# A Sustainable Polygeneration System for a Residential Building
## 1 Introduction  
In Europe, buildings are responsible for 40% of energy consumption and 36% of CO\({}_{2}\) emissions, and similar figures are reported in the United States [1]. The decarbonization of the building sector by 2050 is an ambitious objective requiring the implementation of energy transition strategies. To do so, the actions needed to perform that plan must be helped by regulatory changes and the support for investments in energy efficiency, especially for existing buildings, in which improvements are easier to reach since present references are relatively high in terms of energy consumption. Since 2002, three progressive Directives on Energy Performance of Buildings have been approved (Directive 2002/91/CE [2], Directive 2010/31/EU [3], and Directive (EU) 2018/844 [4]). This has led to a gradual improvement in enclosures to reduce losses, which has been transposed into national regulations. Consequently, from 2020, all new residential, office, and service buildings are expected to be nearly zero-energy buildings. In the next few years, new rules will try to account for many new and existing decarbonized buildings by the year 2050. A new directive is currently being drafted, which would limit the use of fossil fuels for community heating and promote high-efficiency heat pumps. A new energy rating for buildings is to be upgraded in quality in each country depending on the age of the building. To reduce final energy consumption, it is evident that improving the thermal envelope could lead to significant cuts in demand. It could also lead to better thermal comfort [5]. Note that around 50% of the primary energy demands in buildings are linked to heating, ventilation, and air conditioning. Building energy codes (BECs) have been one of the keys in reducing buildings' energy consumption. Unfortunately, their effectiveness is based on their mandatory and enforced nature [6]. This can be shown in comparative studies of BECs in neighboring countries with similar climate conditions [7,8]. On the other hand, energy demands have to be served more sustainably. A sustainable and safe solution for a building is a polygeneration scheme that simultaneously produces electricity, heating, cooling, and freshwater (partly converted into domestic hot water) from a single or several primary resources. Polygeneration schemes have been used in industry, in many cases with products other than those described above, and not always using only renewable energy sources [9,10]. Several studies on providing electricity, freshwater, and heating and cooling for diverse uses have recently presented sustainable solutions in isolated and/or rural areas [11, 12, 13, 14, 15, 16, 17, 18], coming from solar and biomass resources. In the case of these four basic demands for a building, the number of scientific references is already somewhat smaller according to our state-of-the-art review [19, 20, 21, 22, 23, 24]. In all cases, simulations are carried out, including the optimization of their design, their energy, environmental, and economic analysis, and their sensitivity to the externalities of the proposed plant. Still, there are no experimental studies analyzing the feasibility of this integrated approach, given its complexity and the significant investment required. A polygeneration scheme mainly based on RES heat-producing technologies, such as PVTs and biomass boilers, is a natural solution since these are the only available renewable sources in a water-scarce area near the coast. Anyway, at the tail of the scheme, different options to produce cooling and desalinated water should be checked to find the best integration. In other words, consuming heat or electricity will lead to different designs for the same purpose. In most of the previously analyzed papers, only a configuration has been performed for a case study. Therefore, the main novelty of this paper is the analysis of different structures of a polygeneration scheme based on renewable energies, essentially thermal, to cover the five typical demands of a building of 80 dwellings.