Lesson The Temperature Effect

Learning Objectives

After this lesson, students should be able to:

• Describe the effect that different temperatures have on the efficiency of PV panels.
• Explain how engineers might attempt to control the temperature of solar PV panels.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

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NGSS: Next Generation Science Standards - Science

Common Core State Standards - Math

International Technology and Engineering Educators Association - Technology

State Standards

Worksheets and Attachments

Introduction/Motivation

Have you ever noticed how an LCD display, such as your calculator or cell phone screen, changes color when exposed to extreme cold or hot temperatures? Temperature affects how electricity flows through an electrical circuit by changing the speed at which the electrons travel. This is due to an increase in resistance of the circuit that results from an increase in temperature. Likewise, resistance is decreased with decreasing temperatures.

Imagine going for a run in the desert when it is 110 ºF. Do you think your body would enjoy working hard under those conditions? Now, imagine that same run on a nice cool fall evening with a slight breeze. In which weather conditions would your body work the best? In the same way that a human body's abilities change depending on weather conditions, a solar panel's output depends on its working conditions.

Solar panels work best in certain weather conditions, but since the weather is always changing and as engineers are installing solar panels all over the world in different climate regions, most panels do not operate under ideal conditions. That is why it is important for engineers to understand how panels react to different weather conditions. With this knowledge, they can design ways to improve the efficiency of solar panels that operate in non-optimal conditions.

In some cases, they design cooling systems to keep the panels within certain temperatures. For example, solar power plants in extremely hot climates may pass a cool liquid behind the panels to pull away heat and keep the panels cool. This is similar to how your body might sweat as a way to stay cool if you were on that run in the 110 ºF air temperature. Following the lesson refer to the associated activity Photovoltaics & Temperature: Ice, Ice, PV! for students to examine how the power output of a PV solar panel is affected by temperature changes by exposing the panel temperature to various temperatures and recording the resulting voltage output.

Lesson Background and Concepts for Teachers

For each individual PV system, engineers must use specific equipment, such as inverters, to ensure that the system runs at maximum efficiency. Different inverters are rated for different maximum voltages and have higher efficiencies between different voltage ranges. Engineers must carefully size the PV system in different temperature environments to ensure that the output voltage is not too high, which could damage the equipment. A PV system in Arizona will have a maximum system voltage that is lower than the same system in North Dakota (built with the same materials) because of the higher temperatures in Arizona. Because PV panels are more efficient at lower temperatures, engineers also design systems with active and passive cooling. Cooling the PV panels allows them to function at a higher efficiency and produce more power.

Panels can be cooled actively or passively. An active system requires some external power source to run. A passive system is requires no added power. An example passive system might be an array of panels that are set off the roof 2 feet (61 cm) to allow air to naturally flow behind the panels and pull away some heat, or a white-colored roof that prevents the surfaces around the panels from heating up and causing additional heat gain. An active system might have fans to blow air over the panels, or pump water behind the panels to pull away heat. An active cooling system may be used in certain situations in which the added efficiency to the panels is greater than the energy needed to run the system, such as with a solar power plant in a desert. They also may be used in situations in which some additional purpose can be achieved, such as domestic water heating.

An example hybrid photovoltaic thermal (PVT) system is shown in Figure 1. This combined solar PV and water heating system was installed on the roof of a student-designed solar decathlon home. The system runs cool water behind the panels to absorb heat from them, making them more efficient. The heated water is used in the home for showers or heating. Even if the outside temperatures are cold, the dark panels and rooftop become quite hot on sunny days because of all the solar radiation received, making a PVT system a practical solution to increase electrical power production from the PV panels and reduce the heating loads in the home!

While it is important to know the temperature of a solar PV panel to predict its power output, it is also important to know the PV panel material because the efficiencies of different materials have varied levels of dependence on temperature. Therefore, a PV system must be engineered not only according to the maximum, minimum and average environmental temperatures at each location, but also with an understanding of the materials used in the PV panel. The temperature dependence of a material is described with a temperature coefficient. For polycrystalline PV panels, if the temperature decreases by one degree Celsius, the voltage increases by 0.12 V so the temperature coefficient is 0.12 V/C. The general equation for estimating the voltage of a given material at a given temperature is:

V_OC,mod = open circuit voltage at module temperature

T_STC [°C] = temperature at standard test conditions (STC), 25 °C, 1000 W/m² solar irradiance

T_mod [°C] = module temperature

V_OC,rated = open circuit voltage at STC

As an example, for polycrystalline, the equation is:

The effect of temperature can be clearly displayed by a PV panel I-V (current vs. voltage) curve. I-V curves show the different combinations of voltage and current that can be produced by a given PV panel under the existing conditions. Two sample I-V curves at different temperatures for the educational modules are shown in Figure 2. The current, measured in amps (A), is given on the y-axis. The voltage, measured in volts (V), is given on the x-axis. To gather data for this graph, a PV panel was placed flat on the ground in Boulder, CO, at 40° latitude on December 11. The first measurements were taken in ambient temperature conditions, and then they were taken again after the panel was cooled in an ice bath for one minute.

Associated Activities

• Photovoltaics & Temperature: Ice, Ice, PV! - Students examine how the power output of a PV solar panel is affected by temperature changes by exposing the panel temperature to various temperatures and recording the resulting voltage output. They plot the power output and calculate the panel's temperature coefficient.

  Watch this activity on YouTube

Lesson Closure

Today we learned that temperature can affect how electricity flows through an electrical circuit by changing the speed at which the electrons travel. Also, since solar panels work best at certain weather and temperature conditions, engineers design ways to improve the efficiency of solar panels that operate in non-optimal temperature conditions. This might involve designing cooling systems that use outside air, fans and pumps.

Now that we know the effects of temperature on the power output of PV panels, what do you think would be an ideal climate in which to set up a large PV system? (Answer: A cold and sunny climate.) Do you think there are many places in the world like this? If you look up maps on Google images of world temperatures and world solar radiation availability, you will find that most locations are not ideal for solar PV panels. This means engineers have many opportunities to design innovative systems to keep panels cool as solar power plants become more common, because the ideal cool and sunny climate is rare.

Vocabulary/Definitions

active cooling: Using forced water or air to cool the surface of PV panels in order to improve their efficiency.

ambient: Surrounding environmental conditions.

inverter: An electrical device that converts the DC current produced by the PV panel to an AC current used by electrical devices. Inverters can also be used for maximum power point tracking to maximize the efficiency of the PV panel.

open circuit voltage: Voltage available from a power source in an open circuit.

photovoltaic thermal system: An active cooling system in which cool water is used to decrease the temperature of the PV panel while warming the water to be used in hot water applications.

short circuit current: Current drawn from a power source if no load is present in the circuit.

temperature coefficient: Number [V/°C] that one would use to find the open circuit voltage of a PV panel at a temperature other than standard test temperature.

Assessment

Pre-Lesson Assessment

Discussion Questions: Ask the students the following question, and discuss as a class:

• What effect do you think temperature has on how well a photovoltaic (PV) panel works?

Post-Introduction Assessment

Class Discussion: Have students read the Fundamentals Article and engage the class in a discussion of what they have read. On a sheet of paper or in a journal, ask each student or group to record five facts they learned from the article. Have each student or group write one of these facts on the board to be discussed with the entire class.

• As a class, ask students to interpret the graph on page 3 of the Fundamentals Article. (Answer: The resulting graph clearly shows that when the panel is at a colder temperature and a higher voltage, then a higher power output is achieved.)

Lesson Summary Assessment

List & Discuss: From their reading of the Fundamentals Article, ask students to list on paper 10 applications for which solar panels could be used. Next to each application, write the temperature(s) to which the panel might be exposed. Then share answers and discuss as a class. (Example answers: 1. Solar plant in a desert to power a nearby city: very hot, 100 °F or higher; 2. Provide light and power to a research station in Antarctica: very cold, below 0 °F.)

Lesson Extension Activities

Graphing: Given a temperature coefficient and a V_{OC, rated}, ask students to create a graph of output voltage vs. temperature for the yearly temperature range found in their home town.

References

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