Algae grown in the University of Technology, Sydney science faculty © UTS
Increasing energy efficiency is a proven way to mitigate the greenhouse gas emissions associated with buildings – which, including their energy use, account for 40% of the world's total. Renewables offer great potential in this regard, and are set to dominate 21st-century energy production.
In 2017, for instance, bioenergy accounted for 70% of global renewable energy consumption, generating some 15m GWh, so it is no longer in transition in terms of volume. However, building surveyors know little about bioenergy, even though there are a number of innovative applications in the construction industry. Among these is algae building technology (ABT).
PBR pioneer
In 2013, Arup designed the Bio-Intelligent Quotient (BIQ) House in Hamburg, Germany, which is the first project in the world to use ABT. Mounted on the sides of the building that are exposed to the sun is a secondary facade of glazed, storey-height panels, partially filled with algae in water, which are called photo-bioreactors (PBRs). Biomass is grown in the PBRs and harvested for conversion into biofuel for the building's heating system, while the panels also collect solar thermal energy.
There are estimated to be between 300,000 and 500,000 species of algae, and many of these could be grown in the panels. The BIQ House uses Haematococcus pluvialis because it grows in suspension, whereas those algae that grow as biofilms would need to be scraped off the window surface and are not easily harvested from the water.
The BIQ consists of 15 apartments over 4 floors, with 120 PBRs totalling 200m2 integrated into 2 facades. The panels are arranged horizontally to create a thermally controlled microclimate on the apartment balconies, insulating them from external noise and providing dynamic shading.
The external walls are of a low-energy Passivhaus design. However, the construction costs – around €5m – were higher than for conventional apartments, while the PBRs add dead loads to the structure that need to be factored in to the design.
The PBRs are designed to encourage algal growth and require minimal maintenance. They are constructed with 4 glass layers: a central cavity contains the algae suspension, between a pair of double-glazing units that are filled with argon gas to minimise heat loss. Mixing air and carbon dioxide in the PBRs maintains appropriate pH levels and creates the turbulence that causes algae to grow when it is exposed to sunlight.
Heat harvest
The liquid in the PBRs is recovered by a heat exchanger for hot water and central heating. Water temperature is controlled by the speed of fluid through the panels; for example, lower flow rates allow more time for sunlight to warm the water.
Generally speaking, temperature and light are the main criteria for selecting a suitable species of algae. As higher temperatures could harm or kill H. pluvialis, the maximum for the PBRs in the BIQ House is 40C. However, this largely limits the use of the extracted thermal energy to pre-heating other building systems. In a warmer climate than Germany this might not be the case – providing that local algae species suited to these conditions are selected.
The biomass is meanwhile transported by pipework to an on-site energy management centre where it is harvested, and the paste is taken to a separate processing facility for conversion into biofuel or other uses, such as fibre for fabric manufacture or feedstock for animals. The system needs regular maintenance and flushing to ensure constant flow rates for algal growth.
ABT is not yet as effective as other renewable energy sources. PBRs only convert 38% of the sunlight they collect into heat compared to 60–65% in conventional solar thermal sources, while the biofuel they produce only burns with 10% efficiency compared to the 12–15% efficiency of typical PV installations.
However, by sequestering carbon dioxide the algae does offer an advantage that renewables cannot, with the BIQ House's 200m2 PBR facade removing up to 6 tonnes annually. PBRs can also provide energy directly to several building services, and supplementary benefits such as shading during the summer. On a cost–benefit analysis, therefore, the overall outcomes might be positive, although this still needs to be scientifically tested and confirmed.
Excess heat collected by the PBR facade panels is used to pre-heat domestic hot water and for central heating. The PBRs receive around 150kWh/m2 a year, which heats them to around 40C. The solar thermal energy is fed into a heating network through a heat exchanger, or stored in underground geothermal boreholes.
The biomass grown is harvested every 3–4 weeks through an algae separator, and once converted into biofuel generates energy equating to around 30kWh/m2 annually. Around 80% of the biomass harvested at the BIQ House is converted into methane at an off-site, outdoor biogas plant and returned to the building as fuel for electricity and heat generation.
ABT adoption
Acceptance of innovations such as ABT requires owners, users and built environment professionals – including surveyors, project managers, contractors, property and facility managers – to be aware of their benefits. Without this understanding, adoption of the technology will remain limited.
With this in mind, we carried out a feasibility study to ascertain the views of key stakeholders and professionals on the pros and cons of ABT. Our next article will outline their responses, as well as the technical issues associated with the technology.
"Acceptance of innovations such as ABT requires owners, users and built environment professionals to be aware of their benefits"
Algae's role in building materials
Algae also offer potential in the manufacture of building materials. Some species can produce shells, collecting carbon dioxide from the atmosphere and transforming it into calcium carbonate, which means carbon capture technology using algae could be installed in cement plants to produce sand suitable for concrete.
This would help alleviate exploitation of natural sand, which is now being depleted at a far greater rate than its renewal, while its excessive mining from rivers puts stress on their ecology and biodiversity. As carbon dioxide generation from the manufacture of Portland cement is second only to that from fossil fuels, such algal applications would also represent a step towards reducing emissions from this process.
Related competencies: Construction technology and environmental services, Sustainability