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Cranes are a familiar fixture of practically any city skyline, but one in the Swiss City of Ticino, near the Italian border, would stand out anywhere: It has six arms. This 110-meter-high starfish of the skyline isn't intended for construction. It's meant to prove that renewable energy can be stored by hefting heavy loads and dispatched by releasing them.

Energy Vault , the Swiss company that built the structure, has already begun a test program that will lead to its first commercial deployments in 2021. At least one competitor, Gravitricity , in Scotland, is nearing the same point. And there are at least two companies with similar ideas, New Energy Let's Go and Gravity Power , that are searching for the funding to push forward.

To be sure, nearly all the world's currently operational energy-storage facilities, which can generate a total of 174 gigawatts, rely on gravity. Pumped hydro storage , where water is pumped to a higher elevation and then run back through a turbine to generate electricity, has long dominated the energy-storage landscape. But pumped hydro requires some very specific geography—two big reservoirs of water at elevations with a vertical separation that's large, but not too large. So building new sites is difficult.

Energy Vault, Gravity Power, and their competitors seek to use the same basic principle—lifting a mass and letting it drop—while making an energy-storage facility that can fit almost anywhere. At the same time they hope to best batteries—the new darling of renewable-energy storage—by offering lower long-term costs and fewer environmental issues.

In action, Energy Vault's towers are constantly stacking and unstacking 35-metric-ton bricks arrayed in concentric rings. Bricks in an inner ring, for example, might be stacked up to store 35 megawatt-hours of energy. Then the system's six arms would systematically disassemble it, lowering the bricks to build an outer ring and discharging energy in the process.

This joule-storing Jenga game can be complicated. To maintain a constant output, one block needs to be accelerating while another is decelerating. “That's why we use six arms," explains Robert Piconi, the company's CEO and cofounder.

What's more, the control system has to compensate for gusts of wind, the deflection of the crane as it picks up and sets down bricks, the elongation of the cable, pendulum effects, and more, he says.

Piconi sees several advantages over batteries. Advantage No. 1 is environmental. Instead of chemically reactive and difficult-to-recycle lithium-ion batteries, Energy Vault's main expenditure is the bricks themselves, which can be made on-site using available dirt and waste material mixed with a new polymer from the Mexico-based cement giant Cemex .

Another advantage, according to Piconi, is the lower operating expense, which the company calculates to be about half that of a battery installation with equivalent storage capacity. Battery-storage facilities must continually replace cells as they degrade. But that's not the case for Energy Vault's infrastructure.

The startup is confident enough in its numbers to claim that 2021 will see the start of multiple commercial installations. Energy Vault raised US $110 million in 2019 to build the demonstration unit in Ticino and prepare for a “multicontinent build-out," says Piconi.

Compared with Energy Vault's effort, Gravitricity's energy-storage scheme seems simple. Instead of a six-armed crane shuttling blocks, Gravitricity plans to pull one or just a few much heavier weights up and down abandoned, kilometer-deep mine shafts.

These great masses, each one between 500 and 5,000 metric tons, need only move at mere centimeters per second to produce megawatt-level outputs. Using a single weight lends itself to applications that need high power quickly and for a short duration, such as dealing with second-by-second fluctuations in the grid and maintaining grid frequency, explains Chris Yendell, Gravitricity's project development manager. Multiple-weight systems would be more suited to storing more energy and generating for longer periods, he says.

Proving the second-to-second response is a primary goal of a 250-kilowatt concept demonstrator that Gravitricity is building in Scotland. Its 50-metric-ton weight will be suspended 7 meters up on a lattice tower. Testing should start during the first quarter of 2021. “We expect to be able to achieve full generation within less than one second of receiving a signal," says Yendell.

The company will also be developing sites for a full-scale prototype during 2021. “We are currently liaising with mine owners in Europe and in South Africa, [and we're] certainly interested in the United States as well," says Yendell. Such a full-scale system would then come on line in 2023.

Gravity Power and its competitor New Energy Let's Go, which acquired its technology from the now bankrupt Heindl Energy , are also looking underground for energy storage, but they are more closely inspired by pumped hydro. Instead of storing energy using reservoirs at different elevations, they pump water underground to lift an extremely heavy piston. Allowing the piston to fall pushes water through a turbine to generate electricity.

“Reservoirs are the Achilles' heel of pumped hydro," says Jim Fiske, the company's founder. “The whole purpose of a Gravity Power plant is to remove the need for reservoirs. [Our plants] allow us to put pumped-hydro-scale power and storage capacity in 3 to 5 acres [1 to 2 hectares] of flat land."

Fiske estimates that a 400-megawatt plant with 16 hours of storage (or 6.4 gigawatt-hours of energy) would have a piston that's more than 8 million metric tons. That might sound ludicrous, but it's well within the lifting abilities of today's pumps and the constraints of construction processes, he says.

While these companies expect such underground storage sites to be more economical than battery installations, they will still be expensive. But nations concerned about the changing climate may be willing to pay for storage options like these when they recognize the gravity of the crisis.

This article appears in the January 2021 print issue as “The Ups and Downs of Gravity Energy Storage."

• Gravity Batteries, Green Hydrogen, and a Thorium Reactor for China - IEEE Spectrum ›
• Skyscrapers—A Gravity Energy Storage Boon - IEEE Spectrum ›

Samuel K. Moore is the senior editor at IEEE Spectrum in charge of semiconductors coverage. An IEEE member, he has a bachelor's degree in biomedical engineering from Brown University and a master's degree in journalism from New York University.

Its sad, that IEEE published the number of 35 MWh stored in those concrete blocks. Just as a reminder: when lifting 100 tons by 100 meter the amount of stored energy is 100 Mega Joule or somewhat less that 30 kWh. (or 1/3 of a tesla battery);1 KWh is 3.6 Mega Joule. For 30 MWh one would need 100000 tons lifted by 100 meter

Lift Renewable Energy uses a form of gravity battery. To store energy, buoyant gas containers are pulled down into water by a winch, water is in effect lifted hundreds of meters. The cycle is then reversed and electricity is generated as the gas containers rise. Relatively little infrastructure is required, the batteries can be sited near major population centers and round trip efficiency is 85+%. The system can be scaled from KWH's to GWH's. https://lift-re.com/

Always glad to see gravity storage in the news! Terrament is working on a new design of "gravity storage" that can achieve larger scale by digging deep underground using existing mining technology. Our patent-pending design enables us to maximize both of the two simple ingredients of gravity storage: weight and height. Our secret sauce is how we efficiently use the shaft's bedrock to support the enormous loads of our modular weight. Fingers crossed we can get this built asap to better support wind and solar. https://www.terramenthq.com/

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Why not give robots foot-eyes, video friday: humanoids get a job.

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• Published: 16 November 2022

Parametric optimisation for the design of gravity energy storage system using Taguchi method

• Mostafa E. A. Elsayed 1 , 2 ,
• Saber Abdo 1 , 5 ,
• Ahmed A. A. Attia 1 , 3 ,
• El-Awady Attia 1 , 2 , 4 &
• M. A. Abd Elrahman 1 , 3  

Scientific Reports volume  12 , Article number:  19648 ( 2022 ) Cite this article

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• Energy storage
• Mechanical engineering

Gravitational energy storage systems are among the proper methods that can be used with renewable energy. However, these systems are highly affected by their design parameters. This paper presents a novel investigation of different design features of gravity energy storage systems. A theoretical model was developed using MATLAB SIMULINK to simulate the performance of the gravitational energy storage system while changing its design parameters. A parametric optimization study was also conducted using Taguchi and analysis of variance (ANOVA) techniques for optimizing the energy storage rate. Six parameters were studied; three are related to the piston design (diameter, height, and material density). The other parameters are the return pipe diameter, length, and charging/discharging time. Results revealed that the piston diameter and height are the two most significant parameters for the system performance compared to the other parameters, as they contributed by 35.11% and 30.28%, respectively. The optimization results indicated that the optimal piston diameter, height, and return pipe diameter were 0.25, 0.5, and 0.01 of the container height. The outcomes of this paper can significantly improve energy storage and power generation from renewable energy systems as it provides a reliable, economical, sustainable, and durable energy storage system.

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Introduction.

Renewable energy (RE) generation has increased in recent years and is expected to continue to grow over the coming years. Electricity generated by RE is expected to rise from 10% in 2010 to 35% by 2050 1 , 2 . However, renewable resources usually cannot be used as a stand-alone power plant or as a primary source of electricity due to their intermittent nature and significant fluctuation, especially wind and solar energy 3 , 4 . This defect encouraged researchers to develop a solution for this irregular nature. Two immediate solutions have been suggested to address this problem. The first solution is the mixed-use of renewable energy resources, i.e., wind and solar energy. The second is using energy storage devices coupled with renewable energy resources.

There are three critical reasons for storing energy 5 , 6 , 7 , 8 ; the first reason is transferring power from a non-portable energy source to a portable one. The second is controlling the power-to-energy (PTE) ratio of the energy generation source, which means that the generated output can be directed to meet the changes in energy demand. The last reason is using it later whenever needed to satisfy the increase in demand. An energy storage system that fulfills the second and third reasons can be beneficial in overcoming the intermittent nature of renewable energy. It is worth mentioning that the energy storage systems can also provide flexibility for smart electric grids in the future since they can meet the variation in demand.

Different energy storage systems have been studied and developed over the last two decades. Most of the systems introduced were the electrical, chemical, electrochemical, thermal, and mechanical energy storage 9 , 10 , 11 . Mechanical systems, such as flywheel energy storage (FES) 12 , compressed air energy storage (CAES) 13 , 14 , and pump hydro energy storage (PHES) 15 are cost-effective, long-term storage solutions with significant environmental benefits for small- and large-scale renewable energy power plants to overcome energy generation fluctuation 16 .

A relevant study proposed three approaches for combining gravitational storage systems with renewable energy resources 17 . The first was the "Energy Vault Tower", which employs ropes to raise masses using the generated energy. The stored energy can be retrieved by lowering these masses (concrete blocks) while driving an electric generator with ropes 18 . The second method, which can be used in abandoned mine shafts, uses a massive suspended weight rather than multiple concrete blocks 19 . The third method utilizes a heavy piston that moves vertically inside a cylinder by compressing fluid flow through a valve.

According to 20 , the first closed hydraulic circuit was developed by a company called Gravity Power. The main idea was to pump water from a low-pressure side to raise a piston in a closed hydraulic circuit; in this case, this is called the storage phase. When there is a need to recover the stored energy, the piston is allowed to descend by opening a valve, allowing water to flow through a hydraulic turbine and generate electricity. According to Heindl 21 , the efficiency of the round-trip gravitational energy storage system can reach more than 80%.

Gravity storage systems were studied from various perspectives, including design, capacity, and performance. Berrada et al. 22 , 23 developed a nonlinear optimization model for cylinder height using a cost objective function. Their findings demonstrated that the Levelized price of gravity energy storage is competitive with other techniques. Furthermore, the proposed small-scale gravity storage systems could be stand-alone renewable energy storage systems. Berrada et al. 24 also numerically examined the use of various materials in gravity storage systems. They suggested using “iron ore” for the piston and reinforced concrete for the system container.

On the other hand, valuable efforts were made to avoid the use of heavy pistons and improve system performance 25 . Botha and Kamper 26 investigated a waterless gravity energy storage system with a wire rope hoist and drive train technology up to 90% efficiency 27 , 28 .

Statistical analysis of energy storage systems should be considered as they reduce experimental costs, which helps minimize the research cost and time. It also offers a comprehensive view of parameters influencing the system performance 29 . In a relevant study, Elsayed et al. 30 added a fuzzy control system to a gravity energy storage system, employing three fuzzy membership functions, triangular, trapezoidal, and Gaussian, to determine the appropriate design parameters criteria for various sized power plants. Their results showed that the Gaussian membership function best represents the fuzzy model of the storage system.

Statistical methods are also necessary to improve the forecasting and management of supply and demand in energy storage operations. Rehman et al. 31 used recent advances in deep learning methods and compared them with traditional statistics to forecast hourly natural gas demand in five locations in Spain. They concluded that the benefits and drawbacks of both classical and deep learning methods should be considered before deciding on a suitable technique. Traditional methods can outperform deep learning methods when prediction accuracy is highly weighted. Deep learning methods may be a good option if avoiding extreme/negative predicted values is very important 32 , 33 . Generally, there are two optimization strategies; traditional and statistical. The traditional optimization strategy is based on changing one parameter while all other parameters remain constant. The main disadvantage of the traditional method is being time-consuming and costly.

On the other hand, the statistical design of experimental methods provides a straightforward and equally efficient approach. The evolutionary operation, factorial, regression, response surface, and Taguchi methods are the most used for experimental design 34 , 35 , 36 . Ibrahim et al. 37 presented Taguchi optimization of tribological behaviors of composite materials. They concluded that Taguchi and analysis of variance (ANOVA) techniques are promising for predicting tribological behavior and can then be used to guide the design and implementation of tribological materials.

Taguchi's method is superior to other optimization methods because it allows simultaneous optimization of multiple factors. Furthermore, fewer experimental trials can yield more quantitative information. Taguchi's method has been used in various fields, including renewable energy generation and energy storage systems 38 , 39 , 40 , 41 .

The primary literature demonstrates that the capacity of gravity energy storage can be increased by selecting appropriate geometrical design parameters. Furthermore, hydraulic losses can be reduced by efficiently designing the system's components and selecting suitable devices. As a result, more investigation is needed to understand and optimize the parameters affecting the performance of gravity storage systems. This study investigates various design parameters that can affect the performance of a small-scale gravity storage system. It also presents a comprehensive model to optimize these design parameters. Six system design parameters are investigated, including three piston-related parameters (diameter, height, and density), in addition to three other parameters related to system components; return pipe diameter, length, and charging/discharging time.

This paper presents a novel comprehensive model that predicts and optimizes the most influencing parameters on the performance of gravitational energy storage systems. The simulated model using MATLAB-SIMULINK was created and validated against experimental data from the literature before applying the statistical approach. The Taguchi method was then used to predict the contribution of design parameters to system performance and to determine the best combination of parameters to maximize system performance due to its simplicity and dependability.

Research methodology

Figure  1 shows the general components of the gravity storage system investigated in this study. There are two main working cycles in these systems. The first is the charging phase, where a pump uses the available electricity to store a pressurized liquid in chamber B with a heavy-weight piston on the top; the pump pushes the fluid from point 3 to point 1. The second phase is the discharging phase, in which the piston weight drives the flow from point 1 to point 2 while the pump works as a hydraulic turbine. This process uses different flow control valves to manage the charging and discharging rates.

Schematic illustration of gravity energy storage.

This research was divided into six stages. The first stage was performing the mathematical modelling of the system by applying the governing equations. The second stage was the development of a virtual simulation of the system using MATLAB/Simulink. This simulation is used to investigate the system performance.

The third stage was the model validation against existing experimental work from the literature. The fourth is the preliminary analysis used to investigate the effect of the different design parameters on system performance. The fifth step is the design of the experiment (DOE) based on the Taguchi method and obtaining different levels of the parameters for each experimental trial. The levels of parameters were used to optimize the system performance using the simulation model. The final step was the statistical analysis using Taguchi signal-to-noise analysis and the ANOVA analysis. The optimal combination of the design parameters was identified and discussed based on the statistical results. The flow chart of the present algorithm is shown in Fig.  2 .

Flow chart for the presented algorithm.

Mathematical modelling and simulation

The equations describing the systems are applied to numerically investigate the parameters that can significantly affect a gravity energy storage system. As there are different interactions between system components, the motion of the model was built by adopting Berrada et al. in 22 , 30 technique, with some modifications over the main assumptions and strategies.

The initial volume in chamber A, \(V_{A,0} { } = { }0\) ; i.e., empty means the system is fully charged.

The initial volume contained in chamber B, \(V_{B,0} = \left( {H_{C} - H_{P} } \right)A_{B}\) , where \(H_{C} \;and\;H_{P}\) are the chamber and the piston heights, respectively; initially, all the liquid is below the piston.

Applying the mass conservation equation on the system as the piston starts to move a distance \(x_{p}\) . Assuming that the system's volume would not change with the operation and ideal conditions where there is no leakage in the design, the contained volumes at points A and B were calculated using Eqs. ( 1 ) and ( 2 ), respectively.

where D is the piston diameter in (m).

Applying the continuity equation between chambers A and B gives:

\(\dot{m}\) : is the mass flow rate in kg/s.

\(\rho\) : is the fluid density in kg/m 3 .

\(\dot{V}\) : is the volume flow rate in m 3 /s.

As the system is operating at high pressure, the fluid density difference should be considered as there will be energy used in such a system to compress the fluid. The density, as a function of the system's pressure, was estimated using Eq. ( 5 ) as follows:

where P: is the system pressure in Pa.

E: is the bulk modulus of the fluid at the given pressure that was calculated using the following equation 42 :

\(E_{0}\) : The fluid bulk modulus at one atmospheric pressure.

\(P_{0}\) : The standard pressure of 1 atmospheric pressure.

\(K1\;and\;K2\) : Empirical constants of 90 and 3. (Assuming isothermal case).

Combining the continuity equation with the variation of bulk modulus with the pressure change, the pressure change in chambers A and B are expressed as:

After obtaining the pressure as a driving force for the system, Newton's second law was applied as follows:

where \(f_{f}\) is the friction force between the piston and the chamber walls, which is a function of the sealant material that is equal to

where \(\mu\) : is the friction coefficient estimated as 0.1 and N is the normal force exerted by the sealant on the walls calculated using Eqs. ( 12 – 16 ) in 43 .

Rearranging Eq. ( 9 ) for the acceleration of the piston ( \(\ddot{X}_{p}\) )

To avoid the problems associated with the extreme positions of the piston, the logic function shown in Eq. ( 12 ) was used for assigning the acceleration condition.

As the hydraulic energy stored and recovered from the system will be proportionally associated with the hydraulic losses in the design, it was crucial to consider the fluid friction in the pipes. The hydraulic losses related to the fluid flow in the system are estimated using the Darcy-Weisbach equation and the Moody chart 43 .

Reynolds number was determined using Eq. ( 13 ) as follows:

v: is the fluid flow velocity in (m/s).

d: is the flow diameter in (m).

\(\mu\) : is the fluid viscosity in Pa.s.

Then the major hydraulic losses ( \(h_{L,major}\) ) are estimated using the Darcy-Weisbach equation that is expressed by Eq. ( 14 ) in the function of frictions \(f_{1}\) and \(f_{2}\) .

The friction coefficients \(f_{1}\) and \(f_{2}\) are for the piston and the pipe, respectively. They were estimated separately using their diameters from Eq. ( 15 ):

Furthermore, the minor losses were assumed to be 50% of the friction losses owing to the path's numerous bends. Consequently, the hydraulic losses were calculated using Eq. ( 16 ).

Bernoulli equation was applied between point 1 and 2 to calculate the hydraulic head created by the pump, as follow:

The inlet pressure \(P_{1}\) can be calculated by applying the Bernoulli equation between the inlet (1) and chamber B as expressed by Eq. ( 18 ).

Then the outlet pressure \(P_{2}\) can be calculated by applying the Bernoulli equation between the outlet (2) and chamber A as expressed by Eq. ( 19 ).

Finally, the hydraulic power generated by the turbine was calculated using Eq. ( 20 ) as follows:

\(P_{hyd - turbine}\) : is the hydraulic power available for the turbine in (W).

The governing equations of a gravitational energy storage system were used to develop a Simulink model on MATLAB software 30 .

Model validation

A model was created using Matlab/Simulink and was validated against the experimental results reported in 44 . This reference was chosen as experimental data were available for comparison 44 . The physical model data are presented in Table 1 .

The data shown in Table 1 were used as input parameters for the model and then compared with the actual results reported in 30 , 44 . After completing the validation process, different parameter ranges were chosen to investigate their effect on the energy storage performance. Table 2 presents the stimulated ranges used in this study. On the basis of the reference model, these ranges were selected to investigate the effect of increasing and decreasing the design parameters. As the container height is the most significant design parameter, it will be considered as a fixed value, while the others are modified as follows:

Results and discussion

According to the given ranges in Table 2 , the effect of each parameter as a function of the container height is determined. Then, the output values of the Taguchi method were used to investigate the most and the least significant parameters in the design of the gravity storage system. While changing one of the parameters in this section, all other parameters are considered unchanged from the reference values given in Table 1 above.

The first studied parameter was the piston diameter to container height ratio (D p /H c ). Five values (0.05, 0.1, 0.15, 0.2, and 0.25) were simulated versus the energy stored in the system. Figure  3 presents the output power versus the piston diameter/container height ratio change. From the figure, it can be seen that the hydraulic power increases with increasing the piston's diameter. This increase is a result of increasing the weight of the piston as the piston and fluid volume increase. The rise in the fluid volume increases the flow rate considering that the discharge time is constant at 6 min.

Hydraulic power versus diameter to container height ratio.

The second parameter was the piston height to container ratio (H p /H c ) (Fig.  4 ). The range of 0.1–0.5 was selected with a step of 0.1. It can be noticed from the chart that the hydraulic power and the piston height are directly proportional due to the effect of increasing the height on the piston weight and the pressure difference.

Effect of piston height variation on the hydraulic power.

For the return pipe, two design factors are considered: the pipe diameter (d) and the pipe length (L). Figure  5 represents the influence of return pipe diameter (d) on the storage power. As the return pipe diameter to container height grows from 0.01 to 0.02, the resultant energy steadily climbs to the highest value at the highest diameter ratio of 0.02. This occurred because of the fact that increasing the diameter of the discharge pipe lowers the flow rate and, consequently, the friction losses of the pipe.

Effect of return pipe diameter variation on the system's power.

Figure  6 illustrates the influence of return pipe length on the power generated. Within the studied range between a ratio of 0.5–1.5, it can be shown that increasing the length increases the friction losses and, thus, reduces the available hydraulic power.

Effect of return pipe length variation on the hydraulic power.

The output power and available energy extracted from the system at different charge/discharge times are shown in Fig.  7 . As the charge/discharge time rises from 6 to 30 min, the resulting energy gradually increases to the highest value at 30 min. This is due to the increase in the discharge time slows down the volume flow rate and discharge speed. Reducing the discharging speed significantly reduces the hydraulic losses and increases the available power.

Effect of charge/discharge time variation on the power.

Figure  8 shows the influence of the piston density ratio on the power. It can be observed that when the density ratio of the piston to water increases from 2.7 to 12, The resultant power progressively climbs to the most significant value at the density ratio of 12. As the piston density increases, the piston weight increases, giving a higher pressure difference that increases the system's available power.

Effect of density ratio variation on the system's power.

The second step in the current research was to investigate the effect of the design parameters with varying container heights. The container height is varied over the range between 2.2 and 20 m. The interaction between design parameters is shown in Tables 3 , 4 , 5 , 6 , 7 and 8 . Each table shows the resultant generated power considering one variable from the design parameters at different container heights.

Table 3 shows the results obtained after using variable piston diameters versus container heights. It can be seen that increasing the container height can significantly increase the generated power while increasing the stored potential energy. The limitation applied to this case will be determined according to the installation location parameters.

Table 4 depicts the interaction effect of piston height and container height on the generated power. It can be seen that the optimum piston height ratio is 0.4 Hc for container heights less than 5 m, while the best ratio drops to 0.3 Hc for container heights more than 20 m. This change occurred due to the influence of the volume height on the stored fluid capacity and its flow rate, as well as the increased pressure created by utilizing pistons with greater heights.

The data of the interaction between the return pipe diameter and container height on the system power are tabulated in Table 5 . Increasing the discharge pipe diameter can increase the power generated as the losses in that pipe decrease in all of the studied cases. The optimal pipe diameter ratio of 0.02 Hc remains constant while the container height grows from 2.2 to 20 m.

Table 6 shows the influence of the return pipe length ratio on storage power at various container heights. The results reflected that the smaller the pipe length, the more efficient the system would be. This is because of the reduction in the losses of the discharge pipes. For higher container heights, as the length is taken as a function of height, the losses caused were higher than the generated power, so these values are not recommended for the actual case applications due to their inefficiency.

In Table 7 , piston density ratios versus piston heights are presented. The data showed that the optimal piston density ratio of 12 remains constant as the container height grows from 2.2 m to 20 m; however, after the height of 5 m, there is a negative effect on the system performance due to the massive increase in system's weight that causes the flow rate and system losses to increase.

Table 8 shows that the optimal charging/discharging time increases with increasing the container height due to the need to reduce the flow velocity to minimize the friction losses in the discharging pipe.

Based on the previous analysis, it can be concluded that the system's design parameters significantly affect its performance. Consequently, the effect of these parameters should be investigated and optimized. An optimization study for different influencing parameters was done using the Taguchi method.

In the following, a trial is performed to estimate the optimum threshold for each design parameter for maximum power output and determine the most significant control parameters of the system 45 , 46 .

The Taguchi approach yields an orthogonal array as an experimental design. This array contains a series of experiments created by combining different parameter values known as control variables. The control variables and their values should be initially specified in the Taguchi analysis. Optimizing the efficiency of the gravity energy storage system yields hydraulic power. Using Taguchi analysis, six control variables representing the design parameters are defined to optimize the stored energy. Based on the previous preliminary investigation, the range of each factor and its levels can be given as listed in Table 7 . The levels will be used to produce the DoE for the analysis. The current study considered five levels for each parameter listed in Table 9 .

The Taguchi design is produced using Minitab 19 software, as shown in Table 10 . An orthogonal array of L25 is constructed. The simulation model runs the different trials and obtains the corresponding system output. Only one response is considered at a time. Moreover, the response is represented as values of a potential hydraulic power that the system can store. The result of the simulation for each trial is listed in Table 10 .

Taguchi technique produces several trials depending on the control variables and their levels. Furthermore, this approach transforms the response output data to a signal-to-noise (S/N) ratio. Then, the signal represents the desired value, whereas the noise is the unwanted standard deviation from the mean value. The log transformation of mean square deviation (MSD) calculates the S/N ratio Eq. ( 21 ). MSD is considered a more effective tool for comparison than conventional measurements. It may determine S/N ratios in three ways: Larger is better, nominal is better, and smaller is better. If the output response needs to be maximized, the larger is better is used 38 . When attempting to attain current S/N ratios, the nominal is better condition is employed. The S/N ratio was estimated as an Eq. ( 21 ). In which y i is the trial response obtained by the simulation and n is the number of replications for each trial. The S/N is computed as listed in Table 10 for only one reproduction for each trial ( n  =  1 ).

Considering that “greater is the better” is adopted in the current study. Table 11 summarizes the response table of the S/N ratio for the different levels of each parameter. Setting the highest value of the S/N ratio and determining the optimal combination of parameters to provide the largest storage capacity will result in the optimal level for each parameter.

Relying on the parametric analysis results of the S/N, the optimal design parameters for the gravity storage system were identified as piston diameter ratio ( \(0.25{ }H_{C}\) ), piston height ratio ( \(0.5{ }H_{C}\) ), piston density ratio (12 \(\rho_{w}\) ), charging or discharging time (6 min), return pipe diameter ratio ( \(0.01{ }H_{C}\) ), and the return tube length ( \(0.5{ }H_{C}\) ).

Figure  9 also clearly shows that the piston's diameter and height significantly influence the storage capacity. In contrast, other parameters have minimal effect on power generation. Based on these findings, the piston dimension should be considered further throughout the design process. The piston density and charge/discharge time are the following influencing parameters.

Signal-to-noise ratio on each factor.

The optimal combination of the parameters that is obtained from S/N ratio Graph is A5–B1–C5–D5–E1–F5 (A5 = 0.25, B1 = 0.5, C5 = 12, D5 = 6, E1 = 0.01, and F5 = 0 0.5), which requires a confirmation test.

ANOVA of the data was done using Minitab Software for hydraulic power to analyze the influence of (A) piston diameter ratio, D/H c , (B) Return pipe length ratio, L/H c , (C) Return pipe diameter ratio, d/H c , (D) Piston specific density, (E) Charging / discharging time, and (F) Piston Height ratio, Hp/H c . ANOVA allows the analysis of each control factor influence on the total variance of the results.

The ANOVA of the hydraulic power is shown in Table 12 . According to Fig.  10 , the ANOVA results showed that piston diameter, height, density ratio, and charging/discharging time have a significant impact of 35.11, 30.28, 15.30, and 9.62%, respectively. Moreover, the significant effect of these four parameters can be noticed by investigating F-value and P value. The P value is < 0.05 indicates the statistically significant impact of these parameters. The same remark relies on the F-value (select high positive value). The effects of the other two parameters on the energy storage capacity are minor and can be considered insignificant. It is clear from the table that the maximum error value is 1.7%, which confirms the validity of the results.

Contribution percentage of each parameter on the variation of storage capacity.

This study aimed to provide a parametric analysis of gravitational energy storage systems. MATLAB Simulink was used to generate the system's model then the Taguchi method was used to optimize these design parameters. The six studied parameters were the piston diameter and height, the return pipe length and diameter, the piston relative density, and the charging/discharging time. The ANOVA method showed that the piston diameter, height, density ratio, and charging/discharging time have percentage impacts of 35.11, 30.28, 15.30, and 9.62%, respectively, on the system performance. In addition, the pipe parameters (length and diameter) have a relatively minor impact on the power. The optimal combination of the investigated parameters has also been determined. The optimal threshold for each factor was identified as the Piston diameter (0.25 × H C ), piston height (0.5 × H C ), piston density (12 × water density), charge or discharge time (6 min), return pipe diameter (0.01 × H C ), and returns pipe length (0.5 × H C ), where H C is the container height. The results of the current research can be utilized as design guidelines for gravity energy storage devices in future studies. From the perspective of this work, the optimal combinations of the parameters will be used to build an actual energy storage prototype.

Abbreviations

Piston area (m 2 )

Flow rate (m 3 /s)

Flow rate above the piston (m 3 /s)

Flow rate below the piston (m 3 /s)

Pressure (Pa)

Pressure above the piston (N/m 2 )

Pressure below the piston (N/m 2 )

Bulk modulus of water (N/m 2 )

Piston position (m)

Volume above the piston (m 3 )

Volume below the piston (m 3 )

Piston mass (kg)

Turbine hydraulic power (watt)

Signal-to-noise ratio

Total number of trials

Number of replications for each trial

Degree of freedom

Contained height (m)

Piston height (m)

Diameter of the piston (m)

Velocity of the piston (m/s)

Acceleration of the piston (m/s 2 )

Reynolds number

Water dynamic viscosity(kg/ms)

Water density (kg/m3)

Velocity through the pipe (m/s)

Pipe diameter (m)

Pipe friction factor

Head loss (m)

Total head (m)

The response

Average performance

Mean squares

Sum of squares

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Mostafa E. A. Elsayed, Saber Abdo, Ahmed A. A. Attia, El-Awady Attia & M. A. Abd Elrahman

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Combustion and Energy Technology Lab., Mechanical Engineering Department, Shoubra Faculty of Engineering, Benha University, Cairo, 11672, Egypt

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Gravitricity Gravity-based Energy Storage Demonstrator

Gravitricity is an innovative gravity-based mechanical energy storage technology being developed by Gravitricity, an energy storage company based in Edinburgh, Scotland, UK. The novel energy storage system is based on the principle of raising and lowering a heavyweight to store and release electrical power.

Project Type

Gravity-based mechanical energy storage system

Port of Leith, Edinburg, Scotland, UK

Developer/Operator

Gravitricity

Estimated Cost

£1m ($1.25m)

Start of Operation

It is believed that the technology if commercialised, will enable the storage of intermittent renewable energy, grid stabilisation, and rapid frequency response.

Recommended White Papers

Demand Flexibility, Predictive Load Management and the Future of Plant Efficiency

An example of a systems approach at work to solve electrification challenges at scale.

Gravitricity is piloting a 250kW energy storage demonstrator project based on this technology in Edinburg with the start of trial operations and grid-connection expected in 2021. The cost of Gravitricity’s 250kW energy storage demonstrator is estimated to be approximately £1m ($1.25m).

The Gravitricity demonstrator energy storage system will be an above-ground structure to be installed at the Port of Leith in Edinburg, Scotland, UK.

Gravitricity entered into a land lease agreement with Forth Ports, the operator of Port of Leith, in May 2020, to build the demonstrator on an industrial site at the Leith port. The port’s electrical network and grid connections will be utilised by the demonstrator.

Gravitricity energy storage concept

The system operating on Gravitricity technology consumes electric power to lift the weight and generates electric power when the weight is lowered.

A bunch of cables, with the help of a winch system, will suspend the weights weighing up to 5,000t in a shaft. Electrical drives will be used to control the winch system in order to keep the weight stable in the shaft. A tensioned guide wires system will prevent the weights from swinging and damaging the shaft.

The disused mineshafts or newly built shafts can be used for suspending weights. The depth of the shaft ranges from 1500m for disused mineshafts to 150m for purpose-built shafts.

The company claims the Gravitricity energy storage system can offer a 50-year design life and a round trip efficiency in the range of 80-90%. It is also believed to offer a cost-effective energy storage solution compared to lithium batteries.

Gravitricity 250kW energy storage demonstrator

The fabrication of the 250kW demonstrator commenced in August 2020. It will be an above-ground system with the rig height planned to be 16m. The demonstrator consists of two weights of 25t each suspended by steel cables. The length of the stroke will be 7m while the drop time for the full power of 250kW is expected to be 14s.

Gravitricity is planning a two-month test programme on the demonstrator system. It will perform a test by dropping two weights simultaneously for the generation of full power (250kW) and calculate the speed of response. The other tests include dropping one weight at a time to calculate energy output over a longer period.

The test will also validate the frequency response of the system. The test programme will provide inputs for the development of a full-scale 4MW project which is expected to be launched in 2021.

Gravitricity fundraise and grants

Gravitricity received approximately £1.5m ($2m) through crowdfunding. The company was also awarded a grant of approximately £300,000 ($373,000) under Innovate UK’s Energy Catalyst programme in March 2020. The funds will be used for Gravitricity project feasibility work in South Africa in which consultancy firm RESA Energy is a partner.

Gravitricity was also earlier awarded a grant of approximately £650,000 ($907,000) from Innovate UK’s Infrastructure Systems Innovation competition in February 2018, apart from an initial grant of approximately £175,000.

Contracts awarded

Huisman, a Netherlands-based company specialised in lifting and drilling equipment, entered into a contract with Gravitricity to deliver a 250kW gravity-based grid stabilisation concept demonstrator in January 2020.

The fabrication of the energy storage demonstrator started at Huisman’s factory in the Czech Republic in August 2020. It is expected to be completed by the end of December 2020.

Kelvin Power, an engineering company, was contracted for the fabrication of a lattice tower for the 250kW demonstrator.

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Gravity energy storage elevated to new heights

by Geoffrey Ozin | Apr 1, 2022

Renewables are projected to increase from its current 12% of the global energy supply to 90% in 2050. Yet the widespread use of renewables is challenged by the intermittency of solar and wind, and we’re not yet at a place where we can store enough energy to avoid these problems.

As renewable energy supply increases around the world, so to is the demand for grid-scale energy storage. It has been projected that the combined global stationary and transportation annual energy storage market will increase from today’s baseline of around 600 GWh by a factor of four by 2030 to more than 2,500 GWh.

Today, global energy storage capacity is dominated by gravity-based pumped hydro (90%), followed by lithium, lead and zinc batteries (5%), with the remaining capacity alloted to thermal and flow batteries, compressed air, flywheels, and other gravity-based mechanical systems.

Within the framework of large-scale, grid-level energy storage, gravity-based solutions currently dominate the commercial space. Pumped hydro, for example, is a reliable technology with a rapid response time and proven longevity.

It suffers nevertheless from the availability, scalability, and cost of suitable mountainous and water rich land, low round-trip energy efficiency (70%), carbon-intense construction, and the challenge of co-locating solar and wind.

Gravity energy storage

I wrote two ASN articles in 2019 about some exciting new developments in storing renewable energy as gravitational potential energy by lifting and lowering heavy objects ( Gigawatt Electricity Storage Using Water and Rocks and Climate Change Will Require Heavy Lifting ). At the time, a Swiss private company founded in 2017 that caught my attention was Energy Vault . In a demonstration project built and showcased in Switzerland, they showed the first use of cranes to lift and lower heavy composite blocks into massive architectures to respectively store and release significant amounts of renewable electricity.

Importantly, the composite blocks enable the use of alternative materials to replace environmentally-unfriendly substances like concrete, which accounts for 7-8% of greenhouse gas emissions. In addition, the technology can accommodate the recycling of various pre-existing waste materials, which in return helps large utility and industrial companies transform financial and environmental liabilities into infrastructure assets to support their transition to a fully circular economic approach.

For example, coal bottom ash waste and retired wind turbine blades can be re-directed from landfills into the company’s custom-made composite blocks that anchor the gravity-powered systems. By maximizing the use of locally sourced soil, sand, and waste materials — including outputs from fossil fuel production — Energy Vault’s supply chain design reduces the impact of greenhouse gases from the transport sector while increasing jobs for local economies.

The result is an end-of-life solution for materials that are difficult to break down and can have negative environmental consequences. This beneficial reuse eliminates waste and enables the continual use of local resources within the framework of a circular economy.

During lifting, electricity is stored as gravitational potential energy in the blocks, and on lowering, the stored potential energy drives a motor generator to regenerate electricity with as little loss as possible to maximize the efficiency of the process.

The technological performance and commercial potential of this gravity-based system relative to other new entrants into the energy storage space was not apparent at the time, especially the levelized cost of electricity in $/MWh compared to lithium-ion batteries. Somehow, extremely tall cranes that lift and lower massive blocks in huge construction sites did not seem to be a practical global solution to grid-scale renewable energy storage.

Fast forward to today and I have changed my mind. As of April 2022, Energy Vault became listed on the New York Stock Exchange, and with the breathtaking news of its latest gravitational energy storage system, it is one of the most exciting companies to watch.

In just three years it has established an impressive global reach with its advanced gravity storage system on five continents, with more than US$32B earmarked projects over the next five years.

What has changed to elevate Energy Vault to such great heights?

It’s simple: They have simplified their gravity storage system by integrating the lifting-and-lowering of heavy weights into a familiar “elevator” style building design that is compatible with all international building codes. Plus, they have perfected the manufacturing process of their eco-friendly and fully recyclable composite materials.

The Energy Vault system literally can be built anywhere a building can be built. It is scalable on demand with no topological and geographical constraints, having flexible modular construction with the capacity to deliver GWs of power over short and long enough durations to handle solar and wind intermittency shortfalls.

The energy storage system can also withstand harsh and changeable weather conditions, it is resilient to storage capacity degradation over time, not reliant on carbon intensive mining and refining of rare and toxic metals, and is devoid of chemical and fire safety risks.

The round-trip efficiency or the proportion of stored to retrieved electricity is currently 83-85%, rather close to that of comparable power rating lithium-ion batteries, which hold 87-89%. Most importantly, it is purported to offer a lower levelized cost of electricity than any competing technology, particularly 60% of of today’s lithium-ion batteries — by 2025 this is projected to drop to 51%.

I believe this is one of the most promising sustainable solutions to global grid-scale renewable energy storage. It almost certainly will prove to be an indispensable piece of the circular economy puzzle, having a positive ripple effect on creating new clean technology industries and jobs, avoiding environmental liability, ameliorating climate change, and mitigating global warming.

Now that’s what I call heavy lifting!    

Written by: Geoff Ozin and Athan Tountas

Solar Fuels Group, www.solarfuels.utoronto.ca

Feature image: The Energy Vault Resiliency Center (EVRC). Reprinted with permission

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GRAVITRICITY energy storage: Subsystem Testing and Detailed Design for cost reduction

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What is Round Trip Efficiency?

Energy storage systems function by taking in electricity, storing it, and subsequently returning it to the grid. The round trip efficiency (RTE), also known as AC/AC efficiency, refers to the ratio between the energy supplied to the storage system (measured in MWh) and the energy retrieved from it (also measured in MWh). This efficiency is expressed as a percentage (%).

The round trip efficiency is a crucial factor in determining the effectiveness of storage technology. A higher RTE indicates that there is less energy loss during the storage process, resulting in a more efficient overall system. Grid systems engineers strive for energy storage systems to achieve an 80% RTE whenever feasible, as it signifies a desirable level of efficiency and minimizes energy losses.

What Factors Can Affect the Round Trip Efficiency of an Energy Storage System?

The RTE of an energy storage system can be influenced by various factors, including:

1. Technology: Different storage technologies have varying round-trip efficiencies. For example, hydro storage typically ranges from 65% in older installations to 75-80% in modern deployments, while flywheels have efficiencies of about 80% to 90%. Some battery technologies can have round-trip efficiencies ranging from 75% to 90%.

2. Storage duration: Some technologies may experience leakage or energy loss over long-term storage, which can affect round-trip efficiency. It is important to consider the specific characteristics and limitations of the storage technology when evaluating its efficiency.

3. Age and condition of the system: Older storage systems may have lower round-trip efficiencies compared to newer ones. Factors such as wear and tear, component degradation, and maintenance practices can impact the overall efficiency of the system.

4. Charging and discharging rates: The speed at which energy is charged into and discharged from the storage system can affect its efficiency. Certain technologies may have lower efficiencies at high charging or discharging rates.

5. System design and control: The design and control strategies implemented in the energy storage system can influence its round-trip efficiency. Optimal system design, efficient power electronics, and effective control algorithms can improve the overall efficiency of the system.

6. Temperature: Temperature can have an impact on the performance and efficiency of energy storage systems. Extreme temperatures can affect the efficiency of certain storage technologies, such as batteries, leading to lower round-trip efficiencies.

Considering these factors is crucial when evaluating the round-trip efficiency of an energy storage system, as they can significantly affect its performance and effectiveness in storing and retrieving energy.

Must Read: What is Power Conversion Efficiency?

Elliot is a passionate environmentalist and blogger who has dedicated his life to spreading awareness about conservation, green energy, and renewable energy. With a background in environmental science, he has a deep understanding of the issues facing our planet and is committed to educating others on how they can make a difference.

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These 4 Startups Are Revolutionizing Energy Storage by Using Gravity

Current energy storage solutions face challenges such as negative environmental impacts, geographical constraints, scalability issues, and long-term sustainability issues. 

For instance, lithium storage requires rare earth minerals and poses recycling challenges, while pumped hydro requires large reservoirs and significant land use, limiting deployment.

One of the alternatives, Gravity energy storage, emerges as a promising solution, offering a novel way to store energy using the earth’s gravitational force. This method involves elevating heavy weights during excess energy production and releasing them to generate electricity when needed.

The technology is appealing due to its potential for high energy capacity, long lifespan, minimal environmental impact, and the ability to repurpose existing infrastructures such as disused mines and oil wells.

It further offers lower costs over the lifecycle, minimal geographical limitations, and a smaller environmental footprint. As a mechanical storage solution, it also avoids the chemical issues associated with battery disposal and the extensive land use required for pumped hydro.

This article explores five innovative growth-stage startups advancing gravity energy storage technology. These startups have the potential to grow rapidly, are in a good market position, or can introduce game-changing technology to the market in the next 2-3 years. 

This makes them a great option to partner, collaborate, or acquire.

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1. Gravitricity using MineShafts with GraviStore Technology for Energy Storage

Balancing the energy supply and demand using increasingly common intermittent renewable energy sources, like solar and wind, is necessary. The problem lies in storing excess energy generated during peak production times and releasing it efficiently during high-demand periods to ensure a stable energy supply.

To tackle this, Gravitricity developed the GraviStore technology, which harnesses gravity power storage by utilizing existing mineshafts. This system can support thousands of tonnes of mass to store electricity, functioning similarly to pumped hydro storage but with the benefits of a battery. 

The technology has a lifespan of over 50 years without performance degradation. It goes from zero to full power in less than a second and can have high power output and extended operation times. Gravity storage has lower costs than lithium-ion batteries.

Gravitricity’s solution has high efficiency (around 80% round-trip efficiency) and energy/power ratio flexibility, allowing for storage durations from 15 minutes to 8 hours. Its system does not suffer from the depth of discharge limits, maintains high power output without degradation, and offers high availability (97%) with no standing losses. It also has an H2FlexiStore – Hydrogen Storage technology.

The startup is now led by Executive Chair Martin Wright. While this is his temporary position, he has continuously been a co-founder and chairman of Gravitricity since its inception.

His previous experience includes roles in entrepreneurial development, venture capital, marine energy, and maritime technology. He also serves as Chairman of the Association for Renewable Energy and Clean Technology.

Gravitricity’s latest funding of £804.5K was raised on March 29, 2023, from an Equity Crowdfunding round. The UK government and Innovate UK are lead investors.

2. Renewell Energy Developed a Mechatronic Energy Conversion System to StoreEnergy in Oil Wells

Millions of inactive oil wells across North America are sources of methane leaks and other pollutants. These abandoned wells represent an environmental hazard and a significant financial burden due to the high costs of permanently sealing them.

Renewell has developed a unique solution called “Gravity Well” technology, transforming idle oil and gas wells into efficient, green energy storage systems. 

This technology uses a mechatronic energy conversion system to store energy by lowering a weight down the well shaft, converting potential energy into electricity with a regenerative winch mechanism. 

The system is characterized by its low capital cost of $5 per kWh and operational cost, which results in a levelized cost of storage (LCOS) of $63 per MWh.

The startup technology uses existing infrastructure to achieve unprecedented storage efficiency and cost-effectiveness. The Gravity Well system leverages the depth of old wells (averaging about 5,200 feet) to increase energy storage capacity per kilogram of weight. 

By sealing and repurposing these wells, Renewell’s technology mitigates methane emissions and can result in a net-negative carbon footprint. This approach transforms a costly cleanup challenge into a revenue opportunity for well owners.

Co-founder and CEO Kemp Gregory led this startup with his experience as a completions engineer at Shell company. He holds an MS in Energy Engineering from Stanford University School of Engineering.

Renewell Energy raised its latest seed funding on Jul 10, 2023.

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3. Green Gravity and its Gravitational Energy Storage System is a Long-lived Energy Source

Green Gravity developed a gravitational energy storage system that moves heavy weights up and down in disused mine shafts. It utilizes the gravitational potential energy to store and release energy. This system is designed to be low-cost, long-lived, and environmentally friendly. 

It uses no processed chemicals and suffers no performance degradation, which makes it a compelling alternative to chemical batteries and other energy storage methods​​​​.

Green Gravity uses the existing underground infrastructure, specifically old mine shafts, to create energy storage solutions that are both economically and environmentally advantageous. 

The technology leverages the significant depths of these shafts to maximize energy storage potential, making it more space-efficient and cost-effective than constructing new facilities or using above-ground structures. This approach repurposes idle assets and contributes to the circular economy by reducing the need for new constructions and the associated environmental impact. 

CEO Mark Swinnerton led this startup with his experience in Directorial, presidential, and managerial roles in the mining industry. He worked with companies like BlueScope Steel for 7+ years and BHP for 3 years, where he handled multiple management roles.

Green Gravity completed an Early-stage VC funding round on 25 May 2022 to secure $985K.

4. RheEnergise Need Just a Small Elevation with its R-19 Fluid to Create & Store Energy

RheEnergise developed a technology called High-Density Hydro®, which utilizes a fluid called R-19 that is 2.5 times denser than water. This technology allows energy storage systems to be constructed on smaller hills. It reduces the need for large elevation changes and minimizes construction’s environmental and financial costs​​.

High-Density Hydro® operates by pumping the dense R-19 fluid to an upper reservoir during low energy demand and releasing it to lower reservoirs through turbines to generate electricity when demand increases. 

This system can be integrated into existing grid infrastructures and supports the co-location with other renewable projects, offering power outputs ranging from 10MW to 50MW​​.

Using the non-corrosive, environmentally neutral R-19 fluid enhances the technology’s sustainability. RheEnergise’s solution provides more than double the energy output​​​​of conventional low-density hydro systems.

Stephen Crosher is a co-founder and the CEO of this startup. He has director-level experience across three industries: renewable energy, innovation and policy, and architecture and design. 

He has held roles such as MD, CCO, Commercial Director, and Advisor with companies like X-Wind Power Limited and Fleet Renewables.

RheEnergise’s latest Series A funding was on Feb 15, 2023.

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Author: Naveen Kumar , Market Research

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• Dec 11, 2023

Gravity Batteries

In the quest for cleaner and more efficient energy solutions, innovators are exploring alternative methods of energy storage. One such emerging technology is gravity-based energy storage, an idea that leverages the power of gravity to store and release electricity.

While lithium-ion batteries have become the go-to solution for energy storage, they come with limitations. These batteries have a finite number of charge and discharge cycles, typically lasting only a few years before capacity degradation sets in. Additionally, the production and disposal of lithium-ion batteries raise environmental concerns.

Gravity-Based Energy Storage

Gravity-based energy storage systems offer a compelling alternative to traditional battery technology. These systems work by harnessing the potential energy of heavy objects, such as massive weights or blocks, and convert it into electricity.

The basic idea behind a gravity battery system is lifting a heavy object using energy from other sources such as a large mass of concrete or a weight high into the air, to the top of a deep shaft, on a pulley, letting it fall when energy is needed and converting its potential energy into electricity using an electric generator.

This is often done when there is plenty of green energy, the batteries use the power to lift a heavy weight either high into the air or to the top of a deep shaft. Then when the power is needed, winches gradually lower the weight, and produce electricity from the movement of the cables.

For what it is, it is an attempt to improve on an old idea: pumped hydroelectric power storage. Dams often require a reservoir with water pumped to it at times of low demand, usually at night and then released to generate electricity. But hydroelectric dams require specific terrain, expensive infrastructure, and planning approval. In the gravity battery, water is replaced by a solid weight.

Let's take a closer look at how these innovative companies are approaching this concept:

Energy Vault

From Switzerland, Energy Vault takes a different approach by using a tower of 35-ton bricks and a six-armed crane. According to Energy Vault, the bricks are "proprietary cement/polymer-based composite bricks that can be made of ultra-low-cost materials: soil, mine tailings, coal ash, incinerated city waste, and other remediation materials." Each of this brick is designed to weigh 35 metric tons and is engineered to have a specific gravity at least twice that of water and enough compressive agility.

Their original system consists of a combination such bricks and a tall tower. A surplus of power, from either solar or wind used to power a mechanical crane to raise the blocks 35 stories into the air. These bricks then stay suspended there until power is needed again when they are then lowered to work like a hydroelectric dam, that is, pulling on cables that spin turbines, thus producing electricity. According to Energy Vault, the blocks will have a storage capacity of up to 80 megawatt-hours and be able to continuously discharge 4 to 8 megawatts for 8 to 16 hours.

The control system manages the complex choreography, ensuring a constant energy output. In August 3, 2023, Energy Vault announced the completion of its first gravity energy storage system in Jiangsu, China. It is a 25 MW/100 MWh storage system that makes use of the company’s new ribbon-based lifting systems.

Its EVx, the Energy Vault system, installed in 2020 in Switzerland in a demonstration project, performed at round-trip efficiency of about 75%.

The tower is controlled by computer systems and machine vision software that control the charging and discharging cycles. While Energy Vault hopes to reach a range of storage durations from two to 12 hours, this first commercial installation in China will use a 4-hour duration.

Gravitricity

This Scottish startup has developed a unique 'underground' approach to gravity-based energy storage. Their system involves suspending a 50-ton iron weight using cables within a vertical underground shaft. Electric motors can lift the weight, storing potential energy. When needed, the weight is released, and the motors become generators, sending power back to the grid. Gravitricity's system offers rapid response times, making it suitable for balancing the grid during fluctuations in demand.

Gravitricity designed its systems to be housed in old mine shafts rather than towers. In the UK, it could go to depths of 750m (2,461ft) - twice the height of the Eiffel Tower in Paris. In in African countries, they expect to install them in holes underground holes constructed for the systems with depths that could exceed 2km (1.2 miles).

New Energy Let's Go and Gravity Power

The California-based Gravity Power uses an iteration of the hydroelectric dam. Renewable energy is used to pump water under a heavy piston and lift it. When power is needed, the piston weight is released, forcing the water through a hydroelectric generator. German company New Energy Let's Go uses a similar design.

These companies are all developing different types of gravity batteries, but they all share the same goal: to create a more efficient and sustainable way to store energy.

Advantages of Gravity-Based Storage

Gravity-based energy storage has several advantages:

Unlike traditional batteries, winches, cables, and heavy weights can maintain their performance for decades, reducing the need for frequent replacements.

Environmental Benefits

These systems have a smaller environmental footprint compared to lithium-ion batteries, as they rely on readily available materials like steel and concrete.

Cost-Effective

Initial estimates suggest that gravity-based energy storage systems can be cost-competitive over their lifetime when compared to lithium-ion batteries.

While the technology is still in its early stages and faces enormous challenges, such as scaling up and regulatory hurdles, it holds promise as a sustainable and efficient energy storage solution. As the world transitions to renewable energy sources and strives to reduce greenhouse gas emissions, innovative technologies like gravity-based energy storage are expected to play a vital role in creating a more sustainable energy landscape. While these systems are not yet widely deployed, they represent a promising step toward a cleaner and more reliable energy future.

In conclusion, gravity-based energy storage is an exciting and evolving field that has the potential to reshape the way we store and utilize electricity. With ongoing research and development, we may see these innovative systems become an integral part of our global energy infrastructure, helping us transition to a greener and more sustainable future.

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Energy storage is the fundamental element of the new energy system

CHALLENGE – As the world generates more electricity from intermittent renewable energy sources, there is a growing need for technologies which can capture and store energy during periods of low demand and release it rapidly when required.

SOLUTION – At Gravitricity we are developing two complementary technology streams which utilise the unique characteristics of underground spaces to provide an appropriate scale of distributed energy storage.

• Long life (>50 years, >50,000 cycles)
• Competitive LCOS

Performance

• Rapid response (<1s) for high value markets
• High efficiency (~80% round trip efficiency)
• Versatile energy / power ratio (15 mins – 8 hrs)
• No depth of discharge limits
• High power output without degradation
• Very high availability (97%)

Implementation

• Security of supply – limited exposure to geopolitical risks
• Small physical footprint
• No combustible chemicals

H 2 FlexiStore – Hydrogen Storage

Distributed mid-scale buffer storage for Green Hydrogen

Our  H 2 FlexiStore  underground hydrogen storage technology uses the geology of the earth to contain pressurised fuel gas, allowing safe, large-scale storage underground, close to the point of demand.

Key advantages of underground hydrogen storage

Building the future of renewables using gravitational energy storage

Watch full video

Our technology

Green Gravity’s energy storage system moves heavy weights vertically in legacy mine shafts to capture and release the gravitational potential energy of the weights. By simply using proven mechanical parts and disused mine shafts, Green Gravity’s energy storage technology is low-cost, long life and environmentally compelling.

Storing energy in this way uses no processed chemicals and has no performance degradation. Moving weights vertically allows for high Round Trip Efficiency and using legacy mine shafts allows reuse of existing structures, contributing to the circular economy and lowering costs.

Green Gravity’s energy storage technology improves the economics of wind and solar power, leading to a faster and lower cost transition away from fossil fuels. Truly the next generation of ultra-green energy.

Using gravity to store energy

Green Gravity’s energy storage solution harnesses the fundamental principles of gravity and kinetic energy to store and dispatch energy by lifting and lowering heavy-weighted objects. Green Gravity’s innovative technology was inspired by pumped hydro like Snowy 2.0.

Like pumped hydro, we use the gravitational potential energy of a mass moving between two heights. However, rather than water between two dams, Green Gravity requires much less space by using very dense materials. To overcome friction, a vertical height available from a mine shaft is used rather than an incline on the side of a hill.

How Green Gravity's energy storage works

Demonsrating Using The Gravity Lab ™

The Gravity Lab™ is a specialised research facility aiming to gather precise performance data from our proprietary gravitational energy storage system. Green Gravity has partnered with BlueScope Steel to create The Lab in Port Kembla, Australia.  

The Lab enables cutting-edge R&D on gravitational energy storage. It can test the technology’s capabilities by moving 16 weighted objects in a sequence, focusing on  power generation capacity, efficiency, and grid connection dynamics.

How it stacks up

Applications for the Green Gravity energy storage technology

Our solution can provide services at all levels of the electricity system. We make renewable energy cheaper, make the grid more stable and reduce transmission costs, and can help mines and industrial plants reduce carbon emissions. We also have a role in supporting local community energy schemes through firming distributed energy resources.

Power Generation

Managing Intermittent renewable generation

Energy Arbitrage

Peak Shaving

Transmission

Ancillary Services

Transmission Constraints

Inertia Services

Responsive Flexibility Services

Voltage Support

Distribution

Reactive Power

Local Security

Distribution Losses

Power Reliability

Energy Management

DER Firming

Mine Decarbonisation

Frequently Asked Questions

Gravitational storage refers to a process of converting electrical energy into gravitational potential energy through moving an object to a height. The energy is then released back to electrical energy at a later time by moving the object to a lower height, in the process turning an electric motor using the kinetic force of the descending object.

Green Gravity’s system can be deployed in many types of mine shafts. With nearly 100,000 shafts in Australia, this offers many potential deployment locations. Locations with the best economic case include those with shafts built after 1950, with electrical infrastructure sill accessible, and with greater depth. Shafts over 300 metres deep offer very attractive energy economics.

Green Gravity’ energy storage system is fundamentally more sustainable than chemical batteries. Some of the most important differentiating points include:

• Our parts can be locally sourced. Lithium batteries are developed using water intensive processing, combined with rare minerals and are assembled in a long global supply chain.
• We use basic steel cables, motors and recycled and inert materials. Chemical batteries are future landfill liabilities and are hazardous materials. 
• Gravitational energy systems do not leak energy over time, don’t degrade and have very long asset lives. The energy system needs long-term stable clean capacity. Green Gravity can deliver equipment life 3 to 4 times longer than a chemical battery. 
• Green Gravity re-uses existing infrastructure. We take minesites, which are sitting idle today, and convert them into energy storage systems capable of accelerating the uptake of renewable energy.

Green Gravity uses existing proven technology from the steel, mining and energy sectors to build the energy storage centre. We use cables, weighted blocks, mine winders, electric motors and off-the-shelf handling equipment to make our technology work. 

The weights moved depend on the depth and market configuration for an individual storage centre. For a large shaft, we move weights up to 40 metric tonnes, which give us the capability to store up to 10 kWh of energy per 100 metres of depth. For context, an average car weighs 1.3 tonnes, meaning we drop objects weighing the equivalent of 30 cars.

Radically accelerating the world’s renewable transition