Carbon Neutral Urban Block in Athens - 2050

Athens’ extensive urbanisation, lack of green areas and the extreme heat caused by increasingly frequent heat waves indicate the need for actions improving indoor and outdoor comfort, which is closely related to the energy consumption of the buildings. This work’s aim is to create a carbon neutral block in Athens on the 2050 horizon. The optimization of the block’s form based on principles of environmental design and climatic analysis was performed to enhance its environmental benefits. Simulations on the energy performance of the block and calculations on the ability to cover the energy loads by renewables were conducted. Finally, to meet the zero-carbon neutrality, a connection with the neighbouring blocks was established. The results demonstrate the benefits of a bioclimatic, carbon neutral building design in Athens and provide a practical prototype, which can be adapted in other projects, thereby enabling the shift to a more efficient and environmentally friendly built environment.


Introduction
Athens is the third densest city in Europe with only 0.96 m 2 of green area per resident, well below the limit provided by WHO of 9 m 2 per resident [1]. During the first three post World War II decades, the population of Athens more than doubled. The city developed rapidly with no urban planning, mostly by unskilled workers. This resulted in the disorganised placement of similar buildings in the city, also known as 'polykatoikia', which represents 73% of residential buildings in Athens [2]. Thermal insulation was absent, turning the building stock into a high energy consumer, especially during winter. Greece shows a 5%/year increase of energy consumption, which contrasts with most European countries [3]. Uncontrolled urban growth and the fact that the unbuilt space in-between the buildings was considered as a left-over space, led to a degraded urban environment.
The declining population rates show a way towards a solution as the need for housing is predicted to decrease. There could be space in the city to cover other needs such as adding green spaces that the city lacks, areas for renewables and spaces for outdoor activities. Zero carbon solutions are being explored  at an energy consumption level. A zero-carbon city runs entirely on renewable energy. It has no carbon footprint and will in this respect not cause harm to the planet.

Climate
Athens has a hot-summer Mediterranean climate (Csa) with hot and dry summers and mild winters. Looking at the projection of 2050, higher temperatures will be recorded throughout the year, averaging +2 o C, while the dry summer will be extended and last from June till September. Climatic analysis indicates that in the design process both winter and summer seasons should be considered. Strategies should be season-specific and easily retractable to adopt to the different conditions.

Passive Design Strategies
The goal of a well-designed urban block in 2050 will involve resilience to future changes, reduction of energy demand and minimisation of the negative impact on the environment. The optimization of the block's form based on principles of environmental design and climatic analysis was created to enhance its environmental benefits. The site selected is a typical block of 80m by 60m located in the Koukaki district of Athens. Building height restrictions apply here, thus permitting lower density developments (18-21m height) than those of the rest of the city, due to the proximity to the hill of the Acropolis.
Various massing typologies were tested and it was found that, based on initial research, the courtyard urban block works best. Besides the fact that courtyard buildings constitute the typical style of Athenian vernacular architecture [4], courtyards can be a source of fresh air, light and heat or coolness.
The vegetative courtyard can contribute a reduction of up to 48% in energy consumption [5]. An extra canopy    can further improve the micro-climate of the courtyard. If cut-outs in lower levels and water elements are also added, then the courtyard temperature can be 6-10 o C below the outdoor temperature [6].
Height analysis based on VSC criteria results in some changes of the shape, to not negatively affect the surroundings. A cut was made to make sure that all levels receive the appropriate amount of sun.
The key issue of this step is taking advantage of the site's solar potential to reduce heating loads in winter.
Moving further inside the building, the internal distribution was tested. There is a better performance when living rooms face towards the courtyard, than when they face the street. Moreover, the benefits from the courtyard are of great importance. For this reason, balconies which are connected to the living room are maximized facing the courtyard and access corridors facing the street. A typical plan is created in this manner.

Thermal Studies -Unit
The block has three types of units. To analyse and calculate the energy performance of the study block, one typical unit of each type was selected as a representative. In this paper, results for unit B are selected to be shown. Typical unit type B is 80m 2 occupied by 3 people. It is located on the third floor of the north-west wing. The plan layout was chosen to enable cross ventilation in the common areas. The size of the windows is the maximum allowed to take advantage of the whole glazing area. Designing for a hot climate implies minimizing the glass surfaces, which can cause an increase in cooling loads.
To simulate the thermal conditions of the indoor spaces, particular attention was paid to the building elements' construction. Of great importance are the double-glazed windows which have a G value of 0.4 (antelio clear glass) and the ceiling with a time constant of 5.694. The unit is divided in 8 zones. The access corridor and the balcony are considered as external spaces. The bathrooms and the entrance are unconditioned indoor spaces. The remaining areas (living room, kitchen, and the bedrooms) have been tested to calculate the unit's energy demand.

Summer Strategies
Five scenarios were tested for the summer thermal analysis, using the TAS EDSL modelling tool. All summer strategies are retractable. Testing the performance of the unit considering all windows open for the whole day is the base case scenario. The combination of the balcony as a shading strategy, thermal mass, the antelio glazing and the size of the windows result in operative temperatures following the external temperatures, while during some days, they are even lower.
The first scenario can benefit the unit with difference of up to 5 o C. The importance of opening the windows strategically, based on the climate and the micro-climate of the site, is highlighted. Closing the windows when the indoor dry-bulb temperature is 31 o C stops the rising and helps to avoid high peaks. Night-time ventilation takes advantage of the low nocturnal temperatures.
The second scenario is focusing on the optimization of the south-east facade. The spaces adjacent to this facade are related to the courtyard. For this reason, the courtyard is shaded by a horizontal wooden structure. This strategy offers multiple benefits. Not only the unit's temperatures are decreasing by 1 o C, but also the courtyard is showing lower temperatures, providing a cooler micro-climate. Strategies to cool down the courtyard and bring cooler air into the units have already been disgusted previously. Due to software limitations, these strategies (deciduous plants and water features) cannot be simulated. Scenario 3, is focusing on the north-west facade and bedroom 2, resulting in a temperature drop of 4 o C.
In scenario 4, the final one, solar gains are reduced almost by half in the living room and by 5 times in Bedroom 2. Entrance, kitchen and living room are related, showing similar temperatures. Thus, by shading the north-west façade, it is not only affecting Bedroom 2 but also the kitchen and the living room. The combination of the scenarios shows exceptional results, with the spaces to be mostly within the comfort zone when outdoor temperatures do not exceed 35 o C.

Winter Strategies
During the winter period, the access corridors are closing with retractable glazing, turning them into an indoor space with high infiltration. The solar gains in winter can get higher than those of summer and mid-season. The goal of the building's bioclimatic design was to allow access to the low winter sun in the units as much as possible.

Results
Comparing the final heating and cooling demand to the Passive Haus thresholds (12 kWh/m 2 ) [7] it is shown that it is below recommendations (6.3 kWh/m 2 ). The analysis of the summer period strategies shows that the performance of the building is affected by the type and the amount of shading, thermal mass, and windows strategies. Heating loads are lower than cooling loads by more than half.

Net Zero Carbon
Mechanical systems working with electricity are chosen to cover the heating and cooling demand so that the energy required could be covered by the solar panels. The demand calculated previously is translated into electricity by using COP and EER numbers. Film heaters with a COP of 1 are chosen ending up with 22,605 kWh annually. For cooling, a mixed mode with electric ceiling fans and an airconditioning system (EER 4.95) is chosen. This ends up with 13,204 kWh annually. Even though the cooling demand is twice the heating demand, the selected cooling systems are more efficient, demanding less electricity.
The entire roof space is used for solar energy generation providing 414,301 kWh/a of electricity. In buildings, the availability of solar power does not match up with the temporal demands of a building. Excess solar power is sent off and additional energy needed during times of insufficient solar power is supplied by other sources. Balancing these two flows allows a building to become net zero.
The graph below shows the monthly consumption, the supply from the PV panels and the remaining excessive or needed energy. From March till October, the study block generates more electric power than needed. From November until February, the solar radiation is not enough to cover all the block's needs.
To achieve the net-zero target set in this study, four concept scenarios were assessed. The first scenario consists in storing the excessive energy produced in summer to be used in winter, where there is more need. The second scenario consists in sharing the excess energy beyond the boundaries of the building or the block, with a cluster of neighbouring buildings; a so-called "nearby energy community". The third scenario consists in generating off-site renewable energy through a financial assistance to a company working with wind farms. Finally, the fourth scenario consists in sharing with the grid.
The scenario selected is the second one. Approaches that are not only limited to a single building might enable reaching almost zero energy conditions in a "Carbon Neutral Neighbourhood". This approach has the potential to increase energy efficiency, renewable and local resource application and to promote energy resilience and security [8]. Since the study block constitutes an example to be replicated in the city, the new blocks can be linked, sharing energy with each other.