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 thatreaches the earth is many times larger than the energy we use. What ismissing 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 panelsaround the world and experience with these have driven efficiencieshigher, to the 15 percent -18 percent range. The high installed cost ofabout $11,000 per kWh is the major deterrent, besides the need to storethe electricity generated in batteries for use after sunset.
Alternative technologies such as plastic film solar cells are beingdeveloped to drive costs down and to extend the usable surfaces forsolar panels from the rooftops to the vertical surfaces. The other major effort in replacing fossil fuels has been to pursue the development ofbio-fuels. Plants convert Carbon Dioxide from the air into biomass inthe presence of sunlight and water by the photo-synthesis process.
This biomass is converted into ethanol and used as an additive togasoline. There is some thought that bio-fuels could be used not just as additives but as the primary fuel. The major disadvantage of biofuelsis that they need land and water , which could limit their adoption inplaces 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 developedin parallel by researchers in different parts of the world that couldbring about a new approach to these problems. The approach of theseresearch teams to to convert sunlight directly into a fuel that can bestored and transported as easily as the fossil fuels we now use. This is because weight-for-weight, a fuel is a more efficient energy storagedevice than a battery.
A collaborative team from the Universities of East Anglia , York ,Nottingham and Manchester in the UK are working to make a “solarnano-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 solarspectrum.
The quantum dot array , therefore, absorbs more energy than thecrystalline solar cell. The second light absorber is porphyrin molecules which are similar to those used by plants for photosynthesis. Acatalyst is grafted to the quantum dots. When sunlight falls on thisarray, electric current is produced as in a conventional photovoltaicjunction.
This current is used to electrolyze water molecules into Hydrogen and Oxygen. Professor Wendy Flavell from the University of Manchester’sPhoton Science Institute suggests that this Hydrogen could be directlyused 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 toproduce methane . This group aims to capture carbon dioxide emissionsfrom power plants or cement kilns and convert those to methane by solarnano-cells. The methane gas could be piped for home heating or otherapplications using the existing gas transport infrastructure.
Joule Bio-technologies, a Cambridge , Massachusetts based companyhas taken a different route altogether. They use genetically engineeredmicro-organisms called cyanobacteria grown on a flat plastic bed toconvert sunlight and carbon dioxide directly into bio-fuel. Themicro-organisms “digest” the Carbon Dioxide and secrete diesel orethanol-like fuel that can be continuously recovered from the bed.
Joule Biotechnologies claims that it can produce 15,000 gallons ofthis bio-fuel per acre of land which is about 50 times the output fromconventional bio-mass plantations and aim to produce the fuel at $20 /barrel , one-fourth to one-fifth the cost of petroleum fuels. They alsoaim for commercial production from 2012 , scaling up from the pilotprojects already in operation
With the emergence of these new technologies , the dream of putting sunshine into our fuel tanks is closer to realization.