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Solar You Can Count On 
Hybrid Solar/Natural Gas Plants Provide Power When Needed
 

Source: Skyfuel                                                       Source: BrightSource Energy

August 18, 2009
by Craig Severance

By far the largest source of safe, clean energy that will never run out  (i.e. renewable energy) available in the United States is the sunlight falling on the unused deserts of the Southwest.  This attractive source of energy produces no nuclear waste, no carbon dioxide or mercury emissions, and none is imported from foreign countries.

According to the U.S. Department of Energy enough sunlght falls in just the unused, nonsensitive areas of our SW deserts to generate over twice the total kWh's now consumed in the entire U.S..

SW Solar Now.   In June, Interior Secretary Ken Salazar opened up 24 of  the SW's sunniest areas on Bureau of Land Management lands in six  states to begin leasing for installation of up to 100,000 MW of solar power plants. (See here for article on the Interior Department announcement).  The first plants could be operating within 3 to 4 years in these ideal locations, which were chosen for maximum clear sunny days and minimal impact on the environment or other land uses.

Sun Doesn't Shine All the Time.  Although the SW sunshine resource is enormous and largely untapped, critics of solar energy routinely note the sun does not shine all the time.  The implication is that power is needed all the time, and since the sun is not always available, solar opponents say it would be foolish to invest in generating electricity from the sun. 

Grid Can Use Solar.  Utilizing solar electricity when the sun does shine is not really a major problem for the electric grid, until the percentage of power generated by solar reaches high percentages.  This is because roughly 50% of the electrical capacity on the grid consists of load-following power plants (chiefly natural gas and hydroelectric), which can quickly reduce power output when a renewable resource such as solar or wind is available, and increase output when needed.  The ability of the grid to absorb a high percentage of  power from renewables has been documented by the U.S. Department of Energy and was discussed in my article "The Wind does NOT Blow Only 1/3 of the Time" here.

The output from a solar power plant also fits very well with the times when  power is most needed.  Most utilities see increased demand for electricity during daylight hours, with peak demands occurring on hot sunny days when a solar power plant produces well.  By the same token, less power is needed at night.

It is generally agreed, however, that extending the percentage of  our electricity generated by renewable power sources above 20-30% will require means to better regulate the grid (see "Smart Grid" article here),  more efficiently supplement renewable power, or store it for later use. 

Solar Thermal Offers More Choices.  Solar photovoltaics (PV) require storage of their electrical energy output to extend their use into evening and cloudy hours.  Methods the electric grid can use to store electrical energy include batteries, flywheels, pumped hydro or compressed air energy storage.  

The "other" kind of solar power -- Solar Thermal power -- offers more choices to integrate with the grid to provide reliable power. 

Instead of directly converting the sun's rays into electricity, Solar Thermal plants use mirrored surfaces to concentrate sunlight to produce high temperatures.  This is why they are also called Concentrating Solar Power (CSP) Plants.

The high temperatures are used to boil water to produce superheated steam to generate electricity.   This different technology means there are now three different ways that Solar Thermal power plants can provide power when the sun is not shining:

1.   Integrate a back-up source of heat (e.g. natural gas) to produce steam.
2.   Produce excess solar heat during the day, and store that heat.
3.   Grid storage of electrical energy (as with PV or wind).

This expansion of choices means that a Solar Thermal plant can function as a reliable source of "24/7" power to the electrical grid.

Steam Generators Most Common Source of Electricity.   The key to generating electricity for a century has been to produce high temperatures to heat water, superheat the steam (so it will not condense into water droplets inside the steam turbine and damage the blades), and then run this superheated steam past blades in a steam turbine to spin those blades to run a generator.  After the steam passes through the turbine it is then cooled, and the water is re-used. 

This same basic process is used in coal, oil, and most natural gas power plants.  Even today's nuclear power plants are just "a fancy way to heat water".

Concentrating Sunshine to Produce Steam. 
Different Solar Thermal companies use different means to concentrate sunlight.  They each cite their own advantages:  

Troughs.  Solar trough companies such as Skyfuel  use long  "trough" collectors (see picture at top, and immediately below) which rotate east-west during the day, to focus sunlight on tubes carrying hot oils.  The hot oils then pass through a heat exchanger, to heat water into superheated steam.


Source: Skyfuel   (Note trough rotates east to west as day progresses).

Trough supporters point to the long track record of the technology, including some 25 years of continuous production at the SEGS plants in Southern California, which has established clear performance and cost histories.  Skyfuel's key innovation is to develop a highly reflective coating film known as ReflecTech(TM) which eliminates the need for expensive curved glass mirrors for the troughs.

Flat Mirrors Focusing on Compact Linear Fresnel Reflector.  Another "line" approach is typified by Ausra.  In this approach, flat mirrors are ground-mounted and turn to concentrate reflected sunlight upward to a Compact Llinear Fresnel Reflector, which concentrates the sunlight onto a pipe carrying water which is turned into superheated steam:

Source: Ausra

Ausra notes its technology saves costs by requring no curved mirrors, and does not use oil-to-water heat exhangers, as it uses water directly.  Ausra's mirrors are also tightly packed together, harvesting more sunlight per acre of ground.  Ausra CEO Bob Fishman has said, "We can get twice as much steam per acre as power tower and twice as much as trough."

Solar Power Towers.  A radically different approach is the solar power tower, typified by Brightsource Energy and start-up eSolar.   With the solar power tower, the solar field consists of tens of thousands of flat mirrors, each mounted with a 2-axis tracking motor to tilt the mirror in three dimensions to focus intense amounts of sunlight on a boiler mounted on the top of a tower.  Superheated steam is produced in the boiler, and is fed to a ground-mounted steam turbine to generate power.  See Brightsource Energy picture at top, and concept drawing below:


Brightsource Energy's Concept Drawing of Ivanpah, CA project

Proponents of the Solar Power Tower approach argue it has miles less piping and pumping, and the largest towers can operate at higher steam temperatures for better operating efficiency.  They also claim higher kWh output on an annual basis because their mirrors can tilt upward to catch the lower sun in the wintertime.

Steam Plants in the Desert?   While some locations may offer special opportunities to use water cooling, most solar thermal plants will be built with dry cooling. Keely Wachs of Brightsource Energy notes, "For our 410 MW Ivanpah site, the use of dry-cooling technology will reduce the projects' overall water usage by 90%, from 1000 acre/ft to a little less than 100 acre/ft annually. 100 acre/ft is roughly the equivalent of 300 homes' annual water usage.  So we are producing enough energy to power 140,000 homes, while using 300 homes worth of water."

Hybrid Solar/Gas: One Power Plant With One Steam Turbine.  Because Solar Thermal power plants produce superheated steam to generate electricity in a steam turbine, they can be designed to share the same steam turbine generator as a conventional natural gas power plant.    See, for instance, the schematic from Solar Thermal firm Ausra, below:


Source: Ausra

Instead of relying upon a separate power plant miles down the road to guarantee grid reliability to generate electricity when the solar plant cools off,  just one plant can be built, with two sources of heat -- sunlight and natural gas.  

This saves on construction costs because only one steam turbine is needed instead of two.  Also, much of the ancillary equipment such as controls, pumps, valves, etc. are not duplicated.  Perhaps most importantly, duplicate sets of transmission lines are avoided.

Operating costs can be saved with just one team of workers, running one power plant, instead of needing two sets of skilled staff.  

Finally, fuel costs for the natural gas component of operations may be saved by smoothly combining the two heat sources, gradually increasing natural gas use as the solar resource cools.  This is expected to be more operationally efficient than ramping up and down a separate natural gas power plant. 

This "hybridization" of solar thermal and natural gas power plants is an economical "bridge technology" approach to immediately reach fully dispatchable solar plants, providing "firm power" available to meet utility needs --  whether or not the sun is shining. 

Hybrid Solar/Natural Gas "Load Following" Plants.   In the ideal super-sunny locations in the desert Southwest where Solar Thermal plants are being erected, it is expected they will generate from solar, roughly 25-30% of the total kWh's they could generate if they were able to operate 24 hours a day, 365 days of the year.  This percentage is referred to as a "Capacity Factor".   (That's actually very good, when you consider how many hours per day the sun shines.)

Most natural gas power plants in the U.S. actually are under-utilized, operated at an average of only 42% Capacity Factor.  This is because they typically serve a "load-following" function, turned on only when needed, during the higher-demand parts of the day and year.  When demand for power drops to minimum levels, they are turned off because "Base Load" power plants designed to run all the time, are already running all the time to provide this minimum ("base load") demand.  Most "Base Load" power plants are coal or nuclear plants. 

If a "hybrid" solar/natural gas plant were also operated as a "Load Following Plant", it  might also be needed only 42% of year-round time.  However, if 30% of year-round time it's energy came from sunshine, the percentage of energy supplied by sunshine could be very high -- 30% over 42% -- or about 70% or more of the energy supplied.  

This is good news for those seeking to cut fossil fuel emissions from power plants.  Solar power could cut fossil fuel use (and hence CO2 emissions) by load-following power plants by roughly 2/3 compared to current patterns of operation for these plants.

Economics of Hybrid Solar/Natural Gas "Load Following" Plants.   The relatively low annual use of a "Load Following" plant has traditionally favored power plants with low initial construction costs.  Low construction costs are important when you don't use it very much. 

A  Combined Cycle Gas Turbine power plant today costs roughly $1,100/kW - $1,500/kW to build, one of the cheapest power plant options.  However, unlike sunshine, natural gas isn't free, so total generation costs (at $7/MMBtu gas) are likely to be around 11 cents/kWh for a new natural gas "Load Following" plant in the first year of operation.  (WIth no specific "carbon penalty" for fossil fuel.)

Costs for Solar Thermal plants are becoming known as several have already been completed.  The Nevada One plant completed in 2007 was built for roughly $3,600/kW of capacity, using older trough technology with curved glass mirrors.  With technology advancements, new proposals are now being estimated at lower costs.  For instance, planned 20 MW plants in Algeria and Morrocco were recently estimated as costing only $2,500/kW to build. 

Since a Hybrid Solar/Natural Gas plant will not cost as much to build as two separate plants, these cost ranges imply total generation costs of a Hybrid Solar Thermal/Natural Gas "Load Following" plant may run approximately 13 cents/kWh (after today's 30% Federal Tax Credit for solar, and assuming $7/MMBtu natural gas), in the first year of operation.  Since roughly 2/3 of the Hybrid "Load Following" plant's "fuel" is sunshine, the Solar Hybrid plant has a powerful hedge against future increases in fuel costs, including increases driven by "carbon penalties" on CO2 emissions.

What happens when the 30% Solar Tax Credit expires in 2017?  Solar Thermal companies argue that during this time mass production of the mirrors and other components of CSP plants will bring down costs.  At the same time, fossil fuel prices and carbon penalties may increase.  

Possible Costs for Hybrid Solar/Natural Gas "Baseload" Plants.   Operating the same plant as a "Baseload" plant  can lower overall generation costs/kWh because the same capital cost is spread over more kWh output per year.  

A new natural gas power plant operated as a "Baseload" plant, for instance, may cost roughly 9 cents/kWh total generation costs, lower than when the same power plant is used only about half as much in "Load Following" mode.  

Operating a Hybrid Solar/Natural Gas plant as a "Baseload" plant will spread its total capital costs over more kWh's per year, however the extra generation would come entirely from burning more natural gas.  WIth the same assumptions as above but with more usage, a Hybrid Solar/Natural Gas might have total generation costs/kWh of roughly 10 cents/kWh (with no specific Carbon Penalty).

Note the two choices (each seen as One Power Plant) are near parity in total generation costs, but the Solar Hybrid plant would have less exposure to long-term increases in fossil fuel prices and carbon penalties. 

The Solar Hybrid plant can also eventually further reduce its fossil fuel use by adding storage.

Adding Solar Storage.  As natural gas prices rise,  Hybrid Solar/Natural Gas power plants can raise the percentage of energy from sunshine by storing excess solar energy generated during daylight hours.

Storing heat instead of electricity can be very physically efficient.  For instance, Skyfuel notes that heat can be stored, then used later, with a 90% efficiency of heat recovered.  

Adding storage isn't cheap, however. 

First, it generally will require increasing the size of the solar field so that more heat is generated during the day than would be used to generate steam during the day.  Next, this extra heat would be stored in a heat storage fluid, such as molten salts.  This requires heat exchangers and heat storage tanks for the molten salts.

The general consensus now seems that most Solar Thermal plants will be built with little or no molten salt storage. However, Solar Thermal plants automatically can run about 20-30 minutes on existing steam if clouds come over.

Hybridization with natural gas will make sense, and storage can be added incrementally as years go by, and it becomes important to reduce natural gas usage. 

Skyfuel's William Felsher notes, "The optionality is there.  You can add storage and more collectors to increase Capacity Factor later."  With Solar Thermal's modular technology, "enhancements can be made incrementally."

Adding Solar to Existing Power Plants.   With growth in demand flat or even negative, many utilities may currently have no need to build totally new power plants.  However, adding solar to an existing power plant can help the utility meet Renewable Portfolio Standards and gain valuable operating experience with Solar Thermal.

One economical way to achieve a hybrid solar/natural gas power plant is to add a solar thermal collection field to an existing natural gas combined cycle power plant.   The solar field of mirrors, lines, or troughs (or a small Solar Power Tower) would feed superheated steam into the existing steam cycles used by the power plant to generate power, as a supplemental heat source.  Typically no new land is needed and transmission connections are already in place.

Savings of fuel consumption typically in the range of 10-15% may be achieved with a small solar addition.   Skyfuel's FuelSaver(TM) program  encourages utilities to add typically 5 - 50 MW of solar power to existing power plants, to reduce fossil fuel use and gain valuable experience with Solar Thermal power.  Ausra is also encouraging solar retrofits, after reducing coal usage at an existing power plant in Australia.

A Solid Choice for Utilities.   Utility managers seeking to add carbon-reduced firm electricity generation can now look to Solar Thermal as a viable choice.  Decades of experience have proven the technology, and recent advances are reducing its cost. 

American and overseas companies have operating power plants, and are competing for utility RFP's on utility terms that protect utilities from massive cost overruns.  Announced projects for Solar Thermal plants in the U.S. already total over 6,000 MW.

The desert is blooming with power. 



This article was originally published on August 18, 2009.
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