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June 27, 2007

Hybrid Power Solutions

One challenge for solar and wind sources is that they are intermittent — power is only generated when the wind blows or the sun shines. Utilities prefer what they call “firm” power — essentially power that is comes with a future delivery commitment. Because wind and solar are intermittent and the available power may not be known in advance, utilities are sometimes reluctant to assign capacity values to these sources. The result is utilities often don’t consider solar or wind output as firm[1].

This is not a major technical problem today, since these intermittent sources constitute only a fraction of the total electricity provided to the grid. Indeed many experts believe that these sources could exceed 20% of all power on a grid and still be manageable[2]. It is unfortunately still a perception problem however, and sometimes and institutional problem.

There are at least three solutions that help mitigate the fact that renewable sources are often intermittent. I’ll classify all of these as hybrid solutions, although what is being combined is different in each case.

Matching source to use

One approach is to co-locate intermittent power sources with applications that don’t need a firm power source. Such applications include water treatment, water pumping, desalinization facilities, some energy-intensive industrial facilities, and so forth. Consider water pumping; indeed the dutch have using wind power to drain their polders since the 16th century. The point here is that not every energy consuming activity has to be done right now. One reason that a number of these hybrid solutions are associated with water is that the demand for that water is also intermittent, so storing water for later use is common, and nicely complements that fact that your energy supply is also intermittent.

Source/demand synergies

To some extent, this is similar to the above point on matching source to use. Given that peak demand for electricity in warm sunny regions (for air conditioning) coincides almost exactly with peak output from photovoltaic and concentrating solar power sources, these become perfect complements to provide incremental supply when it’s most needed.

Hybrid energy sourcing

Another approach to this issue is to match an intermittent source to a controllable alternative source. This is commonly proposed as an approach for concentrating solar power (CSP). By pairing up CSP with natural gas (which is a relatively simple technical extension as the gas could heat the same working fluid as the solar energy), you can convert CSP into a firm source that essentially behaves like an ultra efficient gas plant. Not, strictly speaking, a renewable energy source, but better than pure gas-fired electricity.

What hasn’t been explored to any extent is hybrid solutions using paired renewable sources, such as wind and geothermal. This likely requires geothermal power technology to move ahead further (it’s still relatively expensive for most locations), but such combinations would provide the best of all worlds, and serve as a long term power source with virtually no negative environmental impacts.

March 19, 2007

The solar variety show

Category: Concentrating Solar,Photovoltaics – Dan 7:38 am

Like many people, what I knew about solar power was limited, and in particular was limited to knowing about flat panel solar collectors. Oh, and one article that I’d read years ago about a “power tower” being built somewhere in the southwestern US with a bunch of heliostats (sun tracking mirrors).

Turns out that solar is an area with not only vast potential, but extensive innovation on a number of different approaches to turning solar energy into useful power. Here’s my list:

  • Flat panel technologies – All flat panel technologies have some key characteristics. They do not require direct sunlight, and generate energy regardless of where the light source is located. As a result, they don’t need to track the sun (although efficiency will be higher if they do), and they generate some power even on cloudy days. There are several types of flat panel collectors:
    • Flat panel crystalline silicon solar cells – These are what most people think of when they think solar. Cystalline silicon is grown into rods, which are sliced into wafers that form the cells. Arrays of the cells are built into flat panels.
    • Thin film technology – The high cost (both dollars and energy) of crystalline solar cells prompted development of a new approach where thin films of semiconductor materials are deposited. Although these generally have lower efficiencies than crystalline cells, the costs are also significantly lower, both in terms of manufacturing the active material, and in terms of the required support structures (thin film cells are much lighter). Thin film cells are also flexible, and can therefore be bonded directly to roofing materials; an example of this is the BIPV Solar Electric Roofing produced by SolarIntegrated.
    • Non-traditional photovoltaics – There is a new generation of semiconductor solar cell devices that work differently than the above two options. Companies doing this development include Konarka Technologies, Nanosolar and Nanosys.
  • Concentrating Photovoltaics – If the most expensive component of a traditional solar cell is the collector itself, then why not use mirrors or lenses to concentrate sunlight on the collectors. The advantage of this approach is lower costs. This does, however, make it more important that the collectors track the sun, and this technology clearly works better in southwest locations that have relatively cloudless skies. Companies working on concentrating photovoltaics include SolFocus, Silicon Valley Solar, and Sharp Electronics.
  • Solar Thermal Technologies – If what you need is heat and hot water, why go to the trouble of converting sunlight to electricity? This is the theory behind various solar thermal technologies. Solar thermal has been around for generations, and unfortunately this seems to be the unglamorous stepsister of solar technology. Solutions range from basic arrays of black tubing on the roof to help heat swimming pools, to modern sophisticated devices using glass evacuated tubes coated with advanced materials and integrated with ethanol-filled heat pipes. This is worth an entire article, so I won’t write more here.
  • Concentrating solar power – If you think big, it’s possible to create facilities that gather sunlight over a large area to create high heat that then is used to generate electricity. The three primary types of facilities here are:
    • Parabolic Trough – Long parabolic trough-shaped mirrors focus sunlight on tubes that heat a fluid to well over 500 degrees F; this heat creates steam to power a turbine generator and produce electricity.
    • Power Tower – A circular array of sun tracking mirrors focus sunlight on a central receiver on tower, heating a fluid which powers a generator.
    • Dish Engine systems – A mirrored dish focuses sunlight on a stirling heat engine at the focal point that directly generates electricity.

March 5, 2007

Concentrating Solar Power – Becoming cost effective

Category: Concentrating Solar – Dan 11:08 am

Based on some comments I received, I thought it would be useful to provide some additional information about Concentrating Solar Power (CSP). You don’t hear much about this technology, which is too bad, because CSP is a practical technology that getting close to being cost effective!

There are three basic approaches to CSP in use today:

  • Parabolic Trough – These use long parabolic trough-shaped mirrors to focus sunlight on tubes that contain a heat transfer fluid. The fluid, heated to well over 500 degrees F, is used to create superheated steam to power a turbine generator and produce electricity.
  • Power Tower – These systems use a circular field array sun tracking mirrors) to focus sunlight onto a central receiver on top of a tower, heating a working fluid which then powers a generator.
  • Dish Engine systems – These systems involve a large mirrored parabolic dish with a stirling engine at the focal point that directly generates electricity.

For details on how these systems actually work, check out the CSP Overview from Sandia Labs.

CSP is proven. Over 350 MW has been operating in California’s Mojave Desert for two decades.[1].

What’s most encouraging however, is that even without subsidies or carbon trading credits, CSP’s costs are becoming competitive with traditional fossil fuel facilities:

  • Installed cost – The installed cost of a modern pulverized coal combustion plant ranges from $1,100 to $1,490 / kW (that is an estimate without any form of carbon capture). With carbon capture, these costs roughly double to $1,940 to $2,580 / kW[2]. The installed cost of concentrating solar power is currently about $2,000 to $3,000 per kW[3] – just little more than coal if carbon-capture is included.
  • Operating cost – The operating cost of the coal fired facility ranges from $37 to $52 / mWh without carbon capture, and this rises to $64 to $87 with carbon capture[2]. Since there is no fuel cost, the operating cost of a CSP plant is low (although CSP plants are estimated to require more labor to operate and maintain than traditional fossil-fuel facilities[4]).
  • The estimated cost of CSP on a per kWh basis is now between 9 and 12 cents/kWh[3].

The bottom line — this is a technology who’s time has come. When you hear people dismiss solar based on the current high-cost of photovoltaic solar cells, please be sure they are aware that there’s another approach to solar energy that is approching cost effectiveness today, especially when you look at operating costs over the long term.

February 28, 2007

Hot Technology Series – Stirling Energy Systems

Concentrating solar makes sense. If you concentrate the sun using mirrors (a well understood and cheap technology), you have more light and heat to work with, and the actual collectors can be smaller.

So check out Stirling Energy Systems (SES)!

Sterling Energy Systems dishSterling Energy System’s product is a 37′ sun tracking mirrored dish with a sterling engine mounted at the center. The sun is focused on the engine, which powers a generator creating electricity. What’s hot about this technology?

  • Standardized manufacturing – As SES ramps up production, they will be able to crank these out by the hundreds. Efficient to produce, and easy to install, these will lower the cost of solar electricity.
  • Proven technology – Mirrors clearly are proven, and sun tracking systems have been around for decades. The sterling engine is also proven and has been in industrial use for many years.
  • Low maintenance – The sterling engine is a closed cycle system that uses a working fluid. There’s no intake, no exhaust, no explosions, or any of the things that cause engines to need intensive maintenance. Maintenance only involves lubricating the tracking mechanism, and washing the mirrors occasionally.
  • Low impact – Besides creating shade, these collectors have no impact. They use no water (except for washing), there no drilling, there are no emissions.

SES has signed two major contracts for installations, one for a 300 MW system with San Diego Gas and Electric, and a second for a 500 MW system with Southern California Edison. In March ’06 they were rated #1 in Fast Company magazine’s “Fast 50”.