Electric hot water tanks play a pivotal role as demand response assets within the UK's energy system by storing heat when energy is inexpensive and delivering domestic hot water when it is required. This role will become increasingly important if non-dispatchable renewable energy sources are to play a bigger part in the energy mix. Historically, the design standards relating to hot water tanks have focused primarily on minimising heat losses. However, in addition to preserving energy, a hot water tank should preserve the availability of heat above a useful temperature for as long as possible to avoid energy usage during peak times when it is costly or carbon intensive. To do this, thermal stratification within hot water tanks must be promoted. Unfortunately, thermal stratification leads to conditions that are conducive to bacterial growth due to the hospitable temperatures that arise during operation. For this reason, question marks have arisen over the extent to which more flexible control strategies, designed to allow for increasing penetrations of intermittent renewable energy sources, might lead to the growth of pathogenic bacteria within hot water tanks. The objective of the work discussed in this thesis was to understand the extent to which there is a conflict between thermal stratification and bacterial growth in practice, whether this conflict can be resolved and the potential implications for electric hot water tanks operating on a time of use tariff. A small field study demonstrated that there is prolific bacterial growth within conventional electric cylinders and that this can be attributed to thermal stratification with a confidence of (P<0.01). Fitting a de-stratification pump, to enhance sanitary performance, resulted in a 19% decrease in the recovery of useable hot water above 43°C. Given that the tanks tested during the field study were made of copper, the consequences of alternative material choices on thermal performance were explored. It was found that the rate of useable hot water loss, due to de-stratification associated with thermal diffusion across the thermocline, could be reduced by a factor of 2.7 by making the tank liner wall from stainless steel instead of copper. Further numerical work indicated that this improvement in stratifying performance was most significant for small tanks with high aspect ratios. In addition to de-stratification that arises due to vertical conduction, de-stratification due to inlet mixing was reduced by up to 30% by installing a spiral diffuser into the base of a test cylinder. In addition, by lowering the immersion heating element to ensure there is sufficient heat transfer to the base of the cylinder, sterilising temperatures could be attained throughout the stored volume of water in the tank during heating. This showed that the conflict between thermal and sanitary performance within electric tanks could potentially be resolved. A bespoke tank, made from stainless steel and fitted with a diffuser, was built and subjected to typical draw cycles that reflect real world operation. These tests showed that more useable hot water could be delivered in comparison to a commercial off the shelf copper tank and consequently the utilisation of the Economy 7 time of use tariff would be enhanced.

Cette thèse s'intéresse au développement de stratégies de décalage de charge pouvant être appliquées à un parc de chauffe-eau Joule (CEJ).On propose une modélisation entrée-sortie du système que constitue le CEJ. L'idée est de concevoir un modèle précis et peu coûteux numériquement, qui pourrait être intégré dans un CEJ intelligent. On présente notamment un modèle phénoménologique multi-période d'évolution du profil de température dans le CEJ ainsi qu'un modèle de la demande en eau chaude. On étudie des stratégies d'optimisation pour un parc de CEJ dont la résistance peut être pilotée par un gestionnaire central. Trois cas de figures sont étudiés. Le premier concerne un petit nombre de ballons intelligents et présente une méthode de résolution d'un problème d'optimisation en temps discret. Puis, on s'intéresse à un parc de taille moyenne. Une heuristique gardant indivisibles les périodes de chauffe (pour minimiser les aléas thermo-hydrauliques) est présentée. Enfin, un modèle de comportement d'un nombre infini de ballon est présenté sous la forme d'une équation de Fokker-Planck. / This thesis focuses on the development of advanced strategies for load shifting of large groups of electric hot water tanks (EHWT).The first part of this thesis is dedicated to representing an EHWT as an input-output system. The idea is to design a simple, tractable and relatively accurate model that can be implemented inside a low-power computing unit embedded in a smart EHWT, for practical applications of optimization strategies. It includes in particular a phenomenological multi-period model of the temperature profile in the tank and a realistic domestic hot water consumption model.The second part focuses on the design of optimal control strategies for a group of tanks. Three use-cases are studied. The first one deals with a small number of smart and controllable EHWT for which we propose a discrete-time optimal resolution method. The second use-case adresses a medium-scale group of controllable tanks and proposes a heuristic which keeps the heating period undivided to minimize thermo-hydraulic hazards. Finally, we present the modelling of the behavior of a infinite population of tanks under the form of a Fokker-Planck equation.

Despite the share of renewable energies worldwide is increasing, which can help in reducing the CO2 emissions, their unpredictability has become a problem due to the mismatch between generation and demand. Among the different alternatives to solve this problem, energy storage is a very interesting solution. Depending on the aim of the storage, there are two types: intra‑day or seasonal. The former corresponds normally to a highly efficient and high‑cost storage, such as the Li‑ion; whilst the latter is a low efficient and low‑cost storage. An example of this second type of storage is the power‑to‑heat‑to‑power storage, whose efficiency is mainly determined by the heat‑to‑power converter, and it can be increased if the waste heat produced in the converter is reused in a combined heat and power system.Given that the residential sector represents a large amount of the global energy use (both electricity and heat), this study has considered a power‑to‑heat‑to‑power storage in a fully electrified residential building with a PV installation in order to increase self‑consumption and reduce the cost. Both the heating, electricity and cooling demand are supplied by the system.As this storage technology is currently under an early stage of development, this project aims to understand the main challenges for this storage and the advantages over a very well settled technology, such as the Li-ion. In order to achieve this objective, a model has been created in Matlab. A parametric study has been conducted in which optimum sizing of the components for several scenarios have been considered, as a means to identify the most important parameters that could hinder the feasibility of the power‑to‑heat‑to‑power storage system.From the optimization it was concluded that the scenarios with a thermally driven heat pump for cooling, resulted in larger installations leading to higher cost due to the low coefficient of performance. Regarding the other scenarios which consider an electrical heat pump for cooling, this technology can surpass the Li-ion performance for heat‑to‑power efficiencies over 20 %. In these cases, the feasibility is clearly hindered by the cost per energy capacity, which must be below 5 €/kWh and could be achieved with silicon; and the cost per power capacity that must be around 300 €/kW. An example of a heat‑to‑power converter could be the TPV technology which is a solid‑state converter, whose efficiency is currently around 30 % and is expected to reduce its cost up to 300 €/kW. In smaller systems, in which the stand‑by heat losses have more impact over the system’s feasibility due to the larger surface to volume ratio, it is imperative to reduce these heat losses, as well as reduce the cost per energy and power capacities. In addition, it is remarkable that there is no significant improvement when increasing the heat‑to‑power efficiencies over certain values. To finish, as this technology increases its feasibility when implemented in large systems, further studies should be done in the industrial and tertiary sector.