Energy storage

obstacles for onshore wind turbines

• trees

• buildings

boundary layer

layer of wind slowed down by friction with the land

what disturbs the boundary layer

• trees

• buildings

• urban canopy

• intermittent tree canopy

how would a mountain/hill effect wind speeds

exposed side - wind often fastest, speed peaks at apex

hidden / leeward side - separation of the wind, recirculation, reduced wind speed

katabatic wind

downslope of wind driven by gravity due to the flow of cold, dense air from high to low elevations in the evening

direction of air flow in mountain/valley topographies in the day

upwards as the air in the valley warms and rises

land-sea interface wind behaviour

there is a sea-breeze driven by cold air displacing warm air.

diurnal

daily, day to night

Weibull probability distribution

b/n [T-y/n]^b-1 exp(-(T-y/n)^b), looks like boltzmann distribution

average power produced from wind turbine compared to maximum power (qualitative)

a fraction

How would you be able to solely use wind power

need a storage system as wind power output is very variable

wind power is proportional to

u³

how are wind turbines arranged

in arrays so they dont block each other

model of wake

r = r(0) + lx (linear)

why is the solar radiation less compared to the edge of the atmosphere

its scattered by clouds, vapour, and aerosols

what technology collects solar power

solar photovoltaics (PVs)

how do PVs work

• photon w/ enough energy is absorbed

• electron emitted from outer shell

• electron hole

• current produced

band gap required for PVs (eV)

1.1 eV

what happens to the excess energy of a photon absorbed by PV

converted to heat

range of efficiency of commercial PVs

18-23%

why is radiation lower at the equator and extreme latitudes

lots of vapour at equator, more atmosphere at poles

how does radiation vary with seasons

less intense in winter, more intense in summer

what relation makes using solar an issue for the power grid

time v power, its not stable

what is needed when the sun sets and solar isn’t working

alternate power supplies

what is needed to calculate power produced by PV cell

incident radiation, efficiency, capacity

what can cause daily fluctuations in solar

clouds, pollution

what is needed to smooth out the supply from PV cells?

high frequency storage to account for the high freq. fluctuations

main issues with solar power

• fluctutations (daily, seasonal)

• Panel orientation

how are panels usually orientated

at the optimal angle, averaged over the year’s power production

optimal panel orientation in the UK

35* to the horizontal

what climate trends have been identified re. solar

• decreasing cloud cover

• increasing radiative flux

How are the fluctuations in renewable power managed?

energy storage that has a rapid response

what storage technologies have a short discharge time? (minutes to hours)

flywheel, batteries

what storage technologies have a medium discharge time (days to months)

compressed air, pumped

what storage technologies have a large discharge time (up to a year)

Hydrogen, methane

what ion batteries exist

Li, Ca

issues with batteries

• extraction of raw materials

• not suitable for long term storage

• not suitabe for very large scale energy storage

how does the Dinorwig resevoir convert PE to power

by allowing the resevoir to drain rapidly

PE of water entering the turbine (no friction)

ro*g*h

rate of doing work in resevoir discharge (no friction)

Q*ro*g*h

efficiency of Dinorwig resevoir

75%

how do flywheels tend to dissipate energy

friction, drag

rotational energy of flywheel

E = ½ ro*h*R⁴*ω²

Why do we use flywheels

• can respond rapidly

• used to provide power for short times to balance intermittency

Limits to a fly wheel

• strength of material

• rotation rate

• size of wheel

how does compressed air act as a storage solution

• compressed air is stored in a salt cavern

• during times of high demand this air is used to drive a turbine and provide more energy

real-life example of compressed air storage

Huntdorf, Germany, stores air up to 100 atm

pressure of air in cavern (ideal gas)

P = ro*RT (ro = n/V like in PV=nRT)

mass of compressed air

m = ro*V

internal energy of compressed air

E = mcT (c = heat capacity)

what happens as the air is compressed

temperature increases adiabatically

what happens to the T of air as it is extracted

cooled

UK electricity supply (GW)

40 GW

what can compressed air be sotred in

• salt caverns

• aquifers

how can an aquifer store air

needs a structural trap, ie bends with peak upwards so the air collects there

volume of air in anticline aquifer

V = 2*pi*r²*h

pore volume in a permable layer of rock

porosity * volume

is an aquifer or salt cavern better at storing compressed air

aquifer

problem with air storage in aquifer

air-water interface can become stable and the air can be trapped / hard to access

how can the flow of air through porous media be controlled

• viscous dissipation of air as it migrates through pores (Darcys Law)

• capillary pressure at a-w interface tends to suppress migration of air into successive pore spaces

what does air do rather than displace water

develop ‘fingers’ through the water

uses of hydrogen for power production

• fuel for H fuel cell

• direct combustion

efficiency of electrolysis

75-80%

overall efficiency of using H for electricity

40%