Projects:2019s1-192 Gravitational Energy Storage

The cost of extending conventional power grids makes it difficult to supply remote locations. There are established solutions, such as diesel-generators, but these are expensive, and are more suited to corporations, or large communities. There are low-cost solutions for small remote communities, in the third-world. These typically involve the use of batteries, but there are problems with batteries: they have finite lifetimes; they use chemicals that are scarce and toxic; they are difficult to repair (using simple tools); they suffer from internal leakage. There are solutions that have been used in remote communities (in Africa) where the storage is gravitational. Gravitational storage is much more convivial. Gravitational storage is very robust, with a long lifetime; it does not use exotic materials, it can be easily repaired using simple tools; there is no internal leakage. The main disadvantage of gravitational storage is low energy-density. It would take a very large mass, falling through a very large distance, to match a car-battery, for example. The loads have to be chosen appropriately, low-power LED lighting, for example. Also energy conversion has to be very efficient. In this project we will examine the possibility of using permanent-magnet synchronous machines, and power-electronics to efficiently store energy in gravitational form, and then to efficiently recover that energy at a later time. The plan is to design, build and test a small-scale solution, as a proof of concept.

Project Team

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Students

• Keilah David
• Kai Yang

Supervisor

• Andrew Allison

Introduction

Gravitational energy storage is that changing the height of the solid matter can be stored or released by a lifting system driven by an electric motor/generator. Compared with fossil fuels, this type of energy is more environmentally friendly. Gravitational storage is very rugged and has a long service life. It does not use special materials and can be easily repaired with simple tools. There is no internal leakage. There will be no other pollutants discharged at the same time. However, this system is not perfect. It needs heavy enough weight and enough height to get enough gravity potential to generate electricity for the motor. This means that the gravity energy storage system needs enough space to run.

Aim

The aim of the project is to develop (design, build and test) a small scale gravitational energy storage solution and effectively and efficiently store and recover energy by using permanent-magnet synchronous machines, power electronics and appropriate loads.

motivation

The motivation of the project is to supply energy to remote communities and third-world countries cost-effectively and provide a much cleaner, cheaper and more robust form of energy supply.

Background

The input power of the motor comes from the gravitational potential energy generated by the weight drop. According to the formula, E= mgh. The gravitational potential is proportional to the mass of the weight and the height. Therefore, heavy enough weight and enough height can get enough gravity potential energy. The pumped hydro is an application of gravitational energy storage.

Case Study

Three Gorges Dam

The Three Gorges Dam is the world's largest hydraulic gravity dam, located in Sandouping Town, Yiling District, Yichang City, Hubei Province, China. The installed capacity is 22,500 megawatts and consists of 32 turbine generators with a power of 700 watts and two small large motors of 50 watts. The dam produced 98.8 TWh and 103.1 TWh in 2014 and 2016, respectively. The Three Gorges Project completed all construction tasks in three stages, with a total construction period of 17 years. The first stage (1993-1997) is for construction preparation. The construction takes 5 years to mark the river closure. The second stage (1998-2003) is the second phase of the project. The construction takes 6 years to mark the initial storage of the reservoir, the first batch of generating units and the permanent ship lock. The third stage (2004-2009) is the third phase of the project. The construction takes 6 years to mark the completion of the complete power generation and hub project. When it comes to the economics, in the early stage of fundraising, the State Council decided to levy a fund of 3 cents per kWh of the Three Gorges Project in 1992. Since 1994, the Three Gorges Fund has adjusted to 4 cents per kWh. On March 18, 1994, the property rights of Gezhouba Power Plant were placed under the China Three Gorges Corporation, and part of its power generation profits could be used for the construction of the Three Gorges Project. The source of funds for the construction of the Three Gorges Project is also solved by using the power generation income, loans, and issuance of bonds of the Three Gorges Power Station. After the completion of the first project, the Three Gorges Corporation issued bonds, 98 Three Gorges bonds (2 billion yuan), 99 Three Gorges bonds (3 billion yuan), and 01 Three Gorges bonds (5 billion yuan) were issued one after another at the Shanghai Stock Exchange and Shenzhen. The stock exchange is listed. By the end of 2001, the Three Gorges Corporation had issued 11 billion yuan of corporate bonds to the society. By the end of 2001, a total of 76.230 billion yuan was invested in the construction of the Three Gorges Project. At the same time, the China Development Bank loaned 2.8 billion yuan, the Swiss export credit agency Bundesrat Exportrisikogarantie loaned $143.1 million to the Three Gorges Dam project, and the Canadian Export Development Cooperation (EDC) funded $38 million. The above funds will be used for the second and third stage. In terms of raw material suppliers, AGRA Monenco signed a $25 million contract in 1994 for project management systems to provide system layout and engineering, testing, operational guidance and training. Dominion Bridge, Inc. A contract worth $64 million was signed with Chongqing and Sichuan Province to supply cement for the dam. HUAXIN Cement also supplies cement to the dam. General Electric of Canada and Siemens and the German engineering company Voith Hydro signed a $320 million contract in 1997 to provide six turbines for the dam. French electrical equipment and engineering company GEC-Alsthom supplied eight turbine generators to the Three Gorges Dam in 1997 with Asea Brown Boveri (ABB) and Kvaerner Energy. Siemens and GE Canada and Voith-Hydro jointly supplied six generators to the dam. In terms of profitability, China Southern Power Grid needs to pay 250 RMB per kWh, and China National Grid needs to pay 250 RMB per kWh. Each province that receives electricity from the Three Gorges Dam will have to pay an additional fee of RMB 7 per MWh. Other provinces must pay an additional fee of RMB 4 per MWh. The Tibet Autonomous Region does not charge any surcharges. It is expected that the dam will be returned after 10 years of full use, but the full cost of the Three Gorges Dam was recovered on December 30, 2013, with a payback period of four years. The construction of the Three Gorges Dam also brought about the relocation of local people and the destruction of the surrounding environment. While building the dam, the Three Gorges Corporation also needs to pay the local residents' moving expenses and restore the local ecological environment.

Kidston Pumped Hydro

Kidston Pumped Hydro (K2H) is proponed by Genex Power Limited, which is located in Kidston, Far-North Queensland. It is used to generate dispatch electricity during peak demand period. The project acts as a natural battery storage, allowing energy to be stored and includes five components: An upper reservoir formed by a 20-meter-high dam around the existing Wises Pit. A lower reservoir utilizing the existing Eldridge Pit. A powerhouse cavern with the capacity to generate 250 MW. A tailrace allowing water to pass from the powerhouse to the reservoirs. A spillway from the upper reservoir to the Copperfield River. K2H need to cooperate with K2S, which is 270 MW Kidston Solar Project and the lifespan is 80 years, which is the stage 2 for the Kidston Renewable Energy Hub. The power generation process is shown in Figure 2. During the day, K2S generates the electricity can pump the water from the lower reservoir to the upper reservoir. During the work and evening peak period, water will be released from upper to lower to generate dispatchable electricity. In the evening, grid power will be used to pump the water into upper reservoir. K2H supports 2000 MWh of continuous power generation in a single generation cycle, which means 250 MW of peak power generation over 8-hour-period. The project will be finished in 3 years by 2021. In terms of funding, the project will cost AUD 330 million. K2H get the government support, one is Designated ‘Critical Infrastructure’ by Queensland State Government and another is from ARENA, which provides AUD 9 million. For the contractors, feasibility completed by Entura, the engineering consultants are from MottMacDonald and ANDRITZ will support hydroelectric turbine. Since there is no public financial plan for K2H, the above is all the funds information that can be found.

Inspiration

The inspiration is from gravity lights, which has been shown in Figure , and it is designed by a team from Kenya and can be used in countries of the third world or in developing countries. Compared with kerosene lamps, gravity lamps are more energy-saving and environmentally friendly. For the selection of raw materials, gravity lamps are not as harsh as kerosene lamps. Only enough stones are needed, and such conditions are easily met in the third world. And they are a substitute for kerosene lamps. First fill the bag with gravel, about 20 pounds, to get enough weight. Place the bag high so that it slowly descends, and the drop time will be controlled for about 25 minutes. The descending weight drives the generator to operate to supply power to the load. The load can be an LED light. The energy generated by the falling weight can maintain the LED lamp for 25 minutes.

Design and Specification

Designed Gravitational Energy Storage

As can be seen from figure, the gravitational energy storage system works similarly to gravity lamps. The gravitational energy storage system has two modes, a work mode and a recovery mode. In the work mode, the clutch between the gearbox 1 and the gearbox 2 will close so that the two gearboxes are connected. The object is placed at a high position, and the released object will slowly drop due to gravity. The falling object drives the two gearboxes to rotate. The gearbox 1 will rotate slowly, while the gearbox 2 will rotate quickly and drive the generator to rotate.

Then Brushless permanent magnet motor generates electricity. Power converts the alternating current into direct current through the inverter, providing power to the 5-watt load.

In the recovery mode, the clutch between the gearbox 1 and the gearbox 2 will open causing the two gearboxes to disconnect. At this time the motor does not work. Manually turning the crank drives the gearbox 1 to return the weight to the original height.

Components

Power Electronics

The inverter is connected to the motor by a cable, and the cable is connected to the 5-watt load on the other side.

Synchronous Machine

In terms of motor, a brushless permanent magnet DC motor will be used, as shown in Figure 5. For this type of motor, the input and output are all DC. It will be used to power the load. For the nameplate value, rated voltage is 48 V, rated current is 4.63 A, rated power is 160 W, the rated speed is 6000 rpm and the peak torque is 75 Nm. For the operational voltage for encoder is 5V. This motor can drive a weight of 100 kg, this is because: The radius of the bicycle front wheel sprocket is about 7.5 cm, and the torque generated by the 100 kg weight is 73.5 Nm. It is much smaller than the peak torque.In terms of driving load, it is sufficient to supply a load of 5 watts for a motor with a rated power of 160 watts.

Gear Box No. 1

The gearbox 1 is connected to windlass by using bicycle chain. A crank is also mounted on the gearbox 1, which allows manual weight recovery to return to a height of 3 meters. The gearbox 1 and the gearbox 2 are connected by metal shaft, but a clutch is mounted on the gears in the gearbox 1 to satisfy the two modes

Gear Box No. 2

One end of the gear case 2 is connected to the gear case 1 by a metal shaft. The other end is connected to the generator by a metal shaft.

Crank & Windlass

The chain is connected to the gearbox 1 using a bicycle chain. This is because the gear in the gearbox 1 is the sprocket of the front wheel of the bicycle, which can be used together.

Weight

The weight is connected to the chain, and the chain is wound around the windlass. This allows the heavy object to hang at a height of 3 meters.

Load

The 5-watt USB is connected to the inverter by wires, and the battery is connected to 5-watt USB on the other side to charge the battery.

Test

Determine Internal Resistance

Internal resistance of the generator was determined by applying a current between phases: phases U&V, V&W and U&W. The applied current was gradually increased and phase voltages were recorded. The sum of adjacent phase resistances were then obtained. The average value for the internal resistance is 1.648 ohms.

Open Circuit Test

Open circuit test was done to determine the induced voltage E in each phase. A resistor that is much larger than the internal resistance was connected to the circuit in order for the output phase voltages of the generator to be very close to E in each phase.

Load Test

The frequency of the generator was adjusted and varied to measure the internal inductance of each phase of the generator. Corresponding voltage and current values were recorded.