New possibilities for reducing the cost of pumped storage backup for intermittent

wind and solar power

by Prof. Dawid E. Serfontein, School of Mechanical and Nuclear Engineering, North-West University

History

The North-West University recently conducted an

academic study on feasible deployment dates of

nuclear units in South Africa, which is also

available from us.

This study devoted a large amount of focus on the

instabilities that intermittent wind and solar-PV

creates on the power grid.

To our great surprise we found that a number of new

technologies now offer affordable solutions to these

grid instability problems.

History (continued)The list of solutions to the grid instability problem

includes:

The price of imported Liquefied Natural Gas (LNG)

has dropped sharply and sustainably, due to new

cheaper technologies that resulted in an

overproduction of gas in the US.

Since the output of gas turbines are very flexible,

they offer a natural solution to the intermittency of

wind and solar-PV.

CSIR wind map discovered reasonable wind cites all

over the country. If wind farmes were to be

distributed optimally, this would substantially

alleviate intermittency.

Trends for US natural gas prices

History (continued)The list of grid instability solutions (cont.):

Almost the whole of the Northern Cape, which

currently is viewed as SA’s solar power paradise, is

also suitable for wind power generation.

Addition of wind farms would mean that the current

solar transmission cable can double up as a wind

power carrier during the times of day when the sun

is not shining.

If wind and solar farms were to be distributed

optimally, this could actually alleviate the overload

on the transmission network.

History (continued)The list of solutions to the grid instability problem

includes:

The possibilities for maintaining grid stability by

optimal deployment of Dynamic Demand Response

(DDR) should be studied:

Time-of-use meters and pricing.

Frequency sensitive trip switches, with

compensation

Deploying variable speed turbine-pumps for

hydro pumped storage.

History (continued)Reasons for lack of deployment of strategies to

combat grid instability:

The capacity expansion computer model of the IRP

represents the international state of the art.

However its model input assumptions contains some

shortcomings, including:

Ignoring external cost of imported energy sources,

including natural gas.

Underestimating the external cost of coal.

Focusing on fixing mismatches between supply and

demand by increasing supply, rather than by decreasing

or shifting demand by means of aggressive time-of-use

pricing and Dynamic Demand Response.

Shortcomings of IRP model input assumptions

(cont.):

Assuming that the role players (producers and

consumers) will automatically do what’s good for

the country, instead of creating incentives to drive

these changes.

Reasons for lack of deployment of strategies to

combat grid instability:

Roll-out of renewables were primarily controlled by

the REIPPPP (who apparently thought that

combating the intermittency of renewables was

unnecessary) and

by Eskom who have publicly stated that intermittent

renewables are unwelcome.

History (continued) Therefore no one actively targeted the necessary

strategies to combat intermittency:

The REIPPPP did not implement time-of-

production or location-of-production prices.

Therefore almost all wind turbines are in the

Western Cape, which greatly increased

intermittency and congestion of the transmission

network.

Eskom did not deploy additional gas turbines, or

DDR.

Ingula received constant speed pump-turbines,

which cannot vary it input/output continuously in

order to combat the continuous variations of the

power output of wind and solar.

Potential of Pumped StorageThe need for pumped hydro storage:

Traditionally pumped storage has been used for

peak shaving:

SA time-of-day power demand

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0 3 6 9 12 15 18 21 24

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Pow

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Time of day (h)

Normalised avg. time-of-day power demand vs. PV-solar and wind power

output curvesSA PowerDemand (1 kWAv.)

Avg. PV-SolarPower Output(1.03 kWp)

Avg. WindPower Output(1.25 kWp)

Potential of Pumped StorageThe need for pumped hydro storage:

Introduction of intermittent wind and solar PV power

is creating the need to short-term pumping when the

wind is blowing or the sun is shining:

Wind and PV-solar will create multiple opportunities

to empty and refill the dam in a typical day, which

will reduce capital cost

Pumped storage has virtually zero variable costs:

multiple extra pumping and power production

sessions will come for free.

Constant vs. variable speed Pumped

Storage With price out of the way, there remain only one

stumbling block to large scale deployment of wind

and solar-PV power: Intermittency

In the US the current glut of cheap shale gas is used

to fuel gas turbines as cheap back-up for wind and

solar-PV.

South Africa has not yet been able to confirm large

amounts of shale gas.

Gas imports also create strategic risks for South

Africa.

Constant vs. variable speed Pumped

Storage These problems create scope for a local clean,

sustainable and virtually CO2-free storage

technology: Pumped hydro storage

Variable speed pump-turbines adds the possibility to

do ultra-short-term frequency control:

SA time-of-day power demand

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 3 6 9 12 15 18 21 24

Norm

alis

ed

Pow

er

Dem

an

d

vs. S

up

ply

( k

W )

Time of day (h)

Normalised avg. time-of-day power demand vs. PV-solar and wind power

output curvesSA PowerDemand (1 kWAv.)

Avg. PV-SolarPower Output(1.03 kWp)

Avg. WindPower Output(1.25 kWp)

Technical aspects of variable speed pumped

storage:

Control pumping power via varying power frequency.

Control turbine power via water by-pass.

More expensive civil works and pumps, less efficient

Twice as profitable

Conclusions and Recommendations

Variable-speed pumped storage may present a

cheaper, cleaner, and more sustainable form

of back-up for intermittent wind and solar-PV

power than the currently planned fleet of LNG

fuelled open cycle gas turbines.

Variable-speed pumped storage can replace

OCGT with the fuel efficient LNG-fuelled

CCGT.

The combination could provide backup that is

affordable, clean and sustainable.

Conclusions and Recommendations

Such backup could substantially increase the

scope for deploying large generation

capacities of cheap new intermittent wind and

solar-PV power.

This could be to substantially lower South

Africa’s greenhouse gas emissions at an

affordable cost.

Thank you!

Any questions?