The case for north-south pitch
GSES investigates the effect of a north-south split pitch on array output and a rethink of optimum design.
Solar installers or system designers in Australia know that orienting a system to the north will result in maximum year-round generation.
For sites where a north-facing pitch is not readily available, an east-west array can be the next best solution.
Intuitively, south-facing arrays do not make sense. However, systems have been designed in which PV modules on southerly roof slopes have been tilted away from the surface to face north.
But what exactly is the reduction in generation? Is a south-facing array to be avoided at all costs, or are there cases in which it may be beneficial to the client?
To find out the effect of different orientations (including south-facing arrays in the system design), GSES investigated using PVSyst software to plot generation figures.
It is expected that arrays with southerly aspects will have higher generation in summer and lower generation in winter, compared with east-west arrays.
However, the precise effect needs to be quantified to provide guidance for installers and designers.
The PVSyst analysis was carried out to compare the output of east-west and north-south arrays in all capital cities in Australia.
The arrays are split evenly between the two aspects, with the output of the whole system calculated at 10°, 15°, 20° and 30° pitch.
The minimum pitch angle was chosen on the assumption that modules will be pitched to a minimum of 10° to aid self-cleaning.
The common pitch angles for Australian homes are 15° and 22.5°. The pitches 15° and 20° (an approximation of 22.5°) are therefore chosen for analysis.
A 30° pitch is chosen as the maximum angle for ground-mount arrays to strike a balance between inter-row spacing and system yield in larger designs.
The results are displayed in specific yield per day (kW/kWp/day).
The results show that at most locations the specific yield is similar at pitches of 10° and 15°. It may be acceptable at 20° for most locations, but it starts to become significant at 30° for all sites other than Darwin.
Darwin, being in the tropics, shows that a north-south array suffers similar losses to east-west arrays regardless of array tilt angles.
It was interesting to observe that in most places north-south arrays have better average specific yield than east-west arrays at higher pitches.
For example, even a 20° pitch north-south array in Hobart has a higher specific yield than a 30° pitch east-west array at the same location.
This finding shows that having a substantial amount of the sub-array facing south may not be as detrimental to system yield as one would intuitively think.
In a north-south layout, the south-facing array has less output in winter months.
However, this is offset by a higher output in summer months, compared with east-west arrays.
This means that north-south arrays may be beneficial for residential systems that experiences high loads in summer – eg: for air-conditioning – but have much lower heating loads in winter.
For instance, south-east Queensland households may not have high heating loads in winter but will have substantial cooling loads in the summer. Additional arrays on a south-facing roof could help reduce the electricity bill.
This is not to discount the fact that east-west arrays (especially west facing) can cope better with afternoon loads, but it highlights that south facing roofs pitched at 15° to 20° may still provide solutions.
The system owner would need to be provided site-specific monthly energy production estimates for arrays facing east, west and (especially) south so as to avoid bill shock.
The analysis finding is applicable to commercial applications for a few reasons.
North-south arrays can increase flexibility of system design and increase the array packing density.
The information could also guide the decision on whether a south-facing roof surface can be used for installation.
Designers may also find that customers can benefit from a greater financial return with flush-mount southerly arrays, compared with tilt-adjusting the arrays to face north. The factors here may be an increase in packing density and reduction in installation cost.
East-west arrays pitched at shallow angles on shallow or flat roofs have been shown to increase packing density, enabling larger systems to be installed in the same roof space.
At 10° pitch, the difference in yield between an east-west array and north-south array is 0.1-0.3%. Not being restricted to an east-west arrangement can allow more flexibility in system design on unusual roof shapes. This allows a balance between available area, packing density and access for maintenance.
Installations on large sheds with a shallow pitch of 8-10° may be able to take advantage of existing roofs with southerly aspects using flush mounting of arrays.
Flush mounting would reduce the installation cost compared with tilt-frame systems. If the options were between flush mount and ground mount, the cost difference would be even greater. This design would particularly benefit manufacturing processes with high refrigeration loads, where the high summer power consumption can be matched with higher generation.
The inter-row spacing of northerly pitched arrays may even be utilised to achieve higher packing density, which would benefit sites with intensive energy use.
When comparing north-south arrays to east-west, south facing arrays should not be completely disregarded as an option.
This knowledge can enable system designers and installers to provide a wider range of PV solutions to perspective customers.
GSES offers feasibility assessment services to determine the technical and financial risks and benefits of adding a renewable energy generator or energy efficiency measure. The company can also carry out preliminary and detailed PV system design services. Visit www.gses.com.au.