3- Pumped-storage hydropower 3.1 Brief considerations Since the electrical grid needs to assure the supply at any time, including the time of high demand, which is in the morning and late afternoon, the rest of the day, and especially during the night, energy is not needed as much. The major goal is to try to uniform this curve, getting the line as close to horizontal as possible, avoiding like that peak demands, which make it necessary to have such quantities of energy available, only for very short periods. In Figure 3.1, one can observe the individual household electricity demand for a one-day period. As it was mentioned before, the peaks that can be seen are the ones that should be minimized in order to facilitate the energy efficiency, and avoid therewith the waste of energy. Figure 3.1 – Individual Household Electricity Demand (University of Oxford). In Figure 3.2 it is possible to learn about the aggregate energy consumption, where again a less desirable peak can be verified. While the aggregate demand smoothes out the individual peaks, the daytime load still doubles the night time load. Figure 3.2 - Aggregate Energy Consumption (Berkeley,2010).
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3- Pumped-storage hydropower
3.1 Brief considerations
Since the electrical grid needs to assure the supply at any time, including the time of
high demand, which is in the morning and late afternoon, the rest of the day, and
especially during the night, energy is not needed as much. The major goal is to try to
uniform this curve, getting the line as close to horizontal as possible, avoiding like that
peak demands, which make it necessary to have such quantities of energy available,
only for very short periods. In Figure 3.1, one can observe the individual household
electricity demand for a one-day period. As it was mentioned before, the peaks that can
be seen are the ones that should be minimized in order to facilitate the energy
efficiency, and avoid therewith the waste of energy.
Figure 3.1 – Individual Household Electricity Demand (University of Oxford).
In Figure 3.2 it is possible to learn about the aggregate energy consumption, where
again a less desirable peak can be verified. While the aggregate demand smoothes out
the individual peaks, the daytime load still doubles the night time load.
Figure 3.2 - Aggregate Energy Consumption (Berkeley,2010).
Concluding, a lot can be done in order to promote energy efficiency, and avoid waste of
energy. Once these measures are all taking place and in a hypothetical case, the demand
curve is horizontal, the electricity price could be constant as well, instead of having
several peaks, as Figure 3.3 shows.
Figure 3.3 - Electricity Tariff (€/kWh) (Vieira e Ramos,2009).
Considering that the goal of a horizontal curve of demand is close to impossible to
achieve, the option, besides not wasting the energy produced, is storage. By storing
available energy during low demand hours, the energy won’t be wasted and can
therefore be used at times of peak and high demand. Since the renewable energy sources
are intermittent, like the sun or the wind, it is not possible to count on it on hours of
shortage of energy, because they won’t guarantee the supply. The solution to this
problem is to store these produced energies, so that they can be used when they are
necessary and not when they are available. Unfortunately the storage of such energies is
not easy or cheap, which leads to the waste of it, if it is not necessary at the time it is
available. This major concern will be addressed in this document, presenting a solution
of storage of renewable energy through hydropower with pumped-storage plants.
Sustainability
“Sustainability is a concept that turned into a principle. It prescribes that the use of
natural resources to satisfy present needs should not compromise the satisfaction of
necessities of future generations.“
It is often considered a “fuzzy concept” that can be represented through three
overlapping circles as presented in Figure 3.4.
Figure 3.4 - The Three Spheres of Sustainability.
The three circles, hereby called spheres, represent the environmental, the social and the
economic sphere. The sphere that applies mostly to this work is the environmental one.
It’s in this area that the pumped-storage as well as the use of hydropower have a goal of
sustainability. The use of the natural resources, without abusing them or damaging
them, is the key to reach a sustainable development. With pumped-storage, renewable
energy from, for example sun or wind, can be stored with the help of water. All the
resources are natural and their use is harmless for future generations, because it doesn’t
affect their source.
In conclusion pumped-storage may contribute to energy efficiency as well as
sustainability of the use of natural resources. It is yet to be discussed if such a solution
can be possible, viable, and competitive.
3.2 Pumped-Storage Power Plants
Historical Review - In the 1890s the first pumped hydroelectric storage in the world
appeared in the Alpine regions of Switzerland, Austria and Italy. The earliest designs
use separate pump impellers and turbine generators. Since the 1950s, a single reversible
pump-turbine has become the dominant design for pumped hydroelectric storage. The
development of these systems remained relatively slow until the 1960s, when utilities in
many countries began to envision a dominant role for nuclear power. Many of these
facilities were intended to complement nuclear power providing peaking power.
In the 1990s, the development of pumped storage power plants (PSPP) significantly
declined in many countries. Many factors may have contributed to the decline. Low
natural gas prices during this period make gas turbines more competitive in providing
peaking power than pumped-storage. Environmental concerns caused the cancellation of
several PSPP projects and significantly prolonged the licensing process. In some
countries, power sector restructure probably also contributed to this slowdown. During
the 1990s, several countries started to restructure the power sector by unbundling
generation and transmission. The nature of PSPP falls into the grey area between
generation and transmission. Because the net electricity output of PSPP operation is
negative, a PSPP facility usually cannot qualify as a power generator. Although PSPP
provides crucial load-balancing and ancillary services to the grid and reduces the needs
for transmission upgrades, PSPP do not typically qualify as transmission infrastructure.
For instance, in the United States, the Federal Energy Regulatory Commission denied a
request from a proposed PSPP project to be categorized as a transmission facility for
purposes of rate recovery. The regulations for PSPP vary from country to country. For
example, in China, PSPP is considered a transmission facility and the Chinese
government charges the state grid corporations with the primary responsibility for
developing PSPP and allows them to recover costs of PSPP through transmission tariffs
(Yang, 2011).
Nowadays, a very big interest in PSPP is taking place worldwide. Many new PSPP are
being built and older ones are being upgraded and renovated. Pumped hydroelectric
storage is at the present moment considered the most established technology for utility
scale electricity storage. Low-carbon electricity resource, like the wind, the sun, or even
nuclear power, can’t adjust their output to match fluctuation power demands, without a
storage component.
Importance, Potential and Technology of a Pumped-Storage Solution
- Importance - Global warming is an issue that concerns the world. The
major concerns are about the happenings over the past decades, namely:,
the world has been warming at a rate that is equally unprecedented, and
these circumstances have been made possible by the similarly unprecedented
magnitude of anthropogenic CO2 emissions, due to humanity’s ever-increasing
burning of fossil fuels such as coal, gas, and oil (McDaniel, 2011).
There is a big necessity of mitigating global warming, and reducing the burning of fossil
fuels is the major mean to achieve this final goal. This burning of fossil fuels may only
be reduced if some kind of energy exists to replace it. The option of renewable energy is
a very popular one, but in order to be competitive and available at all time, it needs to be
stored. Pumped-storage is therefore a piece of high importance in the puzzle of the
mitigation of the global warming.
- Potential - “Pumped-storage is the only viable, large scale resource that is
being broadly used for storing energy, and it offers the best option for
harnessing off-peak generation from renewable sources. With the ever-
increasing investment in variable generating sources, energy storage will be a
critical tool for using our clean energy resources effectively” (Miller and
Winters 2011).
An example of a pumped-storage power station can be seen in Figure 3.5, where the
upper reservoir and the lower water body can be seen.
Figure 3. 5 Pumped-storage Station in Germany.
These kind of pumped-storage facilities are being upgraded and constructed in several
countries in Europe. This fact is due to its capacity of storing electricity. Besides the
storage it still balances the load and can react quickly to change grid conditions (Züber,
2011).
Such facilities are different from normal hydroelectric power generation facilities.
Instead of only generating electricity it uses it as well. Energy gets stored in form of
water pumped from a lower elevation reservoir to a higher elevation reservoir. In hours
of low demand electricity gets consumed and water gets pumped into the higher
reservoir. When the peak hours arrive, and the demand is high, water gets turbined and
generates therefore electricity. This system buys electricity from the grid when this
electricity has a very low price, and can therefore be called a net consumer, but it sells
this same energy later in high demand hours, where electricity prices are high, and
creates therewith profit.
But generating this profit isn’t the only advantage of this system. This technology
provides the possibility of firming the variability of energy generated by intermittent
renewable sources. Since wind and sun are not constant and don’t guarantee they will be
available once they are needed, the excess of energy generated by these green sources
may be stored in pumped-storage systems instead of being wasted, during low demand
hours. These systems present an interesting option, since they have the possibility of
storage and provide significant flexibility regarding start-ups and shut-downs. The
necessary balance between generation and demand is facilitated by the existence of
these systems and are therefore being installed in all Europe. As explained before, the
European Union has a share of renewable energies that is continuously growing. Until