Adaptation of the biomass wood-chip boiler to fluctuating heat loads

I). General Considerations

In the case of the Barony college, the heat load comprises two separate components:
-heating requirements: these will vary from 0 at the height of summer to a large fraction of the energy supply in winter. The heat load variations associated with heating requirements are likely to vary slowly since they will follow the seasonal temperature changes.
-hot water requirements: the load associated with hot water requirements is much more peak-like since the demand will be concentrated at certain times of the day (around lunch and in the evening for the showers for example)
The biomass boiler will have to provide for both of these heat requirements: this will constrain the choice of the technology.

1). Boiler Operation
If the boiler is intended to be run at high efficiency (no or little incomplete combustion gases such as CO) and with acceptable amounts of pollutants (chiefly NOx), some situations must be avoided¹:
-running the boiler below its specified power range. For example, a 50-150kW boiler should not be run at 30kW because the combustion temperature would be too low, leading to large amounts of CO.
-brutal variations of thermal output: transients should be avoided since they also lead to high emissions of CO and NOx.
Given the hot water peak-like load, brutal variations of the energy requirements of the building will inevitably take place. Besides, the summer and winter energy requirements are very different and might not fit in the power range of a single boiler unit. The boiler must therefore be adapted.

2). Boiler Technology Adaptation
-In order to decouple the operation of the boiler from the demand-side fluctuations, the use of a hot water storage unit (accumulator) is essential. There is no way a wood-chip boiler could respond instantaneously to a hot water peak, and even if it could this would be accompanied by inadmissible CO and NOx emissions as explained previously. However, if the water-storage unit is large enough, the boiler can be run at a constant power equal to the average day demand: the water-storage unit would act as a “buffer” between energy production and demand. This feature is especially important in summer where the absence of the water-storage unit would lead to very frequent “on-off” situations since there is no heating.
-If a single furnace is provided with the boiler, it might not be adapted to both summer and winter operation. Suppose that in extreme winter conditions, the building's consumption is around 500 kW and that a 250-600kW furnace is therefore selected. In summer, if the only energy requirement is hot water which amounts to 50kW (spread out on a daily basis), then it would not be possible to run the boiler at constant power since the efficiency of the boiler would be very poor (50<250). The boiler would have to be operated in an on-off way, regularly bringing back the hot water unit to a set temperature. This stop and go operation mode is common with biomass boilers but it could be avoided by using two furnaces (or two boilers), say a 50-150kW and a 200-450kW which could operate either separately or together in case of extreme energy requirements.

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II). Barony College Energy Demand

The energy bills of the Barony college² were used to estimate the global energy requirements during winter and summer operation. The hot water component of the demand was estimated on the basis of the number of boarders, day pupils, staff members and meals prepared. All figures were taken from the Ashrae 2003³. The maximum heating power required was based on meteorological data from⁴: -7°C is the temperature under which outside temperatures do not drop, on average, more than one day a year in Glasgow and Edinburgh. This figure was assumed to remain valid for the Barony College.

-Summer consumption (June, July, August):
The demand is entirely hot-water based and is around 12 kW (spread out on the entire period). Given the nature of hot water demand, the power required will vary from 0 (in the middle of the night) to values much higher values (during cooking or shower periods). The peak hot water demand has been estimated at 100kW.

-Winter consumption (December, January, February, March):
The average heating requirements on a winter day are around 240 kW and the hot water demand is supposed to be unchanged (12kW on the average, 100kW maximum peak). The maximum heating demand for extreme conditions (-7°C) and if no casual gains are available is 630 kW (this figure relies on average degree-days for the north-western zone retrieved from⁵).

Since these figures were calculated on the basis of the previous energy system, they are roughly valid only if the new heating system has a comparable efficiency and comparable energy losses. If the new heating system is entirely centralized, this will probably mean more pipe work which could result in higher losses: a higher power loss might therefore be possible.
Besides, if the buildings were under heated with the previous system, this will be reflected in the previous calculations and may also lead to an undersized heating system.

-Estimated average electricity consumption (for non-heating appliances): 75 kW
Here again, this mean figure is the time average of what is certainly a very fluctuating demand (nearly zero in the night, high values in the daytime).

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Footnotes

1. J. Lundgren, R. Hermansson, J. Dahl.
2. Energy bills 2005-2006
3. Ashrae 2003.
4. The Energy Management Register.
5. CIBSE Guide A.

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References

Experimental studies during heat load fluctuations in a 500kW wood-chips fired boiler, J. Lundgren, R. Hermansson, J. Dahl, Elsevier, Biomass and bioenergy

Ashrae 2003: HVAC operation

Energy bills 2005-2006
-workshop (boilerhouse 3)
-restaurant (boilerhouse 2)
-college (electric heaters)
-Kirkmichael (Gas oil boiler)
-Nith/Cree (Kerosene boiler)
Barony's training centre, the farms and the cottage were not taken into account in the analysis.

The Energy Management Register

CIBSE Guide A

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