Selecting Appropriate PV Array String Sizes
By: Kent Osterberg
Reference: HP online magazine
To appropriately size a PV array to an inverter, it is necessary to make sure that the:
• PV array never generates more voltage than the inverter’s maximum allowed input voltage
• Voltage remains within the inverter’s maximum power-point tracking range
• Current is within the inverter’s maximum current and short-circuit current specifications
• Power rating is appropriate for the inverter.
Equipment Specifications
Module specs can be found on the manufacturer’s data sheet or on the back of the module. The values shown in the table are for a 54-cell module. The manufacturer includes a temperature coefficient for the peak power voltage. If the temperature coefficient for peak power voltage is not published, then the temperature coefficient for peak power, in percent, may be used as an approximation. Similarly, inverter specs may be found on the inverter data sheet or its label.
Temperature Data
Because a PV module’s output depends on its temperature, calculations also must account for the lowest and highest possible site temperatures. One source for record temperatures is weather.com; another is SolarABCs.org, which has an interactive ASHRAE data map. A table of ASHRAE temperature data can also be found in “Expedited Permit Process for PV Systems” at SolarABCs.
For the first computation, which low temperature should you use—the ASHRAE tables’ “extreme minimum” or the record minimum from the local weather station? The “extreme minimum” is a little misleading—half of all years will have at least one day that is colder than this temperature. The “extreme minimum” is actually the average annual minimum temperature. Since inverters should never see a voltage that exceeds the maximum input voltage, I choose to use the more conservative method—the record low temperature. If you use the “extreme minimum” temperature, you may get one extra module in the string, but on some cold winter day, that extra module may result in damage to the inverter or void its warranty.
Calculate the Maximum Number of Modules in Series
The equation to solve for the maximum number of modules in series is:
Using example data from the table:
Since we have to work with whole modules, 15 modules is the maximum that can be wired in series.
Calculate the Minimum Number of Modules in Series
There are two factors to consider—PV module output degradation over time, and the effects of high temperature. Most studies of PV module degradation focus on power, concluding a median value for module power degradation of about 0.5% per year. Recently, the National Renewable Energy Laboratory (NREL) studied 12 mono- and poly-crystalline PV modules to see how the current and voltage parameters change over time. Relative to determining inverter string size, the significant finding was that 10 out of 12 modules showed less than 0.2% per year degradation in Vmp, with Voc remaining essentially unchanged. Since we’re estimating a PV module lifespan of more than 25 years, we need to consider that Vmp may drop by 5% over the system’s life: 0.2% per year × 25 years = 5%.
For the temperature factor, the ASHRAE “annual design dry-bulb high temp” value (either 2% or 0.4%), or the record high temperature may be used. The 2% temperature factor means only exceeding 2% of the time in the months of June, July, and August. The 0.4% temperature factor is only exceeded 0.4% of the hours during the hottest month—but there may be other months that it is exceeded. The temperature selected will determine how many hours every year the inverter may deviate from the maximum power point voltage.
Using the 2% temperature value will result in about 42 hours over the summer when the inverter may operate off its peak power point. With the 0.4% temperature, the inverter may operate off the peak power point for three hours during the hottest month of the year, and probably a similar number of hours for the other two summer months. If the record high temperature is used, the inverter should always be able to track the maximum power point. Since a small energy loss is far less critical than an inverter failure, there is no need to be conservative about which one is used. The sample documents provided in “Expedited Permit Process for PV Systems” use the 2% value. In my Oregon location, to be eligible for incentive money, I’m required to use the more conservative 0.4% temperature, which is used in the following calculation:
The minimum number of modules in series is 13 modules:
Since these modules are sitting in the sun and heating up during the day, the PV cell temperature will be hotter than the ambient air temperature and thus a temperature “adder” is used to estimate the PV module’s cell temperature. Different temperature adders are used depending on the array mounting method (see table). This example assumes a roof-mounted array, for a temperature adder of 35°C.
Find the Maximum Number of Module Strings
The maximum number of module strings in parallel is the smaller of:
No temperature corrections are used because they are extremely small for current and because the inverter will simply clip power (rather than sustain damage) if current is higher than it is rated for.
Sizing the Array
Since PV modules generally operate at lower output compared to their standard test conditions (STC) rating, inverter manufacturers commonly specify a maximum array size up to 125% of the inverter output rating. Given a 4,000 W inverter, up to 5,000 W of PV modules (4,000 × 1.25) could be installed. In this example, the maximum number of modules would be 5,000 W ÷ 190 W per module = 26.3, i.e., 26 modules.
The possible combinations that may be considered for this inverter are: one string of 13, 14, or 15 PV modules or two strings of 13 PV modules. If we go with two strings of 13, we have a final array size of 4,940 W. Note, since one string would be at most 2,850 watts, it would be appropriate to select a smaller inverter for a single string of modules. But keep in mind the calculations will have to be repeated for the smaller inverter.
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Robert Rapier
Reference:Forbes online news (http://onforb.es/1NLawHy)
Several years ago, when I was working on my book Power Plays, I spent a lot of time thinking about the future of energy. One thing I concluded was that solar power would become one of the world’s most important sources of energy – if not eventually the most important source of energy.
There are a couple of reasons I still believe this. But first, I should make it clear that it will be a long time before solar power rivals the consumption of oil in the global energy market.
While solar power is growing rapidly, we still use about 100 times as much energy in the form of oil (and about 90 times as much in the form of coal). Further, even though solar power is growing at a fast rate, the absolute growth in oil consumption from 2013 to 2014 was about 3 times the growth in solar power consumption. In other words, even though solar consumption grew at a 38% rate and oil consumption grew at about a 0.7% rate, this amounted to an increase in solar consumption of 11.6 million metric tons of oil equivalent versus an increase in oil consumption of 32 million metric tons.
So I want to at least offer that perspective, which is often missing in articles about renewable energy sources. Still, what I would say today is that solar is the most important emerging energy source, and over the next one to two decades it will surpass coal and begin to rival oil in the amount of energy supplied globally.
I believe the key drivers behind solar’s continued growth are that the energy conversion efficiency is relatively high (~an order of magnitude greater than photosynthesis) and that the costs for solar PV have become competitive in many markets. Further, those costs continue to fall. Solar is a very attractive option for those seeking to reduce carbon dioxide emissions.
Among all major energy sources, none added capacity as rapidly as the 50% average annual growth rate of solar photovoltaics (PV) over the past five years.
Growth in solar PV over the past decade has resulted in a nearly 50-fold expansion in the amount of deployed solar PV capacity since 2004:
Of course capacity isn’t the same as production. As I pointed out in Don’t Blame Renewable Energy For Dying U.S. Coal Industry, the capacity factors for renewables like wind and solar power are low relative to their fossil fuel competitors. So an installed megawatt (MW) of solar PV capacity won’t produce the same amount of electricity over the course of a year as an installed MW of coal-fired power (for instance). In fact, it could take more than three times the installed capacity of solar to generate the amount of electricity from a given capacity of coal-fired power in a year.
This means that if solar is to make a serious dent in the world’s fossil fuel demand, we are going to be installing a lot of solar panels. Who makes these solar panels?
According to research and consulting firm GlobalData, the Top 5 solar PV manufacturers in 2015 were China’s Trina Solar with 4.55 Gigawatts (GW) of solar PV panels produced, followed by Canadian Solar (3.9 GW), Chinese companies JinkoSolar Holding Co., Ltd. (3.79 GW) and JA Solar Holdings Co., Ltd. (3.38 GW), with South Korea’s Hanwha Q CELLS Co., Ltd. (3.2 GW) round out the Top 5. While China dominates in global PV manufacturing, two U.S. producers in the Top 10 are First Solar FSLR +2.91% and SunPower.
If solar PV growth continues as I expect, the sector should do quite well overall, but SunEdison’s recent bankruptcy show that bad decisions can quickly take down large companies in rapidly growing sectors. It’s important to keep that in mind and do your due diligence before diving into this sector.
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Last year, for the first time, according to a report in the New York Times, renewable energy sources – principally wind and sun – accounted for the majority of new electricity-generation capacity that was added around the world. More than half of the $286 billion invested in wind and solar occurred in emerging markets such as China, India, and Brazil. Ten percent of all electricity generated globally in 2015 came from renewables, which nearly doubled the amount produced by renewables in 2007.
The average global cost of generating electricity from solar panels fell 60 percent between 2009 and 2015. In such sunny countries as India and Dubai, some developers of solar farms have offered to sell electricity for half the global price. Replacing coal-fired power plants with renewables also has major health benefits in heavily polluted cities in China and India, where ground-level pollutants like soot and smog adversely affect breathing.
Here in the Philippines, the establishment of solar farms in Negros Occidental has put that province on the world map. Negros Occidental recently developed seven polar power facilities, making it the Philippines’ solar power capital, with solar generation capacity of 341 megawatts of power.
In 2015, $161 billion was invested in solar energy globally, which is more than what was invested in coal and natural gas combined. For the first time last year developing countries attracted the most investment and poorer countries, such as China and Chile, are getting the bulk of their electricity needs from giant solar parks which have been linked to their national grids. This year the United States hopes to triple its solar capacity and China and India each plan to add about 1,000 gigawatts of solar power in the next six years.
Lower costs have contributed to this expansion. The price of solar panels, most of which come from China, has fallen by 80 percent in the past five years. In Mexico electricity is being offered at $40 a megawatt hour, which is cheaper than natural gas or coal.
Developing countries will need to invest more in building transmission lines to connect the solar power being generated in sunny areas to its users. Makers of solar panels should focus not just on cutting costs but on improving the technology so that more of the sun’s energy is converted to electric power.
Reference:
Romulo, B. D. (May 5). Renewable energy is a global success. Manila Bulletin. Retrieved from http://www.mb.com.ph/renewable-energy-is-a-global-success/
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NV VOGT EYES 200 MW SOLAR CAPACITY IN PHILIPPINES
MANILA, Philippines - German-backed solar developer nv vogt Philippines is eyeing to build 200 megawatts (MW) of solar capacity in the next three years.
Vivek Chaudhri, president of nv vogt Philippines, bared the company’s direction moving forward during the inauguration of the 7.48-MW solar farm in Bgy. Dalayap in Tarlac City last Friday.
“We started in the Philippines about four to five years back and last year, we’ve seen a dramatic [turn for us] and we aim to continue putting up solar plants in the country. We’re building 200 MW in the next two to three years,” he said.
These prospective solar farms will be located mostly in Luzon and Mindanao, where the company has already established projects and could cost around $300 million to install, Chaudhri said in an interview with The STAR.
The firm is also eyeing a solar park concept in partnership with a local government in Mindanao and this effort is being led by its president Reynaldo Casas.
Last December, nv vogt Philippines completed the five-MW first phase of its 10-MW plant in Surallah, South Cotabato. “We already have a plant in Mindanao. So we’re looking at the second phase of the same size that we have developed over there,” Chaudhri said.
In Luzon, the company has constructed the 7.48-MW Dalayap solar farm and the 8.84-MW Armenia solar plant, both in Tarlac.
The contractor for all the projects was ib vogt, the German partner of nv vogt, and the plants in Tarlac were constructed in a record time of two months.
These planned solar projects can partly replace the peak power demand of these locations, especially in Mindanao, Chaudhri said.
“In Mindanao, there’s a power shortage. In addition a lot of the peak power is delivered by diesel. Solar plants generate during the day, so they’re a good replacement for a part of this peak power. We cannot replace the entire amount (as of yet) but this is a start,” he said.
“We’re waiting for the next round of feed-in tariff to be announced. We expect new FIT rate to be slightly below P8, which we think is still sustainable,” he said.
A part of the company’s 200 MW development will target the FIT.
Rivera, D. (May 10). nv vogt eyes 200 MW solar capacity in Philippines. The Philippine Star. Retrieved from http://www.philstar.com/business/2016/05/10/1581546/nv-vogt-eyes-200-mw-solar-capacity-philippines
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