Wikipedia

Nominal power (photovoltaic): Difference between revisions

Browse history interactively
← Previous edit
Content deleted Content added
VisualWikitext
→‎Cost per watt: OR section
Tags: Mobile edit Mobile web edit
 
(17 intermediate revisions by 13 users not shown)
Line 1:
{{Short description|Nameplate capacity of a photovoltaic devices}}
{{broader|Nominal power}}
{{main|Nominal power}}The '''nominalNominal power''' is the [[nameplate capacity]] of [[photovoltaic]] (PV) devices, such as [[solar cell]]s, [[solar module|modules]] and [[PV system|systems]],. andIt is determined by measuring the [[electric current]] and [[voltage]] in a [[electric circuit|circuit]], while varying the [[Electrical resistance|resistance]] under precisely defined conditions. The nominal power is important for designing an installation in order to correctly dimension its [[Solar cable|cabling]] and [[Solar inverter|converters]].<ref name="confusion-watt-peak">[http://www.photovoltaikbuero.de/pv-know-how-blog/die-verwirrung-um-das-watt-peak/ Die Verwirrung um das Watt-Peak], The confusion around watt-peak, 14 August 2009.</ref>
 
The '''peak power''' is generally not the same as the powerreached under actual radiation conditions. In practice, thisactual conditions will beallow for approximately 15-20% lower generation due to the considerable heating of the solar cells.<ref>{{cite web|title name= What does kilowatt peak (kWp) actually mean?|url = http://www.solar-is-"future.com"/faq-glossary/faq/photovoltaic-technology-and-how-it-works/what-does-kilowatt-peak-kwp-actually-mean/|access-date = 11 June 2015|archive-url = https://web.archive.org/web/20150612120201/http://www.solar-is-future.com/faq-glossary/faq/photovoltaic-technology-and-how-it-works/what-does-kilowatt-peak-kwp-actually-mean/|archive-date = 12 June 2015|url-status = dead}}</ref>
Moreover, in installations where electricity is converted to [[alternating current|AC]], such as solar power plants, the actual total electricity generation capacity is limited by the [[inverter]], which is usually sized at a lower peak capacity than the solar system for economic reasons. Since the peak [[direct current|DC]] power is reached only for a few hours each year, using a smaller inverter allows to save money on the inverter while [[Clipping (signal processing)|clipping]] (wasting) only a very small portion of the total energy production. The capacity of the power plant after DC-AC conversion is usually reported in W<sub>AC</sub> as opposed to W<sub>p</sub> or W<sub>DC</sub>.
 
==Standard test conditions==
{{More citations needed section|date=September 2021}}
The nominal power of PV devices is measured under ''standard test conditions'' (STC), specified in standards such as IEC 61215, IEC 61646 and UL 1703. Specifically, the light intensity is 1000&nbsp;W/m<sup>2</sup>, with a spectrum similar to [[sunlight]] hitting the earthEarth's surface at latitude 35°N in the summer ([[airmass]]&nbsp;1.5), the temperature of the cells being 25&nbsp;°C. The power is measured while varying the resistive load on the module between an open and closed circuit (between maximum and minimum resistance). The highest power thus measured is the 'nominal' power of the module in [[watt]]s. This nominal power divided by the light power that falls on a given area of a photovoltaic device (area × 1000&nbsp;W/m<sup>2</sup>) defines its [[Solar cell efficiency|efficiency]], the ratio of the device's electrical output to the incident energy.
 
== Units {{anchor|Watt-peak}} ==
{{More citations needed section|date=September 2021}}
 
The [[International Bureau of Weights and Measures]], which maintains the [[International System of Units|SI-standard]], states that the [[physical unit]] and its symbol should not be used to provide specific information about a given [[physical quantity]] and that neither should be the sole source of information on a quantity.<ref>{{SIBrochure8th|page= 132}}</ref> Nonetheless, colloquial English sometimes conflates the quantity power and its unit by using the non-SI unit '''watt-peak''' and the non-SI symbol W<sub>p</sub> prefixed as within the SI, e.g. '''kilowatt-peak''' (kW<sub>p</sub>), '''megawatt-peak''' (MW<sub>p</sub>), etc. As such a photovoltaic installation may for example be described as having "one kilowatt-peak" in the meaning "one kilowatt of peak power". Similarly outside the SI, the peak power is sometimes written as "P = 1&nbsp;kW<sub>p</sub>" as opposed to "P<sub>peak</sub> = 1&nbsp;kW". In the context of domestic PV installations, the kilowatt (kW) is the most common unit for peak power, sometimes stated as kW<sub>p</sub>.<ref>{{cite web |title= What does kilowatt peak (kWp) actually mean? |url= http://www.solar-is-future.com/faq-glossary/faq/photovoltaic-technology-and-how-it-works/what-does-kilowatt-peak-kwp-actually-mean/ |website= solar-is-future.com |year= 2011 |access-date= 11 June 2015 |archive-url= https://web.archive.org/web/20150612120201/http://www.solar-is-future.com/faq-glossary/faq/photovoltaic-technology-and-how-it-works/what-does-kilowatt-peak-kwp-actually-mean/ |archive-date= 12 June 2015 |url-status= dead }}</ref>▼
In the context of domestic PV installations, the [[kilowatt]] (symbol kW) is the most common unit for peak power, for example P<sub>peak</sub> = 1&nbsp;kW.
▲The [[International Bureau of Weights and Measures]], which maintains the [[International System of Units|SI-standard]], states that the [[physical unit]] and its symbol should not be used to provide specific information about a given [[physical quantity]] and that neither should be the sole source of information on a quantity.<ref>{{SIBrochure8th|page= 132}}</ref> Nonetheless, colloquialColloquial English sometimes conflates the quantity power and its unit by using the non-SIstandard unitlabel '''watt-peak''' and the non-SI (symbol W<sub>p</sub>), possibly [[SI prefix|prefixed]] as within the SI, e.g.in '''kilowatt-peak''' (kW<sub>p</sub>), '''megawatt-peak''' (MW<sub>p</sub>), etc. AsFor suchexample, a photovoltaic installation may for example be described as having "one kilowatt-peak" in the meaning "one kilowatt of peak power". Similarly outside the SI, the peak power is sometimes written as ("P = 1&nbsp;kW<sub>p</sub>" as opposed to "P).<sub>peak</sub>ref name= 1&nbsp;kW". In the context of domestic PV installations, the kilowatt (kW) is the most common unit for peak power, sometimes stated as kW<sub>p</sub>.<reffuture">{{cite web |title= What does kilowatt peak (kWp) actually mean? |url= http://www.solar-is-future.com/faq-glossary/faq/photovoltaic-technology-and-how-it-works/what-does-kilowatt-peak-kwp-actually-mean/ |website= solar-is-future.com |year= 2011 |access-date= 11 June 2015 |archive-url= https://web.archive.org/web/20150612120201/http://www.solar-is-future.com/faq-glossary/faq/photovoltaic-technology-and-how-it-works/what-does-kilowatt-peak-kwp-actually-mean/ |archive-date= 12 June 2015 |url-status= dead }}</ref>
However, in the [[International System of Units]] (SI), the [[physical unit]] (and its symbol) should not be used to provide specific information about the conditions assumed for measuring a given [[physical quantity]].<ref>{{SIBrochure8th|page= 132}}</ref>
 
== {{anchor|Watt-peak AC}} Conversion from DC to AC ==
 
Solar power needneeds to be converted from [[direct current]] (DC, as it is generated from the panel) to [[alternate current]] (AC) to be injected into the power grid. Since solar panels generate peak power only for few hours each yearday, and DC to AC converters are expensive, the converters are usually sized to be smaller than the peak DC power of the panels. This means that for fewsome hours each yearday the peaks are "[[Clipping (signal processing)|clipped]]" and the extra energy is lost. This has very little impact on the total energy generated throughout a year, but saves considerable amount of [[balance of system]] (BOS) costs.
Due to under-sizing of converters, AC ratings of solar plants are generally significantly lower than DC ratings, as much as 30%. This in turn increases the calculated yearly capacity factor of the plant.
The downrating of peak power and the related clipping is different from the losses incurred in the conversion from DC to AC, which happen at any power level and are usually relatively small.
 
Most countries refer to installed nominal [[nameplate capacity]] of PV systems and panels by counting [[Direct current|DC]] power in watt-peak, denoted as W<sub>p</sub>,<ref name=epia-2014>{{cite web |title = Global Market Outlook for Photovoltaics 2014-2018 |url = http://www.epia.org/fileadmin/user_upload/Publications/EPIA_Global_Market_Outlook_for_Photovoltaics_2014-2018_-_Medium_Res.pdf |website = http://www.epia.org |publisher = EPIA - European Photovoltaic Industry Association |access-date = 12 June 2014 |archive-url = https://wwwweb.webcitationarchive.org/6QGSvAF7w?url=web/20140625154728/http://www.epia.org/fileadmin/user_upload/Publications/EPIA_Global_Market_Outlook_for_Photovoltaics_2014-2018_-_Medium_Res.pdf |archive-date = 1225 June 2014 |page = 15 |url-status = dead}}</ref> or sometimes W<sub>DC</sub>, as do most manufacturers and organizations of the photovoltaic industry, such as [[Solar Energy Industries Association]] (SEIA), the [[European Photovoltaic Industry Association]] (EPIA) or the [[International Energy Agency]] ([[IEA-PVPS]]).
Some grid regulations may limit the AC output of a PV system to as little as 70% of its nominal DC peak power (Germany). Because of these two different metrics, international organizations need to reconvert official domestic figures from the above-mentioned countries back to the raw DC output in order to report coherent global PV-deployment in watt-peak.<ref name="iea-pvps-snapshot-1992-2014">
{{cite web
Line 28 ⟶ 32:
|page=11
|date=30 March 2015
|archive-url=https://wwwweb.webcitationarchive.org/6XPpb1fai?url=web/20150407023056/http://www.iea-pvps.org/index.php?id=92&eID=dam_frontend_push&docID=2430
|archive-date=307 MarchApril 2015
|url-status=live
}}
</ref>
 
In order to clarify whether the nominal power output (watt-peak, W<sub>p</sub>) is in fact DC or already converted into AC, it is sometimes explicitly denoted as MW<sub>DC</sub> and MW<sub>AC</sub> or kW<sub>DC</sub> and kW<sub>AC</sub>. The converted W<sub>AC</sub> is also often written as "MW (AC)", "MWac" or "MWAC". Just as for W<sub>p</sub>, these units are non [[SI]]-compliant but widely used. In California, for example, where the rated capacity is given in MW<sub>AC</sub>, a downrating of 15 percent in the conversion from DC to AC is assumed.<ref name="Gipe-DC-AC-conversion">{{cite web|url=http://www.wind-works.org/cms/index.php?id=38&tx_ttnews%5Btt_news%5D=208&cHash=bebbcb4dacd986e38d771a9bd496bdfb|title=Solar PV DC Conversion Factor for AC kW|last1=Gipe|first1=Paul|date=20 November 2009|website=Wind Works|archive-url=https://wwwweb.webcitationarchive.org/6SLA3c7xv?url=web/20130915213336/http://www.wind-works.org/cms/index.php?id=38&tx_ttnews%5btt_news%5d=208&cHash=bebbcb4dacd986e38d771a9bd496bdfb|archive-date=415 September 20142013|access-date=23 January 2020|url-status=live}}</ref>
 
== Cost per watt ==
{{More citations needed section|date=September 2021}}
{{OR section}}
Although watt-peak is a convenient measure, and is the standardized number in the [[photovoltaic]] industry on which prices, sales and growth numbers are based, it is arguably not the most important number for actual performance. Since a solar panel's job is to generate electric power at minimal cost, the amount of power that it generates under real-life conditions in relation to its cost should be the most important number to evaluate. This [[cost-per-watt|cost per watt]] measure is widely used in the industry.
 
It can happen that a panel from brand&nbsp;A and a panel of brand&nbsp;B give exactly the same watt-peak in laboratory test, but their power output is different in a real installation. This difference can be caused by different degradation rates at higher temperatures. At the same time, though brand&nbsp;A can be less productive than brand&nbsp;B it may as well cost less, thus it has a potential of becoming financially advantageous. An alternative scenario can also be true: a more expensive panel may produce so much more power that it will outperform a cheaper panel financially. An accurate analysis of long-term performance versus cost, both initial and ongoing, is required to determine which panel may lead the owner to better financial results.
 
== Power output in real conditions ==
{{More citations needed section|date=September 2021}}
The output of [[photovoltaic system]]s varies with the intensity of sunshine and other conditions. The more sun, the more power the [[PV module]] will generate. Losses, compared to performance in optimal conditions, will occur due to non-ideal alignment of the module in tilt and/or azimuth, higher temperature, module power mismatch (since panels in a system are connected in series the lowest performing module defines performance of the string it belongs to), aging factor, soiling and DC to AC conversion. The power a module generates in real conditions can exceed the nominal power when the intensity of sunlight exceeds 1000&nbsp;W/m<sup>2</sup> (which corresponds roughly to midday in summer in, for example, Germany), or when sun irradiation close to 1000&nbsp;W/m<sup>2</sup> happens at lower temperatures.
 
Nominal power rating of solar PV plants is not comparable with the name plate [[Maximum Continuous Rating|MCR]] rating of conventional power plants as there is wide difference between its DC and AC ratings (30% to 40%). The net power that can be fed by a coal fired / nuclear power plant is around 90% of its name plate MCR after deducting internal consumption. Similarly for [[Combined cycle power plant|CCGT]], around 97% of site rating and for [[Gas turbine power plant|GTPP]] or [[Hydroelectricity|Hydro electric plant]] above 99% of site rating. The installation cost/MW generally given for Solar PV based on DC capacity is skewed when compared to net MCR of other power generation sources (including [[wind power]] and [[Concentrated solar power|solar thermal]] ). The actual installation cost of solar PV is around 50% more to compare with net MCR of other sources for giving equal input (MW) to the AC power grid.
 
== References ==
Line 54 ⟶ 49:
{{Photovoltaics}}
 
[[Category:Renewable energy economyeconomics]]
[[Category:Units of power]]
[[Category:Photovoltaics]]
Retrieved from "https://en.wikipedia.org/wiki/Nominal_power_(photovoltaic)"