Demonstration 2

Dipping Into Solar Ponds

Objective: Students will observe the process of forming a solar pond and understand the use of solar heat generated from a solar pond.

Review of Scientific Principles:

A solar pond uses the principles of energy transfer by convection to heat water. The bottom of the pond is lined with a dark material that absorbs the sun's rays. The water above the liner consists of three layers of varying salt concentrations. The bottom is very salty. The middle layer changes from salty to fresh. The top layer is fresh water. Due to the difference in densities of the three zones, the gradient zone acts as an insulator, so it doesn't allow heat to be passed by convection currents from the saturated middle layer to the fresh water layer. To be more explicit, the gradient zone is a non-convective layer. The bottom layer is a supersaturated salt solution that reaches temperatures of 60-80^o C and has convection currents. The top layer of cool fresh water is also convective due to daily heating and cooling from the sun and the wind. The convection currents circulate only above and below the concentration gradient. Cool water is pumped through a pipe placed into the hot supersaturated salt zone. There the water is heated as it passes through the surrounding salt water. From there, the water may be pumped through surrounding homes or barns for heat, or it may be converted to steam for electricity production.

Materials and Supplies:

• 1 large hot plate
• 1 L supersaturated salt solution
  • Pour 500 ml of water into a 1 L beaker. Dump salt in until it precipitates. Repeat procedure until you have 1 L of supersaturated salt solution.
• 2 2000 ml beakers
• 2 thermometers
• 1 Styrofoam disk (size of inside beaker)
• 1 sheet of dark construction paper
• 1 paper clip

Procedure:

1. Fill one beaker with 1800 ml of cold water.
2. Fill other beaker with 600 ml of supersaturated salt solution.
   
3. Record initial temperature of each.
4. Bend a paper clip as shown and insert into disk to make handle.
   
5. Place Styrofoam disk in the top of the beaker with the salt solution very carefully. The disk should float!
6. Slowly pour 600 ml of the fresh water over the Styrofoam disk, DO NOT allow solutions to mix. (This will begin to set up the concentration gradient.)
   
7. Carefully remove Styrofoam disk.
8. Both beakers should have equal volumes.
9. Place a dark sheet of construction paper behind both beakers to show the different concentration levels. The supersaturated salt solution will remain on the bottom with a concentration gradient separating it from the salt solution.
10. Place both beakers on the hot plate for approximately 15-20 minutes.
    
11. Record the temperature at the top and bottom of each beaker. The temperature should be uniform throughout the fresh water beaker and very warm to the touch. The salt solution will not have a uniform temperature. The top layer will be quite cool to the touch and the bottom with the salt solution will have a much higher temperature.
12. Have students come up, one-by-one, and place their finger into the fresh water beaker and then in the top layer of the salt solution beaker. They will be quite surprised at how cool the top layer of the salt solution beaker is.

    NOTE: Caution the students against stirring the solution when they put their hand into the water. Mixing will destroy the gradient you have set up and burn them.

13. To save the salt solution, use gloves to carefully pour off the fresh water into the beaker of fresh water until the solution appears to be cloudy. The remaining solution is the supersaturated salt solution and is very hot.

Video Clip

Suggestion to teachers: Time permitting, this procedure would make a good laboratory experiment. Or, alternatively, having a few students set this up would be instructive.