Technical Field
The present invention relates to a gravitational potential energy storage system capable of storing and regenerating energy by using a mass suspended from a cable.
Related Art
From the earliest days of the electric power system, energy storage has been considered an important technology for managing the grid. Today, the changing ways in which electric power is generated and used are making storage even more attractive than before. Energy storage can improve asset utilization, enhance the network reliability, enable more efficient use of base-load generation, and support a higher penetration of intermittent renewable generation. Large-scale stationary applications of electric energy storage can be divided in three major functional categories:
Power Quality Stored Energy: in these applications, is only applied for seconds or less, as needed, to assure continuity of quality power.
Bridging Power Stored Energy: in these applications, is used for seconds to minutes to assure continuity of service when switching from one source of energy generation to another.
Energy Management Storage Media: in these applications, is used to decouple the timing of generation and consumption of electric energy. A typical application is load levelling, which involves the charging of storage when energy cost is low and utilization as needed. This would also enable consumers to be grid-independent for many hours.
Although some storage technologies can function in all application ranges, most options would not be economical to be applied in all three functional categories.
Viable electrical power storage installations can therefore provide significant benefits for grid connected renewable energy sources which are totally reliant on the variable nature of wind, sun or ocean waves. When considering the current and projected installed capacity figures for wind power alone it is evident that its significance as a feasible global energy resource is now well established.
Electricity storage can enhance the value of energy from renewable generation in at least two fundamental ways. Storage can “firm-up” renewables' output so that electric power (kW) can be used when needed. Similarly electric energy (kWh) generated during times when the value is low can be “time-shifted” so that the energy can be sold when its value is high. One option would be to charge existing storage with electricity from wind generation as well as from the grid. Another would be to install additional storage at the renewable site.
Wind energy penetration refers to the fraction of energy produced by grid connected wind compared with the total generation capacity that is available to the grid. The limit for a particular grid will depend on the existing generating plants, pricing mechanisms, capacity for storage or demand management and other factors. An interconnected electricity grid will already include reserves of mostly carbon fuelled generating and transmission capacity to allow for equipment failures and the varying power generation produced by wind and other renewable sources.
In particular geographic regions, peak wind speeds may not coincide with peak demand for electrical power. A wind energy penetration figure can be specified for different durations of time. On an annual basis, as of 2011, few grid systems have penetration levels above five percent. To obtain 100% from wind annually requires substantial long term storage. On a monthly, weekly, daily, or hourly basis—or less—wind can supply as much as or more than 100% of current use, with the rest stored or exported.
Both long and short term stored energy increases the economic value of wind power since it can be deployed to displace higher cost generation during peak demand periods, these potential revenue gains can assist to offset the costs and losses of the storage system and allow base-load suppliers to run their plants more efficiently.
Energy storage exists in many electrical power systems. In the United States, about 2.5 percent of electricity that passes through the network has been stored. Pumped hydro facilities, the form of large-scale storage most familiar to utilities, represent most of this storage. Pumped hydro allows the storage of enormous quantities of energy, though it requires a huge initial investment.
However limitations for pumped storage installations would include the availability of the necessary geographical resources to viably achieve this together with the associated visual/environmental impacts.
Compressed air energy storage (CAES) is a less-widely implemented technology that uses off-peak renewable electricity to compress and store air, which can later be used to regenerate the electricity. Such techniques could be used to store renewable energy for convenient dispatch at later times.
Short-duration storage technologies such as ultra-capacitors and flywheels have uses in other applications, such as those in which power and energy requirements are not large but when the storage is expected to see a great deal of cycling. Such technologies can be used to address power-quality disturbances and frequency regulation, applications in which only a few kilowatts to megawatts are required for a few seconds or minutes.
A great deal of effort has gone into the development of electrochemical batteries. Utilities are familiar with lead-acid batteries which are extensively used for backup power in substations and power plants. In larger-scale applications, however, other battery chemistries such as sodium sulphur and vanadium redox flow batteries are more effective.
Flywheels might be used to provide minute-to-minute frequency regulation while large-flow batteries provide more large-scale ramping over several hours. Strategically placed sodium-sulphur batteries could ease bottlenecks in the distribution system through peak shaving, while reducing demand charges to customers. Ultra-capacitors placed at substations could mitigate the effects of momentary interruptions on distribution feeders.