Optimal electrical and thermal energy management of a residential energy hub integrating renewable energy, demand response and energy storage system

Abstract

The energy consumption of residential buildings contributes approximately 42 % towards the total electrical energy production on a global scale. In residential buildings, the energy consumption could be further sub-categorized into the electrical energy required for space heating, space cooling and water heating, which accounts for 61.2 % of the total energy consumption, thereof. These systems utilized in large residential buildings, are vital for the occupants, to meet their thermal comfort needs, with regards to the residential building air temperature and hot water temperature. Some conventional means of performing space heating and water heating technology, is the boiler type, where hot water is transferred to the various rooms within the residential building and, thereafter, transferred to the spaces in the building, by means of a radiator. Another conventional space heating and water heating technology, is an electric element, which is used predominantly in water heating applications, as well as the wall mounted electric radiators. These conventional technologies are extremely ineffective due to their elementary operating principle. Therefore, in this study, an air-to-air heat pump was selected, to provide space heating and space cooling to the residential building and an air to water heat pump, which provides hot water to the residential building. This type of technology generally consumes approximately three times less energy, compared to the conventional technologies, whilst performing at similar levels. However, the initial investment of the air-to-air heat pump and air-to-water heat pump type technology is significantly higher in comparison and becomes economically feasible over an extended period. The air-to-air heat pump and air-to-water heat pump could be further improved through various methods, from an operational energy cost and energy efficiency perspective, such as the addition of renewable energy, energy storage systems and effective control techniques, to the residential energy hub, as well as thermal energy recovery. The residential building was integrated with renewable energy, which in this case is solar photovoltaic (PV) technology and an energy storage technology, known as hydrogen. Effective energy management techniques, making use of advanced optimization techniques, become essential when it comes to further energy efficiency optimization and operational energy cost minimization of a residential energy hub. Therefore, in this study, various methods to minimize the operational energy costs and improve the energy efficiency of the space heating, space cooling and water heating equipment are applied to a large residential building. These methods, firstly, include the addition of solar PV technology with hydrogen energy storage and, secondly, apply optimization techniques to the large residential building. The main objective is to minimize the operational energy cost of the utility grid, with respect to the Time-of-Use (TOU) tariff structure, to the electrical load and the polymer electrolyte membrane water electrolyser (PEMWE), which is responsible for producing hydrogen gas. The other main objectives, were to maximize the electrical energy supply to the load produced by the solar PV modules, optimal switching statuses of the air-to-air heat pump and the air to water heat pump. These main objectives outlined, were achieved by making use of the optimization algorithm, known as the OPTI-Toolbox, in the MATLAB software. These optimization problems are known as mixed integer nonlinear optimization problems (MINLP) and are solved using the (solving constrained integer programs) SCIP solver in the optimization toolbox of MATLAB. The first model was developed for the optimal switching control of an air-to-air heat pump space heating and space cooling system, to provide space heating and space cooling to a large residential building. A mathematical model of the proposed system was developed, after which simulations were conducted to reveal the performance, as well as the economic viability, thereof. A second model was developed, of a hydrogen PEMWE water heating system, after which the optimal control algorithm was applied and simulated. A third model was developed of the residential energy hub integrating renewable energy, demand response and energy storage system. The proposed model was simulated, in terms of its operation to evaluate the performance and the economic feasibility, thereof. Two baselines were established for the first model, to thoroughly evaluate the performance and economic feasibility of the air-to-air heat pump space heating and space cooling system. One baseline was established, for the second model, to thoroughly evaluate the performance and economic feasibility of the hydrogen PEMWE water heating system. Two baselines were established, for the third and final model, to thoroughly evaluate the performance and economic feasibility of the proposed residential energy hub integrating renewable energy, demand response and energy storage system. The proposed air-to-air heat pump space heating and space cooling system, depicted a daily operating energy cost saving of 27.63 % and 14.73 %, compared to the first and second baseline, during the selected winter day, respectively. Furthermore, a potential daily operating energy cost saving of 16.91 % and 12.30 %, could be achieved, compared to the first and second baseline, during the selected summer day, respectively. Additionally, the study focusing on the proposed hydrogen PEMWE water heating system, revealed that the hydrogen PEMWE water heating system, with optimal switching control, produced slightly less hydrogen energy and, in turn less hydrogen, compared to the standard PEMWE system. However, a daily maximum of 67.32 kWh of thermal energy was recovered during summer. Furthermore, by recovering the generated heat from the PEMWE, the time for the membrane to degrade to a thickness of 50 %, could be prolonged by 1.02 years. Thirdly and finally, the proposed large 270 occupant residential energy hub integrating renewable energy, demand response and energy storage system, revealed a potential breakeven point of 5 years, compared to the first baseline, with a potential cost-saving of 5 640 043.28 USD over a 20-year life cycle, was observed. The optimal control case revealed a potential break-even point of 5.74 years compared to the second baseline, with a potential cost-saving of 5 102 634.70 USD, over a 20-year life cycle, was observed. Furthermore, the proposed optimal control model achieved an annual operational grid energy cost-savings of 83.59 % and 82.37 %, compared to the first baseline and second baseline, respectively. The results of the various proposed models, by applying the proposed optimal control techniques, illustrated that an improved feasibility, in terms of energy efficiency and economic feasibility of high initial capital investments may be achieved. These operational energy costs and energy efficiency incentives become of great importance when it comes to the national energy grid security and greenhouse gas emissions.