PV Power Systems
The Centre for Sustainable Energy Systems does not give specific advice
about the design and installation of photovoltaic power systems. However
the following information should be useful.
General Information
Rebates
For domestic and community building systems there are rebates available
under the Photovoltaic Rebate Program (PVRP), administered in each state
and territory on behalf of the Australian Greenhouse Office. For details
of this program see: http://www.greenhouse.gov.au/renewable/pv/index.html
For systems in rural areas in some states (mainly Queensland, Western
Australia and the Northern Territory) there is also funding assistance
under the Rural Remote Power Generation program (RRPGP). This also administered
in each state on behalf of the AGO. For details see: http://www.greenhouse.gov.au/renewable/rrpgp/index.html
Professional Advice
The Business Council for Sustainable Energy (BCSE) coordinates an accreditation
program for designers and installers of photovoltaic power systems. Industry
personnel accredited under this scheme have been through a rigorous process
to assess and reward their competence to design and install such systems.
See the BCSE web site for a list of accredited personnel.
http://www.bcse.org.au/home.asp
Stand Alone Systems
Load
Minimise the electrical load by using the most appropriate energy form
for the different energy services in the house. E.g. use LP Gas for
cooking and hot water boosting and use direct solar heat gain through
north facing windows for space heating (in an energy efficient house).
Calculate the load by multiplying the load power (in Watts) by the
operating time (hours) to determine the load in Wh or kWh.
System Voltage
This is a bit difficult to get it just right, but if the load is less
than 1kWh per day use 12V, if the load is between 1 and 3 kWh per day
use 24V, if it is between 3 and 6 kWh per day use 48V and if it is
above 6 kWh per day use 110V. Remember that if you expand the system
at any time you will need to select items with compatible system voltages,
so if you anticipate expanding then go for a higher system voltage.
Battery Bank
- Divide the load by the system voltage to convert the load into ampere
hours (Ah).
- Divide the load by the inverter efficiency (typically 85%)
- Multiply this adjusted load by the number of days of autonomy (typically
5) and divide by the maximum allowable depth of discharge (typically
70%) to get the battery bank capacity.
Note that new identical batteries of large capacity should be used and
try to resist building up the battery bank capacity by using parallel
strings. With parallel strings a preferred current route will develop
and lead to premature failure of the battery bank.
PV Array
- Divide the adjusted load by the battery efficiency (typically 90%) to
determine what output the PV array must deliver.
- Identify the number of peak sun hours (PSH) for your location at the
tilt and orientation of the array. This varies from month to month and
an annual average figure is generally used. Remember that your main load
will most likely be in winter so the array is generally installed at
your latitude angle plus 10-150 and will face solar north (different
from magnetic north – your installer can determine this for you).
- Select the modules that you will use and note the output current at maximum
power (this is very close to the current under normal operating conditions).
It doesn’t matter much which module you select, but the installation
is generally easier the larger the modules.
- Monocrystalline, multicrystalline and amorphous modules are all commercially
available and their efficiencies and prices vary. However the cost per
unit of output power is approximately the same for all modules.
- Multiply the module current by the PSH to get the output of each module.
Divide the system voltage by the nominal voltage of each module to determine
the number of modules in series.
- Divide the array output required by the output of each module to determine
the number of parallel strings of modules in the array. Remember you
need a whole number of strings so, round the answer to this division
up or down.
Inverter
- The inverter converts DC electricity from the battery bank into AC electricity
to operate the lights and appliances in the house.
- Inverter voltage must be the same as the system voltage.
Inverter continuous power should be capable of running those load items
that could conceivably be turned on at the same time.
- The inverter will have half hour and surge ratings. These should be checked
against those larger load items that may run for short periods (half
hour) and the starting power and current requirements of and load items
which incorporate motors (fridge, vacuum cleaner, power tools etc.).
Charge Regulator
- The battery bank may be damaged if it is overcharged. The regulator monitors
the state of charge of the battery bank and will vary the charging
current as the state of charge increases. Initially the charge should
be delivered to the batteries as quickly as possible (higher current),
but then slow down as the batteries near full charge. If the batteries
overcharge they may “gas” (evolve hydrogen) as the water
in the electrolyte is broken down into hydrogen and oxygen. This leads
to a potentially explosive build up of hydrogen gas (hence the need
for a well ventilated battery bank location) and a loss of electrolyte
from the battery bank.
- The charge regulator must have the same voltage as the system voltage
and be able to handle the maximum array current. Use the short circuit
current of the array to size the regulator current capacity.
Back Up Generator and Battery Charger
These should be sized to be capable of charging the battery bank and
powering any dedicated load items running directly from the battery
bank at the same time. Chargers are typically in the range of 40, 60
or 80A and generators will vary from 2 to 10 kVA.
Installation
To be eligible for a rebate the system will have to be installed by an
accredited tradesperson. The key to the installation is that all system
components are compatible and they are connected in such a way to minimise
energy loss in the system. i.e. thin cables will lead to voltage drop,
power loss and wasted energy and may render the system dangerous and
/ or inefficient.
All 240V AC installation must be carried out by a licensed electrician
as must all installation designated as Low Voltage – as opposed
to Extra Low Voltage. The demarcation for ELV / EV is 110V DC, so larger
system will almost certainly be LV and must be installed by a licensed
electrician.
Installing the PV Array
The array can be either roof or ground mounted or roof integrated (replacing
tiles or roofing iron). The output of the array will be maximized if
the internal cell temperature of the solar cells in the modules is
as low as possible. For this reason an array installed on a frame above
the roof allows a cooling breeze behind the modules and this keeps
the temperature as low as possible. Roof integrated arrays are more
difficult to keep cool, but it is possible with perforated roof battens.
The array should be installed as close as is practical to the battery
bank to minimise the length of cable and voltage drop, power loss and
inefficiency of the system.
Standards
There are Australian Standards which are required for the design and
installation of PV systems. These include:
- AS4509.1,2 and 3 – Stand Alone Power Systems
- AS 5033 – PV Arrays
- AS 2676.2, AS 3011.2, AS 4086.2 – Batteries
Your accredited designer / installer will work to these standards.
Cost
There are many variables in the price as the design should be customised
to your requirements and no estimate should be treated with any certainty.
However, the retail cost of PV modules is around $10 per watt so a
1kW array will cost around $10,000 Therefore you should expect a well
designed and installed 1 kW system to cost between $20 and $25,000
when all balance of system components and installation are included.
The maximum rebate for a domestic system is $8,000
Grid Connected Systems
These systems are cheaper than stand alone system because they don’t
need such equipment as battery banks, battery chargers and back up generators.
Electricity is produced by the array during the day and any not consumed
in the household is fed back to the local electricity grid via a bi-directional
inverter. At night the house is powered from the grid in the normal way.
If day time production is high and consumption low and might time consumption
from the grid is low there may be a net export from the system to the
grid. This will result in lower electricity bills, which will help offset
the cost of supply and installation of the system.
Load
The household electrical load should be minimised in the same way as
for a stand alone system. It is more likely to be successful for a
grid connected system because natural gas is available in many urban
areas, hence enabling gas to be used for heating, cooking and hot water
boosting at an economical rate.
The system may incorporate a change over switch such that during the
day the load is run directly from the PV array and at night from the
grid.
System Sizing
There are three methods of sizing the system:
- To meet the load. Work out the load and make sure that the output
of the array is greater than or equal to the load requirement.
- To suit what can be afforded or to maximize the available rebate.
The maximum rebate is $4,500 which is available for systems of 1kW
PV array size. Many system of this size have been installed.
- To suit the sized array which will fit on the available suitable
room space. Bear in mind that the output will vary with changes to
the orientation and inclination of the array. Hipped roofs in particular
are often limited in the area of suitable roof space.
Orientation and Inclination
The maximum load may occur in the mid afternoon in summer time. If the
array is to meet the load at the time of the maximum load then a westerly
facing orientation may be preferable. If the array is to deliver maximum
all year round energy output then a solar north orientation would be
better.
You must also take into account the shading effect of trees and neighbouring
buildings when siting the array.
Inverter
The inverter is easier to size in a grid connected system than in a stand
alone system, because the load (the grid) is constant. Hence inverter
efficiencies are generally higher in grid connected systems because
the inverter can be kept running at close to optimum load for more
of the time. E.g. a 1kW array would only require an inverter of about
900W capacity, since the array will only reach maximum output during
the middle of the day on the sunniest days of the year. If there is
lost output during these maximum times because the inverter is undersized
this is made up for in a higher efficiency for the rest of the time.
Installation
Because it is being connected to the electricity grid the system must
be installed by a licensed electrician. Grid connected system often
include a single series string of PV modules to keep the voltage high
and current low (this minimises losses). Hence the system voltage will
most likely be over the Extra Low Voltage / Low Voltage limit (110VDC)
and must be installed by a licensed electrical tradesperson.
Cost
A 1kW system with appropriate inverter installed to all relevant standards
by a licensed electrician will probably cost in the order of $12 – 14,000
and the maximum rebate of $8,000 is applicable.
Large PV Systems
Electricity utilities have an obligation under the Mandatory Renewable
Energy Target to source cumulatively 9,500GWh of electricity from renewable
energy sources by 2010. This has been the stimulus for many large scale
renewable energy projects, not the least of which are large grid connected
photovoltaic systems.
Such systems are generally designed and installed under contracts issued
by the electricity utility and are generally outside the scope of designers
and installers accredited with the Business Council for Sustainable Energy.
These systems are generally ground mounted as it’s difficult to
obtain sufficient roof space to install such large arrays, although one
exception is the large array on the Queen Victoria Market building in
Melbourne. The modules in the array are wired together such that system
losses are kept to a minimum. This means that there will be several modules
in each string so that the voltage is as high as is practicable. This
keep the current low and losses minimal.
The PV modules for such systems are bought in bulk and the price is
often significantly lower than that paid by consumers when purchasing
their domestic rooftop system.
Electricity utilities and the department of energy in each state can
provide details of such projects carried out in their areas.
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