Most inverter manufacturers give a series of power ratings. Common ratings include:
·Maximum rated power of photovoltaic array: the maximum rated power of photovoltaic array, usually in kWp or Wp.
·Maximum DC input power; the maximum amount of DC power that the inverter can convert into AC power (due to the power loss of the photovoltaic array, this value is usually lower than the maximum photovoltaic array power).
·Maximum AC output power: the maximum amount of AC power that the inverter can output.
For most inverters, the maximum power (Wp) of the photovoltaic array is greater than the maximum AC output power of the inverter. The reason is system loss (that is, the array will not output at rated power).
In order to calculate the total number of photovoltaic modules connected to the inverter, the maximum rated power of the photovoltaic array connected to the inverter must be divided by the rated power of the components. If the maximum rated power of the photovoltaic array connected to the inverter is not given, the maximum DC input power should be used. In some cases, the voltage or current characteristics may not allow the maximum number of components to be connected to the inverter. Figure 1 shows an example of the maximum array power configuration of an inverter.
Case: The maximum DC power of the inverter is 3200w, and the maximum power of the module is 185W, so 3200 W/185 W = 17.3.
The maximum number of components that can be connected to the inverter is 17.
Case 1: Sydney, Australia
Assuming that the inverter’s voltage specification limits that only 17 components can be connected in series, and the inverter power specification limits the entire system to 17 components, the only option for the array is a branch of 17 components. The overall rated power is 17 x185Wp=3145Wp.
Case 2: Berlin, Germany
For the German case system, there are two possible configurations, one branch with 16 components. The total rated power is 16 x 185 Wp=2960Wp.
When components and inverters are matched, voltage, current, and power need to be calculated. If the array rated power is much smaller than the inverter power, the inverter efficiency may decrease.
① Loss of grid interactive photovoltaic system
There are a large number of factors that prevent the photovoltaic system from working at maximum efficiency and reaching its rated output power. These factors are taken into account in the “derating” process. The derating factor of the main loss source is highlighted here.
②The temperature of the photovoltaic module
The working temperature of the photovoltaic module has a great influence on its output power, and high working temperature will cause power loss. The effect of temperature on the output varies from component to component, and can be calculated using the temperature coefficient provided by the manufacturer’s data sheet. In order to estimate the system output as realistically as possible, the derating factor is very useful. It represents the overall efficiency when temperature is taken into account.
Case 1: Sydney, Australia
Sharp components are installed in Sydney (average ambient temperature is 23℃
(73.4F)) The derating factor is calculated as follows:
| The battery temperature can be calculated from the ambient temperature | 23℃＋25 ℃ =48 ℃ |
| Then calculate the difference between the battery temperature and the standard test condition (STC, 25°C) | 48 ℃ ﹣25 ℃ =23 ℃ |
| If the unit of temperature coefficient is %/℃, it must be converted into decimal representation | 0.485%/ ℃ =0.00485/ ℃ |
| The temperature coefficient is multiplied by the difference between battery temperature and STC | 0.00485/ ℃ ×23 ℃ =0.11155 |
| In order to calculate ftemp, the number must be subtracted from 1 | ftemp =1﹣0.11155=0.88845 |
| Therefore, the comprehensive temperature coefficient is 88.8% |
Case 2: Berlin, Germany
Calculation of derating factor when Sharp components are installed in Berlin (average ambient temperature is 9.8℃ (49.64T))
as follows:
| The battery temperature can be calculated from the ambient temperature | 9.8 ℃ ＋25 ℃ =34.8 ℃ |
| Then calculate the difference between the battery temperature and the standard test condition (STC, 25°C) | 34.8 ℃ ﹣25 ℃ =9.8 ℃ |
| If the unit of temperature coefficient is %/℃, it must be converted into decimal representation | 0.485%/ ℃ =0.00485/ ℃ |
| The temperature coefficient is multiplied by the difference between battery temperature and STC | 0.485%/ ℃ ×9.8 ℃ =0.04753 |
| In order to calculate ftemp, the number must be subtracted from 1 | ftemp = I﹣0.04753=0.95247 |
| Therefore, the comprehensive temperature coefficient is 95.25% |
From these two cases, the temperature derating factor changes significantly with location (in the case of Sydney 88.8%, Berlin is 95.25%), the colder the climate, the less temperature-related losses. However, photovoltaic systems in cold climates do not necessarily have more power output, because warmer climates also have better solar radiation, while colder climates tend to have shorter days and less sunshine.
③Dust and pollution
The loss caused by the accumulation of dust on the components is often referred to as “contamination” loss. If dust and waste (such as fallen leaves) accumulate on the surface of the photovoltaic module, they will block the module and reduce the output power. The reduction of output power largely depends on the site and the factors that affect pollution. These factors include the inclination of the module installation (if the modules are installed horizontally, it is difficult for rain to wash away the accumulated dust), the level of dust on the site, and possibly on the glass. The local pollution that forms the film, the frequency of rainfall, and whether the environment is salty (ie close to the coast). The derating factor caused by pollution can only be estimated. The more common method is, for very dirty sites, fdirty = 0.90 (10% reduction in output due to pollution); for relatively clean sites with frequent rainfall, fdirty = 0.95 (5% reduction in output due to pollution). Contamination losses can be reduced by cleaning the components.
④Tolerance provided by the manufacturer
The manufacturer’s tolerance takes into account the small changes between the output of a single component provided by the manufacturer and the general specification of the component data sheet. If the manufacturer’s tolerance is a negative loss, such as -3%, the peak power of the component may be 3% less than the value given on the data sheet. For example, the minimum peak power of the 315W component produced by the manufacturer may be 305.55 W (that is, 315W reduced 3%). Some manufacturers also give a positive tolerance. For example, +5% means that the output peak power of some components will be higher than the value given in the data sheet (for example, in the example in Figure 2, 315W is increased by 5%, or 330.75W)
⑤Block
Shading has a great impact on system performance loss, mainly due to the following two reasons:
· Shading causes a voltage drop. If a large number of photovoltaic arrays are shaded, the voltage may drop outside the inverter voltage range, causing the inverter to shut down and the system does not output power.
·In the case of multiple modules in series and parallel, the occurrence of obstruction on one module string will also affect other module strings. This is because the current and voltage are directly affected by the received radiation, and when the component is blocked, it will not receive too much solar radiation.
If a component string is blocked, its output voltage will be lower than other unblocked component strings. The output voltage of the unshielded component strings connected in parallel will also be reduced, so that all parallel component strings output the same voltage. One way to avoid this problem is to use multi-branch inverters.
The best way to avoid these problems is to install the photovoltaic array in an area that will not be blocked. Some experienced photovoltaic array designers may design the system around the problem of partial shading, but this approach is generally avoided or only used by experienced professional designers.
⑥Component orientation and inclination
The installation orientation and inclination of the photovoltaic array will cause some loss. In practice, the influence of these angles on the output power can be calculated using a table based on test data.
⑦Voltage drop
Generally speaking, designers minimize the voltage drop by using larger-capacity cables, because the cost of purchasing large cables is significantly lower than the cost of compensating for power loss by increasing the number of components in the photovoltaic array: for example, the voltage drop of a 3000 wp array It is 5%, which is equivalent to the output power of 105 Wp photovoltaic modules (the direct relationship between voltage drop and power loss is P=IV). The standard specification specifies the maximum allowable voltage drop: Australia is 5%, but industry standards and many designers aim for a voltage drop close to 1% for the reasons mentioned above. The National Electrical Code (NEC) does not have absolute requirements for voltage drop, but the North American Council for Energy Practitioners Certification (NABCEP) recommends that the overall voltage drop of the system be limited to 2%-5% of the circuit working voltage, and the voltage drop of the DC conductor less than 1%. In addition, as the size of the cable is increased, the voltage drop can be further reduced through careful planning and reduction of the cable length. When designing a system for areas that enjoy the feed-in tariff policy, any contribution to reducing the voltage drop of the system can be calculated quantitatively, and its cost can be calculated, so as to determine whether there is an economic advantage by increasing the cable specifications to avoid power loss.
⑧Inverter efficiency
Power loss also occurs inside the inverter due to the heat loss of the electronic components and the transformer (therefore, the efficiency of the transformerless inverter tends to be higher). The power loss is reflected by the inverter efficiency on the manufacturer’s data sheet.
As mentioned earlier, the inverter efficiency also depends on the actual input power and operating temperature. It is important to install the inverter in a cool, well-ventilated place, not in a place directly exposed to the sun. The ventilation requirements of the inverter will be given in the manufacturer’s installation recommendation manual.