University of Strathclyde Biomass Installation Feasibility Tool

Thermal Buffer



Thermal buffer

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Due to the particularities of the biomass boiler, it is very usual to have thermal buffers as a part of a biomass heating system. Actually, in many cases, the suppliers sell these tanks together with boilers. The energy stored in the buffers will be important during the start-up process, supplying part of the energy used to dry the fuel and start the ignition process. Also, especially during the periods of low demand, such as summer, the buffer may prevent the boiler from working with low efficiency (being constantly turned on and off and at low power).

There is a large variety of buffer configurations and installation methods, allowing two or even three different components such as boilers, solar panels or electrical heaters to be connected at the same time. The main principle of its operation is simple, though. The water stored inside of the buffer tank is heated, through heat exchangers or even direct input of hot water and, when requested, this warm water is supplied to the heating circuit or hot water supply. This causes the mean temperature inside the tank drop. Then the heat sources provide energy to reestablish the higher temperature. Knowing the allowed temperature variation inside the tank and the volume of water stored inside, one can easily calculate the amount of energy stored.

The ideal size of the boiler depends of several factors, because it is hard to define one single way of doing it. The heating circuit and how it is controlled, kind and nature of the backup system are all factors that must be taken into account. For example, for the same circuit, the user may decide and program the heating system to (during the start-up of the boiler) not use energy from the buffer, but from a fossil fuel source. This may drastically reduce the size of the buffer, but cause the fossil fuel boiler to be used more often. This is usual with boilers on the range of MW, where a huge amount of water would be necessary to supply the start up energy.

The picture below shows an 8 000L buffer system:

Thermal buffer system