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Optimization of the photovoltaic-hydrogen supply systemof a stand-alone remote-telecom application

Guillermo Gomeza, Graciano Martıneza, Jose L. Galveza,*, Raul Gilab, Raquel Cuevasa,Jesus Maellasa, Emilio Buenob

aNational Institute for Aerospace Technology (INTA), Renewable Energy Department, Ctra. Ajalvir km 4, E-28850 Torrejon de Ardoz,

Madrid, SpainbPolytechnical School – Alcala de Henares University, Electronics Department, Campus Universitario, Ctra. De Madrid-Barcelona Km 33.600,

Alcala de Henares, Madrid, Spain

a r t i c l e i n f o

Article history:

Received 2 February 2009

Received in revised form

4 May 2009

Accepted 4 May 2009

Available online 28 May 2009

Keywords:

Photovoltaic hydrogen

Hybrid system simulation

Stand-alone application

Fuel cell

Electrolysis

* Corresponding author. Tel.: þ34 915201446.E-mail address: galvezmjl@inta.es (J.L. Ga

0360-3199/$ – see front matter ª 2009 Interndoi:10.1016/j.ijhydene.2009.05.014

a b s t r a c t

Hydrogen is considered as the optimal carrier for the surplus energy storage from

renewable resources. Although hydrogen and its application in fuel cell is considered as

a high-cost energy system, some cost-efficient solutions have been found for their use in

stand-alone applications, which usually depend on the variability of renewable sources

that have to be oversized in order to reduce their dependence on external energy sources.

This paper shows the results from the simulation of several alternatives of introducing

hydrogen technologies to increase the independence of a remote-telecom application fed

by photovoltaic panels. Hydrogen is obtained by electrolysis and it is used in a fuel cell

when the renewable energy source is not enough to maintain the stand-alone application.

TRNSYS simulation environment has been used for evaluating the proposed alternatives.

The results show that the best configuration option is that considering the use of hydrogen

as a way to storage the surplus of radiation and the management system can vary the

number of photovoltaic panels assigned to feed the hydrogen generation, the batteries or

the telecom application.

ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

reserved.

1. Introduction In the other hand, great barriers to hydrogen penetration are

Research in new energy generation systems is being focused

on climate change mitigation, safety in energy supplies and

increasing the energy independence of regions. In this

context, hydrogen appears as the preferred energy carrier in

the mid-long term [1], as its heating value per mass is higher

than conventional gasoline or diesel, its application in a fuel

cell provides higher performances than any other alternative,

it can be generated in situ at fuel stations and it can solve the

seasonal variability concern of the renewable energy sources.

lvez).ational Association for H

foreseen, as the needed infrastructures or the high production

costs. These issues make the hydrogen application to be far

from the current energy scenarios.

However, some stand-alone applications can be cost-effi-

cient when hydrogen technologies are employed [2]. Nowa-

days, there are remote-telecom applications without any

possibility of being fed from electrical grid connection because

they are installed at inaccessible areas or there is not a cen-

tralised electricity generation, as islands case [3]. These

applications are usually fed with photovoltaic panels, using

ydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Nomenclature

AC Alternating current

ctrlBat2 Group 2 of batteries control

DC Direct current

Ely_on Electrolyser state

FIRST Fuel cell Innovative Remote System for Telecom

I Current intensity, A

Isc Short Circuit Current Intensity, A

PoToH Power used by electrolyser, W

PresGas Pressure of hydride tanks, bar

PV Photovoltaic

slpm Standard litres per minute

SoC State of Charge

TRNSYS Transient System Simulation Program

V Voltage, V

Voc Open Circuit Voltage, V

V_to_tank_H2 Hydrogen production, m3 h�1

Welec Power consumed by the electrolyser, watts

Wapp Power consumed by the telecom application,

watts

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 5 3 0 4 – 5 3 1 0 5305

batteries as the energy storage system. Their configurations

require to oversize the system capacity and provide an

auxiliary power unit, as a diesel generator, especially in

locations where there are great differences in solar radiation

between summer and winter [4].

One of the preferred solutions comprise the use of excess

photovoltaic energy production in summer to produce

hydrogen, which can be stored for its further use in winter,

when the PV energy production is not enough to feed the

application. In reference [4], Yilanci et al. describe several

applications of hydrogen through the integration of photo-

voltaic panels, electrolysers, batteries and fuel cells. From the

integration engineering point of view, there are three alter-

natives for combining the solar electricity production and the

hydrogen generation through electrolysis: direct coupling;

storage and DC/DC conversion to a DC electrolyser; and

storage and DC/AC conversion to an AC electrolyser. The

direct coupling has several difficulties because of the high

response time of the electrolyser and the mismatch with the

electrolyser curve, although this option has applications at

small scale [5]. The conversion of the current produced by the

PV panels is the common option between the assessed alter-

natives. This work shows the results from the simulations of

a DC/AC integration system, called HIDROSOLAR_H2. The

main objective is to optimize its configuration and to deter-

mine the most energy-efficient option for the solar hydrogen

production, avoiding the system oversizing and minimizing

the maintenance of the installation.

Fig. 1 – Proposed scheme in Fuel cell Innovative Remote

System for Telecom (FIRST).

2. System description and modelling

2.1. Description

HIDROSOLAR_H2 is a spanish spin-off project from European

project FIRST (Fuel cell Innovative Remote System for Tele-

com). FIRST project [6,7] proposed the photovoltaic-fuel cell

integration for enhancing the reliability of a remote-telecom

equipment (Fig. 1).

The same PV panels and fuel cell installation employed in

FIRST project will be used in the HIDROSOLAR_H2 remote-

telecom application. The main difference of the new system is

the introduction of a hydrogen generation/storage system, as

shown in Fig. 2. The solar installation consists of two fields of

photovoltaic panels with 6 each one (model ATERSA A-127P of

127 W and 12 V). These fields feed two groups of batteries with

12 batteries (Tudor type 6 EAN 55 of 396 Ah) each group. The

group 1 of batteries feeds the remote-telecom load, while the

second group is used to feed the electrolyser (VENTUS NM-H2)

for hydrogen generation. If batteries of group 1 are empty, the

second group would feed the telecom application and when

the two groups of batteries are empty, the fuel cell would feed

the application. To lengthen the life of batteries, deep charge

and discharge cycles have to be avoided: once the batteries are

discharged, i.e., their State of Charge (SoC) is 52%, they will not

work again until they are charged to 100% of their capacity.

The water AC electrolyser is a VENTUS NM-H2 that

consumes 300 VA (220 V) and produces 0.5 slpm of pure

hydrogen (99.99%) at 10.5 bar. The size of the AC electrolyser

was the smallest found among the available commercial

electrolysers. Hydrogen is fed to four metal hydrides bottles,

where it is stored. The maximum H2 storage pressure is 17 bar

and the whole capacity of the storage system is 2.56 Nm3. The

fuel cell is a BALLARD Nexa with 1.2 kW of maximum power

and a maximum hydrogen consumption of 18.5 slpm. Two

compressed hydrogen cylinders (8 Nm3) are added to the

hydrogen line as the auxiliary system. This auxiliary hydrogen

can be consumed during the assumed two years of energy

independence of the system.

The whole system is commanded by a central controller

that manages the energy system. The way this controller is

configured is one of the objectives of the optimization in the

simulations that have been carried out. The design of the

controller must take into account the performance require-

ments of HIDROSOLAR_H2:

Fig. 2 – Hybrid system proposed in HIDROSOLAR_H2.

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– To feed the remote-telecom application for two years

without gaps.

– To minimize the maintenance.

– Fuel cells have to work less than 2 h in two years.

– Batteries have to do less than 600 cycles of charge and

discharge in two years.

2.2. Modelling

TRNSYS was used to simulate the HIDROSOLAR_H2 system.

TRNSYS is a complete, open, modular and extensible simu-

lation environment for the transient simulation of systems,

including multi-zone buildings [3,8]. It is used by engineers

and researchers around the world to validate new energy

concepts, from simple domestic hot water systems to the

design and simulation of buildings and their equipment,

including control strategies, occupant behaviour, alternative

energy systems (wind, solar, photovoltaic, hydrogen systems),

etc. The fundamental unit of a TRNSYS simulation is called

‘Type’, which is the unit model for each system component.

As a previous step to the whole system modelling, every

component was simulated and validated:

1) Type109 for meteorological conditions of the environment

where HIDROSOLAR_H2 will be installed. The hourly

meteorological data fed to the program are collected in the

National Institute of Aerospace Technology, located in

Torrejon de Ardoz, Madrid (Spain).

2) Type14 h for the remote-telecom application, with

a medium consumption of 145 W and maximum of 197 W.

3) Two fields of 6 photovoltaic panels each one. Each panel

has the following technical characteristics: 127 W and 12 V,

with an ISC¼ 7.95 A and a VOC¼ 22 V and a point of

maximum power at I¼ 7.28 A and V¼ 17.48 V. Type94a was

used.

4) Type 47a for two groups of 12 batteries connected in series.

Each battery has a capacity of 396 Ah and a nominal voltage

of 24 V.

5) A DC/AC converter of pure sinusoidal wave, with an effi-

ciency of 93%, a nominal power of 800 VA and an input

voltage of 24 V. Type175b was used to simulate this

element.

6) An electrolyser, fed at 220 V, with a consumption of 300 VA.

This element produces a flow of 0.5 slpm of hydrogen at

10.5 bar. To simulate its behaviour following Equations (1)

and (2) were used:

PoToH ¼ 300$ltðPresGasÞ,ctrlBat2 (1)

V to tank H2 ¼ Ely on,0:03 (2)

In equation (1), power used by electrolyser is equal to 300 W,

when the pressure in the hydride tanks is under 4.2 bar and

the batteries are not charging. In equation (2), the Electro-

lyser Hydrogen Production is equal to 0.03 m3/h when the

electrolyser is On (Ely_on¼ 1).

7) Type164b for two tanks for hydrogen storage, one of them

with a maximum capacity of 2560 l of hydrogen in standard

conditions to simulate metal hydride tanks and the other

one with a capacity of 8000 l in standard conditions to

simulate the auxiliary hydrogen tank.

8) A 1.2 kW PEM fuel cell with a consumption of 18.5 slpm

(standard conditions litres per minute) of hydrogen and

a nominal voltage and intensity of 26 V and 46 A, respec-

tively. Type170a was used to simulate the behaviour of this

element.

Our own central controller was created to manage the

whole system. This is implemented with two Type2d and two

Type93 modules.

3. Results and discussion

Once the simulation was validated, six configurations were

assessed to determine the most efficient one.

Simulation 1: In this case the simulated system is the same

as described in the previous chapter. There are two fields of 6

panels each one, which are distributed forming 3 parallel

groups of two panels in series in each group. These panels

feed 2 fields of 12 batteries each one. The field 1 of batteries

feeds the application in a straight way, while the second field

is used to feed the electrolyser and to feed the application if

the first field of batteries is empty. The fuel cell is used as

auxiliary energy system, i.e., it would work when both battery

fields are in charging state.

Fig. 3 shows system’s State of Charge for the two sets of

batteries and pressure of H2 tanks during the first year.

Auxiliary hydrogen is not enough to feed the fuel cell during

the first year. The high variability of the batteries state of

charge and the generated hydrogen indicate that the system’s

performance is not good and the application is fed frequently

from auxiliary hydrogen, although the electrolyser is working

almost continuously. The fuel cell works more than 2000 h

during a year and the telecom application would have to stop.

Then, this configuration is rejected as it does not fulfil any of

the HIDROSOLAR_H2 requirements.

Simulation 2: The number of panels is increased to 26. Now,

the first group of panels consists of 6 parallel subgroups of two

panels in series in each subgroup, while the second group is

Fig. 3 – Batteries State of Charge and Tanks Pressure for Simulation 1.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 5 3 0 4 – 5 3 1 0 5307

composed of 7 parallel subgroups of two panels in series in

each subgroup.

Fig. 4 shows the system’s evolution. As observed, the use of

the auxiliary hydrogen is negligible with this new distribution,

and the fuel cell is only used 753 h per year. Furthermore, the

remote application is fed and the maintenance is minimized.

To achieve this, the PV field has been highly oversized. Then,

this configuration is far from initial objectives and must be

discarded.

Simulation 3: Sixteen panels are distributed in two fields of 8

panels each field with 4 parallel groups of two panels in series

in each group. The fuel cell only works 1 h per day if the

batteries are not in charging state.

Fig. 4 – Batteries State of Charge and

The evolution of the State of Charge and the hydrogen

pressure during one year is shown in Fig. 5. The consumption

of auxiliary hydrogen is less than 1/3 of its whole capacity.

However, the fuel cell works 1018 h per year, which is the

proposed limit for the use of the cell. During the winter period,

it is observed that auxiliary hydrogen is consumed, while the

electrolyser is able to supply the needed H2 when the solar

irradiation is higher, i.e., the summer period. The charge/

discharge cycles of the batteries are also affected by the sun

radiation received by the panels: deeper cycles are observed in

the winter period. Although this configuration is suitable to

HIDROSOLAR_H2 requirements, 4 extra PV panels should be

introduced to the FIRST solar installation.

Tanks Pressure for Simulation 2.

Fig. 5 – Batteries State of Charge and Tanks Pressure for Simulation 3.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 5 3 0 4 – 5 3 1 05308

Simulation 4: In this case, there is not generation of

hydrogen. The system has 12 photovoltaic panels feeding the

load, except when the fuel cell works, i.e., 2 h per day. From

the integration point of view, FIRST project arrangement

(Fig. 1) is being simulated.

As seen in Fig. 6, the application is always fed and 52% of

the whole auxiliary hydrogen. The fuel cell works during 916 h

per year. The FIRST installation was correctly designed as it is

able to feed the remote-telecom application and the mainte-

nance is also minimized. But the application would not be

autonomous during the period required for the application

(2 years).

Simulation 5: The number of panels is 12, but the distribu-

tion of these panels is not constant during a year. When the

Fig. 6 – Batteries State of Charge and

solar radiation is low, the whole array of panels feeds the

group of batteries which feed in a straight way the application.

In summer, when the solar radiation is higher, six panels get

energy to feed the load and the other six produce hydrogen.

With this configuration, remote-telecom application is

always fed, as shown in Fig. 7, and the system only uses the

34% of whole auxiliary hydrogen, while the fuel cells works

968 h per year.

Simulation 6: As simulation 5, the number of panels is 12 but

the number of panels per group is not constant. The system

controller is able to decide what number of panels will be used

to feed the batteries which are directly connected to the

application, and what number of panels will be used to

produce hydrogen. The number of panels to produce

Tanks Pressure for Simulation 4.

Fig. 7 – Batteries State of Charge and Tanks Pressure for Simulation 5.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 5 3 0 4 – 5 3 1 0 5309

hydrogen is calculated as a function of the solar energy

absorbed by the panels and the state of charge of batteries

directly connected to the application.

Fig. 8 shows the system evolution for this configuration.

The remote-telecom application is always fed, the system

only uses the 30% of whole auxiliary system and the fuel cells

only works 917 h per year.

As can be seen in the previous point, the best option is the

6th simulation. Both simulations 5 and 6 will lead to lowest

maintenance, lowest fuel cell use and no oversizing of FIRST

installation was assumed. Fifth simulation also produces good

results and its controller is simpler than assumed in simula-

tion 6, although auxiliary hydrogen consumption is higher. It

has also been shown the importance of the optimization of

Fig. 8 – Batteries State of Charge and

the solar radiation management. This is the same discussion

made in reference [9]. In this work, the optimal management

of the energy depends on the availability of energy sources:

when the amount of renewable source is high, the remaining

energy should be used to electrolyse water, but when the

available renewable source is not enough, the electrolyser

should be fed from the surplus of energy stored in oversized

batteries. The correct management of the solar radiation is

also dependent on the location: Zhou et al. [10] compared two

different locations, i.e. Inssbruck (Austria) and Singapore. The

higher radiation in Singapore made the configuration to be

well balanced (low fuel cell use) when the management

system consist of giving priority to the available route with

higher yield for electricity production. In the case of the lower

Tanks Pressure for Simulation 6.

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radiation of Innsbruck, higher use of the fuel cell is needed

because the availability of the solar resource is not as high as

in Singapore. When our system is compared to those proposed

in reference [10] there is an intermediate situation if the PV

panel area per kWh is compared, HIDROSOLAR_H2, with

9� 10�3 m2/kWh, is placed between Singapore (8� 10�3 m2/

kWh) and Inssbruck (1.2� 10�2 m2/kWh). On the other hand,

the needed power in the electrolyser is quite low in the Madrid

case: 2 Welec/Wapp versus 5.9 in Singapore and 7 in Inssbruck.

This difference is due to the different objectives of the two

cases: HIDROSOLAR_H2 minimize costs reducing the fuel cell

use and the size of the electrolyser, while the installations

assessed by Zhou et al. aim to avoid the use of auxiliary

hydrogen or, even, to produce it.

Once the reliability of the several options has been

confirmed through the achieved simulations, the future work

will comprise the comparison of the results of this paper with

the real performance of HIDROSOLAR_H2 installation. A

special focus will be done on the hydrogen flow behaviour

which can differ from simulated because of the low response

time of the fuel cell and the electrolyser [11] or due to the low

adsorption kinetics onto metal hydrides observed in other

installations [6].

4. Conclusions

The use of hydrogen technologies, i.e. electrolysis and fuel

cell, and its use in stand-alone remote-telecom applications

has been simulated. The surplus of solar photovoltaic energy

is stored as hydrogen, which is used in the fuel cell when the

energy production from the solar panels is not enough to feed

the application. TRNSYS simulation environment has been

used to model the whole system.

The hybrid system configuration allows hydrogen to be

used as the way to storage surplus of radiation, avoiding the

oversizing of the system. The most efficient performance has

been obtained when the management of energy is able to

decide how many panels are feeding each stage of the system.

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