Heat networks — how low can you go?
Maintaining the low system temperatures required for efficient operation of heat networks presents a design challenge for building-services engineers. Silas Flytkjaer of SAV Systems highlights the importance of effective temperature and pressure control.
It is now well accepted that district-heating networks will play an important role in reducing the carbon emissions associated with heating systems as part of the UK’s carbon reduction strategy. Heat networks are also central to the European heating and cooling plan.
Building-services engineers need to address a number of important criteria if they are to optimise efficiency, as highlighted in the CIBSE publication’ Heat networks: code of practice for the UK (2015)’, which recognises that low system temperatures are essential.
They also need to ask themselves, ‘How low can we go?’ CIBSE currently recommends flow and return temperatures of 70/40°C to improve efficiency compared to the more traditional 80/60°C. Evidence now suggests that efficiencies will be further improved by going even lower.
International Energy Agency (IEA) studies based on long -term measurement show the network heat loss in a low temperature network is about 25% of that in a conventional medium temperature network (defined as 80/40°C) — underlining the technical and economic feasibility of low-temperature heat networks.
For example, two residential district-heating schemes in Denmark, operating with flow/return water temperatures of 55/35°C, have each seen a fall in heat losses from the distribution network of around 75% due to lower system temperatures and improved pipework insulation. Such low temperatures also increase the efficiency of condensing boilers and facilitate use of low-carbon heating technologies such as heat pumps, which operate at lower temperatures.
Whether the design aims for 70/40°C or 55/35°C, achieving and maintaining these low system temperatures requires excellent control throughout the system. This includes control at the heat interface units (HIUs) serving each space and the adjustable valves within the spaces, such as thermostatic radiator valves (TRVs).
|How the consumer side of a heat interface unit is controlled affects the primary system.|
For instance, CIBSE AM12/2013 ‘Combined heat and power for buildings’ recommends variable-volume control for district-heating systems, with low flow rates to maximise heat exchange at the terminal unit. This helps to maintain a high differential between flow and return temperatures, ensuring that return temperatures remain low under part-load conditions, while also reducing the energy consumed by pumps.
However, variable-speed pumping will result in varying pressures at different points of the system, such as at the HIUs and TRVs. Attempts to compensate for these variables often result in high system temperatures, low ∆T, unmanaged pressure variations in the system and compromised performance.
Control at the HIU
One part of the answer is to include differential-pressure control valves (DPCVs) within the HIUs — in both the heating and hot-water circuits. DPCVs respond dynamically to peaks and troughs in pressure, maintaining a constant pressure differential and enabling the 2-port control valves to operate as designed.
Incorporating effective temperature and pressure control in the HIU also tackles the common problem of fluctuating hot-water temperature. Historically some systems have varied by as much as ±15 K. HIUs with differential-pressure control can control water temperature to within ±2K. An integral idle temperature controller in the control valve will ensure that water in the supply pipe remains warm.
Control at the TRV
It is equally important to ensure effective control of radiator circuits; as indicated earlier, standard TRVs are not designed to function with variable pressures. Even when HIUs are used to provide hydraulic separation between the heated space and the heat network, the return-water temperature of the distribution system is still dependent on the return temperatures from the heated spaces.
A key issue here is that if a radiator is not controlled properly it may become an unintentional bypass. One unintended bypass in the system can lead to elevated return-water temperatures and poor control of differential pressure. The inevitable result is uneven heat distribution through the system, resulting in an overall reduction in efficiency of the entire heat network. Poor performance can be further compounded by adjustments to TRVs made by occupiers or facilities managers, making the situation even more complex.
|TRVs in systems served by HIUs with variable-speed pumps should be both pressure-compensated and temperature-compensated — such as SAV’s PT40.|
The solution is to ensure that the TRVs are both pressure-compensated and temperature-compensated, using an integral differential-pressure control to ensure that only the required flow passes through each radiator in both full and part load conditions. This provides a responsive system where radiator circuits are dynamically balanced, regardless of pressure variations or changing demand.
Such TRVs should be designed to operate with low flow rates and incorporate integral adjustable apertures that can be pre-set to the required flow rate. This will prevent any single radiator acting as a system bypass.
It is probably true to say that ‘perfect’ control of each system element is rarely achievable, but it is possible to get close to this nirvana by addressing these issues and incorporating appropriate pressure and temperature control in the system.
The key is to adopt an holistic design process and ‘think low’ when it comes to system temperatures.
Silas Flytkjaer is product manager for Danfoss FlatStations at SAV Systems.