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.