In building parlance, the skin of a structure is called its ‘thermal envelope’. In a passive solar building in cooler climates, this must be optimized to retain the solar heat you’ve captured. In a hot climate, a similar practice keeps out the heat and reduces cooling bills. This involves:
• a continuous air barrier;
• a completely continuous, high level of insulation – no gaps; • high-performance windows and doors;
• minimal thermal bridges;
• preserving or enhancing breathability – the ability of the walls to absorb moisture on the inside.
Figure 2.37 The south-facing (a) and north-facing (b) sides of a new Passivhaus standard
dwelling in Wales, UK, a three-bed property, called ‘Larch House’ after its locally sourced Larch cladding. Note the large windows on the equator-facing side and small windows on the cold side. It also features closed-panel timber framing to minimize draught, superinsulation, local stone for thermal mass, and thermal and photovoltaic modules. It achieved outstanding levels of air-tightness, exceeding the Passivhaus standard of less than or equal to 0.6 air changes per hour at 50 Pascals by recording on average 0.197 air changes per hour.
Source: BRE
behind grilles in the facade, or tilted windows, are opened automatically to allow heat to escape and be replaced by cooler night air.
Figure 2.39 The Solar
XXI building in Lisbon, Portugal, which functions as a combined office and laboratory at the National Energy and Geology Laboratory (LNEG).
Source: International Energy Agency (IEA)
Figure 2.40 Plan and sectional view showing distribution of the buried air pre-cooling system. Source: IEA
Buried pipes Vegetation
ventilation pattern was arranged to suit this. The building has high thermal capacity using external installation on the walls and roof. The south facade supports 100m² of solar PV modules and the majority of the glazing. Additional
space heating is provided by 16m² (172ft2) of roof-mounted solar thermal collectors
that also supply hot water, which can be supplemented by a gas boiler. The 18 kilowatt-peak (kWp; the rated power output under standard test conditions) grid-connected PV arrays supply electricity; further panels are located in a nearby car park where they also provide shade. The entire system satisfies heating requirements of 6.6kWh/m² and cooling requirements of 25kWh/m². Annual electricity use for the building is about 17kWh/m², of which 12kWh/m² is supplied by the PV arrays, leaving 30 per cent to be drawn from the national grid.
Natural lighting is encouraged – in the centre of the building is a skylight providing light for corridors and north-facing rooms on all three storeys. The installed artificial lighting load is 8W/m². There is no active cooling system. Venetian blinds are outside the glazing to limit direct solar gain. Natural ventilation is promoted through the use of openings in the facade and between internal spaces, together with clerestory windows at roof level, which help create a cross wind and stack effect. Assisted ventilation is provided by convection due to the PV module heat losses. To supplement this in the cooling season, incoming air can be pre-cooled by being drawn by small fans through Figure 2.41 Compound
parabolic concentrating (CPC) thermal collectors on the building roof.
Source: IEA
Figure 2.42 Cross and
vertical ventilation systems acting together with the buried pipe system.
Source: IEA
Figure 2.43 Method of
operation of the heat output of the PV modules to supplement ventilation.
distance apart so the inlet is not sucking in expelled air. The grilles are situated for the incoming and extracted air inside the room using the same principle. Ducts from each lead to the heat exchanger unit, which may lie beneath a panel in the floor for easy access. Humidistat sensors (e.g. above the shower and toilet) operate the fans when the humidity reaches a certain level.
Figure 2.44 Sensor and an
insulated heat exchange unit in a bathroom single-room ventilation with heat recovery system: (a) heat exchanger; (b) intake, situated above shower, removing warm, humid air; (c) output from the heat exchanger, inputting fresh air with recovered heat.
(a) (b)
Whole-house ventilation
A whole-house ventilation system may be installed if a blower test for airtightness
gives a result of 5m3/hr/m2 at 50 Pascals. Ducting takes air – usually from the
wet rooms – to the heat exchanger in the loft, where a low-wattage pump pushes the heated, clean, incoming air down (an)other duct(s) into a lower room. Incoming air is taken from a vent in the roof. The pump can be powered by a PV module, but then will only work when the sun is shining.
Figure 2.45 Whole-
house system layout for mechanical ventilation with heat recovery.
Source: © EST
Figure 2.46 Insulated
heat exchange unit in an attic – whole-house system.
are brought inside. As electricity is being used for this purpose, it should preferably be renewably sourced. If it is not, and the mains power is at least 70 per cent derived from burning fossil fuels, then using a modern 95 per cent efficient boiler running on a fossil fuel such as gas for heating instead would be just as efficient, due to efficiency losses on the way from the fuel in the power plant to the pump. The exception to this is if a coefficient of performance (COP) of 4.0 or above for the ground source heat pump to underfloor heating could be consistently achieved.
In the ventilation system illustrated in Figures 2.47 and 2.48, air is taken underground from an intake outside, and run through a coil buried outside, at least 3m (10ft) underground. The air is then drawn into the house, and let out through grills beneath the large, equator-facing windows. It then rises through the building using the stack effect. As the temperature of the air is not high (16–24°C;
61–75°F), the efficiency of the system is excellent. Depending on the outside temperature, this can have either a cooling or a heating effect, as the temperature
Figure 2.47 Trench with a coil for
a ground source collector for a heat pump, prior to burial.
Source: © John Cantor
Figure 2.48 Air intake in a garden outside, taking air to an underground heat exchanger, and
underground will be more consistent throughout the year. For a better cooling effect, the system can be specified to work in reverse in the summer, removing hot air and transferring the heat to the ground. Heat pumps have a long life expectancy (typically 20–25 years for the equipment and up to 50 years for the ground coil).
Solar heat and light control
A dwelling with too much equator- or west-facing glass can result in excessive winter, spring, or autumn day heating and too much glare. Using the sunshine for day-lighting as well as heating and cooling, while avoiding too much glare, is a design challenge. Although the sun is at the same altitude six weeks before and after the solstice, the heating and cooling requirements before and after the solstice are significantly different. Modelling must account for this, using software called THERM – Two-dimensional building HEat tRansfer Modelling and Passivhaus Planning Package (PHPP) – see the Resources chapter for more information. If the building has window overhangs to provide midday shade Figure 2.49 Adjustable solar chimneys can be used to modulate the temperature and prevent
overheating, as in the Inland Revenue building in Nottingham. The centre, completed in 1995, was a pioneering ‘green’ project in the UK. At night, the inherent thermal mass of the concrete is exploited and purged with fresh air to pre-cool the structure. At the corners of the buildings, the air within the glass block stair towers warms and rises on sunny days, giving extra drive to the ventilation system. Fabric umbrellas on the tops of the towers act as large dampers, lifting to exhaust hot air and closing, on cool days, to conserve heat. The office buildings were extensively prefabricated. The local bricks of the load-bearing piers were laid in a factory, around steel lifting rods, in storey height units.
Figure 2.50 The ÉcoTerra™ house, built by Alouette Homes, is a 240m2 (2600ft2) prefabricated
home assembled in 2007 in Quebec, Canada. It integrates passive and active solar systems and energy-efficiency technologies. It features high-performance south-facing triple-glazed windows to maximize the amount of solar radiation entering the house while minimizing heat loss; and ventilated concrete slabs on the main and basement level to absorb incoming solar radiation and slowly release it throughout the day and into the night. Solar-heated air from a building-integrated photovoltaic and thermal (BIPV/T) system is used for space heating, domestic hot water preheating and clothes drying. Surplus electricity is exported to the national grid.
Source: IEA
Heat recovery from space under PV roofing. Supplies dryer, domestic hot water and basement floor space
PV laminated roofing
Air supply to each room
Concrete materials act as thermal storage Under-floor air circulation Heat-recovery ventilator Geothermal well
Water-heat recovery
Triple-pane argon windows
they should be of an appropriate depth relative to the building’s latitude. However, these can still be bypassed when the sun is low in the sky. One permanent and fixed solution is to specify a ‘solar control’ coating on the inside of the first pane of glass, which can be precisely calibrated to reflect a given proportion of the sun’s radiation back out and prevent it from entering the room. This might be appropriate in an equator- or west-facing conservatory, which tends to overheat. Such glass might be installed in a roof and reflect, say, 70 per cent of the heat.
Variable solutions are available for occupants to adjust according to conditions, such as:
inside: window quilts, bi-fold interior insulation shutters, manual or
•
motorized interior insulated drapes and shutters;
outside: shutters, roll-down shade screens or retractable awnings.
•
Shading devices have been used for – in some cases – centuries in Mediterranean countries. They can help control daily/hourly variations. The adoption of different types of shading devices can reduce the energy requirement for cooling from 50 per cent to 100 per cent. Automated systems are on the market that monitor temperature, sunlight, time of day and room occupancy and control motorized window-shading-and-insulation devices. While it is important to take care that the energy cost of these, including the embodied energy of manufacture and installation, is not greater than that saved by reducing the cooling demand, research on buildings in Europe has discovered that the adoption of such automatic systems allows much greater solar protection and correspondingly greater reduction of energy needs for cooling. It is advantageous for the designer to consider that combining solar protection, ventilation and thermal insulation in one building component, such as a window or facade, in new-build or refurbishment, offers the potential for great savings.
Figure 2.51 External shutters, internal shutters, overhangs from balconies (visible through the window),