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International Journal of Hydrogen Energy 29 (2004) 15711586

www.elsevier.com/locate/ijhydene

Review

Design and analysis of stand-alone hydrogen energy systemswith dierent renewable sources

Massimo Santarelli, Michele Cal, Sara Macagno

Dipartimento di Energetica, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy

Received 20 October 2003; received in revised form 7 January 2004; accepted 21 January 2004

Abstract

One of the most interesting developments of energy systems based on the utilization of hydrogen is their integration withrenewable sources of energy (RES). In fact, hydrogen can operate as a storage and carrying medium of these primary sources.

The design and operation of the system could change noticeably, depending on the type and availability of the primary source.

In this paper, the results obtained considering a model of a stand-alone energy system supplied just with RES and composed

by an electrolyzer, a hydrogen tank and a proton exchange membrane fuel cell are exposed. The energy systems have been

designed in order to supply the electricity needs of a residential user in a mountain environment in Italy during a complete

year. Three dierent sources have been considered: solar irradiance (transformed by an array of photovoltaic modules),

hydraulic energy (transformed by a micro-hydro turbine in open-ume conguration) and wind speed (transformed by a

small-size wind generator). It has been checked that, in that specic location, it is absolutely not convenient to use the wind

source; the solar irradiance has a nearly constant availability during the year, and therefore the seasonal storage of the RES

in form of hydrogen is the lowest; the availability of the micro-hydro source is less constant than in case of solar irradiance,

requiring a higher hydrogen seasonal storage, but its advantage is linked to the higher eciency of the turbine and the fact

that the RES is directly sent to the user with high frequency (for these reasons it is the best plant option).? 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Keywords: Photovoltaic; Micro-hydro; Wind; Hydrogen; Stand-alone energy system

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1572

2. Description of the components and structure of the plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1573

2.1. General description of the plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1573

2.2. Electricity request of the user . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1573

2.3. RES inputs of the energy systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1573

2.4. Description of the components of the plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1573

3. Design of the plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1577

4. Analysis of the plant operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1578

4.1. Energy analysis of the plant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1578

4.2. Electricity management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1580

4.3. Hydrogen management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1583

4.4. Brief cost considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1583

4.5. Notes on the integration of solar and hydro sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1585

5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1585

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1586

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1586

Corresponding author. Tel.: +39-011-564-4487; fax: +39-011-564-4499.

E-mail addresses: [email protected] (M. Santarelli), [email protected] (M. Cal), [email protected] (S. Macagno).

0360-3199/$ 30.00 ? 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijhydene.2004.01.014
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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1572 M. Santarelli et al. / International Journal of Hydrogen Energy 29 (2004) 1571 1586

Nomenclature

A constant of Tafel overpotential in PEMFC

characteristic V

A constant of the PV characteristic, 1/V

B constant in PEMFC characteristic, V

B coecient in the electrolyzer characteristic,

V

Cp capacity factor of the wind generator

E energy ows, MWh/yr

Er reversible voltage of the PEMFC, V

HS hydrogen system

I current (A)

I0 exchange current of the electrolyzer (A)

Iph photovoltaic current in the PV modules (A)

I current density in PEMFC (mA=cm2)

i limiting current density in PEMFC

(mA=cm2)

in internal and fuel crossover equivalent cur-rent density in PEMFC (mA=cm2)

I0 exchange current density in PEMFC

(mA=cm2)

L energy losses (MWh/yr)

LHV lower heating value (kJ/kg)

MHHES micro-hydro hydrogen energy system

PEMFC proton exchange membrane fuel cell

R cell resistance in the electrolyzer ()

RES renewable energy sources

Rd rotor diameter (m)

Rs series resistance in the PV modules ()

Rsh shunt resistance in PEMFC ()R area specic resistance in PEMFC

(k cm2)

SHES solar hydrogen energy system

TRY test reference year

The tower hub height (m)

V voltage (V)

Vc voltage of the single cell in PEMFC (V)

V0 reversible voltage in the electrolyzer (V)

VOC open circuit voltage in PV modules (V)

WHES wind hydrogen energy system

wcut-in cut-in speed (m/s)

wcut-out cut-out speed (m/s)

ws survival wind speed (m/s)

Greek

a alternator eciency of the micro-hydro tur-

bine

m reduction gear mechanical eciency of the

micro-hydro turbine

t eciency of the micro-hydro turbine

1. Introduction

The aim of this paper is the design and analysis of

stand-alone energy systems serving the electricity needs

(3 MWh=yr), during a complete year of operation, of

an isolated residential building situated in a selected site

(Valle dellEugio-Locana, North-West Italy). The systems

operate using a combination of a renewable source of

energy (RES) and a hydrogen system (HS: electrolyzer,

hydrogen storage, proton exchange membrane fuel cell or

PEMFC) without integration of traditional energy devices

based on fossil fuels. Over the past decade several RES-H2plants have been discussed in literature, both in technical

and economic terms [17]. Most of the RES-H2 demon-

strations to date have focused on photovoltaic (PV)-H2systems. Moreover, some operating experiences have been

developed through demonstration projects. The most im-

portant experiences found in the literature are: SAPHYS

project (I) [8], hydrogen generation from stand-alone wind-

powered electrolysis systems (I) [8, paragraph 9], SCHATZ

solar hydrogen project (USA) [8, paragraph 5], Markus

friedly residential house (CH) [8, paragraph 4], Phoebus

Julich demonstration plant (D) [9], PV-hydrogen system

for FC buses (E-USA) [10].

In this paper, three types of RES have been consid-

ered: solar radiation, hydraulic power and wind energy.

Each system uses as a primary source just one of the

mentioned RES. The rst system (SHES: solar hydrogen en-

ergy system) utilizes the solar radiation as primary source,

through an array of PV panels. The second system (MHHES:

micro-hydro hydrogen energy system) utilizes as primary

source the water ow of a creek (through a micro-hydro tur-

bine in open ume conguration). The third system (WHES:

wind hydrogen energy system) utilizes as primary source

the wind speed (used by a small-size wind generator). The

type of RES used determines the dierent design and per-

formance of the stand-alone system, and these results will

be discussed.

A developed simulation program allows us to design and

to analyze the models of the energy systems. After the de-

sign, the analysis has been developed for every hour of a

complete year of operation. The dierent hours of operationhave been considered as a succession of steady states. The

timestep of one hour is considered sucient for an analysis

developed over a whole year; this is because the RES pri-

mary sources of the plant do not change signicantly over

one single hour. Moreover, we do not have considered the

operation transitory among the steady states, as the paper is

devoted to the preliminary analysis of the management of

the systems over a complete year.

The purpose of the analysis is to highlight the dier-

ences between the systems using the three dierent types of

RES. The electric power produced or used by some compo-

nents will be shown as month and year cumulative values.


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M. Santarelli et al. / International Journal of Hydrogen Energy 29 (2004) 1571 1586 1573

Moreover, the hydrogen production and utilization will be

considered for each month of operation, along with the de-

sign of the storage tank dimension. Concerning the economic

considerations, a complete economic analysis of the SHES

has been discussed in another paper [11]. In this paper, only

the investment costs of the main components are provided

and discussed.

2. Description of the components and structure of the

plants

2.1. General description of the plants

The three plants here considered use a hydrogen energy

system (HS) to store the RES. The system is composed by

a variable number of electrolyzers (disposed in parallel), a

system used to dry the produced hydrogen, a pressurized gas

storage tank, a PEMFC, a battery pack and some auxiliaries.

The hydrogen is produced by the electrolyzers when theelectricity supplied by the RES is higher than the requests of

the user. The hydrogen stored is then utilized in a PEMFC

when the primary source of energy is not sucient to supply

the electricity needs of the user. The battery pack as well

is utilized for short-term energy supply. In this way, the

plants are designed to cover the whole electricity requests

in a complete year of operation [6,7].

The system model is shown in Fig. 1. It is identical for

the three plants; the only dierence is in the primary RES

and the device which converts it into electricity.

2.2. Electricity request of the user

Concerning the user, the prole of the requests of elec-

tric power has been dened using statistical data for a

small residential building of 500 m3 situated in the Valle

dellEugio-Locana (north-west of Italy in a mountain en-

vironment). The electricity requests have been evaluated

along one complete year of operation; for each month a

reference day has been constructed, and in the reference

day the electricity requests are given for every hour of op-

eration. The behavior of the electricity requests is described

in Fig. 2.

2.3. RES inputs of the energy systems

The solar irradiance input of the SHES plant (see Fig. 3)

is based on the test reference year (TRY) of the European

Community [12], which can be applied (with a negligible

approximation) to the specic location because the data

refer to the very close city of Torino. The solar irradiance

has been evaluated at an angle of 30 (optimum value of

PV inclination maximizing the electricity production in a

complete year).

The hydraulic source (see Fig. 4) is based on the evalu-

ation of the rainfall, of the snowmelt and of the hydrologic

behavior of the basin. The data have been supplied by the

Statistical Oce of the Regione Piemonte (Italy). The hy-

drograph represents the water ow input of the MHHES.

Concerning the wind speed source (see Fig. 5), the

data for the construction of the wind time series diagram

have been supplied by the Statistical Oce of the Regione

Piemonte (Italy). This represents the input of the WHES

plant.

2.4. Description of the components of the plants

The model of the PV modules refers to the wiring diagram

with one diode [13,14]; the characteristic currentvoltage

(IV) curve is expressed as

I= Iph I0{exp[a(V + RSI)] 1}

V + RSI

Rsh

: (1)

The parameters of the characteristic curve have been ob-

tained with measurement procedures (a 2-yr monitoring of

the PV array situated in our university), using the BP Solar

585/F PV module with a power of 85 Wp in standard con-ditions (solar irradiance 1000 W=m2 and cell temperature

25C).

The micro-hydro turbine selected in the MHHES is the

Neptune model of WaterTurbine. It is designed to work

in open ume conguration with low water ow rates. Its

runner type is Francis. We have used the characteristic curve

and other technical data given by the company selling the

WaterTurbine (IREM, an Italian company near Torino).

The wind generator considered in the design of the

WHES plant is the Fuhrlaender. It is three-blades and

horizontal-axes. We have used the characteristic curve and

other technical data given by the producer.The electricity produced, used and distributed among the

dierent components of the system is managed using a bus

bar with a voltage of 48 V, xed by the battery voltage. The

rationale of the management of the plant is based on a con-

trol system. The electric power produced from the RES is

distributed to the dierent elements of the plant following

a pre-dened priority order. The power request of the user

has the rst priority, and when available the RES power is

sent directly to the inverter. The excess of power is sent to

the other components of the plant: rst to charge the bat-

tery pack, and after to the electrolyzer to produce hydrogen.

When the PEMFC operates, the power produced is sent by

the control system directly to the inverter. The electricity is

dispatched among equipments by means of switches con-

trolled by the control system. Moreover, the use of some

DC/DC converters (placed between the bus bar and the

electric components of the system) has been considered, to

improve the operation of the components.

The characteristic currentvoltage (IV) curve consid-

ered for the electrolyzer is

V = V0 + b ln

I

I0

+ RI: (2)

The parameters have been dened using the experimental

data of a Norsk Hydro electrolyzer, communicated by Norsk


4/16


CONTROL

SYSTEM

PV

HYDRO

WIND

TECHNOLOGIES

RENEWABLE

SOURCE

1

FINAL

USERINVERTER

3

34

ELECTROLYZER

4

5

8

6

7

BATTERY

PACK

33

32

HYDROGEN

COOLER

11

12

HYDROGEN

DRYER

14

9

HYDROGEN

COMPRESSOR

16

18

HYDROGEN

TANK

17

HYDROGEN

HUMIDIFYER

20

DCDC

DCDC

2

DCDC

DCDC

27

DCDC

DCDC

PEM

FUEL CELL

36

AIR

HUMIDIFYER

24

23

2226

35

WATER

TANK

MEMBRANE

WATER

SEPARATOR

28

29

30

3137

19

25

2113

10

15

DC-Busbar 48 V

FLOW TYPOLOGY FLOW TYPOLOGY FLOW TYPOLOGY FLOW TYPOLOGY

1 Primary Energy 11 Air 21 H2O 31 Air

2 Electrical energy 12 Air 22 H2 + H2O 32 Electrical energy

3 Electrical energy 13 H2O 23 Air 33 Electrical energy

4 Electrical energy 14 Heat flow 24 Heat flow 34 Electrical energy

5 H2O 15 H2O 25 H2O 35 H26 O2 + H2O 16 H2 26 Air + H2O Heat flow

7 Heat flow 17 H2 27 Electrical energy 37 H2O

8 H2 + H2O 18 Electrical energy 28 Air + H2O

9 H2 + H2O 19 H2 29 Air + H2O

10 H2O 20 Heat flow 30 H2O

36

Fig. 1. Schematic model of the plant.

Hydro [15]. The type of electrolyzer modeled is a bi-polar

lterpress model. We do not consider the start-up transitory

of the electrolyzer, because the design and analysis is madeon an hourly basis; therefore, the operation temperature is

considered xed at 80C. The plants use small-size elec-

trolyzers (1 kWe each), which are disposed in parallel to use

the frequent low value of current due to the dierent avail-

ability of RES during the year. The drying of the hydrogen

produced by the electrolyzer is made in two steps: rst, the

stream is cooled to 30C and after it is sent to a drying sys-

tem based on chemical absorption with an absorber bed of

Al2O3; it uses two reactors: one is regenerated (heat ow

14 in Fig. 1) while the other absorbs the water [15]. The

dry hydrogen (stream 16 in Fig. 1) is then compressed to

30 bar and stored in a pressure tank.

The fuel cell modeled is a PEM; it uses air and hydrogen

humidied to 80C in two gurgle tanks to hydrate the Naon

membrane. Concerning the PEMFC, we have used litera-ture data [16,17]. The characteristic currentvoltage (IV)

curve is

Vc = Er (i + in)r A ln

i + in

i0

+B ln

1

i + in

i

: (3)

The excess hydrogen is circulated again. The output air and

water mixed ow (stream 28 in Fig. 1) is used for the

pre-heating and the external humidication of the input air

stream 23 (Fig. 1), through a membrane which allows the


5/16


0

200

400

600

800

1000

1200

1400

0 2 4 6 8 10 12 14 16 18 20 22

hour

Wh

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Fig. 2. Electricity requests of the user every month.

Fig. 3. Annual time series of the hourly solar irradiance in locana at 30.

transmission of water [17]. Then its residual water content

is recuperated (membrane water separation) and circulated

again to the gas humidiers (gurgle tanks); the excess wa-

ter produced by the PEMFC is nally sent to the water tank

connected to the electrolyzer.

In the wiring diagram of the systems there are, between

the components and the bus bar, DC/DC converters that

separate electrically the single components and the bus bar

xed at 48 V by the battery voltage. For example, the PV

operation point is xed in its maximum power point, and

then transformed through a DC/DC converter. The operation

voltage of the electrolyzer is evaluated through its charac-

teristic curve given the amperage, and therefore there is a

DC/DC converter between it and the bus bar. The operation


6/16


Fig. 4. Daily water ow (hydrograph) of the creek in Locana.

Fig. 5. Annual time series of the hourly wind speed in Locana.

voltage of the PEMFC depends on the electricity request,

and then is converted at 48 V through a DC/DC converter.

The battery pack is composed by two lead acid batteries

of 24 V each, with an average lifetime of 5 yr. We have

chosen 48 V because the power of the systems is limited,

and two batteries in series for safety; it represents also the

nominal inverter voltage. The battery pack is used to impose

the voltage to the bus bar (48 V), to supply power to the


7/16


Table 1

Technical data of the components of the plants

Photovoltaic module

Iph 4.6 (A) Photovoltaic ( short circuit) current

Rs 0:009() Series resistance

Rsh 100() Shunt resistanceVoc 21 (V) Open circuit voltage

a 1:46E 5(1/V) Characteristic constant

Micro-hydro turbine

t 0.75 Turbine eciency

a 0.8 Alternator eciency

m 0.95 Mechanical eciency

Wind generator

Rd 13 (m) Rotor diameter

The 27 (m) Tower hub height

wcut-in 2.5 (m/s) Cut-in speed

wcut-out 25 (m/s) Cut-out speed

ws 67 (m/s) Survival wind speed

Electrolyzer

V0 1.189 (V) Reversible voltage

b 0.4857 (V) Coecient of Tafel line

I0 2.946 (A) Exchange current

R 48:5E 6() Cell resistance

PEM fuel cell

Er 1.2 (V) Reversible voltage

in 2(mA=cm2) Internal and fuel crossover equivalent current

density

i0 0:067(mA=cm2) Exchange current density

i 900(mA=cm2) Limiting current density

A 0.06 (V) Constant of Tafel overpotential

B 0.05 (V) Constant of diusion overpotential

r 30E 6(k cm2) Area specic resistance

DC powered hydrogen compressor (energy more stable and

not dependent on the variable behavior of the electricity

produced by the RES), and to supply the load during the

night hours and during the electric transitory.

In Table 1 the technical data of the main components of

the plants are shown.

3. Design of the plants

The main criteria followed in the design activity have

been to supply the requests of the user every single hour of

the complete year without external integration, minimizing

at the same time the residual hydrogen mass stored at the

end of the year (to prevent an overdimension of the plant).

On this basis, the various components and the whole plant

have been designed.

We have written a code which simulates the systems;

though the code, we have developed rst the design and

after the analysis of the behavior of the systems.

The design has been made through mass, energy and

entropy balances written for every component and for the

whole system. The balances have been veried for every

single hour of a complete year.

The system operates in dierent conditions every hour

(dierent load and dierent RES production), and therefore

the various components operate always at partial load: their

operation point depend on their characteristic curve (e.g.,Eqs. (1)(3)).

The three systems use small size electrolyzers (1 kW

each), which are disposed in parallel and which operate de-

pending of the electricity supplied by RES. This solution

has been chosen in order to produce hydrogen also with

low values of electricity supplied by the RES. In fact, it

is well known that the electrolyzer cannot operate under

the 1520% of its nominal power (for safety reasons); this

fact could determine a large waste of electricity in case of

a single-stack large-size electrolyzer driven by RES with

frequent low values of electricity production. This solu-

tion determines a higher investment cost concerning the


8/16


Table 2

Design of the plants

SHES MHHES WHES

RES technologies PV array Micro-hydro turbine Wind generator

Number of modules: 75 Net head: 2:14 m Fuhrlaender

Total surface: 47 m2 Design water ow: 111 l=s Power: 30 kWPower: 6:37 kW Power: 1:40 kW

Electrolyzer Number: 5 Number: 2 Number: 34

Power: 1 kW=each Power: 1 kW=each Power: 1 kW=each

Surface area: 170 cm2 Surface area: 170 cm2 Surface area: 170 cm2

Number of cells: 20 Number of cells: 20 Number of cells: 20

PEMFC Number: 1 Number: 1 Number: 1

Power: 3 kW Power: 3 kW Power: 3 kW

Surface area: 150 cm2 Surface area: 150 cm2 Surface area: 150 cm2

Number of cells: 50 Number of cells: 50 Number of cells: 50

Storage of H2 Volume: 15 m3 Volume: 16 m3 Volume: 21 m3

(pressure 30 bar)

electrolysis section, but allows the reduction in size of other

components of the systems (in particular, the hydrogen com-

pressor and storage tank), because the hydrogen production

is more continuous. The choice of the size of each elec-

trolyzer has been done observing the distribution in fre-

quency, during the year, of the low values of electricity

ows produced by RES: we decided to dimension the elec-

trolyzer in order to obtain that the 20% of its nominal power

(wasted by the electrolyzer and the plant) would be a re-

duced value. After the choice of the model of electrolyzer,

its number depends on the system: it is due to the necessity

to produce enough hydrogen for the seasonal storage of the

RES to ensure the supply of the user needs.

The fuel cell size has been chosen to ensure the typical

power installed by contract in a residential user, which in

Italy is 3 kW. This size of the cell is sucient to ensure

the satisfaction of the user requests (maximum value in

the order of 1:5 kW); moreover, the cell operates far

from the maximum power point of its characteristic curve,

therefore with low current density (in the range 300

600 mA=cm2) and higher eciency.

The dimensions of the H2 storage has been dened ana-lyzing the cumulative curve of the hydrogen production and

utilization throughout the year, and considering the mini-

mum and the maximum point of the cumulative (classical

methodology for the dimension of tanks). We chose a stor-

age pressure of 30 bar to obtain a good compromise be-

tween the volumes of the tanks for a stationary user and the

electric power consumed by the hydrogen compressor.

The dimension of the hydrogen compressor has been de-

ned considering its peak power during the whole year,

and assuming a percentage increase for safety. In the dier-

ent plants, the compressor sizes are SHES 300 W, MHHES

100 W, and WHES 2 kW.

As described above, the battery packs is composed by

two batteries of 24 V in series. Its main utilization is to

supply energy to the hydrogen compressor. The capacity

of the batteries depends on the system (lower in the SHES

and MHHES, higher in the WHES), and has been dened

controlling the state of charge of the battery in every hour

of the year.

Table 2 presents the design results of the three considered

systems.

As we see, the WHES needs an enormous wind genera-

tor to supply the system (30 kW), compared to the capacity

of the PV array and especially of the micro-hydro turbine.

As we will discuss further, this is linked to the distribu-

tion and availability of the three primary RES in Locana.

Moreover, linked to the same reason, the WHES needs the

largest amount of small-size electrolyzers in parallel (34),

compared to the other two plants which require only 23

electrolyzers in parallel, and it needs the larger hydrogen

compressor (2 kW).

4. Analysis of the plant operation

4.1. Energy analysis of the plant

Fig. 6 and Table 3 show the cumulative annual energy

ows of the main components of the system. The ows

indicated with L represent the energy losses.

The ows E1, E2, E3, E4, E5, E6, E7 are electricity ows.

The ows E8 and E9 are the energy associated to the hydro-

gen ows, and have been evaluated using the LHV of hy-

drogen (3 kWh=Nm3). With Table 3 it is possible to evalu-

ate the average annual eciency of the main components of

the systems, shown in Fig. 7. The average annual eciency

of a component is evaluated as the ratio of its cumulative


9/16


E1

L1

E5

L4

E9

E4

L3

E4

E2

E8

E3

E7E6

L2

E0

L5

RENEWABLE

SOURCE

PV/MICRO-

HYDRO/WIND

TURBINE

CONTROL

SYSTEMINVERTER

PEM

FUEL CELL

HYDROGEN

ENERGY

CARRIER

ELECTROLYZER

BATTERY

PACK

HYDROGEN

COMPRESSOR USER

Fig. 6. Energy ows in the plants.

Table 3

Year cumulative of the energy ows

Year cumulative of the energy ows (MWh/yr)

Flows SHES MHHES WHES

E0 63.50 8.17 41.7

E1 6.96 6.51 8.38

E2 1.34 2.00 0.76

E3 5.16 4.22 7.05

E4 1.95 1.36 2.55

E5 2.98 2.98 2.98

E6 1:1E 1 7:38E 5 6:14E 4

E7 3:23E 2 0.26 0.42

E8 3.86 3.22 5.20

E9 3.73 2.57 4.80

L1 56.60 1.66 33.32L2 1.34 1.00 1.85

L3 1.83 1.21 2.25

L4 0.31 0.38 0.33

L5 2:2E 2 1:47E 5 1:23E 4

annual product and its cumulative annual energy input: it

represents not an instant value, but a mean value of the whole

year. Its denition depends on the component: as an exam-

ple, in case of PEMFC, the product is the electricity, and the

energy input is the hydrogen consumption times its LHV.

The rst column of Fig. 7 shows the average eciency ofthe device used in each system to convert the RES to elec-

tricity. The wind turbine works frequently during the year

with low values of wind speed. In fact, the wind regime does

not t well for a wind generator: it has low values for nearly

all the hours of the year, lower than the cut-in speed of many

commercial wind generators. Moreover, the wind turbine

operates frequently in the rst interval of the turbine power

characteristic, with the lowest values of electric power pro-

duction. This causes the low value of its average eciency

(the capacity factor Cp of this wind turbine is 12%).

The electrolyzers in the WHES system have an eciency

lower than the others system because these components

operate for a limited number of hours during the year, and

therefore with high values of current: this increases the over-

voltage losses.Even if the dierences in PEMFC eciency are very low,

it could be noted that the PEMFC in the system SHES has

a slightly lower average eciency because it operates more

frequently with higher values of current density to supply

the user (because it operates always during the high-energy

consuming evening hours) and this causes an increase of

overvoltage losses.

The average annual eciency of the chain of H2 (E3

E8E9E4) is in the order of 38% (not considering the

compressor energy), and this represents the eciency of the

storage of RES in hydrogen form.

The fourth column of Fig. 7 shows the ratio E5/E1, which

represents the wire-to-wire eciency of the whole plant. It is

an important parameter, because it shows how eciently the

renewably generated electricity is utilized inside the plant,

and which does not depends on the eciency of the renew-

able energy generator (aected by the type and availabil-

ity of renewable source, and therefore not controlled by the

plant technology). It shows that the WHES uses less amount

of primary energy directly, needs more H2 storage, and has

therefore the greatest wire-to-wire losses. The MHHES is

the most ecient because the renewable electricity is used

more directly.

The last column of Fig. 7 shows the total eciency of

the system starting from the primary renewable energy andthe nal product (electricity to the user), therefore taking

into account also the eciency of the renewable energy

generator. It adds an information which could be interest-

ing: how much of the solar irradiance, hydraulic energy of

a creek, wind speed is nally transformed in useful elec-

tricity in a complete year. The high value of the annual

eciency of the system MHHES (36.4%) is due to the high

value of the eciency of the micro-hydro turbine and the

nearly constant availability during the year of the primary

source (water ow of the creek) that allows to supply in

many occasion the user without storing the RES in form

of H2.


10/16


Annual average efficiency

0

10

20

30

40

50

60

70

80

SHES 10.9 74.0 51.0 42.8 4.7

MHHES 79.7 76.0 52.0 45.8 36.5

WHES 20.0 73.0 53.0 35.6 7.1

Device RES-Electricity

Electrolyzer PEMFC E5/E1 Total

(%)

Fig. 7. Annual average eciencies of the main components.

Fig. 8. Year cumulative of electric energy used and produced by the components.

The energy consumption of the auxiliaries (water pumps,

air blowers) are negligible compared to the other compo-

nents, and therefore they are not discussed.

4.2. Electricity management

The electricity produced or used during the complete year

(cumulative) by the main components of the three systems

is shown and compared in Fig. 8. The cumulative is the

sum, over the complete year, of the electricity ows used or

produced by the dierent components.

The results shown in Fig. 8 can be explained consider-

ing the utilization of the primary renewable energy by each

plant: percentage of primary energy sent directly to the in-

verter (E2), and percentage of primary energy sent to the

electrolyzer and the hydrogen system (E3). These results are

shown in Fig. 9. Moreover, to better understand the results

obtained, in Table 4 the operating hours during a completeyear of the main components of the systems are shown.

The dierence of the ow E1 (annual production of elec-

tricity from the RES) among the systems is due to the more

direct, and therefore more ecient, use of the RES by the

MHHES and the less direct/ecient use of the RES by the

WHES. As a result, the WHES requires more primary en-

ergy (E1) to meet end use demand. The minimum E1 value

(6:51 MWh=yr) is in the MHHES. This means that the en-

ergy system needs less energy from RES to cover the same

requests. In fact, the micro-hydro source, when available,

could be used more continuously than solar irradiance (for

example during the night) or the wind. In many occasions


11/16


RE-Electrolyser77.9%

SHES



MHHES



9 .2%


RE-Inverter

20.2%

RE-Inverter

30.2%

WHES

Fig. 9. Utilization of the primary RES inside each plant.

during the year, the micro-hydro electricity can therefore besent directly to the user, and it does not need to be converted

and stored in the form of hydrogen (see Fig. 9: the amount

of the RES electricity sent directly to the inverter is 30.9%,

compared to the amount sent to the electrolyzers: 65.1%).

This improves the electric eciency of the whole system.

The system WHES has the maximum value of the ow E1.

This is essentially due to the low wind speed values and the

distribution of the wind speed during the year. In fact, the

cut-in speed is achieved only 3125 h=p yr (35.7%), situated

in particular periods (rst months of the year). This obliges

the system to convert and store the RES in form of hydro-

gen, reducing the eciency of the whole system (as shown

Table 4

Operating hours for the main components of the systems

SHES(h/yr) MHHES(h/yr) WHES(h/yr)

RES technologies 4583 5688 3125

Electrolyzer 2871 4791 1730

PEMFC 3092 2703 6487

in Fig. 9, the amount of the RES electricity sent directly

to the inverter is 9.2%, compared to the amount sent to the

electrolyzers: 85.7%). The SHES has a behavior between

the other two plants: the annual amounts of electricity sent

to the inverter and the electrolyzers are, respectively, 20.2%

and 77.9% (see Fig. 9). The same results can be seen observ-

ing the PEMFC operation during the year. As we have noted

above, there is not a signicant dierence in the PEMFC

annual average eciency among the three plants, the dier-ence is in the PEMFC utilization. In the case of the SHES,

59% of the electrical load required from the nal user will

be given by this component, in the MHHES this value is

45%, while in the WHES is equal to 77%. This means that

the WHES needs a large conversion of primary RES into

hydrogen, reducing the system eciency; at the other side

there is the lower utilization of hydrogen made by the MH-

HES. (One could say that it is better to use as much as pos-

sible a high capital cost component such as the PEMFC. But

in these plants, the PEMFC is not considered the main en-

ergy producer, rather a necessary integration of the RES in

a seasonal scale, when the batteries are not su

cient, and itoperates when the RES is not available, not as much as pos-

sible. Its lower utilization means that the RES are directly

sent to the user with a higher frequency, allowing a better

design of the whole plant.)

Monthly cumulative data (Fig. 10) allow a better under-

standing of the problems related to the energy production

from uctuating renewable resources during a year.

In the SHES, in July the PV array is able to produce about

1100 kWhe (more than 1/3 of the annual needs of the user),

but in November, December and January it produces just

about 200 kWhe each month, less than the user needs in the

same months. In the MHHES, the character of seasonality

is emphasized: in the winter months the micro-hydro pro-duction is nearly absent (due to the formation of ice); con-

versely, it is very high in particular months (May, due to the

snowmelt; August and September, due to the rainfalls; nearly

95% of the annual user needs); therefore, the system needs

a seasonal storage of the RES and, as we will explain in the

Hydrogen management section, in this system the volume

of hydrogen stored in the tank is higher than in the SHES

in order to supply the energy system in winter month. The

seasonal behavior is even more accentuated in the WHES:

the electricity production is nearly absent in November and

December, and it is concentrated in the rst three months

of the year (50% of the annual production; 140% of the


12/16


Fig. 10. Monthly cumulative of electrical energy used and produced by the components.

annual user needs); therefore, the hydrogen seasonal stor-

age is fundamental, and the volume of hydrogen stored will

be the highest. Obviously, the variability of the behavior

of RES between the three plants in

uences the PEMFC

operation in the various months. In the SHES, the PEMFC

operates with a probably preferable more constant behav-

ior along the year, because of the solar radiation availability

and because during the evening hours the PV array never


13/16


produces electricity (in our design of the plant, the battery

pack does not meet part of the evening load, but this choice

has to be analyzed in order to optimize the plant). In the

MHHES and WHES, the operation of the PEMFC has more

uctuations, with high values during the winter months and

very low values in the summer months. This is due to the

high seasonality of the RES sources. At the same time, the

micro-hydro turbine and the wind generator operate also in

hours when the PV does not have electricity production (in

the evening hours), and therefore the PEMFC has to pro-

duce just a little amount of electricity. As a conclusion, the

PEMFC in case of SHES works during the whole year to al-

ways cover the important evening requests, operating there-

fore frequently with high value of current density (with in-

creases of voltage losses and consequently a loss of electric

eciency) that reduces its eciency and increases the need

of hydrogen (produced by the PV array, which therefore has

to be designed with a higher size). Moreover, higher current

densities likely mean less durability. To get out of this prob-lem this component could be substituted in the SHES with a

higher size PEMFC, or it could be better to increase the size

of the battery pack (to partially cover the evening requests)

instead of the PV array. Therefore, it could be interesting

the development of a mathematical optimization routine of

these models, using an economic objective function written

in order to minimize the sum of the costs of the four main

components of the system (PV array, electrolyzer, PEMFC

and battery pack).

4.3. Hydrogen management

Fig. 11 shows the hydrogen produced and used each

month in the systems.

The production of hydrogen varies greatly throughout the

dierent months. It depends directly on the availability of

the RES. In the SHES in the winter months the production

is low (on the order of 25 Nm 3=month), while in the sum-

mer months the production has an important increase (the

maximum production is in July, when it reaches the value of

225 Nm3=month). Also in the case of MHHES in the win-

ter months the production is low (less than 20 Nm3=month),

while in August the production has an important increase

(about 185 Nm3) due to rainfalls. In the WHES the max-

imum value of the production is obtained during March(about 307 Nm3), while in December the H2 is not produced

at all. Also the hydrogen consumption seasonal variation

is interesting. In the SHES and WHES, the hydrogen con-

sumption seasonal variation is less signicant because the

PEMFC operates with a more constant load throughout the

year. In the SHES, the hydrogen consumed has an average

value of 100 Nm3=month during the year, with higher val-

ues in winter months. The lower consumption occurs in July,

when the PV panels operate more and the PEMFC is used

at a lower load. In the WHES, the hydrogen consumption

has an average value of 130 Nm3=month with higher values

in autumn months. The lower consumption occurs in Febru-

ary when the average wind speed is higher than the other

months. In the MHHES, the hydrogen consumption has a

higher variability throughout the year. It reaches a maxi-

mum value in January (189 Nm3=month), and a minimum

value in August when the RES supplies the user without the

integration of the PEMFC.

Fig. 12 shows the cumulative curve of hydrogen stored

in the tank during the year.

First, it has to be noted that we consider the systems

operating in stationary conditions in a generic year, that

is, we do not consider the initial lling of the tank before

starting the rst year; in this way, the curves have to be seen

as periodic over every year. The net balance at the end of

the year is positive in all the systems (criteria imposed in the

design): the balance of hydrogen is 44 Nm3 in the SHES,

217 Nm3 in the MHHES and 133 Nm3 in the WHES. The

indicative dimensions of the hydrogen storage tanks (peak

storage volume at 30 bar) are, respectively, 15 m3 (SHES),

16 m3 (MHHES) and 25 m3 (WHES). Also, the cumulativecurve of the MHHES and especially of the WHES is less

regular than the SHES. This is due to the greater variability

of the RES for MHHES and WHES.

The hydrogen volume at the end of the year is dierent. In

fact, the size of the RES generators (PV array, micro-hydro

turbine and wind generator) is not continuous but discrete,

and therefore it is impossible to guarantee the same hydrogen

volume at the end of the year. It is interesting to note that the

primary generator for the SHES is the most modular. This

allows careful sizing of this system which leads to ecient

use of the hydrogen storage (i.e. nearly all of the stored

hydrogen is used up by the end of the year). In the MHHESand WHES, it is harder to size the primary generator to

closely match system needs. The primary generator must

be oversized and this results in excess stored hydrogen and

therefore a less ecient use of the hydrogen storage. This

aspect has to be optimized in the nal choice of the best

solution.

4.4. Brief cost considerations

In this paper, only the investment costs of the main

components are provided and discussed in order to obtain

the total cost of investment of the systems. A detailed

thermoeconomic analysis of the SHES has been performedin [11]. In the present Italian market situation, the invest-

ment costs of the main components considered are: PV

array 6200 =kWe (data of several Italian PV distributors);

micro-hydro turbine 2500 =kWe (data of a micro-hydro

distributor near Torino, named IREM); wind turbine

2500 =kWe (data coming from Fuhrlaender); electrolyzer

3000 =kWe (data coming from small-size electrolyzer

of Norsk Hydro); PEMFC 6000 $=kWe (data coming

from Italian distributor of Avista PEMFC); hydrogen tank

1470 =m3 (data coming from Italian producer of industrial

gases Sapio). Table 5 resumes the investment cost of the

three systems.


14/16


SHES

0

50

100

150

200

250

300

350

0

50

100

150

200

250

300

350

0

50

100

150

200

250

300

350

1 2 3 4 5 6 7 8 9 10 11 12

month

month

month

produced consumed

produced consumed

produced consumed

MHHES

1 2 3 4 5 6 7 8 9 10 11 12

WHES

1 2 3 4 5 6 7 8 9 10 11 12

Hydrogen(Nm

3/mo

nth)

Hydrogen(Nm

3/month)

Hydrogen(Nm

3/month)

Fig. 11. Monthly cumulative of hydrogen consumed and produced.

The value shown in the last line of the Table 5 repre-

sents the total investment cost of the system. The cost of the

WHES is clearly higher than the cost of the other systems.

This is not due to the cost of the wind technology (that in

the present market is competitive) but to the operation of

the plant: as already mentioned the availability of the wind

source is very low during the year with high wind speed

values in few hours; this causes an oversize of the plant


15/16


-400

-200

0

200

400

600

800

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

hour

SHESMHHESWHES

Nm3

Fig. 12. Cumulative curve of the hydrogen in the tank.

Table 5

Plant investment costs

Cost of investment k

Component SHES MHHES WHES

RES technologies 40 3.5 75

Electrolyzer 9 6 102

PEM fuel cell 18 18 18

H2 tank 22.05 23.52 36.75

Total 89.05 51.02 231.75

forcing the adoption of a high power wind turbine and a high

number of electrolyzers (see Table 2). For these reasons

this plant is not competitive compared to the SHES and the

MHHES. But also the SHES and MHHES are not compet-

itive compared with traditional plants. It has to be consid-

ered that the systems are stand-alone and placed in a remote

area, eliminating the costs of the distribution medium volt-

age lines. Moreover, the investment cost of the components

(especially PV modules and PEMFC) could decrease in a

developed market situation. Finally, the systems have lower

problems of pollutant emissions: in case of internalization

of the external cost due to pollution (carbon tax, market

creation of trading of pollution permits) maybe the com-

petitiveness of these systems would increase compared totraditional systems based on the combustion of fossil fuels.

4.5. Notes on the integration of solar and hydro sources

In another paper [18], it has been discussed the integra-

tion of solar and hydro source. The addition of a PV array

to a micro-hydro system may reduce the need for a seasonal

storage, as the solar irradiance to some extent is compli-

mentary to the hydroelectric source: as an example, even

if in some winter days the hydro source is completely ab-

sent, the solar source is always available (at lower values);

besides, during the evening hours, when the solar irradi-

ance is absent, the hydro source is available. Moreover, the

use of micro-hydro generators can provide electricity at a

more economical cost than the PV. The design and anal-

ysis of the plant is described in the paper [18]. Moreover,

considering the case study of this paper, the examination of

Fig. 10 shows that in months 1 and 2, when wind electrical

energy production is high, solar electrical energy production

is low, and similarly in months 49 when wind electrical en-

ergy production is low solar electrical energy production is

high. Consequently, a hybrid system may be advantageous.

These considerations deserve a further study.

5. Conclusions

1. For every system, we have considered how the electricity

produced from RES is distributed inside the plant (the

quantity sent directly to the user, the quantity stored inform of hydrogen), both in the whole year and for every

month.

2. For every system, we have discussed when the hydrogen

is produced and used on a month basis, and also in a day

basis (for example, in the SHES it is used every evening

in the whole year, and this is not the same in the other

two systems).

3. For every system, we have considered the hours of oper-

ation of every component in the plant, and therefore its

mean utilization.

4. For every system, we have discussed the utilization of the

PEMFC in the plant (as an example, the PEMFC in caseof SHES works during the whole year to cover the high

evening requests of electricity, operating therefore with

high values of current density, with increase of voltage

losses and consequently a loss of electric eciency, in-

creasing the need of hydrogen produced by the PV array,

which therefore has to be designed at higher size); more-

over, higher current densities likely mean less durability.

5. WHES: In that specic location, it is absolutely not con-

venient to use the wind source to supply the user because

the average value of wind speed during the year is too

low. This causes an oversize of the system to exploit this

source (e.g. the largest number of electrolyzers in paral-

lel), low average eciency of the plant and consequentlyhigh investment costs.

6. SHES: The solar irradiance has a nearly constant

availability during the year, and therefore the required

seasonal storage of RES in the form of hydrogen is the

lowest; its problem is the very low eciency of the PV

modules (requiring a high PV surface and investment

cost, and a higher number of electrolyzers in parallel

compared to MHHES); moreover, the evening user re-

quests are fed by the PEMFC every day of the year,

reducing the whole system eciency (see conclusion 8).

7. MHHES: In that specic location, the availability of

the micro-hydro source is less constant than in the case


16/16


of solar irradiance, requiring a higher hydrogen seasonal

storage. The important advantages are: the higher e-

ciency of the turbine; the RES is directly sent to the user

with high frequency (highest E2 value and therefore low-

est E1 value). For these reasons, and the lowest cost, it

is the best plant option.

8. In all the plants modeled, in a complete year, the electric-

ity produced by the PEMFC is 38% of the input electric-

ity sent to the electrolyzer; this value indicates the mean

annual eciency of the conversion chain of the produc-

tion and utilization of hydrogen. The value is higher in

case of batteries, but batteries are not appropriate for sea-

sonal storage.

9. In the MHHES, the PEMFC has an operation behavior

very variable during the various months: it operates in

winter months, and it is inactive in summer; this com-

ponent works in winter with high values of current den-

sity, causing an increase of voltage losses and a loss of

electric eciency of the whole system; moreover, highercurrent densities likely mean less durability. Maybe, it

could be interesting to increase the size of the PEMFC in

this system, to make it operate at lower current densities.

10. The discussion has been developed on a case study to

quantify the energy analysis; but the principal aim of the

paper is to explain a methodology of analysis (see Fig.

6) which is general and could be used in every case of

design and analysis of such energy systems.

Acknowledgements

The authors wish to thank the Ministero Italiano

dellUniversita e della Ricerca Scientica e Tecnologica

(PRIN, 2001) for the support of the present work.

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