Commercial heating
Giving CHP its share
Getting the best performance from CHP involves maximising its percentage share within a system designed to operate at low temperatures. Beata Blachut of SAV Systems explains
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t is increasingly common for an energy centre to include combined heat and power (CHP) in its mix of technologies and this trend is set to continue. In such cases it is important to maximise the CHP’s share of the heat and electrical energy supplied to the building. The bigger the energy share from the CHP, the greater the carbon and cost savings, making the CHP % share figure an important component in SAP/SBEM calculations. Achieving this necessitates a move away
from traditional fixed-output CHP 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 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. Similarly, if it’s configured to match the site’s base electrical load this will limit the contribution to meeting heat loads.
Tracking demand Avoiding these constraints requires CHP units that can modulate their output and dynamically track site demand. In this way, the power output of the CHP is aligned to 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. Moreover, load-tracking CHP is able to ‘self-learn’ the building’s loads and adapt to changing conditions. For example, if the lighting is upgraded to LED the building’s power requirements will change and the CHP will adapt accordingly. In contrast, it’s not unknown for fixed-output CHP to be turned off after such measures have been implemented but electrical demand becomes too low for the CHP to be viable. Dynamic, load-tracking CHP can also be used in a modular/cascade configuration
March 2017
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.
Low temperature 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 temperatures for radiator circuits to be 70°C flow and 40°C return for new district heating systems/heat networks. The recommended maximum return temperature from instantaneous domestic hot water heat exchangers is 25°C. Low return temperatures help to
improve the efficiency of the CHP and are also beneficial for other heat sources. For instance, the optimum primary circuit temperature differential (Delta 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 CHP is the need to operate at a constant Delta T, while LTHW systems operate with a different delta T than CHP. To overcome this, a heat distributor incorporated into the modulating CHP units will ensure 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 60 - 80°C by using a two-port valve to vary the volume
Beata Blachut
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. 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.
A LoadTracker CHP unit from SAV Systems
Taking control If flow temperatures are constant and return temperatures are to be kept 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 (HIUs) in the spaces to be optimised for both pressure and temperature control by incorporating differential pressure control valves (DPCVs). This guarantees precise and stable temperatures at the taps and also reduces 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 % share and deliver optimum efficiency. • Beata Blachut is Technical Manager for LoadTracker CHP with SAV Systems
www.heatingandventilating.net
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