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 fire: a source of

radiation with health benefits

Bri que

creuse

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

Arima,T. (1991): Contribution of Wood Products to Environmental Preservation. Tokyo University. Rinkei

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

Naturverbrauch. Droemer Knaur, München.

Zürcher, E. (2006): La Forêt et le bois: des caractéristiques et des propriétés exceptionnelles face à

l’effet de serre - quelques arguments décisifs. Schweizerische Zeitschrift für Forstwesen 157 (2006) 11:

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.

Zürcher, E., Rogenmoser, C., Soleimany Kartalaei, A., Rambert, D. (2012): Reversible Variations in

Some Wood Properties of Norway Spruce (Picea abies Karst.), Depending on the Tree Felling

Date. In: Spruce: Ecology, Management and Conservation. Eds. Nowak, K.I. and Strybel, H.F.

Nova Science Publishers, Hauppauge, New York 2012; 75-94.