Using molten salts as a thermal storage phase change material for solar concentrators
PHYS 400: Advanced Research Topics
Research Advisor: Prof Pete Schwartz
Research Student: John Sekerak
Background: Solar concentrators are great at generating a large amount of heat at a focal area. However, they are disadvantageous when heat is required and the power source isn’t available (night, clouds, inside, etc.). To increase their functionality and effectiveness, it would be beneficial to have a way to store the excess heat generated during the day for use at other times. Phase change materials provide a means of doing this.
In the figure above, we can see the temperature of the phase change material (water in this case) as a function of the energy added to the system. The flat plateaus in the chart indicate the phase change temperatures, where all of the energy goes towards converting the material to a different phase. This works well for thermal storage, as large amounts of energy can be stored and then released again when needed for use. We are going to test out salt as a thermal storage phase change material.
Looking online at some commercially available thermal storage salts, we have selected a mixture given by the research paper:
Raade JW, Padowitz D. Development of Molten Salt Heat Transfer Fluid With Low Melting Point and High Thermal Stability. ASME. J. Sol. Energy Eng.2011;133(3):031013-031013-6. doi:10.1115/1.4004243.
Who designate a salt mixture comprised of Sodium Nitrate, Potassium Nitrate, and Calcium Nitrate.
We have acquired 30g of each of these from the Cal Poly Chemistry Department. We are going to experiment with them using 1:1:1 ratios. There will be two methods tested for this experiment. Structurally, we would like the thermal storage material to be contained inside of another material such as concrete. However, we are not sure if the salt can be distributed throughout the concrete and still function effectively.
The first setup will take 10g of each salt in a bag, set it into a 50g block of concrete, place some thermocouples into the material and heat it up, hopefully to see the phase change point.
The second setup will take 10g of each salt and mix it in with the 50g of concrete. Thermocouples will also be placed into this block, heat it up, and hopefully see the phase change as well.
The heat source will be a 250 W halogen work lamp. This type of light generates a large amount of heat and power.
Our containment device will be a 5-gallon steel bucket packed with insulation all around the edges and on the bottom. The test specimens are placed inside the insulation and the heat lamp is put on top and turned on.
We are using a REED SD-947 thermocouple datalogger to track the temperature within the test specimens as well as the air inside our makeshift oven.
Test 1: Lumped salt in bag
Looking at the resulting chart from test 1, there does not apear to be any indication that a phase change occurred. We can look at both sides of the heating cycle, the heating up and the cooling down, and there are no significant plateaus which would mark the phase transition.
Test 2: Mixed salt and concrete
Since we didn’t see any phase change in test 1, we didn’t expect to see it in test 2 either, where the salt is more spread out throughout the material. Again, for both the heating and cooling processes there are no plateaus where the temperature remains constant during a phase change.
End of Page