To realise the potential of solar energy will require technological and scientific advances to enable the full energy cycle of capture, conversion and storage to be integrated into an interdependent, synergistic and globally scalable system. By Nathan Lewis, professor of chemistry, Beckman Institute and Kavli Nanoscience Institute, California Institute of Technology
The sun is the champion of all energy sources. Sunlight has provided, through photosynthesis, the energy from which all of our fossil resources were formed, as well as the energy that sustains life on earth and powers the wind, the tides, hydrological flows and the climate. More energy from sunlight strikes the earth in one hour than all of the energy consumed by humans in an entire year.
Global primary energy consumption is around 410 exajoules a year, equivalent to an average thermal power consumption of 13.5 terawatt hours (TWh). The sun provides a staggering 120,000 TWh/y of power to the earth, far more than humans could ever use.
An under-utilised energy resource
Yet sunlight is an under-utilised energy resource. Solar electricity accounts for less than about 0.02%, of total global electricity production. Biomass burned sustainably accounts for 1.5% of our energy
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needs, while that burned unsustainably (by far the largest use of solar energy) accounts for about 10%. The solar production of high-energy chemical fuels such as hydrogen, methane, or alcohol by natural or artificial photosynthesis is negligible. Ethanol produced by fermentation of biomass accounts for less than 30 gigawatt hours (GWh) a year, or 0.2% of our energy needs. Solar thermal conversion for space and water heating accounts for 6 GWh/y of energy, or 0.3% of the global need for space and water heating.
The enormous gap between the potential of solar energy and our use of it arises from cost. Fossil-fuelled electricity costs $0.03-0.06 per kilowatt hour (kWh), compared with $0.30/kWh from photovoltaics (PV) and $0.10-0.15/kWh from solar thermal installations. Direct combustion of biomass is cost competitive in developing countries because it is practised locally and unsustainably, minimising transportation and replacement costs. The global use of biomass for energy is limited by production capacity – land and water restrictions. Solar heating of space and water is marginally competitive with fossil heat, given incentives, but is not enticing enough to displace fossil fuel's established market position.
To provide a truly widespread primary energy source, solar energy must be captured, converted and stored in a cost-effective fashion. The best commercial PV devices, based on single-crystal silicon, are about 18% efficient. Laboratory solar cells based on cheaper dye sensitisation of oxide semiconductors are typically less than 10% efficient and those based on even cheaper organic materials are 2-5% efficient. Green plants typically convert sunlight to biomass with a yearly averaged efficiency of at most 0.3%, with more typical values less than 0.1% to fuel. Ultimately, growth rates are limited to a factor of around two, higher than the best values by a lack of supply of their key growth nutrient, CO2, from the atmosphere to the surface of the earth.
The cheapest solar electricity comes from solar thermal conversion coupled with heat engines, which is 20% efficient on annual average, and can be up to 30% efficient for the best systems. The opportunity for higher efficiency is enormous, a factor of 10 or more for most conversion routes, and factors of between two and five for the best electricity conversion. Increases in efficiency would reduce cost and increase capacity by the same amount, raising solar energy to a qualitatively new level of competitiveness. This will not only require cost-reduction in existing PV manufacturing methods to provide electricity from sunlight, but will also require science and technology breakthroughs to enable, in a convenient, scalable, manufactureable form, the ultra-low-cost capture, conversion and storage of sunlight. Advances in this key area are where nanotechnology will have an enabling role.
A key step is the capture and conversion of the energy contained in solar photons. The total energy provided by the sun is fixed over the 30-year lifetime of a PV module, so once the energy conversion efficiency of a PV module is established, the total amount of useable electricity produced by the module is known for the lifetime of the system. The theoretical efficiency limit for a single band-gap solar conversion device is 33%, and small test cells have demonstrated efficiencies of more than 20%, with the remaining losses almost entirely the result of small reflection losses, grid-shading losses and other 5-10% level losses that any practical system will have.
Costs must fall
Shipped PV modules have efficiencies of around 15-20%. At such an efficiency, if the cost of a module is about $300 per square metre, and including the accompanying fixed costs in the so-called balance of systems – such as the inverter and grid connection – then the sale price of grid-connected PV electricity must be $0.25-0.30/kWh to recover the initial capital investment and cost of money over the lifetime of the PV installation. Utility-scale power generation costs are much less, at around $0.03-0.05/kWh, so for solar electricity to be cost-competitive at utility scale, improvements in efficiency are beneficial, but manufacturing costs must fall significantly.
Improvements in efficiency above the 33% theoretical limit are possible if the constraints incorporated into the theoretical efficiency limit are relaxed. For example, if photons with energies greater than the band gap of the absorbing material did not dissipate their excess energy as heat, but instead produced more voltage or generated multiple, low-energy, thermalised electrons from the energy of a single absorbed photon, theoretical efficiencies in excess of 60% would, in principle, be attainable. However, although many solar photons carry enough energy to unleash several electrons, they almost never free more than one, but squander the energy as heat instead.
Absorbers with a highly quantised band structure, such as quantum wells and quantum dots – semiconductor bits only a few nanometers in diameter – can theoretically produce the desired effects (see Figures 1a and 1b). Recent observations on lead-sulphide quantum dots have demonstrated production, with high quantum yield, of multiple excitons from a single absorbed photon. However, there is, as yet, no method for efficiently extracting the photogenerated carriers from the quantum-dot structure to produce electricity in an external circuit. Materials with mini-bands, or intermediate bands also offer the possibility for ultra-high energy-conversion efficiency. In this approach, different photon energies would promote absorption from different, isolated, energy levels allowing the production of different voltages from different incident photon energies. The approach has yet to be demonstrated in practice.
New manufacturing methods
Even with a dramatic increases in cell efficiency, the value of nanotechnology for new solar-cell materials rests with the potential to enable an entirely different, ultra-low-cost manufacturing process for PV technology. To make a 10% contribution to global primary energy supply by 2050 (to provide 1-2 TW of power), would require the installation of 1m typically sized (2 kW peak-power) roof-top PV systems a week, every week for 50 years. While stabilising atmospheric CO2 concentrations at 550 parts per million would require 10-20 TW of carbon-neutral power capacity by 2050, corresponding to the installation of 1m PV systems every day, for the next 50 years. Without new manufacturing methods, solar electricity is unlikely to make a material contribution to primary energy supply, or a significant contribution to reducing CO2 emissions, in the first half of the 21st century.
The key issue involves the trade-off between material purity and device performance. In a typical planar solar-cell design, the charge carriers are collected in the same direction as light is absorbed. A minimum thickness of the cell is dictated by the thickness of material required to absorb more than 90% of the incident sunlight. However, the required thickness of the material also imposes a constraint on its required purity, because the photoexcited charge carriers must live sufficiently long within the absorbing material to travel to the electrical junction, where they can be separated to produce an electrical current-flow through the metallic contacts to the cell.
Impure absorber materials with short charge-carrier lifetimes can absorb sunlight effectively, but cannot effectively convert that absorbed energy into electricity. In turn, absorber materials with the necessary purity are expensive to produce. Cheaper materials, such as organic polymers or inorganic particulate solids with small grain sizes, have short charge-carrier lifetimes and/or induce recombination of charge-carriers at the grain boundaries of such materials.
Approaches to circumventing this cost/efficiency trade-off generally involve orthogonalisation of the directions of light absorption and charge-carrier collection. High-aspect-ratio nanorods, for example, can provide a long dimension for light absorption, while requiring that carriers move only radially, along the short dimension of the nanorod, to be separated by the metallurgical junction and collected as electricity.
Under widespread investigation
A conceptually similar approach involves the use of interpenetrating networks of inorganic absorbers, such as CdTe tetrapods and/or organic polymeric absorbers, such as poly-phenylenevinylene. Such systems are under widespread investigation and the key is to not only to obtain intimate contact between the light-absorbing and charge-collecting phases, but also to control the chemistry at the interface between the two phases that comprise the device.
Junction recombination is a deleterious-loss pathway even in many planar solar-cell devices and generally becomes dominant in disordered systems that, by definition, have a large increase in their interfacial contact area relative to their projected geometric area for light absorption. Methods for controlling the chemical properties of the surfaces and junctions of such systems, reducing their natural tendency to promote deleterious charge-carrier recombination, are critical. Such methods have been developed for certain well-defined semiconductor surfaces, and will need successful development and implementation for the high junction-area systems to obtain high (more than about 5%) energy-conversion efficiencies from such devices.
A conceptually related system is the dye-sensitised solar cell, in which a random, disordered network of inexpensive titanium dioxide particles is used to collect the charge carriers. Light absorption is performed by an adsorbed-dye molecule and the interfacial contact distance is kept small by use of a liquid or conductive polymer to penetrate the pore structure of the solid and collect the other charge-carrier type to complete the circuit in the cell (see Figure 2). Small champion dye-sensitised solar cells have shown efficiencies as high as 10-11%, although large-area devices typically have efficiencies in the range of 5%.
Improvements in the efficiency of such systems will require improved dyes, better electrolytes and better control over the recombination at the interfacial contact area that dominates, and limits, the voltage produced by such systems to around 50-60% of its theoretical value. Systems stability must also be demonstrated under operational conditions, for extended periods of time (more than 10 years) to allow them to be implemented in the marketplace. Advances in basic science are needed to enable all such nanostructured systems to offer a truly practical, ultra-low-cost, option for solar electricity production.
Tremendous potential for growth
Although there is tremendous potential for growth for PV in electricity generation, solar electricity cannot be a material contributor to primary energy production without cost-effective methods for storing and distributing massive quantities of electricity. The lack of cost-effective large-scale electrical storage capacity is behind calls for development of space-based solar-power systems.
On earth, the cheapest method for electricity storage is pumped-water storage, but even that process does not scale well if every reservoir has to be filled up each day and emptied each night; and a
staggering amount of pumped water would be needed to compensate for the diurnal cycle to provide a material contribution to energy generation. Batteries are a natural approach to electricity storage, but enormous quantities of lead-acid batteries with the cycle life of Li-ion batteries would need to be hooked up to the grid to be cost-effective over the 30-year amortised lifetime of a PV system. Innovative approaches to massive, low-cost storage, including potentially a superconducting global transmission grid, supercapacitors and flywheels could be potentially important enablers of a full solar capture, conversion and storage system.
Perhaps the most attractive method for cost-effective energy storage is in the form of chemical bonds – a chemical fuel. This approach is the essence behind photosynthesis and is the basis for much recent attention devoted to biofuels development. Photosynthesis, however, saturates at about 10% of the intensity of normal sunlight and, consequently, the yearly averaged energy storage efficiency of even the fastest-growing plants is less than 1%, and typically less than 0.3-0.5%, compared with the more than 15% efficiency of PV devices.
One approach is through electrolysis, in which water is split into hydrogen (H2) and oxygen (O2) in an electrolyser. Platinum-based electrolysis in acidic or neutral media is too expensive for this process to be material in global primary energy production. Nickel-based electrolysis in basic aqueous solutions is cheaper, but requires scrubbing the input stream to remove CO2; additionally, even the best fuel cells are only 50-60% energy efficient and the best electrolysis units are 50-60% energy efficient, so the full cycle energy storage/discharge efficiency of such a system is only 25-30%.
Better catalysts required
Better catalysts for the multi-electron transformations involved with fuel formation are needed. Such catalysts exist naturally, with the hydrogenase enzymes operating at the thermodynamic potential for production of H2 from H2O, and with the oxygen-evolving complex of Photosystem II producing O2 from H2O in an energy-efficient fashion. But no man-made systems have been identified that show performance even close to those of the natural enzymatic systems. Development of such catalysts through nanotechnology would provide a key enabling technology for a full solar-energy conversion and storage system.
Whether the fuel-forming system is separate, as in a PV-electrolysis combination, or integrated, as in a fully artificial photosynthetic system that uses the incipient charge-separated electron-hole pairs to produce fuels directly with no wires and with only water and sunlight as the inputs, is interesting from both a cost and engineering perspective. However, the key components needed to enable the whole system remain the same in either case: cost-effective, efficient, capture, conversion and storage of sunlight. Each of these functions has its own challenges and their integration into a fully functioning system will require further scientific and engineering advances.