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WOOD ENERGY and influencing factors
Workshop „More heat with less wood“.
UNECE / FAO Forestry and Timber Section,
Geneva, October 6th, 2015
E.Zürcher, Prof. Dr. wood sciences
Bern University of Applied Sciences, Architecture, Wood and Civil Engineering
2 essential terms for future development
Renewable
When
1
exploitation does not
exceed regeneration
Examples:
Renewable: wood directly produced by
photosynthesis
Non renewable: fossil energies
Sustainable
“A sustainable development
is a development that meets
the needs of the present
without compromising the
ability of future generations
to meet their own needs”.
Examples:
• Naturalistic silviculture
• Organic agriculture
𝑟𝑎𝑡𝑒 𝑜𝑓 𝑒𝑥𝑝𝑙𝑜𝑖𝑡𝑎𝑡𝑖𝑜𝑛
𝑟𝑎𝑡𝑒 𝑜𝑓 𝑟𝑒𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑖𝑜𝑛 ≤ 1
Principle of „sustainability“: origin and signification
• Principle: Activating the capital and
collecting the interest
• Example : In a forest managed according
to the « Plenterwald » -principle, all ages
and developmental stages are mixed in
the same space. The removal of an old
tree gives light for the new generation, but
the forest as a whole seeming
permannent. Such a stand has a growing
stock (or capital) of 400 m3/ha and an
increment of 10 m3/ha/year. This allows
to exploit in 40 years a volume equivalent
to the standing capital, without any
noticeable disturbance of the forest during
this périod. • idw-online.de
• Other option: Coppicing, with new growth
cycles on older root systems (Oak, Ash,
Hornbeam, Birch, Aspen, …)
The origin of wood
energy
1000 kg wood sequester
1851 kg carbon dioxide CO2
Wood = Solar energy
transformed in chemical
energy. Glucose: 692 kcal,
or 2897 kJ / mol
Dessins: D.Rambert
6
The release of wood energy
The reverse process as an accelerated
respiration:
C6H12O6 + 6 O2 6 CO2 + 6 H2O + 686 kcal / mol
1 mol glucose = 180 g
1 kg glucose contains 3811 kcal = 15.9 MJ
1 kg absolutely dry wood (carbohydrates & lignin) contains 19 MJ
1 kg air-dried wood (ca. 15% moisture content) contains 14 MJ
This is the amount of energy needed to heat 170 liters of water
from 10 °C to 30 °C: and take a good bath !
Tree species:
the most
relevant factor
Common name Scientific name Density
kg/dm3 (air-dry)
Firewood-related properties
Hornbeam Carpinus betuls 0.75 – 0.86 Good coppicing, difficult to split, highest
energy content
Beech Fagus sylvatica 0.70 – 0.79 Most common firewood species in Central
Europe, easy to split, high energy content
Ash Fraxinus excelsior 0.68 – 0.76 Good coppicing, easy splitting, burns already
in the fresh state (winter)
Oak, pedunculate &
sessile o. Quercus
pedunculata, Q.
petraea
0.65 – 0.76 Best species for coppicing, needs 2 years for
drying, high energy content
Silver Birch Betula pendula 0.65 – 0.73 Easy to split, bark inflammable, bluish flame,
no sparks
Maple, sycamore &
Norway m. Acer
pseudoplatanus,
A. platanoides
0.61 - 0.66 White and lustrous wood, needs 2 years for
drying
Elm Ulmus glabra 0.60 – 0.68 Very difficult to split, ideal for chopping blocks
Rowan Sorbus aucuparia 0.57 – 0.78 Long lasting embers; taboo in some regions
Scots pine Pinus sylvestris 0.51 – 0.55 Must be very dry for burning, then with a very
bright flame, but producing much soot
Alder, black & grey a. Alnus glutinosa, A.
incana 0.49 – 0.57 Fast growth, air-drying to the lowest moisture
content.
Aspen Populus tremula 0.43 – 0.49 Good coppicing, easy to split, good for starting
a fire
European spruce Picea abies 0.33 – 0.68 Most common in Central Europe, splits well,
gives heat rapidly. Exploding resin pockets
produce sparks
The next step after Felling and
Chopping: the Drying process
Reducing the moisture content of wood to
the level of 12 – 15 % prevents the
degradation by fungi and insects. In
addition, the calorific value will be
noticeably increased.
Calorific value per mass unit (softwoods / hardwoods)
Moisture content in %
Several reasons for preparing firewood in the
winter period and early spring
Figure: Variation during months and seasons of decay sensitivity of
Scots Pine wood (Pinus sylvestris), expressed as weight loss under
fungi degradation in laboratory conditions (Wazny et Krajewski 1984).
This wood is more durable during the period september (09) to
february (02).
Trees are at rest (no
water flow)
Hard, frozen soil
Clean, easy work
Less injuries to
remaining trees
Less root injuries
No fungi spores, nor
insects
Dry air, drying starts
at low temperatures
Followed by a long
warmer drying period
Traditional knowledge validated by science -
Lunar periods are relevant for water movements
↑ Drying: Felling Spruce trees shortly before Full
Moon (vVM) produces wood which has a slightly
higher drying rate than fellings shortly after Full
Moon (nVM)
↑ Hygroscopicity: Felling Spruce trees shortly
before Full Moon (vVM) produces wood which has
a clearly higher hygroscopic water uptake after
drying than fellings shortly after Full Moon (nVM)
A Drying process for free - Natural drying of the
felled trees in the forest
Maintaining the crown 2 or 3 months after felling
of conifer trees enables a noticeable drying of
the stems, by slow evapotranspiration, leading to
a higher wood quality (“mK”)
Must be avoided: limbed stems easily get
attacked by fungi and insects during the
warm seasons - and will never dry out !
Wood: a sink of radiation as well,
… and a new source
Bri que
creuse
An indoor wall built with a material having
a high specific heat capacity
diffuses on the long-term the stored
heat, like an „organic radiator“.
For the same indoor climatic comfort, such
a «warm» wall allows to reduce the room
temperature in the winter period, what
implies important energy-saving.
2 wood properties involved:
• High insulation factor (13
– 18 x better than
reinforced concrete)
• High specific heat
capacity (ability to store
heat and release it slowly)
Too warm
Too cold
Good
climate
Air temperature of the room [°C] W
all te
mp
era
ture
[°C]
Moderately warm wood walls (21 °C) allow
good indoor climate with only 17 °C air
tempeature, with less heating energy than
needed for warm air (20 °C) in cold walls
for the same subjective comfort.
The global
option:
Bri que
creuse
• Heating with
wood energy for
health and
• Building with
energy wood for
heating less
41
259
777
0
100
200
300
400
500
600
700
800
POB + LM
U = 0.16
PBr + Styrop.
U = 0.19
PB100% + Lin
U = 0.15
Temps de refroidissement (en h)
selon les types de parois
Experience on the thermal quality of the habitat
[E. Thoma 2003]
Tree types of wall with similar U-coef.
(corresponding to the Minergie criteria) are
submitted to the following experimental situation:
Winter period, outdoor temperatrure -5°C, indoor
temperature +21°C, leaving of the inhabitants and
heating-stop, drop of the outdoor temperature to
-10°C. Question: After how long will the inner
face of the house-wall reach a temperature of
0°C ?
Wall type Coeff. of
thermal
transmission
U [W/m2K]
Temperature
of 0°C
attained after
[h]
Wood-frame wall,
with mineral wool
insulation
0.16 41 h
Porous brick wall
38cm + 10cm
polystyrene
0.19 259 h
Wall 100%Wood
36.8cm + 10cm
flax insulation
0.15 777 h
Sources
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Shimbum, July 17.
Gabriel, I., Ladener, H. (2006): Vom Altbau zum NiedrigEnergieHaus, 5. Auflage, Staufen.
Mitting, L. (2014): Der Mann und das Holz - Vom Fällen, Hacken und Feuermachen. Insel Verlag, Berlin.
Pollack, G. (2013): The Fourth Phase of Water. Ebner & Sons Publishers, Seattle WA, USA
Rauch, Th. (Hrsg.) (2005) : Nachhaltig handeln. (besonders Kap. W. Winter et al. Ökobilanz
verschiedener Baumaterialien). Hep-Verlag, Bern.
Thoma, E. (2003) : Für Lange Zeit. Leben und Bauen mit Holz. Brandstätter, Wien.
University of Minnesota (1998): The Nature of Wood and Wood Products. Forest Products Management
Development Institute (self-study tool).
Von Weizsäcker, E. U., A. B. Lovins, L. H. Lovins (1996): Faktor Vier - Doppelter Wohlstand – halbierter
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Zürcher, E. (2006): La Forêt et le bois: des caractéristiques et des propriétés exceptionnelles face à
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519 – 522.
Zürcher, E., Schlaepfer, R., Conedera, M., Giudici, F. (2010): Looking for differences in wood properties
as a function of the felling date: lunar phase-correlated variations in the drying behavior of Norway
Spruce (Picea abies Karst.) and Sweet Chestnut (Castanea sativa Mill.). TREES (2010) 24: 31-41.
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Date. In: Spruce: Ecology, Management and Conservation. Eds. Nowak, K.I. and Strybel, H.F.
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