Ten households monitored their grid-connected photovoltaic systems to see if variations such as tilt angle really matters. David Cunnington and Simon Pockley from Merri Solars report on their twelve month study.
The recent boom in solar households means that a lot more comparative data can be collected about how these systems work under different conditions. Take tilt angle for instance. What angle works best for your solar panels at what time of the year? Ten households living within a stone’s throw of each other, collected data over twelve months.
The installation
Merri Solars began as a Sustainability Street program in inner-suburban Melbourne. In late 2007, following a series of water audits and a group purchase of water tanks, the group decided to bulk purchase grid-connected photovoltaic systems. The main incentive was the federal government rebate, which at the time was up to $8000 for a system greater than 1kW, with no means test on eligibility.
The group sought quality and reliability over price and, in keeping with their philosophy of acting locally, chose a local supplier. Thirteen households went ahead and signed up for the installation.
Installations occurred over three weeks in May/June last year, but there were complications with coordinating energy distributors and meter changeovers, so it was mid July before all systems were fully connected. Ten out of 13 households agreed to collect data from their inverters in order to compare the performance of what were very similar systems:
• Siemens 175 watt panels rebranded as Solar World
• Sunnyboy inverters
• All solar arrays face due north (except for one on a north-west compound angle and later another with a north-east orientation).
• Close proximity, therefore minimising climatic variations
Six households installed 1kW systems with six panels. One member opted for seven panels, two members had eight panels and one had ten panels.
Tilt angle
Apart from the number of panels in each system, the main difference was the tilt angle of the panels (15 to 42 degrees). There was vigorous discussion about whether it was better to be optimised for summer (shallow angle) or winter (steep angle). Half the systems needed frames; the other half followed the roof pitch angle.
Comparing performance over 12 months allowed the group to see if tilt angle was a significant factor.
Collecting data
Sunnyboy inverters are designed ‘to go to sleep’ at the end of the day to save energy. When it gets dark, they dump the daily performance data. However, a cumulative total of the number of generating hours (h-total) and a cumulative total of the kilowatt-hours generated (E-total) is retained and displayed when they are ‘awake’.
In winter, when it was nearly dark by 5pm, there were difficulties collecting data because the inverters sometimes shut down by the time people arrived home from work and had not turned on when they left the next morning.
Results
Over 12 months all the 1kW systems (made up of six panels) generated more than 1.2 megawatt-hours of electricity each. The larger 10 panel system generated more than 2 megawatt-hours of electricity.
Graph 1 shows per-panel output of these PV systems over 12 months and the extent to which all the systems were dancing to the same tune in spite of the variations in tilt angle and system size. Tilt angle is not a significant performance factor. There is a difference of only 9.19% (20kWh) between the highest and lowest output. However, the highest overall output came from the system with panels tilted at Melbourne’s latitude (37.5 degrees). The lowest came from the system with the north-west orientation. As expected, the output from the shallow tilt angles raced ahead of the steeper tilts in summer and then fell back in winter.
While shading events and even bird poo reduced output, location (curiously) appeared to be a factor in that the three highest outputs were next door to each other, yet had quite different tilt angles.
As expected, output was highest in late January and lowest in mid June. However, there is a noticeable departure from the expected bell-shaped day length curve. The most significant factors that impact on performance appear to be cloud cover, smoke, dust and temperature.
Graph 2 shows the flip side of these figures by comparing the rate of conversion of the available solar energy over the seasons. Available solar energy information came from Bureau of Meteorology data collected at Melbourne airport. The cooler seasons predictably favour conversion efficiency. The highest reading was 19.61% (July) and the lowest reading was 8.23% (December).
After 12 months householders report that their electricity usage has not changed significantly although many households spawned an ‘electricity nazi’ who turns off unused lights and appliances. Members of the group are confused about the impact of patterns of use on feed-in tariffs, with uncertainty about whether to use large appliances such as washing machines during the day or at night. Most householders reported dissatisfaction with the format, content and veracity of their energy retailer’s power bills.
Perhaps it was inevitable and predictable that there would be more interest from the group in government energy policy, rebates and feed-in tariffs. Overall, the group has not only become more energy conscious but also more politicised.
No regrets?
One of the main difficulties the group encountered at the pre-installation stage was inadequate communication between the government, the supplier and the group about process and timelines.
The supplier held discussions with each householder about the suitability of installing a PV system and the preferred location of the panels, inverter and wiring. All the inspections were completed within a few days but some householders were unable to attend, so nominated neighbours or family to negotiate with the inspection team, causing a few problems with location and framing when the installations began. These were resolved later, but attendance at the inspection is highly recommended.
The matter of RECs had been raised during the negotiations, but a clear understanding of the implications of selling your RECs (see ReNew 105) is important. All households decided to sell their RECs, assisted by the supplier, and were happy to receive a substantial cash payment while awaiting their government rebate. Some now regret the decision.
Graph 1. Comparison of watts per panel generated by each system
Graph 2. Seasonal conversion of available solar energy
This entry was posted on Thursday, September 23rd, 2010 at 4:16 pm