Stand-alone Photovoltaic Systems
The typical scheme of a stand-alone PV system is shown in the Figure and includes the next components:
The PV installation at UMU´s Apes Laboratory is a stand-alone photovoltaic system. This kind of PV systems is not connected to the grid and their main features are:
Stand-alone PV systems are mainly installed for two reasons. Either the PV installation is cheaper than connecting to the closer grid, or it is the more reliable option, for instance, to feed lighting buoys.
Their main applications are:
Photovoltaic solar energy is a way to get electricity from photovoltaic modules.
A photovoltaic cell is an electronic device which converts directly the incident solar radiation into electricity thanks to the photovoltaic effect. When the light, a flux of photons with a specific amount of energy, falls in a semiconducting material, each photon can release its energy to the electrons in the material and promote them from the valence to the conducting band. These excited electrons are able to move freely through the material (an electric current is generated). To extract the current from the material in a useful way, a small potential difference (about 1 Volt) should be generated between the two electrodes of the cell. That is made by doping the material in a selective way (type p/ type n, like it is shown in Figure 3).
The most common PV cells are made of crystalline silicon (Figure 2). However there are a wide variety of technologies, among them it is worth to mention: amorphous silicon, III-V cells, CIS cells and the latest ones: hybrid cells and organic cells.
Figure 2. Crystalline Silicon Photovoltaic Cell
Figure 3. Photovoltaic effect in a solar cell
When several PV cells are electrically connected, they make up a photovoltaic module or panel. The generated energy by the PV module is the sum of the generated electricity from each cell. Thus, the solar cells are connected electrically and encapsulated into photovoltaic modules to protect the cells against the hostilities of the environment and to simplify handling.
The generator should be set in a place without shadows, facing south and tilted in the best angle to catch the maximun radiation posible in that latitud.
The PV generator at the University of Murcia is oriented -20º east from the south (in order to get a better building integration) and tilted 30º over the horizontal. It is made up of 40 modules of 106Wp (Module Peak Power), arranged in 4 series of 10 modules in parallel. It builds up a total peak power of 4.240 kWp. The modules are the ISOFOTON I-106/24 MC model (monocrystalline silicon technology).
<Figure 4. PV generator at the University of Murcia
Normally the photovoltaic generator has an output voltage higher than the nominal voltage of the batteries used in the installation. The charge regulator is an electronic device which adapts the output voltage from the PV generator to the work voltage of the batteries. Also, the use of a charge regulator protects the batteries from overcharge and deep discharges, therefore enlarging the life of the batteries.
The PV facility at the University of Murcia has two charge regulators (Figure 5) ISOTEL 30 SD (ISOFOTON) working in parallel.
Figure 5. Charge regulators of the PV facility at University of Murcia
The bank of batteries is an electrochemical device which transforms the stored chemical energy into electric energy, and vice-versa. A single battery is essentially made up of two electrodes immersed in an electrolyte where the chemical reactions of charging and discharging take place. Single batteries are associated in series to increase the storage capacity and the work voltage.
The main goal of the bank of batteries is the reliability of a stand-alone PV system without an alternative energy source. It supplies the necessary energy during the periods when there are not enough solar energy production to meet the load feeding requirements (nights and cloudy periods). The size of the bank batteries depends on the days of autonomy without solar radiation.
In stand-alone PV systems stationary batteries are the most widely used. These, unlike the traction ones, allows deeper discharging without damages. There are different types of stationary batteries in the market, but the open lead-acid batteries (liquid electrolyte) and the close lead-acid batteries (gel electrolyte) are the most common.
Each open lead-acid battery is made up of two lead electrodes immersed in a sulphuric acid solution (electrolyte). When the battery is discharged, the electrodes are in the state of lead sulphate II (PbSO4 II) embedded in a matrix of metallic lead (Pb).
PbO2 + 2H2SO4 + 2e- --> 2 H 2 O + PbSO 4 + SO 4 2-
Pb + SO 4 2- --> PbSO 4 + 2 e -
Figure 6. Scheme of an open lead-acid battery
The bank of batteries in the PV facility at the University of Murcia is made up of 24 open lead-acid batteries ISOFOTON mod: 2AT2300. Each of them has a nominal voltage of 2 V and a capacity of 2300 Ah(C-100). Its series association makes a bank of batteries of 48V of nominal voltage and 3 days of autonomy.
Figure 7. Bank of batteries in the PV facility at the University of Murcia
In a stand-alone PV facility the electricity from the PV generator and from the bank of batteries is a direct current form. Therefore, if a supply of AC devices is needed, an inverter must be installed. An inverter is an electronic device which transforms the direct current into an alternative current with the characteristics of the common grid.
An inverter used in a PV facility must satisfy the following requirements:
Figure 8. Inverter in the PV Facility at the University of Murcia
We use the term load to name all the electric devices which are fed by the PV facility. The first step for calculating the energetic demand that the PV facility should meet is to know the power requirement and the mean time that the devices are going to be connected on average per day.
Photovoltaic generators supply direct current, because of that, it is possible to supply directly DC devices (DC load) and AC devices (AC Load) using an inverter to convert DC into AC. The most of the electric devices we have at home are AC devices (working with a sinus wave of 220 V and 50 Hz of frequency) The PV facility at University of Murcia supplies only AC devices. The AC Load is made up of 23 lamps of 36 W working 12h/day and 4 lamps of 40W running 24h/day. The whole demand of electric energy is 13776 Wh/day.
Figure 1. Components of a typical stand-alone photovoltaic system.