# Energy Demand and Supply Simultaneous Optimization to Design a Nearly Zero-Energy House
## 1 Introduction  
It is broadly acknowledged that the building sector is responsible for a considerable fraction of energy consumption and CO\({}_{2}\) emissions that can be attributed to a nation. In Europe, these percentages are approximately 40% (primary energy consumption) and 36% (CO\({}_{2}\) emissions) of the annual overall energy outlay and greenhouse gas emissions. As a result, the building sector has been identified as one of the key areas in which measures have to be taken in order to achieve the 20/20/20 targets (20% of greenhouse gas emissions compared to 1990, 20% energy savings by 2020, compared to a business scenario, and 20% share of renewables in 2020) that the Member States have set by common agreement [1]. From this perspective, the first recast of the Energy Performance of Building Directive (EPBD) established the framework and the limits to fulfill these targets. They impose an adoption of measures to improve energy efficiency in buildings in order to satisfy the requirement where, by 31 December 2020, all new buildings should be nearly zero-energy buildings (NZEBs), which are defined as \"buildings whose energy requirement should be very low and significantly covered by renewableenergy sources\" [2]. The second recast of EPBD further encourages NZEB design towards the medium-term target of reducing greenhouse gas emissions 88-91% by 2050 (compared to 1990) [3]. Furthermore, it is recognized that these objectives cannot be pursued without considering financial feasibility, considering that improving the energy performance of buildings is one of the measures with the largest potential for net economic savings in all building types [4; 5]. In this respect, EU established a comparative methodology framework to compare different design alternatives under a cost-optimal perspective [6]. This approach has acted as one of the main drivers for the European scientific community to carry out research related to NZEB design [7]. Many studies performed across Europe demonstrate that efficient NZEB design results from optimal interactions between the many variables impacting energy demand (mainly related to the building envelope), energy supply (related to systems and renewables), and the technical and economic scenarios in which the building is designed. That is why the design of NZEBs can be identified as a complex optimization problem. Considering the complexity of such a problem, growing interest has been reserved to computer-based optimization techniques throughout building research communities [8]. These techniques, also known as simulation-based optimization methods (SBOMs), are based on the combination of a building dynamic energy simulation program and an optimization engine, and they consist of an iterative calculation process performed using a computer-automated model driven by a specific optimization algorithm [9]. Reasons for the success of SBOM are seen in its relatively less expensive design and quicker computation time; this is because of the larger, faster, and therefore more efficient exploration of the design space. That is why SBOM appears to be the most promising method to search for NZEB optimal solutions with the greatest exploration of the design space while maintaining manageable calculations. From a review of recent applications of computational optimization methods applied to building design [10], the design variables that typically affect energy demand are:  
* the shape of the building and its orientation;
* the thermal insulation and inertia of the opaque and transparent envelope;
* the type and size of possible devices to control solar radiation; and
* the optical properties of the transparent elements. Regarding the energy supply side, the following variables may be considered:  
* the type and number of energy generator/converters as well as their sizing;
* the type and size of heating and cooling terminals in thermal zones; and
* the presence and sizing of thermal storages or electrical storage batteries.