Good things come in small packages

It is often said that the best things come in small packages, and nowhere is that more true than in the installation of combined heat and power (CHP). In fact, the most successful CHP installations in the UK have been kept purposefully small. Gary Stoddart, general manager at SenerTec, explains why.

As facilities managers will know, the heating and hot water systems in a commercial building is responsible for the lion’s share of both carbon emissions and energy use, and a large proportion of costs out of the business. However, with a recent report highlighting that average total carbon emissions are 3.8 times higher than those estimated at design stage, it’s clear that the ongoing battle to match up a building’s predicted and actual energy use has to be won if we are to reduce emissions in line with the government’s ambitious carbon targets.

The key for heating and hot water systems largely lies within sizing correctly for each and every application, and CHP is no exception.

CHP can help to achieve carbon reduction either as part of a standalone heating system, or as part of a heat network. It delivers savings in two ways – by producing electricity using natural gas rather than purchasing electricity from the grid, and through the generation of heat.

In comparison to electricity from the grid, CHP electricity doesn’t suffer transmission losses, and costs three or four times less. At the same time, where the ‘waste’ heat from the generation process is lost at gas power stations, the heat generated by CHP can be reused in the heating/hot water systems – normally most effective when CHP is designated as the lead heat source. Overall, it means CHP can be approximately 30 per cent more efficient than traditional electricity.

But in order to achieve the predicted savings, it is essential to first consider whether CHP is appropriate for the building in question, has been specified and installed correctly, and used as intended.

A major consideration for building and facilities managers is the need to rely on the heating/hot water system’s ability to continue to run efficiently, even when demand is low. Because oversized CHP units will not run if the heat demand is not present, and as a result the anticipated electricity will not be generated, this is a particularly important issue for CHP.

Unfortunately, incorrect specification of CHP is all too common. In many cases, the ‘10 per cent for luck’ rule is applied, meaning that many buildings have larger plant than required. The equipment is of course more expensive in the first place, but in addition will not operate efficiently, leading to higher operational costs.

The golden rule for CHP is to keep it as small as possible. Sizing the system so that the base load heating and hot water requirements can be met largely by the CHP unit alone will result in the continuous generation of low cost, low carbon electricity as a by-product of producing base load heating.

According to Building Regulations, CHP must be sized to provide ‘not less than 45 per cent of the annual total heating demand’, which includes space heating and domestic hot water. This is critical for CHP as oversizing for summertime loads can result in either non-operation of the CHP or the heat produced being ‘dumped’ in order to continue generating electricity.

To ensure the CHP runs continually throughout the summer months the hot water requirement should be the initial target for CHP sizing. Here, the risk is that 45 per cent of the total annual heating demand is mistaken for 45 per cent of the central heating load, which is not the case as the hot water loading is all year round and con be a significant part of the annual fuel usage.

Building managers also need to know that the system can react swiftly, accurately and efficiently to changes in heating demand. Cascades are rapidly emerging as an ideal way to deliver these performance benefits, as they can respond rapidly to changes in usage patterns, and extend the operating life and reliability of the equipment.

What’s more, a cascade of CHP units offers end users the option of shutting down parts of the system when there’s a fall in demand or maintenance required, meaning that the remaining units can continue to run at full capacity and maximum efficiency – unlike modulating CHP units.

The best way to ensure that CHP works as intended is for all parties to work together at the early stages of a project. It is also important for the CHP supplier to stay involved with the project after commissioning, to ensure that the system continues to perform and deliver savings. SenerTec offers a range of maintenance and monitoring solutions, meaning we can work with the building operator to ensure the system runs efficiently.

With increasing take-up of CHP technology across commercial building stock and its potential as a low carbon technology to help realise emission targets, it is vital that facilities managers are up-to-date with the latest guidance on how a system should be sized if they are to guard against ill-fitting solutions and costly malpractice.

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