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Buffer Vessels (Thermal Storage)

Buffer Vessel (Thermal Storage) by

This article provides a quick overview of buffer vessel sizing.

A buffer vessel is a cylinder or container that holds water, increasing the overall water volume content of the heating or cooling distribution system. The additional water volume absorbs heat (thermal energy) produced by the heating appliance (or cooling from a chiller) in low load conditions, which the building or system does not yet require principally to prevent plant short cycling.

Why Use A Buffer Vessel?

A buffer vessel or thermal store, captures heat to provide a buffer between load variations in order that biomass boiler appliance efficiency can be optimised, thus alleviating slow response issues. A buffer vessel is designed to provide additional water volume or inertia to the system. A buffer effectively prevents cycling of the boiler when the system demand is less than the boiler minimum output. Reducing boiler cycling is important to minimise harmful emissions of CO and NOx.

There are two vessel variations either a Buffer Vessel used to capture residual heat (thermal energy) on shut-down to improve system efficiency or a Thermal Store which enables a small boiler to serve a system with a higher capacity.

Buffer Vessel:

  • Dissipates heat from boiler on shutdown
  • Protects boiler from overheating
  • Improves overall efficiency
  • Stored heat can be used by boiler on start-up

Thermal Store:

  • Boiler can be sized at less than 100% of the system heat demand
  • Allows boiler to operate continuously for long periods
  • Will serve function of a buffer vessel (takes heat on shut-down and feeds boiler on start-up as required)

The above apply equally for heating boilers or cooling systems with chillers.

Reasons for a Vessel
  • System stability: • a vessel provides sufficient water volume adds inertia to the system, reducing the cycling frequency of heat pumps at low load
  • Management of defrost • stored heat can be delivered during a defrost cycle to provide continuous supply
  • Peak-lopping • large storage volumes can be used to meet demand at peak times, for example during a DHW peak
  • Heat sharing • storing waste heat from earlier in the day
  • Resilience • providing a buffer if a heat pump temporarily fails
  • Energy costs • delivering flexibility, allowing users to take advantage of time-of-use tariffs

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Buffer Vessel Sizing

Vessel size will depend upon the application, heat output, fuel quality, minimum acceptable on-time cycle of compressors (ASHP & GSHP), the operating temperature differential of the [vessel] controls, hours of operation, etc.

The following general definition will determine a vessel size: Vessel capacity = (required system volume) - (actual system volume)

Where the required system volume is that necessary to accommodate the thermal output of the plant and the actual system volume is the water volume in all pipework and heat emitters.

There are a number of rule of thumbs which can be used to determined the vessel size depending on various factors such as the heat source and fuel quality. Here are some variations:

  • Method 1 - Litres per kW: 10 litres x Plant size = storage volume (L) • where 10L·kW-1 is a typical lower limit for biomass.
  • Method 2 – Minimum Operating Time: t= mcΔT/Q • Full explanation in our detailed guide
  • Method 3 – Smallest Zone: m= Qmin - Qzone x t /cΔT • See our buffer vessel guide download for a full explanation - link below.
  • Method 4 – Flow Rate Percentage: 10% x water flow rate = storage volume (L) • for a single compressor unit.

Where the symbols used are:
Symbol Definition
 t Time in seconds (s)
 m Mass of store water (kg) & 1kg = 1 Litre
 c Specific heat capacity of water kJ/kgC
 ΔT Operating temperature differential (°C)
 Q Heat input (kW)
 Qmin Minimum heat output of heat source (kW)
 Qzone Heat demand of smallest zone (kW)

Sizing For Different Heat Sources

Different heat generating plant have different requirements. Here are a few basic tips.

ASHP • BS EN 15450:2007 suggest sizing the buffer storage volume at 12 to 35 L per kW maximum heat pump capacity.

Biomass • Size between 10 and 20 litres/kWth plant capacity (CTG012); a lower value may be used where loads do not not fall to zero, for pellet fuelled boilers which are more responsive and for boilers with a low turndown capability. Where low grade wood with a high moisture content is used, the storage volume may need to be sized at up to 40 litres/kWth.

Chilled Water • A rule of thumb is around 2.5 to 8 litres per kW or 8 to 14 litres per kW when temperature accuracy is critical.

GSHP • Intermittent use heating load (kW) x 25 (L) and continuous use heating load (kW) x 80 (L)

Tank, Vessel or Store?

Term Definition
Accumulator Tank A buffer vessel or thermal store
Buffer vessel A vessel that captures residual heat on boiler shut-down preventing frequent boiler start-ups to improve system efficiency. It will also reduce the energy required from an auxiliary boiler. Usually has a simple on/off control strategy.
Thermal Store A vessel that is charged by the biomass boiler when the boiler output exceeds the load demand enabling a small biomass boiler to meet demand. A progressive control strategy using several temperature sensors will control the boiler output.

Suggested Reading

CIBSE CP1 Heat Networks, Objective 3.11: To optimise the use of thermal storage



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