PhD Theses

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PhD Thesis,
NTNU, Trondheim,

The transition to a sustainable energy system requires a shift to intermittent renewable
energy sources, which call for increased flexibility on the demand side. Heat pumps
offer the possibility to couple the electricity sector and the heating sector, and when
connected to thermal energy storages, they can provide demand side flexibility.
This thesis investigates the flexibility potential of residential buildings in
Scandinavia, and more specifically in Norway. In this regard, three different
boundary levels are considered: power grid level, building level, and heat pump
system level.
At the power grid level, a methodology to evaluate the hourly average CO2eq. intensity
of the electricity mix, while also considering electricity trading is developed. In
general, the CO2eq. intensity of the electricity mix may indicate the share of renewable
energies in the mix. The proposed method is based on the logic of input-output
models and avails the balance between electricity generation and demand. This thesis
shows that it is essential to consider electricity imports and their varying CO2eq.
intensities for the evaluation of the CO2eq. intensity in Scandinavian bidding zones.
Generally, the average CO2eq. intensity of the Norwegian electricity mix increases
during times of electricity imports since the average CO2eq. intensity usually is low
because electricity is mainly generated from hydropower. This hourly CO2eq. intensity
can be used as a penalty signal for demand response strategies applied to residential
heating.
At the building level, the flexibility potential of predictive rule-based controls
(PRBC) in the context of Scandinavia and Norway is studied. For this purpose,
demand response measures are applied to electricity-based heating systems, such as
heat pumps and direct electric heating. In one case study, the demand response
potential for heating a single-family residential building based on the hourly average
CO2eq. intensity of six Scandinavian bidding zones is investigated. The results show
that control strategies based on the CO2eq. intensity can achieve emission reductions
if daily fluctuations of the CO2eq. intensity are large enough to compensate for the
increased electricity use due to load shifting. Furthermore, the results reveal that
price-based control strategies usually lead to increased overall emissions for the
Scandinavian bidding zones as the operation is shifted to nighttime when cheap
carbon-intensive electricity is imported from the continental European power grid.
In another case study, the building energy flexibility potential of a Norwegian singlefamily
detached house is investigated using PRBC. Four insulation levels are studied
for this building: (1) passive house, based on the Norwegian standard for residential
passive houses, (2) zero emission building, based on the LivingLab located at the
Gløshaugen Campus at NTNU, (3) TEK10, based on the Norwegian building
standard from 2010, and (4) TEK87, based on the Norwegian building standard from
1987. The three PRBC investigated aim at reducing energy costs for heating, reducing
annual CO2eq. emissions and reducing the energy use for heating during peak hours.
This last objective is probably the most strategic in the Norwegian context where
cheap electricity is mainly produced by hydropower. It is shown that the price-based
control does not generate cost savings because lower electricity prices are outweighed
by the increase in electricity use for heating. The implemented price-based control
would create cost savings in electricity markets with higher daily fluctuations in
electricity prices, such as Denmark. For the same reasons, the carbon-based control
cannot reduce the yearly CO2eq. emissions due to limited daily fluctuations in the
average CO2eq. intensity of the Norwegian electricity mix. The PRBC that reduces the
energy use for heating during peak hours turns out to be very efficient, especially for
direct electric heating. As an example, for the ZEB insulation level and direct electric
heating, the price-based control reduces the energy use during peak hours by 18%,
and the carbon-based control by about 37%. The control strategy dedicated to reduce
the energy use during peak hours leads to a 93% reduction. For air-source heat pumps,
the control of the heat pump system is complex and reduces the performance of the
three PRBC. Therefore, it is suggested to model a heat pump system with enough
detail for a proper assessment of the building energy flexibility.
The model complexity required to adequately describe the heat pump system
behavior with regards to demand response of residential heating is investigated on
the heat pump system level. In the course of this thesis, the influence of the modeling
complexity of the heat pump system control on distinct energy-related and heat pump
system-related performance indicators is studied. The results prove that the modeling
complexity of the system control has a significant impact on the key performance
indicators, meaning that this aspect should not be overlooked. If the heat pump
operation is investigated in detail and a high time resolution is required, it is shown
that a PI-controller leads to a smoother operation than a P-controller, but tuning of
the controller is highly recommended. It is shown that the choice of the controller (P
or PI) is not crucial as long as the control signal to the heat pump is not of importance
and power is not investigated at very short time scales. Regarding demand response
measures, a strong interaction between the prioritization of domestic hot water and
the control of auxiliary heaters significantly increases electricity use of a bivalent
mono-energetic heat pump system, if demand response is performed for both,
domestic hot water and space heating. The electricity use for heating is only slightly
increased if demand control using predictive rule-based control is performed for
space heating only.
To summarize, energy flexible buildings can play a major role in the transition
towards a more sustainable energy system. The use of the hourly CO2eq. intensity as
a penalty signal for demand response strategies applied to residential heating, can
facilitate achieving the emission targets of the European Union. At the building level,
different objectives of demand response, such as reducing operational costs, reducing
CO2 emissions or increasing system efficiency are often incompatible and thus
difficult to achieve at the same time when using PRBC. When aiming at a realistic
control of the heating system of a single building, it is found that heat pump controller
tuning and DHW prioritization of the heat pump are two significant aspects that
should be considered regardless of the control strategy applied. The combination of
heating system, heat distribution system, system control and building envelope is
always case-specific and it is suggested that future work focuses on the design of a
heat pump system that considers energy flexibility. In this PhD thesis, standard sizing
of a heat pump system that is operated in an energy flexible way is applied.
Keywords
Energy flexibility; hourly CO2eq. intensity; demand response; demand side flexibility;
predictive control; rule-based control; heat pump system; heat pump modeling; model
complexity; direct electric heating; time-varying CO2eq. intensities; time varying
electricity prices; Scandinavian power market