Robert Bosch GmbH, Gerlingen

More economical air conditioning without sacrificing comfort

Bosch can save energy with new induction system

Company Robert Bosch GmbH
Country Germany

The main challenge was to find a solution that would enable energy savings without compromising indoor climate comfort.

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Robert Bosch GmbH, Gerlingen

Tobias Lorson’s aim is to reduce energy consumption while maintaining the same level of air conditioning comfort. The group leader in Facility Management at Robert Bosch GmbH’s Gerlingen-Schillerhöhe site sees the refurbishment of the induction air conditioning system as a promising route to greater energy efficiency. Trials with the manufacturer LTG Aktiengesellschaft from Stuttgart in two test rooms have shown the savings potential: With new induction units, the energy requirement for heating, cooling and ventilation can be reduced by up to 40 %.

In 2020, Robert Bosch GmbH aims to emit at least one fifth less CO2 than in 2007. More efficient building technology should make a significant contribution to this. Tobias Lorson and his team at the Gerlingen-Schillerhöhe site (near Stuttgart) have therefore investigated various approaches. Air conditioning offers great savings potential here. However, the premise is that saving energy should not come at the expense of climate comfort. The office complex on Schillerhöhe is built in the lightweight style typical of the late 1960s. In the 1980s, it was fitted with effective double glazing to minimize heat loss. Automatically controlled external blinds and manually operated slatted curtains are available for shading. Central ventilation systems and parapet induction units in the rooms ensure oxygen-rich and well-tempered air.

Comfortable air conditioning

The ventilation systems in the basement of the building can not only filter and temper the air, but also humidify and dehumidify it, i.e. they are designed as a full air conditioning system. The air exchange exceeds the minimum requirements according to DIN and thus ensures that the CO2 concentration in the room air always remains low. The air in the offices is “changed” several times an hour. This allows employees in individual and open-plan offices to keep a clear head. In addition to quiet rooms and low air movement through the air conditioning system, fresh air and a pleasant temperature also contribute to comfort. These requirements for air quality and acoustic comfort can be met with the induction principle, which is particularly energy-efficient and virtually maintenance-free.

The desired temperature can be set individually for each office. In the open-plan offices, the temperature can even be set by zone via the building management system. One temperature zone is about 25 m² in size. The office and meeting rooms are ventilated and temperature-controlled with the help of induction air conditioning units, some of which date back to the 1960s, while others have since been replaced in the course of partial renovations.

The indoor climate at Bosch is pleasant and employee surveys confirm this: The associates appreciate the comfortable climate. It is therefore understandable that the rate of dissatisfied users is lower than for many comparable properties – and that is how it should remain. “It is still possible to find potential savings without compromising on comfort,” Lorson is certain. “A lot has happened in terms of technology since the building was constructed.”

Adapt air flow to demand

There has even been a huge step forward in induction technology in recent years. In addition to units with a constant volume flow, such as those installed at Schillerhöhe, Stuttgart-based LTG Aktiengesellschaft now also offers units with a variable air flow. They are called HFVsf, where the “sf” stands for “smart flow” (see also HLH 12/2011, page 26 ff.). With this new generation of appliances, the primary air flow can be regulated in two stages or steplessly between 20 and 100 % and can also be switched off. The appliances have adjustable nozzle systems to ensure that the air discharge speed and room penetration remain the same. A two-stage control system is ideal for individual offices, for example, where it is possible to switch to base load operation when the room users are absent. Infinitely variable devices are ideal for installation in open-plan offices and especially in meeting rooms, as the required supply air volume depends largely on the number of people present.

The equipment of the HFVsf devices opens up savings opportunities that are denied with older induction devices, as the latter blow the same amount of air upwards via nozzles along the window surface during the entire system operating time (tangential flow). Due to the strongly directed air jet, they manage with relatively little primary air, but require a high pressure to develop this type of flow. For temperature control, they draw in three to six times the amount of air (the secondary air) from the room using the induction principle – similar to a Venturi pump. This secondary air flows through a heat exchanger before it mixes with the primary air flow. One shortcoming of these devices is that they always had to be designed for the maximum air requirement. In addition, changes in the room such as ceiling fixtures and beams impair the effect, as they interfere with the formation of the room air roll required for tangential ventilation. This disadvantage was unknown to the displacement ventilation units introduced in the 1980s, which were designed for installation close to the façade, as they spread the air close to the floor in the room. However, blowing them towards the center of the room means that room users can literally get cold feet at high cooling capacities.

Improved room penetration

The HFVsf devices put an end to the disadvantages of the old induction devices, as they combine the advantages of the principles described above: With mixed-source ventilation, the air is not blown out against the window front in normal operation, but diagonally upwards into the room. In cooling mode, this leads to a strong mixing of the cold supply air with the warm air near the façade, so that the target temperature can be reached even at a low air velocity. Mixed displacement ventilation also counteracts temperature stratification. However, if particularly intensive ventilation, cooling or heating of the room is required – for example during a meeting – the appliances can also blow out air via the front of the appliance. The “Boost mode” works even more intensively, opening an additional row of nozzles and activating all flow paths in the appliance. Because the maximum primary output and also the maximum secondary output of the appliance are called up here, the desired temperature can be reached quickly on Monday morning after a cold weekend, for example. As displacement air flow and mixed displacement ventilation overlap here in layers, good comfort is possible even then.

Promising savings potential

The advantages and possibilities of the new generation of devices have aroused Lorson’s curiosity. “The fact that the primary air flow can be regulated is a significant improvement. With these devices, we can save air, especially when individual offices are empty during business trips and vacations, open-plan offices are not fully used or meeting rooms are not occupied or not fully occupied.” In addition, the new appliances can work with a lower pressure in the ventilation network. The medium-term renovation plans therefore include replacing the old induction appliances with new ones featuring “smart flow” technology. However, before such a step is taken, Bosch wanted to be sure of the savings effect – after all, around 1,500 appliances are to be replaced in the so-called “high-rise” office complex on Schillerhöhe alone. Lorson therefore chose a single office and a meeting room for a test.

The individual office is just under 30 m² in size and has three induction units, each of which can bring in up to 85 m³ of air per hour. This ensures that sufficient supply air enters the room, even for small meetings. Permanent use by several people would also be possible. The meeting room is slightly larger and designed for 14 participants. There is space for six induction appliances. both rooms are not rectangular, but trapezoidal. Daniel Nack, developer in the Comfort Air Technology division at LTG: “Due to the unusual geometry, it was interesting for us to use these rooms for the test.” The measurements in the LTG laboratory were carried out with rectangular floor plans, so the suitability for unusual installation situations could be tested on the other room geometry. The result: even the devices installed at acute angles contribute as desired to an indoor air flow with a mixing zone close to the façade and a low-pulse displacement flow into the depth of the room. No changes had to be made to the arrangement or design of the appliances.

A presence detector is now installed in the individual office. When the office is empty, the air volume provided by the three induction units can be reduced to less than 100 m³/h. This volume is sufficient to dissipate external loads and provide basic ventilation. If the office is in use, the units deliver a good 200 m³/h, which corresponds to the same air exchange rate as with the old units. Due to the optimized distribution of the air volumes (40 % via primary air outlet, 60 % via mixed source air outlet), the maximum air velocity in the occupied zone is
less than 17 cm/s.

A CO2 sensor is installed in the meeting room so that the output of the six induction units can be constantly adjusted to demand. Accordingly, the air volume here varies between around 100 and 500 m³/h. For comparison: the old appliances constantly conveyed 444 m³/h into the room. For the sake of high efficiency, most of the air is brought into the room via the displacement air opening without creating an unpleasant draught. Measurements in the occupied zone showed a maximum air velocity of 16.7 cm/s at head height and a maximum of 21.1 cm/s at floor level. The flow patterns and thermal comfort with the new devices are comparable to the previous state – despite the slightly higher air volumes at full load.

In addition to adjusting the air volume, LTG was able to identify further savings potential in the laboratory: In the system currently used at Bosch, the flow through the heat exchangers of the four-pipe induction units is controlled by pneumatic dampers. However, due to their age, some of these flaps are leaking. Nack on the measurements: “We put an appliance from 1968 on the test bench and found that neither the hot nor the cold water heat exchanger were completely sealed by the air flap. The appliance measured was therefore wasting over 400 watts of thermal power in extreme cases due to simultaneous heating and cooling.” The new generation of appliances works with electrically driven valves on the heat exchanger, ensuring greater reliability. This also offers the advantage that the costly and maintenance-intensive operation of the pneumatic network can be eliminated when switching to the new generation of devices.

Reduce air transportation costs by less than half

The demand-dependent control already offers great savings potential: if the office users are not all present, the air volume can be reduced. Lorson expects the amount of air in the meeting rooms to be reduced by around 30 %. Air consumption in offices would also be reduced, by an estimated fifth. This reduces exhaust air-related heat losses (which also occur with heat recovery) and lowers the power consumption of the central air units. Optionally, the induction units can also be switched off completely. Another advantage of the new “smart flow” devices is that they can be operated with a pressure of just 100 Pa, whereas the old models installed at Bosch decades ago required 260 Pa. The pressure reduction alone halves the air delivery costs. If further optimization measures are included, such as reducing the pressure in the heating and chilled water network, energy savings of up to 40 % can be assumed for air conveying, heating and air conditioning cooling. Lorson is therefore certain: “We will carry out the renovation in the coming years. The plan is to proceed floor by floor from top to bottom. This has the advantage that the pressure in the main line can be reduced floor by floor to the required 100 Pa. “We won’t be able to measure the actual energy savings until a complete floor has been converted,” says the Group Head of Facility Management. Accordingly, the amortization period could also be different from the four years estimated today. “However, we are confident that the new induction appliances will enable us to achieve significant energy savings and at the same time a good return on our investment.”

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