20 July 2020
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Professor Andrew Blakers, Dr Matthew Stocks and Bin Lu
Australian National University
Executive
summary
- With increasing penetration of variable renewable electricity
generation in the electricity grid, there is a need for large-scale energy storage
to assist in demand management. Pumped hydro schemes provide most of this
energy storage around the world and Australia has no shortage of potential
sites that could be used to support the increasing share of renewable
generation.
Australian electricity options are short briefings on the principal energy sources
and storage options being debated in Australia, including: coal, natural gas,
wind, nuclear, photovoltaics (PV) and pumped hydro energy storage (PHES).
The global COVID-19 pandemic and its economic
consequences mean that statements and projections about future demand and
pricing of energy options may no longer be reliable. Readers should note that
some figures quoted in these briefings may pre-date the pandemic. |
Solar photovoltaics (PV) and wind together constitute 60% of
net new electricity generation capacity installed each year worldwide, and
effectively 100% in Australia. Coal, oil, gas, nuclear, hydro and other
renewables provide the balance. The fundamental reason is that the cost of new-build
PV and wind plant is now lower than the cost of new-build fossil and nuclear
plant.
Electricity generation from PV and wind is variable. When the
proportion of renewable electricity rises above about 50%, then significant storage
is needed. Pumped hydro energy storage has a 97%
share of the global storage market, although battery storage is expanding
rapidly. Additionally, demand management and stronger interstate interconnections
smooth out local adverse weather and complement storage.
Pumped hydro energy storage
Pumped hydro energy storage (PHES) constitutes most energy
storage worldwide. When electrical energy is plentiful and cheap, it is used to
pump water from a lower reservoir to a nearby upper reservoir through a pipe or
tunnel. During periods of peak demand, when electricity is expensive, the
pumped water is released downhill through a turbine to generate electricity (see
Figure 1). About 80% of the electricity used to pump the water uphill is
recovered, and 20% is lost.
Figure 1: How pumped hydro works
Source:
Australian Renewable Energy Agency (ARENA), Winning the uphill battle. How pumped hydro could solve the
storage problem,
ARENA website, 20 August 2017
In addition to storing energy, pumped hydro energy storage
has additional capabilities that help support the electricity system. Pumped
hydro energy storage can provide excellent inertial energy (from the heavy
rotating generator) which helps stabilise the system against disturbances; fast
response time (idle to full capacity in one or two minutes); and black start
capability (to restore a collapsed grid). The operational lifetime is 50–100
years, with low operational costs.
Australia already has river-based pumped hydro energy
storage facilities at Wivenhoe,
Shoalhaven
and Tumut
3. Construction of Snowy 2.0 has
commenced—this project would add 2,000 MW of generation to the National
Electricity Market (NEM) and provide about 175 hours of storage. The Kidston
pumped hydro scheme in an old gold mine in Far North Queensland has
received Northern Australia Infrastructure Facility (NAIF) funds. A further six
pumped hydro energy projects have been shortlisted in the Underwriting
New Generation Investments program.
Off-river pumped hydro energy
storage
Pumped hydro energy storage located away from rivers
(‘off-river’) is well-suited to low cost short-term storage. Off-river pumped
hydro energy storage takes advantage of the vastly larger area of land that is
off-river compared with that available around ‘on-river’ sites, which provides
the opportunity to find numerous good sites close to loads and transmission
powerlines.
Unlike conventional on-river hydro power, off-river (closed
loop) requires pairs of reservoirs that are generally 10–100 hectares in size,
rather like oversized farm dams, located away from rivers and national parks in
hilly country. These sites are separated by an altitude difference (head) of
200–900 metres and joined by a pipe or tunnel containing a pump and turbine. In
these systems, water cycles in a closed loop between the upper and lower
reservoirs. Off-river pumped hydro energy storage differs significantly from
conventional river-based hydro in that:
- the reservoirs are small (hundreds rather than thousands of
hectares). Typically, conventional hydro-electric systems are located in river
valleys with lake areas of thousands of hectares and expensive and extensive
flood control measures to cope with once-in-ten-thousand-year floods
- minimal flood control measures are needed because the reservoirs
are deliberately placed away from watercourses that have sufficient catchment
to cause serious flooding
- the heads are 2–5 times larger because the upper reservoir can be
on top of a hill rather than in a river valley. An increased head is
advantageous because a doubled head allows doubling of energy stored and power
developed, while the cost is generally less than doubled and
- there are minimal environmental impacts because reservoirs are
small and river flows are not disturbed.
Energy storage needs
A key point in relation to storage in a grid dominated by PV
and wind is that a relatively small amount of storage is usually sufficient.
Short-term storage (around 20 hours) covers a variety of scenarios, including
high-demand events such as hot summer afternoons and cold winter mornings and
evenings; night-time; periods of low supply caused by wind lulls and cloud
cover; plant and transmission line failure; and the time required to bring a
fossil fuel power station on line or implement demand management if the supply
shortfall is likely to be extended. Additionally, short-term storage improves
the load factor of constrained power lines to delay or avoid their duplication—for
example powerlines connecting wind and solar farms in windy and sunny rural
regions to national grids.
Today the balancing requirement is met through traditional
hydro and low-duty cycle gas power stations. In the future, new pumped hydro
energy storage could increasingly take on this role as PV and wind generation
increases.
The NEM and grid covers eastern and southern Australia but
excludes Western Australia, the Northern Territory and remote regions. Recent
work shows that about 450 GWh of widely distributed storage is
required to stabilise the NEM when renewable electricity reaches 100% (mostly
wind and PV with some existing hydro and bio energy). This corresponds to an
area of off-river pumped hydro energy storage equal to about 4,000 hectares
(upper and lower reservoirs combined), which is a tiny fraction of the Australian
landmass.
Figure 2: AREMI (Australian Renewable Energy Mapping
Infrastructure) synthetic image of potential PHES upper reservoir sites near
Araluen (Canberra district). The lower reservoirs would be at the foot of the
hills. Head is up to 600 m. The sites depicted have enough storage to support
100% renewable electricity in NSW.
Source: M
Stocks, R Stocks, B Lu, C Cheng, A Nadolny and A Blakers,
A global atlas of pumped hydro energy
storage, 28 March 2019.
Image credits: Data 61 and Bing maps
Sites for pumped hydro energy
storage
Potential sites for off-river PHES can be identified from a
geographic information system (GIS) platform, such as ArcGIS, based on
algorithms with defined search criteria. Detailed information such as head,
reservoir area, average dam depth and storage capacity is then derived from the
search results for further analysis.
A study
at the Australian National University (ANU) identified about 3,000 low-cost
potential sites around Australia with head typically better than 300 metres and
storage larger than one gigalitre (see Figure 3). The sites identified have a
combined energy storage potential of around 163,000 GWh. To put this into
perspective, a transition to a 100% renewable electricity system would need 450
GWh of PHES storage. The potential pumped hydro energy storage resource is
almost 300 times more than required. Developers can afford to be very selective
since only about 20 sites (the best 0.1% of sites) would be required to support
100% renewable electricity generation.
Figure 3: distribution of pumped hydro energy storage sites identified by ANU.
Source: M
Stocks, R Stocks, B Lu, C Cheng, A Nadolny and A Blakers,
A global atlas of pumped hydro energy
storage, 28 March 2019.
Image credits: Data 61 and Bing maps
Energy storage in pumped hydro
The energy storage capability of a pumped hydro energy
storage system is the product of the mass of water stored in the upper
reservoir (in kilograms), the usable fraction of that water, the gravitational
constant, the head (in metres), and the system efficiency. By way of example, a
pumped hydro energy storage system might comprise twin 20-hectare reservoirs,
each 20 metres deep, with a usable fraction of 85%, separated by an
altitude difference (head) of 400 metres, and operating with a round-trip
efficiency of 81% (90% for each of the pumping and generating cycles). This
equates to a usable mass of water (when the reservoir is full) of 3.4 GL.
Accounting for pumping and generating losses, the effective energy storage
capacity is about 3 GWh (or 300 MW of power for ten hours). Roughly speaking, 1
GWh of energy storage requires 1 GL of stored water for 400 m head.
Water use
The use of fresh water rather than salt water is preferred
to reduce corrosion of turbines, pumps and pipes and to minimise the risk of
salt contamination of the land environment. Reservoirs can be lined if
necessary to minimise seepage.
Evaporation rates in reservoirs are relatively high at up to 2,500
mm per year. Evaporation suppressors in the form of coverings over the
water reduce evaporation by reducing solar heating of the water, trapping water
vapour and reducing wind flow across the water surface. High-quality
suppressors reduce evaporation by 90%. This means that rainfall exceeds annual
evaporation in most years, and top-up water requirements will be minimal.
Harvesting of small amounts of water from micro gullies located near the reservoirs
provides additional water at low cost. Whether or not evaporation suppressors
are used would depend upon the cost of commercially supplied water or the
availability of local water.
The initial water fill would be required over the next one
to two decades as reservoirs are progressively constructed to support
increasing amounts of PV and wind. The reservoir water requirement amounts to
much less than 1% of the annual Australian commercial water market.
Building off-river pumped hydro
storage
An ideal pumped hydro energy storage site has a large head
because doubling the head doubles the power and energy available from the upper
reservoir, and halves the water requirement for a given amount of storage, but
usually does not double the cost. Another important requirement is that the
pipeline or tunnel connecting the upper and lower reservoirs be short and steep
for a given head. A slope steeper than 1:10 is preferred to minimise cost.
Preferably, the reservoirs are not located below any
significant catchment to avoid the cost associated with coping with occasional
floods. Some potential sites will be unsuitable because of poor geology,
restrictions on allowed land use or poor access. The three common types of site
are:
- turkey nest—the upper reservoir is built at the top of a flat
hill. Earth and rock is scooped from the interior to create a continuous earth
wall 10–20 m high
- head of gully—an earth wall is placed across a small gully near
the top of a mountain to impound water and
- old mine sites—the mining pit can form the lower reservoir, and
the upper reservoir can be a turkey nest reservoir located near the edge of the
pit. An example is the proposed 250 MW Kidston
pumped hydro energy storage project in an old gold mine in north Queensland.
Pipes, pumps, turbines, generators, substations and
powerlines are standard equipment that is widely available from the hydro-electric
power industry. Construction of reservoirs within Australia also draws upon
extensive experience in the construction of farm dams and tailings dams for
mining operations.
Indicative cost
Most of the costs of off-river and
conventional (on-river) pumped hydro energy storage are similar. The main
difference is that off-river pumped hydro energy storage uses relatively tiny
and low-cost reservoirs that have a much larger head and do not require
expensive flood control. Costs of off-river pumped hydro energy storage systems
are relatively predictable because each off-river pumped hydro energy storage
site looks much like another, whereas river valleys vary greatly. Power costs
(pipe, pump, turbine, generator, transformer, control, transmission) comprise
most of the costs and amount to around $800 per kilowatt for a good site. The
energy cost (the reservoirs) amount to about $70 per kilowatt hour. Thus, the
expected cost of a 1,000 megawatt pumped hydro energy storage system with a
head of 600 m and 14 hours of storage is about $1.8 billion.
Conclusion
There are effectively an unlimited number of suitable sites
for pumped hydro energy storage. Off-river pumped hydro energy storage can
facilitate high (50–100%) penetration of variable renewable energy at modest
cost through the provision of low-cost short-term mass-storage.
References and further reading
A Blakers, B Lu and M Stocks, ‘100% renewable electricity
in Australia’, Energy, 2017, 133, 15 August 2017, pp. 471–482.
X Yao, H Zhang, C Lemckert, A Brook and P Schouten, ‘Evaporation
reduction by suspended and floating covers: overview, modelling and efficiency’,
Urban Water Security Research Alliance, Technical Report No. 28, August
2010.
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