Max S. Dunn

Blog-Powering the Future

Posted by: Max Dunn on August 21, 2008 19:00:24

This was the last in the series of outdoor science talks at the Cantor Arts Center (EVT 173). I wasn't able to make it to the others ones - or wasn't interested in the topic. Most of the information was review, but it was interesting to hear about the work Stacey's group is doing with thin-film CZTS technology.

Presented by:

STACEY F. BENT
Professor of Chemical Engineering and Professor

Overview

Meeting the world's growing energy needs in a sustainable fashion is one of the most pressing problems of our time. Although there are multiple possibilities on the horizon for renewable and clean energy, many scientific and engineering challenges remain. This is an exciting time for researchers in the field of sustainable energy. This presentation will introduce the scope of the energy problem and some of the options for sustainable energy, then will focus on two main devices: solar cells and fuel cells. Solar cells convert the energy of the sun directly into electricity, while fuel cells convert chemical energy directly into electricity. Both devices will likely play an increasing role in future energy technologies. In this talk, Stacey Bent will touch upon current research at Stanford that is exploring the use of new materials to develop the next generation of solar cells and fuel cells.

Intro

• 1 gallon gas = 17 lbs CO2 (EPA says gas 19.4 lb.s, diesel 22.2 Reference)
• Niagara falls produces 4 GW of electricity
• World consumption 13 TW, or 3,000 Niagra falls. most is chemical from fossil fuels: coal, gasoline, oil
• Peak oil not as important as global warming
• Gave global warming background: doubling of CO2 produces temperature increase of 3 or 4 degrees
• Some sustainable energy sources: Wind, PV, solar thermal, bio-fuels, wave, geothermal
• Producing ethanol from corn is likely carbon positive
• Making fuel from organics is not very efficient
• Switch grass only about 1/100 as efficient as solar cells
• Recommends: "Earth: The Sequel"

Solar Cells

• One of the most promising approaches
• Nate Lewis says: 100,000 trillion watts which is 8,000 times the worlds consumption. Every hour, there is enough solar energy impacted on the earth to fiill the world's energy needs for a year
• 100 miles by 100 miles square with 10% efficient solar cells would power the US
• 30 miles by 30 miles square would produce enough for California
• PV needs 1/6 the acreage as wind and 1/30 as much as biofuels
• Problem: Intermittent - needs storage or co-ordination between different sources
  • Possible storage solutions: batteries, heat, pumped hydro
• Problem: cost: Coal is 1c to 4c per kWh, PV 17c to 22c. ($4/watt PV would be about 7c kWh if daily factor is 5 and lasts 3 years)
  • Wind is currently 5c to 7c 
• PV single junction theoretical limit is 31%, multi-junction is 42%
• PVs made out of semiconductors. Photons excite electrons to jump band-gap and produces electricity. To prevent recombination, PVs are made out of two kinds of semiconductors
• First Generation PV: Silicon. Used in 90% of PVs. Considered thick but still about 1/10 mm. Efficiencies are 12% to 19%. Needs very pure silicon so are expensive. PVs now use 50% of silicon produced
• Second Generation PV: Thin-film. Made from semi-conductors that absorb sunlight better than silicon, the semiconductor is only 1 micron and can be laid down on cheap substrates. Is cheaper and can be flexible. Some technology is based on CIGS, Cadmium telluride, organic materials
• Third Generation PV: use more light or waste less energy

CZTS Solar Cells

 

• Silicon is very abundant. Cadmium is very toxic and banned in EU. Indium are very rare and expensive at $350 lb. About 10 million times less abundant than silicon.
• Collaborating with Bruce Clemens, Mike McGee on thin-film based on earth abundant and non-toxic materials based on  copper, zinc, tin ($7/lb) and sulfur referred to as CZTS.
• Jeffery King tried to grow CZTS in a solution as a film on glass. Was difficult but has grown some to make sucessful PV.
• Ben Trang and Christine Yakamoto took nano particles of components and is trying to coat them on glass slide
• Varden Chala is trying to grow material from gas instead of liquid using sputter deposition in vacuum chamber. (This is a common process used to coat glass and put foil on inside of chip bag.)
• Japan group has made some PV this way that have 6% efficiencies
• Just starting to make and test solar cells this way

Hydrogen

• Showed demo of toy hydrogen car
• Need energy storage
• Fuel cell more efficient than burning H2
• Fuel cells work by controlling how O2 contacts H2
• Downside is they have to operate at high temperature, currently 1,500 deg F.
• Work going on to reduce thickness of solid oxide to 100 nm to reduce temperation, about 600-700 deg F
• Platinum used for catalyst which is expensive at $1500 per ounce.
• Using atomic layer deposition to reduce amount of platinum needed. Works better with edges, like a grid

Perspective

• Has faith that we can move to sustainable energy to preclude catastrophic global warming