DISTRICT HEATING DESIGN ISSUES
n the world of large-scale residential development, district heating is at an all-time high. Planners and policy makers seeking a low-carbon future have identified
these networks as part of the solution, based on the efficiency of generating low-carbon heat centrally in large plant, such as combined heat and power (CHP) and biomass boilers. However, sometimes a strength can also be
a weakness. Because district heating systems are so large, the effects of failing to design an efficient system are magnified. In a typical system for 100 properties, for example, there maybe 1.5km of flow and return pipework (3km in total) pipes, so even the smallest problem with the pipework design can have wide ramifications. Not least of these is that the operational costs of inefficiency will also be magnified. Where heat is plentiful and cheap, the defects of poor design may be masked, but this is not the case with the systems currently being installed. These costs will have to be paid by the system operator, and ultimately the user. The irony is that this problem arises
because the private-sector clients developing the new wave of networks are – perhaps not unreasonably given the current economic climate – focusing on keeping costs to a minimum. Currently, it is common to procure district heating built to minimal compliance levels under a design-and-build arrangement, where the focus is to deliver a system for a low capital cost. At the same time, to limit liabilities between landlord and tenant or resident, heat interface units (HIUs) are used to provide hydraulic separation at property boundaries. Both of these approaches can have
serious repercussions in terms of long-term operational efficiency and costs. In certain cases, it may even be possible to prove that the inefficiencies result in a ‘high-carbon’ system – the reverse of policy intention. Below are some common scenarios which consulting engineers need to avoid. Many of the issues that lead to operating inefficiencies relate to the system insulation or the design of the distribution system. These include:
Specifying compliant pipework insulation without considering operational losses Depending on the system type, compliance with Building Regulations means following either the Non-domestic or Domestic Building Services Compliance Guide (2010), which are linked to the British Standard BS 5422:2009. Reference is made in the domestic heating guidance to BS EN 253 for pre-insulated district heating insulating pipework. However, although this quotes a maximum lambda
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(conductivity) value for insulation, there are no references to insulation thickness for a given operating temperature. Typically, this may lead to allowable losses of 10-20W/m in distribution pipework. Where the average pipework temperature is higher than the assumed temperature (the standard applies up to 95°C distribution systems), losses are higher still.
Lack of insulation continuity If insulation is not consistent from plant through to pipework and HIUs, it can add significant heat losses. Though it is becoming more common to see insulated heat exchangers in HIUs, pipework is often left exposed. Manufacturers can provide a fully insulated system, but often at additional cost. These two issues alone can lead to constant standing losses in the tens of kilowatts, even for a system serving 100 homes or less. They are further compounded if the distribution system design results in any of the following:
High, fixed operating temperatures, but low temperature differences This reduces the capital cost and space requirements of heat exchangers at the expense of reduced primary plant efficiency, potentially increased pipework losses, pump sizes and pumping energy costs.
Poor pipework layout If the length of pipework required is increased through poor design, it will compound distribution losses. Added to this, pipework is often ‘oversized’, reducing pumping losses but increasing heat loss from a bigger surface area, as can be seen from the higher heat loss allowances in the compliance guide.
Poor pumping and flow control Cheap bypasses mean systems are ‘always on’ at a certain level, regardless of the requirement for heat. These may also have the side-effect of increased return temperatures, further driving up pipework losses. Pressure sensors installed across pumps restrict turndown, increasing electrical use and the circulation of water at high temperatures.
Many of the issues that lead to operating inefficiencies relate to the system insulation or the design of the distribution system
CIBSE is keen to obtain case study examples of measured performance of recently completed residential and mixed-use development district heating projects, in order to gather evidence of scenarios where there are potential performance gaps. The case studies, which can be kept anonymous, can help CIBSE
identify critical issues about the design, application, procurement and operation of community energy systems. Examples of good performing systems would also be welcome. If you are able to provide examples of measured performance, which will be treated confidentially, please email
DHN@cibse.org
August 2013 CIBSE Journal 33
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