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The Clean Energy Project


Why is solar energy important?

It is expected that by the year 2050 the world's energy requirements will double today’s demand. Energy is without doubt a prerequisite for economic stability in both the developed and developing world; despite its current importance, the actual energy system is far from being self-sustainable. Achieving a completely sustainable energy system will require technological breakthroughs that radically change our paradigms on how we produce and use energy. A possible solution to this problem is to use solar energy. Every hour, our sun produces enough solar energy to supply the whole world’s annual energy requirements. Finding the means to convert the incident solar energy into usable forms to maintain the current way of life represents a main objective of The Clean Energy Project team.

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How does an organic solar cell work?

Organic solar cells convert sunlight into electricity. The first step is that light must be absorbed in the organic solar cell. This absorbed light causes the electrons in the material to increase their energy. Second, electrons must travel to a region where they can be collected (i.e., the donor-acceptor interface, see Figure 1). Once the electrons are collected, they can be extracted to give a current, or they can remain in the device to give rise to a voltage. The electrons that leave the organic solar cell as current can deliver their energy to whatever is connected to the circuit.



Figure 1. Illustration of how an organic solar cell works. (1) Light absorption and formation of an exciton (electron-hole pair); this step is followed by the promotion of an electron into the lowest unoccupied molecular orbital (LUMO) of an electron donor semiconductor (i.e., pentacene molecule); (2) electron transfer from the LUMO of the electron donor semiconductor to the electron acceptor semiconductor (i.e., C60 molecule); and (3) subsequent transport of the electrons to the electrodes. Note: HOMO is the Highest Occupied Molecular Orbital.

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How will The Clean Energy Project help find solar cell materials?

Understanding the properties of new materials that are the basis of alternative sources of renewable energy represents one of today’s major scientific challenges. Many of these materials are composed of large organic molecules that contain hundreds of atoms. These atoms can be rearranged in multiple ways to fine-tune the properties of the desired material. With the aid of World Community Grid, researchers will evaluate the conductive properties of at least 100,000 molecular structures (created by combinatorial methods) that are suitable for organic solar cells applications. The results of such an enormous number of computations will be used to create a database of molecular properties for data mining, which will be publicly available.

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What is the efficiency in a solar cell?

Solar cells are commonly characterized by the percentage of the incident solar light that they can convert into electrical power. Thus, the efficiency is given as a percentage. In general, the efficiency is determined by the material from which it is made and by the technology used to construct the solar cell. Efficiencies for commercially available solar cells range from about 5% to about 17%. Although inorganic-based solar cells have reached a maximum efficiency of up to 40%, these are expensive to produce and polluting when thrown away. The maximum efficiency reached for an organic-based solar cell is around 6% as of 2007. Therefore, there is still a lot of work to be done to improve them.

If researchers could find an organic-based solar cell whose efficiency reached 10%, these would be commercially feasible and would revolutionize the field of solar materials. Additionally, if these cells covered 0.16% of the surface of the planet, they would produce about an additional 20 TW (Terawatts, a trillion Watts), which will make up for the estimated increase in energy for the year 2050.

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What other technological applications will be relevant to this project?

The study of solar cells is similar in form to other fields. For instance, the interaction of titania (TiO2) with organic molecules in dye-sensitized solar cells is very similar to (heterogeneous) catalysis, the act of accelerating the rate of a reaction, where a metal particle or surface interacts with an organic molecule or a group of molecules.

Another technological application that will spawn from this project is the study of molecular electronics, where molecules are used for building electronic components. This means that we will potentially provide the means to extend Moore's Law.

Furthermore, the CEP plans to host a range of other calculations for cleaner energy such as work on solar concentrator and fuel cell materials. It is only with your help that researchers can go ahead and try to answer these questions of both pure and applied research.

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Screen saver/Graphics: How are The Clean Energy Project plots on the screensaver created?

The plots on the screen saver show how the energy and temperature of the molecules changes over time during a Molecular Dynamics Simulation (MDS). In this case, MDS are in some respect very similar to “real” bench-type experiments because we can use this computational technique to measure the properties of interest during a certain time interval.

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Screen saver/Graphics: Why are we interested in determining the energy and temperature of a molecular system for The Clean Energy Project?

We are interested in knowing the energy and temperature of a given molecular material because we need to investigate its stability (i.e., finding a the optimal arrangement of the molecules) and performance (i.e., functionality). Also, these two parameters can be later used to evaluate whether or not a particular molecular structure is suitable for alternative energy applications.

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Screen saver/Graphics: What does "kcal/mole" and "K" mean on these plots for The Clean Energy Project?

The energy units are kilocalorie per mole (symbol: kcal/mole), where a kcal is the amount of energy needed to increase the temperature of one kilogram of water by one centigrade degree (1˚C) and a mole is a measure of how many molecules are in the system.

The temperature units are Kelvin (symbol: K), which is a unit of absolute temperature. A change of 1 K corresponds to a change of 1˚C. In fact, 0 K represents the theoretically coldest temperature where all the molecular and atomic motion ceases. On the Kelvin scale, the freezing point of water is 273 (273 K = 0˚C = 32˚F).

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Screen saver/Graphics: Why are we interested in investigating alternative sources of energy for The Clean Energy Project?

Everyone knows that all the oil-based energies are finite and they can be exhausted in the near future, no question about it. But, how can we help to maintain the energy needs of today and the future? To increase our awareness on this problem, the Clean Energy Project team at Harvard University decided to create animated images, or "scipplets" (science applets), that show factoids of the current and future energy needs of today’s society.

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Screen saver/Graphics: What is the green object in The Clean Energy Project screen saver?

This is a small set of atoms that are being simulated on your computer; each atom is represented by a green sphere. This a "green project", after all!

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Screen saver/Graphics: What are The Clean Energy Project "Scipplets" all about?

"Scipplets" stands for Science Applets. In these pictures, we include energy-related facts that represent the main motivation of our project. Also, we expect to incorporate more "scipplets" as the project moves along, so that they become an informative tool for those interested in knowing more about alternative sources of energy.

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Screen saver/Graphics: The Clean Energy Project Scipplets refer to something called gigajoules (GJ). What is a gigajoule?

A gigajoule is a unit of energy. You can break the word into "giga" and "Joule". "Giga" is a greek prefix used to denote 1,000,000,000 and a Joule (J) is a unit of energy (the amount of force needed to move one kilogram of something one meter). So a gigajoule is 1 billion joules.

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Screen saver/Graphics: What does The Clean Energy Project blue plant leaf object represent?

This is our rendition of a solar cell modeled after a plant. It actually represents our goal for the project in the sense that we are working to discover materials that will be able to convert solar energy into a useable energy, as plants do everyday.

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Screen saver/Graphics: How does The Clean Energy Project benefit humanity?

The Clean Energy Project is focused on understanding the fundamental science of how flexible solar cells work, so scientists can design more efficient energy-related technologies. The results of the project will eventually help us reduce our dependence on fossil fuels to lower our carbon emissions, keep our air cleaner, and contribute to the fight against global warming. Our research will facilitate the development of cheap, flexible solar cell materials that we hope will be used worldwide.

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