lecture%15% - university of chicago

GEOS 24705 ENST 24705 ENSC 21100

Lecture 15

Electricity markets, turbines, hydro

Copyright E. Moyer 2016

Electricity system: genera&on, transmission, opera&on separate

What sets the amounts that people pay?

In the old days

The uBliBes owned everything, and would charge customers enough to recover their costs. The state uBliBes commission would approve the rates.

Nowadays

Generator price set by the day-‐ahead market: Sets the hour by hour price that generators receive for power or for capacity.

Wholesale price set by market and by FERC: Sets the markup that the RTO can charge over market. Sets the transmission rates.

Retail price set by state u=li=es commissions: Determines the rates that the uBliBes can charge their customers. Flat rates – no hourly changes.

Cheap natural gas is upending the electricity market

In PJM fluctuaions are fairly small (only 50% maximum change)

PJM has a normal diurnal demand cycle peak in late a;ernoon, amplitude ~ 30% of min in summer

“The duck has landed” in California

“The duck has landed” in California ResidenBal solar wipes out mid-‐day demand. (Home genera&on counts as canceling demand). Then demand spikes as the sun sets – by 2020 power demand will ~ double in 2.5 hours.

Turbines drive modern electricity genera=on all designed to use flowing fluid to rotate something

Features that may matter to turbine design

•  Fluid velocity •  Fluid density •  Pressure drops

if molecules just push on blades: “impulse turbine” if turbine exploits pressure drop: “reacBon turbine” if elements of both: “impulse-‐reacBon”

Features that may matter to turbine design

Wind: low velocity, low density, no pressure drop Water wheel or run-‐of-‐river hydro: low velocity, high density, no pressure drop Dam hydro: low velocity, high density, large pressure drop Gas or steam turbine: high velocity, low density, large pressure drop

Invented in 1884 (Parsons) 80% of world’s electricity today (all external combusBon) Power growth rapid: first turbine 75 kW (1890), by 1912 Chicago (Fisk!) was 25 MW, > 50 MW in Parson’s lifeBme, > 1 GW now

1. STEAM TURBINE

First built in 1903 (Elling). (Conceived of in 1791, but was not buildable) Adop=on slow: no commercializaBon Bl 1918, no rouBne use Bl 1930s.

2. GAS TURBINE

Invented in 1848 (James Francis). For the texBle mills at Lowell, MA – improved efficiency over tradiBonal water wheel design Most common hydro turbine in use today

3. HYDRO TURBINE (Francis)

4. HYDRO TURBINE (Pelton wheel)

Invented in 1870s (Lester Pelton). Improvement on tradiBonal water wheel used in CA gold mining in small streams Preferred hydro turbine in certain condiBons: small flow, large drop

Why isn’t there hydro on the Mississippi?

Why dam a river to get energy from its flow?

dam vs. run-‐of-‐river If the point is just to get kine=c energy from the

river’s flow, why do you need a dam at all? less environmentally damaging if you didn’t dam…

Photo: L. PapaKaster Photography

Where does the poten=al energy of an undammed river go?

Dam increases extractable energy

Water is guided to the turbine via a “penstock” that controls flow and pressure Image: Xeneca Corp.

Raising “head” allows harnessing energy that is otherwise lost during flow of river

Energy flux and power

Power = ε � ρ � A � v

f f

Flux = flow of something per area per Bme

A

l = v * t

Volume flux = V/A � t = (A � v � t)/(A � t) = v Mass flux = volume flux � (mass/volume) = v � ρ

Energy flux = mass flux � (energy/mass) = v � ρ � ε

Energy/mass in a flow: Bernoulli’s equa=on

Energy/mass = ½ v2 + g h + p/ρ

kineBc gravitaBonal pressure gradient

Can swap energy back and forth between these terms. This form is for incompressible fluids only.. Assumes no change in volume or temperature. Gases are compressible -‐ heat on adiabaBc compression, cool on adiabaBc expansion. Need to add in a term for the thermal energy (“internal energy”) of the gas, since energy swaps into and out of heat in gas.

Energy in a flow: wind power (kine&c term)



P = ε � ρ � A � v energy density is kine&c term: ε =½ v2

P = ½ ρ A v3

Flow f = mass/Bme f f

Power = energy/mass * mass/volume * volume/Bme

Energy in a flow: dam hydro (pot. energy term)



P = ε � ρ � A � v energy density is pot. energy term: ε =gh

P = g h ρ � (A � v)



Energy in a flow: dam hydro (recover in pressure term)



P = ε � ρ � A � v energy density is pressure term: ε =p/ρ

P = p � (A � v)



Pelton (1870s) High head, low flow Impulse turbine used with dam

3 main dam hydro turbine designs choice governed by flow rate and pressure

Images: L. Voith Siemens M. Copyright unknown R. Photo V.O. Kanninen

Francis (1848) Medium head, all but highest flows Pure reac<on turbine

Kaplan (1922) Low head, all flows Pure reac<on turbine

Flow differs in three main hydro turbines

Francis turbine: reaction turbine medium head (10 - 500 m), high flow Hydro getting bigger…over 800 MW max Efficiency already near max – but larger scale lowers cost

Francis turbine runner from Three Gorges Dam project, China (700 MW)

Runner is only small part of turbine assembly Inlet spiral is even larger

Francis turbine intake spiral from Grand Coulee Dam

Turbine assembly contains more than just runner

What is the input spiral for? Why does it decrease in radius? What is the funcBon of the drap tube? Why does it fan outwards?

Turbine assembly contains more than just runner

What is the input spiral for? Why does it decrease in radius? The spiral directs water radially to the runner. Because water flows in to center and is lost, the diameter has to narrow to keep velocity constant. What is the funcBon of the drap tube? Why does it fan outwards? To produce a slow-‐down of moBon and drop in pressure. (It’s an “anB-‐nozzle”)

How fast does the turbine rotate?

15 m rotor for 400 MW generator at Son La hydroelectric facility, Vietnam (L). Rewinding the rotor for a small hydroelectric plant (R)

Will a hydro turbine rotate as fast as a gas or steam turbine? No – gas turbines rotate at 60 Hz but hydro turbines rotate more slowly, someBmes ~1 Hz How does it make 60 Hz AC electricity? By increasing the number of poles in the rotor magnet

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