Small Photovoltaic Arrays

Solar Arrays and Systems

A solar array is simply a number of solar panels, or modules, arranged together to form one interconnected generation system. Solar arrays can be very large or relatively small, and due to their modularity can be configured in almost any manner to supply most loads. If an application requires more power than can be provided by a single panel, then larger systems can be made by linking a number of panels together. However, complexities arise because often very large quantities of power are required at specific voltages, and at a time and level of uniformity that cannot be easily provided directly from the panels. In these cases, PV systems are used to customise the output of arrays to better cater for the energy needs of the user, and generally are comprised of the components in Figure 1.

Figure 1 Elements of a PV System.

(a)     a PV panel array, ranging from two to many hundreds of panels.
(b)     a control panel, to regulate the power from the panels;
(c)     a power storage system, generally comprising of a number of specially designed batteries;
(d)     an inverter, for converting the DC (direct current) to AC (alternating current) power (e.g. 240 Volts AC – like most WA homes).
(e)     backup power supplies such as diesel generators (this is optional).

A framework or structure as well as housing for the system is generally required to dependably support and orientate the array towards the sun and keep the other components dry and clean (see Figure 2). Trackers and sensors to optimise the performance of the system are often viewed as optional in a PV system. However to ensure reliable performance, the design and installation of all PV systems in Australia should always be completed according to the following Australian Standards:

AS 4509 Stand-alone Power Systems (SPS) Parts 1, 2 and 3.
AS 4086.2 Secondary batteries for use with SPS - installation and maintenance.
AS 5033 Installation of photovoltaic (PV) arrays.
AS 2676 Guide to installation, maintenance … of secondary batteries in buildings.
AS 3011 Electrical installations - Secondary batteries installed in buildings (LV batteries).
AS 3010 Electrical installations – Supply by generating set.

Figure 2 Tracked PV Array containing 16 panels.

Arrays of panels are being increasingly used in building construction (building integrated), where they serve the dual purpose of creating a wall or roof as well as providing electric power for the building. Eventually as the prices of solar cells fall, building integrated solar cells may become a major source of electric power.
Trackers are used to keep PV panels directly facing the sun, thereby increasing the output from the panels. Trackers can nearly double the output of an array (see Figure 3). Careful analysis is required to determine whether the increased cost and mechanical complexity of using a tracker is cost effective in particular circumstances. A variety of trackers, which will take about 10 panels, are manufactured in Australia.

Figure 3 Graph showing power output for tracked and non-tracked array.

Arrays generally run the panels in series/parallel with each other, so that the output voltage is limited to between 12 and 50 volts, with higher amperages (the amperage are the units used to measure current). This is due to safety issues and to minimise power losses.

Benefits and Performance

Photovoltaic (PV) systems are used to provide power for many applications that require electricity. In remote areas where it is either inappropriate, too difficult, or too expensive to extend the electricity grid, alterative energy generation technology must be used. In many situations small photovoltaic arrays are the most practical and cost effective method of providing this power. Furthermore PV technology is simple, reliable, requires almost no maintenance, and is practical in many parts of the world with enough sun. For an indication of the global solar resource available for photovoltaic applications, see Figure 4.

For more information on solar resource see the RISE Sun File

Figure 4 The World Solar Energy Map showing the global solar resource available to PV applications

PV modules on the market today are guaranteed by manufacturer's from 10 to 20 years, while many of these should provide over 30 years of useful life. It is important when designing PV systems to be realistic and flexible, and not to over-design the system or overestimate energy requirements (e.g., overestimating water-pumping requirements) so as not to have to spend more money than needed. A well-designed PV system will operate unattended and requires minimum periodic maintenance. PV conversion efficiencies and manufacturing processes will continue to improve, causing prices to gradually decrease. However no dramatic overnight price breakthroughs are expected (El Paso Solar Energy Association, 2007). For the past and future trends in component costs of PV systems see Figure 5.

Figure 5 Graph showing component costs of PV system and price reduction over time.

The daily energy output from PV panels will vary depending on the orientation, location, daily weather and season. On average, in summer, a panel will produce about five times its rated power output in watt hours per day, and in winter about two times that amount. For example, in summer a 50 watt panel will produce an average of 250 watt-hours of energy, and in winter about 100 watt-hours. These figures are indicative only, and professional assistance should be sought for more precise calculations.

Like any such commodity, the total purchase price of a PV system is based on all inherent costs of producing the individual components, transporting these to the site and installing them. There may also be associated costs of designing and engineering the system and purchasing land - particularly for large-scale or one-off projects (IEA, 2002).

For more information see the RISE PV Technology page

Constraints

People should be aware of the practical limitations of PV systems and major areas are discussed below.

Technicians and buyers are often unfamiliar with PV technology, and when systems in remote locations do break down, there may be a lack of servicing, spare parts, or trained personnel. In reality, PV is much less maintenance intensive than diesel systems. Because PV only supplies energy when the sun is shining, there is usually a need for storage so there is available capacity when there is no sun. This adds extra cost to the investment. Also the capital costs of PV systems are currently higher than other conventional systems, but the running costs and maintenance costs are much lower. However this up-front outlay is prohibitive in many circumstances, especially for larger systems.

Costs

In February 2006, panels cost between $3 – 10 per peak Watt, depending on the PV technology. That is, a 50 Watt panel presently costs around $200. Ten years ago, this same ‘standard’ panel may have cost about $500 at a cost of about $8 - 10 per Watt.

Energy storage is often necessary when power is required when the sun is not shining - either at night or in cloudy periods - or in quantities greater than can be supplied directly from the array. Specially designed "deep-cycle" lead acid batteries are generally used. Unlike normal batteries, they can discharge about half of their stored energy several thousand times before they deteriorate. Each battery is usually 2V, and the total battery bank usually has many batteries in series and parallel to give the required power rating. Battery banks need to be individually sized to suit the particular applications, depending on total daily solar radiation, total load, peak load and the number of days storage required. Generally, battery storage costs about $250 per kWh of energy stored for domestic sized systems.

Inverters transform low voltage DC power (eg 12V, 24V, 32V or 48V from batteries) into high voltage AC (generally 240 V in Australia). Inverters are necessary if mains-voltage appliances are to be used. In assessing the cost of the total system, it may be more economical to purchase an inverter and mass-produced consumer appliances than to use low voltage DC appliances, which may be more expensive than normal appliances.

Some appliances, such as high efficiency light globes are not presently available for low voltages. In this case, the cost of more panels must be balanced against the cost of an inverter. As a rule of thumb, inverters cost about $1 - $2 per watt of output, depending on size and features. For example a 1.2 kW sine wave inverter with energy management features costs approximately $2500. The major production costs of inverters are derived from the design, development, raw material, and product assembly factors of production. There are several Australian companies that manufacture inverters for the local and export market and currently there is local research focused on substantially reducing the cost of large inverters especially.

Backup or auxiliary power supplies are required when complete reliability of electricity supply must be guaranteed, when it is uneconomical to provide battery storage for infrequent extended cloudy periods, or when some appliances have large and intermittent power requirements that are uneconomical to meet from the PV system. For further information on batteries, inverters and other enabling technologies, see the RISE Enabling Technology page.

Sometimes wind generators are used in conjunction with PV systems if the combination of sun and wind is viable. Small petrol or diesel generators are often used as the backup supply of power. These petrol or diesel generators are relatively cheap to purchase (less than $1000 per kW) but expensive to run. Several Australian companies are developing total hybrid supply systems that optimise the use of each component to the specific conditions of the application.

In terms of average unit energy costs calculated using traditional accounting techniques, PV generated electricity cannot yet compete with efficient conventional central generating plants. Accordingly, the vast majority of PV installations to date have been for relatively low-power applications in locations, which do not have ready access to a mains electricity grid. In such cases, PV has been selected because it offers a secure and reliable power supply, and is often the cheapest power option.

Conclusion

Solar photovoltaic array applications are growing in scope, scale and in economic viability. At present this is mostly limited to off grid applications requiring relatively small amounts of energy. However due to the increasing economies of scale in production and development, and more service infrastructure provision, this area is sure to expand to be one of the pillars of energy generation in a sustainable world.