Biomass boiler sizing
It is very important to understand your actual heat demand pattern in as much detail as possible. Simplistic comparisons with existing equipment sizes can be misleading.
To size your boiler correctly:
- Assess the weekly heat demand across the year and then the pattern of heat use within a day.
- Sizing your boiler to meet peak demand will give the highest carbon savings, but it will also incur excessive capital costs, have reduced use and have lower than optimum performance.
- A good starting point is to size the boiler to meet the base load. Biomass boilers are designed to run continuously at constant output, and if you can achieve this you will get good equipment utilisation.
- Incorporating a heatstore (hot water storage tank) with a biomass boiler installation allows daily and seasonal energy demand peaks and troughs to be managed and boiler size to be optimised to meet as much of the annual energy demand as possible.
Weekly demand (bare minimum) of a 'typical' 5 Ha nursery (click to see larger version)
Sizing your boiler to meet the base load and incorporating a heat store to meet peaks is the optimal solution and represents a balance between energy cost saving and the cost of installation. A heat store becomes an essential component when the demand pattern shows a wide variance between peak demand and base load.
- It ensures that the boiler can continue to operate during periods of low demand as over production goes into storage.
- The heat in the store can then be drawn back out and used at times of high heat demand.
Hourly heat demand on a typical spring day (click to see larger version)
Edible crop producers may already have a heatstore because they are used for the CO2 enrichment system. This store can usually be used as part of the biomass installation. This is not normally the case for ornamental crop growers, so they will have the added cost of purchase to consider.
There are three points to note in relation to CO2 enrichment:
- Using CO2 for enrichment from biomass flue gases is not yet proven. Therefore, an edible crop grower will still need to run an alternative system – typically a gas-fired boiler or CHP. This is an area of ongoing R&D and, as technology develops, it may be possible to use biomass flue gases for enrichment in the future.
- If CO2 enrichment is a priority, you should still consider biomass as an option. A biomass boiler can be used to deliver your heating base load during the winter and early spring. During this time, the crop CO2 demands are small, so selective operation of your existing boiler or CHP can ensure that both the peak heat demands and CO2 requirements are met. During the late spring and summer when CO2 demands are at their highest, you should then use your existing boiler or CHP as the lead heat provider. This will allow you to use the boiler or CHP exhaust gases for CO2 enrichment, plus the generated heat will satisfy the majority of the heat demand. During this summer period, your biomass boiler will only operate at times when heat demands are at their highest.
- Normally the biomass boiler for an edibles production nursery is likely to meet a smaller percentage of the peak demand than an ornamentals nursery. This means that, for edible and ornamental nurseries of the same size, the edibles nursery will have a smaller capacity boiler.
The heat store is vital to optimal boiler sizing and pain free operation. The boiler and heat store should not be sized in isolation and calculations should be carried out simultaneously to ensure that an optimum solution is derived. Also the role of a fossil-fuelled boiler or CHP which may be used alongside the biomass boiler needs to be carefully considered. Good controls must be installed to make sure that the heating demand of the nursery is effectively met by each of the different heat supply sources working as one system.