Cooking exhaust systems for low energy dwellings

Piet Jacobs

Scientist Integrator indoor environment & energy efficiency

TNO, Delft, the Netherlands.

piet.jacobs@tno.nl

Wouter Borsboom

Business Consultant indoor environment & energy efficiency

TNO, Delft, the Netherlands

wouter.borsboom@tno.nl

Especially in airtight low energy dwellings cooking exhaust systems are of utmost importance, as cooking can be a major source of PM2,5 exposure. Dwellings should be designed including facilities enabling extraction of at least 83 dm³/s (300 m³/h) directly to outside. Residents should be able to select an effective hood.

Keywords: Cooking exhaust, PM2,5, NO2, capture efficiency, exhaust flowrate, low energy dwellings

Cooking emissions have long been seen as an odour problem. However recent studies show that Particulate Matter is the main health risk of indoor air (Logue, 2012) and cooking can be a major source of the total exposure (Kluizenaar, 2017). The exposure to Particulate Matter due to indoor sources like cooking can significantly increase in low energy dwellings, due to the airtight construction. Peaks remained for several hours, see Figure 1, in measurements that were part of a small field study in 9 Dutch dwellings (Jacobs, 2016a). In three dwellings, inadequate cooking exhaust caused in the evening an increase of the PM2,5 concentration in the kitchen / living room of about 10 µg/m³. In 2010, the World Health Organization has indicated that the PM2,5 guideline value of 10 µg/m³ yearly averaged is also applicable to indoor spaces. (WHO, 2010).

The two questions which we try to address in this article are:

·         what is the effect of relative short cooking events on the yearly averaged value;

·         which systems are required for adequate cooking exhaust in low energy dwellings.

We use the term cooking exhaust system as the combined effect of type of exhaust hood, exhaust ducting and air supply is of importance.

Figure 1. Typical particulate matter concentrations during a day, the peak concentration is caused by hamburgers frying, note that it takes more than 4 hours to dilute this cooking contamination.

From mixing ventilation to source control

One of the first guideline (TV 187) for kitchen ventilation in Dutch was set up by WTCB (Wouters, 1993). TV 187 does not provide a value for capture efficiency of a hood. The guideline assumes that the hood will not capture all cooking fumes directly and therefore the air in the kitchen will be polluted. Based on this assumption the guideline gives as a rule of thumb that the exhaust flow of a hood in m³/hour should be 6 to 8 times the volume of the kitchen to keep the lag time, the time required to keep the hood on after cooking, within an acceptable time period. This allows, for a closed kitchen, to remove 95% of the pollutants within 20 to 25 minutes after cooking. This rule of thumb is based on closed kitchens with a limited volume. In a typical kitchen of 25 m³ this amounts to an exhaust flow for the hood of 150–200 m³/hour. In the last years, in the Netherlands the layout of dwellings has changed rapidly from closed kitchens towards open ones. The combined floor space of kitchen and living room is often in the order of 40–60 m², leading to volumes around 100–150 m³. Application of the same rule of thumb requires a hood exhaust flow up to 1200 m³/hour (333 dm³/s). This high flowrate is difficult to realize, as it requires sufficient air supply to prevent uncomfortable under pressure in air tight dwellings. Without preheating, this may introduce comfort problems due to draught and a serious energy penalty due to increased heating energy. The conclusion is that in dwellings with an open kitchen it is essential to directly capture the cooking fumes before they diffuse to the large air volume of the living room which is in open connection with the kitchen.

Effect of capture efficiency on exposure

Source strength

Recently, TNO and the university of Nottingham have measured the Particulate Matter source strength of four meals typical for western European cooking under laboratory controlled conditions, see Table 1. The results indicate that frying meat and vegetables at high temperature generates the most particulate matter. The average PM2,5 emission based on these four meals was 35 mg. In literature, large differences can be found with regard to source strength for the preparation of the ‘same’ dish under lab conditions. For example, for frying chicken Dacunto (2013) found 5,7 mg, while Fortmann (2001) found values between 70 and 464 mg. The latest value was obtained as a worst-case scenario: oil very hot and frying during a long time. However, the average emission of 35 mg coincides well with a field study by Chan (2017) in 18 dwellings during two weeks in which the average source strength of the in total 836 events amounted 30 mg.

Table 1. PM2,5 source strength for four typical dishes.

Dilution and deposition

For the decay of the PM2,5 concentration, aside ventilation also deposition plays an role, as shown in Figure 1. Deposition is the precipitation of particulate matter on surfaces. In lab measurements, the deposition rate was of the same order as the ventilation rate. This effect has also been observed during the field test. The decay could be explained with a dilution of 28 dm³/s, of which 40% could be explained on the basis of the ventilation rate. The other 60% was caused by infiltration and deposition. In general, the decay will vary by dwelling type, occupant behaviour, meteorological conditions, etc. As best guess for this study, a decay rate corresponding with a ventilation rate of 28 dm³/s for the kitchen / living room is taken.

Indoor PM2,5exposure due to cooking

Figure 2 shows the increase in PM2,5 concentration to which residents are exposed to during their stay in their dwelling due to cooking emissions. This figure is based on calculations with a 2-zone COMIS model. Cooking was simulated by a 10 minutes emission period, starting at 18.00 hour with an emission of 25 mg/day. After the cooking period, the decay due to ventilation and deposition was simulated with a volume flow of 28 dm³/s. During cooking the exhaust flow through the hood varied between 21 and 83 dm³/s. With higher exhaust flows, the escaped cooking fumes are more diluted. As discussed earlier, this dilution effect is a relatively small effect compared to the capture efficiency itself. The exposure was calculated over the period 18.00–23.00 h, which is a typical duration for stay in the kitchen/living room. It was assumed no exposure during the time that the persons are in the sleeping room (9 hour per day) and during the time they stay in the kitchen/living room before the cooking. Additional no exposure was accounted during 58 hours per week when occupants are outside the dwelling. The additional exposure of PM2,5 due to cooking (∆PM2,5) was averaged over a 15.7 [1]hour stay per day in the dwelling.

Figure 2. Residence time averaged PM2,5 concentration increase in dwelling as function of hood capture efficiency and flowrate.

The exposure in the dwelling also depends on the infiltration of ambient PM2,5. Hoek (2008) measured the relation between indoor and outdoor concentration in 50 Dutch dwellings in Amsterdam. He found an average infiltration factor of 0.39 during winter time. Research of MacNeil (2014) in 50 Canadian dwellings in Halifax, a city near the coast with a similar climate as in the Netherlands, showed that the infiltration due to open windows in summer (0.8) can be much higher than in winter (0.53). Based on these findings, the assumption of a yearly average infiltration coefficient of 0.5 for Dutch dwellings seems reasonable. The yearly averaged Dutch PM2,5 concentration amounts to 15 µg/m³. Based on these assumptions (the infiltration level and the 15 µg/m³ level), the average PM2,5 indoor concentration due to ambient sources would be 7,5 µg/m³. In order to limit the indoor exposure to the WHO PM2,5 guideline value of 10 µg/m³, in the Netherlands a ∆PM2.5 of at most 2.5 µg/m³ would be permitted, although lower values are preferred. Based on Figure 2, for a typical dwelling and cooking behavior capture efficiencies in excess of 80% are required.

Minimum capacities for adequate cooking exhaust

In the TNOkitchen lab, a measurement method for the capture efficiency of cooking hoods in combination with inductive cooking is being developed. Figure 3shows the relation between the capture efficiency and the exhaust flow rate for three typical hoods at 83 dm³/s (300 m³/h). The capture efficiency of the chimney hood is 95%, the X-hood 84% and the slanted hood 70%. With an ‘efficient’ hood, an exhaust flow of 300 m³/h is advised. However, to reach a capture efficiency of 80% with the slanted hood, a two times higher exhaust flowrate is required as with the chimney hood. These differences can be explained by geometrical factors such as the presence of a damp buffer (Jacobs, 2016b) and the coverage of the burners by the overhang (see Figure 4). The chimney hood almost completely (95%) covers the skillets, which are placed on the front burners. With the slanted hood, the exhaust part of the hood is behind the front burners, and here the coverage of the front burners is 0%. The finding that the coverage of the front burners by the overhang is an important factor was also found in a field study in 15 dwellings (Singer, Delp, Price, & Apte, 2012). This finding was confirmed in a later study (Lunden, Delph, & Singer, 2015) under laboratory conditions. Capture efficiencies in this study varied by hood and airflow: 34–38% for low (51–68 dm³/s) and 54–72% for high settings (109–138 dm³/s) with front gas burner use for stir frying. The lower capture efficiencies found by Lunden et al. can be explained by the high gas burner power of about 2300 W used for stir frying. Assuming a heating efficacy of 50% for gas heating, about 1250 W are lost, causing a larger thermal plume compared to the TNO method for inductive cooking, where 400 and 500 W heat is released by the left and right front burner respectively. When cooking on gas, it is advised to have an exhaust capacity higher than 300 m³/h.

Figure 3. Relation between the capture efficiency and the exhaust flowrate for three typical hoods.

Figure 4. Laboratory setup for cooking: left chimney hood with 95% coverage of front burners, right slanted hood with 0% coverage of front burners.

Combustion gasses due to cooking on gas

Gas burning produces pollutants including carbon monoxide (CO), nitrogen dioxide (NO2), formaldehyde (HCHO) and ultrafine particles. Research by Mullen (2016) in 352 Californian homes showed an average NO2concentration increase of 48 µg/m³ in homes with gas cooking without kitchen exhaust compared to electric cooking. A simulation study by Logue (2014) estimated that in homes with gas cooking without coincident use of venting range hoods 62%, 9%, and 53% of occupants are routinely exposed to NO2, CO, and HCHO levels that exceed acute health-based standards and guidelines. Logue’s simulation results suggest that regular use of even moderately effective venting range hoods would dramatically reduce the percentage of homes in which burner generated concentrations exceed health-based standards.

Recirculation hoods are not recommended

Research by Jacobs (2017) suggests that recirculation hoods based on carbon filters remove PM2,5 only for about 30%. Applied with cooking on gas, a fresh carbon filter removes about 60% of the NO2, dropping within a few weeks of cooking to 20%. For both types of cooking it is not recommended to use such recirculation hoods, but to directly discharge the cooking fumes to outside.

Design implications

In the design phase of a dwelling, especially in case of serial housing construction, it is often not known what kind of kitchen and cooker hood will be installed later. In these cases, it is important to offer the possibility to the residents to connect a cooker hood with discharge to the outside. This is of special importance in apartments, were kitchens are often not directly near a façade. There are two possibilities:

·         A motor less extractor hood with a high-quality grease filter (class A EU energy label for cooker hoods) that can be connected to the ceiling extract valve of the ventilation system with a minimum capacity of 83 dm³/s (300 m³/h). The so-called cooking mode of the ventilation system is only used during the cooking. An example for a balanced ventilation system in single-family house is shown in Figure 5.

·         An air duct direct towards the façade or towards the ridge of the house with a maximum pressure drop of 50 Pa at 83 dm³/s (300 m³/h). This should be implemented in combination with a supply air provision that keeps the under-pressure in the kitchen below 10 Pa.

Figure 5. Nero Zero house, motor less hood connected to balanced ventilation system with 180 mm ducting and minimum number of bends. To be visited from March 1, 2018, consult: info@koppenbouwexperts.nl.

Acknowledgements

The VentKook project has been financed with TKI subsidy of the ministry of Economic Affairs for TKI Urban Energy, Topsector Energie, www.tki-urbanenergy.nl.

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