a photon comes in with too little energy the solar cell will not work. Alternatively, if the photon came in with too much energy the excess energy is wasted and can not be used. This excess energy is lost to thermal relaxation. Another problem is orientation. When a photon knocks an electron out of the silicon it must reach the electric field that is created. If it does not reach the electric field the electron won't be shot out and no electricity is produced. Silicon solar cells also are not a popular choice of energy. One reason is that silicon is very expensive and many companies economically can not make the switch. On top of this, solar cells can be interrupted by natural cycles, such as day-night, cloudy-sunny, and winter-summer. While solar cells are a great environmental improvement that are still many constraints that need to be addressed.

Market

The global solar PV panels market size was around 115.24 billion in 2019 and it is estimated that in this year $131.04 billion. The Global market is expected to grow at a rate of 4.3% from 2020 to 2027.

Some of the companies that are manufacturing solar panels:

• JinkoSolar
• SunPower Corp
• Hanwha Q cells
• LG Solar
• Panasonic

Cost

Solar cells are not in the market but solar panel are. For an averaged sized installation in the United States, the installation would be around $11,144-$14,691. The average price/watt ranges from $2.51-$3.31. The average system size is about 6 kilowatts so about around $12,810 would be the cost of an average solar panel cost. Installing solar panels is worth it because you’re saving a lot of money. Especially in the southwest states like California.

Dye Sensitized Solar Cells

What Are Dye Sensitized Solar Cells?

DSSC was a concept first formulated by German professor Michael Graetzel and American chemist Dr. Brian O’Regan.

Dye Sensitized Solar Cells are a specific type of solar cell that can convert any oncoming visible light spectra to usable electrical energy. This light to electrical energy conversion is the basis for photovoltaic (PV) cells. DSSC technology is extremely versatile, having the ability to use artificial and natural light for electrical energy conversion. This allows the cell to function in situations that regular solar cells may not be able to, such as overcast weather (using artificial lighting).

DSSC function is very similar to the natural processes in plants such as photosynthesis. This makes it very versatile in biological application (Ex; mimicking photosynthesis in plant populations).

Composition of DSSC

A DSSC is comprised of a standard solar cell which contains a redox reaction electrolyte, a counter electrode, and a layer of photosensitive dye . The dye is used to capture light (ex: TiO2).

Pros and Cons of DSSC

Pros:

• Functionality is consistent in low lit conditions such as overcast weather
• Ability to absorb and function off of artificial light
• Encased with a thin layer of conductive plastic, which allows for easier heat release to surroundings.
• Functions at low internal temperatures, making it very efficient.
• Cost & function efficient for small scale applications

Cons

• Difficult to implement into large industrial work because of high costs.
• Some contain a liquid based electrolyte that is highly sensitive to temperature changes and may be unstable.
• The electrolyte solution present in the cell can leak and corrode the cell and its surroundings.
• High functioning counter electrodes for the cell are expensive (Ex: Platinum)

DSSC vs. Silicon Based Cells

Silicon Cells

• Absorbs large amounts of light (1,000x more than DSSC)
• Relatively expensive in cost to efficiency ratio.
• Better implementation into large scale processes
• Electrolyte solution and redox reactions assist in transport of excited electrons

DSSC

• Able to function in dim and artificial conditions
• Not as efficient as silicon cells at current stage
• Silicon acts as the source of the excited electrons

Future Restraints

Main Restraints:

1. Loss of energy that could be potentially absorbed
2. Material Cost and Efficiency
3. Location of Solar Panels

Loss of Energy:

-The biggest limiting factor with solar panels at the moment is the low percentage of sunlight that it can absorb. About only 15-22% of sunlight is captured by solar panels comprised of the average silicon-based solar cells.

- What is limiting solar panels from its full potential? Theoretically, by the 2nd Law of Thermodynamics, 100% thermal efficiency is impossible. There is guaranteed heat wasted by the heat engine (solar panel); however, improvements in the designs of solar panels have slowly been increasing the 15-22% threshold.

Material Cost:

- The biggest issue in the growth of solar panels and its development lies in cost. The average silicon-based solar panels have decreased in price throughout the past decade; however, more expensive solar panels that can capture 30-40% of solar energy is rare in the market. As solar panels can be used in different situations, NASA for example, use gallium arsenide-based semiconductors for their solar panels to combat the harsh environment of space.

- Band Gap (Direct/Indirect): A band gap is defined as the energy range required for an electron to become excited, in order to be used in conduction. In this range, no electron states can exist. The most optimal solar cells should have a direct band gap to maximize the amount of photons absorbed. Silicon has an indirect band gap making it less efficient, but more cheap. As such, development must combat the expensive costs of materials with indirect band gaps.

Location of Solar Panels:

- A large disadvantage of solar panels is their reliance on the sun and constant sunlight to store energy. For example, places like the Arctic and Antarctic who receive 24 hour of sunlight/darkness for several weeks at a time fall under this issue. As such, if solar panels could be made to maximize the sunlight to compensate the darkness, solar panels could be efficiently used in harsh environments.

Design Improvements

Life Cycle Analysis: Carbon

Using data from NREL, a national laboratory of the US Department of Energy, data has been collected through life cycle assessments (LCAs) on solar photovoltaic systems (PVs). In table 1, there are measurements on the values of harmonization on crystalline silicon, mono/multi-crystalline semiconductors, etc. which can be used to estimate the life cycle of carbon on the bifacial modules.

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