Guide to the use of solar energy
Aquaculture sites are often located in zones which are totally deprived of electrical energy, either because of their distance from any inhabited zone (pond and marsh), or because they are at sea (floating cages).
However, it is often necessary to have a minimum of energy available, to operate the feed distributors, or to operate an alarm, a pump, or any other electric system.
In our temperate zones, and especially south of the Loire (France), the sun is sufficiently present to provide energy which is often enough to operate such systems.
The main solar energy captors exist in two forms:
The solar heaters, which consist of tubes where a heat transfer agent, water generally, circulates, which is used to absorb infra-red radiation from the sun to heat a room or any other installation (greenhouses...).
Photovoltaic cells, made up of silicon sensors which convert the solar radiation into electric power.
It is these photovoltaic sensors which are the most relevant for the fish farmer. They are able, from a restricted surface area (1 m2) to produce a power which can reach 80 W at 12V.
In general, although solar energy is free, the photovoltaic panels, on the other hand, are rather expensive, but fortunately, they can easily last several decades if they are good quality.
The early solar cells consisted of monocrystalline silicon, characterized by a collection of blue-tinted discs. They gave a very good output, but were extremely expensive to produce, especially as their circular shape made it impossible to optimize the surface used.
Since then, polycrystalline silicon has appeared, whose active cells are blue tinted rectangles and the large crystals on the surface can be easily distinguished. They currently provide the best energy / price ratio, and have an excellent quality which makes it possible to use them for twenty years. Moreover, their rectangular shape makes it possible to fabricate panels of a reduced size.
Lastly, on the market, there are panels of amorphous silicon which are characterized by a grey-blue speckled surface, where the crystals cannot be distinguished. They offer a reduced cost (they can be found in small gadgets for the car or gardening), but they have poor output, and a weak lifespan. They are not practicable in aquaculture, where the conditions of use are too severe.
The arrangement of the solar panels is also very important to recover the maximum luminous flux from the sun. In our regions (France), the optimum is obtained by directing the panel to the south and by inclining it at 45° to the ground. The ideal situation would be to be able to direct them towards the sun throughout the day (like sunlowers)...
The panels generally exist in 12 V or 6V.
This represents the nominal output voltage; in fact, in cold weather and strong sun, the intensity of a panel of 12 V can easily exceed 22 V. Because of this it is necessary, when charging a battery, to anticipate this, and to incorporate a voltage regulator in the circuit. In the same way, during the night, it will be necessary to incorporate a device to avoid the discharge of the battery into the solar panel...!
In order to increase either the intensity or the current provided, the solar panels can be connected in series (increasing the voltage) or in parallel (increasing the current).
Generally, they are associated with one or more batteries which have the role of preserving and better distributing the electric charge throughout the day.It should not be forgotten that the power stated is the maximum pick power which is reached only when the panel is in full exposure - which lasts, at best, a few hours during the course of the day. During the remainder of the light period, the output falls.
How to calculate the energy requirements
To best evaluate the requirements, it is necessary to recall some basic rules of electricity:
P = U x I
P in Watts, U in Volts, and I in Amps.
E= P x T
Energy is expressed normally in Joules, and time in seconds. However, standard electrical practice uses, Watts/Hour and the KiloWatt/Hour (what can be read on an electricity supply invoice) and the duration is mostly by the hour (1 hour = 3600 seconds).
Q = I x T
Although the standard is the 'Coulomb' , the hour is still in use and not the second, and Ampere.hours : Ah. are therefore used.
Thus the capacity of car batteries varies from 30 to 100 Ah.
Equipped with these basic formulas, we can try to determine the energy requirements of an installation, but a reserve margin should always be taken, bearing in mind that the various components that are used are never perfect, and each one has its losses.
Simple example: you want to use a solar panel for an electric motor (air pumps...) which consumes 1A under 12 V for 5 hours per day. The daily energy consumed per day will be:
12 X 1 X 5 = 60 W h.
If you have a unit of 20 W nominal, you will be able to count 18 W for one hour (full exposure), 10 W for 4 hours (semi exposure), 5 W for 4 hours, and 1 W for 2 hours, that is to say approximately 80 W h over the duration of one sunny day. This will be sufficient for the application, and will permit a battery to be charged which must be able to operate for at least a week without sun, and have a minimum capacity of :
5h x 1 A. x 7 days = 35 A.h.
Taking into account any losses from the battery, it is advisable to have a 20 % reserve, that is to say a battery of 42 A.h.
For more complicated examples, contact a specialist.
Corniche du Bois Sacré
83500 La Seyne sur Mer
Tél. : 04 94 10 26 26
Fax : 04 94 10 26 30