3 September 2010The success of the Federal Government’s Solar Bonus Scheme has seen over 30,000 households generating their own solar energy. Following is an outline of scheme and how it can benefit individual homes.

The Australian Government is finally back on the solar bandwagon with its national Solar Bonus Scheme, intended to boost the number of grid-connected photovoltaic system installations across the nation.

This article intends to provide information on the design process of a general grid-connected PV solar system and important issues to consider before deciding on a system. With an understanding of the operation and limitations of your PV solar system, you and your system designer can create a truly effective, or even profitable, solar energy installation.

Accredited solar system design

The versatility of a PV solar system lies in the fact that it can be tailored to your situation and needs. Some people like their house to be self-sufficient or to reduce their carbon footprint while others like to take advantage of the feed-in tariff as an additional income. Regardless of the reasons for installing a PV system, its important to work closely with the designer in order to get the most out of your system.

A grid-connected PV solar system is made up of the photovoltaic panels, electrically connected to an inverter, which feeds your power to the grid via the meter. Before installing, make sure that the system installer and components are Clean Energy Council Accredited, which ensures every stage complies with Australian Standards.

Another reason the system should be CEC Accredited is to ensure that you are eligible for the Renewable Energy Certificates provided by the Federal Government.

Earning credits

RECs are assigned based on the generating capacity of your system. This is calculated from the size of the system you install and your location. The RECs from the first 1.5kW installed are multiplied by five and dubbed Solar Credits. The monetary value of RECs change with the market, and they can be used to towards the cost of your installation by signing the certificates over to your installing company. If the RECs are used to pay for the system, the payback period of your system is significantly reduced thanks to the lower capital cost.

As part of the Scheme, several state and territory governments provide support (at different rates on on different bases) to people who turn to renewable energy by crediting them for electricity generated by their solar panels. The base rate in NSW is $0.60 per kilowatt hour, about three times the average purchase price, but some suppliers will pay more, making solar an investment worth considering. Payment can be in the form of a cheque or credit on your bill depending on your electricity retailer.

Site conditions are vital to the system design and performance.

To gain maximum sun exposure in Australia, it is ideal to have panels facing north with little to no shading caused by nearby objects such as trees or buildings. However, this this is not essential as the system can be designed to accommodate different orientations and small amounts of shading. Building plans and satellite photos are good tools to estimate the amount of roof space and surrounding sources of shading.

Another consideration is the suitability of the roof surface area. Solar can be quickly and easily installed on tin and tiled rooves, while slate and asbestos are difficult and generally not used for solar installations.

A common concern of householders is the load bearing capability of their roof as the average solar panel weighs approximately 16 kilograms, but householders should be pleased to know that all roof types other than fibro, asbestos, or slate roof are suitable as the weight is evenly distributed across the roof. This means the majority of Aussie roofs can support a standard PV solar system.

As well as facing north, for maximum output your panels should be installed at an angle of 30 degrees from the horizontal. The pitch of a roof is normally anywhere from 5 degrees to 35 degrees, hence the roof pitch contributes to the final installed angle of the panels. Those lucky enough to have a roof pitch close to 30 degrees can have panels mounted flush with the roof surface, but a roof pitch less than 30 degrees is easily catered for with tilted framing. Tilt angle is important for orientating your system to collect maxium energy from sunlight. In Australia, the peak irradiance (energy from direct sunlight) is 1 kilowatt per square metre, lasting for four hours in Sydney. Tilt angle can affect the peak sunshine hours and irradiance, so it important to optimise during design.

Installation generally takes a day. The existing screw holes in the roof tiles or corrugated sheets are used to mount the panels. They can be fixed flush to the roof surface or be elevated at a desired tilt angle to improve system output. However, tilting will create shadowing between rows of panels thereby reducing the number of panels you can install in a given area. This is where design by a professional can ensure an optimal balance between using more panels with less tilting or having better tilting that puts more of your roof in shadow.

PV modules – Different types and sizes
There are several types of photovoltaic modules (or panels) currently on the market. They are made from one of three types of silicon: monocrystalline, polycrystalline, or amorphous. Monocrystalline are the most efficient whereas the performance of amorphous modules degrade quickly. However, if there is excessive shading on your roof, amorphous is more suitable as its output is not as greatly affected.

The size of a panel is determined by its rated peak output power in Watts (W). So a 200W module will provide more energy than an 180W module, over the same area, as it may contain more efficient solar cells.

In terms of system size, the industry will talk in terms of peak output power. For example, a 2kW PV solar system, consisting of ten 200W panels may be installed but the maximum actual power may only be 1.7kW. The real output is lower as it takes into account the total system efficiency, impacted by factors such as dirt on the panels, electrical losses in wiring, and the individual efficiencies of the panel technology and inverter.

Generating Electricity
Electricity generated by solar panels is direct current, or DC so for it to be compatible with the electricity distribution network and standard appliances, the electricity must be transformed to alternating current, or AC via an inverter before entering the grid.

As well as feeding the grid, the inverter characteristics can help the system design accommodate the difference in power generation caused by shading. This is achieved by power point trackers in the inverter and good wiring design. PPTs regulate the output of the panels so that they always operate at the maximum power allowable, ensuring inverter efficiency is about 95-98 per cent. Again, its important to design with these features in mind, and therefore important to use CEC accredited designers.

A warning
Take note, your energy provider may not be responsible for connecting the solar system to the grid. Responsibility of the grid belongs to the company that owns that section of the distribution network. This information should be available online or on your bill. The application process varies but a solar meter is provided (usually free of charge) and installed in your existing meter box. The power measured by this meter is that value you are credited with by the feed-in tariff. A good solar provider will include connection to the grid in their service, so be wary that cheaper quotes may not include this vital step.

Installation guides and warranties for panels and the inverter should be provided by the installer in case of faults, but solar panels are known for their resilience. All CEC accredited panels can withstand hail (reasonable size ones – not like the cricket balls we experienced in prior years) and are cleaned by rain, although you can give them a hand by hosing them down once a year.

Generally the warranty period for solar panel material defects is up to five years. The warranty period with respect to power output is for a total of 25 years.
The warranty period for power output of panels is spread out. For example the first five years you may be guaranteed 95 per cent of the nominal power output, next seven years 90 per cent of the nominal power output and the balance of 13 years 80 per cent of the nominal power output. The breakdown periods and its corresponding efficiencies can vary greatly for different manufacturers, but monocrystalline PV will general have a guaranteed life of 20 years or more.

The warranty period for inverters is five to 10 years. This means your panels will outlast your inverter and so it will need to be replaced after a 10 year period. Inverters are a more active electronic devise than the panels and have the shorter lifetime as a result. One advantage of having to replace the inverter is that it allows for the system to be expanded in the future. The existing inverter will be sized for the current system, but if you are considering expanding the number of panels in the future, you could wait until the inverter needs replacing and then replace it with the larger inverter as well as add more panels.

Do your research

It pays to do some research before going ahead with a solar installation. Different electricity retailers are offering different rates for the Feed In Tariff (from $0.66/kWh to $.072/kWh), so swapping retailers could earn you more cash.

Be sure to collect quotes from a range of solar companies, and look out for costs not included in the quotes, such as connection to the grid.

Finally, once you have decided on the best system, you can sit back, relax and enjoy watching your investment silently making you clean, green, renewable power.

Wendy Truong and Edie Mather are sustainability engineers with Blue Green Engineering, Sydney