Solar energy for electricity and fuels

Authors: Olle Inganäs Villy Sundström

Publish Date: 2015/12/14

Volume: 45, Issue: 1, Pages: 15-23

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Abstract

Solar energy conversion into electricity by photovoltaic modules is now a mature technology We discuss the need for materials and device developments using conventional silicon and other materials pointing to the need to use scalable materials and to reduce the energy payback time Storage of solar energy can be achieved using the energy of light to produce a fuel We discuss how this can be achieved in a direct process mimicking the photosynthetic processes using synthetic organic inorganic or hybrid materials for light collection and catalysis We also briefly discuss challenges and needs for largescale implementation of direct solar fuel technologiesA comparison of finite and renewable planetary energy reserves measured in TW/years Total recoverable reserves are shown in TWy for the finite resources and yearly potential for the renewables OTEC = Ocean thermal energy conversion One TWy is 8760 TWh This figure does not include shale gas or shale oil nor the energy available in methane hydrate Source Perez and Perez 2009Photovoltaic electricity from solar cells has undergone a rapid development and is rapidly being used both in minor private systems as well as in largescale installations connected to the national grids Wind energy has also reach maturity and undergoes presently a rapid worldwide implementation The main challenge with the renewable energy is the intermittency requiring major storage or largescale integrationThis article focuses on solar energy identifying the need for breakthroughs in more efficient ways to produce i electricity from more powerful and cost efficient solar cells ii the possibility of direct conversion of solar energy into fluid such as ethanol or methanol or gas forms methane of hydrogen and iii the need for producing significantly higher bioconversion including breeding of special plants and genetic engineering of cyanobacteriaThe annual flow of energy from the sun dwarfs all other nonrenewable energy flows and stocks and is several orders of magnitude above what humankind needs Fig 1 Very major secondary flows of solar energy are concentrated by the thermal engine of the atmosphere supplying flow of water for hydropower and flow of air for wind power From inside the Earth geothermal heat is delivered from nuclear processes All these are very small by comparison to the solar influx The solar influx over land on a horizontal surface varies considerably between different regions and with the diurnal and annual cycle The most favorable areas are in the subtropics where values can reach almost 300 W/m2 in annual average while in parts of northern Europe values can be as low as around 50 W/m2 with very low values during winter The diurnal variation of the solar energy influx matches somewhat but not perfectly the activity patterns of societies There is therefore a great need to store solar derived energy as electrical or chemical energy to be used at some later timea The solar radiation arriving at the surface of the earth is distributed over different wavelengths of light with tails extending far out in the infrared and invisible region With semiconductor materials light with energy higher/wavelengths shorter than the band gap of the material can be absorbed shadow in graph but none below the band gap b The semiconductor is contacted with electrodes and exposed to solar light Photocurrent J sc from the devices depends on absorption and charge generation in the active material the photovoltage V oc basically depends on the materials c The current–voltage curve shows both these parameters and the device is used to deliver electricity at the maximum power delivery point indicated with a starThe science of photovoltaic is a mature field Novel photovoltaic materials arrive and scientists later learn how to accommodate these novel phenomena to the scientific terminology of photovoltaics The thermodynamic limitations analyzed by Shockley and Queisser S–Q in 1961 resulted in a maximum efficiency of slightly over 33  in a singlejunction device using a band gap of 11–14 eV These limitations are due to heat losses from high energy photons above the singleband gap and from loss of photons not absorbed below the band gap It is also based on the equilibrium flux of photons to the photovoltaic device from the sun and the flow of photons to the universe from the photovoltaic device This is a description that is valid for well established materials like crystalline and polycrystalline siliconbased photovoltaic but also for amorphous inorganic semiconductors in thin film format as well as newer organics in wet or dry state conditions and new hybrid organic–inorganic materials Though the science is mature the implications of this science have not always been fully acknowledged The recent improvement to 288  power conversion efficiency PCE of high performance gallium arsenide GaAs singlejunction solar cells has been accomplished by photonic engineering to manipulate the balance between the incoming and outgoing photon flux with the reward coming as voltage from the cell Miller et al 2012Improved energy conversion is found in multijunction solar cells or tandem solar cells where the heat and transmission losses are minimized by use of two three or more different materials of different band gaps and connecting these devices in electrical series These are very high performance solar cells with power conversion efficiencies up to 447  under the direct sun spectrum at very high concentrations These solar cells are mostly used in space or under concentrated solar light on earth The delicate growth of several compound semiconductors on top of each other to create these structures results in a higher material and processing cost Therefore these solar cells are mostly relevant together with concentrating optical elements and thus require high performance lowcost concentrators which only work well in direct sunlight Such concentrator systems are today becoming more widely available and applications in the multi MW range have been realized

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