Electricity is probably the most underappreciated and understated privilege there is on the face of this earth. A twenty-four-hour availability of such a smooth and consistent supply of electricity is a miracle in and of itself.
Is electricity being produced exactly in synchronization with the amount needed for consumption, every second of the day? There must be fluctuation in the consumption of electricity with every subsequent moment of the day, the production can’t possibly keep up with such a variable demand without dispatchable power that can quickly be switched on and off to suit the demand.
Just like a storage unit for excess goods, there must be a storage plant for electricity. However, storing electrical energy is not that straightforward!
Its very difficult to store electricity so other easier forms of energy are stored that can be quickly converted to electrical energy. There are a bunch of options to store energy, however, one of the most common and efficient ways to do so is by utilizing Earth’s gravitational potential.
Using the earth’s gravitational field as a spring, we can store gravitational potential in bodies. In this case – hydropower generation – water is used as the ‘body’.
How do the world’s biggest batteries work?
Gravity is a powerful, complex force that is surrounding us at all times. For most people, it is a hindrance to flight, but for centuries some people have been harnessing its energy as hydropower.
What happens in producing the electricity is no mystery, water flows by gravity through a turbine with a certain amount of kinetic energy making the turbine rotate, the generator uses the rotation with a magnetic field to convert to electricity.
But once the source of flowing water (the upper reservoir) is depleted, there is no water left above with potential energy to convert to electrical energy! conventional hydropower plants must wait for the reservoir to fill up by natural means, rainfall, snowmelt and flow of a river.
However, in pumped storage plants, water can be pumped up with the help of a turbine (or a separate pumping system) from the lower reservoir to the upper reservoir again. Nevertheless, pumping the same amount of water to the same level as before would consume energy overall due to losses in the system! This means there is a net deficit in the production of electricity.
This dilemma is overcome by employing a carefully administered load management strategy. Temporal arbitrage allows Pumped Storage Hydropower Plants to make money by using power when the prices is low and then producing power when the price is high.
Parts of the Pumped Storage Hydropower Plant
What does a pumped storage setup need?
The most important aspect required to construct even a basic layout of pumped storage is the presence of two water bodies close to each other but separated by a significant difference in altitude.
· Two separate reservoirs
· A Francis turbine/pumping hall – As it should be capable of generating electricity while also being able to pump water back up to the upper reservoir.
· Outlet tunnel/Penstock attached to the upper reservoir leading down to the turbine.
· Discharge tunnel beside the turbine to let the water flow into the lower reservoir.
· A station generator consisting of transformers and storage units that connect to the national grid for electricity generation and transmission.
One important thing to note is that the energy produced per unit volume of water is directly proportional to the elevation difference between the two water bodies.
The Entire Process
Pumped storage hydropower automatically provides energy-balancing, stability, storage capacity, and ancillary grid services such as reserves, through the perks of its sole concept.
The difference in elevation of the turbine and upper reservoir gives the water in the upper reservoir certain potential energy which is used at run-time to convert into electrical energy.
Once the stored energy is used and the demand (and price is reduced), the “pumped” part of the storage comes into play!
Although it may take a portion of the overall power generation to pump the water back up to the upper reservoir. A careful load management scheme makes the whole idea work!
The generation of electricity is done when demand exceeds supply. And when supply exceeds the demand, the price of electricity is low, then the water is pumped to the upper reservoir.
What these pump storages essentially do, is make a profit from consuming and selling electricity at the right time. Using this process, the issue – power shortages during the peak (high demand) hours of the day – is solved for the consumers and distributors alike.
For better understanding, we can discuss the following example. Taking an extreme assumption where the energy required to pump the water back up is equal to the energy produced during run-time with the same water. In this case, the pumped storage will produce electricity and transmit it to the grid during peak hours of need.
Once all the water has come down, at night-time, the storage plant can buy electricity from thermal power plants which stay turned on 24/7, hence, have abundant energy leftover during such off-peak hours.
This cheaper bought electricity will be used to pump the water back up. So, the process will continue regularly. This is the old model for pumped hydropower in the 1960s and 70s PSH was paired with base load thermal power stations to transform uniform supply to meet variable demand. The next generation of PSH currently planned and under construction will be paired with renewables and smooth variable supply to match the fluctuating demand.
One interesting fact about pumped storage is that it can be switched on to start production in as fast as a single minute.
Favorable and Non-favorable Conditions
The most important condition for a pumped-storage hydropower plant is the elevation difference between the upper and lower reservoirs.
The more the altitude difference between the two reservoirs, the more power is generated with the same amount of water. This makes steep hilly areas of countries like Norway and Switzerland extremely favorable for such storage projects. On the contrary, countries such as Netherlands cannot avail of this method with such ease as their land is mostly flat.
Another favorable factor can be rain, helping with filling a little extra water in the upper reservoir, or even just compensating for the water lost to evaporation.
Subsequently, it is quite obvious that high temperatures or sunny climates will be less favorable for the system, due to evaporation losses, as mentioned above.
After analyzing the complete system, these storages, just like other renewable sources of energy, are most efficient when placed as a backup for another production plant. Hence, it’s most suitable to construct this project close to another power plant. That plant may be a wind turbine station, a solar-powered plant, though in the past it was most commonly a thermal power plant.
Reliability and Resilience
A stable power supply can be easily secured in presence of the storage and ancillary services provided by pumped storage hydropower plants. The reasons for this include the storage and reserve power with rapid mode changes and the black-start capability of PSH plants.
All of these rationales are crucial to help build the proportion of variable renewable energy (VRE) in grid systems. In this respect, pumped storage is exceptional at long discharge durations while providing high power capacity which shall be vital in avoiding VRE curtailment, reducing costs and emissions, and most importantly, reducing transmission congestion.
One particular advantage that makes PSH more reliable than most other forms of power plants in the long term is the low-lifetime cost and independence from raw material availability of the concept.
How Efficient is this System?
Taking into consideration the evaporation losses from the exposed water surface and conversion losses, energy recovery to be expected in this plant is about 70-80% depending on the methods adopted. However, in case of rain assistance and other excess energy being efficiently utilized, these figures have even gone up to 87%!
These systems are especially economical for everyone as they flatten out load variation on the power grid by helping the base load bearing power plants to continue operating at peak efficiency.
This helps them cope with the large disparity in the electric load curves of consumption and production.
Well, to sum up, the most significant aspect of pumped storage hydropower efficiency is the exceptional load management strategy which makes up for such an impressive economical upside to the plant.
Recapping
All things considered; pumped storage hydropower is the most developed energy storage technology in the world today. And it’s quite safe to say that PSH is the future of power grid management.
Although, one loophole to be catered to is the dependence of PSH technology on a primary power plant.
Combined with Variable Renewable Energy (VRE) resources it can give rise to a thorough solution of one of the world’s biggest environmental issues – Depletion of non-renewable energy resources/ Global power crisis.
With the right terrian Pumped Storage Hydropower Technology can be deployed accross the world to support the transition from fossil fuels to renewables.