Abstract
The 40% of the total energy consumption in the European Union is related to the building use. The 75% of the energy consumed for heating and cooling comes from fossil fuels. Heat pump units are a feasible solution to turn the building energy demand to electricity and to integrate renewable energy sources to reduce the carbon footprint of building use. This thesis investigates the use of solar and heat pump (SHP) systems for heating and cooling supply in residential buildings. The aim of the study is to investigate different solutions to increase the self-consumption of the system, pointing out the benefits and limitations of the proposed applications in relation to different boundary conditions. Several strategies have been proved to be effective to increase the level of self-consumption, such as energy storage (thermal and electric), control strategies (CS), and the activation of the building thermal inertia. However, it is still not clear how a combination of these strategies can affect the system operation and lead to higher levels of solar energy self-consumption. By exploiting the potential of variable-speed heat pump units, this research aims to define low-cost solutions to increase the self-consumption of SHP systems, by combining the aforementioned strategies.
This thesis analyzes two case studies to increase the use of renewable energy produced in loco, considering single-family houses located in cold climates. The first case study consists of an air-source heat pump units combined with a photovoltaic (PV) plant. A rule-based (RB) control strategy is defined based on the actual PV availability. The PV surplus is stored as thermal energy exploiting two water tanks, and the thermal mass of the building, and using an electric battery system. The second case study presents a water-source heat pump integrated in heating system with a seasonal storage system. The heat pump is used to exploit the residual heat in the tank to increase the solar fraction of the system.
In the first case study, the proposed control strategy is able to decrease up to 22% the amount of energy purchased from the grid and to increase the self-consumption in the range of 22-70%, considering different climates and building characteristics. The results show the feasibility of the proposed RB control strategy to increase the rate of PV energy self-consumption of the system without need of electric storage. At the same time, the CS is able to reduce the grid consumption and to stabilize the grid by reducing the peak load demand. Moreover, the exploitation of the building thermal mass, together with a proper control of the heat pump system, is proved to be able to improve the system operation. The integration of a battery system neutralized the impact of the control strategy over the grid consumption, considering sufficient battery sizes. Nonetheless, the control strategy is able to increase the self-consumption level of the system regardless of the electric storage size. Considering economic aspects, the proposed control strategy (CS2+) with thermal mass activation is able to reduce the annualized global cost up to 20%, in comparison to the standard control strategy, considering a reference period of 20 years. The control strategy results a profitable solution in combination with small battery size (0-1.2 kWh). In this case, the CS is able to reduce the annualized global cost by around 6%.
The second case study shows the integration of the heat pump in the seasonal storage system. This solution is able to increase the global solar fraction of the system in the range of 2-8% and to reduce the influence of negative boundary conditions on the system performance. The HP integration leads to an improvement of the system performance which decreases whit increasing tank sizes and solar collector areas. Under the reference boundary conditions, increasing the tank size reduces the benefit of the HP integration. Considering the economic aspects, the integration of the heat pump results not profitable for the analyzed case. The economic feasibility of the HP integration increases with higher energy demand of the system, for example considering worse boundary conditions.