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Cascade configurations for heating systems have the potential to optimise efficiency and make the best use of a range of heat sources, explains Kevin Stones, Engineering and Service Director with Hoval Ltd.
BOILERS & HOT WATER BSEE
A CASCADE OF OPTIONS Taking control of heat sources
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n the early stages of designing a heating system, be it a new installation or plant replacement, specifiers will typically consider a number of factors. Not least is the inherent efficiency of the central plant (eg: boilers) and the ability of the heating system to respond efficiently to variable loads.
As building standards have tightened and new buildings have become more thermally efficient, the ability to operate efficiently at part-loads is of increasing importance. The same is true, of course, of older buildings that have undergone fabric upgrades to improve thermal performance. Such a system should also have the inherent flexibility to respond efficiently to changes in building usage through the life of the building. Changes in occupancy levels, for instance can have a significant impact on demand for space heating and domestic hot water.
An established and proven approach to managing variable heat loads has been to use a cascade arrangement of modular
boilers, each of which is able to modulate. For example, if there are several boilers in a cascade – each capable of 5:1 turndown – the result is a highly adaptable and responsive system. Clearly, there needs to be an effective level of control to ensure the system delivers maximum performance and energy-saving benefits.
Pre‐fabricated
From an installation point of view, cascade systems can also be largely pre-fabricated to reduce the amount of onsite work. For example, they can be supplied as a kit complete with wall-hung boilers, low temperature hot water (LTHW) and gas pipework header(s), inter- connecting flexible LTHW and gas pipework, pumps and valves. Not only does this make installation easier, it also uses less space in the plant room.
Nor are cascade systems confined to boilers, as the cascade concept can be applied to a mix of heat sources. Thus, in recent years there have been many projects that incorporate gas or oil fired condensing modular boilers alongside low carbon heat sources such as combined heat and power (CHP), heat pumps or solar thermal. Where different heat sources are mixed in this way, the control aspects become even more important in making optimum use of each heating technology and maximising the share of the heat load met by renewable and low carbon sources.
In all cascade configurations, a primary purpose of the control strategy is to ensure the design
uA plant room with UltraGas boilers.
temperature differential (ΔT) between flow and return water temperatures is maintained. If not, the boilers will switch on and off (cycle) far too frequently so that energy is wasted, emissions increase and the life of the plant is reduced.
To maintain the design ΔT where a cascade boiler system is in use, the
boilers should come on at low-fire initially, or modulate to meet the heat load, depending on the type of burner being used. The same principle is true when boilers are being used to ‘top up’ other heat sources as times of peak demand. As noted earlier, heating plant using mixed sources is becoming increasingly popular as building operators seek to reduce their energy consumption and emissions to drive down costs and meet their sustainability agenda. Whilst there are various options available, we are finding that a popular, and highly effective, combination is to use gas-fired CHP, supplemented by gas-fired boilers.
In such cases, the CHP will typically be used to meet base heat loads and it’s important that there is a sufficient base load to operate the CHP for a minimum of three hours each time it runs. This optimises both the efficiency of the heat generation by the CHP and the benefits of onsite electrical power generation.
To create opportunities for CHP to be used with lower heat loads while maintaining acceptable run-times, smaller units with, say, capacities as low as 20kWth have now been introduced to the market. Some smaller units also have lower maintenance requirements than higher output CHP units, which has additional benefits for lifecycle costs.
Adapting to change
When the system is first installed, correct commissioning will ensure that the controls enhance the inherent efficiency of the heating plant and the design of the distribution system. Over time, though, building usage will almost certainly change and if efficiency is to be maintained the control strategy must adapt through what
might be termed ‘continuous commissioning’.
Also, if the system was first commissioned to meet the heat loads of a new shell and core building, then it should be re-commissioned in line with changes to heat loads as tenants move in.
This is where the inherent flexibility of the system, referred to earlier, comes into play – insofar as it should be relatively easy to re-commission the system in response to such changes. Routine maintenance should therefore include regular assessment of the control configuration. However, with the growing popularity of systems with multiple conventional and low carbon heat sources, trying to achieve this by simply tweaking traditional controllers on each item of plant will meet with very limited success. To that end there are now controllers available with new levels of modularity and connectivity that make re-commissioning very straightforward, even when multiple heat sources in cascade configuration are in use.
This is further facilitated by the use of remote monitoring, which can be easily implemented for a range of heating plant. This would also be beneficial for those maintenance service providers that have guaranteed the energy performance of the plant they maintain.
Consequently, it is essential that specifiers give due consideration to control aspects at the early stages of designing a heating system. In particular, they need to specify control systems that are able to meet this broader remit by taking control of a wide range of heat sources, and recognise that heating control is a ‘work in progress’ that will continue through the life of the system.
www.hoval.co.uk
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BUILDING SERVICES & ENVIRONMENTAL ENGINEER JULY 2017 19
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