Green Architecture spirit-
environmental classification systems can often be
conceived within a closed loop which accepts current levels of consumption of synthetic materials.
Economic growth requires more and more buildings and raw materials but this can be challenged by the
development of interest in the use of natural materials that are fully renewable with only limited amounts of
manufacturing and processing. A good example of such a natural material is Hemp.23 Hemp is a fibrous
material which can be grown in the fields with a minimal amount of fertiliser and no need for pesticides. It
grows very quickly to enormous heights and the resulting crop can be used in many ways. Oil can be
extracted which has a variety of therapeutic uses, even ice cream can be made from hemp. The fibre can be
spun into material for high quality clothes and at one time was the principle material for rope making. The
left over hurds or straw can be used in building construction and fibres combined with cement or lime. Such
a 
natural material is infinitely renewable and has no known toxic or polluting effect on the environment.
There is no waste and the energy consumed in planting and harvesting is minimal. If we could makebuildings using such materials we can significantly reduce the use of synthetic materials such as cement and
plastics and metals.
Of course the ubiquitous renewable material is timber, but it takes a long time to grow and thus requires
careful management. Hemp and other forms of straw and reeds grow much more quickly. Bamboo is
another material which has similar properties and uses to timber but regenerates and grows much more
quickly. Innovative buildings from bamboo have been developed in various parts of the world.24 It is also
possible to use earth as a building material, but unlike fired bricks or tiles, which require a lot of energy and
processing, earth can be used as it is dug up on site. We can thus imagine the possibility of creating
buildings which are largely composed of materials which are both natural in origin, locally sourced and
resulting in zero or nearly zero emissions. While the use of such materials may seem impractical at present,
the idea has to be seen as a challenge to anyone interested in green building.25
How can we argue that a building is green and environmentally friendly when it is still composed of
materials which have required a lot of energy, processing, waste disposal and transportation to get it into
place? Thus in the future we are likely to see far more discussion of the use of zero emission and natural
materials or at least their incorporation into more conventional buildings. The use of such materials,
particularly hemp and lime, bamboo and earth construction are likely to be subjects for future issues of the
Green Building Digest. Strawbale construction is dealt with in this book and is the best known example of
using a zero emission, fully renewable, virtually waste product as a replacement for materials such as
concrete blocks, giving very high levels of insulation.
What is Straw?
It is a natural raw material, the by-product of the harvesting of wheat, rice, barley, oats and rye. Straw also
comes from maize, millet, sorghum and hemp and other forms of crop such as sugar. Over 750 million
tonnes of straw are produced worldwide annually. About 60% of straw is baled with the rest ploughed in.
Of 17,000,000 hectares of farmland, 3,353,108 are devoted to cereal crops in the UK with nearly 2 million
devoted to wheat (1996). Between 2.75 and 3.5 tonnes of straw are produced per hectare, suggesting 10
million tonnes of straw are produced in the UK annually.6
At one stage straw came to be regarded as little more than an embarrassing companion to the grain crop.7
However straw is not necessarily a waste material. In organic farming straw is very important and it is the
emphasis on petrochemicals and artificial fertilisers that have reduced the need for straw. One authority
argues that the energy contained in straw is twice as great as the farm’s fossil fuel consumption,8 so it is
clearly important to make good use of this embodied energy. Straw has many uses; as a fuel for heating, for
paper and packaging, as a food, though it needs a lot of treatment for this. It was used in straw boards
(Stramit) and traditional mixed with earth for cob and pise construction.9
It is not right, therefore to call straw a waste material and as there are moves towards more organic
farming it will be used more as animal litter and thus as part of compost making. It can be ploughed back
into the soil. A great deal of straw is also used in mushroom farming. However the use of straw bales for
building in the short term is not likely to make a big dent in the supply.
Different kinds of straw?
Most forms of crop straw are suitable for building when baled. There are different strengths of stem, with
wheat possibly being stronger, but there can be great variations depending on weather, soil, levels of
fertiliser, growing time etc. As standards of straw bale building are established, such issues may become
important, but, the density and moisture content of the bales is more important than the type of straw used.
However not all straw is necessarily suitable, for instance there has been some experimentation with the use
ofHemp straw, but anecdotal evidence suggests that hemp is very tough and difficult to use with bale wall
techniques.10
Normally two string straw bales cost around £1–£2, but larger three string bales cost more. The large
round bales which are increasingly seen are of no use, but straw bale enthusiasts might consider acquiring,
borrowing or hiring a baling machine and re-baling straw obtained from whatever source. Often bales which
are normally produced by farmers in the field are not ideal for construction as they are too loosely baled. It is essential that straw is used for construction and not hay. Hay bales are made from plant material that
is green/ alive and not suitable for this application.11 However there are apparently buildings made of hay
bales in the USA and many people confuse the two referring to hay when they mean straw.
WHAT ARE PHOTOVOLTAICS?
Photovoltaic cells convert sunlight directly into electrical energy. The electricity
they produce is DC (direct current) and can either be:
• used directly as DC power
• converted to AC (alternating current) power, or
• stored for later use.
The basic element of a photovoltaic system is the solar cell that is made of a
semiconductor material, typically silicon. There are no moving parts in a solar
cell, its operation is environmentally benign and, if the device is correctly
encapsulated against the environment, there is nothing 
that will wear out.
Because sunlight is universally available, photovoltaic devices can provide
electricity wherever it is needed. Since the power source will last for hundreds
of thousands of years, and it is very hard to interfere with its delivery,
photovoltaic (PV) is widely expected to become a major source of power
worldwide in the long term.
Photovoltaic systems are modular and so their electrical power output can
be engineered for virtually any application, from low-powered wristwatches,
calculators, remote telecommunications systems and small battery-chargers
to huge centralised power stations generating energy only from the sun. PV
systems can be incrementally built up with successive additions of panels
easily accommodated, unlike more conventional approaches to generating
energy such as fossil or nuclear fuel stations, which must be multimegawatt
plants to be economically feasible.
PHOTOV
OLTAICS
HOW PV CELLS WORK
Although PV cells come in a variety of forms, the most common structure is a
sandwich of semiconductor materials into which a large-area diode, or p-n junction,
has been formed. In the presence of light an electric charge is generated across
the junction between the two materials to create a charge similar to that between
an anode and a cathode. The fabrication processes for making the cells tend to be
traditional semiconductor processes, the same as those used to make microchips, by ‘doping’ the silicon with different elements using diffusion and ion implantation
of the elements into the silicon. The electrical current is transferred from the cell
through a grid of metal contacts on the front of the cell that does not impede the
sunlight from
entering the silicon of the cell. A contact on the back of the cell completes
the circuit and an anti-reflection coating minimises the amount of sunlight
reflected back out from the silicon, so maximising the light used to generate electricity,
as shown in Figure 8.1. See 21ADPV for a more detailed account of how a
cell works.
Photovoltaic panels have been commercially available since the mid-
1970s and were initially used to power some early demonstration buildings,
such as those that are still working at the Centre for Alternative Technology
in Wales. However, it was the 1990s that saw the first great boom in PV
buildings around the world. Germany and Japan lead the way with Japan
installing 110 MWp in 2001, Germany installing 77 MWp and USA installing
18 MWp. These three programmes accounted for over half the world PV production
in 2001. The Netherlands and Spain came next in the table of installations.
Some countries are way behind in the solar race. Britain installed
around 300 KWp in 2001.
WHAT IS A PV SYSTEM?
PV cells are typically grouped together in a module for ease of use. A PV
system consists of one or more PV modules, which convert sunlight directly
into electricity, and a range of other system components that may include an
AC/DC inverter, back-up source of energy, battery to store the electricity until
it is needed, battery charger, control centre, mounting structures and miscellaneous
wires and fuses.
WARNING
Direct current (DC) electricity is much more dangerous to handle than alternating
current (AC) electricity, which is typically used for all household appliances. This is
because there is no break in the flow of a DC current and, if you grab hold of an
exposed DC wire, the muscles contract and it is very difficult to let the wire go
again. Great care should always be taken when dealing with DC electricity.
WHY PV IN BUILDINGS?
Even in cloudy, northern latitudes, PV panels can generate sufficient power to
meet all, or part of, the electricity demand of a building. The Oxford Ecohouse
(see page 330), for example, incorporates 48 PV panels on the roof that generate
enough energy to lower the household electricity bills by 70 per cent.
The flexibility of PV enables its use in many building products, such as solar
roof tiles, curtain walls and decorative screens, which can directly replace conventional
materials in the building fabric. These products serve the same structural
and weather protection purposes as their traditional alternatives but offer
the additional benefit of generating the power to run the house.
WHAT’S GREEN ABOUT PV?
The electricity produced by every square metre of PV can effectively displace
emissions of more than two tonnes of CO2 to the atmosphere over its
lifetime. Few now dispute that CO2 emissions can continue to increase at
current rates without dire consequences, such as global warming. Wider use
of PV power in buildings can help to reduce such environmental impacts of
buildings that are responsible for generating over 50 per cent of all emissions
of greenhouse gases globally.
Let us use the Oxford Ecohouse as an example. In order to calculate the
environmental impacts of the PV system it is necessary to know the UK
energy generation conversion values, the amount of CO2 released into the
atmosphere for every unit of energy delivered to a house. It has been estimated
that an average energy conversion efficiency for thermal electricity
generation plants in the UK is around 37 per cent. This results from an electricity
mix generated from 65 per cent coal, 15 per cent gas, 22 per cent
nuclear and 9 per cent oil. For the PV manufacture assumptions see Energy
Technology Support Unit (1996).
Based on the monitored data, the PV system produces 3093 kWh per
year, that is around 77 000 kWh in its 25-year life cycle.
The Oxford Ecohouse PV system avoids the release of 1.84 tonnes CO2 per
year. These values can be extrapolated to give the avoided emissions in the case
of a massive programme of installing PV on residential building. A system oneeighth
of the size of the Oxford Ecohouse would avoid 230 kg CO2 per annum.
WHAT WILL IT COST TO USE PV IN BUILDINGS?
Solar electric PV systems are now an economic and viable technology in many
parts of the world. More than that, they are a sensible economic investment for
ordinary householders who want to begin to protect themselves from future
changes related to energy and the climate. They should begin to consider
the following:
• Climate change is driving the move towards carbon taxes that will make
energy more expensive.
• Fossil fuel depletion will push up oil and gas prices. We have around 40
years of conventional oil reserves left and around 60 years of gas left. By
2020 oil and gas scarcity will make future energy prices very unpredictable.
• Climate change may well make heating and cooling our houses more
expensive in energy terms as the climate gets warmer or colder.
• Security of energy supply. PV systems can provide electricity during conventionally
produced electricity blackouts resulting from poor supply conditions
or bad weather. There is already a range of uses for which a secure
energy supply should be essential; these include, water pumping, electric
garage doors and gates, lift safety systems, smoke and fire alarms, emergency
lighting and security systems, computer UPS systems and communications
systems.
Investment by people now in their high-earning years in energy efficiency
and renewable energy will pay dividends in, say, 10 years when they retire
and must inevitably face higher energy bills they are less able to afford.
Anyone with a £500-a-year electricity bill would be wise to envisage at least
a doubling of electricity costs in 10–15 years time. Will your pension cover
£1000 a year for one bill?
Costs of installing PV systems today vary significantly according to the
technology used and the application and the efficiency of the system.
Capital costs of PV panels are broadly similar to prestige cladding materials,
ranging from £350 to £750 per m2 depending on the technology and its
detail. Prices are expected to fall significantly over the next decade as
demand grows and the PV industry achieves economies of scale in production.
In parts of Germany and the USA (Sacramento municipality) the cost of
installing one watt of PV power into a home has already fallen to around
£2.75 per watt, which is very low compared with current UK estimates of £6
per watt for an installed system. In those countries, the impact of early
investment in the technology by national and local government bodies has
paid dividends for consumers while people in countries such as the UK have
to suffer because of short-sighted investment policies in this, one of the
most important technologies of the twenty-first century.
Your own investment decision should also take account of the marginal cost
of the PV system (capital cost minus the cost of the alternative material) and
power output. PV systems are not difficult to install and, if maintained properly
(annual washing), have an expected lifetime of around 25 years.
What is certain is that today PVs should be an essential feature of a real
ecohouse, because ecohouses are setting the agenda for building in a changing
climate and helping to prepare society for the ‘post-fossil fuel age’. PVs
have a very important role to play, like solar hot water systems, in the new
agenda for buildings; the earliest PV ‘pioneers’ in the twentieth century often
installed PV systems for ecological reasons rather than economic ones.
However, in some farsighted cities, such as Aachen in Germany, a green tariffon every electricity bill enabled the local utility company to pay every householder
with a PV roof DM2 per kW exported. This enables householders to
pay back the installation costs in around 10 years for systems that will last
for 20 years.
But it is no use placing a PV system on an energy-profligate building and
expecting it to solve the problems wrought by the building designer. This is
just throwing good money after bad. Forget PV for air-conditioned buildings
for the foreseeable future. PVs will work well with low-speed fan-assisted
passive cooling systems, such as earth-coupling and the night cooling of
buildings (see Chapter 5 on ventilation).
To use PVs properly the building electricity loads should be as low as possible,
and only then should the system be designed to meet part or all of
those loads to give you a magic building that generates its own energy.
ADVANTAGES OF PHOTOVOLTAICS AS A DOMESTIC
SOURCE OF ENERGY
• It is a clean green energy source. It does not produce CO2, NOx or SO2
emissions.
• The silicon PV panels are non-toxic in production.
• The energy payback (the time for the PV to produce as much energy as is
required for manufacture) is 2–5 years, while the working life of a PV
panel can be well over 20 years.
• Energy is generated on site so there are very few losses in transport,
unlike remotely generated supplies relying on long supply lines.
• It is reliable. You just put them on the roof and they work. Panel warranties
are now typically for 20 years.
• They are silent.
• They are low maintenance. Once installed they will simply require their
surfaces to be cleaned, especially in dusty environments.
• They can provide power in locations remote from the grid.
• PVs are a transportable technology and can be moved between buildings.
• They can provide power during blackouts.