Energy_Storage_Technologies.pdf

# There are two factors that control the power and

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There are two factors that control the power and energy rating of the system: the height difference between the reservoirs (known as the “head”, and the volume of the reservoirs (the “ﬂow”) [8]. The larger the volume of water available and the greater the height, the more energy can be stored. The greater the ﬂow rate through the pipes, the more power can be produced. This comes from the basic physical principle that potential energy due to gravity is proportional to mass times height, with the constant of proportionality being acceleration due to gravity: E = mgh . Since power is the time rate of energy, or the derivative of energy, and since gravity and height are constant with time, power can be defined as: P = dE dt = dm dt gh . So to increase the energy capacity of the system, increase the volume of water and height differential, to increase the power capacity increase the ﬂow rate of water and height differential. Siting these facilities is complicated in that they require two large reservoirs to be present in close proximity, one higher than the other (and the greater the height differential the better). This type of geologic occurrence is more prevalent in mountainous regions, but in such regions it’s diﬃcult to build these systems, and is typically further from connections to the power grid [8]. If the distance is too great between sites then the connection between the two sites will have to be longer, and likely at a smaller angle (if the reservoirs were side-by-side, the penstock would be nearly vertical) and this would mean more friction between the water and the pipe, losing energy. Upper Reservoir Lower Reservoir Intake Pump/ Generator Discharge Charge Figure 3.1: Overview of Pumped Hydroelectric Storage (adapted from [7]) Variations A few design alternatives have been proposed: Underground Pumped Hydroelectric Storage In this design the lower reservoir is constructed by excavating rock as far as 300 m underground [8]. The generator/motor is placed in the excavated region underground, and water is pumped from the underground reservoir to the above-ground reservoir. This 10

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Energy Storage Technologies Pumped Hydroelectric Storage design allow for the water to ﬂow vertically, minimizing losses due to friction. The environmental impact of this design on the surface is less because it only requires one reservoir that affects the surface [11]. This design requires specific geographic and geologic structures to be in place for this to be viable at a given site. Pumped Seawater Hydroelectric Storage In this design the lower reservoir is the sea, the rest of the design remains unchanged. One benefit is that this can be implemented in many more locations than other designs since there is so much coastline available. At the same time the use of seawater may lead to corrosion of the equipment, and adding seawater to the upper reservoir may negatively effect the upper reservoir’s ecology [8].
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