By Jack Ward, CEO of Soltra Energy
In what direction should roof-mounted solar photovoltaic (PV) panels be sited for optimum energy production?
Conventional wisdom says the panels should be orientated towards north in the southern hemisphere to gain maximum benefit from the sun’s arc. More specifically, solar panels should be pitched between 25 and 35 degrees (approximately equal to the site’s latitude) to allow for the most efficient power generation.
The result, from a power production standpoint is a true ‘bell curve’ reflecting power increases throughout the day peaking at midday and gradually falling again to zero at sunset.
However, in certain circumstances it may be advantageous to point the panels eastwards – or westwards.
In one such instance – a grid-linked hybrid application in Johannesburg – an east-west orientation was found to be advantageous. The motivation was the limited electrical power supply from the local provider which was unable to meet full demand at the business facility. Another challenge presented to the solar PV design team was the need to reduce the aggregate electricity cost for the facility.
An iterative evaluation process was undertaken which focused on testing various solar PV system configurations. The tests initially evaluated power production from traditionally north-facing panels. Subsequently, various directions were tried culminating in a convention-breaking east-west configuration.
The initial goal of the test programme was to match energy production to the measured load profile, while the second objective was to maximise the financial benefit.
For simulations conducted with an equal number of panels and inverters, the east-west configuration showed immediate advantages as it provided power production earlier in the day, with a slightly lower peak. It was thus a better match for the facility’s demand curve (see graph).
While a more advantageous match was achieved, concerns were nevertheless raised about the loss of yield. A number of different angles of inclination were subsequently tested and eventually an angle of 20 degrees from the horizontal was chosen as the best compromise between ‘flattening’ the production curve and loss of yield.
That said, the east-west facing solution, with a 20 degree inclination, showed a total loss of yield of approximately 2.5% when compared to the north-facing alternative.
Nevertheless, when this is compared with the facility owner’s electricity tariff/ billing structure from power producer Eskom, the east-west installation demonstrated the following financial performance:
- In summer months a nett R6-50 per day saving was achieved.
- In winter months a nett R8-50 per day loss was returned.
It is noteworthy that, as the summer tariff benefits apply for nine months of the year, the result is an approximate R1050-00 per year additional saving over a traditional north-facing system.
When the capital cost of the system is considered, this is a relatively small saving and could be considered as a ‘break-even’. However, there are a number of other advantages presented by the east-west orientated installation.
- The lower peak output of the system means that the inverters can be over-panelled by approximately 10% without any change to the inverter system or balance of plant. In a typical rooftop system the panels and installation comprise 70-80% of the turnkey cost.
- The east-west installation results in approximately 5% lower installation cost because the brackets and mounting material are used more effectively.
- The panel density on the roof can be as much as 30% higher, allowing for a higher yield per square metre.
When the above factors are taken into account, it is clear that the east-west oriented system is comparable to a north-facing system on a ‘cost of energy’ versus a ‘kilowatts peak (kWp) installed’ basis. (kWp is essentially the rate at which a solar PV installation generates energy at peak performance.)
Furthermore, when the added advantages of over-panelling the inverters at a fraction of the cost of the entire system is maximised, and the lower cost of installation is taken into account, an east-west orientated installation is seen to provide significant cost advantages over a north facing system.
One of the keys to the success of similar installations going forward lies with their management. Sophisticated ‘smart’ power management solutions can now be installed and tailored to users’ needs.
These systems will, for example, complement grid power with solar power when necessary (at peak times), divert excess solar power to possible battery storage for later or after-hours use as appropriate, and fire up a petrol or diesel generator to integrate seamlessly into the power supply grid should battery storage become depleted.
A range of micro smart-grid solutions that measure the generated solar power on a minute-by-minute basis, compare it to current grid power availability and assess current load states is available.