Resumen de Economic and environmental viability of central solar heating plants with seasonal storage in the european residential sector: a systematic multi-objective optimization approach
The global tendency for changing the world energy map is a booming topic, and more efforts should be scaled up to shift the current energy production systems towards the use of cleaner and less carbon-intensive sources. Currently, fossil fuels share about 80% of the primary energy use. Their high specific energy density and combustion temperatures up to 2500 ºC make them excellent energy carriers capable of meeting extreme energy demands. But unfortunately, large amounts of fossil fuels are used inefficiently to cover energy demands below 260 ºC, a large fraction of which belongs to the residential-commercial sector.
According to the European Environment Agency, in 2013 this sector represented 40% of the total final energy consumption. In order to improve its energy efficiency, this sector should use alternative energy sources, particularly for space heating. On top of that the International Energy Outlook forecasts a significant increase in the world energy demand over the next decades. It is projected that the global energy consumption will evolve by 48% in 2040 with a growth in the usage of crude oil and natural gas by 30% and 53.2%, respectively. This outlook trend leads to serious environmental problems such as more greenhouse gas emissions and the subsequent impact on the climate.
Europe is one of the relevant players in this scenario contributing 21.6% to the overall energy consumption. Additionally, in the European Union the building stock accounts for about 40% of the total energy demand, while the residential sector consumes 63% of this energy. According to estimations of the US Energy Information Administration, the energy consumption demand for the residential section in the EU increases by an average of 0.9% per year. Along with all of these figures, the residential buildings are the fourth most important source of GHG in the EU and it accounted for about 10% of the total GHG in 2016. In response to this challenge, the EU has adopted the 2020 climate and energy package which includes a set of requisite legislation to tackle the environmental concerns and support the energy security and independence. The package sets three main targets: (i) reduce by 20% the GHG emissions compared to the 1990 levels, (ii) increase the renewable energy share and (iii) improve its energy efficiency by 20%. In 2013, the EU approved a new ambitious framework for the climate and energy between 2020 and 2030. This strategy plans to cut the GHG emissions by 40%, to achieve a share of at least 27% of renewable energies, and to improve the energy efficiency by at least 27%.
Over the past decades, various technologies based on renewable energy sources have been put forward as viable alternatives to the use of fossil fuels, including wind power, hydropower, waste energy, geothermal energy, bio energy, solar energy and energy storage. In the residential-commercial sector, and especially in large cities or inner city areas, these technologies can become even more competitive if they are integrated in an existing district heating network.
Among all of the renewable energy resources, the solar thermal energy obtained a considerable attention since it is a CO2 neutral and it can be used for both space and water heating. Apparently, the solar thermal technologies could satisfy substantially the heat demand in the residential sector in many countries. Furthermore, it has several advantages which include (i) savings in the primary energy consumption at the end user and country planning level, (ii) increase in energy security against the fluctuations in the prices of the conventional energy resources, (iii) decrease the dependency on the electricity from the network, and (iv) contribute to the network stabilization. These solar thermal energy systems continue to increase their market share across whole Europe. More than 1.2 GW(thermal) was installed within 2015 to raise the total installed capacity to 34.4 GW(thermal).
However, the solar thermal systems are facing a great challenge of intermittency and predictability, which cause a gap between the supply and the energy demand. The thermal energy storage (TES) systems can effectively solve this issue. There are three main categories of the TES. These categories include the sensible TES through a temperature gradient, the latent TES based on the phase change materials, and the thermo-chemical TES through chemical reactions. Currently, sensible storage is the most common system to be used in the residential sector, while latent and chemical systems are promising technologies under development.
In order to maximize the benefits from the central solar heating plants with seasonal and short-term storages in the residential sector, the optimal sizing of the system components and their operation should be planned properly. This can turn into a computationally requesting task.
The aim of this work was to develop a systematic multi-objective optimization (MOO) framework for optimizing the design of CSHPSS plants considering economic and environmental aspects simultaneously. To this end, a simulation-optimization methodology was developed based on a CSHPSS plant modeled in TRNSYS 18 that was optimized by a generic optimization tool (i.e. GenOpt) according to economic and environmental indicators. The latter objective was assessed through LCA principles, which quantify the impact caused in all of the stages in the life cycle of the energy system. The inspiring numerical results showed that improvements in cost and environmental impact can be achieved simultaneously, comparing to a conventional heating system. With this knowledge we amplified the range of our study and investigated the optimal configurations of CSHPSS in different EU member states and identified forecast models for the cases which predict reductions in the cost of the installation of such systems in the near future. Moreover, with the expected growth of the prices of the primary non-renewable sources makes this type of plants even more attractive with the years.
In summary, the proposed methodology can serve as a supportive tool for decision-makers helping them assess the potential of the CSHPSS plants in Europe and subsequently, promote a clear statement towards the possibility of achieving the 2030 European climate and energy framework targets, and more sustainable energy future.
