In the current Danish energy system, the majority of electricity and heat is
produced in combined heat and power (CHP) plants. With increasing shares
of intermittent renewable power production, it becomes a challenging task to
match power and heat production to its demand curves, as production capacity
constraints limit the efficient operation of the CHP plants. Heat pumps (HPs)
can be used to decouple such constraints, but current state of the art are
not competitive all things considered. Methods to improve the high energy
efficiency are required to match the politically agreed carbon emission goals.
The presented study investigates the possible introduction of HPs from both a
thermodynamic and a system/operation management perspective, in order to
find optimal integration schemes in both current and future energy scenarios.
Five generic configurations of HPs in district heating (DH) systems were identified
and compared based on a thermodynamic analysis. The operational
performance of the configurations were investigated at both local and system
level considering different DH network temperatures, different fuels and different
production technologies in the DH network. The analysis show that
three configurations are particular advantageous, whereas the two remaining
configurations result in system performance close to or below what may be
expected from an electric heater. One of the three advantageous configurations
is required to be positioned at the location of the heat demand, whereas the
two remaining can be located at positions with availability of high temperature
sources by utilising the DH network to distribute the heat.
A large amount of operational and economic constraints limit the applicability
of HPs operated with natural working fluids, which may be the only feasible
choice in Danish conditions. The limitations are highly dependent on the
integration of heat source and sink streams. An evaluation of feasible operating
conditions was carried out considering the constraints of available refrigeration
equipment and a requirement of a positive net present value of the
investment. Six vapour compression heat pump (VCHP) systems were considered
along with the ammonia-water hybrid absorption compression heat pump
(HACHP), corresponding to an upper limit of the sink temperature of up to
150 °C. The best available technology was determined for each set of heat sink
and source temperatures. The results showed that five different HP systems
propose the best available technology at different parts of the complete domain.
Ammonia-water HACHP and ammonia VCHP systems utilising either low or high pressure components are preferable very broad range of sink temperature
and temperature lifts. With the considered economic constraints in place, the
requirements in terms of sink temperatures and temperature lift are not met
for many DH networks considering the configurations which heat to forward
The specific performance for two DH HP configurations were studied in detail,
using the finite temperature levels of a range of common DH networks.
Eight systems were examined in terms of applicability, and the systems were
optimised for each operating condition using exergoeconomic theory. The HPs
were compared based on cost of heat. The results show that including the
practical applicability of components causes a significantly increased cost at
high temperature lifts, compared to the most competitive thermodynamic cycle.
At high and medium temperature lifts cycle efficiencies of 45 - 50 % of
the theoretical maximum (Lorenz cycle limit) can be achieved, whereas for low
temperature lifts, efficiencies as low as 36 % may be expected.
Three frequently used operation optimisation methods were examined, in order
to investigate their impact on operation management of energy system technologies.
By focussing on the physical representation of a CHP-plant, it is clear
that a simple representation allows infeasible production. Using MIP or NLP
optimisation, the number of operation hours and the total production of heat
from HPs are significantly increased, as the HPs may be used to shave the load
patterns of CHP units in significantly constrained energy systems.
A MIP energy system model was developed with focus on the detail level of
features for representation of CHP and HP units. Two energy scenarios were
considered, one current, which is a validated model for 2011, and a future scenario,
as proposed by energy planners for 2025, where reductions in carbon
emissions for heat is of major interest.
The changed distribution of electricity generation technologies may suggest a
reconsideration of optimum for DH network temperatures, in order to achieve
low cost and minimum carbon emissions. The developed energy system model
was used to investigate the changed operation. Production curves from typical
CHP-plant technologies were used to represent the changed power and heat
production for changed DH temperatures. The results show that both primary
fuel consumption and cost can be reduced approximately 5-7 % at DH forward
temperatures of 60 - 70 °C in 2025 scenario. Further reduction results in contrary
tendencies as hot tap water requires increasing amounts of electricity to
reach required temperatures. The results are network specific, as they represent
the specific DH utility technologies and network constraints, but similar
trends can be expected for other large DH networks.
|Series||DCAMM Special Report|