The school kitchens were equipped with run-around heat-recovery coils, with a glycol mixture circulating between a coil in the extract air and a coil in the intake air. This was calculated to have a design heat recovery efficiency of 50 per cent, which was close to meeting the supply air heating demand.
The fluid circuit includes a mixing valve so that proportion of heat fed to the supply air can be modulated on air temperature control. The system includes a heater coil from the main heating system because of uncertainty of maintaining comfort temperatures in winter design conditions and during food preparation when cooking heat gain is minimal.
Performance in use
The two schools opened in October 2011. There was no specific funding for in-depth monitoring so Clarke and Grant used the standard types of monitoring available in the school
building management systems, plus site visits with interviews with kitchen staff and energy measurements of refrigeration using plug-in kWh meters.
The monitoring from Oak Meadow school is shown in figure 1. At around 09:30 the hot cupboard is turned on to start warming the food containers. At 10:00 the ovens are used for batch cooking. The heat recovery was more effective than anticipated at maintaining a comfortable room temperature.
The heat recovery efficiency was inferred from temperature rise and seen to reach a maximum of approximately 45 per cent, modulating down to 10-15 per cent as hood temperatures rose.
This modulation is clearly important for control of room temperature.
With good airflow control the additional heating may not be needed. However, the kitchen staff have been observed to control the fans at a particular rate for several days, and do not adjust them until they feel noticeably too warm or too cold (the on/off time is controlled automatically).
The induction hob was new to the kitchen staff, but they are said to be very happy with its performance. Previously the handles of pans would get too hot to touch - now they are cool and easier and safer to lift.
Clarke and Grant's energy monitoring showed that the fridges and freezers (a 600 litre upright fridge and freezer in the kitchen, and a single chest freezer in a store) use a total of 5 kWh/day. This represents around 1 kWh/m2 per annum of electricity.
Over a two-week menu cycle, the average daily electrical power consumption for Oak Meadow was 55 kWh for 170 meals. Taking the cooking energy use as running from 08:00 until 13:00 (the hot cupboard and heated servery are still in operation at this time) and subtracting refrigeration, the daily power consumption ranged from 43 kWh to 49 kWh, with average of 46 kWh. This gives an average figure of 0.27 kWh per meal.
Hot water use is not separately metered so has been estimated on the basis of the increase in gas consumption by the hot water generators for this period. The total electrical consumption
ranged from 7 kWh to 11 kWh, with an average of 9 kWh. Hot water ranged from 4 kWh to 13 kWh and averaging at 8 kWh.
Clarke and Grant's investigations show that the use of cooking equipment with low heat loss to the kitchen enables the use of lower ventilation rates, with associated reduced need to heat incoming air and reduced plant size and fan power. A low efficiency heat recovery system using air-glycol heat exchangers is also evidently sufficient to provide most of the heat that the supply air requires.
This article is based on a paper delivered by Alan Clarke and Nick Grant to the International Passivhaus Conference in 2011, and conversations with the author. For more information contact firstname.lastname@example.org and email@example.com.