28-11-2012, 04:32 PM
Energy analysis of batteries in photovoltaic systems. Part I:
Performance and energy requirements
Energy analysis of batteries in photovoltaic systems.pdf (Size: 248.54 KB / Downloads: 106)
Abstract
The technical performance and energy requirements for production and transportation of a stand alone
photovoltaic (PV)-battery system at different operating conditions are presented. Eight battery technologies
are evaluated: lithium-ion (Li-ion), sodium–sulphur (NaS), nickel–cadmium (NiCd), nickel–metal hydride
(NiMH), lead–acid (PbA), vanadium-redox (VRB), zinc–bromine (ZnBr) and polysulfide-bromide (PSB).
In the reference case, the energy requirements for production and transport of PV-battery systems that
use the different battery technologies differ by up to a factor of three. Production and transport of batteries
contribute 24–70% to the energy requirements, and the PV array contributes 26–68%. The contribution
from other system components is less than 10%. The contribution of transport to energy requirements is
1–9% for transportation by truck, but may be up to 73% for air transportation. The energy requirement
for battery production and transport is dominant for systems based on NiCd, NiMH and PbA batteries.
The energy requirements for these systems are, therefore, sensitive to changes in battery service life and
gravimetric energy density. For systems with batteries with relatively low energy requirement for production
and transportation (Li-ion, NaS, VRB, ZnBr, PSB), the battery charge–discharge efficiency has a larger
impact. In Part II, the data presented here are used to calculate energy payback times and overall
battery efficiencies of the PV-battery systems.
2004 Elsevier Ltd. All rights reserved.
Introduction
In many types of stand alone photovoltaic (PV) systems, batteries are required to even out
irregularities in the solar irradiation and concentrate the solar energy to higher power. Today,
lead–acid and nickel–cadmium batteries are commonly used in PV systems. Some emerging battery
technologies may also be suitable for storage of renewable energy, such as different types of
redox flow batteries and high temperature sodium–sulphur batteries. Identification of the important
parameters in PV applications can be used to direct research and product improvements, and
comparison of different battery technologies can be used to guide battery choice for specific user
conditions.
For energy technologies, the energy requirement for producing equipment is an important performance
parameter. Large energy requirements in comparison to energy output will limit the
range of possible applications to small niches. Energy requirements for producing PV modules
have been studied and debated since the early 1970s, while batteries have gained less attention.
In a study of solar home systems, Alsema [1] concluded that lead–acid batteries contribute significantly
to the energy requirements. Rydh [2] compared the energy requirements for lead–acid and
vanadium redox flow batteries for stationary energy storage, but other battery technologies have
not been assessed in this context.
The purpose of this study is to provide an energy analysis to enable comparison of different battery
technologies in renewable energy applications. By quantifying energy efficiencies and the energy
requirements for manufacturing the different systems, increased awareness may lead to
improved energy management of energy storage systems. This paper presents the background
data [3] to the calculation of energy payback times and overall battery efficiencies of PV-battery
systems that are presented in Part II [4] of the study.
Goal and scope
The goal of this study is to assess the indirect energy requirements for production and transportation
of different battery technologies when used in a stand alone PV-battery system at different
operating conditions. The contribution of different PV-battery components to the gross energy
requirement and important parameters are identified for each battery technology.
The following battery technologies are evaluated: lithium-ion nickel (Li-ion), sodium–sulphur
(NaS), nickel–cadmium (NiCd), nickel–metal hydride AB5 (NiMH) and lead–acid (PbA). Three
types of redox flow batteries (regenerative fuel cells) are included, namely polysulfide-bromide
(PSB), vanadium-redox (VRB) and zinc–bromine (ZnBr). The battery parameters investigated
are battery charge–discharge efficiency, service life, gravimetric energy density and energy requirements
for production and transport of the batteries (see Section 4).
Method
To enable aggregation of energy of different qualities, the different forms of energy have been
converted to the same energy currency, primary energy equivalents. Primary energy is defined as
the energy content of energy carriers that have not yet been subjected to any conversion. The conversion
efficiency clearly differs between different forms of primary energy. Therefore, we have
consequently assumed that data for primary energy refers to primary fossil energy equivalents
(indicated by the index pf). To convert electricity used for component production to primary fossil
energy equivalents, we have used the factor 1/0.35.
Results and discussion
To present the contribution of different components to the energy requirement, the following
section presents the results for the reference case (Case 1) when the battery service life is limited
by cycle or float life (t3,limit) and the temperature is 25C. It is assumed that the batteries are produced
from 100% recycled materials and that the different components are transported 3000km
by heavy truck.
Production and transportation of batteries contributes 24–70% of the energy requirements of
the PV-battery system, also underlining the energy related significance of batteries in PV systems
(Fig. 2). The relative contribution from the production of batteries is lowest for the ZnBr battery
and highest for the NiMH battery.
Depending on the efficiency of the battery, the power rating of the PV array is 42–71kW, corresponding
to an area of 320–600m2. The contribution of production and transport of the PV
array is 26–68% (NiMH–ZnBr). The highest absolute energy requirement for PV array production
is 88–96GJ/yr for the redox flow batteries due to their relatively low efficiency, resulting in
the need for a larger PV array and charge regulator. Production and transport of the charge regulator
and inverter contribute 2–4%, respectively.
The contribution of transport of all the components to the gross energy requirement is low
(0.9–8.9%) for 3000km transport by heavy truck. The lowest energy requirement for transport
is for the ZnBr battery due to its high energy density and the possibility of recycling the electrolyte.
The transport of PbA batteries contributes 8.9% to the gross energy requirement since these
batteries have a relatively low energy density and cycle life, and therefore a larger mass of batteries
has to be transported.