Natural and hybrid ventilation principles based on buoyancy ... - rehva

Natural and hybrid ventilation principles based on buoyancy ... - rehva

Articlesong>Naturalong> ong>andong> ong>hybridong> ong>ventilationong> ong>principlesong>ong>basedong> on buoyancy, sun ong>andong> windPeter van den EngelDelft University of Technology ong>andong>Deerns consulting engineersp.vd.engel@deerns.nlRemco KempermanDelft University of Technology ong>andong>Deerns consulting engineersAbstractAn outline is given of basic design ong>principlesong> or naturalong>andong> ong>hybridong> ong>ventilationong> related to a moderate climatesuch as in the Netherlong>andong>s. Experiences fromthe past should be used to improve the design of naturallyong>andong> ong>hybridong> ventilated buildings in order to improvethe confidence in these systems. However, with a betteruse of buoyancy, wind ong>andong> sun, the combined pressuredifferencescan be increased, which will result in a morerobust system. The potential improvements of these naturalforces are explored with CFD- ong>andong> mass-flow-simulations.The simulation results of an advanced designednaturally ventilated building are discussed. The resultsshow that the reduction of fan-energy can be substantialwhich could be applied on low-pressure mechanicalventilated buildings as well. The results of the researchcan be applied on new as well as on existing, to refurbishbuildings.This research is inspired by ong>andong> ong>basedong> on the researchproject “Earth, Wind & Fire – Air-conditioning poweredby Nature” (Bronsema 2010).IntroductionGeneralong>Naturalong> ong>andong> ong>hybridong> ong>ventilationong> systems have many advantagesong>andong>, used in an advanced way, will lead to:• more satisfied occupants;• better air quality;• less energy consumption.Heleen DoolaardDelft University of Technology ong>andong>Deerns consulting engineersMany studies show that more or less advanced naturalof ong>hybridong> ventilated buildings have a higher occupantsatisfaction than mechanical ventilated buildings.Another advantage is reduced energy consumption(Bordass, 2001, Hellwig, 2006, Brager, 2008).However, it is not always clear yet what the key success-factorsare, because there are ong>hybridong> or naturalventilated buildings that have been adapted only dueto a poor climate control (draught, too hot, poor airquality). It should also be noted that many fully airconditionedbuildings have comfort-problems as well.Consequently, for natural ong>andong> ong>hybridong> ong>ventilationong> systemsmore insight in effective design ong>principlesong>, bettercalculation models ong>andong> a more robust control strategyare essential.More satisfied occupantsAdvanced designed natural or ong>hybridong> ventilated buildingshave the potential of a high appreciation by the occupants.Especially operable windows are essential, notonly for physical but also for psychological reasons. Thedirect supply of outdoor air can lead to the followingimprovements:• Improving air quality, depending on the localenvironment of the building.• Cooling of the building, by using the outdoortemperature.• Increasing the local ong>andong> average air velocity,when the indoor temperature is high (comfortcooling).Occupant satisfaction is one of the starting points of thedesign of buildings with second skin façades. In buildingcertification systems like BREEAM ong>andong> LEEDS operablewindows will receive special credits. In the programof requirements of the Dutch Government BuildingsAgency operable windows are already design-stong>andong>ardfor years.REHVA Journal – August 2012 25

ArticlesBetter air qualityAir quality can be measured by assessing the CO 2 -levels,but there are other important physical ong>andong> subjectiveelements as well, such as the scent of a tree or theexperience of nature. Much of it is difficult if not impossibleto measure.Less energy consumptionFan-energy contributes, together with energy of appliancesong>andong> lighting, for a major part to the energy-demong>andong>of a building. A well isolated ong>andong> solar protectedbuilding with efficient energy generation neither needmuch heating nor cooling energy.Fan energy can be reduced by:• Reduction of the resistance of all the componentsof a ong>ventilationong> system. An air velocity of 1–2 m/s is recommended (Gräslund, 2011). • ong>Naturalong> ong>ventilationong> during only a part of the dayor the year when the outdoor circumstances arefavorable. • The design of buildings with natural air supply,exhaust or both.• Making use of natural elements like internal orexternal thermal pressure differences (buoyancy),sun ong>andong> wind.The ultimate goal is a building that is, as far as possible,completely naturally ventilated with a minimum use ofheating ong>andong> cooling energy. This is possible by:• Air flow control of the inlet ong>andong> outlet, aftermeasurement of the CO 2 -levels, ong>andong> ong>ventilationong>only during working hours.• Increase of ong>ventilationong> only when the outdoortemperatures are not too high or too low.• Heat recovery with a low air resistance, if necessaryconnected with a heat pump (Christensen, 2010).When the temperature-differences (betweenextracted ong>andong> supplied heat) are low, the efficiencyof the heat pump will be high.EconomyFor a cost-benefit analysis several parameters areimportant.Costs will increase due to the following factors:• ong>Naturalong> or ong>hybridong> ventilated buildings have verylow energy consumption only if the air flowsFor natural systems a velocity 0.5 m/s is most adequate for the primarily design stage (Lomas,2007). Critical elements are filters, sound-absorbing air ducts ong>andong> heat-recovery systems.However, these elements can be integrated in natural systems as well.This is the common occupant behavior with operable window, but windows are not always intelligentdesigned for both large ong>andong> minimum air flows.are effectively controlled. A control system isexpensive.• The size of ducts is generally larger in case ofnatural ong>ventilationong>.• Hybrid systems incorporate a mechanical backupsystem.• A façade with operable windows is moreexpensive than without. The opening of awindow during a hot ong>andong> humid summer daywill lead to the increase of the cooling capacityof a central air hong>andong>ling unit. An option is awindow that can be closed either automaticallyor by the occupant as a result of a warning signal.Benefits are:• ong>Naturalong> ong>ventilationong> ong>andong> operable windows willcontribute to the productivity of occupants ong>andong>reduce sick-leave.• Fan ong>andong> cooling energy will be reduced.• The total length of ductwork can be reduced(depending on the design-principle).To a new type of risk assessmentong>Naturalong>ly ventilated buildings are more vulnerable toair flow disturbances ong>andong> draught than mechanical ventilatedbuildings, however, it should be noted that thisis also a matter of assessment, which is often too general.Consequently, naturally ventilated buildings needa special type of risk assessment, which is not availableyet. For instance, the kind of turbulence produced bya window is different from cooled mechanical suppliedair. Air supplied via a window has another size ong>andong> frequencydistribution of eddies, which requires anotherkind of draught-evaluation. Moreover, the comfort-expectationof the occupants can be different.Points of attentionGeneral points of attention are:• In a completely naturally ventilated building asingle operable window may disturb the wholeong>ventilationong> system due to the large air flows whenthe pressure-differences are high. However, whenthe air quality of a building compartment as awhole remains well, a different air flow patternwill not always be a problem.• Mechanical systems are often able to solve airpressures due to open windows, but it is notalways clear enough what the real limitations are.What are acceptable pressure differences?• In natural ventilated buildings an operablewindow will not interfere with other fresh-airflows when a building compartment or space hasits own air inlet ong>andong> exhaust (Short, 2004).26REHVA Journal – August 2012

ArticlesFigure 1. Diagram of the basic-TRNSYS/TrnFlow-calculation model evaluated in CFD (Phoenics) as well.Moreover, other chimney-types are evaluated, see Figure 2 ong>andong> 3.28REHVA Journal – August 2012

ArticlesIncreased under-pressure due to the venturi-effect in case of a centralized chimneya. The velocity of 5 m/s is increased to circa 8 m/s due to theshape of the roof.b. The resulting under-pressure is 30 Pa.Over- ong>andong> under-pressure of decentralized chimneysc. Zone with overpressure for air inlets.This is about 2 m below the rooftop.d. Under-pressure of decentralized chimneys, integrated in thefaçade, which can be used as solar chimneys as well.Figure 2. Advanced use of over- ong>andong> under-pressure for natural ong>ventilationong> systems, several options.The following setpoints ong>andong> parameters are used:• Minimum temperature 20°C• Maximum temperature 25°C• Opening of the building from 7:00 am – 19:00 pm• The ong>ventilationong> system is shut off when thebuilding is not in use• Internal heat load 35 W/m²• Insulation closed parts of the façade U = 0.23 W/m²K• Insulation glass + window-frame U = 1.6 W/m²K,g-value = 0.67• Glass percentage 30%• Sunshade, g-value = 0.40• Infiltration rate 0.1 h• Ventilation, > 50 m³/h per person• Size of the building 13,050 m² gross floor area• Size of the solar collector, width 7 m, height28.5 m (East, South ong>andong> West)CFD-resultsThe exhaust system has one or three chimneys. The positionof the air inlet is near the top of the building in aREHVA Journal – August 2012 29

ArticlesTable 1. Air flows ong>andong> total driving pressure differences for the different storeys [required Q v = 0.5 m³/s per office floor]StoreyAverageMaximum airflowAverage airflowRelativeStong>andong>ardDeviationAveragemaximumdrivingpressuredifferenceAverageminimumdrivingpressuredifferenceAveragedrivingpressuredifferenceRelativeStong>andong>ardDeviation[m³/s] [m³/s] [%] [Pa] [Pa] [Pa] [%]0 th 3.19 1.26 28 329 3.3 39.3 701 st 3.03 1.16 29 326 4.0 39.4 692 nd 2.79 1.03 30 329 4.1 39.4 693 rd 2.59 0.92 31 328 4.0 39.4 694 th 2,91 0.98 33 329 4.0 39.5 695 th 3.15 1.00 36 329 3.9 39.6 696 th 3.22 0.95 41 330 3.8 39.6 697 th 3.29 0.88 49 331 3.8 39.8 69average 1.02 39.6Table 2. Part of occupation time with sufficient ong>ventilationong> [Q v > 0.5 m³/s per office floor]StoreyNorth wingW orientation of solar chimneySouth-East wingSW-SE orientation of solar chimneySouth-West wingSE-SW orientation of solar chimney[%] [%] [%]0 th 99.3 99.5 99.51 st 99.1 99.3 99.32 nd 98.3 98.8 98.73 rd 96.8 97.7 97.44 th 97.8 98.6 98.45 th 97.7 98.7 98.46 th 93,5 97.0 95.87 th 78.1 86.7 83.7average 95.1 97.0 97.0Table 3. Energy consumption.Heating energy[MJ/(m²•y)]Gas consumption forheating[m³/(m²•y)]Building total 226.9 7.2Fan energy consumptionBuilding caseFan energy consumptionbuilding case [kWh]Fan energy consumption in 100%mechanical drive case [kWh]Energy savings relative to 100%mechanical drive case [%]Buoyancy, wind ong>andong> sun 10 Pa 11 927 99Sustainable (low pressure system) 200 Pa 10,635 18,792 43Conventional 1.000 Pa 75,796 93,960 2030REHVA Journal – August 2012

ArticlesTable 4. Average driving pressures of physical elements.Average Totaldriving pressure,average storeyBuoyancy, averagestoreyWind, averagestoreySolar contribution to stack pressure in asolar chimney with South-East/ South-West orientation, average storey∆P average [Pa] 39.6 19.6 18.5 2.1Pa > 0 during occupation [%] 100.0 98.2 100.0 71.2Pa > 5 during occupation [%] 99.9 92.1 62.0 12.3Pa > 10 during occupation [%] 98.0 79.4 45.4 0.3Pa > 20 during occupation [%] 80.1 47.2 30.0 0.0Table 5. Contribution to the driving pressure of the individual components.Stack pressure in Solarchimney plus shunt ductSouth-East / South-WestorientationSolar contribution tostack pressure in a solarchimney plus shunt ductSouth-East / South-WestorientationUnder pressure onexhaust due to ‘venturi’roofUnder pressure onexhaust due to local‘venturi’ exhaustsdirectly on the chimneys∆P average [Pa] 21.7 2.1 18.3 9.2Pa > 0 during occupation [%] 100 72 100 100Pa > 5 during occupation [%] 98 17 68 51Pa > 10 during occupation [%] 89 0 51 31Pa > 20 during occupation [%] 55 0 31 13zone with overpressure (Figure 2c). The position of theexhaust on the roof is in the centre or near the façade.The under-pressure in the exhaust can be increased bya venture-shape (Figure 2a ong>andong> b). The exhaust systemcan – if located near a façade – make use of solar energyas well.The CFD-simulations show that near the centre of thebuilding there is always the possibility to add an inletwith a positive pressure. TRNSYS/TrnFlow-resultsTable 1 shows that the average pressure difference iscirca 39 Pa, which makes it more interesting to addother components such as heat recovery or electrostaticfilters.From Table 1 ong>andong> 2 can be concluded that the buildingis on average 2 times over-ventilated ong>andong> with a maximumof circa 6 times. This is due to the combination ofvery low outdoor- temperatures ong>andong> much wind.The low pressures near the roof may partly be the effect of the coarse grid of CFD-simulation.In order to reduce the complexity of the simulationmodel,the openings are designed with a fixed size. Thisresults in relatively high air flows in winter with outsidetemperatures around zero. Another point of attentionis simultaneously reduction of heating ong>andong> cooling.Additional reduction of energy is possible with heat recoveryin the exhaust.With a better flow- ong>andong> temperature-control ong>andong> heatrecovery the calculated heating energy of 7 m³ naturalgas equivalent per m²/y (Table 3) could be reduced significantlyong>andong> will probably result in a heat consumptionclose to the passive stong>andong>ard of 1.5 m³ natural gasequivalent per m²/y.However, one of the most striking improvements ofnatural ong>ventilationong> is the reduction of fan energy. Mostof the energy-savings are possible when a high pressureong>ventilationong>-system is changed in a low pressure system(Gräslund, 2008). Comparing a mechanical ventilatedbuilding with a very low pressure-difference of 10 Pa,natural ong>ventilationong> with a solar chimney ong>andong> a venturiroof,can reduce the energy consumption already with99%. Comparing a low pressure mechanical system ofREHVA Journal – August 2012 31

Articles200 Pa the savings are still substantial (43%). Finally,the application of solar chimneys ong>andong> a venturi-roof canreduce the energy consumption of a 1,000 Pa (improvedstong>andong>ard) system with 20% (Table 3).The following table presents what the contribution isof buoyancy, wind ong>andong> sun separately related to the requiredpressure difference in the system:Figure 3. Two options of decentralized chimneys. Theventuri-shape gives the best results.When a decentralized venturi-roof (Figure 3) is appliedthe positive effect of the wind will be smaller, but willstill be significant.The contribution of buoyancy, wind ong>andong> sun on thedifferent components are presented in more details inTable 5.Discussion1. The research shows the high capacity of natural forces to reduce fan-energy, even for completely mechanicaldriven systems.2. Most of the savings are possible by designing a low pressure ong>ventilationong>-system.3. For a medium sized building in a moderate climate an average natural pressure difference of 39 Pa isachievable. The contribution of each of the forces has to be assessed individually.4. Depending on the control-qualities of the ong>ventilationong>-system, ong>andong> the availability of heat recovery, alow energy consumption for heating ong>andong> cooling is possible, near the level of passive-stong>andong>ards. In orderto assess this in detail more research will be required.5. Integration of operable windows still needs more attention. Design-possibilities are return valves, moreflow-controllers in the system or separate inlets ong>andong> exhausts for each building compartment (Short, 2004).Literature1. Bordass B, Cohen R, Song>andong>even M, Leaman A. Assessing building performance in use: energy performance of the PROBEbuildings. Building Research ong>andong> Information 29 (2) (2001) 114-128.2. Hellwig R.T., Brasche S., Bischof W.. Thermal Comfort in Offices – ong>Naturalong> Ventilation vs. Air Conditioning, Proceedings ofcongress Comfort ong>andong> Energy Use in Buildings – Getting it Right, Winsor 2006.3. Brager G., Baker, L., Occupant Satisfaction in Mixed-Mode Buildings. Proceedings of Conference: Air Conditioning ong>andong> theLow Carbon Cooling Challenge, Cumberlong>andong> Lodge, Windsor, UK, 27-29 July 2008. London: Network for Comfort ong>andong> EnergyUse in Buildings.4. Lomas KJ. Architectural design of an advanced naturally ventilated building form. Energy ong>andong> Buildings 39 (2007) 166–181.5. Krausse B, Cook M, Lomas K. Environmental performance of a naturally ventilated city centre library. Energy ong>andong> Buildings 39(2007) 792–801.6. Khan N, SU Y, Riffat SB. A review on wind driven ong>ventilationong> techniques. Energy ong>andong> Buildings 40 (2008) 1586 – 1604.7. Blocken B, Hooff T van, Aanen L. Bronsema B. Computational analysis of the performance of a venturi-shaped roof for naturalong>ventilationong>: venture-effect versus wind-blocking effect. Computers & Fluids 26 April 2011.8. Christensen MS. ong>Naturalong> Ventilation with Heat Recovery ong>andong> Cooling (NVHRC). Rehva Journal September 2010.9. Gräslund J. Simple ong>andong> reliable constant pressure ong>ventilationong> for nZEB. Rehva Journal May 2011.10. Short CA, Lomas KJ, Woods BA. Design strategy for low-energy ong>ventilationong> ong>andong> cooling within an urban heat islong>andong>. BuildingResearch ong>andong> Information 32 (3) (2004) 187-206.32REHVA Journal – August 2012

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