Hands-on Activity Heat Transfer in Solar Dryers

Learning Objectives

After this activity, students should be able to:

• Define conduction, convection and radiation.
• Describe at the molecular level how heat is transferred by conduction, convection and radiation.
• Explain the time differences on each process as they transfer heat from a source to an object or a substance.
• Show how conduction, convection and radiation is applied in a solar dryer and give examples for each process.

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).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

State Standards

Materials List

Each station needs:

An infrared thermometer

Station 1 – Conduction:

• hot plate
• cup of water
• timer dropper or pipette

Station 2 – Convection:

• hair dryer
• cup of water
• petri dish covered with aluminum foil or small ceramic or metal plate that can hold a drop of water

Station 3 – Radiation:

• heat lamp (about 200 to 300 W)
• cup of water
• dropper or pipette
• petri dish with aluminum foil

Worksheets and Attachments

Pre-Req Knowledge

• An understanding of basic chemistry, including definitions for molecules and particles
• An understanding of the concept of heat

Introduction/Motivation

What dried fruit or food have you eaten or tried? Let’s take a look at some example fruit and state the differences between them. Here is a dried piece of [insert name of fruit here] and here is an example of a normal piece of the same fruit.

What is the difference between a dried fruit/food and the real thing? How does a normal piece of fruit turn into a dried one? What was removed/what was done to the normal fruit for it to become dried. What do you think was the process used? What energy was involved in the process? Were there any scientific or engineering concepts that were used to help this process?

In this activity, we’ll be exploring one key process – heat transfer – to understand how the concepts of conduction, convection and radiation work in a unique engineering application. Let’s get started!

Procedure

Background

(Have students read the Heat Transfer Article and fill out the Intro to Heat Transfer Worksheet. Alternatively, go over the background with the students.)

Solar drying technology was invented in the 1950’s as a solution to a common issue related to the scaling of commercial farms, since commercial operations could not rely on open sun drying alone. Solar drying technology was given even more attention later following the oil crisis in 1973, when nations became interested in in finding an alternative system with a sustainable energy source for grain and fish drying.

Solar drying also became a popular process in agricultural applications in tropical countries like Asia and sub-Saharan Africa, where the solar drying potential is higher.

More advanced solar drying systems are being developed through time and today based on the commodity nature, location, cost, and other factors that are of concern we can choose what is best for our purpose.

Here’s a brief introduction to several basic types of solar dryers.

In active dryers, airflow inside the dryer is made by a fan powered with electricity while passive dryers use natural convection to create airflow. Further, dryers are categorized into 3 main types namely Direct, Indirect, and Mixed-mode.

Direct Type Dryers

Direct type dryers are like a cabinet with a compartment and a glass roof. Solar rays directly fall onto the commodity (the crop) through the glass and use the greenhouse effect to create heat. The airflow carries away the moisture as shown below. Tunnel, greenhouse, and cabinet-type dryers fall into this category.

Indirect Type Dryers

Indirect type solar dryers have separate chambers for drying and heating airflow. In here there’s no direct contact between solar rays and the crop as shown in the figure below. Airflow entering from solar collector gets heated up and carries away the moisture from drying chamber.

Mixed-mode Dryers

Mixed-mode dryers are a combination of above two types. The structure is similar to an indirect dryer but the drying chamber is made of transparent material such that the solar collector and drying chamber both get heated up by the greenhouse effect.

[Have students fill out the Solar Dryer Heat Transfer Worksheet as an assessment after the background section.]

Before the activity

Set up each station with its respective materials. Hand out the Heat Transfer Lab Worksheet to be completed during the activity and the Heat Transfer Comparison Worksheet to be completed after the activity.

With the Students

Station 1:

1. Plug the hot plate and adjust the temperature knob to medium.
2. Using the thermometer, measure the temperature of the hot plate.
3. Set the timer and place a drop of water using the dropper or pipette directly on the hot plate.
4. Measure the time until the drop of water is completely dried out.
5. Adjust the temperature knob of the hot plate to the maximum setting.
6. Repeat steps 2 and 3.
7. Record the time in the data section.

Station 2:

1. Plug the hair dryer.
2. Using the thermometer, measure the temperature of the petri dish.
3. Set the timer and place a drop of water on the petri dish and cover it with foil.
4. Turn on the dryer and set it to maximum.
5. Point the dryer directly on the top of the foil
6. Measure the time until the water is completely dried out.  
7. Record the time in the data section.
8. Record the time.

Station 3:

1. Plug and turn the heat lamp on.
2. Using the thermometer, measure the temperature of the hot plate.
3. Set the timer and place a drop of water on the petri dish.
4. Point the heat lamp directly on the drop of water at about a 3 cm away from the water.
5. Measure the time until the drop of water is completely dried out. 
6. Record the time in the data section.

Vocabulary/Definitions

conduction: Heat transfer that involves contact, applies to solids.

convection: Heat transfer by moving particles carrying heat, applies to liquids and gas.

heat transfer: A form of energy transfer through conduction, convection, and/or radiation. Heat transfer occurs any time there is a temperature difference between two objects and occurs in the direction of decreasing temperature, meaning from a hot object to a cold object.

solar dryer: A device that use solar energy to dry substances, especially food. Solar dryers use the heat from sun to remove the moisture content of food.

Assessment

Pre-Activity Assessment

The teacher will ask what dried fruit or food they have eaten or tried. If the budget permits, the teacher can bring some dried food for the students to try, like dried mango (or another dried fruit) and a normal mango. It can also be done by showing a picture of a dried mango and a normal mango fruit and the teacher can ask what the difference between the two. Then the deeper questions can be asked – how does a normal mango turn into a dried mango? What was removed/what was done to the normal mango for it to become dried mango. What does the students think was the process used? What energy was involved in the process? This will lead to a robust discussion and using leading questions like, how do farmers dry their food?

After the discussion and the background section, have students complete the Introduction to Heat Transfer Worksheet and the Solar Dryer Heat Transfer Worksheet.

Activity Embedded (Formative) Assessment

Students complete the Heat Transfer Lab Sheet.

Post-Activity (Summative) Assessment

Students complete the Heat Transfer Comparison Worksheet.

Investigating Questions

The teacher will show a picture of a conventional food solar dryer.

and include the following questions:

How do farmers preserve food by using heat form the sun? (Answer: Produce can be dried by using solar radiation by removing water/moisture from the food. Solar (sun) drying is cheap and designs have progressed to even some applied automation in the drying process.)

How are convection, conduction and radiation shown or applied in solar dryers? Show examples. (Answer: Radiation – direct sunlight provides radiation to the moving air in the solar air collector. If the drying chamber clear material, the radiation from the sun also adds to the heat inside the chamber. Convection – there an air flow that transfer heat caused by the fan that moves heat up to the drying chambers. Inside the drying chambers there is also some air flow that transfers heat. Conduction – The trays are directly touching the food and are also heated by either convection or radiation and they transfer the heat they have by conduction to the food.)

Safety Issues

• Use goggles for eye protection
• Use gloves or tongs when picking up hot objects

Copyright

Contributors

Supporting Program

Acknowledgements

This curriculum was developed under National Science Foundation RET grant no. 1855147— Research Experience for Teachers in Advanced Design and Manufacturing at the University of Houston. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.