As we learned last week, light from the sun (or in your case, an incandescent light bulb) can be transformed by a solar cell into usable electrical energy which can, in turn, be used to power things like a light bulb. However, as soon as the sun goes down (or the light goes out), the electricity stops flowing. Can we store energy for use later?

In this lab, you will design a system for capturing energy from the lamp to use to run your light bulb when your “sun” goes down. You will study how energy is changed from one form to another, and calculate the efficiency of each step in the process.

An example of a real-world gravitational potential energy “battery” (sort of like what you are going to build today) is this new project in Switzerland! (Maybe turn the computer volume down or off… there are no words, just music.)

By the end of this lab, you will have…

• … observed the transformation of energy between different types (light, electrical, gravitational potential, kinetic, rotational, etc.);
• … stored energy for later use in a water reservoir “battery”;
• … practiced engineering and experiment refinement by building towards a design goal; and
• … calculated efficiency stage-by-stage for a multi-step process.

You will again work in groups of three. The TA will introduce the equipment available and guide you through the lab. This time, the focus of the experiment is on designing and modifying equipment to achieve a goal – namely, to store energy which can be used to light a bulb after the “sun” has gone down. You will be working with solar panels to power a hydraulic pump. The pump is used in turn to store water in a “reservoir” which can later be released to turn a turbine and light an incandescent bulb or LED.

It will be best to work on this project in steps: get the first piece of the design working before moving on to the next. At each stage you can consider the transfer of energy from one form to another. Remember that sometimes you measure power, P, (e.g. electrical power, P = IV, where I is current and V is voltage), and sometimes you are measuring energy, E, (e.g. gravitational potential energy, E = mgh, where m is mass, g is the acceleration due to gravity, and h is the height.) Be sure to keep these quantities straight, remembering that they are related by P = E/t or E = Pt, where t is time.

Calculate efficiency, $\eta$ at each step, as the ratio of energy out to energy in, or the ratio of work done to work put in: $\eta = E_{out}/E_{in} = W_{out}/W_{in}.$ In addition to calculating the efficiency at each step, calculate the final overall efficiency.

Again, don’t forget to record everything you do in your lab notebook, and to prepare you most important results on the whiteboard for discussion at the end of the class.

Remember to write down everything you see and do in your group lab notebook.

• Make sketches, do calculations, take notes, and record observations. Your TA may ask to see your notebook during lab, and will read over it afterwards to assign a grade to the group.
• Use the digital group lab report to communicate the process and results from your experiment.
• All members are expected to contribute, read over, and comment on the report and all members of the group will receive the same report grade.

Consider: How does each section of the setup transmit energy to the next section? What different ways have you learned to measure work and power in previous labs that might be applicable here?

Consider: What changes can you make to the setup to produce more power at the end? Consider each step individually. What parameters will increase efficiency at each step or increase overall power delivered?

Consider: Are you choosing the right quantities to monitor? When do you need to record the time of different processes, and when is it fine to work only with power?

Consider: How can you represent your results? What have you learned about the efficiency of this energy storage method? Do you trust these results? Why or why not? Do you believe that the efficiency results you measured here represent the efficiencies of real energy storage systems in use today? Why or why not?

The blue “Watts Up” meters are designed to measure voltages on the order of a few volts and currents on the order of a few amps. They have a resolution of 0.01 V and 0.01 A, and therefore they are unable to read values of voltage or current lower than that.

The solar cells we use in this lab typically produce voltage differences on the order of several volts (so there is no problem there), but they typically produce small currents in the milliamp range: 0.001 - 0.100 A. Therefore, the Watts Up meters may not give accurate measurements of the current (or therefore accurate measurements of the power, $P = IV$). To make better current and voltage measurements, you may need to use the larger black multimeters. A suggested schematic (and photo) showing how to wire things up is provide below.

A schematic (top) and a photo (bottom) showing how to wire your circuit in order to measure both current and voltage with the meters. In this example, the first meter reads a current of $I$ = 5.0 mA and the second meter reads a voltage of $V$ = 1.23 V. (Click on the photos to expand.)

Also, as you explore, you may want to look at what values change when you add solar cells together. There are two possible ways to wire the solar cells together:

• In series: To wire the cells in series, connect the black wire from one cell into the red wire of the next cell. You can continue to make a chain (black to red… black to red… black to red) for as many cells as you wish.
• In parallel: To wire the cells in parallel, connect all the black wires together and then connect all the red wires together. You can stack as many cells together this way as you wish.

Photos of two cells connected in series and two cells connected in parallel are shown below.

Two cells connected in series (left) and in parallel (right). (Click on the photos to expand.)

At the end of the lab, you will need to record your final conclusions (about 1 or 2 paragraphs) in your lab report summing up the important results and take-away points from your experiment. Remember that you should only draw conclusions which are supported by the data, so be ready to back up any statements you make!

When you're finished, save your file as a PDF and upload it to Canvas. (Only one student needs to submit the report, but make sure everyone's name is on it!) If you make a mistake, you can re-submit, but work done after the end of the lab period will not be accepted.