The search for clean sources of energy to replace the fossil fuels we burn inevitably leads us to look at the sun. The solar radiation that reaches the earth is many times larger than the energy we use. What is missing is our ability to harness this energy effectively. A lot of work has focused on photovoltaics to convert sunlight into electricity.
This work has led to the installation of rooftop solar panels around the world and experience with these have driven efficiencies higher, to the 15 percent -18 percent range. The high installed cost of about $11,000 per kWh is the major deterrent, besides the need to store the electricity generated in batteries for use after sunset.
Alternative technologies such as plastic film solar cells are being developed to drive costs down and to extend the usable surfaces for solar panels from the rooftops to the vertical surfaces. The other major effort in replacing fossil fuels has been to pursue the development of bio-fuels. Plants convert Carbon Dioxide from the air into biomass in the presence of sunlight and water by the photo-synthesis process.
This biomass is converted into ethanol and used as an additive to gasoline. There is some thought that bio-fuels could be used not just as additives but as the primary fuel. The major disadvantage of biofuels is that they need land and water , which could limit their adoption in places around the world where these are scarce.Photosynthesis conversion efficiencies are less than 0.5%
There is however news of some technologies that are being developed in parallel by researchers in different parts of the world that could bring about a new approach to these problems. The approach of these research teams to to convert sunlight directly into a fuel that can be stored and transported as easily as the fossil fuels we now use. This is because weight-for-weight, a fuel is a more efficient energy storage device than a battery.
A collaborative team from the Universities of East Anglia , York , Nottingham and Manchester in the UK are working to make a “solar nano-cell” which consists of two types of light absorbers. One of these are quantum dots of semiconductor material which has the advantage that it can be engineered to absorb the different colours of the solar spectrum.
The quantum dot array , therefore, absorbs more energy than the crystalline solar cell. The second light absorber is porphyrin molecules which are similar to those used by plants for photosynthesis. A catalyst is grafted to the quantum dots. When sunlight falls on this array, electric current is produced as in a conventional photovoltaic junction.
This current is used to electrolyze water molecules into Hydrogen and Oxygen. Professor Wendy Flavell from the University of Manchester’s Photon Science Institute suggests that this Hydrogen could be directly used as the fuel for a Hydrogen car. Solar Fuel GmbH, a Stuttgart , Germany based technology group , takes this concept a step further.
The hydrogen gas gas produced is reacted with carbon dioxide to produce methane . This group aims to capture carbon dioxide emissions from power plants or cement kilns and convert those to methane by solar nano-cells. The methane gas could be piped for home heating or other applications using the existing gas transport infrastructure.
Joule Bio-technologies, a Cambridge , Massachusetts based company has taken a different route altogether. They use genetically engineered micro-organisms called cyanobacteria grown on a flat plastic bed to convert sunlight and carbon dioxide directly into bio-fuel. The micro-organisms “digest” the Carbon Dioxide and secrete diesel or ethanol-like fuel that can be continuously recovered from the bed.
Joule Biotechnologies claims that it can produce 15,000 gallons of this bio-fuel per acre of land which is about 50 times the output from conventional bio-mass plantations and aim to produce the fuel at $20 / barrel , one-fourth to one-fifth the cost of petroleum fuels. They also aim for commercial production from 2012 , scaling up from the pilot projects already in operation
With the emergence of these new technologies , the dream of putting sunshine into our fuel tanks is closer to realization.
Via: RoyalSociety
