pwr_july-2024 - 27

COMBINED HEAT & POWER
have CHP that is four times as large as if
you were feeding the heat directly. During
operation, 75% of the heat output is
buffered in storage for later dispatch.
When considering conventional baseload
operation, many district heating systems
are too small to be the basis for a
CHP plant, but this changes significantly
when considering on/off operation instead.
With the help of thermal storage,
it is possible to build a much larger plant
for a better cost-to-scale ratio, while at
the same time taking better advantage
of the volatile electricity price pattern
and possible capacity payments. This
new approach to using thermal storage
can unlock a very large CHP potential,
because there are many small- and medium-sized
district heating networks that
are not yet utilized for CHP at all.
Combining CHP, Heat Pumps, and
Storage
The two options of CHP or heat pumps
(Figure 1) may seem to be competing alternatives,
but there is a third avenue in
between. For example, if you go halfway
with the CHP option, such as installing a
CHP system with perhaps twice the district
heating capacity requirement, then
you can also install heat pumps to cover
the rest. Each system then has half the
capacity of either of the more extreme
first two options, so the investment for
the combined plant itself is perhaps 10%
to 20% higher than for a CHP-only or
heat-pump-only system. However, the
required size of the thermal storage is
significantly less because you now can
supply more heat directly without storage
and charge the storage from both
ends of the electricity price scenarios,
both from the CHP operation and from
the heat pump operation.
You could go from a capacity of two
months of storage to just two days. However,
storage should still be very beneficial
because you can allow full load from either
of the two systems when electricity prices
are at either of the extremes, and you can
avoid operating when prices are somewhere
in the middle and neither system
provides much revenue. By bridging shorter
swings in electricity prices and demand,
you can avoid switching too frequently between
the systems; this means reduced
maintenance costs due to less wear and
tear from start cycles. Adding seasonal
variations to the equation, a combination of
CHP, heat pumps, and storage can flexibly
adapt to the shifting proportions of renewable
energy and residual demand situations
combined with changing heat demand.
July 2024 | POWER
2. Pairing a combined heat and power system with thermal energy storage and heat pumps
allows much greater flexibility without a vast increase in investment. Courtesy: Siemens Energy
Self-supported Heat Pump Operation
If you have both CHP and heat pumps
installed, you can operate both simultaneously,
meaning that you supply the electricity
from the CHP to run the heat pump.
The efficiency (fuel to heat) in this operating
mode is about 200%, far better than
a conventional boiler. This operating mode
could be used for a plant's baseload, or
simply when the price of electricity does
not justify either selling or buying power
from the grid. There is often a spread between
the purchase price of electricity and
the sales price due to grid tariffs and taxation,
which makes this kind of self-supported
operation an attractive proposition.
Many sites also face grid capacity
constraints, which can make it impossible
to ensure the continuous availability
of grid power. Feed-in tariffs are also
poor for many industrial users. The ability
to become self-supported regarding
electricity during periods of grid congestion
could help gain approval for a new
industrial plant to connect to the grid or
increase electricity usage related to a decarbonization
transition.
In regions where renewable energy is
not available, a self-supported operating
mode will cut fuel consumption in half if
the installations replace a boiler. If, over
time, more renewable energy becomes
available, the solution can adapt to that
new situation.
Location of Components
The heat pumps that interplay with CHP
do not necessarily have to be located
onsite with the CHP plant; it may be a
better choice to locate them where the
waste heat to be recovered is found.
For example, a heat pump could be located
at a wastewater treatment plant to
recover heat from the outgoing treated
wastewater or it could be located at an
industrial site.
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Heat pump units often feature a
modular design so that a large installation
is made up of a number of heat
pumps. These heat pump units can be
connected in parallel or cascaded to
optimize the coefficient of performance
depending on the operating conditions,
whether the full temperature of the
district heating supply needs are to be
met directly or only after a downstreamsituated
CHP plant or storage. The thermal
heat storage does not have to be
located near the CHP plant or the heat
pumps either; it can be located anywhere
along the main lines of the district
heating circulation (Figure 2).
Policies and Regulations
There are three ways policymakers
could help. First, they need to acknowledge
that cogeneration plants should
be combined with heat storage and be
oversized in capacity compared to heat
demand to ensure the energy efficient
supply of residual load to the grid. Secondly,
new cogeneration plants should
be required to be future proof in terms
of both fuel and operational flexibility.
In addition, heat storage should be integrated
into the supported heat distribution,
allowing the cogeneration plant
to operate at full load for up to at least
two days without heat demand. Lastly,
the expansion of district heating should
be prioritized to enable the efficient
use of large-scale heat pumps and the
recovery of waste heat from industries
and from power generation in flexible
CHP plants. That's why small-scale heat
pumps should not be supported in areas
that are suitable for district heating
distribution and where large-scale solutions
can be used. ■
-Anders Stuxberg is a specialist in
power plant process integration
with Siemens Energy.
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