BSEE


ith tighter environmental goals and important policy changes ahead, from the introduction of Streamlined Energy & Carbon Reporting to the Committee of Climate Change’s Net Zero roadmap to carbon neutrality in the UK by 2050, energy efficiency will be top of the agenda for forward-


COMBINED HEAT & POWER


Maxing returns with CHP W


thinking organisations.


Increasing the energy efficiency of the UK’s building stock will make important strides towards achieving our environmental goals, and heat is a natural starting point for improvement.


As each building will have its own unique requirements, the building services industry is well placed to advise on how best to reduce energy use while ensuring a comfortable building environment.


As the focus on increasing energy eciency in businesses intensifies, Ryan Kirkwood, specificaon manager for Remeha, looks at the role of CHP in improving building energy performance in a welldesigned system


While equipment such as heat pumps will increasingly be specified to provide low carbon heat in efficiently designed new properties, it’s our notoriously inefficient older buildings that present the real challenge. Due to the nature of the existing heating system, higher temperature heating solutions may prove to be more suitable in buildings like these. Added to which, on a practical, pragmatic level, there simply may not be the budget to upgrade the electricity supplies to meet the requirement of heat pumps.


Compelling economic benefits


So what options are there for improving heating efficiency in older properties? One technology that continues to offer compelling economic benefits in sites with high, constant demand for high grade heat is Combined Heat and Power (CHP).


CHP works by generating lower-cost electricity and high-grade heat on site in one highly efficient process. The potential energy and cost savings can be significant, with CHP capable of delivering a 30 per cent reduction in primary energy usage and emissions by up to 20 per cent compared with traditional generation.


But for the CHP to generate maximum returns, it’s important to ask the right questions at the design stage.


Good design


Firstly, is the CHP providing as much usable energy as possible? CHP is frequently paired with high efficiency condensing boilers to optimise system performance. Accurate sizing is critical. We recommend sizing the CHP on base load to maximise run hours and prevent cycling of the CHP unit, with the boilers meeting peak or steady state demands in winter months. When it comes to system design, ensuring good integration is essential to enable both components to perform at optimum efficiency. There are two main methods of integrating a CHP with condensing boilers – in series or in parallel. With both, the design must ensure that the CHP unit operates as the lead ‘boiler’ and that the condensing boilers are not brought on too early. Typically, this means using the CHP as a preheat for the boilers so that the CHP sees the load first.


Now let’s consider the implications of the design. Is the CHP condensing? Are the boilers condensing? In the case of condensing boilers, for full condensing to occur – where the boilers are most efficient – the return water temperature needs to be below dew point, which is normally at or below 54°C.


Depending on the temperature difference, boiler efficiency could be increased by as much as 9.5 per cent. Even lowering the return water temperature by just 10°C would increase boiler efficiency by around 4.5 per cent (see chart below). The location of the hydraulic connection could therefore have a major impact on the operation and seasonal efficiency of both the boilers and the CHP.


uFull load  boiler and CHP receive system load return temperature


Take the examples below. These designs show the CHP sized at approximately 10 per cent of thermal peak load of the building and connected in parallel with the boilers, but with a separate return used exclusively for the CHP.


On complex CHP hybrid system designs, early engagement with manufacturers or suppliers is beneficial as they not only know their products but how to maximise their effectiveness. Added to which, they will often be able to provide exemplar designs that could prove especially useful on multi- component heating designs


uPart load  no li in return water to boilers


Using this design method, the boilers’ return temperatures are unaffected by the CHP, so both the CHP and the boilers can operate in full condensing mode throughout the entire load range. The success of this design requires that the CHP acts as lead boiler, that the buffer tank is configured in either two or four port pending system loading, and that the boilers are not fired too early during periods of high demand. A good control strategy is also key as it will ensure that the CHP runs as the lead boiler, maximising its operating hours and the financial returns.


Buffering the savings


A further advantage of using a split return parallel method like this is that the buffer or thermal store can be used to support peak loads, thereby increasing the overall heat output delivered by the CHP. This makes it easier for the CHP to track the building’s load during the day. The ability to discharge and charge the buffer as the loads rise and fall will hold off boiler operation as much as possible, enhancing the energy and commercial savings and performance for the end user.


Good collaboration


The renewed focus on improving energy efficiency, and heating efficiencies in particular, offers huge opportunities for the business services sector. For many organisations, CHP will offer one of the most significant and financially compelling single opportunities to reduce energy costs and NOx emissions. At Remeha we look forward to supporting designers and installers with our in-depth product and technical knowledge to achieve a best practice approach to CHP system design and ensure that the full benefits can be reaped.


Condensing the design


The holy grail is to achieve a design that enables both the CHP and boilers to operate in condensing mode, without sacrificing the efficiency of either unit.


20 BUILDING SERVICES & ENVIRONMENTAL ENGINEER OCTOBER 2019 remeha.co.uk Read the latest at: www.bsee.co.uk


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