# What is the cost of energy in purchased wood?

Purchase made 1 mp of beech wood for the price 85 zloty. what is the cost of energy in purchased wood?

Calculations:

You can count the volume of each log using the dimensions, diameter and length. You can also make respect, how many percent of the volume of the pile is occupied by wood and how much air.
For the solution of the task, we accept, that 1 mp = 0,65 m³. Which means, that the price of wood is 85 PLN / mp : 0,65 m³ / mp = 130 PLN / m³.

If we dry the wood to 20% humidity (18 months under a roof), we read it from the table, that the calorific value is 10,49 GJ/m³, which means, that we are in possession:

## 1 mp x 0,65 m³ x 10,49 GJ/m³ = 6,8 GJ energy

price 1 GJ of energy in purchased wood is: 85 zloty : 6,8 GJ = 12,5 PLN / GJ

 • If we burn wood in a boiler with efficiency 50% we will get 6,8 GJ x 0,50 = 3,4 GJ of useful energy, so the price 1 The GJ of useful energy will be: 85 zloty : 3,4 GJ = 25 PLN / GJ • If we burn wood in a boiler with efficiency 80% we will get 6,8 GJ x 0,80 = 5,44 GJ useful energy, that is, the price 1 The GJ of useful energy will be: 85 zloty : 5,44 GJ = 15,6 PLN / GJ

The calculation results according to the above example for firewood, pellets and briquettes are presented in the table below:

 Fuel Fuel price The price of energy in fuel The price of heat with transport and chopping costs humidity humidity 10% 20% 50% 10% 20% 50% boiler efficiency, Þ 50% 80% 50% 80% 50% 80% PLN / mp PLN / m³ PLN / t PLN / GJ PLN / GJ wood fuel 30* 75 7,1 7,9 14,2 8,9 15,8 9,9 65 85 100 120 100 130 150 200 9,5 12,5 14,7 17,6 10,6 13,8 16,3 19,5 19 25 29 35 12 15 18 22 21 28 33 39 13 17 20 24 briquettes and pellets 200 300 400 500 600 11,8 17,6 23,5 29,4 35,3 24 35 47 59 71 15 22 29 37 44

* price excluding transport and chopping costs

The energy value of dry wood is greater than that of wet wood. in addition, burning wet wood causes a decrease in boiler efficiency.
From the results of the calculations in the table you can see, that the cheapest heat will be available to the user, who bought a cheap branch branch for 30 PLN / mp and he transported it himself, he chopped it up and burned it in a good cauldron (8,9 PLN / GJ), and the most expensive – user, brought wet, chopped wood to the house behind 120 PLN / mp. The price of heat from such wood can reach 39 PLN / GJ.

The second method of assessing the energy value of fuel is the method that takes the fuel mass and moisture as the basis for calculations. The dependence of the calorific value on weight and moisture content is presented in the table and the diagram below.