Despite this industry-wide push, however – and engineering calculations that indicate usage will be lower – consultants and contractors are hesitant to trust the efficiency of all-electric systems. They are also inexperienced in determining accurately the peak electrical loadings for all-electric buildings.

They do not understand, or are not ready to believe in, the efficiency of these recent technologies. This is partly because they are not willing to trust the calculations made in the design stages and tend to overdesign margins. This is a significant barrier to widespread adoption of all-electric designs – not just in schools, but in all types of buildings.

As the first wave of all-electric schools start to reach completion, it is up to building services engineers to bridge the gap and help educate the industry, so we all have confidence in the calculations. We need to look at how these first all-electric schools perform in operation by regularly measuring and reporting the performance.

This is not something we have historically been good at as an industry, and it requires a step change to get everyone on the same page. Once you start looking at these schools, numbers are clear, and they go a long way to helping understand actual demands and usage. They can then be fed back into calculations in the design stages of future schemes.

Recently, Cundall was part of the project team that completed West Coventry Academy, a 12,000m² secondary school with a sports block and swimming pool. Part of the DfE’s School Rebuilding Programme and Net Zero Carbon in Operation Pathfinder scheme, it was handed over in September 2023, and is all-electric in operation except for a small gas boiler that serves the pool’s water-heating system. The school has air source heat pumps (ASHPs) for heating and kitchen hot water, and direct electric hot water for everything else. It also has LED lighting and controls, and a small photovoltaic (PV) array (100% PV array was not specified in the brief, but there is room to add this in the future).

The building has a well-insulated and airtight façade, featuring Innovare structural insulated panels and high-performance glazing. The design peak heating load is 280kW, just 23W/m². Ventilation is hybrid, incorporating mechanical ventilation units with heat recovery for all occupied spaces for tempered fresh air in winter, and natural cross-ventilation via circulation atria roof turrets for summertime ventilation. The only cooling is to the server room, and some classrooms have exposed ceiling fans to aid thermal comfort.

The school had taken part in the DfE’s ‘pathfinder scheme’ and, in 2021, had employed the then new DfE Output Specifications and technical annexes that had not been put into practice on any scheme previously, and had continued as a test location for these new specifications. When we designed West Coventry Academy, we called on our experience with a previous Priority School Building Programme project, finished in 2018, that we knew had resulted in a maximum in-use electrical demand of just 276kVA (17.2W/m²).

Many did not believe we had achieved such low figures, but we had the electric bills and meter readings to validate this over several years. With this in mind, we designed West Coventry Academy to have a maximum in-use electrical demand of 550kVA (48.2W/m²), and we were confident our design would generate results similar to previously acheived. When the design and build subcontractor took over at RIBA Stage 4, however, they were not so confident in our calculations and an 800kVA supply was installed.

Six months on from West Coventry Academy being handed over, the school has experienced a winter that included a particularly icy -8°C cold snap, on 18 January 2024. The Cundall team visited the site a few weeks later to gather data and discovered that the maximum demand recorded was just 398kVA (33.7W/m²) – lower than even we had predicted for maximum energy demand, and far, far less than the 800kVA supply that had been installed.

This is a clear demonstration of the efficiency of all-electric designs, even in extreme conditions. It also highlights the potential cost implications of overdesigning maximum electrical demand and the associated infrastructure and low-voltage supplies.

It is a habit that we, as an industry, have acquired after years of designing low-efficiency, poorly insulated gas-fired schools. This overdesign not only adds cost, but also carbon – in construction and for the school itself, which, potentially, is left with a lifetime legacy of higher standing charges for a capacity it will not use.

On a lot of new or replacement schools, this can lead to expensive substation upgrades and high-voltage network reinforcement, which, again, may be unnecessary for the school in the long term.

Another reason the industry tends to overdesign schools is that the BSRIA BG9 2011 design guide suggested an electrical services load of 35W.m^-2 for naturally ventilated schools (or 50W.m^-2 for mechanically ventilated ones), and this assumes that heating and DHWS energy is fed by gas or other non-electric sources. The guidelines are based on 12-year-old data from gas-fired heating and hot water, and haven’t been updated to account for all-electric schools. (BG 9 has now been superseded by BG 84/2024 ‘Space and Weight Allowances’ ).

Typically, total connected loads are established based on good design data, and percentage-based diversity factors are then applied to equipment, ASHPs, domestic hot water systems, and other loads. These percentage based diversities are no longer appropriate.

In recent years, we have seen electrical infrastructure designs presented of typically 120W/m² – nearly four times the maximum demand recorded at West Coventry Academy on the coldest day of the winter. Some designs are even higher, and this is clearly based on guesswork, not empirical data or good practice.

We cannot really blame designers for this; the guidelines have not kept up with the current rate of change when it comes to best practice on how we design our schools.

To counteract this problem, Cundall has established a school daily demand profile tool (see graph, left). The peak loads have been taken from actual meter readings of all-electric schools that we have designed. The daily data around these peaks is based on metered annual consumption data, with some adjustments applied depending on school specific facilities for design and technology or computing. BIN weather data is employed to simulate annual profiles based on the available data. When a full set of seasonal meter readings are available they will be used to improve the model.

West Coventry Academy has air source heat pumps for heating and kitchen hot water

The central premise is that the sum of all hourly and daily loads can be used as a check and equated back to an annual load. We must design schools to achieve specific DfE energy use intensity targets, expressed as kW/annum/m², and, clearly, if these are exceeded in this check sum, you have overestimated a particular load.

This data can be extrapolated and used on any other school type to assist with future design estimates, and is a far more accurate way of calculating maximum in-use electrical demand until the guidelines catch up.

My hope is that we can speed up that process and replace the simplistic rules of thumb by feeding the irrefutable data we have collected already into upcoming design specifications.

In the meantime, the key for everyone in the industry, but especially building services engineers, is to keep measuring and reporting regularly on the in-use operational performance of the buildings we design. 

• Peter Hazzard is a partner and schools sector lead at Cundall

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It is fitting, therefore, that McDonald’s designed and built the world’s first net zero energy fast-food restaurant on the Walt Disney World campus. The all-electric building has achieved more than 105% net positive energy for 12 consecutive months, thanks to a roof-mounted solar photovoltaic array.

Quick-serve restaurants have very high energy use intensity and, with a 24-hour operation, some creative engineering was required to minimise loads.

Cooking appliances consume 55% of the building’s annual energy, while building ventilation is the second-largest load. The majority of ventilation is replacement air for the cooking hood exhaust system, which runs 24 hours per day.

To minimise kitchen exhaust, the kitchen hood exhaust system uses cooking demand-based ventilation controls. These monitor heat, grease and smoke, and adjust exhaust rates to maintain effective capture and containment.

Quick-serve restaurants have large fluctuations in service and have to provide meals to customers on demand. A typical restaurant will keep their cooking lines hot 24 hours a day, so they can react instantly to an influx of customers. This is not only a major energy consumer, but it also adds significant cooling load and keeps the kitchen hood demand-control system from going to minimum flow.

At the Walt Disney World restaurant, McDonald’s deployed a new technology in quick-serve cooking that allows an entire line to go into idle mode and rapidly switch to ‘ready’ when required. This location handles an extraordinarily large number of customers given its proximity to the theme park and has a very large kitchen to handle the demand. There are three cooking lines and being able to bring them to an idle state during lower demand times significantly reduces ventilation and energy load.

Kitchen pressure is held at a 5% negative to adjoining spaces to keep kitchen odours contained. Having a variable exhaust system means the kitchen make-up air unit, and the dedicated outdoor air unit for the adjoining dining room space, must react in sync to maintain a proper pressure balance. Both the make-up air unit and dedicated outdoor air unit have variable speed drives and space-pressure sensors for control. Commissioning this system was complex and required simulation of many scenarios of cooking and occupant loads to tune properly.

The most unique ventilation strategy was the incorporation into the dining room of a natural ventilation system. While this complicates the pressure control of the kitchen, it provides significant energy savings and enhanced occupant comfort.

Orlando is a subtropical climate, classified under the Köppen climatic classification as Cfa. The area has two seasons: hot and rainy, and warm and dry. May to September is considered the hot and rainy season, with high temperatures typically around 32^oC and average lows between 18^oC and 23^oC, with frequent heavy rain. The warm and dry season spans from October through April, with average high temperatures between 20.5^oC and 28.8^oC and lows between 8.8^oC and 18^oC. The warm and dry season experiences about half as much rain as the hot and rainy one.

The operative temperature range for the dining area is between 20^oC and 25.5^oC. Because the restaurant operates 24 hours per day, natural ventilation is available most often at night, when the seasonal temperature ranges are within the operative temperature range. With the wide operative temperature range, the store has 3,800 hours of natural ventilation per year (43%).

The natural ventilation system was commissioned not only to monitor dry bulb temperature, but also enthalpy, wind speed and precipitation. When the weather conditions are within all ranges, operable glass louvres that line the entire south and west façades of the dining area open, while the variable refrigerant flow system providing space conditioning, and the dedicated outdoor air unit providing ventilation, shut down. Natural ventilation fans draw air through the space and help maintain the pressure balance to the kitchen.

Having the natural ventilation glass louvres at the customer level posed a safety concern that had to be accounted for carefully. Louvres could have an item placed in them, or a customer may put their hands in and get them caught.

To address safety concerns, the inside is screened; this also keeps pests or debris from entering the restaurant. On the exterior, a laser system creates a field covering the entire surface of the louvres, and instantly disables actuation of the system if an object is detected. Actuation starts again when the object is removed. An audible message also alerts customers when the louvres open or close, and an internal safety mechanism detects added pressure and prevents complete closure. The commissioning authority tested this and still has all his fingers!

Comfort is maintained in the outdoor area 58% of the time annually between 6am and 6pm

Outdoor dining is not typical with quick-serve restaurants, but this location made that experience the centrepiece of the design. Covered by a sweeping roof made of custom glass panels with amorphous silicon solar photovoltaics, the outdoor dining area is shaded and high-volume, low-speed fans modulate to maintain a comfortable environment.

The shaded environment is comfortable and maintains a similar operative temperature range as the indoor environment, even during some of the hotter temperature conditions. A shading study estimates that comfort is maintained 58% of the time between 6am and 6pm annually. The shaded area also helps precondition the natural ventilation air when that system is active.

The flagship McDonald’s restaurant was originally scheduled to open in April 2020, but this was delayed because of the onset of the Covid-19 pandemic. In July 2020, the restaurant opened for drive-thru service only. The commissioning team took advantage of this opportunity and evaluated and tuned control strategies for the kitchen ventilation systems.

The dining room opened to service in autumn 2020 and the natural ventilation features performed better than anticipated, as temperature and enthalpy ranges were expanded over design setpoint. (See Figure 1). Energy data is tracked live using a monitoring-based commissioning system and is performing better than design. Issues with inverter failures delayed net zero energy performance in the first year of operation. Currently the restaurant is seeking net zero certification through the International Living Futures Institute.

Benjamin Skelton is president at Cyclone Energy Group, which was the commissioning authority and energy expert for the McDonald’s project

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