Distributed Communal Heating
Distributed, or De-Centralised, District Heating has a number of benefits:
- Boilers located near flue position rather than plantroom position.
- Higher redundancy
- Better efficiencies
- Lower cost
At present we tend to combine multiple boilers into one large boiler system that then goes through a single pump set and off to drive a network. We then have problems matching very low loads.
Better to keep each boiler completely independent, with no low loss headers, and no larger pumps than a heat source requires.
Storage allows the network to cope with the lowest, highest, and most rapid of instantaneous loads.
Storage sites 'off' the network, drawing and injecting heat depending on the situation.
Each boiler and thermal store acts as a separate 'boiler' in a larger sequenced boiler system. Rather that a central system orchestrating sequencing, each unit can operate independently using system DP for signals as to the network status.
The larger boilers will have the highest head pumps, so can always take priority by driving up pump head.
The thermal stores have a maximum pump head of 150kPa, so are unable to override a boiler pump capable of 180kPa or greater.
So, when system DP goes over 150kPa we know a boiler is active, and any active stores can ramp down pump head and turn off, or switch to recharge mode if system DP is over 170kPa.
Store pumps can ramp down after a random number of seconds (30s + 1 to 60s) after coming online. This is to detect if they are doubling up where not required. If DP is maintained is will be from another store, so this store can turn off. The last system to ramp down will see a drop in DP as is does so, and can postpone 'last-man-alive' test.
Priority will also be given to stores that are full over those that are empty. This is achieved by linking the pump DP to energy stored. The store with the highest storage level, and hence the highest DP, with remain on when last-man-alive test is performed.
As a store approaches empty the pump head ramps down, and another store (or more) will activate once DP drops low enough (below 140kPa). If all stores are empty then DP will drop below 130kPa and boilers can fire up.
