Energy analysis of batteries in photovoltaic systems Part II. Energy return factors and overall battery efficiencies

Journal article, 2005

Energy return factors and overall energy efficiencies are calculated for a stand-alone photovoltaic (PV)-battery system. Eight battery technologies are evaluated: lithium-ion (nickel), sodium-sulphur, nickel-cadmium, nickel-metal hydride, lead-acid, vanadium-redox, zinc-bromine and polysulphide-bromide. With a battery energy storage capacity three times higher than the daily energy output, the energy return factor for the PV-battery system ranges from 2.2 to 10 in our reference case. For a PV-battery system with a service life of 30 yr, this corresponds to energy payback times between 2.5 and 13 yr. The energy payback time is 1.8-3.3 yr for the PV array and 0.72-10 yr for the battery, showing the energy related significance of batteries and the large variation between different technologies. In extreme cases, energy return factors below one occur, implying no net energy output. The overall battery efficiency, including not only direct energy losses during operation but also energy requirements for production and transport of the charger, the battery and the inverter, is 0.41-0.80. For some batteries, the overall battery efficiency is significantly lower than the direct efficiency of the charger, the battery and the inverter (0.50-0.85). The ranking order of batteries in terms of energy efficiency, the relative importance of different battery parameters and the optimal system design and operation (e.g. the use of air conditioning) are, in many cases, dependent on the characterisation of the energy background system and on which type of energy efficiency measure is used (energy return factor or overall battery efficiency). © 2004 Elsevier Ltd. All rights reserved.