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Energy management on a stand-alone power system for the ...

18 th European Symposium on Computer Aided Process Engineering – ESCAPE 18

Bertrand Braunschweig and Xavier Joulia (Editors)

© 2008 Elsevier B.V./Ltd. All rights reserved.

ong>Energyong> ong>managementong> on a stand-alone power

system for the production of electrical energy with

hydrogen long term storage

Dimitris Ipsakis a,b , Spyros Voutetakis a , Panos Seferlis a,c , Fotis Stergiopoulos a ,

Simira Papadopoulou d , Costas Elmasides e , Chrysovalantis Keivanidis e

a Chemical Process Engineering Research Institute (C.P.E.R.I.), CEntre for Research

and Technology Hellas (CE.R.T.H.), P.O. Box 60361, 57001, Thermi-Thessaloniki,

GREECE

b Department of Chemical Engineering, Aristotle University of Thessaloniki, P.O. Box

1517, 54124 Thessaloniki, GREECE

c Department of Mechanical Engineering, Aristotle University of Thessaloniki, P.O. Box

484, 54124 Thessaloniki, GREECE

d Automation Department, Alexander Technological Educational Institute of

Thessaloniki, Thessaloniki, GREECE

e Systems Sunlight SA, 67200, Neo Olvio, Xanthi, GREECE

Abstract

This paper deals with the importance of an efficient energy ong>managementong> strategy on

stand-alone power systems that use renewable energy sources for the production of

electrical energy friendly to the environment. Due to the intermittent energy source of

the renewables, part of this energy is used to split the water for the production of

hydrogen which is also used for the production of energy in a PEM fuel cell in case of

high energy demands. The developed energy ong>managementong> algorithms were compared to

each other in order to determine the most efficient strategy as far as hydrogen

production is concerned. Parametric sensitivity was also a major issue which was

studied extensively. All the results and outcomes of such an analysis are considered as

the basis for the optimization and control study on stand-alone power systems.

Keywords: Renewable ong>Energyong> Sources, Stand-Alone Power System, PEM Electrolyzer,

PEM Fuel Cell, Hydrogen Production

1. Introduction

Global warming is considered as one of the most critical problems that the environment

will be faced with, in the oncoming years. The use of integrated systems that exploit

renewable energy sources for the production of hydrogen through water electrolysis and

use in fuel cells constitute a very promising solution for the production of eco-friendly

energy. Solar and wind energy is abundant, free, clean and inexhaustible and the

advantages of using photovoltaic systems and wind generators also include the long

lifetime and low maintenance requirements. In addition, the production of hydrogen

through water electrolysis is free from carbon dioxide emissions which are usually met

at the reforming of hydrocarbons like methanol or methane [1]. Renewable energy

systems (RES) offer off-grid energy supply for various applications such us the

electrification of rural and remote areas with problematic grid connection, the powering

of telecommunication stations, the desalination of water that requires large amounts of

energy, the water pumping for irrigation or drinking and many other facilities. Hybrid


D.Ipsakis et al.

systems for the production of electrical energy were studied extensively in the past

where each subsystem was modelled and validated with experimental data [2,3].

However, these research activities dealt with the each subsystem and not with the

integrated system. In this paper, three different energy ong>managementong> strategies were

developed and used in order to study the behavior of the system during a typical four

month period and mainly how the total hydrogen stored was affected by these strategies.

Furthermore, the effect of key variables to the system was examined in order to gather

all the information needed for future optimization studies and control analysis.

2. Description of the Stand-Alone Power System

An application utilizing solar and wind energy with hydrogen production through water

electrolysis, storage and utilisation in fuel cells is currently under development at Neo

Olvio in Xanthi in the framework of a research project with the participation of

Chemical Process Engineering Research Institute and Sunlight Systems S.A. The RES,

consists of a PV array with an installed capacity of 5 kW p and three wind generators

rated at 3kW p totally. Surplus energy is to be supplied to a PEM electrolyzer, rated at

4.2kW p after the demands of a 1kW constant load have been met. The hydrogen

produced is to be stored in cylinders under high pressure. In case there is a lack of

energy, a PEM fuel cell rated at 4kW p is to be used to provide power. Also, in order to

account for short-term needs, a lead-acid accumulator with a total capacity of 3000Ah-

48Volt, is also used and charged by the renewable energy sources or the fuel cell

depending on the availability of the renewable energy sources. A back up unit (diesel

generator) could be used in order to cover the electrical needs that can’t be met by the

system. Its potential use however, would be identified during the study of the system.

Furthermore, power electronic converters are employed for power ong>managementong> and for

the integration of the various subsystems. Fig. 1 represents a layout of the system

developed.

Fig. 1: Block diagram of the proposed Stand-Alone Power System

3. Simulation of the stand-alone power system during a typical four-month

period

The design and analysis of such a complicated system requires the existence of detailed

mathematical models for each subsystem. Due to the large number of equations that

such a system comprises, mathematical model will be given where is possible. The

theoretical study and simulation of the stand-alone power system was performed by

using the MATLAB ® simulation program tool.


ong>Energyong> ong>managementong> on a stand-alone power system for the production of electrical

energy with hydrogen long term storage

3.1. Renewable energy system (RES)

The output power from the PV-array is given by:

P

= V pv

⋅ I

⋅η

pv pv conv

(1)

where P pv denotes the output power from the photovoltaic array in Watt, I pv the

operation current in A, V pv the operation voltage in Volt and η conv the efficiency of the

DC /DC converter (~90-95%) [2].

In a similar way, the output power of the wind turbine is given by the following

equation [4]:

ρ ⋅ Α

w 3

P = c λ , β ) ⋅ v

(2)

m

p

(

wind

2

where P m denotes the mechanical output power of the wind turbine in Watt, c p the

performance coefficient of the turbine, ρ the air density in kg/m 3 , Α w the turbine swept

area in m 2 , v wind the wind speed in m/s, λ the tip speed ratio, and β the blade pitch angle

in deg. The relationship for c p is based on the characteristics of the turbine [4].

From the above equations, the output power of each subsystem of the renewable energy

system was calculated and the results are shown at figures 2a and 2b.

Output Power, Watt

5000

a)

4000

3000

2000

1000

Output Power, Watt

1800

1600

1400

1200

1000

800

600

400

200

b)

0

0 500 1000 1500 2000 2500 3000

Time, h

0

0 500 1000 1500 2000 2500 3000

Time, h

Figure 2: a) Output power from the photovoltaic system during a typical four month

period b) Output power from the wind generators during a typical four month period

3.2. Operation strategies for the stand-alone power system

The output power from the RES, P res , has been calculated as the sum of the output

power from the photovoltaic system and the wind generators. The power demand for the

load, P load , is constant throughout the year at 1kW. Therefore, the shortage or surplus

power is calculated as:

P = P RES

− P load

(3)

Based on the above equations and with the developed energy ong>managementong> algorithms

all the subsystems were studied simultaneously. Two limits for the state-of-charge

(SOC) were used: The minimum limit, SOC min (84%), where energy should be supplied


D.Ipsakis et al.

to the system from the fuel cell in case there is shortage of energy and the maximum

limit, SOC max (91%), where the operation of the water electrolysis is possible in periods

that excess of energy exists. It is highlighted, that the output power of the fuel cell is

constant at 1kW (unless stated otherwise) and the upper limit SOC max is possible to be

surpassed if excess of energy exists for long time period. The electrolyzer is able to

operate from the 25% (P min,elec ) to 100% (P max,elec ) of its nominal power. The basic steps

of the three algorithms are the following:

1 st ong>Energyong> ong>managementong> strategy

If SOC min


ong>Energyong> ong>managementong> on a stand-alone power system for the production of electrical

energy with hydrogen long term storage

3.4. Effect of key variables to the operation of the stand-alone power system

The study of the integrated system requires the analysis of the effect of the limit SOC min

as well as of the hydrogen constraint and diesel use. Table 1 shows that, reduction the

SOC min causes less hydrogen to produce and to consume but the total hydrogen is

higher. Furthermore, there was part of energy that was lost during the analysis of the 3 rd

strategy at 7.75, 3.15 and 2.62kWh for SOC min at 84, 80 and 76% respectively.

H 2 ,-Elec, Νm 3 H 2 -FC, Νm 3 H 2 –Tot, Νm 3

1 st Strategy 92.1 102.8 49.8

SOC min

84%

SOC min

80%

SOC min

76%

2 nd Strategy 106.7 144.4 22.8

3 rd Strategy 118 185.6 -7.2

1 st Strategy 75.2 53.9 81.8

2 nd Strategy 75.9 58.6 77.8

3 rd Strategy 74.6 61.5 73.6

1 st Strategy 71 38.3 93.2

2 nd Strategy 71.2 41.2 90.5

3 rd Strategy 69.3 42.3 87.5

Table 1: Results for the hydrogen production and consumption during a typical four

month period for various values of SOC min and of fuel cell output power

From the analysis of the three strategies, it was found that there were cases where the

demand for hydrogen was more than it was stored in the storage tanks. Especially for an

increase in the output power of the fuel cell the consumption was higher and the stored

hydrogen was less [5, 6]. This outcome resulted in the use of commercial hydrogen with

an additional cost in the system. For the next case study, a hydrogen constraint is used

in all algorithms. If the hydrogen stored is higher than the 95% of nominal capacity of

the storage tanks the excess of energy is used to charge the accumulator (until 100%)

and if excess energy still exists then it is lost (driven to the dump load). In case that

shortage of energy exists then the fuel cell meets the energy demand without taking into

consideration the SOC of the accumulator. Similarly, when the hydrogen stored is lower

than the 5% of the nominal capacity of the storage tanks and shortage of energy exists

(SOC


D.Ipsakis et al.

4. Conclusions and future work

In this paper, the simulation results from the operation of a stand-alone power system

were presented. The case studies revealed that there were periods of high energy

demand and the fuel cell consumed hydrogen to meet the energy deficit. This situation

resulted in high hydrogen consumption and the storage tank were empty. For that

reason, in order to prevent the system from extreme hydrogen starvation, the back-up

unit (diesel generator) should be used to meet the enegy demand. For future work, new

energy ong>managementong> algorithms will be developed to achieve an efficient and reliable

operation of the system, under variable supply and demand conditions. The optimization

of the whole system is another critical issue that is going to be studied extensively. All

these results will be used as the basis of developing end evaluating a ‘‘plant-wide’’

model based control strategy.

5. Acknowledgements

The financial support by the General Secretariat for Research and Technology of Greece

through the Ministry of Development is gratefully acknowledged. This study is

conducted in the framework of the research project ΑΜΘ-9.

References

1. Dimitrios Ipsakis, Panagiotis Kechagiopoulos, Christina Martavaltzi, Spyridon Voutetakis,

Panos Seferlis, Prodromos Daoutidis, Fotis Stergiopoulos, ‘’Study of an integrated system for

the production of hydrogen by autothermal reforming of methanol, ESCAPE17, 27-30 May

2007, Romania

2. Ulleberg Ø. (1998), Stand-Alone Power Systems for the Future: Optimal Design, Operation &

Control of Solar-Hydrogen ong>Energyong> Systems. PhD thesis, Norwegian University of Science and

Technology, Trondheim

3. Kolhe M., Agbossou K., Hamelin J. and Bose T.K. (2003), Analytical model for predicting

the performance of photovoltaic array coupled with a wind turbine in a stand-alone renewable

energy system based on hydrogen, Renewable ong>Energyong> 28, pp. 727–742

4. Siegfried H., Grid Integration of Wind ong>Energyong> Conversion Systems. John Wiley &

Sons Ltd, 1998

5. Dimitris Ipsakis, Costas Elmasides , Fotis Stergiopoulos, Spyros Voutetakis, Panos

Seferlis, ‘’Simulation of a stand-alone power system using renewable energy sources

and hydrogen storage’’, 10th Conference on Process Integration, Modelling and

Optimisation for ong>Energyong> Saving and Pollution Reduction, ISCHIA Island Gulf of

Naples 24- 27 June 2007, Italy

6. Dimitris Ipsakis, Fotis Stergiopoulos, Spyros Voutetakis, Costas Elmasides, Panos

Seferlis, Simira Papadopoulou, ‘’ Study of an autonomous power system based on

solar and wind energy with hydrogen as the intermittent energy source for future

use’’, 4 th Dubrovnik Conference on Sustainable Development of ong>Energyong> Water and

Environmental Systems, June 4-8 2007, Dubrovnik, Croatia

7. Ghosh PC. Cost optimization of a self-sufficient hydrogen based energy supply system PhD

thesis, Forschungszentrum Julich in der Helmhotltz-Gemeinschaft, Institut fur Werkstoffe und

Verfahren der Energietechnik Institut 3:Energieverfahrenstechnik, (Diss. Aachen, RTWH,

2003).

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