A recent addition to pumped storage technology is pumped underground storage that involves the construction of storage reservoirs at depths of up to 2000-feet or 600m below the surface of the upper reservoir. A small version of this technology involves the use of a piston pump moving a relatively small volume of water over extreme elevation. A Kaplan turbine such as the pumping turbine developed by Voith Hydro can move large volumes of water over elevations of 600m.
Pumped hydraulic storage involves a considerable loss of output/input efficiency, especially when measured from the point of power generation. There are several alternatives by which to use existing technology to raise the efficiency and productivity of pumped hydraulic storage in smaller hydroelectric installations. There is also scope for further technological development to raise the output/input efficiency of pumped storage.
Direct Drive Option:
In traditional pumped storage, a thermal engine drives an electric generator that operates at some 94% efficiency and supplies power to an electric motor of 94% efficiency that drives a turbine pump that operates at 91% efficiency. During the generating cycle, water flows through a turbine of 93% efficiency and drives an electric generator of 94% efficiency. A direct drive system between thermal engine and water pump would raise overall storage efficiency from 70% to 79%.
At the present time, direct drive of hydraulic pumps may be restricted to small-site and low-head power dams. Early windmills were used to pump water in Holland and many modern wind turbines do directly drive water-pumping technology. A modern vertical-axis wind turbine of several thousand kilowatts output can directly drive a modern water pump and transfer water from low elevation to a higher elevation using a propeller or piston pump. Kite-based power technologies may also directly drive positive-displacement water pumps.
There are locations where a reservoir may be located next to a river or ocean coast. Ocean wave technology would be able to pump water to higher pressure and transfer water to higher elevation. CETO of Australia already offers such technology that can drive a shore based turbine and electrical alternator of supply pressurized water to a reverse-osmosis desalination installation. Free-flow kinetic turbines can also drive water pumps instead of electrical generation equipment and transfer water from a tidal strait or river into a storage reservoir.
There are numerous small-site power dams around the world where boat propellers may be used to pump water uphill into a storage reservoir. The development of mini-nuclear power of 10MW to 50MW output from companies such as Toshiba, NuScale Power, Adams Nuclear (air-cooled reactor) and Hyperion Power allows for combined-cycle nuclear-hydroelectric installations. Small biomass power stations may also be included in such installations. Small-scale water pumps can operate at speeds of up to 1000-RPM and be directly driven by any of several steam powered engines. The exhaust heat from the thermal engines may be used to energize water-based, steam-vacuum refrigeration technology for district cooling or the energy may be directly used to provide district heating.
The Kaplan turbine resembles a ship propeller and can operate as both a turbine as well as a water pump. In high-head dams, these turbines will rotate at between 70-RPM to 100-RPM or within the same speed range as large marine diesel engines, the largest of which is the Wartsila-Sulzer RTA96-C engine of some 81MW output. It is a 2-stroke engine that can be adapted to burn other fuels. There may be scope to adapt the engine to operate as a uniflow steam engine, with the exhaust steam leaving via the ports near the bottom of the cylinder walls. An engine-driven vacuum pump would assist in evacuating exhaust steam from each cylinder.
It may be possible to rapidly inject ultra-critical steam (4000-psia, 1200 F) into the each cylinder when the piston is at top dead center and in a way that mimics the rise in pressure and temperature as occurs with the combustion of diesel fuel. Such a conversion would ensure high thermal efficiency and allow the engine to pump water to higher elevation in dams of up to 100MW output. However, over the long-term future it would be desirable to develop very compact, positive-displacement engine that could directly drive the pumping turbine during the off-peak hours. Clutch mechanisms could disengage the electrical alternator(s) and engage the engine to directly drive the pumping turbine.
Combined Thermal and Hydroelectric Generation:
There are several research groups and companies that are testing prototypes of positive-displacement rotary engines, most of which can be adapted to operate on steam. The development challenge for the engine builders would be to develop large versions of these compact rotary engines that can operate at 70-RPM to 100-RPM and drive large-scale Kaplan turbines of 20MW to 100MW capability. The development challenge for the engine builders would be to ensure that their engines would be capable of operating on ultra-critical steam to assure high thermal efficiency.
There are several rugged candidate engines for such development including the Quasiturbine engine from Quebec. Another candidate engine would be the rotary vane engine reinforced with tension reaction levers to enable curved vanes to withstand the extreme rise of pressure resulting from rapid injection of super- or ultra-critical steam. These engines would be built to extreme width (or length) compared to diameter to deliver the desired power output and to fit vertically into a modified hydroelectric power station. Multiple injectors would simultaneously and inject high-pressure ultra-critical superheated steam into these engines. Combined thermal and hydroelectric power stations would include insulated steam lines leading to steam engines that drive the pumping-hydraulic turbines during off-peak periods.
The vertical axes of rotation for both hydraulic (pumping) turbine and rotary steam engine will be co-linear. During peak hours, both engines would rotate in opposite directions to each other while driving separate alternators. At the onset of the off-peak period, clutch mechanisms that use positive engagement would disengage the alternators and engage the rotary steam engine. That engine would directly drive the pumping turbine in the opposite rotational direction to power generation for it to operate as a pump that pushes a massive volume of water to higher elevation.
Productive Use for Elevated Storage Water:
While direct drive between an engine and a hydraulic turbine would raise overall storage efficiency, there are other areas by which to increase the productivity of hydraulic power conversion. The temperature of the water moving between the upper and lower reservoirs will determine alternative additional services. If the water temperature in the upper reservoir is warmer during winter than deep level ground temperature, heat pumps may be used to extract heat from the flow of storage water to provide heating inside buildings during peak periods.
There are numerous locations around the world where during winter, river water or seawater may be warmer than geothermal ground water and warmer that the air temperature. The water may serve as the thermal reservoir for heat pumps that transfer heat into buildings. During northern summers, there are northern locations where southbound cold ocean currents would be cooler than the surrounding air, with the option of using seawater being pumped into storage as the heat sink for district cooling systems.
Productive Use for Underground Storage Water:
Such operation can form the basis of a district heating system for groups of large buildings that would be located near the hydroelectric turbines. An equivalent volume of seawater has some 3600-times the heat capacity as air at atmospheric pressure, while fresh water has some 3500-times the capacity. During winter in New York City, the temperature of the Gulf Stream current that flows by the city is several degrees warmer than the temperature of the groundwater or bedrock. That water would be the basis of operation for an underground pumped hydraulic installation that may be built along the East River in New York City.
During wintertime power generation cycles, heat pumps would cool the incoming seawater and transfer heat into nearby buildings through a district heating system. Cooling the temperature of the incoming flow of water from 60 F to 50 F would provide over 70,000-BTU/sec or over 100MW of heating capacity. The cooled water would still be warmer that the surrounding bedrock at 46 F. The water could be further cooled when it is pumped to the surface during the overnight off-peak period to recharged for the next duty cycles.
The thermal energy may be distributed to nearby large buildings via a district heating system. Some of the thermal energy may also be heated pumped into thermal storage chambers located either at a district heating system or in the basements of large buildings. Cooling the warm ocean water that enters and leaves the underground reservoir every winter day could provide up to 200MW of heating capacity to buildings and incur energy savings over traditional geothermal winter heating.
There is an option in New York City to combine underground pumped storage with a thermal power plant that operated during peak periods. It may be possible to use a long, multi-section drive shaft to connect the thermal engine to the pumping turbine so as to enhance energy storage efficiency. The flow of seawater into and from pumped underground storage during summer may serve as the heat sink for the power plant to condense water for re-use. The daily movement of warm water into the underground chamber would warm the surrounding bedrock and enhance its potential to provide district heating during winter.
There are numerous coastal cities around the world where hard, impervious rock occurs at depths of below 1000-feet. That rock would allow for the development of underground pumped storage plus the possible development of a companion thermal power station. The temperature of the water that flows into and out of the underground reservoir can serve as a thermal reservoir for heat-pumped district heating or as a heat sink for district cooling.
There is at present and future potential to combine hydroelectric power generation with a companion power generation technology such as thermal, wind or a complimentary hydroelectric technology. During off-peak periods, the companion technology may directly drive a water-pumping technology to transfer water to a higher elevation at higher efficiency and perhaps at lower cost. There may be additional scope to make productive use of the temperature and thermal capacity of water that flows to and from a storage reservoir at small installations.
Overall storage efficiency would increase from 70% efficiency using electrically driven pumps to 79% efficiency using directly driven pumps, while using the thermal capacity of water could raising the equivalent combined overall efficiency to over 81%. There may be scope to develop large-scale, combined-cycle or hybrid hydroelectric power plants at locations that are distant from main population centers. A combined-cycle installation would require the development of compact thermal engines of high output and low rotational speed to match the power levels and rotational speeds of the pumping turbines. The thermal capacity of the moving water may serve as the heat sink that provides cooling for condensing operations at the thermal power station.