1. EE 394V New Topics in Energy Systems Distributed Generation Technologies Fall 2012

2. 2 © Alexis Kwasinski, 2012 Clarifications from Course Introduction Course Description: Graduate level course. You are already engineers or hold some other baccalaureate degree. So you are expected to show a proactive and independent attitude towards your work. You are expected to also be inquisitive about everything the instructor says. So please, do ask questions in class. The course’s 2 nd goal supports this approach. The 10 % of the grade for “participation” also supports this course concept. The use of a recommended text supported by papers and references in class are also in line with the course’s concept. Goal #1: To discuss topics related with distributed generation technologies. Goal #2: To prepare the students to conduct research or help them to improve their existing research skills.

3. 3 © Alexis Kwasinski, 2012 Microsources Most common microsources: Microturbine (<100 kW/unit) Fuel Cell (<400 kW/unit) Reciprocating Engine – e.g. diesel generator (<100 kVA/unit) PV Module (<250 W/module) Small wind turbine (<10 kW/turbine)

4. 4 © Alexis Kwasinski, 2012 Real microsources http://static.flickr.com/39/101004887_6525c88bfc.jpg Wind turbines + PV modules Microturbines PAFC MCFC

5. 5 © Alexis Kwasinski, 2012 Characteristics: For a voltage (current) source, the internal impedance is zero (infinity): No internal losses. Instantaneous dynamic response. For an ideal voltage source, current has no effect on the voltage output: The output voltage value and waveform are always the same regardless of the load. For current sources replace current by voltage in the last statement. An ideal capacitor with an infinite capacitance behaves as a dc voltage source. An ideal inductor with an infinite inductance behaves as a dc current source. Ideal sources

6. 6 © Alexis Kwasinski, 2012 Fuel Cells Basics Fuel cells convert chemical energy directly into electrical energy. Difference with batteries: fuel cells require a fuel to flow in order to produce electricity. Heat is produced from chemical reaction and not from combustion. Types of fuel cells: Proton exchange membrane (PEMFC) Direct Methanol fuel cell (DMFC) Alkaline fuel cell (AFC) Phosphoric acid fuel cell (PAFC) (*) Molten-carbonate fuel cell (MCFC) (*) Solid-oxide fuel cell (SOFC) (*) (*) Suitable for microgrids.

7. 7 © Alexis Kwasinski, 2012 Fuel cells operation Example: PEMFC The hydrogen atom’s electron and proton are separated at the anode. Only the protons can go through the membrane (thus, the name proton exchange membrane fuel cell). Hydrogen Oxygen Water Heat Membrane (Nafion) Catalyst (Pt) Anode (-) Catalyst (Pt) Cathode (+) dc current

8. 8 © Alexis Kwasinski, 2012 Fuel cell thermodynamics The first law of thermodynamics: The energy of a system is conserved In conservational fields, potential functions change depend only on initial and final values. Hence, For a closed system (control mass system), such as a piston (The total energy change equals the sum of the change in internal energy, the change in kinetic energy, and the change in potential energy) Change of heat provided to the system Change of work provided by the system Change of system’s total energy

9. 9 © Alexis Kwasinski, 2012 Fuel cell thermodynamics For an open system with mass flow across its boundaries (control volume), such as a steam turbine pV represents the work to keep the fluid flowing (p is pressure and V is volume). Hence, if a magnitude called enthalpy H is defined as Then, If we use the 1 st law of thermodynamics for a stationary control volume (i.e. the kinetic and potential energies are constant in time, then Thus, the enthalpy is the difference between the heat and the work involved in a system such as the one defined immediately above.

10. 10 © Alexis Kwasinski, 2012 If the change in enthalpy is negative, heat is liberated and the reaction occurs spontaneously (contrary to endothermic reactions that requires to apply heat in order for the reaction to occur). In the anode: In the cathode: Hence, in a PEMFC, 285 kJ/mol are converted into heat (Q) and electricity (W). How much electricity W can we ideally obtain? Entropy: it is a property that indicates the disorder of a system or how much reversible is a process. This last definition relates entropy to energy “quality”. In a reversible isothermal process involving a heat transfer Q rev at a temperature T 0, the entropy is defined as Fuel cell thermodynamics

11. 11 © Alexis Kwasinski, 2012 In all processes involving energy conversion or interactions ΔS is non- negative. ΔS is zero only in reversible processes.. For any process then The “=“ in the above relationship will give us the minimum amount of heat Qmin required in a process. From the enthalpy definition a fuel cell can be considered as a system like the following one Fuel cell thermodynamics ΔHΔH QQW

12. 12 © Alexis Kwasinski, 2012 Fuel cell thermodynamics The maximum possible efficiency for a fuel cell is, then An alternative derivation involves using “Gibbs Free Energy” The definition of entropy is relates with the 2 nd Law of Thermodynamics. One of its interpretations is that it is impossible to convert all the energy related with irreversible processes, such as heat or chemical energy, into work. Hence, it is possible to define a magnitude with units of energy called Gibbs Free Energy that represents the reversible part of the energy involved in the process. Hence, for fuel cells, the electrical work represents the Gibbs Free Energy and the maximum possible energy conversion efficiency is

13. 13 © Alexis Kwasinski, 2012 From tables: In the anode: In the cathode: And from slide #10 ΔH equals 285 kJ/mol. Thus, The Gibbs Free Energy can also be used to calculate the output voltage of an ideal fuel cell. Since the Gibbs Free Energy equals the electrical work, and the electrical work equals the product of the charge and voltage, then where F is the Faraday constant (charge on one mole of electrons) the factor of two represents the fact that two electrons per mole are involved in the chemical reaction. Fuel cell thermodynamics

14. 14 © Alexis Kwasinski, 2012 Fuel cell thermodynamics Thus, and since F = 96,485 C/mole and ΔG = -237.2 kJ/mole, then E 0 is also denoted by E r, the reversible voltage. This is the voltage that can be obtained in a single ideal PEMFC when the thermodynamic reaction limitations are taken into account. I.e., this is the output voltage of a single ideal PEMFC when it behaves as an ideal voltage source. However, additional energy loosing mechanisms further reduce this voltage.

15. 15 © Alexis Kwasinski, 2012 The Tafel equation yields the cell’s output voltage E c considering additional loosing mechanisms: The first term is the reversible cell voltage (1.23V in PEMFCs) The last term represents the ohmic losses, where i is the cell’s current density, and r is the area specific ohmic resistance. The second term represent the losses associated with the chemical kinetic performance of the anode reaction (activation losses). This term is obtained from the Butler-Volmer equation and its derivation is out of the scope of this course. In the second term, i 0 is the exchange current density for oxygen reaction and b is the Tafel slope: PEMFC output: Tafel equation

16. 16 © Alexis Kwasinski, 2012 In the last equation R is the universal gas constant (8.314 Jmol -1 K -1 ), F is the Faraday constant, T is the temperature in Kelvins, n is the number of electrons per mole (2 for PEMFC), and β is the transfer coefficient (usually around 0.5). Hence, b is usually between 40 mV and 80 mV. The Tafel equation assumes that the reversible voltage at the cathode is 0 V, which is only true when using pure hydrogen and no additional limitations, such as poisoning, occur. The Tafel equation do not include additional loosing mechanisms that are more evident when the current density increases. These additional mechanisms are: Fuel crossover: fuel passing through the electrolyte without reacting Mass transport: hydrogen and oxygen molecules have troubles reaching the electrodes. Tafel equation also assumes that the reaction occurs at a continuous rate. PEMFC output: Tafel equation

17. 17 © Alexis Kwasinski, 2012 PEMFC electrical characteristics Maximum power operating point E r = 1.23 V Activation loss region Ohmic loss region (linear voltage to current relationship) Mass transport loss region E r =1.23V b=60mV, i 0 =10 -6.7 Acm -2 r=0.2Ωcm 2 Actual PEMFCs efficiency vary between 35% and 60%

18. 18 © Alexis Kwasinski, 2012 PEMFC electrical characteristics This past curve represent the steady state output of a fuel cell. The steady state output depends on the fuel flow: Amrhein and Krein “Dynamic Simulation for Analysis of Hybrid Electric Vehicle System and Subsystem Interactions, Including Power Electronics”

19. 19 © Alexis Kwasinski, 2012 A very good dynamic model of a PEMFC is discussed in: Wang, Nehrir, and Shaw, “Dynamic Models and Model Validation for PEM Fuel Cells Using Electrical Circuits.” IEEE Transactions on Energy Conversion, vol 20, no. 2, June 2005. Some highlight for this model: R ohm : represents ohmic losses R act : represents the activation losses (related with 2 nd term in Tafel equation) R conc : losses related with mass transport. C: capacitance related with the fact that there are opposing charges buildup between the cathode and the membrane. PEMFC electrical characteristics Basic circuit

20. 20 © Alexis Kwasinski, 2012 PEMFC electrical characteristics Model for the internal fuel cell voltage E where, Comments: The voltage drop related with fuel and oxidant delay is represented by E d,cell. The fuel cell output voltage depends on hydrogen’s and oxygen’s pressure The fuel cell output voltage also depends on the temperature. The time constants for these chemical, mechanical, and thermodynamic effects are much larger than electrical time constants. Equal to E r

21. 21 © Alexis Kwasinski, 2012 PEMFC electrical characteristics E d,cell can be calculated from the following dynamic equation: where τ e is the overall flow delay. In steady state, both derivatives are zero, so E d,cell = 0. But when the load changes, di(t)/dt is not zero, so E d,cell will be a non-trivial function of time that will affect the fuel cell internal output voltage. When considering fuel cells dynamic behavior, they all tend to have a slow response caused by the capacitance effect in slide 19, the flow delays, the mechanical characteristics of the pumps, and the thermodynamic characteristics. Thermodynamic characteristics were introduced in the model through an analogous electric circuit, as shown in the next slide.

22. 22 © Alexis Kwasinski, 2012 PEMFC electrical characteristics Simulation model and equivalent electric circuit for the thermodynamic block: Fuel cells have a slow dynamic response, as shown in the next figure that evaluates the response of a fuel cell to multiple fast step load changes:

23. 23 © Alexis Kwasinski, 2012 Hydrogen production Hydrogen needs to be produced, and sometimes it also needs to be transported and/or stored. Hydrogen is not a renewable source of energy. Hence, FC are alternative sources of energy. Methods for hydrogen production: Methane Steam Reforming (MSR) It uses natural gas Two-step process: 1) endothermic reaction (needs heat) 2) exothermic reaction (provides heat) 75 % to 80 % efficient. Partial oxidation (POX) It also uses natural gas or other hydrocarbon and/or POX is compact and has faster dynamic response than MSR, but MSR provides higher hydrogen concentration.

24. 24 © Alexis Kwasinski, 2012 Hydrogen production More methods for hydrogen production: Electrolysis of water Water molecules can be separated using electricity. But we use electricity to produce hydrogen to produce electricity again. Pure water is in many places an scarce resource. The electricity for the electrolysis needs to be produced and the water needs to be purified (soft de-ionized water is needed). Reaction: Electricity can be obtained at a large scale from nuclear reactors but the hydrogen needs to be stored and transported, and nuclear fuel is not a renewable source of energy. At a VERY small scale wind or solar power can be used, but this energy is available only when there is wind or sunlight. Gasification of Biomass, Coal or Wastes These methods are still a long way into the future.

25. 25 © Alexis Kwasinski, 2012 Hydrogen Storage Hydrogen atoms are the lightest and smallest of all elements. For this reason, it is very difficult to keep hydrogen from escaping confined environments such as tanks or pipes. Since an “effort” (i.e. work) needs to be done to keep hydrogen stored, storing hydrogen implies loosing efficiency. Some storage methods: Pressure Cylinders: Some efficiency is lost in the compressing process Liquid Hydrogen: it requires lowering the hydrogen temperature to 20.39 K. This process already reduces 1/3 of the efficiency. Metal Hydrides: These are compounds of hydrogen and Magnesium, titanium and other metals. Efficiency is low to medium and lot of heat is generated when the hydrogen is released, but these compounds are very easy to store in the form of soils. Carbon nano-fibers: New technology.

26. 26 © Alexis Kwasinski, 2012 PEMFC Technology and issues Expected life of PEMFC is very short (5,000 hours) and not suitable for DG. The most commonly used catalyst (Pt) is very expensive. The most commonly used membrane (Nafion – a sulfonated tetrafluorethylene copolymer is also very expensive). PEMFCs are very expensive. CO poisoning diminishes the efficiency. Carbon monoxide (CO) tends to bind to Pt. Thus, if CO is mixed with hydrogen, then the CO will take out catalyst space for the hydrogen. Hydrogen generation and storage is a significant problem. Additional issues to be discussed when comparing other technologies: dynamic response and heat production.

27. 27 © Alexis Kwasinski, 2012 The main advantage is that they use a liquid fuel. Reactions: Anode Cathode Voltages: 0.046 V at anode, 1.23 V at cathode, 1.18 V overall. Methanol has high energy density so DMFC are good for small portable applications. Issues: Cost Excessive fuel crossover (methanol crossing the membrane) Low efficiency caused by methanol crossover CO poisoning Low temperature production Considerable slow dynamic response Direct Methanol Fuel Cells (DMFC)

28. 28 © Alexis Kwasinski, 2012 One of their main advantages is their long life in the order of 40,000 hours. The phosphoric acid serves as the electrolyte. The reactions are the same than in a PEMFC. Hence, the reversible voltage is 1.23 V The most commercially successful FC: 200 kW units manufactured by UTC They produce a reasonable amount of heat They support CO poisoning better than PEMFC They have a relatively slow dynamic response Relative high cost is an important issue Phosphoric Acid Fuel Cells (PAFCs)

29. 29 © Alexis Kwasinski, 2012 The main advantage is that their cost is relatively low (when considering the fuel cell stack only without “accessories”. Reactions: Anode Cathode Developed for the Apollo program. Very sensitive to CO 2 poisoning. So these FCs can use impure hydrogen but they require purifying air to utilize the oxygen. Issues: Cost (with purifier) Short life (8000 hours) Relatively low heat production Alkaline Fuel Cells (AFCs)

30. 30 © Alexis Kwasinski, 2012 One of the main advantages is the variety of fuels and catalyst than can be used. Reactions: Anode Cathode They operate at high temperature. On the plus side, this high temperature implies a high quality heat production. On the minus side, the high temperature creates reliability issues. They are not sensitive to CO poisoning. They have a relatively low cost. Issues: Extremely slow startup Very slow dynamic response Molten Carbonate Fuel Cells (MCFCs)

31. 31 © Alexis Kwasinski, 2012 Solid Oxide Fuel Cells (SOFCs) One of the main advantages is the variety of fuels and catalyst than can be used. Reactions: Anode Cathode They operate at high temperature with the same plus and minus than in MCFCs. They are not sensitive to CO poisoning. They have a relatively low cost. They have a relatively high efficiency. They have a fast startup The electrolyte has a relatively high resistance.

32. 32 © Alexis Kwasinski, 2012 Comparison of the most common technologies All fuel cells occupy a lot of space. Much more than any of the other types of microsources PEMFCDMFCAFCPAFCMCFCSOFC FuelH2H2 CH 3 OH H2H2 H2H2 H 2, CO, CH 4, hydrocarbons Electrolyte Solid polymer (usually Nafion) Solid polymer (usually Nafion) Potasium hydroxide (KOH) Phosporic acid (H 3 PO 4 solution) Lithium and potassium carbonate Solid oxide (yttria, zirconia) Charge carried in electrolyte H+H+ H+OH - H+H+ O 2- Operational temperature ( o C) 50 – 10050 - 9060 - 120175 – 2006501000 Efficiency (%) 35 – 60< 5035 – 5535 – 4545 – 5550 – 60 Unit Size (KW) 0.1 – 500<< 1< 55 – 2000800 – 2000> 2.5 Installed Cost ($/kW) 4000> 5000< 1000*3000 – 3500800 – 20001300 - 2000 Fuel cell technologies * Without purifier