Solar energy is most economically captured and stored by using a grid-connected photo voltaic (PV) system if the 60cent/kWhr Feed in Tarrif (FIT) can be used to maximise the return for the generated electricity which is fed into the grid and "stored" as cash $.
When heat is needed, a heat pump uses one kWhr of electricity to deliver 3kWhr of heat.
A wood fire is best used mainly for its ambience and for the pleasure of watching the flames. Wood will become increasingly expensive and I am doubtful about the environmental friendliness of what comes out of chimneys.
The highest quality slow combustion heater burns at ? 75% efficiency.
Growing wood contains 80% water.
Wood is "dry" when its water content has reduced to 10%.
Dry firewood typically contains 17MegaJoules of heat energy per kilogram of firewood.
Burned at 75% efficiency, this dry firewood delivers 13MegaJoules of heat per kilogram of firewood.
Because one kiloWatthour = 3.6 MJ, 13MJ of heat = 3.6kWhrs of heat.
Wet firewood delivers much less heat because much heat is wasted to evaporate the water.
In Adelaide the cost of firewood is increasing every year.
In 2011 firewood is costing ? $240/tonne = 24cent/kg.
At this price firewood provides heat at 24/3.6 = 6.7cent/kWhr.
An additional cost of firewood is the labour to stack the purchased wood in the woodshed, to carry the wood into the house, to feed the fire and to remove the ashes. My estimate for the labour I use to harvest our home-grown firewood totals 16 hours per dry-tonne to cut, collect, cart and stack the fire-wood. Additional daily labour is needed to cart (wheelbarrow) and feed the wood into the fire 5 times per 24 hours.
A reverse cycle air conditioner delivers about 3kWhr of heat from one kWhr of electricity.
If electricity costs 21cent/kWhr, the heat from a reverse cycle air conditioner costs 21/3 = 7cent/kWhr and the labour is reduced to ZERO.
After 20 years of chopping and handling firewood, flicking the switch of our new air conditioner is making me a very happy chappie.
Posted Monday 18 Jul 2011 @ 2:07:25 pm from IP
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