Load Shedding and Shifting on Heat Networks
Relates to: Hot Water Priority, Communal Heating, District Heating, Load Shedding, Load Shifting, Remote Management
With remotely connected HIU's it becomes possible to manage ongoing central heating loads, thereby reducing peaks in load and allowing for reduced pipe sizing and buffer sizing. Where boilers are sized to meet the peak loads, then the function also impacts on boiler size significantly.
Peak load is calculated from peak diversified hot water loads, plus peak central heating loads. The hot water loads are generally fixed - you can't ask someone not to run a tap - but the heating loads require significant examining. It is most likely the case that the peak heating load has not been taken from the steady state losses of the building, but rather are based on initial heat-up times with every radiator drawing peak design load. As such the design peak may be five times the realistic peak loads.
Having the ability to manage the heating loads means one can 'flatten the curve' on heating loads, preventing radiators from taking excessive start-up loads. There is no need to over-size systems just in case everyone turns heating on at the same time, but instead the loads can be limited to a reasonable building heat up rate.
Furthermore, it is no longer necessary to add together both peaks, but rather one should size for the larger of the peaks, in the knowledge that any load above this can be delayed until after the peak.
It is estimated that in the right circumstances this function can reduce network sizing by 50%.
In essence, the function uses the thermal mass of a building in the same way as central buffer storage, using it to meet the peaks in demand.
Meeting Exceptional Loads
The graph below is a site of 150 HIU's on a Heatweb remote management system. At three times in the day we have told the system to turn off all central heating in order to focus all boiler output on current DH load and on recharging the buffer store.
The load is interrupted for 5 minutes, making no difference to comfort levels, but providing a virtual 1500 litres of additional storage.
This function is typically linked to a temperature sensor near the top of the central buffer store - as the store approaches empty, due to high loads, the system activates to prevent further depletion of storage.
The following graph shows the same site with a 100kW load limit in place. It can be seen that as the load goes over 100kW a calculated amount of central heating loads are temporarily turned off to prevent excessive peaks in load. With this function in place the highest load seen is roughly 118kW, however without the function the peak can be estimated at about 160kW.
As the load goes over the 100kW, the system calculates the overshoot and within 10 seconds removes that amount of central heating load. The loads removed are the hottest heating systems - i.e. those that are up to temperature and will see the least impact from loss of services. These systems have underfloor heating so it should be possible to suspend heat input for 20 minutes without a problem, however the off time here is limited to 5 minutes as peaks in load tend to be very brief.
In reality, with redundant boilers, the function rarely activates.
However, it has been seen to be especially useful in times where boilers fail and the design input is not available. At these times the function manages the system to run with the available heat input so occupants do not suffer from loss of hot water services.
If boilers experience a catastrophic failure - such as a flue problem - then it may be necessary to halt all central heating loads, instead reserving the remaining stored heat for hot water. This way it may be possible to maintain services for a number of hours (depending on buffer size) while the problem is attended to. Otherwise, the ongoing central heating load may deplete the store in less than an hour leaving everyone without hot water. Yes, the building will gradually cool down without heat input, but this is slow and may not even be noticed if boilers are fixed quickly enough.