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Heat Storage for Solar Heating

The purpose of this lab is to teach you about:. heat storage devices (necessary for implementation of solar energy) Calorimetry measuring H fusion (for sodium thiosulfate pentahydrate). Heat Storage for Solar Heating. Look carefully at Figure 1. .

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Heat Storage for Solar Heating

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  1. The purpose of this lab is to teach you about: • heat storage devices (necessary for implementation of solar energy) • Calorimetry • measuring Hfusion (for sodium thiosulfate pentahydrate) Heat Storage for Solar Heating • Look carefully at Figure 1. 2. Over the past 25-30 years world energy demand has doubled. Over the next 25-30 years it is expected to double again. In this period, nuclear power has provided a relatively constant 20-25% of US electricity. What happens as we have more hybrid and plug in electric cars?

  2. Concentrating Solar Power (CSP) http://en.wikipedia.org/wiki/The_Solar_Project 1995 Solar One was converted into Solar Two Energy storage medium : molten salt Energy Production : 10 MW Decomissioned in 1999 Nevada Solar one (www.nevadasolarone.net) World’s third largest plant (nominal capacity of 64 MW 400 acres) Online in June 2007

  3. Uses 760 parabolic concentrators and more than 180,000 mirrors that concentrate the suns rays onto 18,240 solar receivers The solar receivers heat a transfer fluid to 735° F, which passes through a heat exchanger, changing water into steam to drive a conventional turbine connected to a generator that produces electricity Utilizes proprietary tracking technology to concentrate the sun’s rays and track the sun’s location during peak demand hours

  4. Problem Heat storage !

  5. Storing heat Newton’s Law of cooling states that a hot object transfers heat to its surroundings (cools) at a rate proportional to the difference in temperature between the two. In other words, the hotter you make your heat storage medium the faster it will cool. (Not what you wanted to hear) Phase transitions At constant pressure!

  6. Heat capacity C = (heat absorbed)/(increase in temperature) = dQ/dT The amount of heat that must be added to an object to raise its temperature by 1°C. Specific Heat capacity The amount of heat that must be added to one gram of an object to raise its temperature by 1°C. Units: J/(g-°C) or J/(g-K) Molar Heat capacity The amount of heat that must be added to one mole of an object to raise its temperature by 1°C. Units: J/(mol-°C) or J/(mol-K) The heat can be added in different ways. For example, it might be added under conditions of constant pressure or it might be added under conditions of constant volume. (with heat capacities denoted Cp and Cv respectively.) In doing wet chemistry we are working under conditions of constant pressure. Qp = H

  7. Hfusionis the heat necessary to convert one mole of a solid to its liquid phase. Where is Hfusion on the diagram above? What is the slope of the line in the solid and liquid phases above related to? Why does the temperature not change as we go from solid to liquid? What is the difference between the temperature a solid melts at and Hfusion? Notice that the process is reversible! Hfreezing = -Hfusion

  8. Calorimetry : Quantitative measurement of heat flow Calorimeter: A device for measuring heat flow. This heat flow is measured between the sample and a “reservoir” whose temperature change is monitored. Necessarily as you want all the heat to flow to the reservoir, the heat flow to the rest of the world must be minimized. You will use a temperature probe interfaced to the computer. What is the value of this? How will you test the temperature probe to ensure that it is working?

  9. Procedure (write your own. This is NOT meant to be a detailed list!) • Build the calorimeter (record mass of calorimeter BEFORE and AFTER adding water) • Prepare the molten crystals • Weigh a 4 dram vial WITH its screw cap. • Melt two large test tubes ½ to ¾ full of Na2S2O3.5H2O. DO NOT LET ALL THE CRYSTALS MELT! • Fill the vial with molten sodium thiosulfate ¾ full (USE TONGS) • Immerse the temperature probe into the liquid making sure that it is not touching the walls of the vial. • Rotate the vial (to allow mixing)without lifting it until the temperature is 45°C (below the melting temperature of 48°C. (super-cooled liquid). There must be NO crystals visible in the liquid or else you will have to re-start this part of the experiment.

  10. Measuring the heat released by freezing the crystals • Begin recording the temperature in the calorimeter • Drop one small seed crystal in the melt (vial). Except for the seed crystal all of the sodium thiosulfate must be liquid at this stage. Otherwise you will need to re-start. • Gently slide the vial into the water, put the lid on and begin measuring the temperature. • The temperature will initially rise and then begin to decrease. • When the rate of cooling is constant for at least 7 minutes and at least 2.5 °C below the maximum, you can stop recording. • Remove the vial from the calorimeter and dry it (removing the cap). • Weigh the vial with the crystals (and the screw cap). • Clean up!!!

  11. Data analysis

  12. System = salt ; surroundings = water (ideally) qsystem = - qsurroundings qfreezing = - qwater qwater = mwatercwaterT Hfusion = -qfreezing (in kJ) / # of mol Additional reading for those of you that might be interested: Solar EnergyVolume 80, Issue 5, May 2006, Pages 616-625

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