What is the energy content of your pellets?
Our A1-grade pellets are guaranteed to have a net energy content (aka Calorific Value, CV) of 4.6 MWh/tonne (as-received basis), or 5.0 MWh/tonne (dry basis). In practice, they typically have an energy content of 4.8 MWh/tonne (as-received basis).
Watch out for fuel suppliers claiming high CVs
Some fuel suppliers will tell you that their pellets have a higher energy content. They will usually be misleading you by false comparisons, using different ways of stating the CV to compare apples with oranges.
Energy content can be measured in a number of ways. Wood pellets still contain a little moisture (less than 10% for A1-grade pellets), and more moisture is generated in the process of combustion (the carbohydrates combine with oxygen to produce water and carbon dioxide). Energy is used in evaporating this moisture, and is not recovered unless the vapour is condensed.
Energy content is stated in terms of the energy divided by the weight (e.g. MWh = energy, tonne = weight). Both the energy and the weight can be described in different ways to give a higher or lower figure.
The energy content can be measured as though the energy in the water vapour were recovered (Gross Calorific Value or Higher Heating Value) or as though it were lost (Net Calorific Value or Lower Heating Value).
The energy content can be stated relative to the weight of pellets in the form that they were burnt ("wet", or "as-received basis"), or with all the moisture driven off first ("dry basis").
The Gross CV (dry basis) will be significantly higher than the Net CV (as-received basis). Typically, someone quoting higher CVs for their pellets will be trying to mislead you by taking advantage of this difference to compare apples (their Gross CV) with oranges (their competitors' Net CV).
If not that, then you need to check carefully what went into the pellets. A high CV may indicate that substances with higher CVs, like plastics, got into the pellets. It may well be a sign that the pellets contain recovered materials and are therefore (a) suitable only for use in boilers that are designed to be compliant with the Waste Incineration Directive, (i.e. not most pellet boilers), and (b) may contain variable quantities of nasties like arsenic and lead which you really don't want coming out of your flue. You are likely to invalidate your warranty and increase your maintenance costs if you use pellets containing recovered wood in a boiler that is not designed for the purpose.
Watch out for equipment suppliers claiming high efficiencies
Equipment suppliers often play a similar trick. The efficiency of their boiler is measured as the amount of usable heat produced relative to a given quantity of energy input. That energy content can be stated in terms of the Gross or Net CV of the fuel. The efficiency will appear higher if it is calculated relative to the Net CV.
For a condensing boiler, which recovers some of the energy in the moisture, it is not uncommon for suppliers to quote absurd efficiencies of greater than 100% (i.e. perpetual-motion machines), because they compare the energy recovered (including the energy recovered from condensing the moisture) with the Net CV of the fuel (i.e. the energy content assuming the energy in the moisture cannot be recovered).
There are no condensing biomass boilers on the market, so you should not see 100% efficiency quoted for any boiler, but you may still get a misleading impression if a fuel supplier quotes you the Gross CV of their pellets, and you combine that with a high claimed efficiency of the boiler calculated relative to the Net CV of the fuel.
Specified vs analysed energy content
There is also a difference between the specification of pellets (which is the minimum standard guaranteed) and the actual measured values of any sample of the fuel.
To allow for the small amount of variation that can occur between one batch of pellets and another, and between one sample and another even from the same batch, it is normal for the analysed values to be a little better than the specification. If they were the same, there would be a serious risk that another sample would not meet the specification.
So whilst the A1 specification states that pellets will have a minimum Gross CV of 5 MWh/tonne and a minimum Net CV of 4.6 MWh/tonne, lab analysis of a sample of A1 pellets will typically give a Net CV of around 4.8 MWh/tonne and a Gross CV of around 5.2 MWh/tonne.
Calculating your system's real energy efficiency
Putting it all together, the relevant energy content for you to look at is the Net CV, as-received (or wet) basis. You can then combine this with the efficiency figure quoted by your boiler supplier, which will almost certainly be based on the Net CV of the fuel, to give you the energy output of the boiler relative to the fuel consumption.
It wouldn't be unusual for a pellet boiler's net efficiency to be around 85% and the Net CV (as received) of A1 pellets to be 4.8 MWh/tonne. If so, the amount of energy you should expect to get out of your boiler is around 4.1 MWh for every tonne of pellets that it uses.
(The same logic applies to wood chip, and with bells on because the moisture content of wood chip is so much higher than the moisture-content of wood pellet.)
Then you need to allow for any losses in the heat distribution network, if the boiler is not in the building where the heat will be used. Losses from well-insulated, well-installed district heating pipes should be very low per metre, so the main factor in the extent of the losses will be the distance that the heat is distributed, and the amount that is circulated relative to the amount that is used.
The heat has to be continuously circulated, if you want to have hot water on demand. As hot water demand is variable, the heat may circulate round the pipes several times before it is used. A little heat is lost in each pass. A good district heating system will aim to adjust the circulation to limit the amount of heat that is lost when demand is low.
If the boiler is in the building where the heat will be used, the relevant efficiency is effectively the efficiency of the boiler (from the perspective of incentives like the RHI). If district heating is involved, you will have to allow for further losses of perhaps between 1% and 20%. At 20%, the net efficiency of the whole system is down to 68%, and you will only get around 3.3 MWh of usable heat from one tonne of pellets.
This will make a big difference to the economics of your project. If you are using district heating, it is important that you get a clear indication from your equipment supplier of the anticipated losses. It is probably also worth putting a heat meter on the boiler as well as on each building where the heat is used (the latter is required for the RHI anyway), so you can calculate the distribution losses to see if your installer had been truthful with you.