A flexible approach to CHP

The flexibility to respond to the needs of the building is critical to achieving maximum benefit from CHP. Beata Blachut explains how this principle can be applied to both large and small projects

Sustainability continues to be a key driver for many public and private sector organisations, with energy consumption and carbon emissions often being the sustainability components that receive most attention. This focus on energy is reinforced by the Building Regulations, while the 2011 version of BREEAM (BRE Environmental Assessment Method) places greater emphasis on reducing carbon emissions than the 2008 version.

Participants in the Carbon Reduction Commitment Energy Efficiency Scheme also have a strong incentive to reduce carbon emissions, thereby minimising the cost of carbon allowances and improving performance in league tables. In parallel, local planning requirements often require inclusion of low or zero carbon (LZC) technologies.

So the scene is clearly set for the wider use of LZC technologies and combined heat and power (CHP) is proving to be an increasingly popular choice. Nor is CHP confined to large projects – a common misconception. With modern modular, modulating systems many smaller projects can also reap the rewards of lower energy consumption and carbon emissions. Typical examples of smaller facilities include multi-residential housing, schools, nursing homes, leisure centres and hospitals.

Typically, CHP will be at the heart of a central energy centre that also incorporates other heat sources, which together serve a large or small district heating system with hot water for space heating and domestic hot water (DHW). Power generated by the CHP may be used locally, exported to the grid or a combination of the two.

According to the Carbon Trust, CHP technology can potentially reduce carbon emissions and energy costs by 30 per cent, compared to using gas-fired heating plant and mains electricity. There are two key reasons for this. First, CHP captures the heat generated by power generation and uses it for heating water. Second, a proportion of the site’s electricity requirements are met using mains gas, which is more cost-efficient than mains electricity.

The key point here is to ensure the ‘potential’ savings are turned into real savings. And this will only happen if the CHP system is designed and configured to deliver maximum efficiency. That may sound fairly obvious but overall efficiency doesn’t just come down to the ability of the CHP plant to generate heat and power efficiently. It is also influenced by the alignment of the CHP plant’s operation to the needs of the building(s) and how the electricity and hot water are used. This is where the flexibility referred to above comes in.

For example, large-scale fixed-output CHP systems need to be carefully matched to the anticipated electricity and heating loads. As a result of this inflexibility, they are usually sized to match the site’s base electrical load and do not contribute to the site’s electricity usage beyond that base load. Consequently they may only achieve relatively small reductions in overall energy usage at the site.

An alternative is to use CHP units that are able to modulate their output, using small-scale CHP units in a modular configuration (in the same way that groups of modular boilers are more responsive than large, single boilers). For example, a CHP unit that can modulate down to 40 per cent of its maximum electrical power output will ensure that the electricity generated never exceeds demand, so there is no need to ‘dump’ heat or sell electricity back to the grid at unfavourable rates.

To put this into perspective, we recently evaluated the options for a small leisure centre. With conventional CHP sized to cover base electrical demand only the CHP would only provide 39 per cent of site electricity usage, resulting in energy and carbon savings of around 10 per cent. In contrast, using a modulating CHP system to track site demand and modulate output accordingly, 80 per cent of the site’s electrical demand could be met.

At the heart of such a system is the monitoring and control of the system, tracking the requirements of the load demand in real time and ‘learning’ as the system operates to ensure consistent optimal performance.

Sophisticated controls also allow the overall operating strategy to be aligned to the building’s most significant demands to maximise savings. So, in the example above the system was optimised for electricity demand, but systems may also be optimised for heating/DHW demand. They may also be ‘economy’ led, based on actual fuel and electricity prices to gain maximum economic benefits.

Experience shows that a modulating CHP system will consistently deliver energy savings over and above those delivered by a fixed-output CHP system.

In parallel with ensuring the central plant is efficient and responsive, the hot water for space heating or DHW also needs to be optimised. Here, use of pre-insulated heat interface units using patented valve technology will enable both heating and DHW needs to be met directly from the hot water generated by the CHP, without the need for separate hot water storage in the spaces requiring DHW.

In this configuration the heating circuit is designed for direct generation of heat, with individual temperature control in each room/space. This can also be combined with timer control for individual spaces or zones, if required.

DHW is heated via a heat exchanger in the heat interface unit, cooling the water from the central heat source to a safe temperature for DHW, while compensating for variable loads, supply temperatures and differential pressures. Heat interface units can also be fitted with integrated idle temperature controllers to ensure that DHW is responsive at times when space heating loads are low.

It is this attention to detail, from the flexibility and responsiveness of the central plant to the smart use of hot water distributed for heating and DHW that will enable CHP to deliver maximum financial and environmental benefits.

• Beata Blachut is CHP product manager with SAV Systems


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