Buffer Store Control

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The new Heat Network Guidance from CIBSE, along with CP1, have been clear about the correct way that buffer stores should be implemented in systems.

The methods laid out in guidance require boilers and heat sources to be sequenced from storage, rather than temperature error , and they must also have loading valves. These functions can prove a challenge to the best BMS installers, so the use of a standardised open-source controls library and hardware interface makes a great deal of sense, allowing the finer detail to improve (evolve) over time, rather than trying to reinvent the wheel on every installation based on an individual's understanding of both guidance and BMS software, within contract time and budget constraints.

This controller function is suitable for domestic and commercial storage systems. It can be used stand-alone, in groups, or as an interface for BMS systems.


Functionality

  • Heat sources are assigned an order, representing the preferred order in which they fire, with 1 being the lead.
  • Uncontrolled heat sources, such as solar thermal, has an order of 0. Heat will be taken if available and safety controls allow.
  • Heat sources with the same order will be run in duty/standby and rotated, pulling in the next in order as required.
  • Heat sources are fired up according to store depletion, both on volume remaining and rate of depletion.
  • Heat sources may be exercised after a period of inactivity, regardless of order.
  • Modulation will be applied to heat sources where possible to match loads and minimise cycling.
  • Low load buffering mode to minimise cycling.
  • Timing functions can be applied.
  • Includes functions for override of non-critical loads, such as central heating, when store close to empty.
  • Includes functions for activation of heat dump circuits for solar thermal and biomass.
  • Includes functions for boiler loading valve and pump control (with DP switch or sensor).
  • Includes functions for network tempering valve control.
  • BMS integration via Modbus RTU or IP.
  • Includes functions for M-Bus/Modbus heat meter reading.
  • Includes alarm functions for low/high temperature, low/high pressure, low storage, failure of heat sources.
  • Makes possible the implementation of storage management and sequencing based on volatile fuel prices using predicted loads

Hydraulic & Wiring Layouts

The following wiring diagram can be generated from the hydraulic layout above.

Concons1.png

Q&A Design

This application starts with a definition of the system to be controlled. This is done through a schematic design that in turn leads to control points, logic, and a parts list.

Q&A Modules

We arrive at the definition through a series of questions and automated calculations, using a standardised dictionary to record answers.

All open controls on this wiki follow the same open controls dictionary, so data collected can feed directly into software, and be used with different Q&A modules.

Questions can range from simple yes/no through to spreadsheets where schedules can be input, for example...


Hncalc listloads.png

Each question is referenced by a key. In our standardised dictionary (for example), nBuildings is the key for the number of buildings. It is a network design variable so comes under network/design/nBuildings as an MQTT topic or network.design.nBuildings as a JSON object.

We are in the process of refining Q&A modules to suit different applications, however the Q&A Designer for heat networks covers buffer store and heat source selection in detail and is ready to use.


Saving and Referencing Designs

Designs can be saved and opened via a simple URL link that holds the data and is provided by the Q&A. However this can get unusably large on more complex designs. To overcome this designs can be exported as a JSON file, or can be uploaded to the IPFS file system to provide a short URL that links to a saved JSON file (of any size and complexity) online. e.g. https://heatweb.mypinata.cloud/ipfs/QmegcRKHhunR6ZR4Vvo65NEMHzau8c9CNrR1GMi7YfZT3N

The CID provided by IPFS, in this case QmegcRKHhunR6ZR4Vvo65NEMHzau8c9CNrR1GMi7YfZT3N, is unique to the content. The same content uploaded from any system to any server will be given the same CID, however any change in the content will result in a different CID. As such the CID is a true unique identity of the original data set, however large.

The following schematic was generated from https://hw7.ddns.net/ui/hndesign?loadCID=QmegcRKHhunR6ZR4Vvo65NEMHzau8c9CNrR1GMi7YfZT3N

Hnbrschem1.svg

Example Q&A Input

The following table is a summary of the Q&A for the bivalent system above.

Schedule of loads [loadSchedule] {table}
Number of buildings [nBuildings] 1 buildings
Peak network flow temperature [tPeak] 65 °C
DHW network return temperature [tPriRtnDHW] 19 °C
DHW load profile [profileDHW] EST
Central heating emitter [typeEmitter] underfloor
Central heating connection [connectionCH] indirect
External temperature at peak load [tXPeak] -5 °C
Central heating network return temperature [tPriRtnCH] 35 °C
Central heating diversity [divCH] 70 %
Base temperature [baseTemp] 16
Central heating degree-days at base temperature [degDays] 1850 °days
Boilers [goBoilers] true
Air Source Heat Pumps [goASHP] true
External Heat Network Supply [goHN] true
Water Source Heat Pumps [goWSHP] false
Cooling Source Heat Pumps [goReclaim] false
Combined Heat & Power [goCHP] false
Solar Thermal [goSolar] false
Boiler fuel type [boilerFuel] gas
Boiler Emissions [boilerEmissions] 0.216 kgCO2/kWh
Available boiler outputs [listBSizes] 30, 50, 75, 100, 250, 500, 750, 1000 kW
Boiler selection [selectBoilers] 4 x 250 n x kW
Electrical Tariff [tariffHP] flat
Electrical Supply Emissions [elecEmissions] 0.233 kgCO2/kWh
ASHP refrigerant [fridgeASHP] R744
Available heat pump outputs [listASHPSizes] 30 kW
ASHP selection [selectASHP] 6 x 30 n x kW
Buffer volume [vBuffer] 2000 litres
Schedule of pipes sizes [pipeSizes] {table}
Flow pipe sizing maximum velocity [maxVelocity] 1.5 m/s
Return pipe sizing minimum velocity [minVelocity] 0.5 m/s
Save design [goSave] false


Example Calculations

From the design above, the following calculations are arrived at:

Total Properties 149 properties
Total People 444 people
Total of central heating outputs 603 kW
Average people per property [pPP] 2.979 people
Average people per building (density) [density] 444 people
Typical HIU rating for DHW [kwDHWEst] 37.5 kW
People per standard DS439 property [peepDS439] 2.3 people
Equivalent number of DS439 properties [eqPropDS439] 193.043 properties
Peak DHW Load [kwDS439] 509 kW
Volume DHW used per property per day [vPPEST] 40 litres
Volume DHW used per person per day [vPHEST] 28 litres
Average temperature Rise on DHW [tRiseEST] 35 °C
Volume drawn per day for DHW (tap) [vDHWEST] 18392 litres
Energy used per day for DHW [kwhDHWEST] 751.006 kWh
Volume used per day for DHW (primary) [vPDHW] 13993.913 litres
Buffer Storage for DHW (based on 9%) [vBuffer9] 1259.452 litres
Peak diversified (steady state) central heating load [kwCH] 422.1 kW
Energy used per day for CH [kwhCH] 10130.4 kWh
Volume used on peak load day for CH (primary) [vPCH] 289440 litres
Peak energy used per day [kwhP24] 10881.405 kWh
Peak volume used per day (primary) [vP24] 303433.913 litres
Peak load [kwPeak] 931.1 kW
Average 24h peak load [kwP24] 453.391 kW
Primary flow rate at peak DHW load [m3hDHWPeak] 9.484 m3/h
Primary flow rate at peak central heating load [m3hCHPeak] 12.06 m3/h
Primary flow rate at peak load [m3hPeak] 21.544 m3/h
Primary return temperature at peak load [tPriRtnPeak] 27.955 °C
Weighted primary return temperature on peak load day [tVWART24] 34.262 °C
Buffer storage peak energy content [bufferkWh] 86.438 kWh
Boiler quantity [boilerQty] 4 boilers
Boiler unit power [boilerkW] 250 kW
Boiler peak power (n) [boilerkWPeak] 1000 kW
Boiler power (n-1) [boilerkWxN1] 750 kW
ASHP quantity [ASHPQty] 6 ASHPs
ASHP unit power [ASHPkW] 30 kW
ASHP peak power (n) [ASHPkWPeak] 180 kW
ASHP power (n-1) [ASHPkWxN1] 150 kW
Peak power available from all sources [kWInPeak] 1180 kW
Peak power available from (n-1) sources [kWxN1] 930 kW
Excess power availiable (n) [sparekW] 726.608 kW
Oversizing (n) [ovsersizing] 160.261 %
Oversizing (n-1) [ovsersizingN1] 105.12 %
Calculated degree days per year [dDaysCalc] 1749.5 °days
Peak load day degree days [dDaysPeak] 21 °days
Annual days of full load heating [eqDaysPeak] 83.309 days
Annual energy for central heating [kwhCH365] 212738 kWh
Annual energy for domestic hot water [kwhDHW365] 274117 kWh
Annual energy utilised [kwhUsed365] 486855 kWh
Annual distribution heat losses (est at 20%) [kwhDistLoss365] 97371 kWh
Annual energy required [kwh365] 584226 kWh
Pipe bore at peak load and velocity [borePeak] 71.272 mm
Nominal main line pipe size (max velocity) [dnMain] DN80
Nominal main line pipe bore [boreMain] 73 mm
Velocity at peak load using nominal size [velocityPeak] 1.429 m/s
Maximum pipe bore at peak load and minimum velocity [borePeakMax] 123.447 mm
Maximum main line pipe size (min velocity) [dnMainMax] DN100


Sample PDF Designer Output

Click image to open PDF

QmegcRKHhunR6ZR4Vvo65NEMHzau8c9CNrR1GMi7YfZT3N.pdf

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