Economic and environmental analysis of central solar heating plants with seasonal storage for the residential sector

Résumé

Buildings represent 40% of the Union’s final energy consumption; the member states should establish a strategy to improve the energy performance in buildings and reduce the consumption of non-renewable primary energy. In Spain, the implementation of the Technical Building Code (CTE) compels to install solar thermal collectors in new buildings providing a minimum solar contribution of domestic hot water (DHW). In north and center European countries, e.g. Denmark, Germany and Austria, new installations also supply heat for the space heating needs. The approach of central solar heating plants with seasonal storage (CSHPSS) is the storage of solar thermal energy from the period of higher offer (summer) to be consumed in the periods of higher demand (winter). These installations are integrated into district heating systems that supply heat for a large number of dwellings and reach a solar fraction of 50% or higher. In this thesis the experience gained in Europe on centralized solar district heating systems with seasonal storage will be transferred to the Spanish situation, in order to establish the conditions and criteria for installing these systems in Spain in the midterm. The main objective of this thesis is the proposal and design of CSHPSS that could be able to provide a high fraction of thermal energy demand with solar thermal energy for different climatic areas. These systems should be feasible from a technical viewpoint, economically acceptable, and with a low environmental impact. That is, this thesis unveils the requirements for the feasibility of CSHPSS and is intended to foster their development in Spain. In order to reach this objective, it has been performed a revision of the state of the art of district heating systems, with emphasis to: i) solar district heating systems and CSHPSS; ii) design and calculation methods that could be used for new systems in Spain; iii) economic data and results from existing solar district heating systems and CSHPSS in Europe and worldwide; and iv) environmental assessment methodologies and analysis performed for solar thermal components and systems. An original calculation method for the analysis, design and evaluation of these installations from technical, economic and environmental points of view has been developed. The variation of solar radiation along the day and the year and the monthly distribution of the residential sector demand are considered. The main advantage of the method developed, compared to other methods, is the simplification of the calculation process and the utilization of simple climatic and demand data. The method developed has also been used to perform parametric analyses that have served to obtain new design criteria for different locations. The technical viability of these installations is not enough argument to motivate their development. The investment cost of these installations has therefore been analyzed according to the main design parameters (area of solar collectors and seasonal storage volume) and validated with results from real projects in north European countries. Moreover, this thesis analyzes the environmental impact of these installations using the Life Cycle Assessment (LCA) methodology. This impact assessment not only considers the consumption of fuels and electricity for the production of energy but also the consumption of materials for the construction of the plant. Three different environmental assessment methods have been used to determine the impacts generated and avoided by a CSHPSS: i) emission of greenhouse gases and their contribution to the global warming; ii) consumption of primary energy; and iii) environmental indicator IMPACT 2002+, which encompasses a significant range of environmental burdens. Based on the previous thermal, economic and environmental models and analyses developed, appropriate design criteria for CSHPSS in different geographical areas have been established. It has been concluded that design criteria are strongly dependent on the local climatic and demand conditions. Therefore, CSHPSS designs for north European countries cannot be applied in south Europe. Furthermore, it has also been concluded that CSHPSS have a considerable potential in Spain; i.e. it is interesting to build CSHPSS in those regions of Spain with significant heating demand, because they can supply heat to large communities at a competitive cost with a low environmental impact. Finally, from the calculation and analysis tools developed in the thesis, a software application with a friendly user interface has been developed to pre-design CSHPSS. The software is mainly oriented to European locations and provides the thermal performance, economic cost and environmental impact of the evaluated CSHPSS.