Academics have patented new concepts on electric power energy storage using gravity.
The concept of gravity energy storage using soil batteries is being promoted by University of Nottingham academics Professor Saffa Riffat, Fellow of the European Academy of Sciences and President of the World Society of Sustainable Technologies, and Professor Yijun Yuan, Marie Curie Research Fellow.
Why Energy Storage
The world’s requirement for electric power is growing rapidly and according to the International Energy Agency estimates, an additional 250 Gigawatts of power will be required annually between now and 2050. Renewable energy technologies including wind, solar, wave and tidal can provide clean energy but these technologies are intermittent, often producing power when grid utility or users don’t require them. With the growing use of renewables, peaking power, able to provide the surge in power as needed, require the worldwide reserve capacity to be increased by up to 10% per year in order to handle peak power usage periods.
Existing grid-scale energy storage systems include batteries, compressed air storage, and pump storage hydropower (PSH) with PSH being the main player in grid-scale storage. The difficulties with PSH include siting, land/water usage, construction time and capital cost required prior to a facility can be deployed. The key challenges to providing a successful grid-scale storage are low capital and running costs, quick construction, high energy storage density, scalability, low maintenance requirements, and long lifetime.
Gravity Power Technologies
The gravity storage technology concept is similar pumped hydropower systems. In pumped hydro systems, water flows down via gravity from an upper reservoir to a lower reservoir, passing through a turbine/generator making power. Water is then pumped back up from the lower to the upper reservoir using electricity and the process is then repeated.
In gravity storage systems, a heavy weight block is moved from a lower point to an upper point which represents energy storage ‘charging’, and then, when needed, the mass returns from a higher to a lower point where the kinetic energy of the descending block powers a generator, ‘discharging’, creating electrical power when the grid requires. A simple way to illustrate the gravity storage concept is to use a person’s arm lifting and lowering a heavy object. As shown in Figures 1a, the object is trying to pull the arm down due to the force of gravity. As gravity pulls down on the object, it causes a rotational force at the shoulder joint. This force is torque and the muscles of the shoulder must then be activated to overcome this force in order to hold the weight from moving down. The rotational force at the shoulder can be represented by a pulley/motor-generator linked to a weight. The generation process occurs by moving the arm downwards (Figure 1a) while the charging process is accomplished by moving the arm upward (Figure 1b).
In recent years several gravity power technologies have been proposed. These include Energy Vault using six-arm crane to lift concrete blocks up and down a 33-storey building, mine shafts using heavy lifts, ARES shuttle-train system, moving heavy rail cars from higher to lower locations and back, generating power through the electric train motor-generators, Sink Float Solution using ocean gravitational energy storage and Energy Cache storage system using buckets on a line that picks up gravel at the bottom of a hill, and moves the gravel to the top of the hill; when the process is reversed the gravel moves back down the hill and powers a generator to produce energy. All these technologies will play important roles in renewable energy storage.
A new concept is proposed by University of Nottingham academics Professors Saffa Riffat (President of the World Society of Sustainable Technologies) and Yijun Yuan (EU Marie Curie Fellow Fellow), who filed a patent application in May 2019 claiming a novel gravity energy storage technology based on drums filled with soil.
The basic concept of GravitySoilBatteries is shown in Figures 2 and 3. The technology uses storage cores (large drums filled with compacted soil) that could be shifted between lower and higher points. The soil for the storage device can be obtained locally by digging the ground to create deep channels for the system. The soil is also used as a filler for the central concrete support structure. Pulleys are mounted on the top of central concrete structure. The drums are fitted with axial shafts and bearings and are mounted on a metal frame similar to tarmac rollers. The drums could be then pulled on the sloped central concrete structure using cables and motor/generator. The motor/generator is mounded on the ground to provide a good stability and ease of maintenance. When heavy drums moved down, they release potential energy (i.e. electricity generation) to the main grid system (Figure 2). During the discharge phase, the drums are moved upward to store energy supplied by photovoltaic solar power or wind turbines, using power when not needed by the grid, storing the energy for later use as shown in Figure 3.
GravitySoilBatteries can be used for a large-scale storage in conjunction with main grid systems. The technology is environmentally friendly and simple to construct. The estimate cost of GravitySoilBatteries is about $50/kWh or lower depending on the depth of the channels and height of the central support structure. The cost of PSH storage (without consider land cost) is about $200/kWh while the cost battery storage is about $400/kWh.
The energy storage capacity of GravitySoilBatteries for a small-scale storage could be 300kWh while for a high-scale storage could be 30,000kWh (or above) depending on the drum weight and stack height with an estimated system efficiency of approximately 85%. GravitySoilBatteries can be applied widely with simple siting and construction. Figure 4 shows an example of a large-scale application of GravitySoilBatteries with the system extended over several kilometres.
The main benefits of GravitySoilBatteries include:
- Safe and reliable technology
- Use locally sourced soil as the storage material
- Constant high efficiency of over 80% compared to PSH efficiency of 50-70%
- High energy storage density of up to 8 times that of PSH
- The storage capacity of the system could be between several hundred kWh to thousands kWh
- Unlike PSH no requirement for water availability
- Lower cost than existing energy storage systems as local soil material can be used as storage media.
- Higher efficiency than pump storage hydropower and battery technology
- Generate no waste materials
- Requires significantly less area than PSH
- Scalable as required
- Short construction time
- Long life time
Figure 1a. Concept of gravity energy storage using arms movement – Generation
Figure 1b. Concept of gravity energy storage using arms movement – charging mode
Figure 2. GravitySoilBatteries – electricity generation process
Figure 3. GravitySoilBatteries– electricity charging process
Figure 4. (Very top image) Large-scale energy storage using GravitySoilBatteries
For more information please contact Professor Saffa Riffat:
Fellow of the European Academy of Sciences
President of the World Society of Sustainable Energy Technologies
University of Nottingham
Nottingham, NG7 2RD