Making school kitchens energy efficientFebruary 2013

Engineers specify low energy heating and lighting for schools, but often forget the catering kitchen. Roderic Bunn reports on more efficient ways of cooking for kids.

Catering kitchens are full of kit that consumes energy. Cooking ranges, fridge freezers and bains-maries are obvious gas guzzlers, to which one can add dishwashers and a wide variety of food preparation equipment. In a hotel kitchen one expects equipment to be of industrial strength. But surely in a primary school things can be at a more domestic scale? For some reason, they rarely are.

Energy consumption - and by association the carbon dioxide emissions - caused by school kitchens is therefore not insignificant. As energy use is driven down elsewhere in a school the power consumed by the kitchen equipment begins to stand out. Not only that, the heat generated by fridges, freezers and under-counter coolers can contribute to overheating, a problem that increasingly seems to bedevil new schools, particularly well-insulated ones.

Take the average dishwasher. A passthrough dishwasher operating at 85°C can come with a 2 kW element plus a rinse tank with 7 kW boost heater. Larger units with power input requirement of 3-9 kW are not unusual.

Another major consumer of energy in school kitchens is the ventilation hood. With gas cooking in particular, the ventilation hood performs a multiple role: it removes heat from cooking equipment, it removes vapour and particulates generated during cooking, it removes the products of combustion, and finally it provides fresh air for gas combustion. The cooker hood is also interlocked to the gas supply valve which ensures the hood will run whenever the gas is on, thereby providing protection in the event of a gas leak.

UK designers follow the guidance of the HVCA's DW/172 Specification For Kitchen Ventilation Systems. This quantifies ventilation rates required for various items of equipment depending on the type of equipment and usually the horizontal area.

For equipment under the hood, the aim is to capture the plume of hot air that rises naturally from the hot surface. By extracting in excess of this quantity of air from the hood, heat and vapour do not spill out into the kitchen. This requires higher ventilation rates for equipment with hotter surfaces. Being insulated, ovens have lower ventilation rates than hobs. Grilles and deep fat fryers have high ventilation rates.

Energy engineer Alan Clarke and Passivhaus consultant Nick Grant thought through the problem with respect to two Passivhaus schools: Oak Meadow and Bushbury Hill, completed
in Wolverhampton in 2011. Oak Meadow is a 2400 m2 school for 420 pupils while Bushbury Hill is a 1900 m2 school serving 240 pupils. For these projects Clarke and Grant were the Passivhaus consultants to architect Architype and services consultant E3 Consulting.

To achieve the Passivhaus Institute heat demand target of 15 kWh/m2 per annum both schools have mechanical ventilation at 18 m3/h per person, with 80 per cent heat recovery for the
classrooms and hall. At Bushbury, for example, the ventilation rate was 5400 m3/h. The standard kitchen design requires another ventilation unit of 3600 m3/h with no heat recovery.

In simple terms this additional ventilation heat loss alone could add up to 10 kWh/m2 per annum to the school heating demand, which would be caused by the need to ensure the cool fresh air supply to the kitchen causes no discomfort.

The first step was to investigate whether the ventilation rate could be reduced by thoughtful procurement of the catering equipment. The first step in minimising heat output was to go all-electric. The switch to electric appliances made it possible to reduce the ventilation rate further at times of low heat output, whereas with gas cooking the airflow must remain at the design level to ensure it will always dilute combustion products. This strategy reduced the total ventilation rate to 2400 m3/h.

The designers also opted for an induction hob. Although the capital cost is higher, there are significant running cost benefits. For example, when cooking the heat lost to the kitchen from a gas hob is around 100 per cent of the heat used, whereas the heat to the kitchen with an induction hob is only 25 per cent of the useful heat. At idle the heat gain to the kitchen from some hobs, such as an iron range or gas left burning can actually be higher than when in use, but is close to zero for an induction hob.

To aid understanding of the heat flows in the kitchen Clarke and Grant built a steady-state spreadsheet model to combine the main heat sources and the air movement. This drew on detailed measurements of actual heat emissions of various appliances and equations developed by the kitchen ventilation specialist, Halton.

The model assumed a percentage of maximum cooking power, with around 15 per cent radiant heat to the kitchen, and 15 per cent convective heat extracted via the hood. 100 per cent of other heat gains from lighting, refrigeration, occupants and hot cupboards go to the general kitchen area. A large fraction of cooking heat input is retained by the food, and some
goes to generating vapour in addition to the convective heat. The ventilation rate was variable, from design maximum down to a minimum set by the fan characteristics.

An air supply temperature of 12°C was considered the minimum for comfort directly below the supply air terminals. At full cooking load the gains to the kitchen are such that a 12°C supply air temperature was judged to lead to comfortable working conditions. At an external ambient of 20°C and fans running at maximum airflow, the model indicated that operative temperature would peak at 28°C - an acceptable limit. At times when there is no cooking, reducing the air flow rate to around 30 per cent obviated the need to raise the supply temperature above 12°C.

Schools with Passivhaus levels of insulation and airtightness should not require background heating in their kitchens. Clarke and Grant reasoned that its should be possible to deliver all the required heat via the supply air. (There is some demand for space heating in winter when outside air temperatures can fall below zero, but as the supply air only needs to be heated to 12°C, the heat demand can be calculated for a much lower heating degree hours than for general space heating.)

The total heat production of the kitchen was deemed sufficient for the extract to be used to pre-heat the incoming fresh air. A counter-flow heat exchanger was not thought robust enough for the hot, moist and greasy kitchen air, but a simple air-to-water heat exchanger was a good alternative.

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 alan@arclarke.co.uk and nick@elementalsolutions.co.uk.