Keywords

Solar modules; encapsulation; inverter; grid synchronization; off-grid; grid support; power point tracking; balance of plant components

Cell and Module Outputs

The output of a solar cell or solar module varies with the load that is applied to it. When it is open circuit, the output voltage will be at its highest but it will be supplying no current. When a load is applied to the output, the output voltage will fall as the current increases. Initially the voltage will fall gradually, and linearly, as the current increases from zero. Then, at a certain inflection point the rate at which the voltage falls will increase. This inflection point is called the operating point of the cell, and it is the point at which the cell provides its maximum power (see Fig. 10.2). The power at this point is called the “peak power output,” and this figure is used as the rating of the solar cell or module. Peak power output is affected by the temperature of the cell, and at higher temperatures it falls off slightly.

A typical silicon solar cell will have a surface area of 15 cm². A commercial cell of this size will have an output of 6 A at 0.55 V, or a peak power output of around 3 W. Module sizes vary, but a commercial rooftop module might contain 60 individual, interconnected solar cells providing an output of 250 W at peak output, at a voltage of around 30 V and a current of 8.3 A. Typical outputs for commercial panels vary between 175 W and 315 W, although most are around 200–220 W.

Table 10.1 compares the characteristics of solar modules made from different types of solar cells. The module efficiencies follow the material efficiencies already discussed in earlier chapters, so that single crystal (monocrystalline) silicon modules are the most efficient, with a typical efficiency range of 15–18%. These cells generally produce 110–140 W for each square meter. Polycrystalline modules are slightly less efficient at around 13–15%, and produce 110–130 W for each square meter. Meanwhile, amorphous silicon modules have a much lower efficiency, 6–8%, and generate only 50 W to 80 W/m². Cadmium telluride modules are more efficient than this, with an efficiency range of 9–11% and an output of 80 W–99 W/m². Meanwhile, copper indium diselenide has an efficiency of 10–12% in module form and produces 90 W–110 W/m².

Inverters

In some situations it is possible to take the DC output from a solar panel and use it directly to provide power. For example, the panel might provide a dedicated power supply to a specific piece of electronic equipment. Virtually all modern electronic equipment requires a DC supply, and if supplied from the grid, the AC supply must be converted to DC with a consequent energy loss, so a direct DC supply can be more efficient. Further, a solar module could provide power directly to charge a battery. Solar modules destined for this type of stand-alone operation can be sized to match the needs of particular battery or electronic equipment.

This type of direct DC connection from solar module to power consumer is feasible, but in most situations it is actually more efficient to use an interface between the solar module and the equipment or battery being supplied. This interface, which is designed to optimize power capture from the solar module under varying light conditions, is called an inverter.

An inverter takes the power from the solar pV device and conditions it for use. In a typical stand-alone application an inverter might control the output of a solar module being used to charge a battery. In this case it will take a varying DC output from the array of solar cells and convert it into a stable DC supply. To do this, the DC power is converted into a high-frequency AC supply and then back to a stable DC voltage suitable for the battery.

More often a solar power system will be required to provide an AC output at the grid frequency so that the solar system can be integrated into a grid-supplied system. In this case the inverter takes the DC output from the solar panel or from a set of solar panels and converts it into an AC output. In doing so the inverter has to cope with a continuously varying output from the solar devices as the solar conditions vary, and yet still produce a stable output at grid frequency and at grid voltage.

There are a range of different types of converters available to suit different applications. These include stand-alone inverter systems where the solar output is not connected to a grid-supplied system. Meanwhile some hybrid inverters send DC power to a battery bank to keep an energy storage system charged while the remainder of the power from the solar system is used to supply power to the grid or to a domestic or commercial establishment that is grid-connected. Finally, the most common type of inverter today only provides an output that is grid-connected and grid-synchronized.

The inverter itself is an electronic module that uses solid-state power devices to condition the power output from the solar panels. For smaller domestic and commercial solar pV installations there are two types of inverters in common use. One type, called a micro inverter, handles the power from an individual solar panel. The second type, called a string inverter, takes its input from a number of panels. Each has its advantages and disadvantages. The string inverter is the most well-established type of inverter for solar pV, and it is the cheapest and potentially the easiest to install. However, each string inverter must handle the power from multiple panels so they must have a high power capacity, and this can affect the cost and reliability of the components from which they are made. In addition, a string converter treats all the panels connected to it in the same way. The inverters use an adaptive technology to try and obtain the maximum output from the solar panels, but if one or more of the panels is shaded or soiled, or if a panel fails, this can affect the performance of all the panels. String inverters also have limited flexibility when it comes to expanding a solar array as they are normally sized for the initial number of panels. This makes it difficult to add more panels at a later stage.

Micro inverters are designed to manage the power from a single solar panel. There will therefore be a lot more of them in a system that involves more than one or two panels, and they will usually be more expensive than a single string inverter. However, since each panel is managed independently, the effect of the shading of one panel, or a panel failure, will be confined to that panel. Micro inverters also handle, individually, much lower levels of power than string inverters, and can be made from potentially smaller and more reliable electronic components. Advocates claim that they are capable of improving overall efficiency of a solar power system by between 5% and 20%. Micro inverters are potentially more flexible than string inverters because new panels can easily be added to expand a system. Some solar panel manufacturers are beginning to supply panels with integrated micro inverters, making installation even simpler.

Utility-scale solar pV power plants generally use large string inverters because the cost of micro inverters would be too high; their advantages are much less clear. Utility plants normally have tracking solar modules and the units are cleaned regularly to maintain performance. In addition, they are sited so that modules are never in the shade, except as a result of the Sun setting. Therefore, being able to isolate the performance of each panel is not so important. Expanding a utility plant would involve the addition of multiple modules too, so there is no advantage to be gained from being able to expand one module at a time. While inverters for rooftop applications may be able to handle several kilowatts, for utility plants the inverters have capacities of up to 500 kW each. In a typical plant, individual inverters deliver their power to external transformers that feed into a medium-voltage power collection system connected to a central substation where a substation transformer steps the power up to grid voltage before delivery into the system.

Maximum Power Point Tracking

In order to get the best from solar cells and solar panels, most modern inverters use a technique called maximum power point tracking (MPPT). The technique is employed to optimize power capture from solar cells under varying conditions. The output of a solar cell varies depending on the intensity of the light striking it, the angle of incidence at which the light strikes the cell, and the temperature of the cell. For each set of conditions, that solar cell has a specific maximum power point, a specific voltage and current for which its power output is at its maximum. MPPT invokes systems in the inverter that are able to sense how the actual output of the cell compares to its optimum as the conditions vary, and then adjusts its own operating conditions so that the cell can produce its maximum power.

Some of these MPPT inverters use simple approaches such as a fixed output voltage to control the solar cells. However, others use a much more sophisticated approach such as “perturb and observe,” where the output conditions (current and voltage) of the cell are altered slightly at regular intervals, by the inverter, and the power measured; if the new power is higher, then the conditions are moved further in that direction until the power starts to drop off, when the optimum point has been found.

Gains from MPPT will vary. However, it may be possible to increase performance by 10–20% during the summer and 30–40% during the winter by using the technique.

Grid Interfacing

While some solar pV systems operate independently of any grid system, the majority of pV systems in use are grid-connected. Large- and medium-sized utility power plants based on solar pV generation feed all their power directly into the grid, and they have to be able to meet grid codes of performance when they do so. However, many solar pV systems are much smaller rooftop systems that supply power either to individual dwellings and to commercial operations, or operate as distributed generation systems supplying power to a group of small users. In each of these cases the system will usually have a dual role, to supply power to its local consumers and to supply power to the grid. These, too, have to meet grid standards.

Grid connection is controlled by a stringent set of codes and conditions to ensure that a system connected to the grid does not degrade the grid supply by affecting grid voltage, grid frequency, or by adding unwelcome noise or harmonics to the grid. Depending on its size, a solar installation may also need to be able to provide support for the grid, either by being able to increase or reduce its output on demand from the grid controller, or by helping stabilize the grid voltage and frequency. Further, any pV system connected to the grid will have to respond in a controlled way to the grid power supply failing, a situation known as islanding.

The inverters used to convert the DC output from a solar panel into grid frequency AC are capable of controlling the frequency and voltage they produce for the grid as the input voltage from the solar system varies. The control systems are often fast acting and their output stable. This type of output can help strengthen the grid and reinforce it against fluctuations caused elsewhere in the grid. However, for this to be effective, there needs to be some form of communication between the power supply unit and the central control room that manages the grid.

Communication systems of this type are relatively expensive and are not yet widespread. If such communication systems are established, it becomes possible to modulate the output of the solar unit to suit grid needs. If this is coupled with accurate weather forecasting, which helps predict how solar output will vary during the day—potentially on an hourly basis—then even rooftop solar installations can be used for grid support services as well as supplying power. This will become more important as the proportion of solar pV on the grid increases in coming years.

It is also important that power systems connected to the grid behave in a specific and predictable way when the grid supply fails. If the main supply trips as a result of a problem somewhere across the system, a pV system will normally continue to generate power. If this pV system is connected to the grid, then it will behave as an island of live power when the rest of the grid has lost power. This can cause problems on the grid side when technicians are trying to rectify the fault, and on the power plant side if it is feeding power into a dead grid. To avoid this, grid codes usually require that a solar pV system that is grid-connected be switched off or disconnected from the grid in the event of a grid-side failure.

Solar Photovoltaic Installations

Solar cells can be used in a wide variety of situations, and this flexibility distinguishes them from any other form of power generation. If one excludes the smallest applications, in consumer electronic devices, for example, the application of solar pV is usually broken down into three or four categories. The main groups are distributed solar pV—including the many rooftop installations now in use—utility power plants, and off-grid applications. The distributed category can be further subdivided into domestic rooftop pV, commercial and industrial rooftop installations, and some commercial and industrial ground-mounted solar pV facilities.

The sector that drove solar pV during the 1990s and the first decade of the 21st century was the domestic and small commercial rooftop use of solar panels. In several countries this was supported with government grants, and these programs led to a massive expansion in the use of solar pV. The most important of these early programs were in Japan—where a program ran from 1994 until 2005—and in Germany and the United States. Subsequently, incentive schemes of various types were launched in many countries, and many of these continue today. However, the level of support is beginning to tail off as the cost of solar cells falls to a level where subsidy is no longer needed.

The early incentive programs were vital in driving up the volume of solar cell production and bringing the price down to a level at which they now approach grid parity. In 2010 residential solar pV installations accounted for 63% of global solar pV capacity according to the International Energy Agency (IEA).

Commercial and industrial solar pV represent a much smaller part of the total installed capacity. These installations are generally nothing more than larger versions of the typical domestic rooftop installation, although some of the largest are approaching utility-scale. Trends include companies covering the roofs of their distribution centers with solar panels. At the end of 2010 commercial installations accounted for 11% of global capacity.

Off-grid use embraces a wide range of applications. There are, for example, many different types of stand-alone installations, from air pollution monitoring stations to traffic monitoring and remote relay stations, that rely exclusively on solar cells for their power. There are also large numbers of people, particularly in the developing world, who have no access to grid power. Solar pV installations are increasingly being used to supply power to off-grid communities, and as the cost of solar cells falls, the economics become easier. There are also a wide range of individual dwellings in the developed world that are still remote from the grid, or choose to remain off-grid and use solar power, perhaps with wind power, to supply electricity.

While all these sectors continue to be of importance, the fastest growing sector in the second decade of the 21st century has probably been utility-scale power plants based on solar cells. The first megawatt-scale plant of this type was the Lugo plant in California with a generating capacity of 1 MW. It started generating in 1982. The Lugo project was an outlier as was a 6 MW plant that was built in the United States in 1985. After these two, large-scale pV plant construction was abandoned until early in the first decade of the 21st century because the economics were not sustainable. Since around 2003 the capacities of both individual plants and the aggregate global utility plant have risen dramatically. The world’s first 10 MW plant was erected at Pocking in Germany in 2006. A plant with a nominal capacity of 97 MW was built in Canada in 2010, and in 2011 a 200 MW plant was built in China. More recently, a 550 MW plant was built in California in 2013, and in 2015 this was overtaken by a 579 MW plant, also in California. The total capacity of utility-scale solar pV power plants at the end of 2014 was 36 GW according to Wiki-Solar, or roughly 20% of the total global solar pV installed capacity.

As solar pV capacity continues to grow, the main market segments will remain the same, although proportions may change. The IEA has predicted that in the period from 2014 to 2040, utility-scale and rooftop systems will each take roughly half the market.⁴