Historically, buildings have been passive nodes in the electric grid system with one-way power flows. However, with the recent market development of solar photovoltaic (PV) and stationary behind-the-meter battery storage systems, buildings are now transitioning to active nodes, offering bi-directional power flows. Various system topologies and modelling aspects are of interest for these active nodes and their viability. This thesis compares internal building direct current (DC) distribution with the conventional alternating current (AC) distribution for single-family and office buildings. For both building types, the geographical location is altered to examine the effect of PV and load correlation on the DC performance. The energy loss over a year and the loss distribution across various components are examined for three DC topologies, including one with constant power electronic converter (PEC) efficiencies, to quantify the loss discrepancy to experimentally derived PEC efficiency characteristics. Using constant efficiencies for a single-family building underestimates the annual losses by 34% (63 kWh/a). With load-dependant PEC efficiencies and without battery storage, DC operation shows no performance enhancements compared to AC. Depending on the studied case, DC operation results in loss savings, −16.3 to −43.6% with PV and battery.

Two methods are proposed to reduce the grid-tied converter (GC) losses from partial load operation. One method—a modular GC design consisting of a smaller and a larger GC—is modelled for two cases: a single-family building and an office building, and presents an optimal GC size configuration of 15/85%. The loss savings relative to AC operations for a 15/85% configuration are 26% for the single-family building and 15–40% for the office. The savings depend on the office’s location and system design (PV and battery sizing). For the offices, the effect on DC loss savings is examined via a parametric sweep by varying PV and battery sizes, with resulting savings up to 40% (−12.8 MWh/a) compared to AC operation. The results highlight the effect of GC sizing on the DC performance, the effect of battery storage, and how the PV and load correlation affects the DC performance.

Furthermore, a battery model is derived from experimental measurements of the cell’s current—resistance and open-circuit voltage (OCV)—state-of-charge (SOC) dependencies. The battery model is verified against the measured voltage with good compliance (RMSE<7 mV). Three representations—including the round trip efficiency approximation—are compared for annual battery system losses. The results indicate that the cell’s losses—making up 22–45% of losses for the examined case—and that the internal resistance’s current dependency is essential for an accurate representation. The loss discrepancy for the round trip approximation varies between −5% to 29%, relative to the experimentally derived representation, depending on the modelled battery size.

The role of PV and battery storage for an airport micro grid is examined in a forward-looking case with electric aviation (EA) and electric vehicles (EVs). Seven scenarios are studied, including four with battery storage and different operation algorithms. One of the algorithms is a novel operation combining self-consumption (SC) and peak power shaving. Compared to the current situation, the techno-economic evaluation shows a significant increase in energy (89.4%) and power (+1 MW) demands from EA and EV. For the nominal battery price and peak power tariff (Ct), the novel operation shows the shortest Payback Period (PBP) of 4.8 years for the battery scenarios. With varying battery prices and peak power tariffs, the sensitivity analysis shows that Ct can significantly affect the PBP.

Lastly, the effect of PV module operating temperature on performance is empirically evaluated and quantified for seven arrays from annual operation. For the Building–Applied PV (BAPV) c-Si modules, the elevated operating temperature adds 1% to the total losses and 2% for the c-Si Building–Integrated PV (BIPV). Examining the results of SC and self-sufficiency (SS) verifies the correlation between SC and power rating and introduces the correlation between SS and annual yield, considering the effect of system design, level of roof integration and PV cell type. For this case study, comparing two systems with and without battery storage shows the weekly variation in SS and SC and highlights the drawback of single-objective dispatch.

This work quantifies the techno-economic performance of AC and DC residential building distribution. Two methods, utilising software and hardware configurations, are showcased to improve DC distribution: (i) a novel rule-based battery dual-objective operation (DOO) and (ii) a modular Master/Slave design of the grid-tied converter (GC). Both methods use the GC’s load-dependent efficiency characteristic, eliminating partial-load operation and enhancing energy efficiency. The work uses measured annual PV and load data to evaluate the performance of the methods compared to AC and DC references. The techno-economic analysis includes the annual net electricity bill and monetised battery degradation. The results show that the DOO eliminates GC partial-load operation at the cost of increased battery usage, resulting in marginal net savings. In contrast, the modular converter design significantly reduces losses: −157 kWh/a (−31%) and −121 kWh/a (−26%), respectively, relative to the DC and AC references. For a parametric sweep of electricity price and discount rate, the Lifetime Operating Cost (LOC) comparison shows savings from DC of up to USD 575 compared to AC.

This work compares and quantifies the annual losses for three battery system loss representations in a case study for a residential building with solar photovoltaic (PV). Two loss representations consider the varying operating conditions and use the measured performance of battery power electronic converters (PECs) but differ in using either a constant or current-dependent internal battery cell resistance. The third representation is load-independent and uses a (fixed) round trip efficiency. The work uses sub-hourly measurements of the load and PV profiles and includes the results from varying PV and battery size combinations. The results reveal an inadequacy of using a constant battery internal resistance and quantify the annual loss discrepancy to −38.6%, compared to a case with current-dependent internal resistance. The results also show the flaw of modelling the battery system’s efficiency with a fixed round trip efficiency, with loss discrepancy variation between −5 to 17% depending on the scenario. Furthermore, the necessity of accounting for the cell’s loss is highlighted, and its dependence on converter loading is quantified.

This work presents a comparison of alternating current (AC) and direct current (DC) distribution systems for a residential building equipped with solar photovoltaic (PV) generation and battery storage. Using measured PV and load data from a residential building in Sweden, the study evaluated the annual losses, PV utilization, and energy savings of the two topologies. The analysis considered the load-dependent efficiency characteristics of power electronic converters (PECs) and battery storage to account for variations in operating conditions. The results show that DC distribution, coupled with PV generation and battery storage, offered significant loss savings due to lower conversion losses than the AC case. Assuming fixed efficiency for conversion gave a 34% yearly loss discrepancy compared with the case of implementing load-dependent losses. The results also highlight the effect on annual system losses of adding PV and battery storage of varying sizes. A yearly loss reduction of 15.8% was achieved with DC operation for the studied residential building when adding PV and battery storage. Additionally, the analysis of daily and seasonal variations in performance revealed under what circumstances DC could outperform AC and how the magnitude of the savings could vary with time. © 2023 by the authors.

Following the societal electrification trend, airports face an inevitable transition of increased electric demand, driven by electric vehicles (EVs) and the potential rise of electric aviation (EA). For aviation, short-haul flights are first in line for fuel exchange to electrified transportation. This work studies the airport of Visby, Sweden and the effect on the electrical power system from EA and EV charging. It uses the measured airport load demand from one year’s operation and simulated EA and EV charging profiles. Solar photovoltaic (PV) and electrical battery energy storage systems (BESS) are modelled to analyse the potential techno-economical gains. The BESS charge and discharge control are modelled in four ways, including a novel multi-objective (MO) dispatch to combine self-consumption (SC) enhancement and peak power shaving. Each model scenario is compared for peak power shaving ability, SC rate and pay-back-period (PBP). The BESS controls are also evaluated for annual degradation and associated cost. The results show that the novel MO dispatch performs well for peak shaving and SC, effectively reducing the BESS’s idle periods. The MO dispatch also results in the battery controls’ lowest PBP (6.9 years) using the nominal economic parameters. Furthermore, a sensitivity analysis for the PBP shows that the peak power tariff significantly influences the PBP for BESS investment. 

In this paper, the performance of a direct current (DC) distribution system is modelled for a single-family residential building and compared with a conventional alternating current (AC) system to quantify the potential energy savings and gains in photovoltaic (PV) utilisation. The modelling is made for two different climates to quantify the impact of the geographical location. Results show that the system losses are reduced by 19–46% and the PV utilisation increased by 3.9–7.4% when using a DC distribution system compared to an AC equivalent, resulting in system efficiency gains in the range of 1.3–8.8%. Furthermore, it is shown that the geographical location has some effect on the system's performance and PV utilisation, but most importantly, the grid interaction is paramount for the performance of the DC topology. © 2021 The Authors.

Energy from solar photovoltaic (PV) are generated as direct current (DC) and almost all of today’s electrical loads in residential buildings, household appliances and HVAC system (Heating Ventilation and Air-conditioning) are operated on DC. For a conventional alternating current (AC) distribution system this requires the need for multiple conversion steps before the final user-stage. By switching the distribution system to DC, conversion steps between AC to DC can be avoided and, in that way, losses are reduced. Including a battery storage–the system’s losses can be reduced further and the generated PV energy is even better utilised.

This thesis investigates and quantifies the energy savings when using a direct current distribution topology in a residential building together with distributed energy generation from solar photovoltaic and a battery storage. Measured load and PV generation data for a single-family house situated in Borås, Sweden is used as a case study for the analysis. Detailed and dynamic models–based on laboratory measurements of the power electronic converters and the battery–are also used to more accurately reflect the system’s dynamic performance.

In this study a dynamic representation of the battery’s losses is presented which is based on laboratory measurements of the resistance and current dependency for a single lithium-ion cell based on Lithium iron phosphate (LFP). A comparative study is made with two others, commonly used, loss representations and evaluated with regards to the complete system’s performance, using the PV and load data from the single-family house. Results show that a detailed battery representation is important for a correct loss prediction when modelling the interaction between loads, PV and the battery.

Four DC system topologies are also modelled and compared to an equivalent AC topology using the experimental findings from the power electronic converters and the battery measurements. Results from the quasi-dynamic modelling show that the annual energy savings potential from the suggested DC topologies ranges between 1.9–5.6%. The DC topologies also increase the PV utilisation by up to 10 percentage points, by reducing the associated losses from the inverter and the battery conversion. Results also show that the grid-tied converter is the main loss contributor and when a constant grid-tied efficiency is used, the energy savings are overestimated.

In this paper, the performance of a direct current (DC) distribution system is modelled and compared fora single-family residential building with a conventional alternating current (AC) system to quantify the potential energy savings and gains in PV utilization. The modelling is also made for two different climates to quantify the impact of the geographical location. Results show that the system losses are reduced by 19-46% and the PV utilization increased by 3.9-7.4% when using a DC distribution system compared to an AC equivalent, resulting in system efficiency gains in the range of 1.3-8.8%. Furthermore, it is shown that the geographical location has some effect on the system's performance and PV utilization, but most importantly the grid interaction is paramount for the performance of the DC topology.

The performance of a conventional ground-source heat pump (GSHP) has been measured in the laboratory with alternating current (AC) and direct current(DC) operation using the standardised points fromEN14511:2018. The results from these measurements have been used to modify a variable speed heat pump model in IDA Indoor Climate and Energy (ICE) and the annual performance of AC and DC operation have been simulated for an entire year's operation at two geographical locations in Sweden. Results show that the energy savings with DC operation from laboratory measurements span between 1.4{5.2% and when simulating the performance for an entire year's operation, the energy savings vary between 2.5{3.4%. Furthermore, the energy savings from the simulations have been compared to the bin method described in EN14825:2018.