Decentralised heating plant was once the order of the day, but it is increasingly common to create centralised energy centres using a variety of heat sources. Geoff Lord explains
Some years ago there was a trend in building services to replace large central boiler rooms serving a number of buildings with smaller, decentralised heating plant distributed around the site. The reason for this was that smaller boilers were deemed to be more responsive and offer higher efficiencies, particularly when dealing with variable heating loads.

Now we are seeing this situation reversed as many projects that serve more than one building are turning to central energy centres to generate the heat for space heating and domestic hot water and distribute it via a district heating network. Using modern plant, this approach offers a number of benefits.
First, modern gas, oil and biomass boilers are now much more efficient than their predecessors, even when dealing with variable loads and particularly when combined with advanced control systems. The same is true for other heat sources that may be used, such as combined heat and power (CHP).

Low carbon energy sources

Secondly, most such projects are required to include additional low carbon energy sources to comply with planning regulations or other legislation such as Part L or the CRC Energy Efficiency Scheme. Furthermore, many companies are now very keen to adopt a sustainable approach to the management of their buildings.

In addition, there are financial advantages to using heating technologies that qualify for the Renewable Heat Incentive (RHI). Under the RHI, the Government pays users of heating technologies such as biomass, solar thermal, heat pumps and qualifying CHP installations for the amount of heat they produce. This has important implications for the design of heating systems and the return on investment calculations that inevitably accompany them.

Consequently, a modern district heating system may derive its heat from a number of different heat sources and these could include CHP, gas, oil and biomass boilers, solar thermal heating and heat pumps. Clearly, it is much easier to control these and optimise the performance of each at a central location.

Nor is this approach confined to large projects - even relatively small commercial complexes, retail centres, industrial parks and even housing developments can take advantage of the benefits of centralised energy centres.

The important thing is to ensure that each of the heat sources is used to optimum effect, and balancing the heat loads requires a good understanding of projected building usage and seasonal variation and of the various heating technologies that may be deployed. For example, a commercial development will usually experience the highest heating and hot water demand during the day, with only limited demand at other times.

In contrast, a mixed retail and housing development will, overall, have a much more even demand, switching from the retail premises during the day to the residential properties in the evening.

However, at weekends during the winter there will be high demand from both retail and residential properties so the system needs to be able to cope with all of this variation while maintaining optimum efficiency.

In the case of the space heating, for instance, it will make sense to meet base heating loads using technologies that offer maximum efficiency when operating at full capacity, such as CHP and/or biomass boilers.

Meeting peak loads

It may then be most efficient to use a gas-fired condensing boiler to meet peak loads as these offer high efficiency and a constant flow temperature at variable heat loads. Thus, a typical scenario might use a CHP unit sized on its thermal output to meet a constant base load throughout the year, with the power generated being exported to the grid or used on site.

As the demand for heating increases during colder weather, biomass boilers may be used to meet this higher base load, so that variation in base loads throughout the year is met with a very efficient solution that also generates revenue through the RHI. Peak loads can then be met by fully modulating gas or oil boilers to achieve optimum efficiency at low firing.

In the case of domestic hot water (DHW) - where, again, demand will vary through the day - stainless steel buffers fed by minimum storage, high output calorifiers or plate heat exchangers will provide a flexible solution. The cold mains water may be pre-heated using solar thermal or heat pump systems and then 'topped up' using high efficiency boilers.

All of this adds up to an increasingly complex situation for building services engineers, where meeting heat loads efficiently is now only part of the picture. The requirement by building operators to achieve a good BREEAM rating, cut their carbon emissions and reduce their spend on carbon allowances - all while ensuring compliance with legislation - are powerful drivers for mixed source, centralised energy centres. It is the skill of the system designer combined with the experience and expertise of specialist companies that will make it all work.

• Geoff Lord is national business development manager at Hoval