The integration of renewable energy sources and heating electrification strategies in households is an unavoidable component of the energy transition. Unfortunately, the pace at which such integration occurs has proven challenging for the distribution system operators. On the one
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The integration of renewable energy sources and heating electrification strategies in households is an unavoidable component of the energy transition. Unfortunately, the pace at which such integration occurs has proven challenging for the distribution system operators. On the one hand, system operators design low-voltage networks for low, unidirectional power flows at the connection points with the consumers, with expected lifetimes of several decades. On the other, the decreasing prices, together with the economic and environmental incentives to install (mainly) rooftop PV systems and heat pumps, create highly stochastic, bidirectional and potentially high-power power flows at the connection points of the former consumers, transforming them into prosumers. This thesis studies the misalignment between the interests of system operators and prosumers, proposing realistic alternatives that are achievable in the short term. This thesis hypothesizes that the aggregation of residential multi-carrier energy storage systems would be capable of bridging the interests between prosumers and distribution system operators. To validate the hypothesis, this thesis is comprised of five research topics.
Distribution system operators commonly address the grid congestion through infrastructure reinforcements, which is slow and expensive. Chapter 2 studies how energy storage systems with different carriers can provide a collaborative solution involving prosumers as ancillary services providers at the distribution level. Specifically for the European urban context, this chapter analyzed renewable energy sources, batteries, supercapacitors, hydrogen fuel cells, thermal energy storage, and electric vehicles through a thorough review of successful implementations. The correlations found between individual energy storage technologies and ancillary services provided insight into the flexibility opportunities each technology can provide to the grid. It was concluded that multi-carrier systems would provide the most robust yet flexible solution.
Based on the previous premise, Chapter 3 evaluated four multi-carrier energy system configurations for a Dutch household. The chapter also provides analytical models for every component (including the thermal losses from the thermal storage to the ground) and the space heating and electrical demands. The results suggest that using a heat pump combined with a photovoltaic system and a battery provides the best trade-off for the prosumer. The photovoltaic-thermal system alone could not supply the thermal demand required for comfortable space heating nor reach temperatures high enough to charge the thermal storage. Combining the thermal storage with the heat pump allows a certain degree of flexibility for the heat pump activation at the cost of COPs between 0.8 and 1.38 when used to charge the thermal storage, thus increasing energy consumption and equivalent emissions considerably.
Chapter 4 then elaborates on different energy management strategies to control the multi-carrier systems as proposed above. Two adaptable energy management system strategies were proposed for any system architecture with a reduced number of constraints. The first strategy uses genetic algorithms with a discrete-continuous approach for the power setpoints, maximizing thermal comfort and minimizing energy cost and CO2equivalent emissions. The EMS employs random forests for short-term predictions of the PV generation and electric and thermal demand. The results demonstrate that the strategy can solve the power allocation problem in the order of 1 s, including forecasting 60 minutes. This strategy, however, is too computationally demanding for complex distribution systems with multiple houses. Therefore, the second strategy uses a policy-based heuristic method to control the multi-carrier system, minimizing energy costs and maximizing thermal comfort. Also, this strategy allows the EMS to follow, or not, an external power setpoint from an aggregator, resulting in control decisions in the order of 30 ms. In addition, an ageing-aware EMS was briefly introduced, demonstrating the importance of ageing the BESS during operation.
Chapter 5 investigates, from a cost perspective, what conditions can make it attractive for individual prosumers to participate in a low-voltage ancillary service market, specifically power curtailment and peak shaving. For the former, it was shown that there are conditions where curtailing power does not significantly reduce the system's revenue but greatly reduces the peak power injected into the grid. However, it was also shown that curtailing might affect the power electronic components of the solar converter, potentially reducing its expected lifetime compared to a normal operation without curtailment. Similarly, an estimation of the degradation of the batteries for the cases with and without providing peak shaving was done using a semi-empirical ageing model, concluding that doing peak shaving to ensure a fixed power exchange with the grid will drastically reduce the life of the battery. Therefore, following an external setpoint to reduce occasional peaks would extend the battery's life. The results suggest that power curtailment and peak shaving can be attractive for prosumers, thus creating opportunities for ancillary services business models at the residential scale.
Chapter 6 incorporated households with single- and multi-carrier energy storage in a low-voltage distribution network to quantify the benefit of aggregation for the prosumers and system operators. The aggregator is generally assumed to have full observability and controllability of the assets, which is unrealistic in many cases. For this reason, this chapter considered separate controllers for the prosumers and the aggregator. Using a real 301-node low-voltage residential distribution network in the Netherlands, it was demonstrated that aggregated multi-carrier energy storage can ensure the voltage conditions established in EN50160 for penetrations of PV systems coupled with heat pumps up to 80 %. In contrast, aggregated single-carrier storage can reach 60 % and centralized storage only 40 %. Despite generating an economic benefit while supporting the grid, the high investment costs for both single- and multi-carrier storage result in unattractive conditions for prosumers compared to a case with only PV and heat pumps, requiring compensations for around half of the energy purchase costs for the single-carrier storage and higher than the total energy costs for the multi-carrier.
In summary, it was proved that, from a technical perspective, aggregated residential multi-carrier energy systems are a robust yet flexible solution for the voltage problems caused by the energy transition in residential low-voltage distribution networks. However, the current state of thermal storage makes the technology too expensive to be economically attractive.