Maximising biomass boiler performance
One of the biggest challenges facing facilities and building managers that use biomass boilers as part of their energy management strategy is how to control their building’s heat. Buffer tanks represent an effective way to optimise the performance of biomass boilers. They not only can improve energy efficiency, but also lengthen the lifespan of biomass boiler systems. Paul Clark, Technical Director, of our biomass specialist Rural Energy explains how to make buffer tanks an integral part of your strategy.
A buffer tank can dramatically improve the efficiency and longevity of biomass boilers by storing heat and delivering it when required.
If you want an efficient biomass boiler, a buffer tank is essential writes Paul Clark of Rural Energy A Myriad CEG Company.
Buffer tanks store heat when it is available from your boiler and delivers it when it is required. They are used with biomass boilers for a variety of reasons but the principal one is that they regulate fluctuations in demand by insulating the boiler from what is happening in the building.
In control
A building’s heating control system will switch pumps on and off during the day to suit the building’s heating demand and is often switching much quicker than the biomass boiler’s response times. As with any biomass boiler, to get the optimum combustion efficiency you must try to limit the number of starts the boiler experiences and allow the boiler sufficient time to build the fire up and allow time for it to reduce.
The boiler’s only job is to ensure that the buffer is always charged with heat. Incorporating a buffer tank into the system reduces the cycling on/off of the boiler. A high cycling rate will reduce the seasonal efficiency, increase the risk of condensation in the heat exchanger, reduce the boiler lifetime and create much higher levels of smoke.
Perhaps the best way to think about the benefits of a buffer tank to a biomass boiler is by making an analogy between boiler cycling and driving a vehicle in an urban environment (with frequent stops/starts) and on a motorway (where we typically drive at a continuous speed). Motorway driving gives better fuel efficiency and increases the life of the vehicle. This is common knowledge and the principle is the same with biomass boilers.
Modern biomass boilers with good combustion control (lambda, temperature control etc) can easily reach 90% efficiency when working in their modulating range (typically 3:1 turndown) but boilers that are allowed to cycle on/off frequently can see their efficiency reduce by up to 40%. The key is to create a design that limits the cycling whilst still providing the responsiveness that thebuilding requires. This is where we can help. Myriad CEG has over a decade’s experience and can help with these fundamental questions.
Size matters
So how should a buffer tank be sized? There are many products on the market with competing ‘design philosophies’. Some are ‘fast response’ engineered with a small thermal mass; others the opposite with a high thermal mass. Boilers in the first category can modulate very quickly, while those in the latter can burn wetter fuels but a large buffer volume is required to dissipate heat effectively. In our experience, the primary consideration must be that the buffer tank size is adequate to allow the boiler to dissipate the residual heat once the boiler is switched off. For low thermal mass boilers this is typically around 10ltr/kW and 20ltr/kW for high thermal mass designs.
No buffer guff
So what’s the argument against using a buffer tank? Some installers claim that they are an unnecessary addition to the system. That really is a load of hot air! An investment in a buffer tank will shorten the payback period of your biomass boiler because it improves the efficiency of the boiler. Without one in the system the biomass boiler will be cycling on and off and producing lots of smoke and excessive wear and tear, which ultimately will shorten its life and reduce its operational performance.