How do pho­to­vol­taics work?
Solar cells are based on the pho­toe­lec­tric effect that was disco­ve­red by the French phy­si­cist Ale­xan­dre Edmond Bec­que­rel in 1839, which inte­re­sted many other great resear­chers inclu­ding Hein­rich Hertz, his stu­dent Wilhelm Hall­wa­chs and even Albert Ein­stein. In 1907, Ein­stein sub­mit­ted a quan­tum theory expla­na­tion as to why light gene­ra­tes elec­tri­cal char­ges in cer­tain mate­rials. But it wasn’t until the 1950s that Ame­ri­can labo­ra­to­ries pro­du­ced the first solar cells made from the semi-con­duc­tor sili­con with an elec­tri­cal output that could be used in a tech­ni­cal appli­ca­tion. In this case, the emer­ging aero­space indu­stry, but also as a decen­tra­li­sed power supply for tele­phone ampli­fiers, for exam­ple. The same elec­tri­city that solar cells gene­rate can be used imme­dia­tely, stored in bat­te­ries or tran­sfor­med into alter­na­ting cur­rent and fed into the grid.

Sili­con as base mate­rial
Sili­con, the mate­rial used in most of today’s solar cells, is a semi-con­duc­tor. This raw mate­rial is common in the earth’s crust in the form of sili­con dio­xide (quartz, sand), making it avai­la­ble in almost unli­mi­ted quan­ti­ties. Mono­cry­stal­line, poly­cry­stal­line and amor­phous sili­con can be pro­du­ced from high purity sili­con. These base mate­rials are in turn used to pro­duce solar cells with vary­ing pro­per­ties. Solar cells made from amor­phous sili­con offer low effi­ciency at a cor­re­spon­din­gly low cost; solar cells made from mono­cry­stal­line sili­con may be more costly, but they are also more effi­cient. The right cell type depends on the spe­ci­fic appli­ca­tion. Lumi­nai­res with a decen­tra­li­sed power supply require a com­pact design and high effi­ciency, which is why hei solar lumi­nai­res from Selux use highly effi­cient solar cells gene­rally made from mono­cry­stal­line sili­con.

Nomi­nal power and yield
The nomi­nal power of pho­to­vol­taic instal­la­tions is mea­su­red in Wp (Watt-peak). Wp refers to the per­for­mance under test con­di­tions that appro­xi­mate the maxi­mum solar radia­tion in Ger­many. These stan­dard test con­di­tions (STC), used to com­pare dif­fe­rent solar modu­les, are defi­ned as a cell tem­pe­ra­ture of 25°C, radia­tion of 1000W/​m² and an air mass of 1.5. A typi­cal PV-instal­la­tion on the roof of a family home (with 40m²) yields around 4 – 5kWp; the PV cells on a hei-solar lumi­naire from Selux have nomi­nal power ratings in the range of 100 to 250Wp. Pho­to­vol­taics are ideally suited to regions within what we call the solar belt, where there are high levels of solar radia­tion irre­spec­tive of the season. These include sou­thern areas of Europe and North Ame­rica, Cen­tral and South Ame­rica, Africa, Asia and Austra­lia. Pho­to­vol­taics are also a cost-effi­cient alter­na­tive in nei­gh­bou­ring regions such as Cen­tral Europe, as has been demon­stra­ted in recent deca­des. Seve­ral cities and regions in Ger­many and elsewhere are recor­ding pho­to­vol­taic yields in solar land regi­sters.

Finan­cial fea­si­bi­lity
Solar power is not only sustai­na­ble from an eco­lo­gi­cal point of view, it is now also finan­cially com­pe­ti­tive. Over recent deca­des, tech­ni­cal advan­ces have impro­ved the per­for­mance of solar cells, as well as other system com­po­nents such as inver­ters, con­trol and char­ging elec­tro­nics and bat­tery sto­rage. At the same time, the eco­no­mics of sca­ling pro­duc­tion mean costs have fallen con­si­de­ra­bly: solar cell prices today are 90% lower than in 2010. In many regions with cor­re­spon­ding light inten­sity, pho­to­vol­taics are already con­si­de­red the chea­pest way to gene­rate elec­tri­city. In appli­ca­tions such as making road or path lighting self-suf­fi­cient (»off-grid«), addi­tio­nal fac­tors posi­ti­vely impact eco­no­mic effi­ciency and the eco­lo­gi­cal balance sheet: not only are there no elec­tri­city costs per se, nei­ther are there any line char­ges or wiring instal­la­tion costs.