There is rapidly increasing understanding of the need to reduce use of fossil fuels. People are becoming more aware of the possibility that petroleum supply is close to peaking, and of the implications of the greenhouse problem for use of fossil fuels. However it would be difficult to find a more taken for granted, unquestioned assumption than that it will be possible to substitute renewable energy sources for fossil fuels, while consumer-capitalist society continues on its merry pursuit of limitless affluence and growth. I think there is a very strong case that this assumption is seriously mistaken.

The argument is detailed in my book Renewable Energy – Cannot Sustain Consumer Society, to be published by Springer early in 2007 (referred to as RE below.) The core themes will be summarised here.

The limits to renewable energy have been almost totally ignored. There has only been one book published on the topic, Hayden’s The Solar Fraud, 2004. No one wants to think about the question. Everyone is eager to assume that we can move from fossil fuels to renewables without any threat to ever-rising affluent living standards and limitless economic growth. Unfortunately the renewable energy experts are the last people to throw critical light on this question of possible limits. They always give the most optimistic pronouncements on their pet technologies, reinforcing the impression that they could solve all the problems, if only we would give them more research funding.

Energy is only one of the alarming global problems now threatening us. Chapter 10 of RE explains that consumer society is grossly unsustainable and unjust, and that we should be trying to reduce our rich world per capita resource consumption to perhaps 10% of present levels. The problems over-consumption is generating cannot be solved without vast and radical change from consumer-capitalist society. Our goal has to be transition to what I refer as "The Simpler Way."

It is necessary to divide a discussion of renewable energy potential into two parts, one to do with electricity and the other to do with liquid fuels. Liquid fuels set the biggest problem.

Electricity.

Many sources could contribute some renewable electricity but the big three are wind, photovoltaic solar and solar thermal. Several other technologies are valuable but would not seem capable of contributing much to very large scale electricity production.

Wind.

An examination of wind maps indicates that the annual quantity of wind energy that is available in the US, Europe and SE Australia could well be considerably greater than demand. Off-shore potential is less clear because much depends on the water depth limit assumed. The maximum water depth for windmills at present is around 18 metres. Czisch (2001, 2004) estimates that if they could go to 55 metres then 500,000 square kilometres of the North and Baltic seas could be farmed. If all this was used it might provide twice Europe’s 2,100 TWh/y demand. Mills can be mounted on floating platforms but the cost and the movement set difficulties.

However on land usually only a fraction of the suitable area can take wind farms, for reasons such as prior uses. This is especially so in densely populated Europe where the fraction could be under 10%. In off-shore regions this is not such a significant problem, but other uses still reduce useable areas surprisingly. The fraction would be much higher in Australia and the US.

The variability problem.

The major limitation with all renewables is their intermittency or variability. Sometimes there is little or no wind. In the past it has been generally agreed that for this reason wind might be able to contribute up to 25% of demand. In fact there is reason to think that the figure will be much lower. The Germans, with far more wind mills than any other country, and the Danish with the world’s highest ratio of wind output to electricity consumption, have run into "integration" problems while wind is supplying only about 5% of demand. (See Sharman, 2005, E.On.Netz, 2004, 2005.) (Denmark’s output is equivalent to c.18% of demand but most of this is not used locally and is exported.)

Another significant problem is that because the wind sometimes does not blow at all in a system in which wind provided a large fraction of demand there might have to be almost as much back-up capacity from other sources as there is wind generating capacity. E.On Netz has emphasised this problem with respect to the

German experience. So if we built a lot of wind farms we might have to build almost as many coal, gas or nuclear power stations to turn to from time to time. This means that renewable sources tend to be alternative rather than additive; the winds tend to be strong in winter when there is little solar energy, so it is not a matter of having each renewable source carrying a small part of the load all the time. This means we might have to build two or even four separate systems (wind, PV, solar Thermal and coal/nuclear) each capable of meeting much of all of the demand, with the equivalent of one to three sitting idle all the time. This would obviously be very expensive.

In addition electricity distribution grids would have to be reinforced and extended, especially to cope with the new task of enabling large amounts of power to be sent from whatever region in which the winds were high at that time. Centralised coal or nuclear generators do not have this problem.

One aspect of the variability problem is the seasonal difference in wind strength. Czisch ( 2004, Fig. 5.) shows that in February Europe gets almost 5 times as much wind energy (not mean speed; energy is proportion to speed cubed) as in May, so if we built a system big enough to meet demand in February it would only do 20% of the job in May.

These problems of variability and integration could be overcome if electricity could be stored in large quantities, this can’t be done and satisfactory solutions are not foreseen. (See below on "pumped storage", using dams

There are schemes for connecting vast intercontinental regions into the one wind energy system, e.g., from Morocco to the Sahara and Kazakhstan. (Czisch and Ernst, 2003) This would considerably reduce the variation problem because when the winds were low in Western Europe they would probably be up in some of the other regions. This is the phenomenon of relatively low correlations between wind strengths when mills are spread over large distances; there is always likely to be wind somewhere.

The important point however is that even though correlations could be zero and there will always be some wind somewhere, there will still be many times when the average wind across the whole system is low, and that means the wind system as a whole will not be producing much. As Hayden (2004, p. 150) says, "There are times when the wind is calm everywhere."

Let’s assume that the wind is always good in Morocco, or Kazakhstan or Siberia or Western Europe. If we are to have a system that always reliably meets demand from one of these regions we will have to build four entire systems each big enough to meet demand…along with their highly costly 4000-5000 km transmission lines to Europe (losing perhaps 16% of energy generated.)

Note that most of these regions are well to the East of Europe so it will be night time there when European demand is highest, during the day. Winds tend to be low at night. How is that huge quantity of energy going to be stored?

Sharman (2005) reports that even in Denmark in 2003 the average output of the wind system was about 17% of its peak capacity and was down to around 5% for months at a time. The E.On Netz (2004) report for Germany, the country with more wind mills than any other, also says that in 2003 system capacity was 16%, and around 5% for months. They stress that 2003 was a good wind year.

Davey and Coppin (2003) carried out a valuable study of what the situation would be if an integrated wind system aggregated output from mills across 1,500 km of south east Australia. Coppin points out that this region has better wind resource than Europe in general. Linking mills in all parts of the region would reduce variability of electricity supply considerably, but it would remain large. Calms would affect the whole area for days at a time. My interpretation of their Figure 3 is that the aggregated system would be generating at under 26% of capacity about 30% of the time, and for 20% of the time it would be under 20% of capacity Clearly a very large wind system would have to be backed up by some other large and highly reliable supply system, and that system would be called on to do a lot of generating.

Even if 20% or 50% of electricity demand could come reliably from wind, where would the rest come? Note also that if wind enthusiasts are right and 25% of electricity demand can be met by wind without variability becoming a problem, this is only so only if there are other sources that can provide the other c. 80%, and sometimes provide 100% of demand. Remember the focal question RE is concerned with is could we run everything on renewables.

Why not build more windmills than we normally need, so we will have enough when the winds are low? It can make sense to move in this direction, but as Figure 3 from Davey and Coppin makes clear, no matter how many mills you have there is a probability that some of the time supply will fall well short of demand. Doubling the number of mills halves this chance, it does not eliminate it.

Wind farm costs are usually quoted as c. $(US)1000/kW, but this is misleading. Firstly the recent cost of some Australian systems has been up to $(A)2,400/kW(e). More importantly these figures are for peak output and average capacity at a good site might be only usually 35%+ of peak output. Again the German system in 2003 averaged less than half this figure. (hus we should focus on the performance of the system, not of individual millsz; the system involves other factors and losses.) If we take the recent Australian cost and a system with 25% capacity, the European average, then the capital cost of electricity is over 3 times that for a coal fired station plus fuel for its lifetime. To this would have to be added revisions to the grids and the back up plant required.

Wave energy.

The energies in waves are huge, but so are the difficulties, especially to do with storm damage. In view of the energy per metre of wave front devices would have to be very lengthy to harvest a lot of energy. It is significant that despite many years of experimentation no commercial plant had been put into operation before 2004. Ross (1995) does not think wave power can make a significant difference. The UK Department of Trade and Industry study estimated UK potential at equivalent to about one third of a 1000 MW power station. The UK Carbon Trust estimates that waves could theoretically generate 14% of UK electricity, if all the suitable sited could be connected to the grid. (Black, 2006.)

According to a source within the industry (personal communication) there are 16,000 km of coast around the world with excellent wave energies, i.e., 30 kW/m, and three times as much energy again if sites down to 20 kW/m are used. Devices built into the shore would not be so vulnerable to storm damage but as these would take so much coast line, large scale wave harvesting would have to use floating devices moored out to sea. The problem then would be that these would have to be very robust, and therefore expensive, to withstand storm damage.

Industry sources believe 40% efficiency can be achieved, meaning output of 12 kW/m at the best sites. If 10% of these ideal sites could be used and if 40% efficiency is achieved, output would be equivalent to 18 power stations. The equivalent of a 1000 MW power station would be 80 km long. Hayden (2004, p. 210) derives a similar figure, 130 km, from another experimental project assuming 25% efficiency. Adding the estimate for 20 kW/m coasts suggests a total roughly equal to 76 power stations. This would be a welcome contribution, but industry sources consulted do not think wave power will exceed 5 — 10% of world demand. World electricity supply at rich world rates of consumption for the present total world population would equate to roughly 9,000 power stations.

Renewable energy advocates often assume that the gaps can be filled by electricity generated when renewable sources are available and stored as water pumped into dams to be used to generate later. This works well but the capacity is very limited. World hydro generating capacity is about 7 — 10% of electricity demand so there would often be times when it could not top up supply. To increase generating capacity would again be to build alternative plant to sit idle much of the time.

Are solar sources the answer? Not in mid to Northern Europe in winter. Winter solar incidence in Scandanavia can be 3% of summer incidence. However when winds are much lower in summer PV and solar thermal are at their best, but this again involves having two or more alternative systems.

The big problem with PV is that it too is an intermittent source and its possible contribution to a wholly renewable energy system is therefore very limited without the capacity for very large scale storage. It is fine ( though costly) when it can feed surpluses from house roofs etc., into a grid running on coal, while drawing power from that grid at night. But this only works when a lot of coal or nuclear power plants are running all the time to act as a giant "battery" PV can send surpluses into.

Solar Thermal Electricity:

After wind, Europe’s best option for renewable electricity will probably be solar thermal plants located in the Sahara region. These will impose significant transmission losses but their big advantage is their capacity to store energy as heat to generate and transmit electricity when it is needed. However the magnitude of the potential is uncertain, and especially doubtful in winter. Solar thermal trough systems do not work very well in lower solar incidence. Even in the best locations output in winter is about 20% of summer output. The winter incidence of solar energy in the Sahara it is not that impressive, perhaps 6 kWh/m/d towards Libya and Egypt and a long way south of the Mediterranean.

A "polar axis" arrangement of troughs, where one end is raised to set the trough parallel to the earth’s axis, increases winter performance, but seems to be quite impractical for large scale application at significant distances from the equator, because the shortness of the troughs would produce significant end loss" effects (sunlight reflecting out of the ends of many short troughs.)

As with wind and PV there will be significant gaps, e.g., when it is cloudy for some days in the Sahara and there is little wind in Europe. Again we would be building a solar thermal system, and a wind system, along with the coal/nuclear systems for back up.

The capital cost of solar thermal plant is rather high. Sargent and Lundy (2003) put it at $(US)4,589/kW ($(A6,556) for the near term future compared with $(A)3.7 billion for coal plus fuel over plant lifetime. But note that these figures are for peak outputs and the average output from a coal plant is c. .8 of peak whereas for a solar thermal plant it is around .25 of peak capacity. Thus capital cost per delivered kW from solar thermal plant is over 6 times as great as for coal including fuel. As we become more dependent on renewables we will have to get used to much higher energy costs, but could our economies cope with such a multiple when we would also have to pay for the other expensive systems? The transmission lines (under the Mediterranean sea) from the Sahara would probably add more than 33% to generating plant capital costs. (Czisch, 2001, 2004.) (…not taking into account the loss of 15% of energy sent.)

Solar thermal dishes perform better than troughs in winter, but they cost more and their big disadvantage is that because each tracks the sun it is difficult to take heat via flexible couplings to a central generator or store. They are being developed with Stirling engine generators at each focal point, meaning that energy can’t be stored to generate electricity when it is needed. Central receive or tower systems can store, but suffer a "cosine" problem similar to troughs, reducing winter performance.

It is likely that solar thermal systems will be located only in the hottest regions, will have to supply major demand centres by long transmission lines, and will not be able to make a large contribution in winter.

Could the gaps left when there is little sun or wind be filled by use of coal without risking the greenhouse problem? Unfortunately the gap is too big. The IPCC emission scenarios indicate that to keep the carbon concentration in the atmosphere to a safe level world per capita fossil fuel use should cut world average per capita carbon use to about .11 tonnes p.a. This amount would generate about .03 kW…which is about 3% of the rich world per capita electricity consumption rate. Far more than that would be required to meet electricity demand when wind and/or solar sources were low. (So even if we aimed for a 3 GT limit fossil fuels could generate only about 10% of electricity demand, leaving no fuel for transport. See bellow.)

Combine renewable souces?

Being able to draw on a range of renewables reduces the gap problem, but unfortunately there are times when both the main two sources, sun and wind, are down for days at a time. This is especially problematic where I live in eastern NSW. My house is dependent on windmills and solar panels and autumn and winter are always times of inconvenience, worrying about battery damage, and use of the back up petrol generator.

Note that if we want to draw on a range of renewables which tend to alternate in their contributions rather than add, then we are moving towards a situation in which we might have three very expensive systems capable of more or less meeting demand while the other one or two sit idle, and in addition we must keep a coal or nuclear system capable of meeting demand when all the renewables are down. What would electricity cost then?

These have been some of the reasons why it is far from clear how much electricity from renewable sources we are likely to be able to afford, and why it is probable that present demand will not be met. It is very unlikely that renewables will be able to generate so much electricity that much or all of our transport energy can also be provided via electric or hydrogen vehicles.

To this we must add the fact that electricity demand is rising all the time, and fast. Australian peak demand is increasing at 2.9% p.a. At this rate it would be more than 4 times as great as it is now by 2050.

What about the "hydrogen economy"…won’t we run everything on hydrogen some day?"

Chapter 6 of RE outlines the weighty reasons why we are not likely to have a hydrogen economy. Several significant problems derive from the physical nature of hydrogen. If you make hydrogen from electricity you lose 30% of the energy that was in the electricity. If you then compress, pump, store and re-use the hydrogen the losses at each of these steps will result in something like only 25% of the energy generated being available for use, e.g., to drive the wheels of a fuel-cell powered car. In fact plausible assumptions can make the figure 10%.

Bossel, (2003, 2004 and undated) details these and other difficulties. For instance a 40 tonne tanker can deliver 20 tonnes of petrol, but it would only deliver 320 kg of compressed hydrogen. Much energy would have to go into pumping hydrogen to move significant quantities of energy, because the gas is so light and diffuse. Special pipelines would be desirable, because present gas lines are too narrow for efficient pumping, would need plastic liners to prevent leaks and embrittlement, and these pipes are in any case already in use for natural gas. The new pipes could cost $1.5 million per mile. To pump to Europe from the Sahara would take most of the energy going into the pipe line at the start.

In Australia transport takes twice as much energy as there is electricity consumed so to run transport on hydrogen generated from wind electricity would require generation of 7 to 8 times as much electricity as we would need just to meet electrical demand. Use of electric vehicles might halve the task.

So even if there is much more wind energy harvestable than would be needed for electricity it is anything but clear that it could be stored as hydrogen to deal with intermittancy, let alone to also meet transport demand.

Liquid fuels.

The situation regarding the idea of meeting liquid fuel demand via renewable energy sources is much more clear cut than the question of meeting electrical demand — and impossible.

Any very large scale scenario will have to be via ethanol produced from woody biomass. There is nowhere near enough waste, oil crop potential, or corn/wheat input material for biodiesel or ethanol production on the necessary scale. The current view among the main researchers and agencies is that in future it will be possible to produce about 7 GJ of ethanol from each tonne of biomass. (Fulton, 2004.) This is a net figure, i.e., the amount after all energy costs of production have been paid, but it does not deduct the (unknown) energy content of co-products, so the figure for output of liquid/gaseous fuel would be lower.

People in rich countries such as Australia use about 128 GJ of liquids (oil plus gas) per year, so to provide this via ethanol would require 16.3 tonnes of biomass each year.

Biomass can be produced at 20 t/ha/y, and more than 35 t/ha/y as sugar cane (dry weight), but only in special conditions. Very large scale biomass energy will have to come from such very large areas that the average yield will be far below these figures. World forest growth is c. 3 t/ha/y. I will assume that for very large scale biomass production the yield will be 7 t/ha/y. This would mean each person would need 2.6 ha of land growing biomass to provide for their liquid and gas consumption (in the form of ethanol net, not primary energy amount.) To provide the 9+ billion people we will probably have on earth by 2060 we would therefore need 24 billion ha of biomass plantations.

This is a slight problem here … because the world’s total land area is only 13 billion ha, and the total forest, cropland and pasture adds to only about 8 billion ha, just about all heavily overused already. So vary the above assumptions as you wish (e.g., assume 15 t/ha/y for willows grown in Europe) and there is no possibility of explaining how all people could ever have something like the present rich world liquid fuel consumption from biomass.

There are many reasons why the potential for biomass production will decline in future years, including increased pressure on land for food and building materials as energy-intensive materials become very expensive, and especially the effects of the greenhouse problem. For instance the water resources of the Murray-Darling river system in Australia could be halved this century.

There is no doubt that the potential for energy saving is large, both in terms of wasteful practices and the potential for developing much more energy-efficient devices. A common claim is that energy use can be cut by 50%, by eliminating waste and designing more efficient machines and ways. This is plausible. Lovins and von Weisacher (1997) have argued that a "Factor Four " reduction is achievable, i.e., halving resource and environmental loads while doubling GDP. Most of Lovins’ (valuable) arguments and cases indicate 50 — 75% reductions. For instance hybrid cars could cut petrol consumption in half and Lovins believes future possibilities might halve that again. So why can’t we solve the problem if we just keep up this effort?

We should note firstly that not everyone agrees with Lovins regarding the scale of the possible reductions. ABARE (2006) offers an estimate of the overall probable conservation achievement by 2050 which is much lower than the expectation often encouraged by tech-fix optimists offering theoretical analyses of what might be achieved. They estimate that we are on a path to a total global carbon emission rate p.a. that is an alarming 2.6 times as high as it is now, and that conservation effort will reduce the resulting 15 GT figure by only 23%. (Remember that a sensible carbon emission limit might be 1 GT/y.)

We are in an era when the easiest conservation gains are being made. We are "picking the low hanging fruit". For instance to cut 10 kg from the annual carbon emission from the national car fleet average in 2006 would be far easier than cutting off another 10 kg in 2030. US oil intensity fell in the 1985-2005 period at half the rate that it fell in the previous 15 years. (Lovins, et al., 2005, p. 43.) Gains in aircraft flight distance per litre of fuel are falling, because the easiest gains have been made. (Lovins, et al., 2005, p. 80.)

Another point enthusiasts about conservation and technical advance easily overlook is the "Jeavons " or "rebound" effect. Often technical advances enable savings in energy required and therefore reductions in the price, which promptly leads to greater demand. This has to be understood in relation to the fundamental imperative in a consumer-capitalist society, which is to maximise output, wealth, consumption and GDP all the time. Any firm that finds its energy costs cut by better technology will immediately increase production of cheaper goods, or pass the saving to customers who will have more money to spend on something else. If we find we can travel for half the cost, we double our travelling.

The costs of savings also have to be accounted. Insulating a house saves energy, but it takes energy to do it. Very light cars use less energy, but the materials they are built from are very energy-intensive to produce. In fact Matega (2000) reports that because of their sophisticated electrical systems, over their lifetimes hybrid vehicles take 30% more energy than the average car, and in some cases 5 times as much. The popular Prius takes 142% more energy than the average car.

Also the full balance sheet needs to be filled out. For instance energy used in US corn production fell 15% between 1959 and 1970, but that was only energy used on the farm. When all inputs were taken into account energy use actually rose 3%. (Hienberg, 2003, p. 162.)

If the magnitude of our overshoot were not so great these efforts might be capable of solving the problem, but we have to make perhaps 90% reductions. Let us assume that energy use and other resource and environmental impacts must be halved (…although solving the greenhouse problem would require far bigger reductions.) Now if by 2070 we have 9+ billion people on the "living standards" we in Australia would have by then given 3% growth, total world economic output would be 60 times as great as it is now. How plausible is it that by then we can also reduce impacts by 50%, meaning a Factor 120 reduction, not Factor 4.

Clearly system change is needed. The problems cannot be solved by more conservation effort on the part of individuals and firms within consumer-capitalist society. They are being caused by an overshoot that is far too big for that, and they are being caused by fundamental structures in this society. Consequently much of the effort being made to "save the planet" is a waste of time. Most irritating are the "What you can do in your own home" campaigners urging us to buy biodegradable wash up liquid, use a low-flow shower head, recycle our bottles, buy a smaller car, etc. Such efforts can make no more than a small difference to household impact, when we need something like a 90% reduction in national consumption. Nothing remotely like this is possible within a consumer-capitalist society committed to affluent lifestyles and limitless economic growth. It is only possible through dramatically reducing the volume of production and consumption and therefor by changing from such a society.

Therefore most of the rhetoric on sustainability and sustainable development is transparent nonsense, designed to give the impression that steps in the right direction are being taken while carefully avoiding any suggestion that there is any need to question affluence and growth. Unfortunately most of the green agencies reinforce this message, along with the business world, economists and politicians.

The answer?

In Chapter 10 of RE it is argued that there is no possibility of solving the many huge global problems confronting us unless the commitment to affluence and growth is abandoned. Consumer-capitalist society is grossly unsustainable and unjust. It involves rates of resource use and environmental impact that are far beyond sustainable levels, and could never be extended to all the world’s people. It is possible for the rich countries because they are taking most of the world’s resources, thereby condemning the Third World majority to far less than their fair share. This commitment to affluent "living standards" This inevitably creates problems of geopolitical conflict and war. In addition the obsession with growth and affluence is damaging the quality of life and social cohesion in even the richest societies.

Despite the fact that the present levels of production and consumption are unsustainable, the top priority is economic growth! This will multiply the magnitude of the problems many times in coming decades. It is no surprise that many now predict major global breakdowns.

The only way out of this absurd and alarming situation is some kind of Simpler Way, which Chapter 11 of RE outlines. This must involve non-affluent (but quite sufficient) material living standards, mostly small, highly self-sufficient local economies (and therefore the end of globalization), economic systems under social control and not driven by market forces or the profit motive (although there might be a place for markets and private firms), and highly cooperative and participatory systems. Obviously such radical system changes could not be made without profound change in values and world view, away from competitive, acquisitive individualism.

There are good reasons for thinking that we have neither the wit nor the will to face up to changes of this order, especially given that they are not on the agenda of official or public discussion. A major factor that has kept them off the agenda has been the strength of the assumption all wish to believe, that renewable energy sources can substitute for fossil fuels.

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For information on these themes, especially the nature of The Simpler Way, see

Australian Bureau of Agricultural Economics, (ABARE), (2006), Technological Development and Economic Growth, Jan. 12.

Bossel, U., (2003), "Efficiency of hydrogen fuel cell, diesel-SOFC-hybrid and battery electric vehicles", European Fuel Cell Forum, Morgenazvcherstrasse2F, CH-5452 Oberrohrdorf.

Bossel, U., (2004) "The hydrogen illusion; why electrons are a better energy carrier," Cogeneration and On-Site Power Production, March — April, 55 — 59.

Bossel, U., (Undated), "Towards a sustainable energy future", www.efcf.com

Czisch, G., (2001), Global Renewable energy potential; approaches to its use, http://www.iset.uni-kassel.de/abt/w3-w/folien/magdeb0030901/

Davy, R. and P. Coppin, (2003), South East Australian Wind Power Study, Wind Energy Research Unit, CSIRO, Australia.

Fulton, L., (2004), Biofuels For Transport; An International Perspective, International Energy Agency. (No source.)

Hayden, H. C., (2004), The Solar Fraud, (Second Edition), Vales Lake, Pueblo West.

Heinberg, R., (2003), The Party’s Over, Gabriola Island,New Society.

Lovins, A. M., E. K. Datta, O. J. Bustnes, G. Kooey and N.J. Glasgow, (2005), Winning the Oil End Game, Colorado, Rocky Mountains Institute.

Lovins, A and E. Von Weisacher, (1997), Factor Four : Doubling Wealth - Halving Resource Use : A New Report to the Club of Rome, St Leondards, Allen & Unwin.

Mateja, D., (2000),"Hybrids aren’t so green after all", www.usnews.com/usnews/biztech/articles/060331/31hybrids.htm

Sargent and Lundy (Consulting group), (2003), Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts, NREL.