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As renewable heat sources become more viable and more affordable,
one of the main challenges is how to integrate them with each other and
with conventional technologies. Ian Dagley of Hoval considers some of
the key factors.
Without a doubt, renewables are now at the top of many sustainability
agendas – in terms of meeting local and national emissions targets as well as
reducing costs.
When renewables are included as part of the heating solution, the real
challenge is getting all of the systems to work in harmony, particularly when
the renewable element varies on a daily or even hourly basis (e.g. solar
thermal). Solar irradiation levels in the UK can range from less than
100Wh/m2 of collection area on a cloudy day to over 1000Wh/m2 on a sunny
day. As a result, any solar heating system must be backed up by 100%
auxiliary heating. This, of course, could be biomass boilers as they work well
in conjunction with solar heating and there may also be opportunities to
include heat pumps – both of which would enhance the low carbon content of
the project.
When considering renewables the key is to decide which will work well
together in a controllable fashion and which overall solution gives the best
results for that project. Solar heating, for instance, is particularly good for
projects with a large heat sink such as swimming pools, or it may be used for
meeting some or all of the domestic hot water (DHW) requirements. In larger
projects, which inherently have significant shifts in demand, it is usually more
cost-effective to use solar to pre-heat the DHW and then top it up to the
required temperature using a biomass or conventional boiler.
It is important to note that the volume of any pre-heated, stored potable water
should be kept to a minimum, as this may have to be regularly pasteurised as
part of the anti-legionella regime. In these circumstances it makes sense to
store the solar energy in a thermal storage vessel and pre-heat the cold feed
water through a suitable heat exchanger.
As mentioned above, biomass boilers are often chosen as a suitable back-up
heat source to solar, or they may form the main renewable element of the
project. When introducing biomass, though, it’s vital to be aware of all the
implications – from the engineering design through to the day-to-day
practicalities of living with a biomass heating system.
For example, in summer when there is no space heating requirement, it may
not be cost-effective to run a biomass boiler just for the hot water system. It
may make more sense to make use of solar thermal, heat pumps, smaller
gas-fired condensing boilers or a combination of any of these. A suitably-sized
buffer vessel, perhaps capable of managing with several different heat
sources, can also help to optimise operation.
With biomass, it’s also important to choose the most appropriate type of wood
fuel – pellets or chips – and this, in turn, will determine the volume of fuel
storage required, the fuel handling mechanisms, the combustion controls, the
type and size of vehicles that need access for delivery, the frequency of
deliveries and the training of maintenance staff. All of these need to be
balanced to arrive at the most appropriate and cost-effective solution for each
project.
Similarly, inclusion of ground source or air source heat pumps will introduce
another energy-saving aspect to the project – but a variable one that will need
to be controlled efficiently to achieve optimum performance and emissions
reductions.
At the same time, it’s important not to become so focused on low or zero
carbon technologies that other, potentially more efficient, configurations are
ignored. For example, in some circumstances inclusion of high efficiency
condensing boilers in such a scheme may provide the best solution –
particularly where there are low return water temperatures that will maximise
condensing.
In fact, this illustrates the key consideration for integrating renewable
technologies into an energy project. Namely that, without in-depth
understanding of the pros and cons of each technology, it will not be possible
to achieve the best mix of technologies and control strategies for each project.
Ends
Reader enquiries:
Ian Dagley
Hoval Ltd
Northgate
Newark
Notts
NG24 1JN
Tel: 01636 672711
Fax: 01636 673532
E-mail: hoval@hoval.co.uk
Web: www.hoval.co.uk |
