EIBI 03.2012
combined heat
and power
Beata Blachut is CHP
product manager with
SAV Systems
A flexible approach to CHP
The flexibility to respond to the needs of the building is critical to achieving maximum benefit from CHP.
Beata Blachut explains how this principle can be applied to both large and small projects
S
ustainability continues to
be a key driver for many
public and private sector
organisations, with energy
consumption and carbon emissions
often being the sustainability
components that receive most
attention. This focus on energy
is reinforced by the Building
Regulations, while the 2011 version
of BREEAM (BRE Environmental
Assessment Method) places greater
emphasis on reducing carbon
emissions than the 2008 version.
Participants in the Carbon
Reduction Commitment Energy
Efficiency Scheme also have a
strong incentive to reduce carbon
emissions, thereby minimising
the cost of carbon allowances and
improving performance in league
tables. In parallel, local planning
requirements often require
inclusion of low or zero carbon (LZC)
technologies.
So the scene is clearly set for the
wider use of LZC technologies and
combined heat and power (CHP)
is proving to be an increasingly
popular choice. Nor is CHP confined
to large projects – a common
misconception. With modern
modular, modulating systems
many smaller projects can also
reap the rewards of lower energy
consumption and carbon emissions.
Typical examples of smaller facilities
include multi-residential housing,
schools, nursing homes, leisure
centres and hospitals.
Typically, CHP will be at the heart
of a central energy centre that also
incorporates other heat sources,
which together serve a large or
small district heating system with
hot water for space heating and
domestic hot water (DHW). Power
generated by the CHP may be used
locally, exported to the grid or a
combination of the two.
According to the Carbon Trust,
CHP technology can potentially
reduce carbon emissions and energy
costs by 30 per cent, compared to
using gas-fired heating plant and
mains electricity. There are two key
reasons for this. First, CHP captures
the heat generated by power
generation and uses it for heating
18 ENERGY IN BUILDINGS & INDUSTRY
Sophisticated controls also allow
the overall operating strategy to
be aligned to the building’s most
significant demands to maximise
savings. So, in the example above
the system was optimised for
electricity demand, but systems
may also be optimised for heating/
DHW demand. They may also be
‘economy’ led, based on actual
fuel and electricity prices to gain
maximum economic benefits.
Experience shows that a
modulating CHP system will
consistently deliver energy savings
over and above those delivered by a
fixed-output CHP system.
Many smaller projects are now reaping the rewards of combined heat and power
‘It's an attention to detail, from the flexibility and
responsiveness of the plant to the smart use of hot
water that will enable CHP to deliver its benefits’
water. Second, a proportion of the
site’s electricity requirements are
met using mains gas, which is more
cost-efficient than mains electricity.
Making real savings
The key point here is to ensure the
‘potential’ savings are turned into
real savings. And this will only
happen if the CHP system is designed
and configured to deliver maximum
efficiency. That may sound fairly
obvious but overall efficiency doesn’t
just come down to the ability of
the CHP plant to generate heat and
power efficiently. It is also influenced
by the alignment of the CHP plant’s
operation to the needs of the
building(s) and how the electricity
and hot water are used. This is where
the flexibility referred to above
comes in.
For example, large-scale fixedoutput CHP systems need to be
carefully matched to the anticipated
electricity and heating loads. As a
result of this inflexibility, they are
usually sized to match the site’s base
electrical load and do not contribute
to the site’s electricity usage beyond
that base load. Consequently they
may only achieve relatively small
reductions in overall energy usage
at the site.
An alternative is to use CHP units
that are able to modulate their
output, using small-scale CHP units
in a modular configuration (in the
same way that groups of modular
boilers are more responsive than
large, single boilers). For example, a
CHP unit that can modulate down
to 40 per cent of its maximum
electrical power output will ensure
that the electricity generated never
exceeds demand, so there is no need
to ‘dump’ heat or sell electricity back
to the grid at unfavourable rates.
To put this into perspective, we
recently evaluated the options
for a small leisure centre. With
conventional CHP sized to cover
base electrical demand only the CHP
would only provide 39 per cent of
site electricity usage, resulting in
energy and carbon savings of around
10 per cent. In contrast, using a
modulating CHP system to track
site demand and modulate output
accordingly, 80 per cent of the site’s
electrical demand could be met.
At the heart of such a system is
the monitoring and control of the
system, tracking the requirements
of the load demand in real time and
‘learning’ as the system operates
to ensure consistent optimal
performance.
Efficient and responsive
In parallel with ensuring the central
plant is efficient and responsive, the
hot water for space heating or DHW
also needs to be optimised. Here, use
of pre-insulated heat interface units
using patented valve technology will
enable both heating and DHW needs
to be met directly from the hot water
generated by the CHP, without the
need for separate hot water storage
in the spaces requiring DHW.
In this configuration the heating
circuit is designed for direct
generation of heat, with individual
temperature control in each room/
space. This can also be combined
with timer control for individual
spaces or zones, if required.
DHW is heated via a heat
exchanger in the heat interface unit,
cooling the water from the central
heat source to a safe temperature
for DHW, while compensating for
variable loads, supply temperatures
and differential pressures. Heat
interface units can also be fitted
with integrated idle temperature
controllers to ensure that DHW
is responsive at times when space
heating loads are low.
It is this attention to detail, from
the flexibility and responsiveness
of the central plant to the smart use
of hot water distributed for heating
and DHW that will enable CHP to
deliver maximum financial and
environmental benefits. z
ENQUIRY No. 121
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