Optimal energy management strategy and system sizing method for stand-alone photovoltaic-hydrogen systems

ARTICLE IN PRESS I N T E R N AT 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

33 (2008) 477 – 489

Available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/ijhydene

Optimal energy management strategy and system sizing method for stand-alone photovoltaic-hydrogen systems Keliang Zhoua,b,, J.A. Ferreirab, S.W.H. de Haanb a

School of Electrical Engineering, Southeast University, Nanjing 210096, China EPP, Faculty of EWI, Delft University of Technology, Mekelweg 4, 2628 CD Delft, The Netherlands

b

art i cle info

ab st rac t

Article history:

Addressing the mismatch between the intermittent solar irradiation and the time-varying

Received 26 August 2006

load demand, the optimal energy management principle with its corresponding control

Received in revised form

scheme for stand-alone PVH2 system is investigated to achieve high energy efficiency.

10 September 2007

Based on the proposed optimal energy management strategy, a systematical system sizing

Accepted 27 September 2007

method is developed for determining the minimum capacity of the system components.

Available online 26 November 2007

Therefore the system hardware cost can be well assessed using the developed system

Keywords: Photovoltaic hydrogen Stand-alone power system Energy management System sizing

1.

sizing method. Case studies are carried out to verify the effectiveness of the proposed energy management strategy and system sizing method. Simulation results show that the investigation in this article provides a good straightforward solution to the pre-design and operation of stand-alone PVH2 systems. & 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

Introduction

Renewable forms of energy exist almost everywhere. However the problems of most renewable energy sources (RES) are the intermittence and the insufficient sureness of supply, and consequently the impossibility of fitting the time-varying load demand of the users in different locations. An alternative to overcoming the intermittence of RES, such as sun and wind, is to develop a stand-alone system where the excess electrical energy could be converted and stored efficiently. Hydrogen ðH2 Þ that has a high mass energy density can be stored for long periods without energy loss. This property makes H2 well suited for energy storage. H2 derived from the RES is a suitable fuel in places where conventional fossil fuels are expensive, e.g., remote islands, and there are decentralized electricity supply systems. A stand-alone system placed in the remote areas, such as a photovoltaic-hydrogen ðPVH2 Þ system, usually includes both short-term and long-term energy storage. A battery bank is usually used for short-term energy

reserved.

storage due to its high round-trip efficiency, fast charging/ discharging capacity for smoothing the fluctuations of both the load and the RES. However, since the battery bank has low energy density, self-discharge and leakage, it is not suitable for long-term energy storage. Therefore the combination of a battery bank with a H2 storage bank can significantly improve the performance and reliability of the stand-alone RES systems. The stand-alone PVH2 systems have been widely investigated around the world [1–9] over the past decades. Since the intermittencies of both the RES and the power demand significantly depend on their locations and seasons, it is a challenging issue how to optimize system energy management and appropriately size the system components for achieving high energy efficiency, rational system cost, etc. The simulation results in [10] show the importance of the control strategy in the stand-alone PVH2 systems: the electrolyzer in variable current mode yields significantly higher energy efficiency than the electrolyzer in fixed current

Corresponding author. Now at: School of Electrical Engineering, Southeast University, Nanjing 210096, China. Fax: +86 25 83792696.

E-mail address: [email protected] (K. Zhou). 0360-3199/$ - see front matter & 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.09.027

ARTICLE IN PRESS 478

I N T E R N AT 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

Nomenclature

SoCH2 SoCBT ZA ZB ZC ZD ZPV ZMP ZD1 ZD2 ZEL ZFC ZBT ZIN IS ISmax ISYavg PS

state of charge of H2 tank state of charge of battery transmission efficiency of route A transmission efficiency of route B transmission efficiency of route C transmission efficiency of route D PV panel efficiency Conversion Efficiency of MPPT efficiency of DC/DC converter 1 efficiency of DC/DC converter 2 electrolyzer efficiency fuel cell efficiency round-trip efficiency of battery efficiency of DC/AC converter solar irradiation maximum solar irradiation over a year average solar irradiation over a year

PFC PFCr PEL PELr PLoad PLMmax PLYavg ELup ELlow FCup FClow APV EBT VHT MBT MPV MEL MFC MHT

33 (2008) 477 – 489

output power of fuel cell rated power of fuel cell input power of electrolyzer rated power of electrolyzer end-use load power demand maximum monthly averaged load average load power over a year upper SoCBT for electrolyzer lower SoCBT for electrolyzer upper SoCBT for fuel cell lower SoCBT for fuel cell area of PV panel energy capacity of battery volume of H2 tank margin coefficient for battery margin coefficient for PV panel margin coefficient for electrolyzer margin coefficient for fuel cell margin coefficient for H2 tank

power generated by PV panel

mode does; the lower output power setting of the fuel cell leads to better efficiency, etc. But it is not addressed how to develop an optimal energy management strategy for achieving high energy efficiency in the stand-alone PVH2 systems in [10]. Only based on the daily power and energy estimation, Chapter 12 in [11] presents a simple method for sizing PV panel and battery. However, due to the serious mismatch between the RES and the demand, it is difficult to properly size the system components in the stand-alone PVH2 systems. In this article, taking the mismatch between the intermittent solar irradiation and the time-varying load demand into account, an optimal energy management strategy is proposed for the stand-alone PVH2 systems. Based on the optimal energy management, a system sizing method is developed. Case studies are provided to verify the validity of the proposed methodology.

2.

System description

2.1.

Configuration

A very possible stand-alone PVH2 system is shown in Fig. 1, which consists of a PV panel with maximum power point trackers (MPPT) for solar energy conversion, a pressurized advanced alkaline electrolyzer with a DC/DC converter for H2 production, a pressurized tank for seasonal H2 storage, fuel cells with a DC–DC converter for H2 utilization, a lead-acid battery bank for daily electricity energy buffer, and a DC/AC inverter for the user load [4–6]. PV panel converts solar irradiations into electricity. For the increase of overall system efficiency, the DC/DC converter with MPPT enables the PV panel to work at the maximum power point in the highly fluctuated environment. The conversion efficiency of most prevalent silicon solar cells is about 10–30% at ambient temperature 25  C [5,12].

Since the large fluctuations in power are associated with the solar irradiation, a battery bank serves as an instantaneous and daily energy buffer for storing the fluctuating power coming from the PV-array. The battery bank smoothes the PV output, and eliminates the intermittence. What is more, the battery bank provides electricity for the daily operation of the control unit and auxiliary devices. The round-trip efficiency of the deep-cycle lead-acid battery is up to 90%. Overcharge (e.g., SoCBT 495%) and over-discharge (e.g., SoCBT o20%) should be avoided to protect the battery from being damaged. For an electrolyzer, the H2 generation rate is proportional to the current into the water electrolysis. For space saving and better system performance, H2 will be produced and stored under high pressure. The efficiency of the pressurized electrolysis is usually up to 88% with current density of several hundred mA=cm2 . In addition, if input power is less than 15–20% of its rated power, the electrolyzer is apt for yielding a mixture of H2 and O2 . For safety reasons, the minimum input power for the electrolyzer should be above 15–20% of its rated power. Electrolyzer can be operated with fixed or variable input current (or power) [5,10]. If neither the PV panel nor the battery can provide sufficient electricity, the fuel cell will utilize H2 to produce electricity for the load. The fuel cell needs a DC/DC converter to transform its output voltage level to the DC bus voltage. Since the fuel cell cannot promptly follow a sudden load change and its output voltage changes slowly, it behaves as a constant current/power source. The efficiency of the fuel cell is usually about 45–65%. A microprocessor based control unit is used to monitor the status of all PVH2 system devices, control and protect them, and coordinate the overall system operation. The efficiency of the DC/DC converters is up to 95%. The efficiency of DC/AC converter is up to 90%. Note that the electrolyzer and the fuel cell will not be operated simultaneously.

ARTICLE IN PRESS I N T E R N AT 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

Control Signals

Solar

Monitor Signals Control Unit and Auxiliaries

PV Panel

DC-DC Converter (MPPT)

479

33 (2008) 477 – 489

Battery

Power User

DC-AC Converter

Electricity DC BUS Compressed Air DC-DC Converter

DC-DC Converter

Fuel Cell

H2 H2

Water

Hydrogen Tank

Electrolyzer

Fig. 1 – A stand-alone PV-hydrogen system.

2.2.

System objective

The primary objective of such a PVH2 system is to provide sufficient and reliable electricity to meet the end-use power demand, and store the excess energy into the battery and H2 . Furthermore, for the sustainability, after one or several years’ continuous operation, without any operation collapse, the final energy residue stored in both the battery and H2 should exceed its initial value. For a sustainable stand-alone PVH2 system, all the energy required by user load comes from the solar irradiation via the PV panel. Excluding the partial solar power for meeting the certain load demand, the excess solar power will be eventually stored in both battery and H2 tank. In practice, the energy capacity of the H2 tank is usually much higher than that of the battery. Therefore, the final amount of H2 in the storage tank is the key performance index for energy efficiency [10], i.e. the larger the amount of H2 in the storage tank is, the higher the energy efficiency is. The energy efficiency is significantly determined by the energy management strategy for the stand-alone PVH2 systems.

3.

Optimal energy management

Fig. 2 shows the corresponding diagram of energy flows within the stand-alone PVH2 system. The direct transmission, the battery bank for short-term energy storage and the H2 tank for long-term energy storage should be complementarily operated to provide sufficient and reliable electricity from the solar energy to the end-use load. Obviously, how to

regulate the energy flows has significant impact on the energy efficiency of the overall system.

3.1.

Optimal energy management principle

In the PVH2 system as shown in Fig. 2, from the solar energy to the end-use load, there are four routes A–D for energy flows that are plotted in Fig. 3. According to the efficiency of each system component, the transmission efficiency Zi (subscript i ¼ A, B, C, D) for the energy flow routes can be ranked as: ZA 4ZB 4ZC 4ZD , where ZA ¼ ZPV  ZMP  ZIN , ZB ¼ ZA  ZBT , ZC ¼ ZPV  ZMP  ZD1  ZEL  ZFC  ZD2  ZIN , ZD ¼ ZC  ZBT . Therefore, in order to keep the overall system operation efficiency as high as possible, the optimal energy management strategy for the stand-alone PVH2 system can be described in the following three scenarios encountered in the system operation: (1) When the instantaneous electrical power PS generated by the PV panel exceeds the demand of the end-use load PLoad , route A will be employed to transfer electricity from the PV panel to the end-use load. At the same time, if the battery is under-charged (i.e. SoCBT is low), the excess power will be used to charge the battery via route B; if the battery is fully charged (i.e. SoCBT is high), the excess electricity would be inputted into the electrolyer to produceH2 via route C or D. (2) When PS is insufficient for the demand of PLoad and SoCBT is high, PS will also be directly sent to the end-user via route A. And the battery will provide the electricity to compensate the deficit power via route B.

ARTICLE IN PRESS I N T E R N AT 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

Solar Irradiation

ηIN

PSD Direct Transmission PS (W)

MPPT

IS (W/m2)

ηMP

PV Panel with Area APV (m2)

ηPV

33 (2008) 477 – 489

Solar Power

DC/AC Converter

480

PBT

PSB

Battery Bank

PFB

PFD

ηBT

PSE

Pload (W) End-Use Load

PFC (W) ηD1

DC/DC Converter 1

DC/DC Converter 2

ηD2

ηEL

Water Electrolyzer

Fuel Cell

ηFC

Hydrogen Tank Fig. 2 – Energy flows within stand-alone PV-hydrogen systems.

a Solar

ηPV

ηMP

ηIN

PV

MPPT

DC/AC

End-User

Route A

b Solar

ηPV

ηMP

ηBT

ηIN

PV

MPPT

Battery

DC/AC

End-User

Route B

c Solar

ηPV

ηMP

ηD1

ηD2

ηIN

PV

MPPT

DC/DC

DC/DC

DC/AC

ηEL Electrolyzer

Fuel Cell

End-User

ηFC

Route C

d Solar

ηPV

ηMP

ηD1

ηD2

ηBT

ηIN

PV

MPPT

DC/DC

DC/DC

Battery

DC/AC

ηEL Electrolyzer

Fuel Cell

End-User

ηFC

Route D Fig. 3 – Four energy flow routes.

(3) When PS is insufficient for the demand of PLoad and SoCBT is low, the fuel cell will be switched on to utilize the stored H2 to generate electricity PFC to provide the deficit power via route C or D. Generally speaking, in any possible operation scenario, higher priority will be assigned to the route with higher transmission efficiency for the energy flow. Above descriptions reveal the basic principle of the optimal energy management strategy for the stand-alone PVH2 systems.

3.2.

Corresponding control scheme

In practice, a double hysteresis loop control scheme [10,13] as shown in Fig. 4 is usually used to regulate the energy flows of the PVH2 system. The hysteresis loops are used to prevent the electrolyzer and the fuel cell from being switched on/off too frequently. Obviously the on/off switching actions of the electrolyzer and the fuel cell will be determined by SoCBT, where ELup , ELlow , FCup , and FClow are the key control parameters. Furthermore, since the fuel cell behaves as a

ARTICLE IN PRESS I N T E R N AT 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

constant current/power source, it would be operated in the fixed output current/power mode. And the fixed output power setting PFC for the fuel cell is another key control parameter. It is very clear that the depth of discharge (DoD) of the battery is ðELup  FClow Þ. The available active energy capacity of the battery is determined by ELup and FClow . Therefore, within all the operation constraints, to fully exploit the battery energy capacity, ELup (less than the saturation level, e.g., 95%) would be set as high as possible, and FClow (greater than the over-discharging level, e.g., 20%) would be set as low as possible. Usually ðELup ; ELlow Þ ¼ ð0:9; 0:8Þ or (0.85, 0.75) and ðFCup ; FClow Þ ¼ ð0:45; 0:35Þ or (0.5, 0.4) are the appropriate parameters in the applications. The electrolyzer can be flexibly operated in either the fixed or the variable input current/power mode. However, if the electrolyzer is operated in the fixed input current/power mode, high priority would be assigned to the low energy efficiency route C or D whether PS and SoCBT are satisfied with the optimal energy management strategy described in scenarios (1) and (2) in Section 3.3.1 or not. Therefore, following the optimal energy management strategy, the electrolyzer will be operated in the variable input current/ power mode. Simulation results in [10] have demonstrated that the electrolyzer in the variable power mode leads to a higher energy efficiency of PVH2 system than the electrolyzer in fixed power mode does. When PS is insufficient for the demand of PLoad and SoCBT is low, the fuel cell will be switched on to output a fixed power PFC to compensate the fluctuated deficit load power via route C or D. For the safety of the battery, the setting of PFC for the fuel cell should theoretically at least exceed the daily averaged deficit load power. However, if the setting of PFC is too low, the battery could sometimes be over-discharged; if PFC is too high, the overall system energy efficiency will be reduced due to the overuse of route D, and the corresponding cost of the fuel cell will be unnecessarily increased. In practice, for making a trade-off between the overall system efficiency and the safe operation

Battery protection (upper)

Electrolyzer On

ELup

Battery SOC

ELlow

481

of the battery, PFC can be initially set to be equal to the maximum monthly averaged load power PLMmax over one typical year.

4.

System sizing

For designing a stand-alone PVH2 system with a required capacity, it is a must to properly size the system components. Oversized system components will unnecessarily increase the system cost; undersized system components will cause the power blackouts. However, due to the serious mismatch between the fluctuated solar irradiation and the time-varying end-use load, it is challenging for designers to determine the minimum secure system size for providing sufficient power or energy capacity. Based on the above optimal energy management strategy, a systematic system sizing method is developed in the following subsections.

4.1.

Power and energy requirement

It is the first step to estimate the energy consumption for the system sizing. For a specific customer, its statistical load data over a typical year will be collected. The end-use loads normally include lighting, TV, other home appliances, and auxiliaries for the PVH2 system, where PLMmax is the maximum monthly averaged load demand over one year. Fig. 5 shows an example of the daily and seasonally fluctuated load demand. Although the load power is highly timevarying, the yearly averaged load power PLYavg does not change significantly year by year.

4.2.

Battery sizing

In the double hysteresis loop control scheme in Fig. 4, DoD of the battery is ðELup  FClow Þ. The battery energy capacity EBT for short-term energy buffer can be determined by EBT ¼

100%

33 (2008) 477 – 489

PLMmax  24 h MBT ðWhÞ, ðELup  FClow Þ

(1)

where the margin coefficient MBT is usually chosen to be 1–3 for effectively smoothing the daily mismatching between the solar power PS and the load power PLoad for a short term (e.g., several days). If MBT becomes small, fuel cells will be switched on/off more frequently; if MBT is large, the battery can provide long-time energy buffer but at correspondingly increased cost.

Electrolyzer Off

4.3.

PV panel sizing

Fuel Cell Off FCup

FClow Fuel Cell On

Battery protection (lower) 0% Off

On

Fig. 4 – Double hysteresis control scheme.

To sustain its continuous operation, the stand-alone PVH2 system must at least maintain the energy balance over years. It means that the final total energy stored in both the battery and H2 should be at least equal to its initial value. Besides the time-varying load demand, the input solar irradiation to the PV panel is not only highly intermittent daily, but also significantly varies with both locations and seasons. Fig. 6 shows a fluctuated solar irradiance profile. The PV panel size

ARTICLE IN PRESS 482

I N T E R N AT 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

33 (2008) 477 – 489

Maximum Monthly Averaged Load PLMmax

Load Power (W)

Monthly Averaged Load (W)

PLoad

Average load PLDavg

1

Time (Hour)

Yearly Average load PLYavg

24

1

Time (Month)

12

IS

Daily Average Irradiation ISDavg

1

Time (Hour)

24

Monthly Averaged Solar Irradiation (W/m2)

Hourly Averaged Solar Irradiation (W/m2)

Fig. 5 – Fluctuated load power demand.

Maximum Solar Irradiation ISmax IS

Yearly Average Irradiation ISYavg

1

Time (Month)

12

Fig. 6 – Fluctuated solar irradiation profile.

can be determined through the following steps:

1. Assuming that all the end-user load power PLoad come directly from solar power PS via route A, the estimation of the initial area A1 of PV panel will be

A1 ¼

PLYavg PLYavg ¼ ðm2 Þ. ISYavg  ZA ISYavg  ZPV  ZMP  ZIN

(2)

optimal energy management strategy, during the longterm intervals (t0 , t1 ) and (t2 , t3 ) in Fig. 7, the deficit load power since (PLoad =ZIN  PSA1 Þ will be provided by the fuel cell and the battery via route C or D; during the long-term interval (t1 , t2 ) in Fig. 7, the excess solar power (PSA1  PLoad =ZIN ) will be converted to H2 and the battery energy via route C or D. Note that the short-term mismatch is smoothed by the battery via routes B and D. The required PV panel area APV can be calculated by

0

APV

1 1 R t2 ð1  Z  Z  Z  Z ÞðP  P =Z Þ dt SA Load D1 EL FC D2 IN 1 t¼t B C 1 P C    LYavg  þ t3  t0 ¼ MPV B @ A 1 1 ZC ZD ISavg  ISYavg  ðZA þ ZB Þ þ 2 2 ZIN ZIN 0 1 1 R t2 ð1  ZD1  ZEL  ZFC  ZD2 ÞðPSA1  PLoad =ZIN Þ dtC t¼t B 1 MPV t  t0 C ðm2 Þ,  BA1 þ 3 ¼ A ISavg  ZPV  ZMP  ZD1  ZEL  ZFC  ZD2 ð1 þ ZBT Þ=2 @

2. However, due to the mismatch between PS and PLoad , there are additional energy losses in the round trip energy transfers via route B, C, or D. Based on Fig. 5 and initial area A1 , the mismatch between solar power PSA1 and load power PLoad is shown in Fig. 6. According to the proposed

ð3Þ

where PSA1 ¼ IS  A1  ZPV ZMP ; the margin coefficient MPV is greater than 1.1 to ensure the sufficient capacity of the PV panel and the sustainability of the PVH2 system. MPV ¼ 1:0 means the calculated result in (3) is the minimum required size of the PV panel, that is, the final SoCH2 in the H2 tank

ARTICLE IN PRESS I N T E R N AT 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

Solar Power PS = IS⋅APV⋅ηPV⋅ηMP

Solar Power PSA1 = IS⋅A1⋅ηPV⋅ηMP

Load Power PLoad /ηIN

t1

Time

Average Load Power PLMavg

Power (Watt)

Power (Watt)

Yearly Average Load PLYavg

t0

t2

t3

Load Power PLoad / ηIN

t0

t'1

1 Year

after one typical year’s operation will be approximately equal to its initial value.

Electrolyzer sizing

The rated power PELr for the electrolyzer can be calculated by PELr ¼ MEL  ISmax  APVr  ZPV  ZMP  ZD1 ,

(4)

where the margin coefficient MEL ¼ 0:720:9. The maximum current density of commercial electrolyzer for efficient operation is normally less than 1 A=cm2 . And the rated current density of electrolyzer is usually 0:320:5 A=cm2 [6,13–15]. MEL o1 means overload capacity of the electrolyzer and less component cost. Since the input power of the electrolyzer should exceed 15–20% of its rated power, the maximum rated power of the eletrolyzer will be less than ðISmax  APVr  ZPV  ZMP  ZD1 Þ=ð0:1520:2Þ.

4.5.

Fuel cell sizing

(5)

where the margin coefficient MFC is usually 2–3.

4.6.

t'2

t3

Fig. 8 – Mismatching between solar power and end-use load. where HPT is the storage pressure of H2 tank in bars, the margin coefficient MHT will be greater than 2–3 for accommodating the seasonal SoCH2 fluctuation and initial H2 storage. The above system sizing method can also be employed into other stand-alone renewables-H2 systems. With the proposed system sizing method based on the optimal energy management, we can appropriately design the stand-alone PVH2 system and exactly assess the hardware cost of the system. Assume the unit prices are hx=m2 for PV panel, hy per Wh for battery, hz per W for electrolyzer, hm per W for fuel cell, hn per m3 for H2 tank, and h p for power converters and the control unit, the total hardware cost of a stand-alone PVH2 system will be Cost ¼ x  APV þ y  EBT þ z  PELr þ m  PFCr þ n  VHT þ p ðEurosÞ.

5.

According to the optimal control scheme in Section 2.3.2, the initial output power setting for the fuel cell could be equal to PLMavg , therefore the rated power PFCr of fuel cell could be determined by PFCr ¼ MFC  PLMmax =ZIN ðWÞ;

Time 1 Year

Fig. 7 – Initial estimated solar power and end-use load.

4.4.

483

33 (2008) 477 – 489

Hydrogen tank sizing

According to the ideal gas law, the heat density of H2 is 237 kJ/mol or 65.8 Wh/mol, and 1 molð¼ 2 gÞ H2 gas at 25  C is 22.4 L. From Fig. 8, according to the optimal energy management strategy, the volume of H2 tank can be determined by VHT ¼ 0:0224MHT R t02 ððIS  APV  ZPV  ZMP  PLoad =ZIN Þ  ZD1  ZEL Þ dt t¼t01  ðm3 Þ, 65:8HPT

Case studies

Both the load power demand and the solar irradiation vary with the locations and seasons. To verify the proposed energy management strategy and the system sizing method, Singapore ð1 NÞ and Innsbruck, Austria ð47:27 NÞ, are chosen as the two distinct locations for setting up self-sufficient houses with stand-alone PVH2 systems. TRNSYS 16 is employed to be the simulation platform. In the simulations, the efficiency of silicon PV panel is ZPV  10%, the efficiency of alkaline electrolyzer is ZEL  0:85, the efficiency of alkaline fuel cell is ZFC  0:55, the efficiency of lead-acid battery ZBT  0:9, and other efficiencies ZD1 ¼ ZD2  0:95, ZIN  0:9, ZMP  0:95. For the double hysteresis loop control scheme, the control parameters are chosen as: ELup ¼ 0:9, ELlow ¼ 0:8, FCup ¼ 0:5, FClow ¼ 0:4.

5.1.

ð6Þ

ð7Þ

System sizing

Fig. 9 shows the collected solar irradiation data over one typical year for the two places. Since the solar irradiations are

ARTICLE IN PRESS 484

I N T E R N AT 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

highly intermittent, the monthly averaged solar irradiation is plotted in Fig. 10 for simplifying the system sizing calculation. Fig. 11 shows the monthly averaged residential house

33 (2008) 477 – 489

electricity demand (including auxiliary power for the operation of the PVH2 system itself). The key data in Figs. 9–11 are listed in Table 1. Following the system sizing method in

Singapore

Solar lrradiation (W/m2) 0

1100 1000 900 800 700 600 500 400 300 200 100 0

1000 2000 3000 4000 5000 6000 7000 8000 Time (Hour)

0

1000 2000 3000 4000 5000 6000 7000 8000 Time (Hour)

Fig. 9 – Solar irradiation over one typical year.

Singapore

Innsbruck, Austria 220 Monthly Average Solar Irradiation

180

Solar Irradiation (W/m2)

Solar Irradiation (W/m2)

200 160 140 120 Average Solar Irradiation

100 80 60 40 20 0 1

2

3

4

5 6 7 8 Time (Month)

9

220 200 180 160 140 120 100 80 60 40 20 0

10 11 12

Average Solar Irradiation

1

2

3

4

Monthly Average Solar Irradiation

5 6 7 8 Time (Month)

9

10 11 12

Fig. 10 – Monthly averaged solar irradiation.

Innsbruck, Austria

Singapore

700

700

600

600

Monthly Average Load Power

500 400 Average Load Power

300

Load Power (W)

Load Power (W)

Solar lrradiation (W/m2)

Innsbruck 1000 900 800 700 600 500 400 300 200 100 0

Monthly Average Load Power

400 300

200

200

100

100

0

Average Load Power

500

0 1

2

3

4

5

6

7

8

Time (Month)

9

10 11 12

1

2

3

4

5

6

7

8

Time (Month)

Fig. 11 – Monthly averaged residential house electricity demand.

9

10 11 12

ARTICLE IN PRESS I N T E R N AT 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

485

33 (2008) 477 – 489

Table 1 – Load demand and solar irradiation Item

Innsbruck, Austria

Singapore

517 680 4530

630 685 5520

Average irradiation ISavg ðw=m2 Þ

140.8

183.7

Maximum monthly irradiation ðw=m2 Þ

210.7

222.2

Minimum Monthly irradiation ðw=m2 Þ

50.6

170

Maximum irradiation ISmax ðw=m2 Þ

968

1050

Innsbruck

Singapore

65

65

PV panel APV ðm Þ MPV ¼ 1:0 MPV ¼ 1:121:3

57 69

45 51

H2 tank VHT (200 bar) with MHT ¼ 2:5 ðm3 Þ

12

6

Electrolyzer power PELr with MEL ¼ 0:8 (kw)

5

4

1.5

1.5

Load power Average load (including auxiliaries) (w) PLMmax (w) Total energy consumption per year (kw h) Solar irradiation

Table 2 – System component size Component size Battery capacity EBT with MBT ¼ 2 (kw h) 2

Fuel cell power PFCr with MFC ¼ 2 (kw)

Area of PV Panel = 57 m2

Area of PV Panel = 57 m2 1

700

0.9

600

Fuel Cell Output Power

500

0.7 Power (W)

State of Charge

0.8 0.6 0.5 0.4 0.3

SoC of H2

0.2

400 300 200

SoC of Battery

100

0.1 0

0 0

2000 4000 6000 8000 10000 12000 14000 16000

0

2000 4000 6000 8000 10000 12000 14000 16000 18000

Time (Hour)

Time (Hour)

Area of PV Panel = 69 m

Area of PV Panel = 69 m2

2

1

700

0.9

600

Fuel Cell Output Power

500

0.7 Power (W)

State of Charge

0.8 0.6 0.5 0.4 0.3

SoC of H2

0.2

SoC of Battery

400 300 200 100

0.1 0

0 0

2000 4000 6000 8000 10000 12000 14000 16000 Time (Hour)

0

2000 4000 6000 8000 10000 12000 14000 16000 Time (Hour)

Fig. 12 – Two typical years’ operation with different PV panel size APV at Innsbruck (a) MPV ¼ 1, PFC ¼ PLMmax ; (b) MPV ¼ 1, PFC ¼ PLMmax ; (c) MPV ¼ 1:21, PFC ¼ PLMmax ; (d) MPV ¼ 1:21, PFC ¼ PLMmax .

ARTICLE IN PRESS 486

I N T E R N AT 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

Section 4, the sizes of the system components are figured out and listed in Table 2, where the MPV ¼ 1 means the corresponding PV panel has the minimum size for meeting the load demand, and MPV ¼ 1:121:3 means the corresponding PV panel has 10–30% extra capacity for producing H2 .

5.2.

Simulation results

Figs. 12–15 show the simulation results of the stand-alone PVH2 systems with different parameters at both Innsbruck and Singapore. All the key simulation result data are summarized in Table 3. In Figs. 12 and 13, we notice that when the PV margin coefficient MPV ¼ 1, the final SoCH2 are approximately equal to the initial SoCH2 after two years’ continuous operation at both locations; when MPV 41:1, the final SoCH2 are significantly greater than the initial SoCH2 after two years’ continuous operation at both locations. It means that MPV ¼ 1 is corresponding to the minimum PV panel size for the system energy sustainability. The PV sizing results in Table 2 provide the exact estimation of the required system size. Moreover, no operation constraints of the system components are violated in the operation. In Figs. 12–15, the final SoCH2 with PFC oPLMmax in Figs. 14 and 15(a) will be a little higher than the final SoCH2 with PFC ¼

PLMmax in Figs. 12 and 13(c); the final SoCH2 with PFC 4PLMmax in Figs. 14 and 15(c) will be a little lower than the final SoCH2 with PFC ¼ PLMmax in Figs. 12 and 13(c). That is to say, lower PFC leads to higher energy efficiency. But lower PFC may lead to lower SoCBT (o0:35) in the operation as shown in Figs. 14 and 15(a), and lower SoCBT may damage the battery. Therefore the simulation results indicate that PFC ¼ PLMmax is an appropriate setting for the fuel cell, which brings a good trade-off between the efficiency of the overall system and the safe operation of the battery. Note that the initial SoCH2 ¼ 0:4 in pressurized H2 tank is used to cope with the SoCH2 fluctuations and ensure the continuous operation of the system. And since the solar irradiation at Singapore is less seasonally fluctuated than that in Innsbruck, SoCH2 at Singapore is less seasonally fluctuated, and the fuel cell at Singapore is less frequently switched on/off too. Moreover, the initial SoCH2 at Singapore can be much less than that at Innsbruck.

6.

Conclusions

In terms of energy efficiency, the optimal energy management principle for stand-alone PVH2 systems is addressed: in any possible operation scenario, higher priority will be

Area of PV Panel = 45 m2

Area of PV Panel = 45 m2 1

700

0.9

600

Output Power of Fuel Cell

0.8 500

0.7 Power (W)

State of Charge

33 (2008) 477 – 489

0.6 0.5 0.4 0.3

400 300 200

0.2 SoC of H2

0.1

100

SoC of Battery

0

0 0

2000 4000 6000 8000 10000 12000 14000 16000

0

Time (Hour)

Time (Hour)

Area of PV Panel = 51 m

Area of PV Panel = 51 m2

2

1

700

0.9

600

Output Power of Fuel Cell

0.8 500

0.7 Power (W)

State of Charge

2000 4000 6000 8000 10000 12000 14000 16000 18000

0.6 0.5 0.4 0.3

300 200

SoC of H2

0.2

400

SoC of Battery

0.1 0

100 0

0

2000 4000 6000 8000 10000 12000 14000 16000 Time (Hour)

0

2000 4000 6000 8000 10000 12000 14000 16000 18000 Time (Hour)

Fig. 13 – Two years’ operation with different PV panel size APV at Singapore (a) MPV ¼ 1, PFC ¼ PLMmax ; (b) MPV ¼ 1, PFC ¼ PLMmax ; (c) MPV ¼ 1:13, PFC ¼ PLMmax ; (d) MPV ¼ 1:13, PFC ¼ PLMmax .

ARTICLE IN PRESS I N T E R N AT 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

487

Area of PV Panel = 69 m2

Area of PV Panel = 69 m2 1

600

0.9

Output Power of Fuel Cell

500

0.8 0.7 Power (W)

State of Charge

33 (2008) 477 – 489

0.6 0.5 0.4

400 300 200

0.3 SoC of H2

0.2

SoC of Battery 100

0.1 0

0 0

2000 4000 6000 8000 10000 12000 14000 16000

0

2000 4000 6000 8000 10000 12000 14000 16000

Time (Hour) Area of PV Panel = 69 m

Time (Hour) Area of PV Panel = 69 m2

2

1200

1 0.9 0.7 Power (W)

State of Charge

Output Power of Fuel Cell

1000

0.8 0.6 0.5 0.4

800 600 400

0.3 SoC of H2

0.2

SoC of Battery 200

0.1 0

0 0

2000 4000 6000 8000 10000 12000 14000 16000 Time (Hour)

0

2000 4000 6000 8000 10000 12000 14000 16000 18000 Time (Hour)

Fig. 14 – Two typical years’ operation with different output power PFC of fuel cell at Innsbruck (a) MPV ¼ 1:21, PFC oPLMmax ; (b) MPV ¼ 1, PFC oPLMmax ; (c) MPV 41:21, PFC ¼ PLMmax ; (d) MPV ¼ 1:21, PFC 4PLMmax .

assigned to the energy flow route with higher transmission efficiency. The corresponding control scheme with key parameters selection is elaborated:

 ðELup  FClow Þ is the DoD of the battery. The available active energy capacity of the battery is DoD*EBT .  The electrolyzer should be operated in variable power mode.  The fuel cell should be operated in fixed power mode. PFC ¼ PLMmax is a good setting for the fuel cell.

Based on the optimal energy management strategy, taking the mismatch between the intermittent solar irradiation and the time-varying load demand into account, a direct system sizing method is developed for pre-designing the size of the system components: PV panel, electrolyzer, H2 tank, fuel cell, and battery. In terms of hardware cost, the proposed system sizing method provides a quick and simple way to assess the economic viability of PV/H2 systems. Case studies are carried out to verify the effectiveness of the proposed energy management strategy and system sizing method. Simulation results show that our study provides a good solution to the general system design and optimal

operation of stand-alone PVH2 system in various parts of the world. It should be pointed out that, for more detailed system design and further optimization of system operation, more factors should be taken into consideration in detail, such as advanced control strategy, thermal impacts on the operation efficiency of critical components (e.g., PV, electrolyzer, battery, and fuel cell), optimum operation pressures for system components (e.g., electrolyzer, storage tank, and fuel cell), and so on. For example, an intelligent control algorithms, which is based on solar radiation and load forecasting, system state variables (e.g., component temperatures), etc., can be developed to further improve the overall energy efficiency. Moreover, in order to have an accurate techno-economic optimization, the overall system cost should be calculated on the basis of investment costs, operating costs, life times of the main components, etc. Furthermore, the proposed methodology could be employed to other stand-alone renewables-H2 systems. However, validating the proposed methods for such renewablesH2 systems, for instance, wind/H2 system, would require much more sophisticated system control tools because of the highly fluctuating wind energy resource and much more complicated dynamic characteristics of wind turbines.

ARTICLE IN PRESS 488

I N T E R N AT 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

33 (2008) 477 – 489

Area of PV Panel = 51 m2

Area of PV Panel = 51 m2

1

300

Output Power of Fuel Cell

0.9 250

0.7 Power (W)

State of Charge

0.8

0.6 0.5 0.4

200 150 100

0.3 SoC of Battery

0.2

50

SoC of H2

0.1 0

0 0

2000 4000 6000 8000 10000 12000 14000 16000

0

2000 4000 6000 8000 10000 12000 14000 16000 18000 Time (Hour)

Time (Hour) Area of PV Panel = 51 m

Area of PV Panel = 51 m2

2

1

1200

Output Power of Fuel Cell

0.9 1000

0.7 Power (W)

State of Charge

0.8

0.6 0.5 0.4

800 600 400

0.3 SoC of Battery

0.2

200

SoC of H2

0.1 0

0 0

0

2000 4000 6000 8000 10000 12000 14000 16000

2000 4000 6000 8000 10000 12000 14000 16000 18000

Time (Hour)

Time (Hour)

Fig. 15 – Two typical years’ operation with different output power PFC of fuel cell at Singapore (a) MPV ¼ 1:13, PFC oPLMmax ; (b) MPV ¼ 1:13, PFC oPLMmax ; (c) MPV 41:13, PFC ¼ PLMmax ; (d) MPV ¼ 1:13, PFC 4PLMmax . Table 3 – Simulation results in Figs. 12–15 Component

Innsbruck

Singapore

PV APV ðm2 Þ MPV

57

69

45

51

1.0

1.21

1.0

1.13

Fuel cell PFC (W) PLMmax

676 ¼

532 o

676 ¼

1016 4

676 ¼

270 o

676 ¼

1016 4

H2 tank Initial SoCH2

0.4

0.4

0.4

0.4

0.4

0.4

0.4

0.4

Final SoCH2

0.44

0.81

0.82

0.79

0.36

0.92

0.9

0.9

Battery SoCBT o0:35

No

Yes

No

No

No

Yes

No

No

R E F E R E N C E S

[1] Dutton AG, Bleijs JAM, Dienhart H, et al. Experience in the design, sizing, economics, and implementation of autonomous wind-powered hydrogen production systems. Int J Hydrogen Energy 2000;25:705–22. [2] Agbossou K, Kolhe M, Hamelin J, Bose TK. Performance of a stand-alone renewable energy system based on

energy storage as hydrogen. IEEE Trans. Energy Conver 2004;19(3). [3] Stantarelli M, Macagno S. Hydrogen as an energy carrier in stand-alone applications based on PV and PV-micro-hydro systems. Energy 2004;29:1159–82. [4] Stantarelli M, Calı` M, Macagno S. Design and analysis of stand-alone hydrogen energy systems with different renewable sources. Int J Hydrogen Energy 2004;29:1571–86. [5] Barthels H, Brocke WA, Bonhoff K, et al. Phoebus-Ju¨lich: an autonomous energy supply system comprising photovoltaics,

ARTICLE IN PRESS I N T E R N AT 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

[6]

[7]

[8] [9]

electrolytic hydrogen, fuel cell. Int J Hydrogen Energy 1998;23:295–301. Vidueira JM, Contreras A, Veziroglu TN. PV autonomous installation to produce hydrogen via electrolysis, and its use in FC buses. Int J Hydrogen Energy 2003;28:927–37. Hollmuller P, Joubert JM, Lachal B, Yvon K. Evaluation of a 5 kWp photovoltaic hydrogen production and storage installation for a residential home in Switzerland. Int J Hydrogen Energy 2000;25:97–109. Szyszka A. Review of the Neunburg vorm Wald solar hydrogen demonstration project. Power Eng 1996;10:226–32. Goetzberger A, Bopp G, Griesshaber W, Stahl W. The PV/ hydrogen/oxygen-system of the self-sufficient solar house Freiburg. In: IEEE 23rd photovoltaic specialists conference; 1993. p. 1152–8.

33 (2008) 477 – 489

489

[10] Ulleberg Ø. The importance of control strategies in PVhydrogen systems. Sol Energy 2004;76:323–9. [11] Patel MR. Wind and solar power systems. Boca Raton: CRC Press; 1999. [12] Klein SA, Beckman WA, Mitchell JW, et al. TRNSYS—A transient simulation program. Solar Energy Laboratory. University of Wisconsin-Madison; 2004. [13] Kreuter W, Hofmann H. Electrolysis: the important energy transformer in a world of sustainable energy. Int J Hydrogen Energy 1998;23:661–6. [14] Casper MS, editor. Hydrogen manufacture by electrolysis, thermal decomposition and unusual techniques. Noyes Data Corporation; 1978. [15] Barbir F. PEM electrolysis for production of hydrogen from renewable energy sources. Sol Energy 2005;78:661–9.