Product

Hybrid Solar Energy Systems

Prototype Systems

Advanced Double Skin Façades Integrating Semi-Transparent PV

By Zissis Ioannidis, Concordia University, Canada

Product Description

Brief Concept Description

Advanced Double Skin Façades (DSFs) can be identified as one of the most promising responsive building elements. In the last years, they have gained a rising attention due to their high potential to achieve energy, comfort, aesthetic and structural advantages, as well as to provide space for integrated PV panels. DSFs are one of the buildings key new features and their use in combination with PVs (DSF-PV) become particularly favourable in case of high-rise buildings where the wide exposed envelope area can be used to harvest solar energy.

DSFs typically consist of an exterior and an interior skin separated by a ventilated air cavity used as air channel and thermal buffer. This solution applies to new construction but also to refurbishment actions, where an external skin is added to the existing envelope. The air cavity of the DSF creates a microclimate around the building, enhancing its climate resilience and its adaptability to weather changes. Additionally, the temperature differences inside the air cavity can facilitate natural/hybrid ventilation or can be exploited for heat recovery purposes, reducing in this way the space heating or cooling loads of the building. Moreover, the addition of an external layer is a valid opportunity to integrate photovoltaics, which is normally a cost-effective investment to further improve the energy performance of the building.

Architectural and Technological Integration into the Envelope

In DSFs, the exterior skin acts as the exterior layer of the envelope, protecting it from the weather phenomena, while the cavity provides pressure equalization and keeps the water from entering the envelope. There are different ways to integrate the semi-transparent PV in the envelope of the building to form a double skin façade. The most practical way is by utilizing a curtain wall approach, where the semi-transparent PV modules are secured on the mullions with pressure plates and the width of the mullion is the width of the DSF-PV cavity


Figure 74. Architectural integration of DSF-PV in a curtain wall design.

Integration into the Building: System and Comfort

In order to operate, the use of fans and dampers for the DSF-PV is required. The fan cools down the semitransparent PV panels and drives the pre-heated air towards the mechanical ventilation system of the building and for these reasons is one of the most important elements for the coupling between DSF and building (Figure 75). The development of suitable strategies to control fan and dampers of the DSF-PV system can increase the efficiency of the system, but, if such strategies are not thoroughly investigated, occupants might perceive discomfort. For example, a fan working over the appropriate set-point might generate excessive noise disturbing the occupants, cooling down the interior glass of the DSF-PV to a point where occupants experience thermal discomfort. 
In addition, the electrical connection between PV panels and inverters is a challenging task that has to be considered when integrating photovoltaics.


Figure 75. On the left, a two story DSF-PV system in a curtain wall design. On the right, photo and rendering of the experimental test-hut located at Concordia University depicting a DSF-PV.

SWOT Analysis

Strengths

  • PVs can be integrated in the large exposed envelope areas of high-rise buildings and generate electricity
  • The semi-transparent PV can control the solar heat gains of the building and still allow the penetration of daylight
  • The airflow within the cavity of the DSF-PV can cool down the PV panels and increase their efficiency
  • Heat can be recovered from the preheated air and supplied either directly to the building or to the HVAC system
  • DSF-PV can facilitate natural and cross ventilation in the building
  • DSF-PV has a significant potential for daylight control and energy savings through the use of shading devices like louvers or blinds.
  • The acoustic comfort can be improved by the addition of the external skin
  • The double skin protects the building from wind or rain penetration by applying pressure equalization
  • Operable windows can be used in high-rise buildings
  • The temperature of the interior surface of the glazing of the DSF-PV is higher during the heating season, improving the thermal comfort of the occupants

Weaknesses

  • The fans used for mechanical ventilation consume electricity
  • The electrical connection between PV panels and inverters is challenging
  • Larger technical rooms are needed to accommodate inverters and heat recovery units
  • Predictive controls should be applied for an optimal operation of the DSF-PV
  • The upfront investment cost is higher than for traditional envelope solutions
  • Currently, there is not a wide range of commercially available semi-transparent PV products that can be integrated in the building´s envelope
  • The view to the outside can be compromised if a low-transparency PV panel is selected
  • Overheating can occur during the summer if the control strategies are not thoroughly considered

 
Opportunity

  • More and more PV companies enter the building sector and could be interested in disposing of such system
  • This solution could benefit from new policies that promote Net-Zero Energy Buildings
  • The price of PV panels is still dropping rapidly, making this solution more costeffective
  • The solution is suitable for both new constructions and retrofits

 
Threats

  • There is no standard to assess the performance of BIPV/Ts, in this case PV + heat removal from the air flow
  • The price of aesthetically pleasant PV panels is still high
  • There are not enough published studies or books that promote and spread awareness about the integration of PVs in the building's envelope

Lessons Learned

  • BIPV must be aesthetically pleasant in order to be adopted by architects.
  • Colder climates seem to benefit more from passive Double Skin Façade operation, whereas warmer climates seem to benefit from an active Double Skin Façade operation, as cross or natural ventilation are facilitated.
  • Active ventilation mode removes heat from the semi-transparent PV but also from the building.
  • Wiring is an issue in markets that are not familiar with this kind of installations.
  • A path-way for cleaning and removing dust should be considered at the design stage.
  • Semi-transparent PV based on c-Si presents higher panel efficiencies and peak power per square meter.