• Explore solar energy and hydrogen-based energy.
• Understand efficiency and how it affects the power output of a machine.
• Construct I-V curves.
• Become familiar with using equation fitting in MATLAB
• Test data acquisition skills.

Please complete the following worksheet during the first week (10/19-10/23) of this lab.

There will be no worksheet for week 2 of this lab. Lab report guidelines and rubric can be found at this link.

This 2 week laboratory will analyze the performance of a hydrocar system. Performance of each individual subsystem will be carefully investigated to predict the efficiency of the entire hydrocar system. Students will gain a stronger understanding of how photovoltaic (PV) and hydrogen fuel cells work and the underlying processes that affect their efficiency. Students will use their Photons and their data acquisition skills they gained from the previous labs to take measurements and make conclusions on the efficiency of different subsystems. Finally, students will present their findings in a lab report.

In your hydrocar kit, there will be a short instructional booklet. Please follow the instructions provided in the booklet to assemble your hydrocar.

A PV cell, or solar panel, is responsible for converting light energy to electrical energy. The underlying principle that dictates the operation of solar panels is the photovoltaic effect which is where electrons can be excited to another energy level due to interaction from a photon. Most common solar panels are made of silicon but the silicon used is usually impure, meaning it is mixed with other materials. Silicon has 4 valence electrons. When silicon is mixed, commonly known as doped, with a material like phosphorus that has 5 valence electrons, it is known as n-doped silicon. If silicon is doped with a material like boron that has 3 valence electrons, it is known as p-doped silicon. N-doped silicon has an extra electron and p-doped silicon is missing an electron and this missing electron is normally referred to as a hole. When n-doped and p-doped silicon is combined, the extra electrons from the n-doped side diffuse to the holes on the p-doped side. There will then be holes closer to the n-type and extra electrons closer to the p-type sides of the junction. The junction between the two opposite doped material is known as the depletion region. Figure 1 shows what the junction of the two materials looks like. Across the depletion region, an electric potential will build as the electrons and holes flow from one side to the other. Electrons and holes will flow backwards as well, but the electric potential will get to a level in which there will be a balance between the number of electrons/holes flowing across the depletion region.

Figure 1: Creation of the depletion region at the junction of p and n-doped silicon

Once the solar cell is exposed to light, the photovoltaic effect comes into effect. Photons will be excited to their next energy level and are then free to move into the material. All throughout the material, electrons are excited, but not all of them. At the depletion region, thanks to the presence of an already existing electric potential, the extra electrons are excited from the p-type side of the junction to flow to the positively charged n-type side of the junction. The holes on the n-type side of the junction flow to the negatively charged p-type side of the junction. So, electrons flow from the p-type side to the n-type side and vice versa for the holes. Since current flows in the direction of positive charge (therefore in the direction of the holes), current flows from the n-type side to the p-type side. The current extracted by this process can then be used as electrical energy. Figure 2 shows how the photovoltaic effect creates a current in PV cell.

Figure 2: Flow of current in a PV cell due to the photovoltaic effect

Using the solar panel to power the electrolysis process will require a large amount of time (on the order of hours). It will be difficult to measure voltage with the Photon and current with a multimeter by hand for that long period of time. Instead, we can construct an IV curve in which the voltage is measured using the Photon when there are different resistor loads. Ohm's law will then allow us to solve for the current. To create the IV curve for the solar panel, assemble the circuit shown in Figure 3. You will vary R_load by swapping in different resistors. 15 resistors is a good amount of resistors to test. Once you have collected the IV dataset, it is important to now fit a function to the data. A function of the form aexp(bx)+cexp(dx) tends to work well. But always double-check! With this function, it will now be much easier to determine the current for a measurement measured by the Photon during the electrolysis process. Use the HydroCar_IVCurve.m to create the IV curve for your solar panel. Make sure you record the coefficients for your fit.

Figure 3: Solar cell circuit for acquiring the IV curve while varying R_load

While you can try and take your measurements outside, it will be difficult to ensure a constant amount of solar energy for hours at a time. It is recommended to use a desk lamp, one where the bulb can get extremely close to the PV cell. If you use a desk lamp, make sure you record the wattage of the bulb and the type of light bulb. Your light bulb will most likely be either an incandescent or an LED. Incandescents are notoriously inefficient where only 10% of the energy is converted into light energy and the rest is converted into heat. An LED is much more efficient and typically has an efficiency of 80%. For this lab, it is safe to assume that the light power produced by your light bulb is constant over the course of the experiment. Using the time it takes for your measurements, the bulb wattage, and the bulb efficiency, you can estimate the amount of light energy that your solar panel was exposed to.

The fuel cell provided for this lab is responsible for conducting the electrolysis process and the forward fuel cell process. Electrolysis is the chemical process in which water molecules are split into oxygen and hydrogen molecules using an electric potential. In the case of this lab, the electric potential is provided by the PV cell or the battery. A diagram of the electrolysis process can be found in Figure 4. The steps are as follows:

1. H₂O reacts at the anode to form O₂, H⁺ (hydrogen ions), and electrons.
2. The presence of a potential difference allows for electrons to flow through the circuit and only hydrogen ions flow through the membrane to the cathode.
3. At the cathode, hydrogen ions combine with electrons from the circuit to form H₂.

Figure 4: Electrolysis Process

There is reaction that occurs at the anode: 2H₂O->O₂+4H⁺+4e^-
and a reaction that occurs at the cathode: 4H⁺+4e^-->2H₂

The net reaction from the electrolysis process can be summarized as: 2H₂O->2H₂+O₂

But how can we use this reaction to measure the efficiency of the electrolysis process? Reactions can be divided into two groups, endothermic (require energy) and exothermic (expel energy). From our understanding of how the hydrocar operates, we can hypothesize that the reaction is endothermic because the fuel cell requires energy from the solar cell to operate. To measure the energy required for this reaction, the bond energies associated with the bonds that are broken/created in the reaction. Broken bonds are exothermic and created bonds are endothermic. Bond energies for some common chemical bonds can be found at this link. However, make sure you divide your final answer by 2, because the right hand side assumes 2 mols of H₂0.

In the previous section, the energy required for the endothermic electrolysis reaction was calculated. However, the fuel cell had to consume a greater amount of energy for this reaction to take place. The form of energy that fueled the electrolysis process is electrical energy and through the role of providing a potential difference to the circuit, will electrolysis take place. The equation for electrical power associated with a portion of a circuit is: P=IV. Application of Ohm's law can lead to different forms of the equation. At this point in the course, students should be familiar with how to take voltage measurements using their Photon, therefore, half of the electrical power equation can be solved for. You can use the HydroCar_VoltAcq.m MATLAB code. A model circuit diagram of the setup can be found in Figure 5. Once the desired amount of hydrogen and oxygen gas has been stored, students will have a time history of the voltage in the circuit. To get a time history of the current in the circuit, students will need to use their IV curve and fitted function to determine the current. Since electrical power is the time derivative of the electrical energy consumed by the circuit, students will have to take the integral of their electrical power measurements. The integral of your discrete power-time measurements can be taken using the trapezoidal rule.

When the solar panel is connected to the fuel cell as explained in the instructional booklet, the hydrogen and oxygen gas will displace an equal amount of water inside the beakers, causing the water level to rise. After the water inside the hydrogen beaker starts to bubble up, disconnect the solar panel from the fuel cell. The total amount of hydrogen and oxygen gas produced during the process is equal to the difference in the water level before and after electrolysis. Use 0.082kg/m^3 and 1.429kg/m^3 as the densities for hydrogen and oxygen gas, respectively, to determine the mass of each produced. Next, use a periodic table to convert from kg to mols for each gas. Add the two quantities together to determine the net number of mols of product produced from this reaction. Use the energy per mol calculation from earlier and the total number of mols to determine the net energy required for the electrolysis reaction.

Figure 5: Circuit diagram for taking the voltage measurements necessary to calculate the electrical power of the circuit during the electrolysis process

With the stored hydrogen and oxygen gas, the fuel cell can now be used to power motor on the hydrocar. The action that converts the chemical energy stored in the hydrogen and oxygen gas into electrical energy takes place within the fuel cell. A diagram of a fuel cell is shown in Figure 6. The steps of the fuel cell reaction are:

1. Hydrogen gas is supplied at the anode of the circuit and reacts with a catalyst at the anode. The reaction between the hydrogen gas and the catalyst separates the hydrogen gas into hydrogen ions and electrons.
2. Electrons flow from the anode to the cathode.
3. Hydrogen ions flow through the membrane and combine with the oxygen gas and the electrons to create water molecules.

Figure 6: Fuel Cell Process of converting hydrogen and oxygen gas into water

There is reaction that occurs at the anode: 2H₂->4e^-+4H⁺
and a reaction that occurs at the cathode: O₂+4e^-+4H⁺->2H₂O

The net reaction from the fuel cell process can be summarized as: 2H₂+O₂->2H₂O

The net reaction of the fuel cell is the opposite of the reaction from the electrolysis process. Therefore, the same energy/mol calculation that was made from the electrolysis portion can be used but it is now the negative of that calculation because the reaction was reversed.

By this point, you should have hydrogen and oxygen gas in your hydrocar reservoirs. In the same manner as measuring the electrical energy consumed by the fuel cell during the electrolysis process, the electrical energy that is produced by the fuel cell and delivered to the motor must be measured. Once again, an IV curve will need to be constructed, but for the fuel cell this time. Use the same HydroCar_IVCurve.m code from before. When collecting data for your IV curve, you may run out of hydrogen and oxygen. If this happens, use the battery to refill your hydrogen and oxygen supplies. Instructions for this are found in the hydrocar booklet that came with your kit. Once refilled, continue with collecting data for your fuel cell IV curve. A circuit diagram for how to take these measurements is shown in Figure 7.

Figure 7: Circuit diagram for acquiring voltage measurements for the IV curve of the hydrogen fuel cell

With your generated IV curve and equation fit, you are now ready to acquire voltage measurements for the fuel cell when its powering the motor. Assemble the circuit shown in Figure 8. You can use the HydroCar_VoltAcq.m code from earlier, but you can change the time between measurements, dT, to something smaller, such as 5 seconds. Make sure you record the amount of hydrogen and oxygen before and after each test as the car may stop before all of the hydrogen has been depleted. It might be helpful to take the voltage measurements while the car is not moving. You can use the battery pack to lift up the front of the car to keep it from running away from you while you take measurements. Once you have your voltage measurements, you can then use your fuel cell IV curve to determine the current for each measured voltage. Then calculate the power and energy using the trapezoidal rule. The energy calculated represents the amount of electrical energy converted from the chemical energy through the use of the fuel cell.

Figure 8: Circuit diagram for acquiring voltage measurements while the fuel cell is powering the motor