Elsevier

Energy

Volume 40, Issue 1, April 2012, Pages 196-209
Energy

Sizing, techno-economic and generation management analysis of a stand alone photovoltaic power unit including storage devices

https://doi.org/10.1016/j.energy.2012.02.004Get rights and content

Abstract

Due to the mismatch between the load demand and the intermittent solar energy, a stand-alone photovoltaic-hydrogen system and an optimal control scheme are designed to maintain the high system efficiency.

Based on meteorological and the load demand data, a system sizing technique is proposed to establish the minimum capacity of the system components, which are a photovoltaic (PV) panel, a proton exchange membrane fuel cell (PEMFC), a battery bank and an alkaline electrolyzer (Elz).

An accurate energy management scheme that is utilized during power transfer is proposed to meet the economic requirements. Case studies are used to verify the efficiency of the energy management strategy and system sizing technique. Simulation results illustrate a simple solution to the design and processing of stand-alone PV-hydrogen (PV-H2) systems.

Highlights

We study a stand alone PV-hydrogen system comprising renewable devices. A PV generator, a battery bank, a fuel cell and an electrolyzer are modeled. Power management taking into account the economic arrangement and the operating conditions to ensure energy availability is proposed.

Introduction

Renewable energy sources (RESs) are becoming widely used to overcome the lack of fossil energies and to satisfy the residential electrical energy demand. Photovoltaic generators are considered among the most promising RESs for their availability and inexhaustibility. However, the weaknesses of this type of energy remain the intermittence of solar irradiations depending on locations and seasons and the time varying load demand. To overcome this problem, the PV panel is always associated with other storage systems. These systems absorb the excessive energy, store and deliver it at an adequate time and a suitable rate. The most common design is the H2 system, which comprises an electrolyzer as a hydrogen producer and a PEMFC as a hydrogen consumer to produce electrical energy. This H2 system acts as long-term energy storage. Another device can also be used in such a combination as fast energy storage. For example, a buffer may be a rechargeable lead acid battery due to its high efficiency, quick charging/discharging ability and capacity for smoothing source fluctuations [1].

The use of such a system leads to certain critical questions:

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    What is a suitable size for these components?

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    At a given excess/deficient power, what is the best economical combination of energy storage devices to absorb/exude the energy?

This study aims to answer these questions for the PV–H2 system.

Ulleberg [2] reported the simulation results of a PV–H2 system consisting of a battery, an electrolyzer, a PV generator and a fuel cell. The author presents a control scheme based on the on/off switching of the fuel cell and the electrolyzer that uses the battery state of charge (SOC). This work presents the simulation results without presenting the models and sizing of the mathematical components.

Maclay et al. [3] developed an empirical model consisting of a PV panel, a reversible fuel cell, a battery bank and an ultracapacitor. The authors present the power evolution of multiple combinations without presenting battery and ultracapacitor SOC evolution, which are critical to control.

Li et al. [4] developed a dynamic model of a stand-alone photovoltaic power system using an H2 device and a battery bank as energy storage devices. This work presents a dynamic model of all system components, makes a cost evaluation of the system based on size and describes an energy management strategy based on the hydrogen tank and battery SOCs. The load and the photovoltaic powers said to be used in the energy management are not described in the paper, and the authors are content with presenting the annual battery SOC evolution.

Zhou et al. [5] build on the work of Ulleberg with the battery SOC as the control key for the electrolyzer and fuel cell on/off switching. This study gives the sizing equations for different system components basing on load consumption and irradiation curves over a year. Suggestions for the optimum size of the hybrid system are given for two different locations. This work lacks an economic analysis.

The use of RES and H2 systems for residential applications has increased significantly [6]. This work expands on the combination of energy storage devices for residential PV systems. Then, we focus on a sizing method depending on PV and residential consumption curves. The current study contributes to strategy design for optimal energy management based on economic challenges and measurable SOCs of different storage devices. Mathematical models of PV–H2 system components are built using MATLAB/Simulink, and the second scale evolution of different source powers is detailed.

A scheme of the stand-alone PV–H2 system presented in the current work is shown in Fig. 1. This model contains the following components: a PV generator with maximum power point tracker (MPPT), residential power demand, a battery bank with a DC/DC converter for fast energy transitions, a PEMFC with a DC/DC converter to supply electricity, an alkaline electrolyzer with the DC/DC converter to produce H2, a compressor, a pressurized tank for H2 conservation and a DC/AC converter for use at the end.

A PV panel collects solar irradiations and converts it into usable electricity. The DC/DC converter behind the PV generator is controlled by an MPPT module to allow more solar energy extraction.

The energy produced by the RES must first be used to satisfy the residential load. If the PV energy is greater than the demanded energy, the excess power can be used in two ways: to charge the batteries or produce H2 in the electrolyzer using a process called charging. To choose where to send the spare energy, we must evaluate the energy cost of each device (the device used to charge the batteries and the device used to form H2 in the electrolyzer). Thus, the system will choose the method that uses the least cost.

If, on the contrary, the consumer demand is more than the photovoltaic supply, then the lack of power can be satisfied in two ways: switching on the fuel cell or the battery using a process called discharge. Thus, when choosing a certain device (PEMFC or battery), we have to estimate the cost of power generation in each system and choose the less costly device.

The proposed system can be used as a stand-alone power system in remote areas where there is no access to the grid and as a support power medium to cover electricity deficiency during special situations, such as natural disasters.

Section snippets

System size

Sizing an autonomous PV–H2 system is a complex task because different resources are employed. A properly sized system is a system that responds to different energy requirements to reach a techno-economic autonomous hybrid system [7]

Based on the energy management strategy described above, a system sizing method is developed in the following section.

Photovoltaic panel modeling

Photovoltaic panels have been studied for more than 20 years [9]. The most common empirical model is the one with a diode, as presented in Fig. 6.

The empirical output current of a PV panel is described by the following equation:IPV=ILIDIsh=ILI0[exp(U+IRSa)1]UPV+IPVRSRsh

Battery model

Currently, battery technology is being considered in research and development. Thus, different battery models, such as lithium-ion and lead-acid, show promising results for use in power applications. In this study, lead-acid

Hybrid system control strategy

The purpose of the optimization process is to minimize the total net present cost (NPC) of the system presented in Fig. 1. The NPC is the purchase cost plus the investment cost during the lifetime of the system. To estimate the NPC of the system, we simulate the system during its lifetime, based on assumed component lifetimes, the energy produced and consumed by the components and the operating and maintenance costs.

As a fundamental control rule, the power produced by renewable source PPV

Charge process

When more photovoltaic power is available than what is demanded by the end user load, the spare energy will be used to charge batteries and/or to produce hydrogen using the electrolyzer. The selected charge mode depends on which of the two has the lowest cycling energy cost for a certain power.

Simulation results and discussion

Most studies include a simulation based on one-day data or hourly data over the course of a year. However, because of the dynamics of the FC systems and batteries, the data should be evaluated at the minutes or seconds scale to better estimate the energy management system efficiency. Meteorological data are available at an hourly scale; thus, a more precise analysis is necessary. Therefore, an instantaneous power flow distribution is considered for the load demand supply.

Simulation results were

Conclusion

According to the optimal energy management strategy that takes into account the difference between the intermittent solar irradiation and the varying load demand, a system sizing method is developed to design the following system components: the PV panel, battery bank, PEMFC, electrolyzer and H2 storage tank. In terms of device cost, the optimal system provides a simple method to evaluate the economic feasibility of a stand-alone photovoltaic system.

Energy management requires monitoring and

Glossary

a0
Output battery voltage when SOC = 0%
AElz
Electrolyzer active area (m2)
AFC
FC active area (m2)
ai
Parametric coefficients of an electrolyzer i = {1…7}
Aini
Initial PV panel area (m2)
APV
PV panel area (m2)
B
Parametric coefficients used in concentration losses
C
Equivalent electric capacitance of an FC (F)
Cbat
Battery bank acquisition cost (€)
Cbulck
Bulk capacitor (F)
CElz
Electrolyzer acquisition cost (€)
CFC
FC acquisition cost (€)
CN
Nominal capacity of the battery bank (Ah)
CO&M
Operation and maintenance cost(€)

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