Energy and the Environment

1. Energy and the Environment Fall 2013 Instructor: Xiaodong Chu Email：chuxd@sdu.edu.cn Office Tel.: 81696127

2. Flashbacks of Last Lecture • The Otto cycle • The Otto cycle is associated with fossil-fueled engine (reciprocating internal combustion engine (ICE))in the automobile, which does not depend upon heat transfer to the working fluid from an external combustion source • The fuel is burned adiabatically inside the engine, and the products of combustion produce more work during the expansion stroke than that is invested in the compression stroke, giving a net power output

3. Flashbacks of Last Lecture • The vapor compression cycleis the use of mechanical power to move heat from a lower temperature source to a higher temperature sink • The process is the reverse of a heat engine in that power is absorbed rather than being produced, but it still observes the restrictions of the first and second laws of thermodynamics

4. Flashbacks of Last Lecture • You should master processes of the Otto cycle and the vapor compression cycle on page 53 and pages 56-57

5. Thermodynamic Principles: Fuel Cells • Fuel cells provide a more efficient way to convert fuel energy to work than direct combustion of the fuel with air • In a fuel cell, an electrolyte fills the space between two electrodes that are chemically similar but one is supplied with a fuel and the other with an oxidant, generating an electric potential difference between them

6. Thermodynamic Principles: Fuel Cells

7. Thermodynamic Principles: Fuel Cells • The surface reaction at the anode is • At the cathode, the oxidizing reaction is • The net effect of both of the electrode reactions is

8. Describing a fuel cell’s performance and efficiency Basic energy conversion of a fuel cell was described as: Chemical energy of fuel = Electrical energy + Heat energy Hydrogen Energy = ? Electricity Energy = V·I·t Heat (byproduct) Water (byproduct) FuelCell Oxygen Energy = ? The input energy is that produced during reactions at the electrodes. We will describe the above energy balance in more detail using the first and second laws of thermodynamics.

9. Performance (cont.) The performance of a fuel cell is governed by its Polarization Curve. This type of performance curve shows the DC voltage delivered at the cell terminals as a function of the current density (current per unit area of membrane) being drawn by the external load. One measure of the energy conversion efficiency of a fuel cell is the ratio of the actual voltage at a given current density to the maximum voltage obtained under no load (open circuit) conditions.

10. Thermodynamic Analysis: 1st Law Control Volume Electrolyte -E Fuel Oxidant +E 1stLawforacontrolvolume: ΔH = Q - W For a fuel cell, the work is obtained from the transport of electrons across a potential difference, not by mechanical means, such as turning of turbine blades.

11. Defining the work term Electrical work is, in general, described by the relation: W = EIΔt where E is the cell voltage and I is the current In a fuel cell reaction, electrons are transferred from the anode to the cathode, generating a current. The amount of electricity (IΔt) transferred when the reaction occurs is given by NF, where N = number of electrons transferred F = Faraday’s constant = 96,493 coloumbs So the electrical work can be calculated as: W = NFE The First Law then becomes: ΔH = Q - NFE

12. Thermodynamic Analysis: 2nd Law Will consider the fuel cell to be ideal for now, meaning that it is reversible and thus behaves as a perfect electrochemical apparatus: Recall that the heat transferred during a reversible process was expressed as: Q = T ΔS Combining the First and Second Law analysis, we get: ΔH = TΔS - NFE

13. Gibb’s Free Energy (chemical potential) From our previous result for a cell operating reversibly: dH = TdS – FEdN Undertheseconditions: • thelossesareminimal • theuseful workobtainedismaximized This maximum work is represented by the Gibbs free energy: df = -FEdN So the thermodynamic expression for the maximum useful work obtained from a fuel cell becomes: df = dH - TdS

14. Physical Interpretation of df = dH - TdS dH represents the total energy of the system. TdS represents the “unavailable” energy (that which cannot be converted to useful work). Therefore f represents the “free” energy or the energy available to do useful work.

15. Maximum Voltage Produced by a Single Cell The reversible open circuit voltage (i.e. the maximum voltage that could be generated) can be calculated based on Δf0as: E = Δf NF o For example, in the previous reaction where Δfo was 237 kJ/mol, the open circuit voltage would be: E = 237,000(2 mol H2)/(4 electrons)(96,493) = 1.23 volts

16. Fuel Cell Vs. Carnot Cycle Efficiency The efficiency limit of a Carnot heat engine is defined as: η = 1− TL carnot T H So the higher the hot temperature source, the higher the efficiency. If, for example, one wanted to calculate the maximum efficiency of a steam turbine operating at 400° C with the water exhausted through a condenser at 50° C, it would be: = 1− 325 = 0.52 675 ηcar Undertheseconditions,the turbinecouldbenomorethan52%efficient.

17. Fuel Cells Vs. Carnot Engines (cont.) • Fuel cells, on the other hand: • Operate isothermally – no temperature cycling. • Operate with less energy lost in maintaining the temperature of the “hot source.” • •Are inherently less irreversible. • Fuel cells are not limited by the Carnot efficiency limit.

18. Fuel Cell Efficiency Since fuel cells use materials that are typically burnt to release their energy, the fuel cell efficiency is described as the ratio of the electrical energy produced to the heat that is produced by burning the fuel (its enthalpy of formation or Δhf). From the basic definition of efficiency: η = W / Qin where W is given by Δf (or NFE) Qin is the enthalpy of formation of the reaction taking place. Since two values can often be computed depending on the state of the reactant, the larger of the two values (“higher heating value”) is used (HHV). ΔfNFE = = HHV HHV

19. Maximum Fuel Cell Efficiency The maximum efficiency occurs under open circuit conditions (reversible) when the highest cell voltage is obtained. Δfo NFE o = = HHV HHV For the hydrogen fuel cell reactions shown previously where Δfo was 237 kJ/mol and ΔHo was 286 kJ/mol, the maximum efficiency of the fuel cell would be 83%.

20. How does a Carnot engine match up? A Carnot engine would have to have a high temperature of 1753 K, with a corresponding low temperature of 298 K, to achieve an efficiency of 83%! However, the work done by a Carnot engine increases with increasing temperature. The reverse is true for the Δf based fuel cell work (and hence efficiency) because Δf decreases with increasing temperature.

21. Fuel Cell Vs. Carnot Efficiencies As can be seen, there exists a temperature above which the fuel cell efficiency is lower than the Carnot efficiency. This temperature is approximately 950 K for a H2-O2 system.

22. Losses Associated With Fuel Cell Operation In reality fuel cells achieve their highest output voltage at open circuit (no load) conditions and the voltage drops off with increasing current draw. This is known as polarization. (ref. 2) The polarization curve shows the electrochemical efficiency of the fuel cell at any operating current.

23. Classification of Losses in an Actual Fuel Cell3 Activation Losses: These losses are caused by the slowness of the reaction taking place on the surface of the electrodes. A proportion of the voltage generated is lost in driving the chemical reaction that transfers the electrons. Ohmic Losses:The voltage drop due to the resistance to the flow of electrons through the material of the electrodes. This loss varies linearly with current density. Concentration Losses:Losses that result from the change in concentration of the reactants at the surface of the electrodes as the fuel is used. Fuel Crossover Losses:Losses that result from the waste of fuel passing through the electrolyte and electron conduction through the electrolyte. This loss is typically small, but can be more important in low temperature cells.

24. Thermodynamic Principles: Fuel Cells • In the overall reaction, for each hydrogen molecule two electrons flow from the low anode potential to the high cathode potential in the external circuit, producing electrical work • The electrical work per unit mass of fuel is • That is

25. Thermodynamic Principles: Fuel Cells • According to the second law, the work cannot exceed the Gibbs’ free energy change in the oxidation reaction • So the electrode potential difference is limited • The higher efficiencies of fuel cells, compared to heat engines utilizing the direct combustion of fuel with air, stems from the electrode processes where the electrostatic energy binding molecules can be converted directly to electrostatic energy of the ions and electrons that move in the cell

26. Thermodynamic Principles: Fuel Cells • The thermodynamic efficiency of a fuel cell can be defined as the ratio of the actual electric work delivered by the cell to the maximum value (Gibbs’ free energy change) • Fuel cell efficiencies are about 45%

27. Thermodynamic Principles: Fuel Cells • To maintain the fuel cell temperature at a fixed value, heat must be removed • From the first law, the heat removed per unit mass of fuel is

28. Thermodynamic Principles: Fuel (Thermal) Efficiency • A practical measure of the efficiency of converting fuel energy to work is the ratio of the work produced to the heating value of the fuel consumed, which is called the fuel efficiency or thermal efficiency • Usually the lower heating value of the fuel is used to measure the practical amount of fuel energy available • We can calculate the fuel mass consumption rate if its fuel efficiency is known

29. Thermodynamic Principles: Fuel (Thermal) Efficiency

30. Thermodynamic Principles: Synthetic Fuels • A synthetic fuel is one that is manufactured from another fuel so as to enhance its usefulness while retaining as much of the original heating value as possible • Oil produced from coal, oil shale, or tar sands • Gas from coal, oil, or biomass • Alcohols from natural gas or biomass • Hydrogen from coal, oil, or natural gas • Liquid fuels, such as gasoline, are partially synthetic in that the refining process produces components that are synthesized from petroleum constituents and added to the natural fractions of petroleum that ordinarily comprise the liquid fuel

31. Thermodynamic Principles: Synthetic Fuels • Where the synthesizing reactions are endothermic, heat must be added to maintain the reactor temperature • Practical synthesis results in a reduction of heating value in the synthesized fuel and an increase of carbon emissions per unit of synthetic fuel heating value

32. Thermodynamic Principles: The Hydrogen Economy • Hydrogen has been promoted as an environmentally friendly synthetic fuel that can be used in a fuel cell to generate electrical power at high efficiency while emitting no air pollutants • Two possible sources of hydrogen fuel are the reforming of methane and the electrolysis of water

33. Thermodynamic Principles: The Hydrogen Economy • Both of these reactions require additional energy to bring them to completion • The first requires combustion of additional methane to supply the heat needed for the reforming of methane to hydrogen • The free energy increase in the second reaction is provided by external electric power • Producing electrolytic hydrogen is very energy inefficient when the electricity is generated by burning a fossil fuel, because the heating value of the hydrogen will be less than one-third of the heating value of the fuel burned in the electric power plant

34. Thermodynamic Principles: The Hydrogen Economy • Does the use of hydrogen as a substitute for fossil fuel reduce CO2 ? • If hydrogen is produced by reforming a fossil fuel, there is no net reduction in carbon dioxide emissions; in most circumstances there will be an increase in emissions and costs • If electrolytic hydrogen is produced by electricity, there is no reduction in carbon dioxide emissions as long as some electricity is produced in fossil fuel plants • However, hydrogen production from a fossil fuel provides a path for CO2 recovery and sequestration • A fossil fuel is converted to a noncarbon fuel, H2, while the CO2 formed in the conversion process can be recovered and sequestered under ground or in the ocean, preventing its emission into the atmosphere

35. Thermodynamic Principles: The Hydrogen Economy

36. Thermodynamic Principles: The Hydrogen Economy