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Designing a photovoltaic system: suggestions and optimization techniques

Orientation, slope, possible shadings and temperature. Let’s introduce all the factors that can limit the solar radiation taping affecting the panel efficiency

While working in the field of power production through solar emission photovoltaic conversion, many factors should be taken into account in the designing and assembling procedures. The comprehension of such a problems allows to optimize the solar radiation taping and to increment the system life time and efficiency. Whenever no limitations are present in the area availability, the maximization of economic incomes is equivalent to the maximization of the system productivity, in terms of produced power per available kWp. On the contrary, whenever the area constitutes a restrictive parameter, the panel efficiency has to be carefully taken into account. As an information, the panel efficiency can range between the 6% and the 19%, depending on the panels. Generally speaking, it is possible to distinguish two different kind of interventions: before installation, i.e., in the designing stage, and after installation, i.e., done on the system in action.

Let’s analyze the parameters that should be taken into account in the design stage in order to optimize the system productivity (expressed in kWh). First of all, it is necessary to analyze data concerning the local solar radiation. Such an information are provided on-line by the PVGIS European atlas and can be referred in respect to the angle with the South direction (Azimuth) and to the panel slope. Such a data, whenever properly surveyed, are pivotal in order to obtain an agreement between the preliminary technical and economical estimation and the actual production of the final system. As a matter of fact, by using excessively optimistic estimations, it is concrete the risk of postponing the system breakeven to the expected time. On the contrary, by underestimating the system potentialities, we take the risk to underrate the economic advantage of the system for the customers or for the banks that could be interested in covering the system costs.

Moreover, from the design viewpoint, it is very important to consider the influence that the environment exerts on the photovoltaic generator. The orientation of the surface that will sustain the panels should be as closer as possible to the south (Azimut 0°) in order to maximize the taping of the sun rays all over the day. If the generator orientation differs from the south, the system can suffer a reduction in the power productivity or an increment in the structural complexity of the plant (whenever panels are anyway oriented to south). We recall that the solar radiation can be divided in three components: the direct radiation, that is perpendicular to the panel surface, the diffuse radiation, that propagates across the air, and the reflected radiation that is a function of the environmental conditions. In order to maximize the system performances it is necessary to choose the panel slope so that the incident radiation could be, as much as possible, a direct radiation, i.e. the component characterized by the highest intensity. It is noteworthy that there are also panels, such as amorphous silicon panels, able to properly employ the diffuse radiation, at cost of a general lower efficiency. Obviously, the panel proper slope is a function of the local latitude; going from the north hemisphere to the equator, the sun rays are less inclined in respect to the observed level. As a consequence, the panel slope should be diminished accordingly. For the Livorno zone, by considering a panel completely oriented to south, the optimal slope is 33 degrees.

A fundamental factor of the environment is the presence of shading objects, such as trees, hills, buildings or chimneys or any other object that can cast a shadow on the panels sometimes during the day. Shadows can affect single modules or even switch off an entire row of modules by limiting, in this way, the entire power production. In order to avoid such an hitch, the dimensioning of the system is based on a series of photometric surveys that verify the shadows that close elements in the environment can generate during a day. Starting from such a dataset, it is created a shadow diagram that accounts for the sun motion, allowing the designer to choose an optimal housing for the photovoltaic modules.

An other important factor is the environmental temperature. The photovoltaic system is subject to temperature-dependent losses of efficacy that depends on the meteorology of the site and on the characteristics and the positioning of modules. In order to obviate such a temperature-dependent losses (that in the Livorno area are the 9%, circa), knowing that it is not possible to shade a photovoltaic system, it is opportune to choose an airy place for the system positioning (and a consequent assembling, too). As a matter of fact, such an expedient can avoid the overheating of the system components. In the case that the architectural integration of the system prevents an adequate ventilation (e.g., by considering a complete integration, with mono- or polycrystalline silicon modules, the most sensible to temperature, adjacent to the pitch with no cavities), it is necessary to modify the system characteristics accordingly. For example, it is opportune to choose modules that are less sensitive to the temperature effects, such as the amorphous silicon modules, by avoiding the widespread mono- and polycrystalline silicon ones.
It is also important to grant a proper ventilation of the room where the inverter is located or of the inverter itself (whenever outdoor). As a matter of fact, an overheated inverter invalidates the overall system efficiency and reduces its average lifetime (usually 10 years).
The commercial inverters can be actively or passively ventilated through fans or dissipators, respectively. Which is the best method for the inverter cooling is a controversial topic but, it should be noted that a fan is very effective but it is more fragile and noisy that a metal fin. In dusty environments or in conditions in which the noise could be a limiting factor, the passive dissipation should be preferred. Finally, it is advisable to prefer inverters provided with a as long as possible warranty, reasonably 15 years.

As for the after-installation actions, the designer and the installer should suggest to the scrupulous customer a regular monitoring of the power production of the photovoltaic system. In fact, such a care is important in order to reveal possible troubles or disconformities of the system and to compare one’s power consumption, production and input in the power supply network. Moreover, it is necessary to periodically inspect the modules, in order to reveal the presence of possible breaks or deposits and, once an year, to clean the modules surfaces. As a matter of fact, the presence of deposits such as rain limestone, leafs, salt power or animal excrements, reduces the taping surface and, consequently, the system power production. It is a duty of the system manager to plan the timing and the form of such an interventions, in relationships with the environmental and meteorological conditions of the plant site. At this purpose, useful checklists are available for the maintenance staff in order to be able to forecast possible power production limiting factors or structural collapses. Such an interventions concern: the photovoltaic generator, the supporting structures (brackets, iron bars, screws, nuts, etc…), the electric panels, the grounding network, the static converter and the electric networking.

by Giacomo Silvestri, Emiliano Niccolai
02 february 2009, Technical Area > Science News