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Maximising CHP percentage share Delivering optimum efficiency for the end client


Using modulating CHP helps to maximise CHP percentage share, while low temperature design ensures optimum energy efficiency when the plant is running. Beata Blachut of SAV Systems explains.


need to maximise the CHP’s share of the heat and electrical energy supplied to the building and to enter this CHP percentage share figure into their SAP/SBEM calculations. Essentially, the bigger the energy share from the CHP, the greater the carbon and cost savings. When traditional fixed-output CHP is being used, this CHP percentage share figure is likely to be relatively low, because the run-times for the CHP will be limited by the system’s inability to cope with varying demand. For example, if fixed- output CHP is sized to match the site’s base electrical load this will limit the contribution to meeting heat loads. When sized to meet a base heat load, the electrical power output is determined by the heating requirements of the site. This can often result in selling electricity to the grid at unfavourable rates. An alternative approach is to use CHP units that are able to modulate their output and dynamically track site demand. In this way, the power output of the CHP is aligned to


W


hen designing systems that incorporate combined heat and power


(CHP), specifiers


changing site requirements, providing effective management of the most expensive utility – the electrical power – to deliver maximum cost savings. Any excess heat is typically transferred to water in a storage vessel to ensure it is not wasted.


This performance is further optimised by the ability of load- tracking CHP to ‘self-learn’ the building’s loads and adapt to changing conditions. A key benefit of this is that if other power-saving measures are implemented in the building, such as upgrading the lighting, the CHP will adapt accordingly. There are many instances where fixed-output CHP has had to be turned off following such upgrades because the site’s electrical demand becomes too low for the CHP to be viable.


Flexibility


Dynamic, load-tracking CHP can also be used in a modular/cascade configuration to provide additional flexibility across a wider range of applications. For example, five 20kWe/38.7kWth units can be combined to provide outputs up to 100kWe/193.5kWth.


It is a very obvious statement that the less we waste, the more we have, and this principle of ‘waste not, want not’ is particularly applicable to the efficiency of CHP systems. One key element in designing CHP systems for optimum efficiency is to design for lower return water temperatures than are traditionally used in UK heating systems.


In its Applications Manual AM12 ‘Combined Heat and Power for Buildings’, the Chartered Institution of Building Services Engineers (CIBSE) recommends operating


uWhen traditional fixed‐output CHP is being used, this CHP percentage share figure is likely to be relatively low. The alternative is to use CHP units that are able to modulate their output and dynamically track site demand.


temperatures for radiator circuits to be 70°C flow and 40°C return for new district heating systems - with a maximum return temperature from instantaneous domestic hot water heat exchangers of 25°C.


These low return temperatures help to improve the efficiency of the CHP and are also beneficial for other


uControl panel display.


heat sources that may be included in the system. For instance, the optimum primary circuit ∆T for gas- fired condensing boilers is 55°C/30°C; for heat pumps it is 40°C/35°C. Additionally, lower flow water temperatures are better suited to the relatively mild UK climate, where heating systems are often ‘over-sized’ for the few very cold days we may experience each year.


Constant flow temperatures


Another engineering challenge with traditional fixed-output CHP is the need to operate at a constant temperature differential (∆T), as variable flow temperatures may result in inconsistent performance. To overcome this issue, a heat distributor can be incorporated into the modulating CHP units, so that heat at a constant temperature of 80°C is always generated, irrespective of the return water temperature.


The flow temperature to the system can then be set in a range from 20 - 80°C by using a two-port valve to vary the volume of 80°C water introduced to the mains and mixing it with return water from the site to achieve the target temperature. A Flow Master controller on the CHP unit determines the volume of 80°C water required, dependent on return water temperature and flow rate. Thus a constant flow temperature is maintained irrespective of the site return water temperature.


This arrangement ensures the CHP always produces high grade heat, minimising or even eliminating (periodically) the need to run back- up boilers.


Taking control


Clearly, if flow temperatures are constant and return temperatures need to be as low as possible the heat loss from the system to the heated spaces needs to be tightly controlled. This requires the heat interface units in the spaces to be optimised for both pressure and temperature control by


incorporating differential pressure control valves (DPCVs). Not only will this guarantee precise and stable temperatures at the taps, it will also reduce system flow rates so that more heat is transferred to the space through the heat emitters resulting in lower return water temperatures. In addition, lower flow rates enable the use of smaller, variable speed pumps, compared to a system with higher flow rates, resulting in lower capital costs and reduced pump energy consumption.


For all of these reasons, the use of dynamic, modulating CHP in a system designed to operate with low return water temperatures is the obvious way to maximise CHP percentage share and deliver optimum efficiency for the end client.


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Combined Optimised Heat and Power... …for maximised CHP % share


www.sav-systems.com 20 BUILDING SERVICES & ENVIRONMENTAL ENGINEER APRIL 2016 VISIT OUR WEBSITE: www.bsee.co.uk


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