Chapter 1

INTRODUCTION

1
 INTRODUCTION Chapter
1

Renewable energy of any form has been observed over time to have high investment costs and
therefore necessitate its operation where its resource is readily available.
However, the major problem after producing the energy is finding the most economical way of
storing it due to its variable nature that most often do not match demand patterns.
Nowadays, energy production sources are expected to be reduced in cost, increased in
reliability and have little or no carbon emission. The global environmental concerns and recent
2016 agreement in Paris have led to strict monitoring on greenhouse gases emission and impact
on the environment. These concerns on climate change have consequently driven the focus of
energy systems towards renewable sustainable energy sources and countless number of
literatures have suggested that tilting dependability towards these renewable resources might
boost the availability of energy globally. But it tends to be less reliable and cost intensive
compared to other generating methods that rely on fuels.
The dependability and efficient operation of the electrical grid are hampered by the high
share of intermittent renewable energy sources. In order to safeguard the supply of energy,
stringent cost requirements have been placed on power systems, which are currently at the
beginning of a new revolution. One of the solutions for ensuring the stability of the electric
grid is thought to be energy storage technologies. There are now only a few storage solutions
available because several technologies are still in the very early stages of research. This
article introduces gravity energy storage, an alternative to pumped hydro systems for storing
energy. In this system, gravitational potential energy is used to store electricity. The method
for sizing gravity storage is presented in this paper both technically and economically. In

order to ascertain the, it does an economic study.

1.1 Existing battery

Several innovative energy storage techniques have been developed in recent years, all of
which may be essentially described as gravitational energy storage using weights. These
systems differ in their design and function, but they all share the goal of increasing the energy
density and scalability of gravitational energy storage by employing a solid substance rather
than water. Here are several examples:

2
1.1.1 Gravitricity (Gravitricity, 2020): Using wire ropes attached to a motor-generator, a
500 to 5000 t weight is suspended in a subterranean shaft. Energy is stored by pulling
power from the grid to lift the weight and released by lowering it.

Fig 1.1.1: Gravitricity

Source: www.pv-magazine.com

1.1.2 Gravity Power (Gravity Power, 2020): A massive piston divides a sealed water-
filled subterranean shaft. A smaller return pipe connects the chambers above and
below the piston, with a pump-turbine linked to the main grid. Water is pushed from
the top chamber to the lower chamber to store energy, elevating the piston. Water is
allowed to flow back via the return pipe to produce power, causing the piston to fall.
Heindl Energy has suggested a similar concept, but with an above-ground reservoir
(Stenzel, 2015).

3
 Fig 1.1.2: Gravity Power
Source: https://netcapital.com/companies/gravity

1.1.3 Energy Vault (Energy Vault, 2020): Cranes is mounted atop a central tower are
utilised to lift and \slower an array of 35 t weights. Energy is stored by stacking
weights higher and released by returning weights to lower levels. Energy Vault has
reused technology from Energy Cache, which was working on a mountainous gravel-
based system that is no longer being pursued (Fyke, 2019)

4
 Fig 1.1.3: Energy Vault
Source: www.handelsblatt.com

1.1.4 Advanced Rail Energy Storage (ARES) (ARES, 2020): Automated shuttle trains
with regenerative traction drives transport up to 45 t per axel on a rail network with an
8.5% maximum elevation gradient. Shuttles go between two storage yards at varying

heights.
Fig 1.1.4: ARES
Source: netl.doe.gov

5
 1.1.5 Gravity Soil Batteries (Riffat, 2020): Motor generators linked to wire ropes and
pulleys raise and lower drums filled with compacted dirt on an inclination. An above-
ground concrete support structure with dug side channels is used to construct the

inclination.
Fig 1.1.5: Gravity Soil Battery
Source: www.wsset.org

1.2 Objective of the Project

1.The aim of this project is to design and develop a gravity power generator using gravitational
potential energy stored in a suspended mass.

2.The final objective of the project is to achieve a design and produce a cheap user-friendly
and environmentally clean, generator that can harness gravitational potential energy.

6
 Chapter 2

LITERATURE REVIEW

7
LITERATURE REVIEW Chapter
2

A literature review was conducted to understand the various works and studies done by the
research workers in the field of GPG. The review tried to focus on the various parameters that
helps in optimization of the performance of the GPG. Following are the literatures that have
been studied and are summarized below.

Elsayed et al., (2022) The purpose of this work was to offer a parametric analysis of gravitational
energy storage devices. MATLAB The system model was created using Simulink, and the Taguchi
technique was utilised to optimise the design parameters. The piston diameter and height, return pipe
length and diameter, piston relative density, and charging/discharging time were the six parameters
investigated. The ANOVA approach revealed that the piston diameter, height, density ratio, and
charging/discharging time affects on the system performance. Furthermore, the pipe specifications
(length and diameter) have a slight influence on the power. The best combination of the evaluated
parameters was also found. The best threshold for each component was determined. The current
study's findings can be used to provide design recommendations for gravity energy storage devices in
future investigations. From the standpoint of this work, the ideal parameter combinations will be
employed to construct an actual energy storage prototype.

Botha et al., (2021) In this study, a gravity energy storage system that uses the consequent-
pole linear Vernier hybrid machine technology in its hoisting mechanism. The suggested
LEM-GES system's economic feasibility is assessed using LCOS analysis. The NSGAII
method is used to do a multi-objective design optimization for the CP-LVHM. The Pareto
front solutions are then utilised to examine the impact of various LEM designs on the
system's LCOS, and an optimal design is derived.

Oluwole K. Bowoto, (2021) This study suggests a combined solar and gravity energy storage
device. The suggested system model's design synthesis and computer modelling were
examined utilising a fixed height but variable mass. Using the chosen design experimental
parameters used in this work, efficiency of up to 62% was attained. This storage system has

8
been characterised in this context utilising energy storage performance metrics such as energy
efficiency value (charge/discharge rate), system capacity, and so on. These factors, as well as
others given in the study, were utilised to determine the practicality of the storage system.

Morstyn and Botha, (2021) The article begins with a review of the key design
considerations that distinguish distinct systems from one another, as well as how these effect
the practicality of various energy storage applications. Then, a methodology for estimating
the levelized cost of storage while taking physical dimensions and energy storage application
into account is described. Case studies for an example single-weight subterranean
gravitational energy storage system are done using this technique. It is demonstrated that the
economics of particular systems are dependent on the physical scale at which they are built,
and that the energy storage application heavily determines which economic and technical
characteristics are most essential for cost estimate. The fact that their power capacity is
decoupled from their energy capacity allows them to be built specifically for a specific
application, and that they can be built in a variety of locations, with different systems suitable
for underground, above ground, and offshore construction, are two key advantages.

Sahil Kak, (2020) This article covers gravity-powered power generation, and the projects
aim to demonstrate how energy may be captured and utilised at a regularly used equipment.
The model concept may be applied in the industrial sector to minimise load shedding during
peak demand and in rural regions to generate electricity with a lower initial investment.

D.K. Chaturvedi, (2020) This study demonstrates the operation of a gravity battery, which
meets the requirement for sustainable energy storage by creating a prototype model in the
Electrical Power System Research Lab.As opposed to lead acid/lithium ion batteries, gravity
batteries have no leakage current. There is also no environmental contamination, unlike lead
acid batteries, which emit acid vapours. These batteries have a long life and no disposal
issues, unlike conventional batteries. The main feature of these batteries is their great
efficiency. The system's efficiency may be increased more in the future. Multiple weights can
be utilised to store more energy. Gravity battery energy storage systems may incorporate a
disaster management strategy.

Sumeet Sagar, (2020) The SolidWorks simulation results show that it is feasible to identify
the design's weak points and further optimise the design. The mesh created for the CAD

9
model will not only serve to enhance the design, but it will also pave the way for further and
more in-depth study. The circuit simulation in MATLAB validates our proposal under ideal
conditions, demonstrating that considerable power may be generated with the proposed
layout. It is possible to harness energy with certain mechanical effort by humans. Such an
approach is both inexpensive and simple to run and create. When compared to chemical
batteries, the approach is environmentally beneficial and has nearly no carbon footprint.

Julian David Hunt, (2019) This study suggests Mountain Gravity Energy Storage (MGES), a
novel storage technology that might cover the required energy storage solutions for various
renewable energy sources. MGES systems transport sand or gravel from a lower storage location
to a higher elevation. Because this technique is bound by the terrain of the region, the bigger the
height difference, the greater the quantity of stored energy in a particular installed capacity. This
new energy storage solution based on mountain gravity is found particularly for grids smaller
than 20 MW.

Ana Cristina Ruoso, (2019) As a novel technique for small-scale application, this research
offered a new gravity-based energy storage device. The operational principles and limits of many
mechanical energy storage devices, particularly those that employ gravitational energy, were also
explained. Furthermore, these technologies were technically contrasted and their applications
were examined.The suggested system is an electromechanical version that requires less area and
does not require water to operate, as well as having fewer rigorous geographical criteria than
typical stockpiles. It also offers installation advantages because it may be deployed in buildings
and off-grid. This unique technology has the potential for high storage density, unloading times
of minutes, a lifetime of 50 years, and an efficiency of about 90%, which would maximise power
supply and use while increasing energy self-sufficiency.

Berrada et al., (2017) This work presents gravity energy storage, a storage solution similar to
pumped hydro systems. This system uses gravitational potential energy to store electricity. This
paper provides a method for sizing gravity storage that is both technically and economically
feasible. It does an economic study to calculate the levelized cost of energy (LCOE) for this
technology and then compares it to other storage options. The results show that gravity storage
has superior operational and economic properties when compared to alternative storage systems.

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 Chapter 3

GRAVITY BATTERY

11
GRAVITY BATTERY Chapter 3
A low budget device is designed that is capable of harnessing energy from an object falling
under gravity. Object is suspended from the device, through a high strength thread, placed at
a certain height. Gravitational potential energy of an object placed at a certain height is
converted into kinetic energy. As the object descends it rotates the wheel which is coupled
with gear train. This gear train further rotates the pinion that is mounted on the generator
shaft. An electrical output is produced at the armature terminals of the generator which is
sufficient for various applications. The surplus energy in grids produced from renewable or
other potential sources are converted into gravitational potential energy that is later converted
into electrical energy. The device can be used as mechanical battery as well and has the
potential to replace the use of chemical batteries used in storing energy produced from solar,
wind etc.

3.1 Types of Gravity Battery

There are mainly two types of gravity battery
3.1.1 Large scale
a) Pumped-storage hydroelectricity (PSH) is the most popular and powerful type of grid
energy storage. PSH) uses pumps to move water from a lower reservoir to a higher reservoir,
where it can be discharged through turbines to produce electricity. A different PSH idea makes
use of a specialised high-density liquid that is 2+12 times denser than water and so requires a
smaller head (elevation), reducing the size and expense of the required infrastructure.

Fig 3.1.1(a): Pumped storage hydroelectric powerplant

12
 Source: www.researchgate.net

b) Gravity Line: As part of the energy-storage-by-rail concept, large trains with heavy loads are
driven uphill when there is little demand for electricity. As they roll downhill, functioning as a
gravity battery, the potential energy is eventually released by applying regenerative braking.
Gravity Line, a utility-scale (50 MW) plant being built by Advanced Rail Energy Storage
(ARES) at the Gamebird Pit gravel mine in the Pahrump Valley of Nevada, is expected to
provide up to 15 minutes of service at full capacity. Construction on the facility started in
October 2020.

Fig 3.1.1 (b): ARES Nevada

Source: netl.doe.gov
c) Lifted weight storage (LWS)- With the use of excess energy, solid weights are
mechanically raised vertically via LWS technology, often using a pulley system. The mass is
reduced when more energy is required, and the pulley then drives a generator.
Using a tower made of 32-ton concrete blocks stacked with 120-meter cranes, Energy Vault is
constructing a LWS system. 20 MWh of energy, or enough to power 2,000 Swiss homes each
day, is anticipated to be stored in a single commercial unit.
Gravitricity's LWS system in an underground shaft employs an electric winch to hoist a 500-
to-5000-tonne weight, which when dropped acts as a generator, turning the winch motor. The
system produces 10 MWh of energy, which is enough to power 13,000 houses for two hours. The
weight can also be abruptly decreased for a burst of power.

13
 .
Fig 3.1.1 (c): LWS
Source: www.energypost.eu

3.1.2 Small scale

Gravity Light is a tiny gravity-powered light that generates energy by physically lifting a bag of
pebbles or sand and then letting it fall on its own. It is intended as a replacement for individuals
who do not have access to power and must rely on kerosene lamps, which are costly, unsafe,
polluting.

Fig 3.1.2 Gravity Light

Source: https://deciwatt.global/gravitylight

14
3.2 Proposed Model
The Gravity Battery comprises a combination of gears trains, chain pulley mass system that is
designed to store the energy in the form of gravitational potential energy taking the help of
gravity.
The device is designed to be used with a variety of renewable energy generation methods.
Here energy provided by these sources is captured using renewable energy collecting devices.
This powers a motor, which is used to lift a fixed weight into the sky using a homemade
gearbox. When needed, the energy is released by dropping the weight, which rotates the
motor, which is now a generator, producing electrical energy. The available power is
determined by the size of the dropping mass, the speed at which it drops, and the distance
travelled.
A cad model has been developed showing all the specifications and design of the prototype
that can be fabricated into a physical working model. The simulation of the model has also
been prepared to better understand the working of the device. Considering of the intricacies, a
simplified design has been framed considering all the aspects keeping its performance intact.
This prototype fully resembles the idea of how a gravity battery works and how the actual
working model will look like after being fabricated.

Pulley 2 3 Rotary
Spar
Wheel Gear Damper

1
4

5 6
Gearset Dynamo LED
Mass

Fig 3.2.1: Schematic Diagram of GPG

15
3.2.1 Working Principle
1) The electric current from the source (solar, wind, etc) is used to run the electric motor.

2) The motor pulls the weight up to the height depending on the power supply.

3) The weight is suspended using a chain attached to the frame containing the gear
arrangement.

4) The suspended weight has the gravitational potential energy stored in it.

5) As the weight drops the gears linked to the generator rotates.

6) The generator in turn produces the current that can be used when it is necessary.

3.3 CAD Model of GPG

16
3.3 CAD Model of GPG

Fig 3.3.1: Isometric view of model

17
Fig 3.3.2: Bottom view of model

18
Fig 3.3.3: Top view of model

19
 Fig3.3.4: Lateral view of model

3.3.1 Components of the model

3.3.1 (a) Mass, Cable & Pulley Wheel

In our example, the drum is required to convert the suspended mass's linear free fall motion
to rotating motion in the shaft. The drum of choice resembles a winch drum and will have
two cables, one of which will wound and the other unwind concurrently. This is done so that
the user can unwind the second cable and coil up the mass once it has descended to its lowest
possible position. The design's diameter is predicted to be around 6 centimetres. A sprocket
and chain were used in the initial design, but this proved to be hefty because the standards
that were accessible were all made of steel. The shaft will have a hole through which a Sprag
clutch bearing will fit.A reasonable selected mass of 10 kg will be hanged at a 2.5m height to
achieve the requisite input power. due to the consideration of both the human factor and

20
safety. The group advised enclosing the material in a bag. This will make it possible to fill it
with a variety of items to reach the 10-kilogramme mass. The height is selected by taking into
account an average normal person’s weight and the average distance between the grounds
and ceiling floor of most rooms.

3.3.1 (b) Sprag Clutch Bearing

A one-way free wheel clutch is the Sprag clutch bearing. This component must be used since
the drum must be disconnected from the system in order to be unwound. It will be situated
between the input shaft and the drum.

3.3.1 (c) Rotary Damper

A rotary damper is utilised in the machine to address the issue of free-fall, or the "g"
acceleration of the mass that is dropping. By increasing counter torque at higher speeds, a
rotary speed damper uses fluid resistance to counter the acceleration. Because the counter
torque will grow if the velocity tends to rise over the marked one, this will result in a constant
velocity from free fall. We can also see from the design that the torque on the shaft is only
affected by the mass and radius of the drum and remains constant.

3.3.1 (d) Spur Gear

A simple spur gear mesh will be utilised to transmit rotational motion from the drum input
shaft to the gearbox input shaft. Spur gears are a form of cylindrical gear with straight teeth
that are parallel to the shafts and have coplanar, parallel shafts. They are undoubtedly the
simplest and most popular kind of equipment since they are simple to make and may be used
for a variety of purposes.

3.3.1 (e) DC Motor

Since a DC motor serves the same role as a dynamo when its functionality is reversed, it will
be employed in place of a dynamo. Due to the LED light's low power requirements, the DC.
This amount of power can be produced by the motor. DC motor chosen operates at 6 V and
1600 mA. No-load speed for the motor is 625 rpm, and stall torque is 0.106 Nm.

21
3.3.1 (f) LED
The project participants want to maximise the brightness of 2 white LEDs. Using a maximum
voltage input of 3.6V and a 20mA current for each LED, it was determined that 0.144watts of
total power was needed to turn on each LED.

3.3.1 (g) Bearing

A bearing is a component of a machine that limits relative motion to only that motion that is
intended and lessens friction between moving elements. The bearing's design may, for
instance, permit free rotation around a fixed axis or free linear movement of the moving part.
It may also serve to prohibit motion by managing the vectors of normal forces acting on the
moving parts. Numerous bearings also help the desired motion as much as they can, for
example, by reducing friction. According to the type of operation, the motions permitted, or
the directions of the loads (forces) supplied to the parts, bearings can be widely categorised.

3.4 Design Calculations

3.4.1 Tangential velocity (vt)
For, height = 3 m
Time taken = 1 min = 60 seconds
Tangential velocity = 3/60
=0.05 m/s
3.4.2 Angular Velocity (va)
We know that,
v = ωr
=>ω = v/r
= 0.05/0.01
= 5 rad/s
(r = radius of pulley = radius of shaft = 10mm)
3.4.3 Speed of shafts
Again, ω = 2πN/60
=>N1 = 60 ω/2π
=>N1 = 47.75 rpm
Speed of alternator, N5 = 1440 rpm

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3.4.4 Velocity Ratio
Velocity ratio = 1440/N1
= 30.16
For D1 = Diameter of the first driver gear i.e., connected to the pulley is 180 mm
D2 = Diameter of the second gear driven by D1 is 52 mm
D3 = Diameter of the third gear connected to the shaft containing D2 is 180 mm
D4 = Diameter of the fourth gear driven by D3 is 52 mm
D5 = Diameter of the fifth gear is 160 mm
D6 = Diameter of the fifth gear is 64 mm

Thus,
D1∗D 3∗D 5
For the compound gear system velocity ratio =
D1∗D 3∗D 5
=29.95
3.4.5 No. Of teeth (T)
Taking gear module, m = 4 mm
We know, m = D/T
Therefore, T = m*D
We have T1 = m*D1
= 45
Similarly, T2 = 13
T5 = 40
T6 = 16
For, D1 = D3, T1 = T3 = 45
Also, D2 = D4, T2 = T4 = 13
3.4.6 Power Output (P)
P = 2 hp for a single-phase alternator
Required power to

23
 Table 3.3.1: Design specifications of the GPG
Sl no. Item Quantity Material Dimension

1. Shaft 3 SAE mild steel 20 mm diameter

2. Deep groove ball 6 Stainless steel 20 mm diameter

bearing SKF=6204

3. Alternator 1 1440 Rpm

220 volts, 2HP

4. Gear of Diameter D1 1 Aluminium Pitch circle

diameter=180mm
No. of Teeth=45
Outer diameter=200mm

5. Gear of Diameter D2 1 Aluminium Pitch circle diameter=52mm

No. of Teeth=13
Outer diameter=60mm
6. Gear of Diameter D3 1 Aluminium Pitch circle
diameter=180mm
No. of teeth=45
Outer diameter=200mm
7. Gear of Diameter D4 1 Aluminium Pitch circle diameter=52mm
No. of teeth=13
Outer diameter=60mm
8. Gear of Diameter D5 1 Aluminium Pitch circle diameter=160
No. of teeth=40
Outer diameter=180
9. Gear of Diameter D6 1 Aluminium Pitch circle diameter=64
No. of teeth=16
Outer diameter=70mm
10. Cable or Rope 1 Nylon Length = 8m
Thickness = 0.5 cm
11. Sprag clutch bearing 1 Stainless steel

12. Mass 1 Water Drum 20kg

13. Body/frame 1 Wooden 400 * 600 * 200 mm3

24
 Chapter 4

METHODOLOGY

25
METHODOLOGY Chapter
4

26
 Chapter 5

CONCLUSION

27
CONCLUSION Chapter
5
Gravity storage batteries, also known as gravity energy stores systems, utilize the potential
energy of a heavy object such as weight or water to store energy. These systems are
considered a form of mechanical energy storage, as they convert surplus electrical energy to
mechanical energy (potential energy of the weight) and back again.

One of the main advantages of gravity storage batteries is their high energy density, as a
significant amount of energy can be stored in a relatively small space. Additionally, they are
relatively simple and durable, with few moving parts and no chemical reactions involved.

However, there are also some limitations to the technology. One of the main challenges is the
need for a relatively large amount of space to accommodate the weight, which can be a
limiting factor in certain applications. Additionally, the efficiency of the system is affected by
the friction and other losses involved in lifting and lowering of the weight.

This system of power storage and generation is possible only by providing elevation of at
least minimum 3 meters. So, performing the experiment at ground level is not feasible.

Overall, Gravity storage batteries have a potential for use in certain applications where high
energy density and simplicity are important, but the technology is still in its early stages of
development and further research is needed to fully utilize its potential.

28
 Chapter 6

REFERENCES

29
REFERENCES Chapter 6

[1]Elsayed, M., Abdo, S. and Attia, A. A. ‘Parametric optimisation for the design of gravity
energy storage system using Taguchi method.’ Scientific Reports. 12. 10.1038/s41598-022-
20514-y

[2]Botha, C. D. and Kamper, M.J. (2020) "Linear Electric Machine-Based Gravity Energy
Storage for Wind Farm Integration," International SAUPEC/RobMech/PRASA Conference,
CapeTown,SouthAfrica,2020,pp.16,doi:10.1109/SAUPEC/RobMech/PRASA48453.2020.90
41100.

[3]Berrada, A., Loudiyi, K. and Zorkani,( 2017) I. Dynamic modeling and design
considerations for gravity energy storage, Journal of Cleaner Production, Volume 159, Pages
336-345,

[4]Hunt, J. D., Zakeri, B., Falchetta, G., Nascimento A., Wada, Y., Riahi, K. (2020)
Mountain Gravity Energy Storage: A new solution for closing the gap between existing short-
and long-term storage technologies,Energy,Volume 190, 116419, ISSN 0360-5442

[5] Ruoso, Ana & Caetano, Nattan & Rocha, Luiz. (2019). Storage Gravitational Energy for
Small Scale Industrial and Residential Applications. Inventions. 4. 64.
10.3390/inventions4040064.

[6] S. Sagar, S. Sondhi and J. Sagar, "Gravity Battery: Storing Electrical Energy In The Form
Of Gravitational Potential Energy," 2020 IEEE International Conference on Computing,
Power and Communication Technologies (GUCON), Greater Noida, India, 2020, pp. 182-
188, doi: 10.1109/GUCON48875.2020.9231091.

[7] Bowoto, O.K., Emenuvwe, O.P. & Azadani, M.N. Gravitricity based on solar and gravity
energy storage for residential applications. Int J Energy Environ Eng 12, 503–516 (2021).
https://doi.org/10.1007/s40095-021-00393-1

[8] D. K. Chaturvedi, S. Yadav, T. Srivastava and T. Kumari, "Electricity storage system: A

Gravity Battery," 2020 Fourth World Conference on Smart Trends in Systems, Security and
Sustainability (WorldS4), London, UK, 2020, pp. 412-416, doi:
10.1109/WorldS450073.2020.9210321.

[9] Morstyn, Thomas & Botha, Christoff. (2021). Gravitational Energy Storage With
Weights. 10.1016/B978-0-12-819723-3.00065-2.

[10]Kak, S. (2020) ‘A RESEARCH PAPER ON POWER GENERATION USING

GRAVITY’ International Research Journal of Engineering and Technology (IRJET) Vol. 07
Issue: 11 pp. 20-23

[11] R S Khurmi, J K Gupta, Theory of Machines, Eurasia Publishing House Pvt. Ltd. ,
Distributors S. Chand and Company Ltd. For Toothed Wheels.

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[12] V B Bhandari, Design of Machine Elements, Third Edition, McGraw Hill Education
(India) Pvt. Ltd. for Rolling Contact Bearings, Pp. 564-596.

[13] Design Data Handbook for Mechanical Engineering in SI and Metric Units 4th Edition,
by K. Mahadevan K. Balaveera Reddy

[14] www.pv-magazine.com

[15]https://netcapital.com/companies/gravity

[16]www.handelsblatt.com

[17]www.netl.doe.gov

[18]www.wsset.org

[19]www.researchgate.net

[20]www.energypost.eu

[21]https://deciwatt.global/gravitylight

31