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Thermal Energy, Chemical Energy IE350

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Outline Thermal Energy Chemical Energy Electrolysis PV and electrolysis Fuel Cells

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Thermal Energy We already know that in order to increase by 1°C the 1 gram of water we need 1 calorie. For any mass and any temperature difference we will have: Q = C·m·t, where C is the Specific Heat

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Specific Heat The Specific Heat measurement unit, c naturally is: cal/(g·°C) = = 4.184 J/(g·°C) or J/(g·°K) Water has a mass-specific heat capacity of about 4.184 joules per Kelvin per gram near 20 °C. … or 1 calorie per kelvin per gram near 20 °C (this is again the calorie definition).

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Heat Storage Assume you have 1 ton of water at 94°C in a room. After some time the temperature decreases to 24°C. How much energy is released to the room? Q = c·m·t c = 4.184 MJ/(ton·°K) m = 1 t t = 70°C Q = 4.184 · 70 MJ = 292.88 MJ = 81.35[5] kWh (1 kWh = 3.6 MJ).

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Table of Specific Heat for Various Materials. Which material is best for heat storage? Remember that water is limited in t, e.g. bricks or granite – not so much. However losses at larger t-s are much higher.

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Specific Heat (C) of H2O Water: J/(g·°K) - gas,100 °C 2.08 - liquid, 25 °C 4.1813 - solid, 0 °C 2.114

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Specific Heat

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Losses Losses are linearly related to the temperature difference t (temperature gradient)!

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Specific Heat of: Fusion and Vaporization Specific Heat of Fusion: Amount of energy needed to turn solid into liquid. Specific Heat of Vaporization: Amount of energy needed to turn liquid into vapor.

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H2O: From Ice to Vapor How much Energy is needed to turn ice into vapor? 5 steps of calculation: Energy needed to reach the melting point; Add energy needed to melt the ice; Add energy needed to reach the vaporization point; Add energy needed to vaporize the water; Add energy needed to reach higher temperature of vapor (analogy with band gap in Si).

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H2O: From Ice to Vapor Energy needed to melt the ice: 333 J/g = specific heat of fusion Energy needed to vaporize the water: 2260 J/g = specific heat of vaporization How this difference is explained?

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Phase change storage!

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Coffee Joulies

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Coffee Joulies

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Enthalpy Enthalpy or heat content (denoted as H or ΔH, or rarely as χ) is a quotient or description of thermodynamic potential of a system, which can be used to calculate the "useful" work obtainable from a closed thermodynamic system under constant pressure, Short definition: Enthalpy is the energy density in heat-mass transfer (transportation) phenomena.

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Enthalpy Enthalpy, H = {Energy content}/mass = E/m measured in J/g or J/kg. Importantly, in many cases H = Q/m.

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Humidity Absolute Relative Absolute Humidity = weight of water in the volume of air, g/m3; … or weight of water in weight of air, g/kg.

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Relative humidity Relative humidity is defined as the ratio of the partial pressure (or density) of water vapor in a gaseous mixture of air and water to the saturated vapor pressure (or density) of water at a given temperature. Relative humidity is expressed as a percentage and is calculated in the following manner: RH = 100% • [p(H2O)]/[p*(H2O)] where: RH is the relative humidity of the gas mixture being considered; is the partial pressure of water vapor in the gas mixture; and is the saturation vapor pressure of water at the temperature of the gas mixture.

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Psychrometer

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Dependence of Relative humidity and Temperature.

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Anti-condensation bathroom mirror

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Anti-condensation bathroom mirror

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Chemical Energy The weight of a proton or neutron is 1.66 · 10-24 g Since the electron weight is too small compared to proton, 1/1837 –th, the weight of atoms is defined by protons and neutrons. NA, Avogadro Number, = 6.022 ·1023mol-1 particles. The unit of amount of substance. number of atoms in 12g of the isotope carbon-12 Interesting is that the volume of 1 mol of ideal gas is always the same. Precisely,

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Avogadro Number’s Holiday October 23 is called Mole Day. It is an informal holiday in honor of the unit among chemists. The date is derived from Avogadro's constant, which is approximately 6.022×1023. It starts at 6:02 a.m. and ends at 6:02 p.m.

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Heat of Formation Reactions can be endothermic – absorption of heat takes place, temperature of ambience is decreased; or exothermic – release of heat takes place, temperature of ambience is increased; Denoted by Hf° - amount of energy per unit amount of substance, kcal/mol, released or absorbed by a reaction – is the reaction enthalpy.

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Exothermic Endothermic

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Exothermic & Endothermic reactions

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Heats of Formation

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Hydrogen and water

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Electrolysis. However, what is the future? Hydrogen Combustion Engines? Hydrogen Fuel Cells? Large Ocean Solar Stations?

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PV and electrolysis. Storage of solar energy is a problem yet to be solved. Hydrogen is one of the best solutions. Electrolysis efficiency is about 80%, with theoretical maximum of 94%. Safety problems: The enthalpy of combustion for hydrogen is 286 kJ/mol, Burning concentration starts from 4% (v)! However, as experience shows, it is safer than e.g. gasoline or methane!

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Electrolysers

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Electrolysers

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Electrolysers

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Fuel cells

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Photoelectrochemical cells In this type of photoelectrochemical cells, electrolysis of water to hydrogen and oxygen gas occurs when the anode is irradiated with electromagnetic radiation. This has been suggested as a way of converting solar energy into a transportable form, namely hydrogen. The photogeneration cells passed the 10 percent economic efficiency barrier. Lab tests confirmed the efficiency of the process. The main problem is the corrosion of the semiconductors which are in direct contact with water. Research is going on to meet the DOE requirement, a service life of 10000 hours. Photogeneration cells have passed the 10 percent economic efficiency barrier. Corrosion of the semiconductors remains an issue, given their direct contact with water.[5] Research is now ongoing to reach a service life of 10000 hours, a requirement established by the United States Department of Energy

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How to store Hydrogen? Cylinders – compressed hydrogen Metal Hydrate Compounds Cryogenic storage Chemical Storage Carbon nanotube storage Glass Microspheres Liquid carrier storage

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Cylinders – compressed hydrogen requires energy to acomplish lower energy density when compared to a traditional gasoline tank same energy content yields a tank that is 3,000 times bigger than the gasoline tank

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Metal Hydrates MgH2, NaAlH4, LiAlH4, LiH, LaNi5H6, TiFeH2 and palladium hydride similar to a sponge, 1-2% of the weight. could reach to 5-7% if heated to 250°C delivering Hydrogen at a constant pressure. it also absorbs any impurities introduced into the tank by the hydrogen. The result is the hydrogen released from the tank is extremely pure, but the tank's lifetime and ability to store hydrogen is reduced as the impurities are left behind and fill the spaces in the metal that the hydrogen once occupied.

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Cryogenic storage Liquid hydrogen typically has to be stored at 20o Kelvin or -253o C. again, necessitate spending energy to compress and chill the hydrogen into its liquid state, resulting in a net loss of about 30% of the energy that the liquid hydrogen is storing. a similar percentage will be due to the temperature gradient losses. t is usually > 270°C! Larger, composite material tanks would be beneficial.

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Cryogenic storage

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Chemical Storage Some examples of various techniques include ammonia cracking, partial oxidation, methanol cracking, etc. These methods eliminate the need for a storage unit for the hydrogen produced, where the hydrogen is produced on demand. Still in the research stage.

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Carbon nanotube storage Carbon nanotubes are microscopic tubes of carbon, two nanometers (billionths of a meter) across, that store hydrogen in microscopic pores on the tubes and within the tube structures. 4.2% - to 65% of their own weight in hydrogen!

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Glass Microspheres Tiny hollow glass spheres can be used to safely store hydrogen. The glass spheres are warmed, increasing the permeability of their walls, and filled by being immersed in high-pressure hydrogen gas. The spheres are then cooled, locking the hydrogen inside of the glass balls. A subsequent increase in temperature will release the hydrogen trapped in the spheres. Microspheres have the potential to be very safe, resist contamination, and contain hydrogen at a low pressure increasing the margin of safety.

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Liquid Carrier (Carbohydrate) Storage This is the technical term for the hydrogen being stored in the fossil fuels that are common in today's society. Whenever gasoline, natural gas methanol, etc.. is utilized as the source for hydrogen, the fossil fuel requires reforming. The reforming process removes the hydrogen from the original fossil fuel. The reformed hydrogen is then cleaned of excess carbon monoxide, which can poison certain types of fuel cells, and utilized by the fuel cell. Reformers are currently in the beta stage of their testing with many companies having operating prototypes in the field.

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Hydrogen Safety The range of explosion proportion in air is rather wide, starting at 4%. Hydrogen is light – it goes up in atmosphere. Hydrogen molecules are small – they penetrate and escape from many situtations.

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Hydrogen Use Internal Combustion Engines PEM Fuel Cells

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PEM Fuel Cells

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PEM Fuel Cells Acts like a battery, delivering electricity with efficiencies around 55%. This “battery” does not need to spend time on recharging! Whenever H2 and O2 (or humidified air) are supplied – it operates. The rest of the energy can theoretically be used – in a form of heat. Excellent way to provide distributed power and integrate with renewable sources.

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PEM Fuel Cells

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PEM Fuel Cells

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Homework Assume that a household needs 3 kW heating power on average of 24 hours during any day, during the 4.5 months of winter period. What kind of seasonal heat storage you may suggest (material, size, controllability, t, price)? Explain why and make the calculation. Calculate the heat content and the daily amount of the hydrogen gas needed to power the daily need to run a fuel cell powered smartphone for 12 hours, 2.5W. Assume conversion efficiency of 43%.