FAÇADES RENEWABLE ENERGY
A long-distance view of the BMW World building’s high-tech façade
dry biomass can be converted on site, via a hydro-thermal processes, into methane, which can be used as fuel to generate electricity or heat on site or to be fed into the community. On average the expected energy gain is around 30 kWh/sq m yr
l Energy generation through production of solar thermal heat: The part of the solar spectrum that is not consumed by
photosynthesis will be absorbed, leading to rising temperature levels inside the bioreactor. Through a heat exchanger, this energy can be captured and stored, resulting in an additional energy gain of about 30 kWh/sq m yr.
Bioreactors Tracking the sun brings further benefits
l Reduction of CO2: Up to 250g of CO2 can be absorbed daily for each square metre of reactor surface, through the growing of biomass. A possible source of CO2 could be local combined heat and power plants or industrial plant, reducing the carbon footprint of the local plant.
A sketch showing different systems of integrating bioreactors into
the building skin
The framing system used wto mount the photobioreactors (PBRs) are mounted on the façade plays a major role, not only for the visual appearance but also for the functionalities of the system. To optimise solar gain and biomass production, it would be beneficial to track the sun’s path by tilting the PBR generally around a horizontal axis (and, in some instances, rotating around a vertical axis may also offer some advantages). During winter the vertical mounting of bioreactors is beneficial due to the low angle of inclination of the sun. A continuous closed skin of PBR has the additional advantage that the closed
32 CIBSE Journal October 2011
air cavity acts as a thermal buffer. During summer a tilt of 30 degrees will lead to the best production rate for a south-orientated façade. The tilt of the panels will allow for natural ventilation of the buffer space to help prevent overheating of the building. To explore the dependencies between energy generation, CO2 absorption, solar shading, thermal insulation, sound absorption and daylight control, standardised PBR configurations need to be modelled and looked at in detail. In principle, three basic
systems have been identified: l Bioreactive façade as a double skin (A): The PBRs wrap around the entire building
and enclose a buffer zone, which can be used as a circulation space or be inhabited by users. The PBRs contribute directly to the thermal, solar and acoustic performance of the
building skin. l Bioreactors as external shading devices (B): The PBRs are located on the outside of the building or in front of glazed areas. This system aims at dynamically regulating solar gains, in addition to generating
energy. l Bioreactors as ventilated rainscreen system (C): The PBRs represent a continuous external shell in front of an opaque insulated wall, improving thermal and acoustic performance.
l Dynamic shading: As the micro-algae absorb light, the PBRs act as devices for filtering the solar radiation. The shading factor depends on the density of cells. With a cell division rate of a maximum of one per day, the shading factor can be increased by 100% within a day if the harvesting process is suspended. In this way the system represents an interactive, adaptive shading system: during times of high solar radiation the density of the cells increase, blocking the light. In contrast, the light transmission can be increased by an intensified harvesting process.
l Dynamic ‘active’ thermal insulation: With PBR representing a continuous external layer, the heat losses of the building can be controlled through the temperature of the medium inside the bioreactor. If the temperature of the PBR is above the temperature of the external macroclimate, the bioreactors present an active thermal insulation. This effect can be enhanced through an air cavity between the skin of PBRs and the thermal insulation of the building. While an air flow could be used to cool the building by ventilating off any excess heat, a stationary air cavity provides a
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