How to choose and run boilers to optimise energy efficiency in heat networks

The UK has committed to an 80% reduction of carbon emissions compared to the 1990 baseline by 2050. To achieve this, the Government has set five-yearly carbon budgets.

The UK has made significant progress on carbon emissions which have fallen by 42% from 1990 to 2016. Interestingly, we have also seen energy demand growth break away from growth in GDP (energy demand usually increases with GDP). Although the Climate Change Committee (CCC) which monitors the emissions, praises the progress, also warned the UK is currently not set to meet the fourth (2023-27) and fifth (2028-32) carbon budget. The Clean Growth Strategy published last year was acknowledged as a good start. However, the CCC called for the policies and proposals set out in the Strategy to be firmed up and additional measures to be identified to close gaps between the carbon budgets.

The Clean Growth Strategy outlines heat networks as one of the core initiatives to reduce carbon emissions in the heating sector. In this article, we’re looking at how modular boilers and correct setup can help to provide a highly energy-efficient solution alongside CHP units to achieve heat network efficiency targets.

Backing up the heat supply

A popular choice in heat networks are Combined Heat and Power (CHP) plants. A CHP unit will be running continuously to meet the base load heating requirements which, due to colder temperatures, are high in winter and lower in the summer. To provide a backup to the CHP, compact modular boilers present an energy efficient solution to cover the baseload present all year. Furthermore, they deliver top up heat to effectively meet shoulder demand (spring and autumn) and peak demands in winter while saving valuable space in the plant room.

Continuous product development over the last few decades has made it possible to achieve an output of 1MW from less than 1 square metre of floor space with modular boiler systems. Good news for cities, as it effectively reduces the room a boiler is taking up which can be used for other purposes (more flats or buildings). Compared to larger high output (and high water content) boilers which used to deliver this kind of output, compact modular boilers benefit from a low water content and hence a quicker heat up time. Due to multiple boilers being used, a continuous heat supply is also ensured should one module break down or be serviced.

Considering costs, space and environmental impact of heating equipment

With prices in parts of London, such as e. g. the West End reaching £1,075 per m² of floor space per year[1] for a rented office, compact heating equipment working alongside a CHP is almost a necessity. Smaller plant rooms have become common in bigger cities, calling for flexible heating solutions with high output requirements.

It is however not just the footprint but also the flexibility and efficiency that benefit buildings equipped with modular boilers.

Access can be an issue, often the only route to the plant room is through tight stairways and narrow doorways.

One of the benefits is that every boiler module can be a separate boiler installed alongside another in a horizontal arrangement, or as a vertical stack. This means a flexible arrangement is possible even in city centre plant rooms. They can be stacked on top of, or next to each other. To address access issues, boiler modules are easily separable and can be moved to the place of use. Compared to larger high output boilers, the use of a crane or other expensive equipment to get the boiler into a building may not be necessary, reducing the cost even further. This may also remove the need for road closures, another win for city centres.

How do multiple boiler arrangements achieve high efficiencies?

Several boiler modules have the advantage of working at lower loads together compared to a large output boiler working at full load on its own

In multiple boiler arrangements, the boilers can be set to operate in different modes. In unison control, the controller attempts to hold as many boiler modules firing at the same time to match the base load of the building. The aim of this setting is to have them all modulate to more efficient low fire together to match the system load which offers higher operating efficiencies and hence lower CO2 emissions. When set to cascade control, the controller attempts to match the base load with as few boiler modules as possible. This means the next boiler module is switched on once the previous reaches 100% of its capacity. The controller alternates operation of the boilers to ensure even use which helps the long-term reliability of the system.

Wide differential temperatures and boiler size evolution

In the case of condensing boilers, operating temperature and return temperature then come into play. Condensing boilers are most efficient when they ‘are able to’ condense. That’s why it is even more important to have a wide temperature differential with low return temperature to ensure it is operating in condensing mode (e. g. 70/40°C). This follows the CIBSE Guidance AM12 – Combined Heat and Power for Buildings recommendation of a minimum of 30°C differential temperature (delta T) for efficient district heating network design. Some modern modular boilers can achieve differential temperatures of up to 40°C. This means maximised condensing operation going hand in hand with savings in energy and costs, whilst being compact enough to fit through a single doorway. A higher operating temperature to compensate for heat losses in combination with a wide temperature differential and low return temperature achieves better condensing performance and is recommended by CIBSE/ADE CP1 Heat Networks: Code of Practice for the UK. A system could then run at 90°C with a return temperature of 50°C to condense most efficiently – given the modular boiler’s maximum operating temperature is 90°C with a delta T of 40°C.

Thanks to product improvements, the formation of thermal NOx can also be decreased significantly. This means NOx emissions as low as below 40 mg/kWh help to achieve maximum BREEAM credits while reducing the impact of heating equipment on air quality.

A modular energy- and space-saving solution for heat networks

Thanks to their ability to operate at more favourable lower load conditions, high operating, wide differential and low return temperatures, modular boilers present a highly energy-efficient and reliable solution to support CHP plants in heat networks. They can overcome access issues in city centre plant rooms, owing to their flexible stacking options and compact size. A small footprint in terms of floor space effectively reduces the space needed for heating equipment. Lastly, low NOx emissions alleviate the negative impact on air quality and reduced CO2 emissions help achieve the Government’s aims as set out in the Clean Growth Strategy and carbon budgets.