New technical solutions for energy efficient buildings

State of the Art Report

Innovative cooling concepts for office buildings

Heimo Staller, Angelika Tisch, IFZ

February 2011

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Background In Europe (and worldwide) the energy consumption for the cooling of buildings is in-creasing dramatically. Studies like “Energy Efficiency and Certification of Central Air Conditioners” (EECCAC)1 and “Energy Efficiency of Room Air Conditioners” (EERAC)2 predict a four-fold growth of energy demand for cooling between 1990 and 2020 in the EU 15 [1].

Fig. 1 Estimated increase of cooling energy in the EU-15 from 1990 to 2020 [1]

The reasons for this development are manifold. As main causes can be identified:

Inefficient building design (e.g. orientation and size of transparent building ele-ments)

Increase of internal loads (IT-equipment, lighting)

Climate change aspects (increase of outdoor temperatures in the summer period)

Tightened requirements on indoor comfort (e.g. caused by standardisation and building directives, comfort claims by occupants)

Misconduct of the occupants (misappropriate ventilation by occupants in the cooling period)

1 Study for the Directorate General Transportation-Energy, 2003 2 Study for the Directorate General for Energy, 1999

Annual cooling energy demand in the EU-15

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Inefficiency of conventional cooling systems (e.g. reduction of indoor temperature with conventional cooling techniques from 21° to 20° consumes approximately three times the energy than heating from 20° to 21°)

As most of the cooling systems for buildings are still vapour compression machines based on electrical energy, an energy source with enormous environmental impacts (high CO2-equivalents and primary energy factors), alternative cooling strategies are of the utmost importance.

In the European building stock the highest ratio of cooling energy consumption can be found in office buildings, therefore this report focuses on this building type, whereas it can be stated that almost all strategies mentioned here are feasible for other building types. Having highest potentials for energy savings another focus is on strategies for refurbishment project.

In addition to the presentation of strategies for the reduction of cooling energy demand, strategies for the assessment of cooling energy in different planning stages (from pre-liminary design to detailed planning) are dealt with, too.

General Strategies As the cooling demand is always a result of the climatic conditions on the building site, cooling strategies have to be adapted to regional climate characteristics. Nevertheless measures and strategies for the reduction of cooling energy mentioned in this document are unique principles to be applied to almost all European climate zones.

In general there are two strategies to reduce the cooling demand in buildings:

Passive cooling strategies (on which will be the main focus of this report)

Active cooling strategies (like solar cooling)

Passive cooling strategies The first step towards the reduction of energy consumption has to be done on the de-mand side. A comprehensive reduction of the cooling load can be realized by following measures:

Building design

Reduction of solar gains – Size and orientation of transparent building elements (ap-plicable mainly for new buildings)

Orientation and size of transparent building elements (windows) have an important influence on the cooling demand. North orientation of offices will generate best results for the cooling energy demand, but worst results for the heating energy demand, so in the Middle European climate pure south orientation is the best orientation for the re-duction of the heating and the cooling energy demand, whereby east and west orienta-tion lead to worst results for the cooling energy demand.

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Reduction of solar gains - Innovative sunblind (applicable for new buildings and re-furbishment projects)

Innovative, intelligent shading elements which are different according to the orientation (e.g. south windows with horizontal elements, west and east windows with vertical ele-ments) are further measures for the reduction of solar gains. Solutions for sunblind al-ways have to be seen with the daylight concept of the building, otherwise an increase of internal loads caused by artificial lighting demand can be counterproductively.

Fig. 2 Innovative sunblind, enabling daylight control and protection against solar radiation. Right picture: RETROlux sunblind with innovative shading and light control concept, RETROSolar Company [2]

=> Potential for energy saving

Depending on the climatic zone and kind of sunblind, reductions for cooling energy de-mand from 10 – 70% are possible. [3]

=> Recommendation for implementation/assessment in the tendering process

Concepts for size and orientation of transparent building elements in combination with intelligent daylight concepts have to be provided by all participants of architectural competitions. Requirements concerning the amount of window area per orientation (e.g. max. 30% of transparent area in the west façade) might be given. Assessment of cooling demand can be done by a simplified method (Ratio of window area to total façade area, or to conditioned gross floor area) or by the use of simplified software tools (with input of geometrical building data by participants).

Mass storage capacity – New buildings

An increase of the thermal mass storage capacity contributes to the reduction of the cooling energy demand. Office buildings with heavy building elements (walls, ceilings) have a better cooling energy performance than light weight buildings. To activate the

Daylight control

Sunblind

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mass storage capacity of building elements, following aspects have to be integrated in the design concept of buildings:

o Avoidance of suspended ceilings (hindering the activation of the thermal storage capacity of massive ceiling components behind).

o Avoidance of double floors (hindering the activation of the thermal stor-age capacity of massive ceiling components behind).

o Minimum thickness of building elements responsible for the primary stor-age capacity (storage through elements being in direct contact with sunlight) should be 10 – 20 cm. The surface area of these building ele-ments should be three times the area of the windows area of the room.

o Minimum thickness of building elements responsible for the secondary storage capacity (storage through elements being not in direct contact with sunlight, contact only by indoor air) should be 7 – 15 cm. Surface area of these building elements should be an eight to ten-fold of the win-dow area of the room.

o Large surface areas of building elements responsible for thermal mass storage capacity should be considered.

=> Potential for energy saving

Depending on the climatic zone, reductions for cooling energy demand from 5 – 10% are possible. [3]

=> Recommendation for implementation/assessment in the tendering process

To generate benefits for the cooling demand, office buildings with high thermal capacity storage should be asked for in the planning stage. In most countries classification (stan-dards, regulation) for buildings regarding their mass storage capacity (e.g. from light weight building to very heavy building) are existing, so in architectural competitions requirements concerning their storage capacity can be stated. Also intelligent concepts for construction and HVAC systems enabling the benefits of mass storage capacity have to be taken into consideration (no suspended walls and ceiling for HVAC installations).

Mass storage capacity – Refurbishment projects

A high ratio of office buildings built within the period of 1950-1980 are light weight con-structions with an inadequate mass storage capacity. In almost all refurbishment pro-jects an increase of mass storage by new heavy building elements like walls and ceilings is not possible. To close this gap, new innovative interior plasters and panels for walls, ceilings and float screeds with high mass storage capacity have been developed. This plasters and panels are based on PCM (Phase Change Materials) working with latent storage principles. Most of the PCM`s are micro encapsulated paraffin. For example 3°cm of PCM interior plaster on paraffin base is equal to 18 cm of concrete or 25 cm of clay brick. Demonstration projects in Germany have shown that it is possible to turn

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light weight office buildings to middle weight or heavy weight buildings (e.g. concrete building) reducing the cooling energy demand dramatically. It is recommended to use natural night ventilation as it enforces the impact of PCM`s. [4]

Of course PCM`s are also applicable for the reduction of cooling energy demand of light weight new buildings.

Fig. 3 Comparison of the heat storage capacity of PCM (product: Micronal BASF) and conventional building materials [5]

Natural cooling with passive night ventilation (Applicable for new buildings and partly for refurbishment projects)

In office buildings up to 2/3 of the total cooling load (around 200 – 250 Wh/m2/day) can be managed by passive cooling without mechanical energy. A Night temperature in summer around 15°C, an adequate thermal building mass and the possibility of natural stack ventilation (e.g. in atriums, see figure below) are requirements for this kind of cooling.

Fig. 4 Passive cooling (without mechanical energy) through stack ventilation in the night, office building W.E.I.Z. II, Weiz, Austria, architects: A plus ZT GmbH Weiz [6]

3 cm of interior PCM plaster

is equal to

18 cm concrete

25 cm clay bricks

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To enable the effective reduction of cooling demand following aspects have to be con-sidered:

Total daily cooling demand has to be under 150 Wh/(m2d)

The building should be a heavy building

In the night at least in a period of 5 hours the outdoor temperature should be under 21°C

The Minimum air change rate should be 2 preferably 4 h-1 (two to four-fold change of the total air volume of the building within 1 hour)

Minimum ratio of suspended ceilings and walls

=> Potential for energy saving

Depending on the climatic zone, reductions for cooling energy demand from 10 - 40% are possible. [3]

=> Recommendation for implementation/assessment in the tendering process

In the tendering documents for architectural competitions the possibility of passive night ventilation should be a specification. Participants have to provide schematic sketches and plans, showing the night ventilation concept of their projects. Assessment can be done only on a qualitative level, as quantitative assessment requires very com-plex and costly simulation tools. In further planning stages complex simulation tools should be used. Problems emerge with performance guarantees, as passive cooling con-cepts are very fragile, depending on a lot of unforeseeable aspects like climatic condi-tions and user behaviour.

Energy efficient equipment (applicable for new buildings and refurbishment projects)

The use of energy efficient lighting and equipment with low rejected heat (like LED lamps, computers, etc.) on one hand contributes to a reduction of electrical energy and on the other hand reduces the internal cooling load of office buildings. Technical equipments in combination with exhaust-air plants are further solution for the decrease of the cooling load.

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Fig. 5 Lamp with exhaust-air, Radolux Gesellschaft für Lichttechnik mbH, Germany [7]

=> Potential for energy saving

Depending on the climatic zone, reductions for cooling energy demand up to 30% are possible. [3]

=> Recommendation for implementation/assessment in the tendering process

Calculations and target values concerning the internal cooling load should be based on energy efficient equipment. Therefore tendering documents for electrical lighting and equipment have to include benchmark (maximum) values for the power input (if possi-ble also specifications concerning rejected heat).

Passive cooling strategies using environmental energy Cooling systems mentioned below are in a way passive systems as there is no produc-tion of cooling energy, whereas energy is only required for the distribution in the build-ing. The following text is et al. based on the results of an Austrian research project called: “Passive cooling concepts for office and administrative buildings using earth-to-air and earth-to-fluid heat exchangers”. [8]

TABS (Thermo-Active-Building-Systems)

TABS cool buildings are using tube heat exchangers integrated in the building structure (ceilings and walls). Most common are water-carrying systems for concrete core activa-tion. The energy source can be environmental energy from the ground, ground water and outdoor air. Depending on the cooling load of the building the only energy needed for cooling is energy for the distribution of the environmental energy (electrical energy for pumps). This way of cooling, called “Free cooling”, is one of the most efficient ways of passive cooling with lowest primary energy demand and lowest cooling costs; further-more the same system can be used for heating. With this system a maximum cooling demand around 480 Wh/m2d can be managed.

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Fig. 6 Thermo-active building systems (TABS): capillary tube systems, concrete core temperature control, underfloor temperature control, and double surface building element temperature control. From the mul-titude of various TABS. Fraunhofer ISE, Freiburg [9]

Fig. 7 After the upper reinforcement has been laid over the modules, the tube mats are fixed in place. Here, the spacers between formwork, lower and upper reinforcement and the wire mesh for alignment of the tubes can be seen. Fraunhofer ISE und Solares Bauen GmbH [9]

Earth to Air Heat Exchangers (EAHE)

For office buildings with mechanical ventilation systems cooling concepts based on air should be considered. These systems use ground heat exchangers for generation of cool

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air, which is distributed in air ducts to the conditioned areas. It is inadvisable to use EA-HEs when the stream of air is in the range of the hygienic air change rate, as the energy demand and the costs are increasing. With these systems a maximum cooling demand of around 300 Wh/m2d can be managed.

Fig. 8 Earth to Air Heat Exchanger (EAHE) [8]

=> Potential for energy saving

Highest potential for savings of primary energy and costs for construction and operation can be realized by TABS (Thermo-Active-Building-Systems) with ground heat exchang-ers with water carrying systems in combination with heavy building elements (concrete ceilings). These systems have limitations concerning their daily cooling demand. For buildings with cooling demands above 480 Wh/m2d (respectively cooling loads over 40 W/m2) other systems have to be chosen.

=> Recommendation for implementation/assessment in the tendering process

The decision for these cooling systems has to be taken into account in early planning stages, as their technical system has strong influences on the design of the building (e.g. air ducts for contribution or uncovered ceilings). In contracts with tenants it should be stated, that only cooling loads up to 40 W/m2 can be provided and that there are no guarantees for standardised comfort conditions over this value (which means for ten-ants to meet planning and standardisation targets concerning internal loads caused by equipment and number of persons/m2 and by standardised user behaviour).

Filter

Heat exchanger

exhaust air/fresh air ventilator

damper

ground heat exchanger

ambient air

exhaust air

Fresh air

exhaust air

exhaust air

ventilation appliance

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Active cooling strategies with solar energy In the last decades promising cooling concepts with solar energy are emerging. One of the main advantages of solar cooling is the fact that the cooling demand and the highest solar gains exist at the same time (summer period). Especially in Mediterranean regions with high solar gains and high cooling demands solar cooling will become more and more an alternative to conventional cooling systems.

Fig. 9 Supply of solar energy and demand over the year in Central Europe, showing the benefits for cool-ing through a better time-match between supply and demand, IEA – International Energy Agency [10]

As there is a separate State of the Art Report (Solar heating and cooling) dealing with this cooling concept, only a short description is given below. In general there are two concepts:

Cooling with PV collectors

A conventional vapour compression machine is operated with electricity provided by PV collectors.

Cooling with thermal collectors

There are three thermal driven systems:

o Absorption cooling with chilled water

o Adsorption cooling with chilled water

o Desiccant Cooling for air based cooling systems

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Fig. 10 Office building CGD in Lisbon, collector area: 1.579 m², capacity of the chiller: 545 kW, company: S.O.L.I.D, Graz, Austria [11]

=> Potential for energy saving/environmental benefits

Solar energy is the most environmental friendly energy source. For example the primary energy consumption of cooling systems with PV collectors is around 1/3 of a conven-tional system vapour compression machines based on electrical energy and the CO2-equivalents are nearly zero.

=> Recommendation for implementation/assessment in the tendering process

The decision for solar cooling systems has to be taken into account in early design stages, as they have tremendous influences on the building design. Huge amounts of col-lector areas have to be integrated into the building design. For architectural competi-tions requirements concerning these solar collectors (e.g. area, orientation, incline or required gains from solar collectors) have to be stated for all participants. Assessments can be verified by the amount of collector areas or by simplified software tools. For ten-ders of HVAC-systems, the performance guarantees for collectors and the whole cooling system should be integrated into the tendering documents.

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References [1] Adnot, J. et al. (2003). Energy Efficiency and Certification of Central Air Conditioners (EECCAC). Study for the D.G. Transportation-Energy (DGTREN) of the Commission of the E.U., Final report

[2] Right picture: RETROlux, RETROSolar Company

[3] Kleindienst, Land Steiermark - Fachabteilung 17 A Energiewirtschaft u. allgemeine technische Angele-genheiten (2008). Gesamtheitliche Planung von Gebäuden – eine Existenzfrage (?)

[4] Kerschberger, Sick (2007). Innovative Sanierung spart Energie. Deutsches Architektenblatt 05/07

[5] Micronal PCM, BASF Company (2011). http://www.micronal.de/portal/basf/ien/dt.jsp?setCursor=1_290798

[6] A plus ZT GmbH, Weiz, architect Heimo Staller

[7] Lamp with exhaust-air, Radolux Gesellschaft für Lichttechnik mbH (2011). http://www.radolux.de/

[8] Fink, Blümel, Kouber, Heimrath (2002). Passive cooling concepts for office and administrative build-ings using earth-to-air and earth-to-fluid heat exchangers . Final report within the research programme Haus der Zukunft on behalf of the Bundesministeriums für Verkehr, Innovation und Technologie, Austria

[9] Pfafferott, Kalz (2007). Bine Themeninfo 1/2007 – Thermo-active building systems. FIZ Karlsruhe GesmbH, Germany

[10] IEA – International Energy Agency (2011). Renewable Energy Essentials: Solar Heating and Cooling. http://www.iea.org/publications/free_new_Desc.asp?PUBS_ID=2192

[11] S.O.L.I.D, Graz, Austria (2011). http://www.solid.at/