Lecture 2

# Lecture 2 - CONVERSION Energy in one form Energy in another...

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1 CONVERSION Energy in one form Energy in another form Losses Efficiency = output / input

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2 Types of converters Rotating machines Solar cells Power electronic converters Fuel cells Nuclear reactor Batteries Flywheels Water - hydroelectric
3 Fuel cells Convert chemical energy to electrical energy. Produces DC Typically about 50% efficient Produces heat If heat is recovered, efficiency rises to about 75% Very old technology, very new materials used “Jury is still out”

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4 Fuel cell equivalent circuit V oc R fc + Example: A PEM fuel cell produces 1.5 v open circuit, and has 50% efficiency at rated power output of 200 mW. Find the equivalent R fc of the cell.
5 Pout = 200 mW Pin = 400 mW Ploss = 200 mW Pin = IVoc = 400 mW = I*1.5 I = 267 mA Ploss = 200 mW = (267 m) 2 R fc R fc = 2.8 ohms

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6 CONVERSION Energy in Energy in another form Losses CONVERSION Losses
7 Overall efficiency is now Efficiency #1 time Efficiency #2 Example Hydrolysis is used to create hydrogen from water. The process is 50% efficient. The hydrogen is used to make electricity in a fuel cell. That process is 50% efficient. Find the number of watt*hours of electricity needed to make enough hydrogen to power a fuel cell where the fuel cell will power a 30 hp motor for 10 hours.

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8 1 hp = 746 watts 30 hp = 22.38 kW 30 hp for 10 hours = 223.8 kWh Overall efficiency = (0.5)(0.5) = 0.25 Therefore need 4*223.8 kWh of electricity = 0.8952 MWh Note: this might be compared to a fuel cell car – the cost of a tank of gas to go Phoenix – L. A. is about 22\$ -- the cost of 0.8952 MWh is about \$44.76 That is for a 30 hp car – for a 60 hp car, double the fuel cell cost.
9 Hydrogen Not a fuel Portable Environmentally friendly Main value seems to be for electric vehicles 20 gallons of gasoline = 0.2 m 3 Equivalent amount of H 2 is 1.2 m 3 at 5000 psi PV = nRT, at 500 psi need a tank of size12 m 3 , at 50 psi need 120 m 3 These figures are at 100% conversion efficiency. If you use 50% conversion efficiency in a PEM fuel cell, double the tank size. You will need a big car!

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10 SOLAR ENERGY
11 As early as the 1980’s, a 354-MW solar power plant was built in the Mojave desert, in California. Here the heat contained in solar rays, concentrated by reflecting troughs and raised to 400 o C, produces steam that runs a conventional power generator. When the sun is not shining, the plant switches to natural gas. The latest generation of this type of plant incorporates new engineering solutions and new scientific principles such as non-imaging optics, which makes it possible to build much more efficient concentrators at lower costs. These developments open new prospects for the technology in the sunniest parts of the world.

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12 Diversity factor: not all loads connected to the grid operate at the same time, the diversity factor is the percent of the connected load that is operative. Generally, D < 33% Example: how many square meters of PV cells are needed to serve a town of 10,000 people with a peak demand of 30 MW?
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