CHP plant can do a power of good

The specialist CHP systems division of Dalkia, which acquired a majority stake in the company in 2007, Cogenco has over 20 years’ experience in combined heat and power, or “cogeneration” as its is often now called, with its extensive range of CHP engines able to operate on a variety of fuels, including natural gas, renewable biogas, biomass, biodiesel and landfill gas. Buying in many of the key components, including the “engine”, the exhaust gas heat exchanger, the plate heat exchanger, and the alternator which produces the electricity, the company undertakes assembly of the CHP systems at a large industrial unit, also its UK headquarters, in Horsham, West Sussex. Here the various components are put together on a recently extended assembly line before each finished unit is rigorously tested for anything up to five days in two special test cells, which check, in particular, for leakage of oils or water as the system is run up to full power. Once testing has been completed the systems are lagged and a metal enclosure craned into place over the top to form a finished system ready for transportation to the customer, be it a hotel, leisure centre, industrial facility, fruit grower or large acute hospital. Although the concept of producing heat and electricity simultaneously is by no means new – Cogenco emphasises that the steam generators used some 200 years ago during the Industrial Revolution were effectively undertaking the same process, albeit in a somewhat less sophisticated way – Giles Vidgeon, a sales support engineer at Cogenco, says he and his colleagues are still surprised at the “relative lack of knowledge” of CHP technology in many sectors where potential users could secure significant financial benefits, healthcare included. While users of Cogenco’s CHP systems range from leisure centres to hotels, and even fruit growers seeking an energy-efficient, environmentally-friendly way to heat their greenhouses, today approximately 20-25% of sales are to the healthcare sector. A quick glance at a list of the 60 or so private and NHS hospitals supplied by Cogenco over the past 25 years underlines the degree of take-up. However Giles Vidgeon, one of whose key responsibilities is producing feasibility studies for potential customers considering installing a CHP system to help them evaluate both the initial cost and ensuing financial benefits/payback time, feels many more hospitals could benefit. He says: “Providing that we are not connecting one of our CHP systems to a heating or hot water system which has not been efficiently serviced and maintained – for instance one where sludge has been allowed to build up in the pipework as a result of inadequate periodic maintenance, then both the energy saving and CO2 output reductions achievable can be very significant, especially at a time when electricity prices show no major signs of falling.”

Car engine similarities

In a typical CHP unit a fuel such as natural gas is used to drive a large engine – akin in many ways to a car engine (it, for instance, incorporates spark plugs), which in turn drives a 3-phase alternator that produces electricity that can be used to provide (depending on the site’s size and overall energy requirements) a significant proportion of an adjacent site’s overall energy demand. Rather than wasting the heat rejected by the electrical generation process the CHP plant’s exhaust gas heat exchanger recovers the heat energy that would normally be lost in engine exhaust gas and engine cooling water to provide hot water. This can then be fed directly, via a plate exchanger, to, a hospital’s own heating system or hot water supply, meaning that, for significant periods of time, the healthcare facility’s other boilers plant may not need to be operating at all. Giles Vidgeon does, however, add a caveat: “CHP systems are not designed as a replacement for existing boilers, but to supplement them, and in the vast majority of the applications we supply the CHP engine will run in parallel with the existing boilers and other plant.” As an approximate rule of thumb, a typical CHP unit will have a thermal output of around 1.5 times its electrical power output, although Giles Vidgeon explains that, as the electrical output rises, thermal efficiency will generally fall slightly. He elaborates: “If you take our smallest units, which have an electrical power output of some 30 kWe, they will generally have a thermal output of around 60 kw, whereas one of our large engines, say with a 2.5 MWe electrical output, might generally produce a maximum thermal output of around 3 MW. “When we undertake a feasibility study we use a purpose-designed template and software system into which we input a whole host of different parameters – from the overall size of the building or buildings to be supplied with heat and electricity to the site’s base load, i.e. the electrical and thermal load experienced during non-peak periods. From this data we can optimally size the CHP engine required. We now supply units with an electrical power output ranging from around 30 kw to 3 MW. A 3 MW CHP engine might typically be needed to supply the energy needs of a large acute hospital.”