Abstract
A Vanadium-Vanadium Redox battery can improve Photovoltaic system performance, reliability and robustness by increasing the energy conversion efficiency of the battery to 87%, by making the battery life, efficiency and ongoing energy capacity independent of state of charge and load profiles and by reducing maintenance requirements. High battery efficiency reduces the required PV while a battery life insensitive to battery usage relaxes system constraints. These advantages are utilised in a demonstration PV system in Thailand that was designed specifically to use vanadium technology. Following a 12 month field testing programme with 4kW Vanadium Batteries, 300 systems consisting of 2-4 kW PV, a 4kW, 15kWh, Vanadium Battery and a 4 kVA grid interactive inverter are intended to be installed in residence in Thailand.
in PV applications requiring energy storage, the selection of the
energy storage system is of primary concern. The electrical
parameters of the storage system constrain and shape the PV system.
The deliverable power determines the maximum size of the electrical
load, the energy storage capacity determines the duration of power
to the load and energy conversion efficiency determines the amount
of extra PV needed to make up the energy lost in the conversion.
Additionally, the reliability of the storage system determines if
the PV system can be used in a critical application and maintenance
scheduling determines when personnel must visit the PV site.
PV systems engineers have traditionally employed electrochemical
storage using lead-acid batteries. Extensive development and use of
lead-acid technology, particularly in the automotive industry, has
allowed the adaptation of that mature technology directly to PV
applications. Lead acid technology is well understood, is reliable,
is in mass production and is readily available; however, lead acid
technology does have inherent attributes that must be designed
around. In order to maintain energy capacity and long battery life,
extra energy must be supplied periodically to the battery to
de-stratify the electrolyte and to equalise the cell voltages. This
process of "boost-charging" causes hydrogen evolution and water loss
from the battery. The additional energy associated with this process
is supplied by the installation of extra PV and periodic maintenance
is used to replace the lost water. The battery life is strongly
affected by how discharged the battery is allowed to get before it
is recharged and, if the battery is allowed to stay in a discharged
state for very long, irreversible damage occurs to the plates of the
battery. A useful battery parameter, the state of charge, is
difficult to determine accurately and after the battery is
installed, it is, in practice, difficult to change the size of the
battery to account for the addition of new loads not specified for
in the original system design.
The constraints imposed by lead acid technology suggest that a more
flexible, higher efficiency and cost effective technology would be a
benefit to PV systems.
A new type of electro-chemical storage developed by the University
of New South Wales (UNSW), the Vanadium-Vanadium Redox Battery [1],
exhibits many of the qualities desired by PV systems designers. This
battery has a very high efficiency, a reasonable electric density,
high charge and discharge rates, a long lifespan independent of
state of charge and load profiles, and low maintenance requirements.
These qualities greatly ease the constraints imposed upon PV system
engineers. It is not neccessary to oversize the battery in order to
maximise battery life or install additional PV for boost charging.
During periods of low sunlight, the battery can be operated
nominally at low states of charge with no effect upon battery life.
Additionally, this battery has a feature which allows for many new
options not available with lead acid technology. It is possible to
simultaneously charge the battery at one voltage while discharging
it at another voltage. This feature can be utilised to make a
minimum cost, high efficiency, maximum power point tracker or allows
to operate the battery as a DC transformer, electro-chemically
transforming a current and a voltage into a different current and
voltage.
The Centre for Photovoltaic Devices and Systems in collaboration
with the UNSW Vanadium Research Group and the Thai Gypsum Products
Co. Ltd., Thailand, has designed and installed a PV system using
Vanadium Battery storage in a demonstration house in Thailand. This
is a pre-commercial prototype version of a residential grid
interactive system intended for installation in 300 houses in
Thailand.
Redox flow batteries employ a different energy conversion method
than solid plate batteries. In contrast to the solid phase chemical
changes that occur on the plates of a lead acid battery, a redox
battery stores energy as chemical changes in two liquid electrolytes
that are hydraulically pumped through the battery stack. Energy
conversion occurs in the battery stack and the charged electrolyte
is stored in reservoirs external to the battery stack. The physical
size of the battery stack determines the power available from the
battery and the volume of the electrolyte reservoirs determines the
kWhrs energy storage of the battery.
picture:vanadi1.gif
Figure 1, Schematic View of Vanadium Cell
The development of the Vanadium-Vanadium Redox Battery at UNSW has
overcome significant technical limitations that have plagued other
type of redox flow batteries. [2] The battery is insensitive to
atmospheric oxygen, has a high 1.4 V cell voltage, has high
longevity, low maintenance requirements and the electrolytes are not
mutually destructive.
In the vanadium battery, identical electrolyte is used initially in
both the positive and negative sides of the battery. During
charging, electro-chemical reactions within the battery stack change
the valance of the vanadium in the two electrolytes with the
negative reaction changing V(III) to V(II) and the positive reaction
changing V(IV) to V(V). This process is reversed during discharge.
If any inadvertent mxing of the charged electrolytes occurs there is
an energy loss as heat but, because the mixed electrolytes revert
back to their uncharged states, they can be recharged next time
through the stack. Thus, cross contamination is not detrimental to
the longevity of the battery. The above reactions do not, under all
normal operating conditions, generate hydrogen.
Because both the valance reactions are permissible to the originial
electrolyte, it is an arbitrary decision as to which side of the
battery is positive and which side of the battery is negative. Only
after initial charging is there a positive and a negative side of
the battery.
The battery stack's electro-chemical reactions are all highly
efficient with the energy voltage and colombic efficiencies ranging
from 90 to 99%. When the energy needed to operate the pumps, which
amount to 2 to 3% of the total battery energy, is also taken into
account the total battery efficiency is a very high 87%.
An accurate state of charge determination is made possible by
measuring the open circuit voltage of a small vanadium cell attached
to the battery with some portion of the electrolytes being pumped
through it.
These attributes of the Vanadium-Vanadium Redox Battery make its use
in PV systems very desirable.
The use of a Vanadium Battery with its very high energy conversion
efficiency and no boost charge requirements directly relates to less
PV being needed for the system. Greater system robistness is
achieved through the battery's ability to be left indefinitely at
any state of charge with no reduction in battery life and, because
there is no hydrogen evolution, there is no water loss from the
electrolytes. Greater system flexibility is achieved with the new
capability of tailoring the kWhrs storage to meet any new loads by
varying the volume of the electrolytes and, because the electrolytes
are stored separate from each other, there is very low self
discharge. The battery itself can supply multiple output voltages --
a valuable advantage in PV systems with DC loads of different
voltage requirements. These features offer new versatility in the
choice of applications that use PV systems. Material redundancy is
minimised by the full power, very deep cycle (100%) capability of
the battery and additionally, because there is no hydrogen
evolution, there is no need for forced ventilation. Maintenance
requirements are low which reduces visits to PV installations.
The 2-3% energy loss associated with the vanadium battery's pumps is
calculated with the battery operating at full power. If the battery
is operating at low power then the pumping power loss is a more
significant proportion pf the system power.
The strategy to minimise this energy loss and improve system
efficiency is to turn the pumps off during periods of low charge or
discharge rates. With no electrolyte flow all of the power going
into or coming from the battery operates directly on the
electrolytes present in the stack.
When the energy level of the stack eletrolytes reaches a threshold,
the pumps are turned on for a period of time which fills the stack
with fresh electrolytes, then the pumps are turned off and the
battery again waits until the treshold it met.
In PV systems where random load profiles are present this feature
allows for an optimisation of pumping energy versus system load
power requirements. UNSW is developing a micro-controller based
vanadium battery controller with strategies for optimising the
efficiency of the battery for these applications.
The ability to easily and accurately determine the true state of
charge of the Vanadium Battery allows for dynamic predictions of the
amount of time that a battery can sustain a load. This allows
greater system diversity and gives the designer the ability to fine
tune the kWhrs storage of the battery for differing load profiles
and load types.
A valuable feature of the Vanadium battery is its ability to have
its charge voltage being different than its discharge voltage. It is
possible to simulateously charge the battery at the 12V tap and
discharge at the 48V tap or visa versa. Used in this manner, the
battery becomes an 87% efficient DC transformer.
picture:vanadi2.gif
Figure 2: Voltage taps for MPPT and differing charge and discharge
voltages.
Voltage taps increase system flexibility as loads with different DC
voltage requirements may be operated from one power source without
the additional conversion losses associated with voltage matching.
A maximum power point tracker (MPPT) is useful in reducing the PV
required in system applications. The relatively high cost of the
power electronics MPPT, however, often reduces the cost
effectiveness of the reduction of PV.
The tap change method presents itself as being a highly cost
effective and efficient method of Maximum Power Point Tracking.
PV array's maximum power point can be matched buring charging by
choosing an appropriate voltage tap on the Vanadium Battery and
changing to another voltage tap as PV array's maximum power point
changes. Unlike the complex power electronics counterpart, there are
no energy conversion losses associated with the tap change method
and the electronic are relatively simple and rugged.
An economic analysis of Vanadium Storage technology has determined
the cost of Vanadium electrolytes to be US$48/kWhr and the cost of
the battery stack components to be US$206/kW. Using a factor of 2.5
to account for the additional costs of storage tanks etc., resulted
in the capital cost for a battery varyinig from US$635/kWHr for a
battery with 1 hour storage capacity at full power discharge (e.g. 4
kW battery with 4 kWhrs storage) to US$146/kWhr for a battery with
20 hours of storage capacity at full power discharge (e.g. 4 kW
battery with 80 kWhrs storage). This cost analysis indicates that
the cost per kWhr is determined by the ratio of the battery's power
output to the number of total hours of full power storage. Thus both
a 1 kW battery with 20 kWhrs storage and a 4 MW battery with 80
MWhrs storage would be US$146/kWhr.
picture:vanadi3.gif
Total Cost of Vanadium Storage per kWhr as a function of Storage Time
Figure 3: Capital Cost of Vanadium Battery per kWhr
Most obvious in this analysis is the dramatic drop in cost per kWhr
as the battery goes from 1 to 5 hours of full power storage.
A major economic advantage that Vanadium technology has over other
technologies relates to the ongoing costs of battery storage.
Because the electrolytes are not damaged by atmospheric oxygen or
cross contamination they have an indefinite life and are considerd
to be a capital cost. Current estiomates indicate that the battery
stack will need to be replaced every five years yielding a very low
ongoing cost, when contrasted with a lead-acid battery where, in PV
applications, the entire battery needs to be replaced on the average
of every seven years.
The first licensee for the commercialisation of the Vanadium Battery
is the Thai Gypsum Products Co. Ltd., (TGP) Bangkok, Thailand. TGP
built a PV & vanadium demonstration house on their industrial estate
at Laem Chabang, Thailand with the opening, lead by HRM Princess
Maha Chacri Sirindhorn of Thailand, on 23 December 1992. This
function has 600 guests from industry, military and the media.
This PV & Vanadium system was installed in the demonstration house
in December 1992 by members of the UNSW Centre for Photovoltaic
Devices and Systems, UNSW Vanadium research Group, and the Thai
Gypsum Vanadium Commercialisation Group.
The demonstration system was designed to operate AC loads and a
small, less than 800 watt "compressor type" air conditioner was
chosen as the load in the demonstration house.
picture:vanadi4.gif
Figure 4: Schematic of PV and Vanadium Battery system installed in Thailand
In Thailand, TGP built the demonstration house and roof mounted 36
Kyocera LA441K63 PV modules giving 2.2kW of installed PV.
In Australia, the UNSW Centre for Photovoltaic Devices and Systems
selected the inverter and other system components and built the
micro-controller for the Vanadium Battery as specified by the UNSW
Vanadium research Group.
The UNSW Vanadium Research Group designer and built a Vanadium
battery rated at 1.2kW, 15kWhrs. This battery has 12 cells, giving a
system voltage of 16.8 Volt, and uses 200 litres in each of the two
lectrolyte reservoirs.
Butler Solar Products, Australia, the designers of the Siemens'
range of SUNSINE inverters, modified an existing 1 kW, 12 Volt stand
alone SUNSINE inverter for the 16.8 Volt PV & Vanadium system. This
required a redesign of the transformer, installation of additional
FET's in the bridge arms and modifications for the Thai requirements
of 220V, 50 Hz output. This inverter is not grid interactive.
The system load is a National CU-700K split system "compressor type"
air conditioner. The starting power required for this air
conditioner was measured to be from 6-11 kW -- a considerable amount
of peak power for a 1 kW system.
The initial charging of the Vanadium Battery was with a power supply
connected to the AC grid.
The demonstration system works as designed. Work continues with this
system giving the Thai Gypsum Vanadium Commercialisationi Group
hands on systems experience that will be directly applicable to
their 300 house project.
TGP is in the process of commercialising thr 4 kW vanadium battery
with the first application of the technology being a 300 house
installation in Bagkok. Each house will have a PV & Vanadium Battery
system consisting of 2-4 kW PV, 4kW Vanadium Battery and a 4
quadrant, 4kVA grid interactive inverter. It is hoped that the first
of the houses will be complete by the end of 1993 and that all 300
will be complete 18 months later. Data acquisition for system
evaluation will be employed.
The Vanadium-Vanadium Redoc Battery offers system performance
benefits through increased system efficiency and robustness, reduced
maintenance requirements, and greater flexibility in both system
design and system application.
The 300 house residential grid connected systems will test this
Vanadium technology in a variety of system configurations.
The Centre for Photovoltaic Devices and Systems is supported by the
Australian Research Council under the Special research Centre Scheme
and by Pacific Power.
Research for the Vanadium Battery development has been funded by
ERDC, NSW Office of Energy and Thai Gypsum product Co., Ltd. the
support of Formica Australia is also gratefully acknowleged as is
the assistance of Michael Kazacos, Rui Hong, Dennis Yan and Jim
Wilson.
Introduction
The Vanadium Battery
![[IMAGE]](vanadi1.gif)
PV Systems with Vanadium Batteries
Pump Losses
Voltage Taps
![[IMAGE]](vanadi2.gif)
Maximum Power Point Tracking
Economic Considerations of the Vanadium Battery
![[IMAGE]](vanadi3.gif)
PV & Vanadium Demonstration System in Thailand
![[IMAGE]](vanadi4.gif)
Construction and assembly of the system was as follows:
300 House Project
Conclusion
Achknowledgements