3.4.1Energy demand
Balanced mechanical ventilation with HR outperformed natural ventilation in terms of energy in all instances but one. When comparing supply air windows driven by natural ventilation (scenario 4) with balanced mechanical ventilation with an HR rate of 75 %, a SFP of 1800 J/m3 and an infiltration rate of 0.13 l/(m2Ãs) (scenario 7) their performance in terms of energy was about equal.
That the two scenarios were equal was largely due to the relatively high level of infiltration included in the scenario with balanced mechanical ventilation.
Lowering the infiltration rate is known to improve performance of balanced mechanical ventilation systems with HR [23,65,84]. When comparing the equivalent scenarios without infiltration (scenarios 4 and 5), the relative performance gap in terms of energy grew from about 0 % to 19 %. The reduction in energy demand is comparable with what is reported in the literature [84]. Dependent of type of housing, un-renovated Danish housing stock (prior to 2006) has average infiltration rates ranging from 0.22-0.28 l/(m2Ãs) [85]. In order to ensure optimal performance, a home must be tightened to the exterior before employing a balanced mechanical ventilation system with HR. Studies have found that it is possible to reduce infiltration rates by 70-80 % in single-family housing [84,86]. This level of reduction in infiltration is enough to make balanced mechanical ventilation with HR net profitable [66]. Alternatively, if tightening the building envelope is either not possible or not wanted, a balanced mechanical ventilation system can minimise harmful exfiltration by setting the exhaust rate higher than the inlet rate.
Results show that fan power is negligible when compared to the reduction in energy demand for heating. For exhaust ventilation, fan power accounts for only 2 % of the combined energy demand for heating and ventilation. For balanced mechanical ventilation with HR, fan power constitutes between 6 % and 8 %.
Choosing a ventilation system with energy efficient fans can lower the ratio
further. Modern ventilation systems have nominal SFP values between 550 J/m3 and 1450 J/m3 [148]. Meanwhile, in practice, due to high pressure losses or malfunctions, actual SFP values can be higher than their nominal counterparts [62]. One study found that centralised ventilation systems in single- and multi-family housing, with nominal SFP values between 1050 J/m3 and 1450 J/m3, averaged a SFP value of 1730 J/m3 [148]. Still, the potential for reduction in energy demand outweighs the possible consequences of implementing a ventilation system with higher than nominal SFP values.
The nominal HR efficiency of a contemporary heat exchanger can be rated higher than 90 % at peak performance (e.g. Genvex [58], Nilan [59] and InVentilate [60]). Though it is possible for a carefully constructed ventilation system to meet the nominal HR efficiency [61], there is a risk that systems will not perform as well as planned. Ventilation systems with HR are vulnerable to leaks and studies have found that systems with nominal HR rates between 70-80 % often do not recover more than 50-70 % [62,63]. In the above simulations, exchanging basic windows with efficient windows and improving the HR rate from 75 % to 80 % in scenarios without infiltration (scenarios 5 and 6) improved relative performance in terms of energy by 4 %. Considering the extent of renovations, the return on investment in form of reduction in energy demand was marginal. Still, the results show that even relatively modest HR rates can significantly reduce energy demand in Danish homes.
3.4.2Supply air temperature
Balanced mechanical ventilation with HR outperform natural ventilation systems in terms of supply air temperature. On average, the supply air window raised the supply air temperature by 3.8 °C. In the cold months from December to February the supply air temperature averaged 4.4 °C. In the same period, when outdoor temperatures averaged 0 °C, the ventilation system with an HR rate of 75 % delivered supply air temperatures averaging 15 °C.
3.4.3Supply air windows in system design
Supply air windows do not recover heat as such. The way supply air windows help reduce energy demand is by reducing the heat loss through the glazed part
of the window. For a temperature difference of 20 °C and flow rate of 4.57 l/s, the effective U-value of the glazed part of the supply air window in this study was 0.33 W/(m2·K). In recent years, windows have become much better. Today, a tripled layered low-E window with krypton gas filling can have a centre of glass U-value of 0.5 W/(m2·K). In terms of energy, the difference is now marginal and supply air windows have lost some of their competitive edge.
Estimates in published literature were that, when compared to natural ventilation, HR with an efficiency of 80 % and supply air windows would reduce energy demand by 22 % [64,65] and 11-24 % respectively [33,34,38]. Where estimates for ventilation with HR were accurate, supply air windows only reduced energy demand by 8 %. Still, it is worth noting how close supply air windows and generic balanced mechanical ventilation with HR ventilation were in performance in terms of energy. It is possible that the Danish climate constitutes a form of critical point: it is highly conceivable that supply air windows can outperform HR ventilation in countries with warmer winters. Conversely, colder winters will probably exacerbate the performance gap that was observed in this study and favour HR ventilation.
The above does not mean that natural ventilation does not have a place in modern buildings in Denmark. Certainly supply air windows can be successfully implemented in energy efficient hybrid designs. For example, supply air windows are good at reducing ambient noise and can be used to facilitate natural ventilation in areas with high outdoor sound pressure levels [149,150]. Also, there are supply air window designs that are different from the one used in this study.
In terms of energy, various different designs will probably perform on par with each other (per unit area), but there are designs that are better at increasing the supply air temperature [38,45]. It is possible to increase the supply air temperature by increasing the time supply air spends in the ventilated cavity (larger area or longer pathway) and by reducing the number of panes separating the conditioned interior from the ventilated cavity. Such a supply air window would work well in a ventilation system where heat recovered from exhaust air is transferred to a reservoir, such as a domestic hot water storage tank, instead of the supply air.
Considering the above, it is concluded that the primary lesson learned from this study is that designers aspiring to design an energy efficient ventilation system in a climate like the Danish should consider some form of HR on the exhaust air.
This work has not considered the carbon foot printing of a given ventilation system. Presumably a natural ventilation system will have lower production costs in terms of materials and energy. Also maintenance and reparations will play a role here. For example, supply air windows probably have a longer life span than a mechanical aggregate. It is wholly conceivable that a life cycle analysis of a building – or more specifically the ventilation systems in a building – can show that supply air windows are more cost effective in primary energy over the useful lifetime of the building than a mechanical ventilation system, despite the energy saving capability of air-to-air HR. This, however, is a question to be answered by future research.
4 Estimate of emission rate of pollution from building materials
This chapter consists of three subsections. The first subsection serves as a brief introduction to the theory behind contemporary physics-based VOC emission models. The following subsection examines how patterns and emission rates predicted by models based on mass transfer theory compare to observations made in practice. The comparison was done using HCHO as an example of a VOC and material properties that are representative for building materials known to contain and emit HCHO. The comparison revealed that the examined emission models overestimate emission rates and underestimate the time it takes to deplete the content of HCHO in building materials.
The second objective of this thesis was to develop a method to estimate the impact building generated pollution has on IAQ, the energy demand and ventilation rates. In order to achieve this, first it was necessary to identify emission models that give accurate estimates of emission rates of pollution. Since the attempts at identifying a suitable emission model in the existing literature failed, it was decided to develop VOC emission models by regression analysis of data collected in buildings in practice.
The regression analysis yielded two emission models for HCHO. One for
‘normal’ emission levels and one for ‘high’ emission levels. The models were based on data gathered in newer detached and semi-detached single-family homes in rural and suburban Denmark. However, data was scarce. This has imposed several limitations on the use of the derived emission models. The third and final subsection of this chapter contains a thorough description of the respective emission models, how they were derived and what their limitations are.
The chapter is based on work presented in Paper 2 and Paper 3. The original material can be found in the appendix, see list on page 112.