More efficient air conditioning without loss in comfort
Tobias Lorson's goal is lower energy input without a reduction in comfort level. The team leader in Facility Management at Robert Bosch GmbH sees a promising path to more energy efficiency in the refurbishment of the old induction system. Operational tests with the manufacturer LTG of Stuttgart, Germany in two test rooms have shown the savings potential: New induction units can lower the energy usage for heating, cooling and ventilation by up to 40%.
Robert Bosch GmbH wants to reduce CO2 emissions at least by 1/5 by the year 2020. More efficient building technology is expected to make an essential contribution to this goal. Therefore, Tobias Lorson and his team examined various approaches. It showed a large savings potential is offered by the air conditioning system. However, the requirement is that energy savings should not be at the expense of the comfort of the user. The office complex was built in the light-weight construction style, typical for the late 1960s. Double-pane windows were installed in the 1980s to reduce heat loss. For shading automatically controlled outdoor roller shutters, plus manually operated vertical blinds on the inside are used. The central air handlers and the induction units in the room provide well-tempered supply air with good oxygen content.
Comfortable air conditioning
The air changes of the existing system exceeds the minimum requirements, which helps in keeping the CO2-level low in the office area at all times. This way, the employees in single and open plan offices will keep a “clear head”. Comfort is increased by quiet rooms, low air velocities, fresh air, and a pleasant temperature. The requirements for air quality and acoustical comfort can be met by induction units and it can be done very energy efficient and almost maintenance-free.
The desired temperature can be controlled individually for each office. In the open-plan offices, it is controlled by zones. Each zone is approx. 250 sqft. Ventilation and conditioning of office and meeting rooms is done with induction units, some still from the 1960s. Others have been replaced in the meantime during partial renovations.
The room climate at Bosch is pleasant, and employee surveys confirm: the comfortable air conditioning system is appreciated by the employees. Therefore, it is understandable that the rate of dissatisfied users is lower than in many comparable objects – and this is how it should remain. "Savings potentials can still be found without reducing comfort," Lorson is certain. "A lot has changed in technology since the building was built."
Adjusting air flow to demand
Induction technology has made gigantic improvements over the last couple of years. Besides units with a fixed air volume, like Bosch has installed, LTG AG from Stuttgart, Germany now also offers units with variable air volume. They are called HFVsf were sf stands for smart flow. With this new generation of units the airflow can be modified in steps or continuously between 20 and 100% and shut-off completely. To make sure the air always leaves the unit with the same velocity, the units are equipped with an adjustable nozzle system. Two-stage controls are advantageous for individual offices, with the user leaving the office the unit switches into base load operation. Continuous controls is predestined for open plan offices or especially meeting rooms, since the required supply air is mainly depending on the amount of people present.
The configuration of the HFV sf units provide savings opportunities not possible with older induction units, since the latter push the same amount of air through the nozzles all the time. They get by with a small amount of primary air, due to the strong directional air jet, but need a high primary pressure to create this form of flow.
The HFVsf-units open up savings options that are impossible to reach with older induction units. Older style induction units blow the same amount of air through nozzles all the time, as long as the primary air system is operating. Due to the strongly directed air jet, they get by with a small amount of primary air, but require high primary pressure to create this type of flow. For heating and cooling, they pull in three to six times the amount of air primary air, as secondary air from the room via the induction principle – similar to a Venturi pump. The secondary air flows through a heat exchanger before being mixing with the primary air flow. One downside of induction units is that they always have to be operated with the maximum primary air demand. Also obstructions in the room such as ceiling installations and girders influence the air flow, since they obstruct the formation of the room air roll needed for tangential ventilation. Displacement induction units introduced in the 1980s did not have this issue. They were intended for installation near the façade and they spread the air in the room close to the floor. Moving slowly towards the room centre without mixing, it leads to room users literally getting cold feet at high cooling outputs.
Improved room penetration
The HFVsf-units do not have the disadvantages of the old induction units. They combine the advantages of the above principles: In mixed displacement ventilation, the air is not blown against the window during regular operation, but diagonally up into the room. In cooling operation, this leads to a strong mixture of the cold supply air with the hot air close to the façade, so that the target temperature can be reached even at lower air velocities. The mixed displacement ventilation also works against temperature layer formation. However, if particularly intense ventilation, cooling or heating of the room is required – for example for a meeting room – the units can supply additional air via the front diffuser. The "boost mode" works even more intensely. Here, an additional nozzle row opens and all flow paths in the units are activated. Because the maximum primary air output and the maximum secondary air output in the device are used, it is possible to produce the desired temperature quickly e.g. on Monday morning after a cold weekend. Since a displacement air flow and a mixed displacement air flow overlap in layers, good comfort is possible even with the high demand.
Promising savings potential
The advantages and possibilities of the new generation unit have made Lorson curious. "The option of controlling the primary air flow is a clear improvement. With these units, we can save air especially when single offices are empty during business trips and during holidays, when open-plan office are not used entirely or when meeting rooms are not or not fully occupied. “Additionally, the new units can work at a lower pressure in the ventilation system. The renovation plans for the medium term therefore include exchanging the old induction units with new ones using the "smart flow" technology. However, before this kind of step was taken, Bosch wanted to make sure that the savings are real. After all this office complex alone has about 1,500 devices to exchange. Therefore, Lorson chose a single office and a meeting room for a test.
The single office is about 320 sqft and has three induction units that can introduce up to 50 cfm air. This ensures that there will be sufficient supply air to the room even in smaller meeting settings. Permanent use by several persons would be possible. The meeting room is a little larger and designed for 14 people. Six induction units fit in here. Both rooms are trapezoid rather than rectangular.
Daniel Nack, product manager at LTG: "Due to the unusual room geometry, it was interesting for us to use the rooms for the test." The measurements in the LTG lab took place with rectangular footprints; therefore, the different room geometry could be used to test suitability for unusual installation situations. The result: Even the devices installed at an acute angle contribute to a room air flow with mixed zone close to the façade and low-impulse displacement flow into the room depth. Nothing needed to be changed at the unit placement or concept.
Only one occupancy sensor is installed in the single office. If the office is empty, the air volume provided by the three induction units can be reduced to below 60 cfm. This is sufficient to discharge external loads and to perform basic ventilation. If the office is in use, the units provide roughly 125 cfm, which corresponds to the same air exchange rate that the old units had. Due to the optimized distribution of the air (40 % via primary air outlet, 60 % via mixed displacement air outlet), the maximum air speed in the occupied area is less than 33 fpm.
There is a CO2-sensor installed in the meeting room, so that the output of the six induction units can be continually adjusted to the demand. Accordingly, the air volume here varies between roughly 60 and 300 cfm. For comparison: The old units consistently supplied 260 cfm into the room. For higher efficiency, the largest part of the air is blown into the room via the displacement air outlet, without causing any unpleasant draft.
Measurements showed a maximum air speed of 32.5 fpm at head level and no more than 41.5 fpm on the floor in the occupied area. The airflow pattern and the thermal comfort with the new units are comparable to the situation before– in spite of the slightly higher air volumes under full load.
In addition to the air volume adjustment, LTG found further savings potential in the lab: The system used at Bosch today controls the flow through the heat exchangers of the four-pipe induction units with pneumatic dampers. Due to old age, however, some of these dampers leak. Nack on the measurements: "We put a device from 1968 on the test bench and found that neither the hot, nor the cold water heat exchanger are sealed perfectly tight by the airflow valve. The measured device therefore can waste more than 1300 BTU/h due to concurrent heating and cooling in extreme cases." The new device generation works with electrically powered valves at the heat exchanger, increasing reliability. Additionally, this offers the advantage that the cost- and maintenance-intensive operation of the pneumatic system may be dispensed with when switching to the new device generation.
Reducing air moving costs to less than half
The demand-dependent control leads to a high savings potential: when not all office users are present, the air volume can be reduced. Lorson assumes an air volume in the meeting rooms reduced by 30 %. The air demand in the offices would reduce. The estimated amount is one fifth. This reduces the exhaust-related heat loss (which occurs even at heat recovery) and reduces the power consumption of the central air devices. Optionally, the induction units can be deactivated entirely as well.
Another advantage of the new "smart flow" units is that they can be operated at a primary air pressure of only 0.4”wg, while the old models installed by Bosch decades ago require 1.2 “wg. The reduction in pressure alone cuts the cost of moving the air on half.
If you take further optimization measures, such as the reduction of pressure in the warm and cold water piping, into account energy savings may amount to as much as 40 % combined. Therefore, Lorson is certain: "We will perform the renovation in the next few years". It is planned to proceed floor by floor from the top down. This has the advantage that the pressure in the main duct can be reduced to the 0.4 “wg that are then needed floor by floor. "Only when the complete level has been retrofitted will the specific energy savings be measurable," says the group manager in Facility Management. Accordingly, the amortisation period may be very different from the currently estimated four years. "However, we are confident that the new induction units will enable us to achieve essential energy savings and at the same time a good return for our investment."