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In the last decades of the 20th century, primarily economic crises -caused by security challenges in supplying energy – and many years later, understanding environmental damages due to improper use of energy, sounded the alarms for scientists, planners and governments. Since the building sector owns a large share in total energy use, it has always been in the focal point of attentions. Consequently, national regulation established to reduce energy consumption and environmental concerns and climate aligned construction became the source of inspiration in architecture and the term ‘sustainable architecture’ was coined. Energy conscious architecture encompasses different aspects and a variety of strategies, ranging from conservation in energy use to applying renewable resources of energy. Regardless of the level of technology in a building, the sun can play an effective role in the energy generation/consumption balance. The cities are growing vertically and the façades have found an undeniable importance for solar energy harvesting in buildings. Integrating solar energy devices with building elements such as walls or roofs, increases the homogeneity of the architecture and improves the overall efficiency of building regarding its life-cycle. Literature Review: The exterior shell of buildings, due to their role in energy gain/loss, has always been the subject of researches with different approaches. Mahdavinejad focused on the influence of roof’s shape on energy balance of a building. He proved that a 60-30 sloped roof, with larger surface facing the sun, shows the best performance, while a dome is the worst in the aspect of thermal behavior. Taban et al. investigated the role of brick ornaments in traditional architecture of historical parts of Dezful, Iran. The results showed that the embossed and engraved patterns made by brick work, increases the shadow cover and plays a role in keeping the wall cool. This research, focuses on the building shell as a solar collecting element. There are many reasons to employ the building envelope to collect solar energy. Limited space in buildings, high price of land, and offsetting the costs of façade construction, are just some of the benefits achieved by integrating solar energy systems with building elements. Methodology: This research focuses on different aspects of the proposed idea and evaluates the system by means of software simulation method. The collector system was modeled, simulated and optimized by TRANSOL® solar analysis program, which is based on dynamic simulation of TRNSYS® engine and uses its validated models. Also, to determine the solar coverage index, a sample room in an office building in Tehran was supposed. Thermal demand was calculated using EnergyPlus® 8.0, while its south façade supposed to be covered with the façade integrated collectors. Results and Discussions: Due to the large angle between the sun’s radiation direction and the normal vector of façades in summer, especially in cities with lower latitudes such as Tehran, a major part of radiation is reflected from the façade surface. Resultantly, if any conventional solar energy collecting solution to be installed on the façade, the efficiency drops extremely and the system may lose its economic justification. To solve this problem, this research recommends that the cover surface of the collector should be oriented toward the sun direction; and to maintain the reasonable thickness of the building shell, this can be achieved by a corrugated geometry. We suggested tubular evacuated solar collectors positioned horizontally todecrease the reflection and increase the annual efficiency of façade integrated collectors. Unlike the vertical array, the horizontal positioning passively tracks the altitude of the sun. Additionally, the proposed configuration benefits from a higher insulation level –vacuum- which results in higher solar energy yield and higher fluid temperature ranges. The higher fluid temperature, results in higher coefficient of performance (COP) of cooling systems driven by thermal resources, such as absorption, adsorption or desiccant chillers. Accordingly, the solar cooling becomes more economical and the solar façade can continue to be used even in summers, at its maximum capacity. For convenience in maintenance and replacement, the heat pipe technology was chosen to transfer heat from the absorber surface of evacuated tubular collector to the circulating fluid of solar collector system. Heat pipe as a container made from copper, contains a drop of a liquid -called working fluid. The liquid -mostly distilled water, gains heat from the lower end of the pipe, and evaporates. The evaporated liquid travels to the other end, and leases its latent heat content to the body of the pipe, and transforms to the liquid form again. For a horizontally positioned heat pipe, a wick structure is needed to return the drop to the evaporation part. This cycle provides a great heat transfer capability. A heat pipe conducts heat to the heat transfer fluid through a dry connection between condenser and manifold, so there is no need for circulating the fluid in the façade collectors; so that, the collector can be installed, repaired or replaced without any sealing requirements. A heat pipe based collector eliminates the vulnerabilities of active solar façade against physical damage risks, extensive fluid pressure drop, leakage and obstruction. The results proved that a horizontally positioned tubular evacuated collector yields much better efficiency all over the year, especially in summer, respectively equal to %62 and %65, while a conventional façade mounted flat solar collector may not show an annual efficiency of higher than %30 and a summer time efficiency of higher than %21. To segregate the effect of vacuum insulation from the impact of geometrical form of the tubular evacuated collector -for the proposed positioning and configuration, a third situation was defined, in which, the flat collector was assumed to have insulation level equal to that of the tubular evacuated collector. The comparison showed that the geometrical form (circular section profile) plays the most important role in increased efficiency during summers, because the sun rays is always perpendicular to the mentioned profile section. On the other hand, the vacuum insulation is mostly effective in winters, in which the temperature difference between the collator and the environment is extremely high, while the lower altitude of the sun rays make smaller angles with the collector surface. Conclusion: Using horizontal evacuated tubular collectors allows penetration of sunlight to the adjacent spaces, provides relative transparency and at the same time, in summers functions like a shading device to maintain thermal and optical comfort of the occupants and decrease cooling load of the building. The study showed that the system may virtually cover the whole demands during the year. Precise sizing is a requisite measure to prevent from problems that may occur due to excessive heat generation, which in turn may cause damages to the system. However, heat pipe principle benefits from an inherent thermal fuse mechanism according to its narrow range of operation temperature. Overall, the research showed that the suggested façade integrated collector can efficiently gather solar radiation and supply a major part of thermal energy demanded for heating and thermally driven cooling. Additionally, the system has other functionalities regarding ease of repair, replacement and low operation risks and costs and long life of service. Keywords: solar façade, tubular evacuated collector, horizontal tubular collector, heat pipe, façade integration.

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