Energy Efficient Office Buildings with Passive Cooling Results from a Research and Demonstration Programme

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1 Energy Efficient Office Buildings with Passive Cooling Results from a Research and Demonstration Programme K. Voss 1, S. Herkel 2, G. Löhnert 3, J. Pfafferott 2, A. Wagner 4 1 University Wuppertal, Pauluskirchstrasse 7, Wuppertal, Germany 2 Fraunhofer ISE, Heidenhofstrasse. 2, 7911 Freiburg, Germany 3 solidar planungswerkstatt, Forststrasse 3, Berlin, Germany 4 University Karlsruhe, Englerstrasse 7, Karlsruhe, Germany ABSTRACT To gain access to information on energy use in office buildings, the German Federal Ministry for Economy launched an intensive research and demonstration programme in In advance of the EU energy performance directive a limited primary energy coefficient of about 1 kwh m -2 a -1 as a goal for the complete building services technology was postulated (HVAC + lighting) for all demonstration buildings. A further condition was that active cooling be avoided. Techniques such as natural or mechanical night ventilation or heat removal by slab cooling with vertical ground pipes as well as earth-to-air heat exchangers in the ventilation system were applied. An accompanying research was established to keep track of the results and the lessons learned from about 22 demonstration buildings realized and monitored until the end of 25. As one outcome this paper summarizes the energy performance of a selection of characteristic buildings together with examples of the summer thermal comfort situations achieved. The research program will proceed during the next five years. Past and future results may be downloaded at: KEYWORDS Office buildings, energy monitoring, passive cooling, thermal comfort, user behaviour INTRODUCTION Energy Use and Thermal Comfort in Office Buildings The left diagram of Fig. 1 gives an impression of a typical energy consumption profile of a fully air conditioned building as a function of the outdoor temperature: In addition to a base energy load, there is a contribution for heating and humidifying below the balance temperature, and for cooling and dehumidifying above it. The balance temperature is defined by that outdoor temperature at which thermal losses are balanced by the internal and solar gains. Due to the decoupling of the room air from the building mass - suspended ceilings, double floors, lightweight walls - and the maintenance of constant indoor conditions throughout the whole year, there are hardly any days when there is neither active heating nor cooling. Today an increasing fraction of office buildings are being constructed or retrofitted which replace almost complete isolation from the weather outdoors by a moderate interaction and individual control of the indoor climate to a large extent. Day lit workplaces and the option for natural ventilation are typical characteristics. However, a combination of integrated measures to achieve the so-called "passive cooling" is a pre-requisite if summer comfort is to be ensured without active cooling, Fig. 1 right diagram (Santamouris, 1997), (Zimmermann, 23). The task is to design buildings such that,

2 even when the weather outdoors varies greatly, the indoor conditions remain within a well-defined comfort zone, which meets the expectations of the occupants. These expectations can be defined according to pren15251 by three comforts classes using an adaptive approach (pren15251, 25). In free running environments the indoor air conditions will vary more than in an air-conditioned building. Research on thermal comfort has shown that this does not have to affect the perceived comfort negatively, as long as a high potential for individual user control of the indoor climate is allowed (Humphreys, 26). Some countries have started taking this into account in there new building codes (USA, UK, The Netherlands). Germany is proceeding in this direction but no code for free running buildings has been established yet total end energy use, qualitative [%] space heating, humidification balance temperature space cooling, dehumidificatio total end energy use, qualitativ [%] balance temperature space heating free floating comfort zone climate independend base consumption weekly average outdoor temperature [ C] weekly average outdoor temperature [ C] Fig. 1: Qualitative profile of the energy consumption of a conventional building (left) compared to a "lean building" (right) in Mid European Climate. RESULTS AND EXPERIENCES Energy Monitoring Table 1 gives an overview of the projects monitored, the passive cooling concepts applied as well as the buildings energy supply systems: 14 office buildings, 4 educational buildings and 4 production buildings. Detailed information together with a comprehensive overview on results and experiences are presented in (Voss, 25). Figure 2 summarises the monitoring results from buildings for which data from at least one year are available. The separation of the electricity use to the type of energy service in particular, requires a very detailed metering concept. In many cases, detailed analysis of the electricity consumption helped to identify weaknesses in the system operation and aid their correction: operation of the heating system pumps outside the heating season, heating of pre-cooled air by an earth-to-air heat exchanger during summer, etc. In large buildings operational faults cause energy consumptions and energy costs in an order of magnitude which is not negligible. From the experiences it can be assumed that these kinds of faults are common practise in the operation of the building stock as a whole. Fig. 2 presents the monitored data on the level of purchased energy, as well as primary energy consumption, taking into account the conversion factors for the specific conditions of Germany as given within the national standard (DIN 471-1, 23). Thirteen of the buildings presented end with a primary energy consumption below or close to the agreed limit of 1 kwh m -2 a -1, eight lie above the limit. It is satisfying to see that the consumption for all of the office buildings is much lower than the comparative values for the building stock (e.g. Weber, 22).

3 Tab. 1: List of the demonstration projects monitored Building Monitoring team net floor area, m² main heat source * eta hx: earth-to-air heat exchanger passive cooling concept eta hx* night ventilation ECOTEC University Bremen 2,941 district heat active slab cooling Wagner University Marburg 1,948 CHP, natural gas passive FhG ISE Biberach, Fraunhofer ISE 13,15 CHP, natural gas hybrid DB Netz Karlsruhe 5,974 natural gas hybrid GIT University Siegen 3,243 natural gas active Office Buildings Lamparter Stuttgart Pollmeier ZUB 3,51 KfW Energieforum Karlsruhe Braunschweig 1, natural gas passive 8,585 wood offcuts wood pellets active passive 2,693 district heat passive Energon Ulm 6,911 district heat TMZ Erfurt 8,976 district heat BOB Cologne 2,72 electricity, heat pump SIC UBA Offenburg Cottbus 13,833 district heat active 32,384 district heat passive Institutional Buildings FH BRS University Dortmund 26,987 natural gas passive NIZ Braunschweig 8,57 district heat passive ZUB University Kassel 1,732 district heat hybrid GMS Biberach 1,65 electricity, heat pump Production Buildings Huebner University Hannover 2,122 district heat passive Surtec Solvis Lebenshilfe University Darmstadt, Passive House Institute Braunschweig Munich 4,423 natural gas 8,215 4,623 CHP, rape oil wood pellets active passive

4 Basis of low energy use is a low heating demand. A measured demand of about 4 kwh m - ² on average well corresponds to the aim defined for the program. Six of the buildings use a high insulation level (U<.2 W m -2 K -1 ), triple glazing and a efficient ventilation heat recovery. They operate with a heat demand of about 2 kwh m -2 a. Despite an average heat demand, some buildings profit from burning biomass as heat source to cut down their primary energy use (Pollmeier: wood offcuts, KfW: wood pellets, Energon: district heating, 4% wood chips fraction). Biomass as largely CO 2 -neutral source is rated with a factor of.2. Drawing heat from a district heating network operating with combined heat and power plant also proved to be favourable in this aspect with a factor of.7 (ECOTEC, ZUB, Energieforum, Energon, TMZ, SIC, ZUB, NIZ). Two buildings use electricity as only energy source (factor 3.). As passive cooling is provided by slab cooling, in direct connection to ground pillows (BOB) or ground water (GMS), the summer heat sink operates as a heat source of heat pumps during the heating season. Due to low heat demands the installed electric power can be kept low (6 to 7 W per m²). Besides energy saving measures and thermal use of renewable energy, some of the buildings apply measures such as combined heat and power plants locally (Wagner, ISE: natural gas motor, Solvis: raped oil diesel engine) or photovoltaics (13 projects) to generate electricity. This energy subsidises grid electricity which, in the other case of no local generation, has to be generated on national average conditions with a mixture of power stations. In the case of a so called zero energy building, the primary energy credits for the subsidised grid electricity fully balance the buildings primary energy consumption on a yearly cycle. Five projects balance more than 1% of the energy use through energy credits, two enter the range of a zero energy building (Solvis, Lamparter). other energy sources electricity end energy use [kwh/m_a] other sources electricity credits CHP credits PV primary energy balance [kwh/m_a] office buildings EcoTec 99 Wagner 1 ISE-Büro 3 DB Netz 1 GIT 5 Lamparter 3 Pollmeier 3 KfW 5 Energieforum 5 educational buidlings production buildings Energon 5 TMZ 4 BOB 5 SIC 5 FH BRS 1 NIZ 4 ZUB 3 GMS 5 Hübner 1 SurTec 2 Solvis 5 Lebenshilfe 5 no electricity data partly occupied, 7% partly occupied, 6% without office lighting partly occupied, 75% without electricity for lighting without electricity for lighting/ventilation office buildings EcoTec 99 Wagner 1 ISE-Büro 3 DB Netz 1 GIT 5 Lamparter 3 Pollmeier 3 KfW 5 Energieforum 5 educational buildings production buildings Energon 5 TMZ 4 BOB 5 SIC 5 FH BRS 1 NIZ 4 ZUB 3 GMS 5 Hübner 1 SurTec 2 Solvis 5 Lebenshilfe 5 Fig. 2: Measured end energy (left) and primary energy coefficients (right). All data refer to the net heated floor area. Data are collected from the monitoring institutions according to tab. 1. The consumption values refer to HVACL. The numbers following the project titles indicate the year of the measurements.

5 primary energy credits [kwh/m_a] primary energy credits [kwh/m_a] credits >1% energy use credits >1% energy use credits <1% energy use credits <1% energy use no credits no credits Solvis Sol vis zero energy zero energy ISE I Lamparter Lampa S rter SICS E Energon I gon C primary energy use [kwh/m_a] primary energy use [kwh/m_a] Fig. 3: Beside a reduced energy use, buildings might gain energy credits by electricity fed into the grid by photovoltaics or combined heat and power stations. So called zero energy buildings balance the energy use on a yearly cycle by equivalent credits. The dashed lines identify buildings with identical primary energy balance. The data points refer to the monitored buildings and data given in fig. 2. Passive Cooling In view of the limited cooling capacity and the long time constants, one of the main design priorities was to restrict the amplitude and dynamics of the heat loads. For this reason, none of the demonstration buildings includes a fully glazed facade. The average ratio of glazing to façade area was 43 % or 27 % referring to the floor area. Almost all of the buildings use external sun-shading devices; the only exceptions being the buildings with slab cooling systems and thereby enhanced cooling capacity (BOB, Energieforum). Experience indicates that the total solar energy transmittance (g tot ) should not exceed.15 for the state with fully activated sun-shading device. This corresponds to performance class 3 according to (pren1451, 24). It has been observed in many cases that blinds were not closed manually until the office workers are disturbed by glare at their desks. A detailed analysis regarding manual blind use in two buildings shows a strong correlation between solar penetration depth and blind occlusion (Reinhart 23). Venetian blinds were seldom closed completely but often operated in the so called cut-off mode. This means that the full potential for improving the indoor conditions with effective sun-shading is far from being realised. Taking this into consideration, g tot -values should be assumed less optimistic during planning as those indicated from the manufacturer s information (Kuhn, 26). The average daily total internal heat load which was observed ranged between 1 and 2 Wh/m². The range refers to the density of occupation, the operation mode of the computer systems and the lighting concepts. Most of the projects from the beginning of the program applied night ventilation in combination with earth-to-air heat exchangers to remove excessive heat in summer (Pfafferott, 24); several of the more recent projects have applied slab cooling in connection with vertical ground pipes or ground pillars due to the increased cooling capacity (Tab. 1). Different criteria were applied to compare the buildings performance regarding thermal comfort based on long term monitoring. As an example for this evaluation Fig. 4 shows the hourly indoor temperature data of the Pollmeier building sorted by the outdoor temperature. According to the Dutch guideline (ISSO, 24), the relevant outdoor temperature is calculated as a floating mean value of the last three days. A

6 detailed discussion for a number of buildings monitored is presented in (Pfafferott, 26) room temperature [ C] floating mean outdoor temperature [ C] Fig. 4: Analysis of hourly temperature data monitored in the Pollmeier open space office in 22. The lines marks the upper and lower limits of the socalled class A, B, C buildings according to the new Dutch guideline ISSO 74. The building applies passive cooling using night ventilation. It mainly meets the strict class-a-criteria for high ambient temperatures (>2 C). CONCLUSIONS AND OUTLOOK The monitored energy performance of the demonstration buildings underline that the primary energy use of new office buildings in Mid European climate can be reduced to about one third of the average building stock. A comparison of the construction costs with current practice shows, on average, no significant cost increase. Consequently the demonstration program will be extended using tightened energy benchmarks. The thermal comfort evaluation of the office buildings demonstrates that during a commonly warm summer in Mid Europe prevalent criteria for thermal comfort in free running buildings are exceeded for less than 5% of the building operation time under realistic user behaviour. References The authors would sincerely like to thank the team members of the demonstration projects for their valuable co-operation and especially for the provision of monitoring data. The accompanying research project was funded by the German Federal Ministry for Economics and Technology. DIN 471, part 1 (23): Energetische Bewertung heiz- und raumlufttechnischer Anlagen, German Institute of Standardization, Berlin, Germany Humphreys, J., Nicols, F., Raja, I. A. (26) : Field studies of indoor thermal comfort and the progress of the adaptive approach, ABER Advances in Building Energy Research, volume 1, in print, ISSO 74 (24) Thermische Behaaglijkeid, Publication 74, ISSO, Rotterdam Kuhn, T. (26): Solar control: A general evaluation method for facades with venetian blinds or other solar control systems to be used `stand-alone' or within building simulation programs, Energy and Buildings, volume 38, pp Pfafferott, J. (24): Enhancing the Design and the Operation of Passive Cooling Concepts, Fraunhofer IRB Verlag, Stuttgart, Germany, Pfafferott, J., Herkel, S., Kalz, D., Zeuschner, A. (26): Thermal Comfort in Low-Energy Office Buildings in Summer, proceedings of Comfort and Energy Use in Buildings, Windsor, UK pren (25):Criteria for the indoor environment including thermal, indoor air quality, light an noise, draft pren 1451 (24): Blinds and Shutters Thermal and visual Comfort Performance characteristics and classification, draft Reinhart, C.F., Voss, K. (23): Monitoring manual control of electric lighting and blinds, Lighting Research & Technology, v. 35, no. 3, pp Santamouris, M., Asimakopoulos, D. (eds.): Passive Cooling of Buildings, James & James, London, 1997, 2 nd reprint 21 Voss, K., Löhnert, G., Herkel, S., Wagner A., Wambsganß M., eds. (25): Bürogebäude mit Zukunft, 2nd edition, Solarpraxis, Berlin, Germany, ISBN , Weber, L. (22): Energie in Bürogebäuden Verbrauch und Energierelevante Entscheidungen, vdf Hochschulverlag an der ETH Zürich, Swiss Zimmermann, M. (ed.) (23): Handbuch der Passiven Kühlung, Fraunhofer IRB Verlag, Stuttgart, ISBN