A Comparison of Two Engines. Benefits of an Electric Motor

Transcription

1 Fuel Cells ( Lecture prepared with the able assistance of Ritchie King, TA 1

2 A Comparison of Two Engines Internal-combustion engine Electric motor Only 33% efficient at best 80-90% efficient Air emissions Zero direct emissions Peaky torque-rpm curve Broad torque-rpm curve (needs a transmission) (does not need a transmission) Power loss in idle No idle Irreversible energy conversion Regenerative braking Big and heavy Small and light (50 hp in 600 lbs = 0.7 kw/kg) (75 kw in 13 kg = 5.8 kw/kg) Noisy Quiet So, why don t we have electric motors in our automobiles today? Because we do not have good enough batteries to store the electricity on board of the vehicle! Benefits of an Electric Motor No idling Direct drive No driveline losses More efficient This number is smaller

3 Hydrogen Fuel Cells A fuel cell is a way to generate electricity on board to power an electric motor Catalytic Electrochemical Process catalyst itself does not change, so unlike batteries, fuel cells don t go bad. However, hydrogen is only a carrier of energy, not an energy source. This means that fuel cells are only as clean as the ultimate energy source used to create the hydrogen. A typical fuel cell ( 3

4 How do fuel cells work? All fuels cells have an anode (+) a cathode ( ) an electrolyte separating the two. A fuel flows to the anode and an oxidant to the cathode the resulting chemical reaction produces electricity. Fuel cells are typically classified according to the type of electrolyte used. Fuel Cell Types Fuel Cell Type Phosphoric Acid Molten Carbonate Solid Oxide Proton Exchange Membrane Alkaline Electrolyte H 3 PO 4 (Na,K,Li) CO 3 YSZ Sulfonated Polymers Aq. KOH Operation Temperature ( o C) ( 4

5 Comparative Advantages of Proton Exchange Membrane (PEM) Fuel Cells Higher Power density (fewer cells needed in stack) No electrolyte corrosion or safety concerns Lower operating temperature allows for instant start-up How do PEM fuel cells work? 5

6 How do PEM fuel cells work? circuit e - e - H air Pressure forces H into catalytic membrane Electronegativity of O attracts electrons water N Technical characteristics 6

7 The complete fuel-cell system is more than the cell stack Examples of PEM fuel-cell vehicles: ( ( 7

8 ( GM s concept 8

9 ... and there even exists a fuel-cell motorcycle! This motorcycle is not just quiet, it is silent! ( Remember Hydrogen is not a source of energy, but rather a carrier of energy. In other words, it takes energy to produce hydrogen fuel. To understand how much energy is needed, we need to look at a little chemical thermodynamics. 9

10 Thermodynamics of Hydrolysis and Water Formation A B 1 H O + energy H + O 1 H + O H O + energy The splitting of water (Reaction A) is an endergenic (endothermic) reaction, meaning that it requires a net input of energy because the products are inherently more energetic than the reactants. Conversely, Reaction B is exergenic (exothermic), meaning that it creates a net release of energy to the environment. However, the energy terms in Reaction A and Reaction B are not equal due to the Second Law of Thermodynamics. The Second Law of Thermodynamics The Second Law of Thermodynamics has been expressed in many different ways over the years. Perhaps the most well known form is the following: The entropy of the universe is always increasing. 10

11 The Second Law of Thermodynamics This means that the universe is becoming more disordered with every chemical reaction. The splitting and recombination of H O is no exception. 1 H O H + O Water, being a single compound, is more ordered than two-component compounds, H and O. It takes some energy to cleave the more ordered water to create the more disordered H and O. The energy associated with creating the disorder, or entropy, ultimately dissipates and cannot be recovered to do useful work. Enthalpy and Gibbs Energy The enthalpy change of reaction ( H) captures the notion of energy and entropy changes for chemical reactions. In contrast, the change in Gibbs energy ( G) of reaction accounts only for the change in usable energy and not the change in entropy. The two are related by the following equation: G = H T S where T is the temperature in Kelvin and S is the change in entropy. Because absolute temperature is always positive and entropy increases with every reaction, the above equation tells us that the change in Gibbs energy is always less than the change in enthalpy, which is precisely what we would expect. 11

12 Back to Water 1 H O H + O For the above reaction, we have: H = 86 kj/mol H O G = 37 kj/mol H 98 K = 5 o C Now, if it takes H to cleave water into hydrogen and oxygen, and we can only get G back through using hydrogen as fuel, then the maximum efficiency we can possibly attain for the entire process, from hydrogen production to automobile propulsion, is: η thermal = kj/mol kj/mol = 0.83 = 83% As you ve probably guessed, actual efficiencies are much smaller. Hydrogen Production Remember: Hydrogen is only as clean as the fuel source used to produce it. Basically, hydrogen can be produced in one of two ways: Through a series of high-temperature chemical reactions By using electricity to split water (electrolysis) In this presentation, we ll be looking at using renewable resources, specifically wind, to generate electricity to be used for electrolysis. 1

13 Chemical reactions for hydrogen production: Steam reforming ( o C) CH 4 + H O (steam) 3H + CO Water gas shift (00 or 350 o C) CO + H O (steam) H + CO Dry reforming ( o C) CH 4 + CO H + CO Partial oxidation of methanol (50 o C) CH 3 OH + H O (steam) 3H + CO An interesting question: What is the minimum number of windmills it would take to produce enough hydrogen to power the US automobile fleet? What would that number be for New Hampshire alone? ( 13

14 What we ll need to know Rate of electrolytic hydrogen production per unit power Fuel economy of hydrogen fuel cell cars Number of miles driven annually in the US Typical capacity of a windmill Note: Often, this kind of information can be found on the Energy Information Administration website: eia.doe.gov Electrolytic Hydrogen Production A typical value for the amount of energy needed to produce hydrogen by electrolysis is 367 kj/mol. (Note that this is appreciably higher than the H value, 86 kj/mol). This means that 1 kw of electricity can produce 1/367 = mol/s or g/s of H. Equivalently, 1 MW can produce kg/s. (Berry, Gene D. Hydrogen Production Encyclopedia of Energy, 6 th ed. Elsevier 004) 14

15 Fuel economy of a hydrogen fuel-cell car: Experience with existing prototypes reveals that a hydrogen fuel-cell car has a fuel economy of 199 km/kg This means that a fuel-cell car can go 199 km (= 14 miles) on 1 kg of hydrogen. Put another way, to have a range of 600 km (= 400 miles), it needs a tank that can hold 600/199 = 3.0 kg of hydrogen. Miles driven/year In 003, US personal vehicles traveled a total of,594 billion miles (4.17 x 10 1 km) 1, equivalent to 8,750 miles per person in the US (everyone, including children, who don t drive). NH residents drove a total of 14,51 million miles (.9 x km) in the same year, equivalent to 10,880 miles per person in the state (including children). 1 Vital Signs 006: Economic and Social Indicators for New Hampshire, Economic & Labor Market Information Bureau. Jan

16 Capacity of average wind turbine Typical wind turbine has nameplate capacity of about 1 MW Nameplate capacity is max capacity of course, average production will be less than this and will depend on local wind patterns. Meeting the US driving demand 1 windmill 1MW 1 MW kg H 1kg H /s 199 km 1year = 11,90 windmills s km 1 year 16

17 Is this possible? 1,000 windmills have a collective capacity of 1 GW. The EPA estimates that the US has over,500 GW of potentially available wind resource, meaning that 5% of total available capacity would be used to power the US automobile fleet. 1 Remember, however, that the value for miles traveled used here was for 003. The EIA projects that by 05, annual vehicle miles traveled will be 3,791 billion, which would require about 178,00 windmills and 7.1% of total available wind resource Is it going to happen? Not without a huge push. The EPA estimates that by 05, the US will have developed a total wind power capacity of 47 GW. If the EIA projections prove accurate, this capacity will only account for 6% of the energy needed. ( 17

18 Meeting NH demand 1windmill 1MW 1 MW kg H 1kg H /s 199 km 1year = 670 windmills s km 1 year Does this make sense? According to the US census, in July 005, the population of the US was about 96.4 million while the population of NH was about 1.31 million. If everybody were driving the same amount everywhere in the States, we would expect New Hampshire to need about 540 windmills. ( 18

19 Some Good Sources Nagamoto, H. Fuel Cells: Electrochemical Reactions. Encyclopedia of Materials: Science and Technology. 006, Pgs Appleby, Anthony J. "Fuel cell", in DOI / , last modified: February 8,