گزینه های تامین انرژی تجدید پذیر در محل یا خارج از سایت؟ تجزیه و تحلیل هزینه چرخه عمر یک ساختمان انرژی "صفر خالص" در دانمارک

The concept of a Net Zero Energy Building (Net ZEB) encompasses two options of supplying renewable energy, which can offset energy use of a building, in particular on-site or off-site renewable energy supply. Currently, the on-site options are much more popular than the off-site; however, taking into consideration the limited area of roof and/or façade, primarily in the dense city areas, the Danish weather conditions, the growing interest and number of wind turbine co-ops, the off-site renewable energy supply options could become a meaningful solution for reaching ‘zero’ energy goal in the Danish context. Therefore, this paper deploys the life cycle cost analysis and takes the private economy perspective to investigate the life cycle cost of different renewable energy supply options, and to identify the cost-optimal combination between energy efficiency and renewable energy generation. The analysis includes five technologies, i.e., two on-site options: (1) photovoltaic, (2) micro combined heat and power, and three off-site options: (1) off-site windmill, (2) share of a windmill farm and (3) purchase of green energy from the 100% renewable utility grid. The results indicate that in case of the on-site renewable supply options, the energy efficiency should be the first priority in order to design a cost-optimal Net ZEB. However, the results are opposite for the off-site renewable supply options, and thus it is more cost-effective to invest in renewable energy technologies than in energy efficiency.

The concept of a Net Zero Energy Building (Net ZEB) implies that on an annual basis the primary energy use of a building is offset by the energy generated from conversion of renewable sources. The technologies, which convert renewable sources, are generally divided into two groups. The first group encompasses the systems installed either on/in the building or on the ground directly attached to the building. The second group includes the systems placed outside the boundaries of the building site, which either are the property of the building owner or the building owner just purchases the generated energy in order to reach the ‘zero’ energy goal. The first group is often labelled as ‘on-site renewable energy supply (on-site RES)’, and the latter as ‘off-site renewable energy supply (off-site RES)’. The above described division is done with focus on the actual location of the renewable technology. Torcellini et al. [1] adopt the same terminology, ‘on-site’ and ‘off-site’; however, they group the systems not according to the location of production but to the origin of used renewable energy source. Generally, the two approaches are very similar. The major difference concerns the biomass/biofuel micro Combined Heat and Power (micro CHP). By adopting the first approach, this technology is an on-site renewable supply option. However, according to Torcellini’s system, the CHP is an off-site supply option because the biomass/biofuel, before being converted to useful form of energy, i.e., electricity or heat, has to be transported from outside the boundaries of the building site. In this paper, the renewable technologies are labelled according to the location of the conversion, e.g., inside the boundaries of the building site – on-site, and outside the boundaries – off-site. According to Marszal et al. [2] and Voss & Musall [3], the most commonly used on-site renewable technologies, primarily generating energy and thus meeting the ‘zero’ energy goal, are photovoltaic (PV) and solar thermal panels. Similar to the international trends, the Net ZEBs in Denmark exploit solely on-site systems [4], [5], [6] and [7]. Also, the Net ZEB definition proposed by the Danish Strategic Research Centre on Zero Energy Buildings includes only on-site and building connected renewable energy technologies [8]. Keeping this approach, Marszal and Heiselberg [9] deployed a life cycle cost analysis to investigate the cost-optimal relation between energy efficiency improvement and on-site renewable energy supply for a multi-storey Net ZEB. The authors concluded that from a private economy perspective, the cost optimized Net ZEB is a building with greatly reduced energy use (around 20 kWh/m2 per year of primary energy, corresponding to the minimum level of energy performance requirements for residences in 2020 in Denmark) and a small on-site renewable energy system. The cost-optimal on-site renewable system is a PV installation in combination with a ground source heat pump. However, taking into consideration the limited area of roof and/or façade, primarily in the dense city areas, the Danish weather conditions, the growing interest and number of wind turbine co-ops [10], the off-site renewable energy supply options could become a meaningful solution for reaching ‘zero’ energy goal in the Danish context. Therefore, by acknowledging that the user/building owner perspective and economy [11] and [12] are crucial factors for a successful adaption of environmental- and climate-friendly technologies, this paper deploys the life cycle cost analysis to investigates the cost-optimal path towards ‘zero’ energy goal in case of off-site RES. The study includes three levels of energy performance requirements, i.e., level 0, level 1 and level 2, with level 0 being the most demanding one. The off-site renewable energy supply options included in the analysis are: (1) private windmill, (2) shares in a windmill farm or (3) purchase of energy from 100% renewable utility grid. By combining the results of this analysis with the results of [9], and additionally by adding the less popular in Denmark micro CHP as on-site RES to the investigations, there are all together 10 different renewable energy supply systems. Thus, this paper provides a comprehensive overview of life cycle cost of different RES from a private economy perspective. Moreover, as the energy use of the Net ZEB is modelled by using a mean monthly-based steady-state calculation tool (Be10) [13] and an hourly-based dynamic simulation tool (BSim) [14], the paper verifies the influence of the resolution of simulations on the energy performance and the life cycle cost of a newly constructed Net ZEB.

The main goal of this paper was to determine the life cycle cost of the on-site and off-site renewable energy supply systems, and to define the cost-optimal relation between energy efficiency and renewable energy generation of these systems for a newly constructed multi-storey residential Net ZEB. The findings of these studies contributes to the on-going discussion of cost-optimal level of energy efficiency for the Net ZEBs, and which renewable supply options should be included in the Net ZEB definition. The analysis has shown that from the private economy perspective and with the current technologies’ cost and energy price, in 4 out of 5 on-site RES options investment in energy efficiency is more cost-effective decision than investment in renewable energy technologies. However, decrease in PV price by 50% changes the life cycle cost trends for the PV-HP and PV-DH alternatives. The off-site RES options have a reverse life cycle cost trend, and for all systems the combination of less demining energy frame and high renewable energy generation is the most cost-optimal path towards Net ZEB. Hence, the off-site renewable energy supply options are not in-line with the Danish initiatives of further decrease of minimum energy performance requirements beyond 2010 regulations. For the on-site and off-site RES options, the cost-effective system is PV-MiCHP(biomass) and SofW-HP or GR-HP, respectively. Moreover, the SofW-HP and GR-HP systems are also the cost-effective systems among all ten renewable energy supply options. However, as presented in Fig. 11, with the PV price reduction by 75%, the PV-HP results to be the cost-effective system. Although, the on-site RES systems have slightly higher life cycle cost than the off-site RES systems, they have a smaller part of the life cycle cost allocated in the energy cost. Therefore, from the private economy perspective, these renewable energy supply options could be seen as more save investment, as it is more robust for the fluctuations of the energy prices. This analysis did not take into consideration such a factor as convenience of various renewable energy supply options. However, it is clear that some options could be seen as more convenient than the others. For example, the off-site RES neither influence the design of a building nor occupy space inside or outside a building. However, having an off-site windmill requires that the building owner has also a peace of ground in a location with good wind conditions. In the case of the on-site RES, the PV panels could be seen as more convenient than the micro fuel cells CHP units. Firstly, they could be easily integrated into the building design, and secondly they do not require any technical room to be place in or storage for fuel, e.g. wood pallets. The results also indicated that, not taking into consideration the future cost, i.e., the cost of replacement, operation and maintenance during the building design process, thus a lack of life cycle cost perspective in the decision making process, leads to a non-cost-optimal building design. Finally, it can be concluded that the increase of resolution from monthly to hourly does not have a significant influence of the life cycle cost results. Therefore, the mean monthly-based steady-state calculation tool Be10 can be used as software for the calculations of energy demands of the building. It should be emphasized, that the authors acknowledge that the modular type of the building construction and analysis of only one Net ZEB topology could be seen as the limitation of this life cycle cost analysis.