Achieving smart system design with CHP

Combined Heat and Power systems are gaining in popularity as businesses seek to balance environmental commitments with the need for profitability. Mike Hefford looks at how to maximise the savings potential of cogeneration technology

Climate change is one of the most pressing concerns of our time. We know that the more energy we generate, the higher our energy costs and the more harmful emissions we emit yet, regardless, our energy demand continues to increase. Buildings are big users of energy with commercial buildings alone accounting for 18 per cent of the UK’s total carbon emissions. Given the environmental and financial implications of high energy use and the tighter legislation around building development and operation, greater sustainability and reduced energy consumption continue to be key drivers for public and private sector organisations. One technology that can significantly reduce both our energy usage and our emissions is Combined Heat and Power (CHP).

CHP, also known as cogeneration, converts a single fuel into both electricity and heat in a single process at the point of use. CHP systems work by burning a fuel, usually natural gas, in an engine to drive the electrical generator and provide power for the building. The hot exhaust gas produced by the engine enters a heat recovery exchanger to feed the boilers and provide LTHW for space heating and HWS generation.

Whereas conventional power stations emit approximately 50 to 60 per cent of the heat created as a by-product of electricity generation into the atmosphere, a CHP plant captures a large proportion of this waste heat and converts it for heating purposes. Electrical energy is also lost in the transmission and distribution of power to buildings. By generating both heat and power on site, CHP systems operate to higher efficiency levels, saving otherwise wasted energy.

For commercial building operators, the environmental and financial advantages of moving to CHP are significant. Most CHP units offer a total fuel efficiency of around 85 to 90 per cent compared with an efficiency figure of around 40 to 45 per cent for traditional generation. This equates to a potential saving in primary energy of around 30 per cent and an emissions reduction of 20 per cent. However, with the arrival of condensing CHP models on the market, such as Remeha’s R-Gen 20/44 and 50/100 NG units, it is possible to achieve even higher total efficiencies of between 103 and 104 per cent with correspondingly greater savings. As both heat and electricity are produced from the one fuel source, the amount of electricity the host site purchases from the grid is effectively reduced, resulting in lower utility bills. Organisations can also reap additional financial savings from carbon-related programmes such as the enhanced capital allowance scheme.

Maximising savings potential

As with all sophisticated technology, in order to achieve the maximum benefits from a CHP system, there are a number of key stages to follow.

The first step is to carry out a feasibility study and site visit. CHP is suitable for both new build developments and existing buildings, however it best suits organisations with year-round demand for space heating and electricity generation, usually running around 5,000 hours annually. This makes it the perfect solution for residential developments, hospitals, care homes, hotels and leisure centres. It is worth noting that CHP plants will still work efficiently in other buildings, however the return on investment will be longer as the financial payback period is dependent in part on the number of operating hours.

Sizing

Accurate sizing of the thermal and the electrical base load and the heat-to-power ratio is essential, although CHP units will offer between 100 and 50 per cent modulation to meet fluctuating demand. The thermal output is a by-product of the electrical output. Sizing above the thermal base load will deliver higher electrical output with the option to export this to the national grid. However, in reality this is not always financially viable as the electricity may have a lower value than that used on site. The recommendation is therefore to size the CHP unit to a true accurate heat demand and specify condensing boilers to provide the supplementary heat when required. The CHP unit operates as the lead boiler and the backup condensing boilers provide the supplementary heat requirements.

Let’s consider next how best to integrate CHP with gas condensing boilers, both in a fully-contained internal package and retrofitting a CHP unit externally onto an existing system as a ‘bolt-on’.

Integration

Once accurately sized, the heating technologies should be fully integrated through good system design to maximise the overall system efficiency. Condensing boilers work most effectively at lower water temperatures where they recover both sensible and latent heat from the flue gases. The hydraulic connection therefore has a major impact on how the system will perform. Connecting the CHP to a low loss header will help with the hydraulic integration of these different technologies. In contrast, connecting the condensing boiler onto the system heating return will preheat the return, raising the temperatures and preventing the condensing boiler from fully condensing.

Controls

Smart controls are critical to achieving the highest efficiencies from a system and can be adjusted to meet the individual requirements of a building by optimising it for heating and hot water demand or for electricity. Adding a full building energy management control system is advisable to enable more accurate monitoring and control of the system and support optimum operation.

CHP is a proven technology that offers a sustainable solution to providing efficient heat and power to organisations with significant, consistent demands for space heating and electricity generation. Nor are the benefits restricted to individual businesses, as increased uptake of CHP can have a wider impact, helping the UK meet its tough 80 per cent emission reduction target by 2050, improving our national energy security and promoting competitive energy pricing for industry growth and a halt to fuel poverty. Supported by manufacturers who are well-versed in this technology, a well-designed, well-controlled CHP system offers both a flexible solution to significant savings and a step forward in the nation’s progress towards a more sustainable future.

For more information on the Remeha CHP range, email us at chp@remeha.co.uk



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