Buffer Store Control

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.

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 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.

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...

We are in the process of refining Q&A modules to suit different applications, however the 'Designer' for heat networks (the Q&A module) covers buffer store and heat source selection in detail and is ready to use. The following schematic was generated using this open-source Heat Network Designer software.

• https://hw7.ddns.net/ui/hndesign - bivalent system
• https://hw7.ddns.net/ui/hndesign - start new design

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

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.

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

And from this the following calculations are arrived at:

otal 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