>by Murray Duffin
[Like previous articles in this series, the following article is somewhat technical. Nevertheless I decided to post it because solar energy is important in the overall discussion of the Energy Challenge. Bill Totten]
The Energy Challenge 2004 – Solar
by Murray Duffin
http://www.energypulse.net (November 04 2004)
Solar energy is our most abundant renewable resource. An analysis of insolation in the USA southwest shows that using only the 1% of the land area considered, that has a slope of less than 1% and more than 7 kilowatt hours per square meter per day insolation, concentrated solar (CSP) can provide power of approximately thirty gigawatts electric. With effective storage (as for solar towers) this potential could provide at least three times the total productive energy of today’s economy.
Solar energy generation can be considered in three categories, solar thermal, solar photovoltaic (PV), and solar thermophotovoltaic (TPV). As TPV is still a far future technology, only the first two will be considered here. 2004 may have been the watershed year for the development of solar renewable energy, although that may not become obvious for several more years. While there has been little progress in installations in some years, technology has continued to improve, and with rising costs of coal oil and natural gas, interest in solar energy is now growing rapidly.
The new economic driver
North American production of natural gas is reported to have declined by more than 3% in 2003 versus 2002, and based on reports by the major producers in America, production in 2004 seems to have declined at least 3% in the first half, and as much as 10 to 12% in the third quarter year-on-year. Hurricane damage can account for less than 40% of third quarter decline, so it seems that decline of mature fields is accelerating. Against earlier forecasts of natural gas prices below $5 per million btu for the second half of 2004, recent cash prices have been above $7, at a time when demand is low, and storage is at record levels.
At the same time export demand for coal has caused prices to more than double, on average, for all but Powder River Basin coal; and we have seen oil prices rise 70% in a few months and are going into winter with heating oil stocks low and prices high. With possible brief respites, these trends appear irreversible.
Solar Thermal Overview
There are 2 major sets of solar thermal, (1) direct heating and cooling and (2) electricity generation.
Each can be split into flat plate and concentrating (CSP) subsets although for electricity generation CSP is the economic choice for all but small-scale applications.
– Direct heating and cooling
The mature technology is water heating for home hot water, space heating and swimming pool heating. Flat plate technology is common, inexpensive and effective and has been used successfully with gradual growth for at least fifty years. Efficiency can be in the 25 to 50% range, depending on design. More recent technology that is growing rapidly, and is the preferred choice for larger installations like hotels and public swimming pools is evacuated tube heating systems which provide much more heat per unit area, remain effective even during light overcasts, and can reach 60% conversion efficiency.
Recently compound parabolic concentrators (CPC) have begun to be industrialized, proving very effective in evacuated tube systems. The advantage of CPC is that, by concentrating sunlight it can raise liquid temperature in pressurized systems to over 300 degrees fahrenheit, enabling economic absorption chillers for cooling systems. CPC is also effective over a very wide angle of illumination, eliminating the need for a tracker in a concentrator system. The California Energy Commission retrofitted and optimized a twenty-ton conventional double effect (2E) LiBr/water absorption chiller to be solar hot water driven, and have estimated that such a system can be supplied commercially for under $4500 per ton, with a net reduction in electricity demand of 1.3 kilowatts per ton. Depending on hours per year of operation and peak electricity costs, an economic payback of 4 to 8 years can be expected.
Direct heating and cooling systems have the effect of displacing electricity, to use Amory Lovin’s term, providing “negawatts” instead of megawatts, and generally at lower cost than increased generating capacity.
– Electricity generation (CSP)
Thermal power generation is being addressed in several ways – and for different sizes of installation:
Solar dish concentrators driving Sterling engine generators.
Trough concentrators heating a liquid to gas system driving a turbine generator.
Solar towers using large reflector (heliostat) arrays to heat molten salts which, through a heat exchanger, drive steam turbines.
Solar chimneys using rising air from a large ground level greenhouse to drive turbines at the base of a kilometer-high chimney.
Dish/Sterling systems tend to be aimed at tens of kilowatt applications for grid connected distributed power, and reach conversion efficiencies near 30%. Cost of electricity is still high, though there is a wide range of estimates. Widespread use seems likely to be well in the future.
Trough concentrators get into the hundreds of kilowatts to tens of megawatts range, good for locally sighted factory power also at attractive efficiencies. The best-known examples are the SEGS series (now up to SEGS 9) in California. Recent projects have been commissioned in Nevada and Arizona. It has been estimated that a 100 mile square in the Nevada desert could provide about 500 gigawatts electric, roughly equal to the USA installed electric power base. Up to now such applications have limited storage ability, so they are unsuitable to 24 hour operation and dispatched power. CPC concentrators might overcome that drawback. These systems approach 14% efficiency today and are projected to get to 17% by about 2015.
Solar towers (Power Towers) are megawatt-sized for utility type supply and have the advantage of retaining heat for 24-hour operation. Solar 1 was operated near Barstow California in the 1980s as proof of concept. Solar 2, a 10 megawatt electric upgrade of Solar 1 operated from 1992 to 1999, demonstrating the feasibility of storing heat for dispatchable power and 24 hour operation. Solar Tres (17 megawatt electric) has been planned for Spain, originally to go into operation by late 2003, but now delayed to 2006, seemingly by bureaucracy. Towers in the 100 megawatt range are projected. Solar towers are about 23% efficient in conversion of incident energy to electricity, and can realize up to 70% capacity factor. Current experience indicates a space demand of ten acres per megawatt, with promise of at least a 20% reduction.
Solar chimneys are only theoretical so far, and seem to have captured most attention in Australia. They can be designed to heat water during the day to provide energy at night. Efficiency is estimated as 3%, but it seems likely that this can be at least doubled. Proposed designs have fresh air drawn into the heating area at ground level. Drawing in air near the tower top would augment generation with the sinking column of cooler air. In dry climates it should be possible to inject water vapor into intake air to further cool the descending air column. Current projected design is for 200 megawatts and requires about 23 acres per megawatt. Capital cost of $2 per peak watt is projected, but seems quite optimistic.
For utility scale electricity generation, the best choices today are trough concentrator and solar tower systems. An excellent 2003 analysis for trough concentrators (based on 2002 data and projections) considers a necessary competitive target price for electricity of $4.50 per million btu, assuming a floor fixed at that level by liquefied natural gas (LNG). We now can be sure that LNG will not be a major factor for at least a decade, and even then will set a floor above $6 per million btu. This analysis showed trough systems becoming competitive at 10 gigawatts electric installed capacity and 6 cents per kilowatt hour. It now seems more likely that 7-8 cents per kilowatt hour will be good which can be reached at 5-6 gigawatts electric installed. Another late 2003 report , using well reviewed data and analysis developed independently be Sunlab and Sargent & Lundy gives present electricity costs of 10 to 12.6 cents per kilowatt hour now, going down to 3.5 to 5.5 cents per kilowatt hour before 2020 for trough and tower systems. Growing fossil fuel shortages seem certain to accelerate progress relative to these studies.
Historically PV has been seen as much too expensive for widespread use, having been represented as “the energy of the future and always will be”. 2003 saw a novel development that should change that conclusion. All of the pieces now seem to be in place for PV to breakthrough all the barriers of demand, cost and capacity that have been holding it back, but it seems that no one in the North American PV or electric utility industries has seen all the pieces yet, let alone put them together to make a picture.
Recent natural gas demand growth is largely for electricity generation. From 1993 through 2003 nearly 300 gigawatts of electrical generating capacity was installed in the USA, about 90% of which is fired by natural gas, both to meet Clean Air Act requirements and to add flexible capability to meet peak loads. Base load demand is estimated to grow at least 1.5% per year (6 gigawatts per year), but seems to have shot up by at least 5% in 2004 versus 2003. Therefore supply fired by natural gas, intended for peaking, is being converted to base load supply, leaving a growing shortage of peaking capacity. Now declining supply of natural gas means that peaking demand growth can no longer be met by adding new capacity fired by natural gas. However peak demand coincides with peak insolation, making PV an attractive alternative.
So, we have demand, at least if the cost is not too high. Can needed costs be met, and can there be adequate supply? In Renewable Energy World, December 2002, Auliche and Schulze estimated worldwide polysilicon feedstock capacity for electronic grade silicon at 26,000 metric tons per year, with production estimated at 14,000 metric tons per year. With such a large excess capacity, polysilicon suppliers have been happy to sell electronic grade silicon for PV production at very attractive prices ($20 to 25 per kilogram), enabling PV producers to lower their prices. As Maycock noted in Solar Today, January/February 2004, PV producers have sold cells and modules at cost, enabling very rapid industry growth in 2001-2003. System quotes as low as $4 per peak watt installed have been mentioned. Total world silicon PV production in 2003 was about 0.7 peak gigawatts, having grown 32% worldwide while actually shrinking in the USA.
Auliche and Schulze estimated that about 2000 metric tons each of “off spec” and “non-prime” electronic grade silicon were supplied to the PV industry in 2000. At 17 metric tons per peak megawatt that was enough to produce 235 peak megawatts in 2000. Maycock shows 2000 production at 288 peak megawatts, which implies another 1000 metric tons from capacity dedicated specifically for PV. With perhaps 8000 metric tons excess capacity in 2000, suppliers have had no incentive to add capacity. However, production of more than 700 peak megawatts in 2003 has surely consumed the excess capacity, even if price may not yet have been attractive for the polysilicon producers. In parallel, while technology is reducing the share of off spec and non-prime silicon being produced, microelectronics demand for silicon is growing rapidly. As a result, in the last twelve months the price of polysilicon has gone from $20 to 25 per kilogram to over $30 per kilogram and is projected to go to $40 to 60 per kilogram. These price increases push bottom prices for PV installations back to the range of $6-7 per peak watt.
While there may still have been some stockpiles from prior years to work off in 2004, it is probably safe to say that PV growth will now be limited by polysilicon capacity and price. To aggravate the situation, during the 2000-2003 period, polysilicon producers experienced very low ROI, making it difficult now to attract the large increments of capital needed for conventional “Siemens process” polysilicon production capacity. Unless there are dramatic technical advances, this condition is likely to persist for several years. John Schumacher has pointed out (Solar Today, January/February 2004), that breakthroughs are needed in both polysilicon capacity capital and production costs, and in ways to get more collector surface per ton of polysilicon. Fortunately, it seems that the technology now exists to meet both needs, and the only delay factor is time to recognition and industrialization.
Schumacher has already operated a “proof of concept” facility for a new polysilicon process that has a capital cost about 40% of that for the Siemens process and projected product price of under $15 per kilogram. Existing, possibly surplus, CZ pullers can be adapted to use the output of this new process with a probable increase in throughput at lower energy input, further lowering the cost of PV wafers.
In December 2003, Origin Energy of Australia , in conjunction with the Australian National University (ANU) announced a new “sliver cell” approach to making PV cells from silicon wafers that is a classic example of “lateral thinking”. Origin claims a twelve-fold increase in collector surface per ton of silicon, and a thirty-fold potential increase in peak watts per wafer. My calculations do not confirm these claims, but taking all yield factors into account, they can probably get to more than six-fold increase in collector surface per ton, which is still a sufficient breakthrough.
In 2004 ANU delivered a paper on sliver cells in concentrator applications, showing a 21% conversion efficiency at 20 suns. The Fraunhofer Institute has also worked with very thin silicon for PV and show 24% efficiency at 60 suns. Even at 20 suns and six-fold yield per ton, polysilicon scarcity ceases to be a restraint.
In writing a National Energy Policy “primer” for the House and Senate Energy Committees in 2001 (which regrettably, but not surprisingly, they totally ignored), I estimated that we would need the output of fifty large factories for twenty years to install enough collector surface at 20 suns to produce 10 quads of PV solar energy per year. The sliver cell will enable 5 quads in twenty years with only four factories. What seemed quite impractical in 2001, now appears quite feasible.
Sliver Cell Whole System Pluses
ANU notes that the cells can readily be connected in series, reducing the need for protective diodes and eliminating the transformer from the inverter. In addition to lowering system cost, these changes would also improve conversion efficiency to alternating current significantly, thus reducing the needed collector area for a given peak watt. Taking all of these factors into account (Schumacher’s polysilicon + six-fold surface increase per metric ton + elimination of diodes and transformer + light weight deriving from thin slivers + system efficiency) it seems likely that PV could get to an installed cost of $1.50 per peak watt before 2010. (ANU has estimated $1.80 peak watt, but it’s not clear that they took all factors into account.)
In a concentrator system, when used for peaking power in conjunction with a Combined Cycle Gas Turbine (CCGT), the concentrator could also preheat water for the steam turbine stage, potentially increasing CCGT output by at least 3%, at no additional cost. If a 500 megawatt CCGT installation needed 100 megawatts for peaking, the extra 15 megawatts of thermal energy would lower the total investment per effective peak watt to about $1.30. With regulated utility type financing (cost of money 3% above inflation) the resulting peaking electricity could be provided at a cost near 13 cents per kilowatt hour. Historic PV electricity cost estimates have typically been quoted (see the Wall Street Journal Special Report September 2001) as 22 to 40 cents per kilowatt hour.
The average retail price of electricity in the USA in 2002 was 7 cents per kilowatt hour, and is surely higher now. Peak electricity price can be at least 3 times higher, making conventional PV historically uncompetitive. (In some California districts, base rates are 12 cents per kilowatt hour and conventional PV is marginally competitive for peak power now). At a base cost of 13 cents per kilowatt hour, even after markup for maintenance and overhead, PV would be attractive for peaking supply across the southern tier. This base cost leaves room for attractive profit margins for everyone. I would expect suppliers of power fired by natural gas to start pushing very hard to have these technologies industrialized as rapidly as possible.
Solar thermal energy for hot water has long been attractive, and recent developments now make it attractive for air conditioning as well. Widespread use could reduce electricity demand in the USA by at least 10%, and this degree of reduction will probably become necessary as supply of natural gas declines.
Concentrated solar (CSP) for electricity production begins to look attractive with rising cost of fossil fuels and very long permitting and construction times for nuclear. The technology is now well understood and poised for rapid development with corresponding cost reductions. We now need an intelligent National Energy Policy, with relatively modest subsidies to kick-start the needed development. We can be very confident of successful exploitation.
A major breakthrough in PV technology has now raised the potential of PV to the level of practicality. Production capacity is still a limiting factor. Lack of awareness is also a barrier. Again, an intelligent National Energy Policy is the key to further progress.
Reliance on imported fossil fuel energy, with its attendant cost, security risk and negative payments balance could realistically be overcome in less than twenty years, with a government driven “Apollo Program” for energy, focused on efficiency, conservation, renewables and nuclear. Renewable solar energy is now positioned to make its contribution.
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