Terms run out of en­ergy

The Weekend Australian - Travel - - Resources -

EN­ERGY pay­back time, en­ergy gain, net en­ergy, en­ergy har­vest ra­tio and en­ergy re­turn fac­tor. Th­ese terms and oth­ers have been used over the years to grasp the en­ergy gen­er­ated by a so­lar pow­ered de­vice.

Bryce Richards, who now lec­tures in re­new­able en­ergy at Ed­in­burgh’s He­ri­otWatt Univer­sity, wants peo­ple to stop us­ing such ter­mi­nol­ogy. The most com­mon con­cept, en­ergy pay­back time (EPT), has per­pet­u­ated the myth that so­lar pow­ered de­vices were un­able to re­pay the en­ergy in­vested in mak­ing them, Richards said.

EPT first arose in the 1970s af­ter pho­to­voltaics suc­cess­ful in space pro­grams were ap­plied on earth. In those early days it was pos­si­ble that a de­vice would never re­coup its fab­ri­ca­tion en­ergy. Richards said EPT was ap­peal­ing be­cause of its sim­i­lar­ity to eco­nomic pay­back times.

‘‘ The draw­back is that EPT does not ac­count for ad­di­tional en­ergy gen­er­ated dur­ing the re­main­der of the eco­nomic life­time,’’ he said. Ap­ply­ing EPT to a mul­ti­com­po­nent pho­to­voltaics sys­tem was also prob­lem­atic. Th­ese sys­tems might in­clude a cur­rent in­verter, bat­ter­ies and mount­ing frames along­side the cells that con­vert light into elec­tric­ity. ‘‘ If an end-user knows what the ex­pected EPT of such a sys­tem is when it is in­stalled, then how should that per­son in­ter­pret any change in EPT when the bat­tery bank needs re­plac­ing af­ter seven years?’’ Sarah Belfield

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