University of PaduaDEGREE DISSERTATION

29° February 2008

Electrical Energy Accumulation System For Ropeways

RELATER: Prof. Andrea Tortella

CORELATER: Ing. Leonardo Sartori

GRADUATED: Rigoni Garola Filippo

RopewaysRopeways1. On monocable ropeways the function

of carrying and hauling the vehicles is taken over by just one rope, the “carrying-hauling” rope. Generally these types are used on short path with high loads.

Fixed grip Detachable grip

2. On bicable ropeways there are one (ore more) carrying rope which the vehicle rolls by means of its carriage, and one (ore more) hauling ropes that propel the vehicles.

Continuous cycle Jig – back or reversible cycle

Jig – Back RopewaysJig – Back Ropeways

If a jig-back ropeways has more then one pylon, the applied force on the hauling rope (vector sum of the forces applied on the vehicle ) is irregular.

Such kind of force generate an irregular resultant diagram that obviously caused an irregular electrical load diagram. This irregularity is not appreciated above all with low voltage power supply and with low short circuit power.

Jig – Back RopewaysJig – Back Ropeways

Accumulation systems applications

Thanks to the installation of a properly electrical accumulation system there is the possibility of load diagram regulation that ensures the following advantages:

1. An increase of the electrical in coming power line quality;

2- cost effective solution in terms of electrical power savings;

3- a resizing of the entire accumulation upstream system;

4- an increase of the global electrical system performances.

Types of accumulation systemTypes of accumulation system1. Multiple conversion

Electrochemical cells Inertial system Potential system

2. Single conversion

Superconductive system (S.M.E.S.)‏

Super capacitors system

The first family is composed by all the systems in which conversions between electrical and a more adaptable accumulation energy, and vice versa, are subsequent. Beside in the second family there are all the systems in which there is only one conversion that uses the energy correlated to the electrical one:• magnetic energy,• electrostatic energy,The traditional transducer: resistance, inductance and capacity are related to their specific energy conversion. Besides for the magnetic and electrostatic energy there are well known transducers. Single conversion systems have a very low specific accumulation energy with the tradition transducers. This is the main reason to choose multiple conversion systems.

1.1 Electrochemical cells 1.1 Electrochemical cells

Lead-acid cells, lead-gel;

Nickel cells:

Ni-Cd

Ni-Mh

Ni-Zn

Ni-NaCl

Lithium cells;

Na-S cells;

Fuel cells.

ADVANTAGES

Well known technology,

reliability,

DISADVANTAGES

emission of dangerous gases,

high maintenance,

short working life,

Low specific features,

The inertial systems use the kinetic energy accumulation. Their key points are the high accumulation capacity and the longevity. Besides their cost is higher due to an higher need of volume and precision realizations. Moreover the high working life caused more cost for maintenance; necessary because there are parts moving with high speed that need cycle security measures.

ADVANTAGES Good specific features, High reliability,

DISADVANTAGES High noise, Possible risks of violence

broken, reduced responsiveness

1.2 Inertial system 1.2 Inertial system

This system use the potential energy accumulation, especially the gravitational one. The system is based on the recovery of geodetic energy, created by a mechanical energy applied to a body scaled to a certain height. The electrical motor is normally used, in this kind of solution, to generate the mechanical energy.

HYDROELECTRIC: facilities where the electrical energy is absorbed during the working gaps and gave back during the peaks.

C.A.E.S. (compressed air energy sorage): facilities where the air is compressed into natural or special cavity.Usually combined to a Bryton cycle to recover the heat, generated during the compression, to produce a cogeneration.

1.3 Potential system 1.3 Potential system

Superconducting Magnetic Energy Storage (SMES) systems store energy in the magnetic field created by the flow of direct current in a superconducting coil which has been cryogenically cooled to a temperature below its superconducting critical temperature. A typical SMES system includes three parts: superconducting coil, power conditioning system and cryogenically cooled refrigerator. Once the superconducting coil is charged, the current will not decay and the magnetic energy can be stored indefinitely.

ADVANTAGES: high specific energy, reduced response time.

DISADVANTAGES: high realization and maintenance costs, hazardous materials used (hydrogen); maintaining the structure at low temperatures; need for special and expensive facilities .

2.1 Superconductor system 2.1 Superconductor system

The super capacitor systems are characterized by an high load and specific capacity. They are made by standard electrolytic capacitor features armors fitted with a porous carbon sink or trough micro carbon tubular structures. This specific solution can ensure from 1000 up to 3000 square meters per gram.

ADVANTAGES: Maintenance absence, Moving parts absence, High temperature range, High performances, High specific power, High reactivity,

DISADVANTAGES: Cells tension reduced, Low specific energy,

2.2 Super capacitor system 2.2 Super capacitor system

Accumulation system comparisonAccumulation system comparison

performance, working life, reaction time, costs, specific parameters

Comparison parameters:

The best accumulation system is the one that uses more independent equipments that cooperate together to reach the same goal, so to have the more efficient solution for the specific application.

The design of a proper system of accumulation, able to regulate the electrical load, starts from the definition of the electrical diagram as the resultant of the strengths on the carrying rope.

There is the possibility to determinate the motor and bus DC power, necessary to define the energy value for the accumulation system , if is known the speed trend.

Accumulation system on jig – back ropewaysAccumulation system on jig – back ropeways

Load uphill – Empty downhill (including masses winch)Speed

Nominal engine power

Average square engine power

POWER OPTIMIZATION: the optimum power is defined as the one needed for the network that is the right technical and economical compromise considering the balance of additional charges and savings generated.

The accumulation system will supply the additional power, to the optimized one, need from the totally carrying plant.

Sizing of the accumulation systemSizing of the accumulation system

Instantaneous power

Optimized power

Average power

If the optimized power is higher then the average power, its network permanence will be lower then the cycle time;

If the optimized power is lower then the average one its network permanence will be higher then one rope loop;

If the two powers will be the same its network permanence will be as the cycle time unless losses.

TECHNOLOGY CHOOSE :1. Low maintenance; 2. High reliability;3. Low weight and small spaces; 4. Low rumors and vibrations;5. High working life in terms of upload and download cycle numbers and

performances;6. High capacity of features maintenance in presence of environment changes,

above all temperature and pressure; 7. No danger to humans and environment, not explosion dangerous, fire and toxic or

dangerous emissions and pollution in general; 8. High response rapidity, even after long periods of stop.

All these considerations bring to the super capacitor system choice but does not rule out the possibility of a flywheels cooperating system increasing the value of specific energy. The idea of using SMES systems is immediately rejected, because in periods of half-season the plant would be stopped and those would caused strong technical complications.

Sizing of the accumulation systemSizing of the accumulation system

DETERMINATION OF ENERGY MANAGED BY ACCUMULATION SYSTEM: This is actually represented from the peak curve of power area that the condenser should provide on the conditions provided.

The peak power diagram is obtain by subtracting from the optimized power the bus DC curve. Its area can be calculated trough numerical methods, however the geometric method should be easier thanks to the curve shape, this involves a lesser effort in the calculation.

Sizing of the accumulation systemSizing of the accumulation system

Instantaneous power

Optimized power

Average power

CAPACITY CHOSSE : 1. Should be considered the manner in which the SC gives energy to the load; 2. Energy should be calculated in addition to the losses dissipated in the SC; 3. Should be considered that the SC has a capacity that is a linear function of the

voltage applied to the heads;4. Should be considered the minimum voltage of SC determined by their current limit

and a possible converter applied.

To calculate the value indicative of capacity it start from considering the maximum and minimum limits of tension that the whole SC system can reach. It then assumes the lower starting capacity the previously one calculated. It determines the performance of the SC voltage corresponding to the loading diagram. It calculates the value of proper capacity that imposing the minimum of tension coincides with the minimum expected.

Sizing of the accumulation systemSizing of the accumulation system

CAPACITY DETERMINATION

The capacity should now be increased taking into account the points 2 and 3 in the previous sheet. In fact the energy that the SC should contain has to consider the losses the linear dependence of capacity despite the tension. The value of final capacity will be:

Wpik is the calculated energy starting from the peak curve area by imposing the minimum voltage; Wj is the energy lost due to various effects (mainly thermal); k is a factor which takes into account the dependence of the capacity from the voltage Vmax Vmin and are calculated taking into account the component limitations (current maximum allowable) and the limitations of the converter applied.

Sizing of the accumulation systemSizing of the accumulation system

The first solution shows that the tension of the SC is bounded by the limits imposed by the converters and carrying line. This implies an expensive design capacity. The range of variability of tension, in this case, goes from 620 V up to 750 V.

The second solution, seemingly less convenient, would provide a much wider variability in the tension of SC thus enabling capacity values sharply lower. The cost of additional converter is offset by a reduced burden on the SC. This solution is adopted in the period under investigation and allows a voltage range from 360 V up to 750 V.

Electrical equipment architectureElectrical equipment architecture

1

2

To validate the theory to practice has been done a simple test that includes the use of:

The test aims not only to verify the reliability of the sizing calculation model, but also wants to verify certain parameters stated by the manufacturer. A great importance is the estimation of performance that has shown excellent results with values higher than 90%.

Load Power

Capacitor Power

Power supply

Vdc Tension

Super capacitor tests Super capacitor tests

a power supply stabilized and current limited,

a rheostat load, a super capacitor a series of measurement

systems.

Diagrams for the first and second parts of the system, required for the DC bus, are reported in the two figures in side. It is shown that the maximum power has been almost halved.

ConclusionConclusion

The system of accumulation can provide service continuity even in temporary absence of power supply, without the help of generators;

The sizing of generators is less expensive because it is based, rather than on peak power, but on the "optimal" one;

The use of accumulation systems ensures a better use of the energy required by the network minimizing the value of losses.

In particular for a rope way the system of accumulation does not involve only the advantages cited at the beginning, but allows other advantages:

ConclusionConclusionData Part 1 Part 2 Dimension

Peak power 365 390 kW

Optimal power on critical part (maximum climb load) 220 200 kW

Total critical part energy 12,6 16,6 kWh

Accumulation system energy 2,14 3,18 kWh

Capacitor voltage limit 750-360 750-360 V

Bus DC voltage 750 750 V

Calculated capacity 35,7 50,2 F

Optimized module BMOD0018-390V BMOD0018-390V V

Number of module 8 12

Number of serial module 2 2

Number of parallel module 4 6

Effective capacity 36 54 F

Indicative capacitors cost 134000 200000 euro

Convenient module BMOD0094-75V BMOD0165-48,6V

Number of module 40 80

Number of serial module 10 16

Number of parallel module 4 5

Effective capacity 37,6 51,5 F

Indicative capacitors cost 123000 165000 euro