Sylvain LEMOFOUET, Alfred RUFER

Swiss Federal Institute of Technology Industrial Electronics Laboratory

Industrial Electronics LaboratorySwiss Federal Institute of Technology, Lausanne

CH-1015 Lausanne - Switzerland

1

Economical Considerations

Overall Efficiency and Possible Topologies

5

Strategy for Power Flexibility: Hybrid Storage

4

3

Principle of Pneumatic Energy Storage

2 Optimisation of the Pneumatic-to-Mechanical Energy Conversion

1 Principle of Pneumatic Storage

1 Renewable Energies: Which storage technologies?

Electrochemical batteries are the today’s main storage technology used in renewable energies’ applications...

Alternative solutions to Electrochemical Batteries are necessary for a sustainable support to renewable sources

An Efficient, sustainable and economical exploitation of renewable energies

Storage Technologies with:High Duty CyclesLow impacts on the

environment

However, they present important limitations…

- limited Duty Cycles- Difficult and costly recycling- Problematic waste

Electrochemical Storage

requires

1 Pneumatic Storage: Potentially suitable for renewable sourcesPneumatic energy storage presents many advantages that

make it highly compatible with renewable energy sources...

+ Air is available everywhere No need to be produced+ High duty cycles More than 15’000+ Low impacts on Environment: No or less problematic waste+ No self-discharge Long storage time+ Based on simple & mature technologies Low cost+ Variety of Applications Stationary and Mobile

This project aims at improving the performances of the pneumatic energy storage technology

High compatibility with renewable energies

However, its performances remain insufficient for that purpose

Main Challenges:- High energy densities- High conversion efficiencies- Low Cost

1 Principle of Pneumatic Energy Storage- Pneumatic Energy Storage relies the “thermo-elastic”

properties of air- Many thermodynamic cycles are possible

The isothermal cycle is the ideal cycle

Cycles:- Joule: A.B.D.E.F- Otto: A.B.C.D.E.F.A- Isothermal: A.D.A- Polytropic: A.B’.D.E’A

1- 2 possible Pneumatic-to-Mechanical conversion systems:

Pneumatic Conversion and Hydro-pneumatic Conversion- 2 possible gas cycles: Closed gas cycle and Open gas Cycle

Pneumatic Storage requires a multiple-step conversion

- Open gas Cycle- Hydro-pneumatic Conversion

BOP-B

- Closed gas Cycle- Hydro-pneumatic Conversion

BOP-A

Energy Conversion Chain in Pneumatic Energy Storage

- Open gas Cycle- Pneumatic Conversion

1The energy density mainly depends on the Gas Cycle, the

pressure and the Heat exchanges with the surroundings

Pneumatic Storage: Expectable Energy Densities

The Open Gas Cycle is necessary when high energy densities are required

Open Gas CycleClosed Gas Cycle

2 Optimisation of the Pneumatic-to-Mechanical Energy Conversion

- Suppressing all the unnecessary sources of losses- Controlling the conversion process so as to avoid all the lossy

operation conditions

The optimisation approach mainly consists in :

2 Pneumatic Conversion Hydro-pneumatic ConversionThe investigations started with air machines, but their conversion

performances are insufficient for storage applications… Alternatives conversion solutions needed to be found

Hydraulic machines appear as good alternative… But they require an Air-to-Oil interface

++ High power densities++ Simple conversion chain

But-- Low conversion efficiencies-- Low pressure ratings

Low energy densities

From Pneumatic Machine

++ High power densities as well++ High pressure ratings++ High conversion efficiencies

But-- Require an Air-to-Oil interface

To Oil-Hydraulic Machine

Variable speed operation is necessary to optimise the efficiency

2 Strategy for Real Time Efficiency OptimisationA Maximum Efficiency Point Tracking (MEPT) algorithm

have been developed for the real time optimisation of the Pneumatic-to-Mechanical conversion efficiency

Principle of MEPT: Efficiency controlled speed variation

Efficiency in Pump Operation

No need for a particular knowledge of the controlled Volumetric machine

No need for additional hardware resources

- Q.I: Quadratic Interpolation

- P.O: Perturb & Observe

The MEPT algorithm combines 2 techniques:

at Start up

for Tracking

2The experimental results show that the MEPT strategy improves

the cycle efficiency of the hydro-pneumatic conversion by about 4% compared to that of a constant speed operation

Performances of the MEPT StrategyC

onst

ant S

peed

(3

000r

pm)

Varia

ble

Spee

d (M

EPT1

)

Quasi-adiabatic Storage-Discharge cycle between 100 et 200bar

Energy Efficiency of the hydro-pneumatic conversion

2 Power constraint of the MEPT strategyBy imposing the speed, the MEPT strategy also imposes

the level of the converted pneumatic power, which may not match the source and/or load powers

A strategy for power flexibility is necessary

Power

Characteristics of the oil-hydraulic machine in motor operation

3 Strategy for Power Flexibility: Hybrid Storage

3 Principle of the Strategy for Power FlexibilityStrategy based on a Hybrid Topology where the

main, Hydro-pneumatic storage is associated with an auxiliary Supercapacitive storage

Hybrid BOP-A & Supercapacitors Storage System

MEPT controlled intermittent operation of the main storage subsystem

The auxiliary storage is used to smooth the output power

The output power can vary freely

3 Principle of output power variation

T 2T 3Tt

Ton Toff

P

Pcv

Pld

Wst

Wrt

Wst = Wrt

onTD=T

The intermittent operation’s Duty Cycle “D” is controlled

through the electro-valve

By regulating the DC bus voltage, the auxiliary storage allows the

output power to vary freely

If Uscap > Umax, Close the valve

If Uscap < Umin, Open the valve

Principle of hydro-pneumatic power modulation

The excess of the converted pneumatic energy is stored in the Supercapacitors and used to supply the load when the main storage is turned off

Result: High quality, freely variable output power

3 Strategy for power flexibility : Experimental evaluationAn experimental setup of the hybrid BOP-A & Supercapacitors

storage system has been realised for the evaluation of the presented MEPT and Power flexibility strategies

Oil Tank

HydraulicPump/Motor

Encoder

+

−

LOAD

RPMS

Motor/Gener. Filter

Flow

Pressure A.

Speed/Position

L

Frequency Converter

AC lineA

B

Rectifier

Main SW Aux. SW

Xformer

Precharge Resistors

C

D

E-Valve

M-ValveS-Valve

t

P

t

P

t

P

Storage Discharge

P

Storage Discharge

MEPTSpeed Regulation

PWMDC Bus voltage

RegulationLoad

Control

INTERFACE BOARD

« SHARC » DSP BOARD : COMMAND & CONTROL

RS 232

MPPT

DC

DC

A

B

PV Pannel

Alternative Source

Line Source

50L 330bar

13kW5cm3

75F60V

100 V

100 V

2kW max

3 Strategy for power flexibility : Experimental evaluationAn experimental setup of the hybrid BOP-A & Supercapacitors

storage system has been realised for the evaluation of the presented MEPT and Power flexibility strategies

3 Performances of the Strategy for power flexibility

The experimental results have confirmed the effectiveness and efficiency of the proposed strategies

Storage Discharge

4 Overall Efficiency Evaluation and Possible Topologies

4 Hybrid Storage System: Overall Efficiency Evaluation

The topology and operating conditions of the hybrid storage system greatly impact its overall efficiency

Sim

plifi

ed p

ower

pro

file

Effic

ienc

y pa

ram

eter

s

Ove

rall

Effic

ienc

y

Assumption: All the units exhibit 95% 1-way

4Many topologies of the Hybrid Storage System are possible,

that allow fulfilling a specific application’s requirements while optimising the performances and cost

Hybrid Storage System: Some Possible Topologies

Non interfaced Supercapacitors banc Higher efficiency and lower cost

but fluctuating DC bus

Topology D

Topology C

Constant DC bus voltageHigh power quality

and flexibility

4

Overall 1-way Efficiencies Comparison

Assumption: All the individual 1-way Efficiencies equal 95%

The presented topologies are also possible with BOP-B

Topology E

Topology F

Many topologies of the Hybrid Storage System are possible, that allow fulfilling a specific application’s requirements while optimising the performances and cost

Hybrid Storage System: Some Possible Topologies

4 Next Step: Open Gas Cycle Hydro-pneumatic Storage System

La topologie hybride est également réalisable avec les Systèmes de stockage à conversion hydropneumatique

Compression - Expansion in Liquid-Piston working chambers with integrated heat exchangers

Nearly isothermal processes

Difficult Air-to-Oil interface (No physical separation)

Complex conversion chain

Open Gas Cycle; Reduce volume of oil High Energy densities (60Wh/l at 400bar)

Hybrid BOP-B & Supercapacitors Storage System

Suitable for almost all kind of application, stationary and Mobil

Target: Open Air Cycle, Water Hydro-pneumatic System

5 Economical Considerations

5 Specifications of the Photovoltaic ApplicationA comparative cost evaluation of 3 storage systems

was made, for a day-to-night power shift, in the context of a photovoltaic home application

DC

Lin

e / L

oad

BOP-A & Supercapacitors BOP-B & Supercapacitors Lead Acid Batteries

129

2318

P (kW)

t16

28kWh

20kWh

10

4 Average

Storage

DischargeAutonomy = 3days 60kWh

Average

7

-5-4

Daily Scenario

MPPTDC

DC

A

B

PV Pannel

PV Supply Specifications

- Storage Capacity: 60kWh

- Peak Power: 10kW

- Duty Cycles: 3’500 min.

- Input/Output Type: DC

(10 years)

5 Layouts of the 3 storage systems for 60kWh – 10kW

The artist impression of the 3 storage systems shows that the BOP-B is the lightest and less bulky system

For hydro-pneumatic systems, the storage unit is completely separated from the conversion package

Higher Design and sizing flexibility

5 Total Cost and Specific Energy Cost for the 3 systems

The cost comparison show that the hybrid, hydro-pneumatic & Supercapacitors storage systems are more cost effective than a Lead acid Battery system in the considered context

BOP-A is more sensitive to storage capacity due to it low energy density (2.5Wh/L)

For hydro-pneumatic systems, there is an offset cost related to the conversion unit

Cost Extrapolation (€)

The high duty cycles of hydro-pneumatic systems allow achieving low specific energy cost

Cost Composition for 60kWh

Hydro-pneumatic storage possesses many technical and environmental advantages that make it suitable for renewable energy sources support…

A Maximum Efficiency Point Tracking strategy is proposed to optimise the Energy Conversion Efficiency.

Some R&D efforts are still going on, in the fields of Thermodynamics and Power Electronics & Control to improve the performances of this technology

A Hybrid Topology is also proposed to improve the Power Quality and Flexibility.