Size matters when it comes to CHP

To benefit from CHP’s potential to increase energy efficiency, it must be correctly sized and appropriate to a building’s heat and hot water demands. Gary Stoddart explains.

The market for combined heat and power (CHP) is on the up as more and more building managers discover the benefits this technology can offer in commercial, industrial and large domestic applications. Its green credentials and ability to significantly reduce energy costs means that it is gaining popularity in premises that have a high demand for heating and hot water.

According to the Department for Business, Energy and Industrial Strategy’s 2016 ‘Digest of United Kingdom Energy Statistics (DUKES)’ report, over 70 per cent (1,522) of the total number of CHP schemes in 2015 were installed in buildings in the commercial, public and residential sectors1. About half of these schemes were in the leisure and hotel sectors. This is not surprising, considering that leisure and fitness centres have a high load requirement for heating and hot water.

CHP delivers savings in two ways – by producing electricity using natural gas rather than purchasing it from the grid, and through the generation of heat. In comparison to buying from the grid, CHP electricity doesn’t suffer losses resulting from moving power over large distances, and costs three to four times less2 . If all the electricity produced by a CHP system is used inside the building, the most cost-effective and energy efficient results can be attained.

With an energy cost ratio of around 3.5/1, the savings are calculated using the cost of energy input to the unit (natural gas), the value of the heat output (thermal) and the electricity produced. Overall, it means CHP can be approximately 30 per cent more efficient than traditional electricity3. Typical payback periods are around seven to 10 years, but five is possible.

To give an idea of how much CHP can save end users, the Sustainable Development Unit (SDU) for NHS England and Public Health England said in its 2016 report, ‘Securing Healthy Returns’, that CHP provides the highest annual potential cost saving in comparison to 18 other energy-saving measures covered in the report.

However, to maximise the predicted efficiency and savings, it is essential to first consider whether CHP is appropriate for the building in question, look at the usage requirements and size the CHP system accordingly. Applying the age old ‘10 per cent for luck’ rule, thereby installing larger plant than what a building needs, not only increases the cost of capital expenditure unnecessarily, but also means that a CHP unit operates less efficiently.

Furthermore, oversized CHP units will not run if the heat demand is not present, and as a result the anticipated electricity will not be generated, leaving building owners with higher operating costs and ultimately disappointed with the performance of their equipment. This is why getting the size right matters because it is key to achieving optimum efficiency, lowering energy bills and reducing carbon emissions.

The golden rule for CHP is actually to keep it as small as possible. Sizing the system so that the base load heating 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. To achieve maximum efficiency, CHP units should be considered in conjunction with condensing boilers, so that additional heat demand can be met by the latter during peak periods when required.

When calculating the size of the system, the Building Regulations state that 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 nonoperation of the CHP or the heat produced being ‘dumped’ in order to continue generating electricity.

A cascade arrangement can be a way to meet fluctuating demands, as this can respond quickly to changes in usage patterns. Cascading CHP units also has benefits when one unit requires maintenance or repair, as there is no interruption to the building’s heating, as an additional unit/s can take on the base load.

The best way to ensure that CHP works as intended and delivers maximum savings is for all parties to work together at the early stages of a project. Indeed, it is important that heating engineers are included in the conversation from the outset so they can feed in the necessary information in choosing the right solution. It is also important for the CHP supplier to stay involved with the project after commissioning, to ensure that the system is being used as intended and is performing efficiently.

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