First published in Cleantech magazine, July/August 2010. Copyright Cleantech Investor Ltd 2010
By Dr Denis Gross
The point beyond zero energy consumption (when solar and other forms of home-based microgeneration balance the external energy imported), the point at which a house generates more energy than it consumes over a period of time, defines the EnergyPlus House.
The energy consumed by different types of housing differs significantly. In the UK, for example, an average semi-detatched house consumes around 22,000kWh/yr. Over 80% of this typically goes on heating; the remainder on electricity. A low energy house, on the other hand, consumes just over one-third of the total energy of the average house, with only 60% of this lower figure going on heat. The heating requirements of higher efficiency buildings, such as the ‘passive house’, are significantly lower again.
The Passive House: The ‘passive house’ concept has, over three decades, achieved reductions in energy consumption of up to 90% in the approximately 20,000 examples built to date. Stemming from the voluntary and rigorous Passivhaus (German) and MINERGIE-P (Swiss) standards for energy efficiency in buildings, ‘passive’ design is integral with the building design process and yields ultra-low energy buildings with minimal energy requirements for space heating or cooling.
Core to passive house design is the use of ventilation air to provide the heat load, and a combination of high levels of insulation, designing-out of thermal bridges, rigorous air-tightness and a ’whole house mechanical ventilation system’ with highly efficient heat recovery – all of which means that traditional heating and cooling systems are not required. Mechanical ventilation with heat recovery (MVHR) conserves energy by recovering heat from extracted air and transferring it to the incoming air. This works both ways, ensuring that if the outside temperature is higher than that inside, the exchanger helps to maintain a comfortable internal environment.
Even with the use of superinsulation materials, walls have to be thicker than in conventional buildings to achieve passive house standards, which either compromises internal space or requires a larger ground area for the building. Other requirements are for sophisticated high R-value window technology, typically triple-plane insulated glass with a krypton or argon filling and low-emissivity coatings, thermally broken window frames, and thorough sealing of the construction joints in the building envelope and service penetrations to provide the high level of air-tightness.
While energy consumption is much reduced, a passive house still requires energy to be imported or harvested.
The Zero Energy Building: The category described as a ‘zero energy building’ (ZEB) or ‘net zero energy building’ refers to a building which demonstrates zero net energy consumption annually. A ZEB is generally optimised to use passive solar heat gain and shading, prevailing winds and the temperature of the earth beneath the building to provide stable indoor temperatures with minimal mechanical intervention. ZEBs employ low energy building techniques, including high levels of insulation, high efficiency windows, solar water heating and heat recovery units on waste water. Lighting during the day is provided by solar tubes and skylights, and at night through LED and fluorescent lighting. The dual-use of energy is typical in ZEBs – for example, using refrigerator exhaust to heat domestic water, or the heat from office machines and electronics to heat the building.
Ideally, ZEBs derive power from a range of microgeneration technologies, including solar PV, solar heating, wind turbines, biofuels, geothermal and microhydro. While such energy harvesting should eventually lead to the off-grid, or autonomous, operation of ZEBs, currently it is common to link ZEBs to the grid to smooth out variations in demand, with the ZEB importing electricity when it cannot harvest sufficient energy, and exporting surplus to the grid. Energy harvesting on a local rather than an individual scale – a collection of ZEBs, or a village – leads to benefits in generation efficiency and the elimination of the transmission and distribution losses of the standard grid. This can be seen in practice in zero-energy neighbourhoods such as the BedZED development in South London.
The EnergyPlusHouse produces more energy from renewable energy sources over a year than it imports from external sources. As with the ZEB, the EnergyPlusHouse relies on low-energy building techniques and microgeneration.
Rolf Disch, German architect, solar pioneer and environmental activist, developed the concept of the PlusEnergyHouse in the 1990s, and built the first example, Heliotrope, in Freiburg in 1994. He sets three goals for a PlusEnergyHouse: 100% renewable energy supply, emission-free operation and a positive energy footprint. All of these are met in the Heliotrope.
Solar is used extensively in Disch’s buildings for power and water heating, and the roof is formed from PV panels. Large triple-glazed windows on the south-facing side permit large amounts of natural light to enter, while infra-red reflecting coatings retain the heat. The outer skin of the house is insulated without a thermal bridge, and the thermal mass of the house is used as a heat store. Aesthetics as well as energy efficiency are a major driving force. For example, it is important that the solar technology used in the building is integrated into the facade.
The core of the Disch building is the ‘Powerbox’, which integrates energy and installations and contains the distribution centre where information on the generation and consumption of electricity is collated. The Powerbox also contains the resource conservation suite of technologies providing MVHR, water and heating.
The Heliotrope building rotates to track the sun, maximising the light, power and heat obtained. A number of energy generation technologies have been used at Heliotrope, including a large dual-axis solar PV tracking panel, a geothermal heat exchanger, a CHP unit and a solar-thermal balcony railing. The building can be a net generator of between four and six times its energy consumption through the year.
After the construction of three Heliotropes – the third in Bavaria to be used as a dental laboratory – the community planning and integration elements of the PlusEnergy concept were employed in the Solar Settlement, a fifty house community in Freiburg. Built between 2000 and 2005, the Solar Settlement was arguably the first development in which the houses produce a positive energy balance, and the community is carbon neutral.
The next step was the Sun Ship, which extended the PlusEnergy concept to a large integrated office and retail building, located next to the Solar Settlement. This 60,000 sq ft commercial, retail and residential building was completed in 2006, and houses a supermarket and other outlets on the ground floor, three floors of offices and work spaces, and nine penthouses on its roof. Technologies employed in this structure include vacuum insulated walls, ventilation with 95% heat recovery, triple-glazing and solar technology integrated into the facade.
Rolf Disch and the PlusEnergy design concept have done much to underpin Freiburg’s Green City status, and show that advanced cleantech principles can be applied to architecture in a cost-effective, aesthetic way to the benefit of a community. We can expect to see many aspects of this concept finding their way into tomorrow’s built environment.
