Energy-effi­cient air-condi­tioning tech­nology for a healthy room climate

The climate and the ambient air quality both have a great effect on concen­tra­tion, health and well­being, so it is impor­tant to have good ambient air, for exam­ples in class­rooms. Window venti­lation used to be common prac­tice, but is effec­tively banned today from the perspec­tive of energy effi­ciency, meaning that climate control systems are needed that provide the neces­sary air exchange and optimum air-condi­tioning even with windows that are closed and air-tight. That by no means applies only to new build­ings. Such concepts can also be inte­grated in conver­sions and reno­va­tions. The result is low energy costs and good ambient air quality. The EC fans employed in modern air-condi­tioning systems are high-perfor­mance and at the same time energy-effi­cient. The appli­ca­tion described below provides an excel­lent example.

In the village of Höchst in the Austrian state of Vorarl­berg, the former secondary school was restruc­tured to the “New Middle School” in 2008. Today, some 400 pupils attend a total of 16 classes. Attached to the building is a public library and a multi­media room. Höchst munic­ipal council, which runs the school, recog­nised early that a very large propor­tion of the council’s expen­di­ture was attrib­ut­able to energy costs. Conse­quently, when the school was rebuilt, high priority was attached to reducing costs and to exploiting energy-savings poten­tial.

Rebuilding as a chance to save energy

With a depth of just 360 mm, each compact unit delivers a air flow of 1500 m³/h and works with a highly effi­cient heat recovery system.

The school building was to become a low-energy building after conver­sion and reno­va­tion. A highly ambi­tious project for which the targets were an energy coef­fi­cient Ew of 40 kW/m²a and a stan­dard­ised air quality with a maximum CO2 concen­tra­tion of 1200 ppm. What do these figures mean? That’s easily explained. The energy coef­fi­cient Ew is the most commonly used compar­ison figure for describing the thermal quality of the building shell. It tells us how much energy is needed per year to air-condi­tion one square metre of floor space. For values less than 50 kW/m²a we speak of low-energy build­ings. For values below 15 kW/m²a, we speak of passive build­ings. The CO2 content in the air has a deci­sive affect on personal well­being. At concen­tra­tions below 0.1% (1000 ppm, parts per million) we feel good; at values above 0.2% uncom­fort­able. A maximum concen­tra­tion of 1200 ppm is thus a good and real­istic limit value for the air quality in class­rooms.

Achieving the targets with respect to energy coef­fi­cient and at the same time main­taining good ambient air quality is only possible with an airtight building and a controlled venti­lation system. Together with the company Bösch, a venti­lation concept comprising centralised and semi-centralised air-condi­tioning units was devel­oped. This was specially tailored to the building’s condi­tions.

Venti­lation concept adapted to the archi­tec­tural features

The flat, ceiling-mounted air-condi­tioning units employ forward curved centrifugal fans with Green­Tech EC tech­nology.

For the air-condi­tioning and fresh air supply to the class­rooms, a total of seven venti­lation and air extrac­tion units were installed in the ceil­ings of the class ante­room on each floor (see Fig. 2). These compact units, which are just 360 mm flat, supply a volume flow of up to 1500 m³/h and work with a highly effi­cient heat recovery system (heat recovery coef­fi­cient > 70%) to guar­antee good reuse of the heat in the exhaust air. At very low outside temper­a­tures, the fresh air is heated up as neces­sary by a PTC elec­tric air heater. Class F7 fine dust filters pursuant to EN 779 with a protec­tion rating of more than 60% provide for a high level of air clean­li­ness in the class­rooms and guar­antee a healthy room climate even when the quality of the air outside is poor.

Because of its different opening hours, and because of the air volume it needs on account of its size, the library is equipped with a sepa­rate, centralised venti­lation and air extrac­tion unit that delivers up to 4000 m³/h. The media room and the other class­rooms in the new exten­sion are also supplied by their own venti­lation and air extrac­tion unit. This is config­ured for a maximum air perfor­mance of 7,000 m³/h. All venti­lation and air-condi­tioning units are inte­grated into a single intel­li­gent central building control system. The air-condi­tioning is controlled according to require­ments in accor­dance with the current CO2 concen­tra­tion. The conver­sion has proven worth­while for the village of Höchst. How much energy the system consumes was closely moni­tored for two full years by the inde­pen­dent “Energie­in­stitut Vorarl­berg” (Vorarl­berg Energy Insti­tute). The low energy consump­tion figures required were confirmed.

Energy-effi­cient centrifugal fans in Green­Tech EC tech­nology

Back­ward curved fans with Green­Tech EC tech­nology provide for the neces­sary air flow in the centralised air-condi­tioning units.

A deci­sive contri­bu­tion to this success can be attrib­uted to the design of the air-condi­tioning units. To generate the neces­sary venti­lation and air extrac­tion flow, two centrifugal fans are used in each of the units. These are highly energy-effi­cient and work extremely econom­i­cally. They are taken from the product range of motor and fan specialist ebm-papst Mulfingen and are specially designed for use in venti­lation and air-condi­tioning tech­nology. Different versions ensure that the right fan solu­tion can be found for every air-condi­tioning unit. For example, Bösch employs forward curved centrifugal fans with Green­Tech EC tech­nology and a diam­eter of 225 mm from ebm-papst in its ceiling-mounted flat air-condi­tioning units (see Fig. 3). The char­ac­ter­istic features of forward curved fan blades are an espe­cially low noise level and rela­tively high air flow with a low static pres­sure increase, plus they require rela­tively little instal­la­tion space. Back­ward curved design versions provide for the neces­sary air flow in the centralised air-condi­tioning units (see Fig. 4). Centrifugal fans with back­ward curved blades are used primarily for intake suction and do not require a scroll housing. They have a high level of effi­ciency. In both cases, the user bene­fits from the numerous advan­tages offered by ebm-papst’s Green­Tech EC tech­nology.

Quiet oper­a­tion according to require­ments

Because the EC motors work so effi­ciently, they consume substan­tially less energy that conven­tional AC drives.

The EC motors that power the fans feature an inte­grated elec­tronic control system that allows the speed of the fan to be adapted contin­u­ously to require­ments. Demand-orien­tated oper­a­tion can be controlled either with an analogue 0-10 V signal or via a digital RS485 inter­face. Because the motors also work with great effi­ciency, they consume substan­tially less energy that conven­tional AC drives (see Fig. 5). However, these poten­tial energy savings are realised not only when oper­ated under full load, but also primarily when oper­ated under partial load. When oper­ating under partial load, EC motors lose far less effi­ciency than asyn­chro­nous motors of the same output. Also very impor­tant with respect to venti­lation systems in schools is the noise level, espe­cially if the units are installed directly in front of the class ante­room.

Noise emis­sions of AC and EC motors in compar­ison.

Here too, EC drives are the better choice, because the motors produce prac­ti­cally no noise (see Fig. 6). In contrast, asyn­chro­nous motors driven by a frequency inverter, partic­u­larly under partial load, produce reso­nance noise that results in the typical unpleasant motor hum. This would certainly not provide a pleasant envi­ron­ment for the teachers and students. The students at Höchst Middle School have nothing to fear. The air-condi­tioning units are equipped with fans powered by pleas­antly quiet Green­Tech EC tech­nology. These are prac­ti­cally inaudible in the class­room. The 28 dBA noise level offi­cially spec­i­fied in the class­room is observed.

Compact design, easy instal­la­tion and long service life

Another advan­tage of these fans is their compact design. The elec­tron­i­cally commu­tated external rotor motor is directly inte­grated into the impeller, which reduces the instal­la­tion dimen­sions. A belt drive between the motor and the fan, which is commonly used other­wise, is not neces­sary. This reduces not only the required instal­la­tion volume – always desir­able for the air-condi­tioning units made by Bösch – but also the asso­ci­ated instal­la­tion complexity. At the same time, fewer parts are required which are subject to wear. This keeps service costs low over the long term and thus saves money for the oper­ator.