DISTRICT HEATING DESIGN ISSUES
The cost of failure Consulting engineers or clients can help protect themselves from the above problems by asking a simple question: what will be the fixed pipework distribution loss of the system we are about to detail/design? If we assume that the insulation of a system
is correctly installed to the limits defined by the compliance guidance, and overlook the
other items highlighted, then heat losses are 10-20W/m per pipe, and therefore double (20-40W/m of network) for separate flow and return pipework, for a 75°C distribution system insulated to BS 5422:2009. Typically, this would be running 24 hours
The installation of district heating networks, like this one in Sweden, has been popular in many European countries where national and regional policies have promoted community heating systems as a means of providing energy security and resource efficiency, if appropriately planned and specified
Without separate metering installed on all thermal plant outlets and inlets, it is difficult to determine how efficiently heating plant is operating and where energy is being distributed
The final problems to consider relate to its performance in use:
CIBSE PARTNERS WITH CHPA TO ASSURE QUALITY
CIBSE and the Combined Heat & Power Association (CHPA) have formed a partnership to assure the quality of heat networks. The partnership aims to be inclusive and to consult widely. Phil Jones, chairman of the CIBSE CHP-district heating group said: ‘Feedback from government, clients and industry indicates that the development of assured quality standards could help the sector grow to the next level.’
Jones welcomes views in the LinkedIn discussion about heat network quality at
lnkd.in/vp_8Av For further information on the CIBSE-CHPA partnership work on assuring quality, stakeholders should contact Dr Jacquelyn Fox at
JFox@cibse.org or Nicola Butterworth at nicola.
butterworth@chpa.co.uk Follow the CHP-district heating group on Twitter at @cibsechpdh
Lack of accurate metering Without separate metering installed on all thermal plant outlets and inlets, it is difficult to determine how efficiently heating plant is operating and where energy is being distributed. As a result, operational difficulties are difficult to diagnose. In homes, metering may only be installed in HIUs, which can show deceptively low individual readings. This is because an efficient building thermal fabric can retain the heat of losses from permanently active pipework and HIUs, resulting in a self-heating – or even overheating – building. Space heating is then rarely required through HIUs, which is reflected in the readings. Without a meter on the building perimeter, this is hard to understand or charge back to occupants or tenants.
Poor initial commissioning, and no seasonal commissioning A district heating system is often as large as – if not larger than – a commercial development. It is not uncommon to recommission commercial systems for the next year using a specialist company independent of the mechanical subcontractor. However, often a district heating system is not commissioned by a third party, and neither is its operation reviewed by an independent professional once operational. High costs are often only apparent after a year of operation. By then, the design and construction team has moved on, leaving the client or occupant with the problem.
34 CIBSE Journal August 2013
a day at operational temperature, which therefore equates to 175,000-350,000kWh of heat energy just to keep 1km of district heating network warm for one year. At 4p/kWh of gas for an 86% efficient gas boiler, this adds up to an annual operating cost of £8,100 to £16,300 per kilometre of district heating network. Feedback from finished projects shows that
by failing to consider this effect during design, the overall system operating efficiencies can fall by 40%. Depending on pipework length this may work out at an end user cost of £110 to £240 per apartment per year. Such problems can be avoided through appropriate design consideration, specification and by using optimisation tools – such as energy demand modelling – to understand how the system behaves, and to anticipate the potential ongoing operational costs. Similarly, performance criteria may be inserted into design and build contracts to incentivise the construction of systems with a low operational cost. This does come at a price in terms of design fees and initial capital investment. But, assuming the pipework heat loss can be reduced by 20%, a three- year payback may provide justification for a £10,000/km increase in capital investment. It is an educated client who asks to understand how the system will operate once constructed, and prepares his lease agreements and charging structures to suit. Sadly, the more common question is, ‘How can capital cost be reduced?’ Of course, this is a valid consideration, but perhaps the following question should be: ‘What is the operational effect of this capital cost reduction?’ The challenge is for engineers to educate
the non-technical user and purchaser, and develop design standards and contracts that help district heating avoid becoming the next low-carbon system to fall from grace. CJ
HUW BLACKWELL is a design management consultant at Hoare Lea
www.cibsejournal.com
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