HEATBANK Minibems District Thermal Store
This article covers the options currently available for Heat Bank systems designed for district heating and fitted with the new Minibems control systems.
- 1 Summary
- 2 Related Articles
- 3 Specification
- 4 Control Features
- 5 Schematic
- 6 Output Capabilities
- 7 Plate Heat Exchanger Selection
- 8 Minibems
- 9 Wiring
- 10 Distributed Heat Input of Any Type
The use of storage within a district system provides a number of advantages, including the ability to let the district heating network go cold during periods of low demand, while been able to deliver decent hot water performance at all times without a need for a keep warm (trickle flow) load on the network.
This system provides 170 litres of primary (district) storage near the point of demand that can be used for both DHW and central heating.
It also makes use on the new Minibems control system, using pump speed control to achieve target temperatures and minimise return temperatures. The Minibems talks to the plant and allows the entire network to be intelligently managed, so that stores only call for heat when needed, and work in unison to they will recharge at the same time.
Removing the DHW peaks from the heat network (now satisfied by stored heat) allows the heat network to be downsized, with pipework now sized for the average load rather than the peak. Furthermore, the very low resulting return temperatures from such a system further reduce pipe sizing.
In addition, with load profiles known in advance and no need to instantaneous response from plant, it is possible to remove the need for centralised storage in plant rooms. It becomes possible to plug in heat sources to the network at any point and manage them intelligently alongside all other heat sources.
This initial model of the system is based around a unit only 800mm in height, with the aim that it can be located in a cupboard while leaving space above it for the remaining domestic service equipment.
The initial requirement is as follows:
- 170 litre buffer store
- 800mm height, 600mm diameter plus 100mm insulation (800mm overall diameter)
- Connects directly to the district heat network
- Provides DHW via a plate heat exchanger
- Provides temperature controlled heating circuit
- Pump speed control on both outputs
- Minibems controller
- Variable control over volumes of water heated up
- Variable control over rate of reheat
- Variable control over flow rate and temperature delivered to central heating
- Return temperature limitation on central heating
- Isolation from primary system upon leak detection
- Drive heating either directly from network, from stored heat, or a combination
- Option to use lower temperature DHW delivery, with periodic sterilisation
- Manage times when stores recover to tie in with peaks in DHW demand
- Run network at weather compensated central heating temperatures
- Allow network to turn off when no central heating demand
- Management of heat input (boilers, CHP, heat pumps) to match demand temperatures
- Management of heat input to match demand flow to overcome need for plantroom storage
- Management of network circulation pumps (duty / assist / standby)
The following schematic shows a refined version where the heating flow and return re piped directly off the input pipework rather than the buffer. This allows the primary input to drop to weather compensated temperatures and drive the heating load without affecting higher grade stored water. The control system will adjust the primary input flow rate to match that drawn by the heating circuit. When the primary is isolates, the heating can still draw from the buffer store.
The calculation below shows how with a primary input of 24kW, approximately 7 litres per minute, we can deliver 9 showers and 2 baths within an hour, with 4 side by side. With diversity this would cover up to 10 properties.
Once the system has recovered following such a heavy draw period, about 10 minutes later, the heat network can shut down, assuming no central heating demand. Or if there is heating it can drop to a weather compensated temperature.
Each store will be able to deliver three more showers before calling for heat, at which point the network will need to respond. One would expect to provide three periods during the day when the primaries are running at normal temperatures, to coincide with peak demands - morning, lunch and evening. Stores will co-operate to ensure they all recover as quickly as available heat allows, prioritising those with heavier loads.
With one HEATBANK Minibems feeding five properties, you save four cupboard spaces, and centralise the system in a central cupboard space to enable access for servicing. Equipment costs go down, primary pipework sizes shrink, and the heat network can sit cold for much of the time instead of running hot 24/7.
Plate Heat Exchanger Selection
The following heat exchanger is selected for low end DHW loads of up to 12 litres per minute from a 70C primary.
The following heat exchanger is selected for higher end DHW flow rates of 30 litres per minute, but can also deliver lower end DHW demands using much lower primary supply temperatures, so is the preferred choice. The capacity to deliver 30 litres per minute at 55C, equates to driving five showers side by side. With DHW diversity this means the unit should be capable of driving the hot water to between 5 and 10 properties, depending on if they are two or one bed.
What is Minibems?
Minibems is a holistic control and monitoring platform which can deliver significant energy and carbon reductions in a complex heat network environment. Minibems is a building energy management system that will monitor and control your heat network to reduce cost, improve efficiency and increase ease of management.
Crucial operating and efficiency data
With remote access to real-time and historical data from key points in the heat network, as well as visibility of energy usage from anywhere with internet access, Minibems makes the invisible, visible.
Energy and carbon saving functionality
Minibems monitors and controls each part of the heat network to deliver energy and cost efficiencies across the board.
Applicable to all heating sources (including biomass, heat pumps, solar thermal and gas condensing boilers), Minibems comes pre-coded with the specific ideal operating conditions for each application, and can adapt usage according to the requirements of the heat network and environment.
Cloud-based heat network control
Minibems controls can be managed remotely from anywhere there is an internet connection. This enables residents to monitor their own energy usage; helps service engineers to monitor heating and hot water usage remotely, as well as diagnose faults without the need to visit site; and gives portfolio managers a single point of data access for all sites under their supervision.
More information can be found on the Minibems Website
Distributed Heat Input of Any Type
Once we have made the break from having to continuously supply high temperature heat for instant DHW generation, it becomes possible to run the network at temperatures better suited to heat sources (providing their is some load that can be satisfied).
This makes it also possible to pull in heat sources such as heat pumps or solar thermal, plugging them into the network where easiest. The following schematic provides a feel for the principle.
In this case we have shown a mix of single property loads, and properties grouped together from a single store. It should be noted that HIUs can also be fed from the storage using something like our SLIM HIU.
The control philosophy in this case would be to use solar input first to satisfy (or preheat) stores, followed by the use of the heat pump and then finally the boilers.
The CHP would typically be on all the time if possible for continuous electricity generation.
For example, the solar input, stored in a separate store, would be fed in after bedtime, when there is little DHW load, and stores are at a reduced temperature from the evening DHW demand. This heat would be fairly rapidly dumped into remote stores, with the system then switching to heat pumps to make use of cheap rate electricity overnight.
Come the morning once DHW load starts up, the boilers can come online to bring all stores up to full temperature and assist with demand.
At all other times, assuming the stores are charged enough to cope with short term demand, the heat source would be selected to match the weather compensated heating supply temperatures if there is a heating demand. If not, then the network is let to go cold, and will wait until a store calls for more heat, at which point all stores that can take heat will be recovered as rapidly as possible to let the network relax again.
Further variations on the theme are naturally possible.