ROOFTOP, BUILDING INTEGRATED PV PANELS AND OFFSITE RENEWABLES

When designing a building for PV there are many considerations. Many are summarised here.

​

For residential solar this solar calculator website will get you off to a good start.   

​

Design basics

The nature of energy distribution in Australia is in the process of total change from centralized energy to distributed energy. In the near future as sharing on-site generated renewable electricity between apartments, tenancies and external buildings becomes more common, the installation of PV panels will be seen not as a cost and return on investment calculation, but a source of free power and potentially revenue on excess power once the investment is paid back.

​

This should make developers eager to plan for solar, and frustrated if their building is not built to accommodate it. Developers may also be able to sell the apartments but retain ownership of the rooftop.

​

Tip 1: Design roof to accommodate the maximum number of rooftop panels.

Tip 2: Most solar projects beyond a single dwelling require a structural engineering certificate to confirm the roof can take the panels. Wind loading is a prime consideration, especially for panels installed on buildings above 4 storeys. The dead weight is also significant. If the strength of the structure is unknown, the number of panels allowed will most likely be restricted.

The greatest wind loads are near the edges and corners of buildings, so for this reason panels may not be installed to the edges of some buildings. Also allow for walkways between panels for access and cleaning, although these might be included already where panels are tilted on a flat roof.

Tip 3: An ideal design should include:

• A dedicated area for:
  ​

  • ​Batteries

  • Inverters to convert the power generated from DC to AC for use in your building

  • A monitor to monitor energy generation, self-consumption, grid consumption and grid exports.

  • An optimiser for if there is some shading of panels

  • Heat pumps - heat pumps are the most efficient form of electric hot water generation. They are currently tall vertical units – taller than a normal HW system as the compressor is on top. Allow approx. 2m

  • other utilities
    ​

• An electrical riser from this area to the PV panels on the roof.

• The main distribution board located near this area

Including this will allow the inclusion of the required PV equipment now, or at some time into the future.

Tindo Solar are the only Australian manufacturer and installer of solar panels, and they install panels Australia wide. Their panel capacities are now at 380W and soon to be 400W per panel, and panels have a 25 year warranty. Due to an automated production process prices are competitive with Chinese supplied panels.

​

Electric vehicles and batteries - Within 5 years, electrical vehicles are expected to become normal and the ability to charge them will become an urgent issue for existing sites. In addition electric vehicles contain large batteries, and cars that are charged during the day will be able to supply power back to building users at night time.

Tip 4: New builds should factor EV charging into the design and the proximity to the main service infrastructure should also be well thought out.

​

Electrical distributers (owners of the street poles and wires) rules

Solar PV is pre-approved for connection for systems with inverter capacity of either 5kW or 10kW per phase depending on which of the five power distributors operates as the monopoly in your area (Jemena, United Energy, Ausnet, Citipower, or Powercor). This is not a limit but a pre-approval. Systems larger than this can often be installed but must have their export limited to either 5kW or 10kW.

For commercial sites larger than 30kW (at the inverter), almost all DNSPs now require network protection relays to be installed. This involves the installation of a “grid-box” near the main switchboard as well as a range of additional fees and mandatory tests such as injection testing. A typical installation between 30kW and 100kW requires about $8-10k in additional costs to implement.

System design rationale

Solar design usually starts with an understanding of the projected electrical load (consumption) of the premises.

​

System sizes and design are determined based on the maximising onsite use of the generated power of the load which gives the greatest rate of return. (As the value of the generated electricity ie the alternative price if you had to purchase from the grid is usually twice the price a site gets for exporting to the grid - the feed in tariff).

​

Since the value of offset is much greater than the value of export, it is often desirable to sacrifice the export value of a system for greater offset. So, installing a larger system than the pre-approved sizes will give a greater offset of heavier loads even though energy exported above the limited size will be cut off.

​

Also, larger than the limit systems are desirable if the system will be fitted with batteries as the lost export will be stored in the battery for later use.

Pitch of panels - Solar PV design works around a series of compromises in order to achieve the best outcome particular to the site. An ideal southern hemisphere system will generate the maximum energy when it faces due north with panels pitched at the latitude for the location minus 10 degrees. In Melbourne this equates to 37degrees minus 10 = 27 degree pitch.

​

On residential buildings, most tiled roof pitches vary between 22 and 30 degrees with an overwhelming number being 22 or 25 degrees. This is a very workable range of pitches with almost negligible losses due to not maximising the pitch on modern panels.

​

Tilting panels on flat roofs - On flat roofs, panels are generally installed on tilt legs or frames that allow setting the pitch at any desired level. Note the inter-row spacing of the panels has to be properly calculated so as the front row does not shade the one behind.

​

Laying panels flat on flat roofs - It is also possible to set panels directly onto a flat roof with no pitch. The loss is relatively small and by laying flat, a much larger system can be installed than a tilt framed system because the spacing to avoid shading is not required. The trade-off is in maintenance.

​

A flat mounted system will accumulate dust and dirt and not be able to self-clean through periodic rain so there is an ongoing maintenance cost in cleaning. A pitch of 10 degrees or greater provides better self-cleaning.

The azimuth of the system (the direction panels are oriented to) is also an important consideration. Prevailing design doctrine is that North is best as it generates the most amount of energy. However, this may not deliver the best outcome. Most residential properties have small loads at midday when North generation is maximum so the bulk of generation will be exported. Often West is a better prospect as household loads are generally higher in the afternoon.

​

Generally, though, EnviroGroup try to install panels on two faces of a property deliver the longest generation. North-East and North-West is close to ideal giving maximum generation for almost the entire day. North and West, or East and West are also good.

​

Batteries change the design priorities. Orientation becomes less of an issue since exported energy will be captured in the battery for reuse, so the time of day generation becomes less important. Note in the near future electric cars will be prevalent source of substantial batteries to absorb exported power.

(Above from discussion with and notes from Brad McPherson of EnviroGroup)