Sunday, February 26, 2006

Energy at the Crossroads

In his 18th book, Vaclav Smil summarizes a lifetime of energy studies in 'Energy at the Crossroads - Global Perspectives and Uncertainties'. The result is a highly accessible book, yet rich in argument.

A chapter on 'energy linkages' looks at the broader energy context (environment, development, war, quality of life, ...). It lies a foundation for a later definition of a necessary energy consumption for essential quality of life requirements.

'Against forecasting' looks back to a century of attempts and often spectacular failures, sometimes by up to an order of magnitude, concluding that the human mind has difficulty assessing the full impact of new technology.

In fossil fuel futures, Smil dares to refer to the Russian-Ukrainian theory of the abyssal abiotic origin of hydrobarbons, calling it 'intriguing' and commenting that it merits more attention that it has received so far in the West.

Before starting the concluding chapter 'possible futures', Smil argues that nuclear energy is likely to have a role, but this role most probably will be limited. Energy efficiency will help as well, but decades of efficiency improvements have seen efficiency gains evaporate for higher demands of energy services. He dares to ask the question of a minimum energy requirements for a decent quality of life, which he estimates at 50-70 GJ/capita, and leaving it to the reader to construct his own scenario's.

In the future energy system, not a single solution will work by itself, and the 'a priori' exclusion of certain options is counterproductive. The path to carbon-free energy is going to take most of the 21st century. The author calls for action of 'complexifying minimalists' rather than 'simplifying maximalists'. We will need a multitude of approaches, flexibility, tolerance and openness.

Finally a personal observation. Sustainable energy may not be an economic or technical problem. The arsenal of solutions is there. At the end of the day, it may be a moral issue.

Energy at the Crossroads: Global Perspectives and Uncertainties

Thursday, February 23, 2006

What can we expect from wood pellets?

Globale Umweltprobleme.By Eike Roth

In partnership with Energie-Fakten

Wood pellets are booming. They have the potential (allegedly):
  • to provide low-cost energy
  • to save exhaustible resources
  • to reduce our dependence from politically instable countries
and above all, they have potential
  • deliver a significant contribution to the reduction of CO2 emissions
And because of this last benefit, they become
  • an important tool to comply with our commitments to mitigate climate change based on the Kyoto Protocol.
Since wood pellets represent another use of solar energy, we should welcome and value their contribution to society.

A reader requested Energie-Fakten for an assessment of the above statements.

We've already taken a position on this subject in a previous answer Can biomass power stations mitigate climate change? Hereby a few additional remarks.

Preliminary note

First of all, we must alert readers to a major distinction: wood pellets can be produced from waste resulting from other wood processing activities - e.g. the furniture industry -or they can be harvested from natural or planted and cultivated trees and bushes (so-called energy plantations). The use of wood residues is basically beneficial, since the raw material for the pellets can be obtained for free (except for the cost of transport, to the extent that the material is not processed on-site) and without a need for further processing. But the volume is relatively small and this type of wood pellets will not make a big difference. In principle, we could produce larger volumes in energy plantations. In this case, we must take into account all expenditures of these plantations (planting, where applicable irrigation, manuring, maintenance, as well as wood chopping and transport) including the machinery, installations and operating supply items required, and also all effects of these energy plantations and their cultivation allocated to the final product - wood pellets. In the latter case, a much more detailed assessment is required.

Technically, equipment for the production and combustion of wood pellets is mature. They operate reliably, without major impact on environment and the equipment for private users (stoves) are sufficiently user-friendly and adequately failure prone. And social acceptance will not be a market barrier.

Low-cost energy

Wood pellets from waste wood are cheap. From energy plantations, despite rising energy prices, wood pellets can still not compete with natural gas, heating oil or coal. When energy prices further increase, economic efficiency will be within reach eventually. We should observe that part of the cost of wood pellets depends on energy costs. With rising energy prices, the cost of wood pellets will increase. Wood pellets will close the cost gap slower than is generally expected. Note as well that transport costs are relatively important, because of the low energy density of wood pellets. Transport from low density areas over long distance to large cities should only be considered on a limited scale.

Clean energy

Waste wood contributes to cleaning energy supply, but its volume does not suffice to make a difference. Energy plantations can make a substantial contribution, but a number of constraints needs to be observed:

First of all, land use for energy competes with land use for food. As long as many hunderds of millions on earth suffer from food shortages, this ethical question cannot be factored out. But even considering sufficient land available for energy use, it still remains to be seen whether wood is the most appropriate application. There are faster growing plants, and a conclusive decision is only possible under consideration of all aspects, for example need for manuring (including environmental impact and impact on total energy demand) and impact on biodiversity. In these areas, we have substantial knowledge gaps.

Land competition is also a factor with technical systems using solar energy: with plants (and trees), the bottleneck is the efficiency of photosynthesis. This is typically 1% (maximum around 5%) and hence substantially lower than the efficiency of technical systems (at present around 10% for solar photovoltaics and around 30% for solar thermal electricity generation, in the latter case only referring to direct solar radiation, since scattered radiation is not usable for these systems; for Germany, scattered radiation represents about 50% of solar radiation, while in tropical areas, its share is substantially lower). Plants grow however by themselves (leaving out manuring and irrigation for a moment), while the technical exploitation of solar energy requires technical facilities (power plants). In the long run, we will see probably a parallel use of plants and technical systems, with technical systems probably taking the larger share.

Finally, we need to point to the need that in the long run, the share of solar energy (in whatever form) and nuclear energy to cover our energy supply needs to be decided on economic grounds, taking into account related requirements, such as security of supply etc. But here as well, the choice is probably and/and rather than either/or.

Fuel dependency from politically instable countries

Since wood pellets inherently represent a domestic energy source, they could very well contribute to the nicreasing our energy independency. A sizeable contribution however depends on the use of energy plantations, whose large-scale application depends on the answer of other questions.

Reduction of CO2 emissions

Combustion of wood pellets releases CO2, exactly in the same amount previously captured. But this does not allow us to conclude that wood pellets are a CO2 neutral energy source. All energy use for the setup and exploitation of energy plantations, the chopping of trees, transport and processing into pellets, as well as all energy consumption for the production and operation of these equipment and installations, produces CO2 emissions. For pellets from waste wood, to the extent facilities are allocated to the main application, these are limited to the pellet production, and transport to the end user. Certified figures are not available, but I estimate that the corresponding CO2 emissions are less than a tenth of what they would be when using fossil energy carriers rather than wood pellets. Wood pellets from waste wood are therefore an appropriate measure for CO2 reduction, to the extent waste wood is available.

For energy plantations, this ratio is clearly less favourable. Also in this case, I do not know of any reliable figures, but when long-distance transports are to be taken into account, the benefit compared to using fossil energy carriers will shrink really fast, and in certain cases can be completely consumed. Wood pellets from energy plantations are probably only practical on a limited scale to adequately reduce CO2 emissions.

Considering CO2 emissions, one should consider whether it wouldn't be better to directly burn wood, rather than converting it into pellets first. This conversion uses energy and releases CO2. In any case, a significant part of the comfort offered by wood pellets would disappear. To a certain extent, we must value CO2 emission reduction against comfort.

Another remark concerning CO2: when considering climate change, the first priority should be to keep CO2 from entering the atmosphere. Wood bonds carbon, and as long as it remains there, it does not enter the atmosphere. We must consider using wood for carbon storage, and cover our energy needs through other (CO2-free) means. This means in principle other forms of solar and nuclear energy. With proper storage, wood can store carbon for a few centuries. By then, we will certainly have much better possibilities than today to protect us against climate change. Wood as medium for carbon storage is likely to be the better strategy compared to wood as fuel, whether we use the intermediate step of wood pellets or not.

Mitigating climate change

Our climate is not only affected by CO2. Also other gasses contribute to the greenhouse effect. Especially N2O (laughing gas). This is a 150 times more potent greenhouse gas than CO2. It originates, among others, from the use of manure containing nitrogen. Our knowledge is still far insufficient about the nitrogen cycle in soil, water, plants and atmosphere caused by manure, but there are those who fear that intensive manuring (as often required for fast growing crop) has a large climate impact than would be mitigated through the energy use of plants. Until we have a satisfactory answer to this question, it is better to proceed with caution. For complying with our Kyoto commitments, pellets from waste wood are a meaningful (though limited) measure, wood pellets from energy plantations rather not.


The properties attributed to wood pellets apply in theory, but in practice there are always constraints that limit their potential. Wood pellets will be part of our future energy system, but will probably play a smaller role than expected by many today.

Eike Roth, Sonnenenergie - Was sie bringt - was sie kostet, Friedman Verlag Muenchen 1999, ISBN 3-933431-05-0

This contribution was originally published on November 17, 2005.

Monday, February 20, 2006

Sustainable Fossil Fuels

Sustainable Fossil Fuels: The Unusual Suspect in the Quest for Clean and Enduring EnergyIn this book, Mark Jaccard doubts our prospects for moving away quickly from carbon fuels to renewable energy sources, and expects an energy system largely dominated by fossil fuels for the 21st century. This however does not need to be incompatible with a sustainable energy system, which the book defines in practical terms according to 2 criteria:
  1. The prospect for enduring indefinitely an adequate level of energy services
  2. Extraction, transformation, transport and consumption of energy must be benign to people and eco-systems.
One of the central themes is a strong move to clean secondary energy - e.g. electricity and hydrogen, with no harmful emissions at the point of use. On the primary energy side, we see a wider use of new emission-free transformation technologies converting fossil fuel into these carriers, or cleaner burning fuels.

In such a scenario, the prospects of energy efficiency would be reduced. Additional transformation steps would reduce the overall efficiency of the energy system. At the same time, world population & economic development will still result in a 3 times larger demand for energy services than today.

As for the nuclear option, as it is disliked by many, a more widespread use of nuclear power is unlikely unless the technology can produce a significant cost advantage.

Sustainable Fossil Fuels would add 25% to the cost of electricity, and the use of hydrogen in cars would increase personal transport by the same. Overall, household spending on energy would increase from 6% to 8% of income.

Thursday, February 09, 2006

Renewable energy and food supply: will there be enough land?

Today, land is used for living, working, recreation and most importantly, food production. A relatively new use is growing biomass for energy use, which requires vast areas of land because of the diffuse nature of sunlight, and the low efficiency of plants capturing it. There is a direct competition for agricultural land between energy and food. Land requirements for both uses depend on consumption and production per square meter, 2 parameters which vary widely. In this paper, S Nonhebel estimates land requirements based on today's consumption patterns and production efficiency, providing a basis for the construction of scenario's.

How much land?

Total land available is 13 Gha (13 billion ha or 130 million square km). Arable land (1.5 Gha), pastures (3.5 Gha) and forests/woodlands (4 Gha) provide total 9 Gha for growing biomass, which in a world of 6 billion people means 1.5 ha (15,000 sq.meter) per person (under the untenable assumption that humans are entitled to 100% of available land).

How much energy?

In 2003, the world consumed 435 EJ of primary energy, or 70 GJ/pp/year. Developing countries use about 35 GJ/pp/year, whereas developed countries consume 200 GJ. Within developed countries, consumption varies significantly between Japan (100 GJ), Europe (150 GJ) and North America (350 GJ).

How much food?

Just like we express energy use in primary energy equivalents expressed in Joules or tonnes of oil equivalent (toe), food consumption can be expressed in kg grain equivalent. A basic menu requires 200 kg/pp/yr, while rich countries consume more like 800 kg (including fooder required for secondary foods such as beef, porc, ...).

Food and energy production

The paper below mentions following conversion factors:
'rich' 'poor'
Grain yield (kg/m2) 1.0 0.2
Energy yield - photovoltaics (MJ/m2) 780 780
Energy yield - biomass (MJ/m2) 27 1.8

Using these ratio's in combination with today's energy & food diets yields following land requirement per person:
  • developing countries, biomass energy: > 20,000 m2
  • developed countres, biomass energy: 8,200 m2
  • developed countries, photovoltaics: 1,000 m2 (of which 256 m2 PV)


The author concludes that for developing countries, the use of biomass points to a critical situation. For developed countries, PV-systems are the only long-term option, though biomass may function as a transition fuel.

S Nonhebel, Renewable energy and food supply: will there be enough land?, Renewable & Sustainable Energy Reviews, 9(2005) 191-201

Saturday, February 04, 2006

Energy yield factors for the generation of electrical energy

By Joachim Grawe

In partnership with Energie-Fakten

The yield factor of a power plant is – in simplified terms – how many times energy generated during plant operation covers the energy used for constructing the plant. The exact definition is: 'The yield factor is the ratio of net energy production during plant life and the cumulative energy used for construction, operation and operating supply items'. The concept is only meaningful in the context of using regenerative energy sources.

The yield factor has been investigated mainly for electricity generation plants in several studies. Sporadic data is also available for solar collectors, heat pumps, district heating systems and insulation measures. The most solid dataset on this subject is available from the 'Institut fuer Energiewirtschaft und Rationelle Energieanwendung' of the University of Stuttgart, that has developed a series of detailed studies on the subject. It is also worthwhile to mention the 'Forschungsstelle fuer Energiewirtschaft Muenchen' and the Chair of Prof Franz-Jozef Wagner, Gesamthochschule Essen.

The literature on this subject is not fully consistent in its treatment of energy uses for operating power plants (for example maintenance works, or fuel consumption) as well as for demolition and disposal of the plant, or in its treatment of energy conversions (for example from final energy to primary energy). Except for strictly scientific papers, many publications unfortunately make only very superficial statements on these issues. In addition, various assumptions relating to lifetime, loading, technology etc have a significant impact on the outcome. As a result, declarations about the yield factor for selected energy generation technologies will fluctuate a lot. There is however some consistency in the declarations of leading institutes.

As a benchmark for current technology, the values in the following table can be used, which give the net energy yield factor, not including energy required for operating the plant, or demolition. Electricity is converted to its primary energy equivalent. For lifetime, typical values of the respective facilities are used. These are based on the constraints defined in the most solid publications on the subject.

This gives following yield factors:

Nuclear power station: 100 – 200
Coal-fired power station: 100 – 150
Large hydro station: 100 – 200
Small hydro station: 40 – 100
Wind power plant: 10 – 50
Solar photovoltaics: 2 – 8

When including the energy required for operating the plant, these values become:

Nuclear power station: significantly below 100
Coal-fired power station: significantly below 80

Another characteristic is the energy amortisation factor (energy payback), i.e the period needed by the power station to generate the energy consumed for its construction. In case of photovoltaics, this period can be as long as 8 years – depending on cell-material used. For small hydro power stations, it is in the range of 2-3 years. For other power plants, the factor is similar between 1-2 years.


This question has prompted me to revisit the subject 'energy yield for fossil, nuclear and regenerative energy generation systems'. This has highlighted an error in my thinking on this subject. The latest comparative reviews at the University of Stuttgart and the Technical University of Munich include for nuclear and coal-fired power plants already the energy required for fuel (coal) respectively fissile material (Uranium), or more accurately: their extraction, transport and disposal is included. Hence, for these 2 electricity generation technologies, the yield factor is more than 100.

We can easily crosscheck with the energy amortisation time. The yield factor corresponds to the lifetime divided by the energy amortisation time. This is the time period until the corresponding system has generated the energy consumed for all stages of its lifetime from 'craddle' (material extraction for construction) to 'grave' (demolition and disposal). Expressed in months, it is on average:
  • hard coal just below 4 months
  • brown coal just below 3 months
  • natural gas (combined cycle) just below 1 month
  • nuclear energy *) just below 3 months
  • hydro power below 14 months
  • wind power **) 7-16 months
  • solar energy ***) 70 – 100 months
*) Pressurised Water Reactor, 1300 MW, direct disposal used fuelrods
**) 1 MW, average wind velocity 4.5-5.5 m/sec
***) 5 kW (roof-mounted installation), polycrystalline and amorphous silicon
We can then derive following yield factors:
  • coal-fired power stations (lifetime 30 years = 360 months): 90
  • nuclear power station (lifetime for old facilities 40 years = 480 months, for new facilities 60 years = 720 months): 160 – 240
If we normalise the expected useful life (as for photovoltaic and wind power plants) to 20 years (240 months), we obtain a yield factor of 60 for coal and 80 for nuclear power plants.

This contribution was originally published on October 6, 2005 by Energie-Fakten. The addendum was added November 24, 2005