Rethinking construction materials
Part one of this blog on climate change draws on a group discussion about energy sources and batteries. Part two reflects the ensuing discussion about promising initiatives the group had heard about aimed at reducing carbon emissions in the built environment. Wendy had read an article about so-called “kinetic pavements”. She understood that a particular material can be laid on pavements which generates electricity from the energy imparted by the tread of pedestrians. She gathered it generates enough power to illuminate the streetlights.
One example of this comes from a company called Pavegen which makes tiles from recycled tyres. These tiles flex by up to 5mm when stepped on. Embedded in each tile are various tiny devices that convert the movement (or ‘kinetic’) energy into electrical energy as the tile is compressed.
Figure 8 Passengers walking over ‘Pavegen’ tiles at Abu Dhabi airport. Image credit: Photogoddle
One type of device inside he tiles uses the piezoelectric effect, in which compression, in certain materials, causes electric charges to flow. A different kind of device involves tiny pieces of metal passing by magnets embedded in the paving material. This phenomenon, known as electromagnetic induction, produces electrical energy. In yet another device, tiny rotors, at the junction of the tiles cause a coil of wire to rotate through a magnetic field as the tile is pressed down. Like the generator in a car or power station, but on a tiny scale, this creates electrical energy. These microgenerators produce about 8 Watts of power during a footstep – enough to light an LED streetlamp for 30 seconds. Bird Street in London is the site of a live trial, as I write in 2022. Perhaps, as regular floor tiles in offices, schools and public spaces, they might have a contribution to make – small but very widely spread.
I should make an important scientific point here about energy. In the foregoing, I have used the words “create” and “produce” to describe energy processes. However, as we are told at school, as a kind of mantra: “energy is neither created nor destroyed”. This reflects one of the most fundamental principles of physics – total energy is always conserved in any process. When things interact, the sum total of their energy is the same before and after, even though it may seem that some has disappeared (e.g. when a moving vehicle comes to rest). What can, and does, happen however is that energy can be transformed from one form to another. Thus, a battery transforms chemical into electrical energy; a hydroelectric installation changes potential into electrical energy, a kinetic pavement turns kinetic energy into electrical form. In all practical cases, some energy will seem to be ‘lost’ or used up in the process; in fact it is usually accounted for as heat energy that gets dissipated invisibly to the surroundings.
Architecture and building material
Building construction is another key area in which changes will have to happen if greenhouse gas emissions are to be significantly reduced. Cement, central to so much of our built environment, is produced in a process that produces carbon dioxide (CO2) as a by-product when the mineral calcium carbonate (limestone) is heated to produce calcium oxide, a key component of cement. Even more CO2 results from firing up the kilns in which this take place.
Alternative building materials are being actively tried and studied. Wood is an obvious choice. As a tree grows, carbon dioxide is captured from the atmosphere in the photosynthesis process. This helps reduce the level of CO2 in the atmosphere – that’s why preserving forests is so important. If the timber were to rot or get burned later on, however, this gain would be lost, as these processes release the trapped carbon dioxide. However, research into the life cycle of wooden buildings suggests that, overall, long-lasting wood, as a construction material, contributes to climate mitigation, by replacing concrete, capturing and storing carbon dioxide for long periods. It also helps by reducing heat losses from buildings through its insulating properties.
Talking of alternative building materials reminded Sarah about a radio programme she’d heard in which hemp was being promoted as a climate-friendly building material.
The UK Royal Institute of Chartered Surveyors explains that ‘hempcrete” – a mixture of hemp fibre, water and lime – is steadily gaining an international following due to its zero-carbon and thermal insulating properties. Hemp is a fibrous plant, part of the cannabis family (though, for better or worse, not a strongly mood-altering member of the family). It has potential as a building material in developing countries where houses can be built using crops communities have grown themselves (figure 9).
Figure 9 Hempcrete
Hempcrete is not strong enough to support large loads alone. Wood, on the other hand is now being used structurally in tall buildings thanks to the use of cross -laminated timbers. These are made of layers (or plies) of wood glued together so that the grains of adjacent laminae are at right angles to one another (figure 10). The result is a structural material stronger than regular wood with a much lower carbon footprint.
Figure 10 Cross Laminated timber
Dalston Works, a ten-storey, all-timber housing project in east London, demonstrates the potential of low-carbon wooden buildings.
Figure 11 Dalston Works wooden housing project.
Image credit: Daniel Shearing, Waugh Thistleton Architects.
Interesting and inspiring as these initiatives are, they are still relatively rare and small–scale. How can building design and renovation contribute more significantly to reducing emissions? As Helen put it in a discussion: what can be done about the massive use of air conditioning in the US and other places with hot, humid seasons? She had heard about tall buildings in Bangkok designed so that hot air rises naturally, escapes through the roof and is then replaced with cooler air below.
Green building design has become a major topic amongst architects and building engineers. The challenge is to minimise energy demand through the design and layout of buildings, as well as through the choice of building materials. To maintain an optimal temperature indoors, heat energy has to be imported through a heating system or expelled through air conditioning according to the season or local climate. Insulation of the walls, floors and rooves helps reduce this loss, either way, by inserting materials that impede the flow in our out. Air is particularly good at resisting the flow of heat energy, provided it is in small pockets, unable to circulate. Styrofoam and synthetic wools trap pockets of air, so make effective insulating materials.
An alternative green option is to ensure that the transfer of heat energy, whether in or out of a building, is not achieved by burning fossil fuels at all, whether directly or via electricity from oil or gas-fired power stations. One way is to use solar panels to generate electricity. These work thanks to materials known as photovoltaics which have a special property: when light falls on them, energy conveyed by the particles of light (known as photons) is sufficient to knock some, negatively-charged electrons out of the atoms of the photovoltaic material. If this is connected to a circuit, these released electrons flow, carrying the energy they had picked up from the photons of light. This electrical energy can be used to power up any devices in a building, including heating and cooling systems.
A new kind of building, perhaps a vision of the future, is shown in figure 12. The shelter is built around photovoltaic panels which generate electrical energy to be fed directly into electric cars. What a difference from the palaces of fossil fuel we use today to energise our vehicles!
Figure 12 Charging station in France using solar energy for electric cars. Image credit: Tatmouss
Another option is the heat pump, which delivers heat energy directly from the surroundings into our homes. Counterintuitive though it may seem, the air around us and the earth beneath us are sources of heat energy – it’s just that usually it’s at too low a temperature for normal use. A heat pump extracts the low temperature heat energy from the air or earth and raises its temperature. It does this by bringing a cold circulating fluid into contact with the cool air or earth outdoors. Heat energy flows from the cool earth to the cold fluid. The fluid then circulates into the building where it gets compressed by an electrically driven compressor. This raises its temperature so that, at the next stage in the cycle, this high temperature energy can be transferred to the interior spaces of a building. In a subsequent stage of the cycle, the fluid is decompressed, cooling it down again below the outdoor temperature, ready to pick up low-temperature heat energy, once again, from the earth or air. Some electrical energy is inevitably used in this process to drive the compressor, but overall, considerably less than in a purely electrical heating system.
Choices made in the basic design and layout of the building can be as important as those made in selecting materials and systems. The orientation of a building impacts directly on heating by the Sun. South-facing walls with plentiful window space clearly favour natural heating; thus, in cooler climates, well-used rooms are best placed in that position. The reverse holds for excessively hot climates. Well-placed awnings also help reduce cooling requirements.
In one group discussion, an architect invited as a guest described a new school in a tropical zone, designed around a corridor running from south to north. The temperature difference between the ends encouraged a cooling flow of air along the corridor, minimising the need for electrical air conditioning. It reminded Helen of the office building, in which she had once worked, which had been designed so the stairwell vented directly to the roof, allowing excess hot air to rise and escape, thus reducing the need for carbon-intensive air conditioning units in summer.
The guest architect deplored the fashion for tall, glass-and-steel buildings because of the seriously high carbon emissions they cause. Infrared radiation from the Sun penetrates the glass, heating up the interior, as in a greenhouse. This ‘solar gain’ has to be countered by air-conditioning units which themselves consume electrical energy unnecessarily.
As figure 13 shows, residential and commercial buildings are a major cause of CO2 emissions – greater even than all of transport.
Figure 13 Carbon dioxide emissions by sector
A pioneering scheme in Islington, North London, makes use of waste heat from underground trains by extracting air from an underground railway and bringing it up to a specially designed building on the surface (figure 14). Here, the heat energy in the air is transferred via a ‘heat exchanger’ to circulating water which is then pumped through nearby blocks of flats, in a district heating scheme.
Figure 14 Greenscies project at Cullinan Studios
The purpose of this blog has been to illustrate some of the innovations currently helping to reduce carbon dioxide emissions and slow the rise in global temperatures. It also offers insights into some of the underlying scientific principles in play. Many novel ideas are being researched, tested and implemented that will reduce carbon dioxide emissions but, so far, on a relatively small scale. The real challenge lies, not so much in technological and scientific know-how, but in political will internationally, to invest in scaling-up innovations. Profound changes in our ways of life are necessary and investment on a grand scale globally is needed to support interventions wherever the risk is highest.
© Andrew Morris 5th December 2022