- Weather & Local Environment
Weather & Local Environment
Local weather can have a dramatic effect on the electricity production from a PV array. The most obvious factor is the amount of sunlight hitting the panels, but air temperature, humidity and wind regime also play a role. Local environmental conditions and rainfall patterns affect the degree to which panels become dusty or otherwise fouled and this, of course, affects energy production.
To get a better understanding of the relationship between weather, its interaction with other environmental factors, and photovoltaic output, weather data is logged and stored. The Live Data Feed shows the current sunlight level, the air temperature, humidity, wind speed and direction data gathered from a number of sensors around the St. Lucia campus. The UQ School of Geography, Planning and Environmental Management also provides on-line access to information from its weather station at St. Lucia. This Weather & Local Environment web page outlines the effects of weather and local environment on the performance of PV arrays.
Weather and Environmental Factors
As everybody knows the amount of sunlight varies seasonally, both in intensity and duration. Summer brings long days and at any given time of day, the sun is higher in the sky and hence more powerful than in winter. The maximum sunlight intensity on a bright summer day may be around 1400 W/m2 whereas it may be less than half that amount in winter.
In any given year, monthly sunshine hours and hence PV output will vary, but on average output dips in winter and rises in summer. A factor to consider is cloudiness. Subtropical areas, like Brisbane, often have more cloud cover in summer than in winter due the development of the monsoon. This factor can counteract the tendency towards increased sun duration and intensity in summer. During the first six month’s operation of the Live Data Display, that is, the second half of 2011, these seasonal factors can be seen at work. The month by month output graph for June to December 2011 is shown in the left most thumb nail screenshot below. Choose “Pick a Year” and “2011” on the RHS of the Live Data Feed to obtain this graph. The sunlight measure shown on the graph is the average intensity for each month (W/m²) on a whole day or 24 hour basis rather than just during daylight hours. It therefore is a proxy for the amount of solar energy potentially available in each month for PV conversion to electricity.
To get the most energy production over a year, solar panels are usually oriented north and tilted at an angle to the horizontal approximately equal to the site’s latitude – in the case of Brisbane this is about 27°30" south – panels are typically tilted at 30° as this angle is easy to set out. In the subtropics, the sun in winter is not as low as say in Sydney or Melbourne. This means that in Brisbane and places further north, panel orientation and tilt is not so critical. Quite often, satisfactory output can be achieved at least cost by simply placing panels flat on low angle roofs, even if this means they do not face north.
On a clear day, the output from a flat fixed panel will rise and fall in line with the movement of the sun across the sky. This results in a typical bell shaped power curve as shown in the middle thumb nail screenshot below. The screenshot shows the power output from the UQ Centre Array at St Lucia on 26 December 2011. This day was the best energy day for UQ Centre during 2011. Interestingly, this day was not the best day for peak power production. The reasons for this are discussed in the section below on temperature. Even though the 26 December was fairly hot, over 30°C (which tends to reduce PV output), the duration and power of the sun made this a perfect day overall for PV production.
As mentioned in the information on PV Modules presented on the PV Features Page, the potential electrical power from a PV panel falls as the temperature of the panels rises. High panel temperatures are usually caused by bright sunshine; overall the high level of sunshine usually compensates for the temperatures de-rating of the panels and PV output rises with increasing sunshine levels. The interplay of sunlight and panel temperature and ambient air temperature is complex, however.
Actual panel temperature is recorded for a number of arrays at the St Lucia campus but the data is not yet accessed by the Live Data Feed. The Live Data Feed taps into ambient air temperature data and to a degree this can be used as a proxy for panel temperature. Furthermore, high ambient air temperatures reduce the transfer of heat gained by panels, from the sun, to the surrounding environment. For a given amount of incident sunlight, PV output will normally drop with higher ambient air temperatures.
Cool yet sunny conditions can occur in Brisbane, especially after southerly wind change has moved through SE Queensland. Such conditions along with a clear sky can be expected to produce high power outputs. The most dramatic effect of temperature can be seen when the sun breaks through on a cool, very cloudy day. As the cool panels are suddenly exposed to bright sunshine, the power output will soar.
This effect is well illustrated for the UQ Centre where the best power day in 2011 was the 18 October, which was generally a cloudy day but with the sun frequently breaking through for short periods (please enlarge the two thumbnail screen shots below). The weather records for this day show a good level of sunlight levels and the same pattern of broken cloud cover (for example, 9 December 2011), but these days had higher ambient air temperatures than in the 18 October 2011. The ambient air temperature when the peak power output was recorded on this day, was only 20.7° C.
The local wind regime can affect PV output in a variety of ways. Its biggest impact is on the location of arrays and the design of the panel support structure. When people see a large area of PV panels on a roof they often wonder about the ability of the roof to support the load; they are usually thinking about the weight (dead load) of the panels. The critical load on a roof with PV panels is, however, usually the wind load created by the panels, not the dead weight of the panels themselves.
Placing panel on tilt frames and orientating these to optimize the PV production can result in large wind loads on a roof. If the strength of the roof is assessed as inadequate to support the wind load created by tilting panels then it may be possible to lay the panels close to and parallel with the roof. In subtropical zones like Brisbane, laying panels flat on gently sloping roof will only slightly reduce PV output, compared with ideal tilt and orientation.
The potential electrical power from a PV panel falls as the temperature of the panel rises. The temperature of a panel will depend on several factors; mainly the amount of sun-light, which in bright conditions will heat the panel; and the ambient air temperature and amount of wind which affects the rate heat is conducted away from the panel.
Panel temperatures in SE Queensland on a bright usually exceed 50oC which is well above the 25oC the panels are rated at. This means that, even on a very hot day, when the ambient air temperature is say 35oC, the more air there is moving over the panels the greater will be their heat loss. The effect depends on panel orientation and tilt with respect to wind direction and the degree to which the panel is exposed to the wind in the first place. Relative humidity of the ambient air also affects the heat transfer rate from solar panels.
Wind can also play another role in PV output. Dust and organic material such as leaves can gather on a panel and effectively shade its PV cells from the sun. Regular moderate winds can help remove light material like leaves and twigs and, to a degree, dust from panels.
Wind direction and speed, as well as relative humidity data, are logged at the St Lucia campus so that they can be correlated with PV array electrical output. The aim is to improve our understanding of the impact of these environmental factors.
Rain associated with clouds and a rainy environment is usually not conducive to good PV production. Infrequent, short duration heavy rain can play a positive role, however. Solar panels are increasingly using special transparency glass, sometimes with special surface treatments, such as anti-reflective coatings. This expensive technology can be undone if the panels become fouled with dust, leaves and other bird droppings.
Even if only a few cells are covered by debris, then the output of a whole panel can be disproportionately reduced. Similarly, an under- performing panel in a PV string can seriously reduce the output of the whole string. Technological development is reducing this problem, but it will still be better to have a lean panel than one that is fouled. Even if only a few cells are covered by debris, then the output of a whole panel can be disproportionately reduced. Similarly, an under- performing panel in a PV string can seriously reduce the output of the whole string. Technological development is reducing this problem, but it will still be better to have a lean panel than one that is fouled.
Tilted panels often have bird spikes along the top edge to reduce the fouling problem. In places like Heron Island, for example, where thousands of birds nest each season, there are simply so many birds “in the air” that the panels are bombarded with guano. Flat to near flat panels are particularly vulnerable to fouling. In the thumbnail photo below, the effect of birds on recently cleaned panels on Heron Island has just started. Regular cloud busts of rain can mitigate this problem. If the local rainfall cannot deal the problem, then a panel washing program will need to be implemented.
The same problem arises with leaves, pollen and other tree sourced debris. Careful pruning of nearby trees can reduce the problem and regular heavy rain can help as well. Once again a panel washing program matched to the local environment may need to be developed. Dust is a particular problem in dry inland areas. Having the PV panels elevated on high roofs helps the situation, but a washing program may be needed. There is research in the field of nanotechnology aimed at developing dust repellent surfaces for glass. This will be of great interest to PV panel manufacturers as well as the managers of high rise buildings where window cleaning is major maintenance issue.