Knowledge: District heating
In the pipeline
With cost remaining a key barrier to the deployment of district heating systems,
the sector must optimise design to reduce cost and improve efficiency, explains
REHAU Renewable Energy’s business team manager, Steve Richmond
he UK has the potential to
become Europe’s fastest
growing district heating
market – the government’s
Heat Strategy supports public
sector investment in district heating and
the Renewable Heat Incentive is proving a
key driver in communal renewable heating
systems.
However, capital costs are often a barrier
for district heating schemes and the crucial
challenge facing those of us within the
industry is to ensure that more of the heat
networks being installed in the future are
optimised so that both costs and heat losses
are reduced.
For installers operating in this sector,
there are four main areas which I would
advise to concentrate on – sizing the
correct heat load, optimising the flow/return
temperatures, using diversity and reducing
installation costs.
Using these four key messages on a
district heating project of 50 houses and a
1.5MW central plant for example, REHAU’s
technical team were able to reduce the
original material list price by over 50 percent
simply by efficient network design.
On retrofit communal heating schemes,
there can be a temptation to estimate heat
loads for individual dwellings which can often
lead to overestimating the total heat load. The
resulting impact of this is the installation of an
inefficient system and a reduction in income
for the scheme operator who is selling the
heat.
Probably the greatest impact on capital
costs comes from successfully optimising
the flow and return temperatures within the
system. In simple terms, the greater the delta
T (difference in flow/return temperatures), the
smaller the pipe size required.
Using a 450kW example heat load and
82/71ºC, a 110mm (OD) pipe size would
typically be recommended. However, if you
can reduce that return temperature to just
T
32 | www.renewableenergyinstaller.co.uk
50ºC, then the pipe size would only need to
be 75mm. With pipe materials accounting
for upwards of 50 percent of the installed
network cost, any savings made in materials
can be significant. Another option is to use
heat pumps linked to district heating which
operate at lower temperatures and hence
have lower transmission heat losses. This has
significant potential in the UK, particularly
using waste heat from infrastructure or
industry.
Reducing the return temperature will
also significantly reduce heat losses. For
example, using 1km of RAUTHERMEX and
the original 110mm pipe sizing at 82/71ºC as
the benchmark, we can show that by using
90mm at 80/60ºC instead, you reduce those
losses by 25 percent and, by reducing the pipe
size still further to 75mm with a 50ºC return
temperature, you reduce the losses by 43
percent.
Diversity of a heat network is an
important topic as it has a large impact on
the size of the central plant. Diversity is the
likelihood of all heat customers using their
heat load at the same time. For small systems
below 10 dwellings it might not be relevant
but for larger networks, it can have a large
impact on the total heat load and hence the
pipe sizing. REHAU has produced bespoke
calculation software to help calculate the
estimated diversity of heat networks.
There can be a temptation
to estimate heat loads
for individual dwellings
which can often lead to
overestimating the total
heat load
Fantastic four: Installers should focus on correct
sizing, optimising flow/return temperatures,
using diversity and reducing costs for the
sector to flourish, says Steve Richmond, REHAU
Renewable Energy’s business team manager
Steel pre-insulated pipes have been
commonly associated with district heating
schemes but the market trend for lower
temperatures is now favouring polymer
pipes such as REHAU’s RAUTHERMEX and
RAUVITHEM due to their high flexibility, low
heat losses and long coil lengths which overall
make them more cost effective to install than
steel.
In many projects, it is also often possible
to use twin rather than single pipe which
works out cheaper than doing two separate
pipe runs and actually has up to 40 percent
lower heat losses overall. Innovative pipe
routing and optimising tee connections can
also help reduce costs.