Understanding the costs and benefits The RHI encourages the long-term delivery of renewable heating in existing buildings, so to fully realise the financial value of your asset you need to analyse the impact it is having on the reduction in consumption of your counterfactual fuel (fossil fuel boilers already onsite may continue to provide back up to the biomass boiler or be used in conjunction with the biomass boiler to meet a peak heat demand on the coldest days). To do this calculation it is essential to understand the difference between fuel going into a fossil fuel boiler and the actual heat delivered. For example, gas is bought and sold based on its Gross Calorific Value (GCV), not Net Calorific Value (NCV). In this case GCV is about 10% higher than its NCV (i.e. when gas is burnt, about 10% of the total energy it contains just goes into evaporating the water that is a product of its combustion). You also need to consider the efficiency of the gas boiler and only then can heat tariffs in terms of pence per kilowatt hour (p/kWh) be compared on a like for like basis.
Biomass tariffs and the RHI are quoted in terms of delivered heat, already taking into account calorific value and boiler efficiency. This key difference between the tariffs quoted on fuel bills and actual delivered heat is often misinterpreted in favour of fossil fuels.
Practicalities – Space and design requirements for biomass heating The next vital step is knowing if you have space for all the equipment. Biomass heating systems are bigger than their fossil fuel equivalents and the footprint depends on the size of the heating demand, the type of biomass fuel and several other site specifics described below.
Biomass boilers can be fuelled by either wood pellets or wood chip. Wood chips are lower cost per kWh of heat delivered than wood pellets, but require greater volumes due to its lower calorific value. Wood chip also has a tendency to form ‘chip bridges’ over equipment and require cajoling and agitating into the boiler through the use of rotary agitators, walking floor mechanisms or hydraulic rams. Chip designs also need an underground fuel store or even a transportable fuel store called a hook bin, otherwise it can become difficult and expensive to re-fuel. Pellets are more expensive per kWh of heat delivered, but have flowing properties and a higher energy density. This means the fuel handling equipment is simpler and has a smaller footprint than the chip equivalent. The fuel store can also be refilled by blowing pellets from a specialised, but widely available, vehicle. In general, we favour wood chip for new developments when we can more easily influence the design of the building to favour wood chip and we encourage pellet for retrofit to existing buildings (where there are usually greater space and design constraints and to minimise significant building modifications).
As well as the large boiler and associated fuel store, some sites may also need an accumulator tank. This
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technology can give many benefits including stopping the boiler inefficiently low load cycling (boiler turning on and off rapidly when there is small heat load). An accumulator tank can also meet a site peak heating demand at a particular time of day and ensure a constant supply of instantaneous hot water. The size and need for an accumulator tank is defined during the project scoping period and can sometimes be an essential part of good design.
Furthermore, depending on the emission requirements defined by the local authority, systems will need to suitably disperse and control emissions. This means particulate cyclones, ceramic filters and flue height options being considered as part of the design.
Finally, all of the equipment described above can be installed within your building or sit externally in a prefabricated plant room such as the ‘Green Heat Module’. If you believe you have the space and the heat demand then this technology is definitely the one for you!
Procurement We believe the RHI is going to drive procurement decisions in favour of ‘best value’ to the end user rather than ‘lowest cost’ meaning quality, efficient biomass heating systems. The track record and expertise of the designer and installer are critical to success. This contract can then be implemented as part of an energy service contract (heat sold on a p/kWh basis), a lease or a direct purchase. Under all scenarios planning permission will need to be sought and Ofgem will need to accredit the installation for RHI eligibility.
Once your RHI project is installed and running efficiently, output needs to be maintained for 20 years. Biomass boilers need regular operation and maintenance, available and responsive biomass engineering expertise and a continuous supply of fuel of the right quality, price and calorific value. An operative will also need to be trained onsite to empty the ash bin, order fuel, submit quarterly meter readings to Ofgem and receive regular fuel deliveries.
This may all seem overwhelmingly complicated, but thankfully there are companies out there that have been planning for the RHI for many years and are now ready to deliver your installation, look after it and fuel it for the next 20 years.
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