MeanGene Rants                                                                             5 August 2003

This Month: Solar With Storage

Cool Stuff

 

·        The Underwater Steam Jet, cleaner, safer, more efficient way to move boats – and it can process food!

·        Power from human blood could make for longer lasting devices embedded in the body, or could lead to sugar fed robots.

·        The first commercial fuel cells should appear next year. No, not cars, but maybe cell phones and laptops powerful enough to last on a long flight.

·        Japan leads in both production of solar panels and in electricity generated from them. The New York Times has the low down. (free registration required)

 

Solar with Storage

One of the many reasons why your house is not yet powered by solar electricity is that solar panels are just not that efficient. According to the  NYTimes article above, the most efficient solar panels today are made by Sharp and they are about 17.4% efficient. BP solar claims 18.3% efficiency, and the NREL has stuff in a lab at 32% efficiency. In all cases, the majority of the energy is lost as heat.

 

Another big reason you probably haven’t bought solar panels is that they only work when the sun shines. The electrons that run your TV at night or on cloudy days aren’t likely to come from the sun.

 

Taking advantage of the fact that it is easier to get heat from the sun than it is to get electricity, and combining that with a way to store excess heat efficiently is the Solar Power Tower. This is not a system for your roof, mind you, but it is possibly better than a photovoltaic system at utility scale.

It Works Like This

  • Position a few thousand mirrors so they reflect sunlight on a special heat receiver.
  • Make the mirrors track the sun and refer to them as “heliostats” because it sounds much cooler than “sun-tracking mirror.”
  • Use the receiver to heat molten salt from 295° C to 565° C. Yes, molten salt.
  • Use the heat from the salt to drive a steam turbine which produces electricity.
  • Store excess heat in the salt until it is needed at night or other times when the sun isn’t shining.

 

 

The result is a system that could generate electricity at full output for up to 70% of the hours in a year. By contrast, PV systems are lucky to generate electricity 25% of the hours in a year and no cost effective storage systems are available to improve that number. Of course, we should point out that the best prototype, Solar Two, operated at only 20% capacity factor.

Salt of the Earth

The salt used in the system is a common fertilizer (sodium and potassium nitrate), is cheap, nonflammable, nontoxic, and stores heat with 97% efficiency.

 

In fact the designers claim that most of the system components, heliostats, storage tanks, and receiver can be built with simple technologies in most countries of the world. Unlike photovoltaic cells, there are few high-tech components with rigorous manufacturing requirements. This makes Solar Power Towers and other similar solar concentrating systems, such as Parabolic Troughs, are more attractive to developing countries.

The Cost

The electricity cost is about 13 cents per kWh which is not quite good enough yet for unsubsidized markets. By comparison coal costs 4-5 cents/kWh, wind 5-6/kWh, and natural gas 4-6/kWh, but it is a far cry better than PV solar which comes in above 25 cents per kWh before tax rebates or other subsidies. The main cost in a Solar Power Tower is the heliostats which have not been manufactured in volume to date. If the market for heliostats grows, overall costs should come down.

 

Land requirements are about 10 acres/MW of power, which is less than hydroelectric and comparable to a coal power plant including the mining operations.

 

Operating costs of a Solar Power Tower dictate that a plant must be at least 30 MW in size to be commercially feasible. Again, you aren’t going to put one of these in your backyard anytime soon.

The Future

According to the US Dept of Energy, there will be more than 500MW of electricity made from solar concentrating systems from a mix of Solar Tower, Solar Troughs, and Solar Dish designs.

 

Who is going to make the first commercial Solar Power Tower?

 

Plans to improve on the prototype Solar Two were underway in Spain where they planned to build a 15MW system they called, Solar Tres. According to Bill Gould at San Francisco based Nexant, the project was put on hold after Spanish authorities reduced their solar subsidy by a third. At 15MW, the system was too small to be commercially viable at the lower subsidy rate.

 

Undeterred, Spain with help from Nexant is considering several other solar thermal projects. Nexant is also studying a site in South Africa for a 100MW Solar Tower system, where local manufacture of the heliostats could be an important part of the equation.

 

Like PV cells, however, the costs of the systems must come down before their use becomes widespread. But at $0.15/kWh today, they have a head start over PV systems.