A sustainable house in Tlemcen, Algeria

Making the most of natural resources to meet the energy needs of a building’s inhabitants is an ancient concept. For example, in the first century BC, Roman architect Vitruvius laid out the principles for building and city planning in order to funnel unhealthy or ‘disagreeable’ winds between buildings and avoid having them blow into building facades.

Northern Africans sought to adequately ventilate their traditional stone-walled buildings, whose high thermal mass trapped daytime heat to warm the cold nights. Thus they oriented their houses to face the prevailing wind and used a two-tower system to direct breezes at the top of the building inside and drive stale air back out.

Today, builders must apply innovative techniques, use resources efficiently, and minimize pollution while satisfying location-specific needs. A study published earlier this year in the Journal of Renewable and Sustainable Energy describes one such project in northern Algeria.

Goals in Tlemcen

Energy physicist Mohammed El Amine Boukli Hacene, of Abou-Bekr Belkaid University in Tlemcen, Algeria, designed a model house (shown below) with the goal of creating a comfortable space for the building’s residents while minimizing the impact of the building on the external environment by reducing energy consumption, using renewable energy, and providing natural ventilation.

Comparing the Tlemcen ecological house design with a conventional western Algerian house, he concluded that although the ecological house would be 15% more expensive to build, the cost would be recovered within 10 years. Furthermore, the building would more fully exploit renewable energy.

‘The Algerian government invests heavily in sustainable development and scientific research, and [promulgates] new laws on sustainable housing,’ said Boukli Hacene. He noted that because Algeria has plentiful oil and natural gas reserves, the price of fossil fuel consumption is already low. The economic savings would be more significant in a country that relies on fossil fuel imports. Efforts to develop ecological buildings demonstrate Algeria’s serious interest in promoting sustainable development.

The Tlemcen house

The Tlemcen ecological house uses cellulose wadding and flagstone within a wooden framework. Wood’s low thermal inertia, low construction cost, and low thermal transmission make it the material of choice. Double-glazed windows and airtight external doors aid insulation, and the south-facing living area enables collection of solar radiation. The house uses solar-powered heaters with exposed sensors on the south side, generates electricity via photovoltaic cells, and makes use of ground cooling. Heating the house is expected to use no more than 15 kilowatt hours per square meter per year (kWh/m²/yr), and the total energy demand is expected to be 50 kWh/m²/yr. That is a small number when compared with the total energy demand of 220 kWh/m²yr for a conventional house.

To put those numbers in perspective, consider the 2004 guide to energy efficiency in buildings published by the UK’s Chartered Institution of Building Services Engineers (CIBSE ). The guide cites energy consumption benchmarks for typical existing buildings as 417 kWh/m²/yr from fossil fuels plus 79 kWh/m²/yr from electricity, for a total of 496 kWh/m²/yr, or almost 10 times the energy requirements of the Tlemcen house. Even the CIBSE ‘good practice’ standards of 247 kWh/m²/yr from fossil fuels plus 44 kWh/m²/yr from electricity, for a total of 291 kWh/m²/yr, are much greater than the energy goal’s of the Tlemcen house.

Energy, economy, and environment

‘Knowing the heat transfer coefficient of each material, the area and volume of each element, we can measure the [energy] loss of each room of the house ... calculating basic losses by heat transmission,’ said Boukli Hacene. The total simulated loss of the Tlemcen house is 183.9 W/hr/°C, a small number when compared with the 3092.2 W/hr/°C loss of the conventional house, whose construction differs only in materials used.

Incoming energy from direct sunlight shining through windows and transmission from hot surfaces in contact with the outside is calculated based on global radiation and sunshine duration. Ninety-three percent of energy needs for the conventional house are saved in the ecological house, thanks to superior insulation.

Other internal thermal contributions include heat produced by the residents, which, when combined with solar heat, can create more energy than is needed during summer months. And while January energy consumption will reach 3 kWh/m²/yr (1/12 of usual consumption), solar and internal contributions can provide only half of that during winter months.

If the excess summer internal heat could be captured and stored, it could potentially compensate for the winter months. Passive annual heat storage simply traps heat in the structure’s spaces and adjoining soil during the summer and transfers it back during winter. Isolated solar gain devices can also capture heat, which is then deposited to a depth calculated to guarantee a certain return time via conduction.

Another concept to consider before deeming a building energy efficient is the amount of ‘embodied energy’ involved, that is, the total fuel, materials, and human resources that were used to make the product. ‘If you believe that putting thermal mass in a building is good for using benefits of night cooling, then you’ll design that in,’ says Shaun Fitzgerald, cofounder and managing director of Breathing Buildings , a company in Cambridge, UK, which developed and commercialized a low energy mixing ventilation system. That system represents a significant cost savings relative to air conditioning. But thermal mass can be achieved by either low-embodied-energy stone or high-embodied-energy concrete. ‘How you design [the building] has a huge impact on how long it takes to get payback,’ he added.

Whereas the total annual cost for heating and electricity to maintain a comfortable temperature in the Tlemcen ecological house is €130, it is nearly twice that in the conventional house. But economic assessments also have to take into account the durability of materials and costs of implementation. The time-of-return curve—how long it takes for the initial investment to gain some energy advantage—reaches a minimum at an energy consumption of 15 kWh/m²/yr. Below that value, materials are so expensive that profitability is decreased.

The broader context

The essential difference between the Tlemcen conventional and ecological houses is the use of materials with low thermal conductivity. ‘But other parameters are important,’ said Boukli Hacene, listing double-glazing, heating and passive cooling, and the house’s orientation. And while the study focuses on a sustainable building particular to Algeria’s equatorial climate, Fitzgerald notes that ‘double-glazing is more important further away [from the equator],’ because insulation needs change as you go farther north. Boukli Hacene agrees that Algeria, with its high sunlight potential, can develop sustainable concepts but the ‘design, orientation, and location of the region’ will have a significant impact on projects elsewhere.

Boukli Hacene is eager to validate the results of his simulated house because such models of sustainable buildings seem promising for predicting performance. Fitzgerald notes that significant disparities between prediction and performance often occur when ‘what actually gets built is different to the drawings.’ Indeed, buildings that are not mechanically ventilated often have more thermal mass and heat absorption capacity than predicted by models because input parameters are conservative.

Boukli Hacene’s next projects will be the optimization of double-flow controlled mechanical ventilation, rainwater recovery, and surplus energy sales. One can only wonder what Vitruvius would have thought if he could see such new developments in sustainable architecture.

Rachel Berkowitz