While single photovoltaic cells can be used directly in certain devices, for solar power generation one usually uses solar modules (also called solar panels), which contain multiple photovoltaic cells. There are also hybrid modules that also generate heat (see below), but these are far less common than pure photovoltaic modules.
Most solar panels have a rigid construction with a rectangular shape and dimensions that are convenient to handle, for example, when mounting panels on a roof. In most cases, they are assembled into larger arrays. They can be mounted in a variety of ways, such as on a roof, in a field, or sometimes vertically on walls.
There are also thermal solar modules, producing only heat, e.g. for hot water generation and/or heating buildings. This article, however, focuses on electricity-generating modules, except for hybrid modules having both functions at the same time.
Functions of Solar Modules
A solar module is a kind of housing which needs to fulfill various functions:
- It provides protection for the cells, especially against weather influences like rain and hail, as well as ultraviolet light. There is usually a glass cover, which is anti-reflection coated to minimize additional light loss, and you also need careful sealing and encapsulation on all sides. Furthermore, the mounting should be done in a way to avoid excessive mechanical stress on the cells, e.g. caused by temperature changes due to thermal expansion.
- Heat dissipation: A solar module should also allow the heat generated to be efficiently dissipated, e.g., to the back plane where it can be dissipated by air circulating under the panel.
- Electrical connection: It provides a single electrical connection (two wires, plus possibly one for grounding) to the internal PV cells.
Serial and Parallel Internal Connection
There are several ways to internally connect the cells in a module:
- Cells can be connected in series so that their generated voltages add up while the same current flows through all cells. It is often advantageous to work with higher voltages (often a few tens of volts from a module) instead of having to deal with higher currents to transport the same electrical power, since you can use thinner wires and still lose less power due to ohmic losses.
- A disadvantage of series connection is that the efficiency can be seriously reduced if only one cell is not producing due to local shading (e.g. by a leaf). This cell may even go into negative cell voltage mode, where it is heated by the current driven by the other cells, and may even be overloaded. This risk of damage can be avoided by adding a passive bypass diode.
- Alternatively, cells can be connected in parallel and then act as one larger cell, delivering the same voltage but the combined current. In this case, shading of a single cell has less effect.
- Often a combination of the two is used: multiple strings of cells are connected in series and these strings are combined in parallel, or alternatively combinations of parallel cells are combined in series.
Some modules have a built-in inverter (see below), but most modules simply deliver DC power, and a central inverter is connected to multiple modules.
In order to convert the DC current provided by the solar cells (or typically modules) into AC current, one or more inverters are required. Typically, one or more inverters are placed outside the solar panels, but there are also modules with an integrated inverter. Some of them are smart modules (see below), where several of them are combined, and in other cases one uses one or a small set of modules for a small household installation.
For more details, see the article on solar power generation.
There are also “smart modules” that include a DC–DC converter with a maximum power point tracker. Such a module can operate with a constant (possibly adjustable) output voltage even under varying light levels and maintain its best efficiency under variable conditions. Smart modules can also be combined in parallel (better than conventional modules), avoiding additional power losses, e.g. when different modules receive different levels of illumination.
Some smart modules may have additional features:
- Some communicate with a central inverter station, making it easier to detect and locate potential problems.
- They can automatically shut down under certain conditions for safety reasons.
Hybrid solar modules, also called photovoltaic/thermal (PV/T) modules, can generate electricity and heat at the same time. Here, the photovoltaic cell releases waste heat to a thermal collector on the back, where water (possibly with an antifreeze agent) transports the generated heat away. Alternatively, air can be used to transport the heat.
The heat is usually more usable at a higher temperature level, while the photovoltaic performance is better at lower temperatures. However, some heating systems do not require more than e.g. 35 °C, while solar cells can work reasonably well and last at somewhat higher temperatures. There are also applications where the temperature can be quite low, even supporting high photovoltaic efficiency; for example, such panels can be used to regenerate ice storage tanks, where only a little above 0 °C operating temperature is needed to melt the ice. A heat pump can take heat from the ice/water tank and deliver it at a higher temperature level, e.g. for a floor heating system. The electricity produced by the modules can be used to power the heat pump during the day.
The disadvantage of the hybrid concept is the higher complexity, especially when water circulation is involved, which typically leads to significant maintenance costs compared to pure photovoltaics.
Of particular interest is the power conversion efficiency of a complete solar module under standard lighting conditions. For various reasons, this efficiency is somewhat lower than that of the photovoltaic cells used:
- The surface of the module cannot be completely covered with cells, so some of the incident sunlight does not hit any cell.
- The cover glass causes some additional light losses, especially through reflection, despite having antireflective coating on both sides.
- There are some conduction losses in the wires used.
- If the heat dissipation is not ideal, the cell temperature is increased, which reduces its efficiency.
For these reasons, module efficiencies are typically a few percentage points lower than cell efficiencies.
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