Passive Cooling Technologies Evaporative Cooling, Radiative Cooling, Night Ventilation, Earth to air heat exchangers, Energy sonds, Groundwater/sea/river/lake water cooling, Energy sonds, Cooling towers
Imprint Published and produced by: Österreichische Energieagentur – Austrian Energy Agency Otto-Bauer-Gasse 6, A-1060 Vienna, Phone +43 (1) 586 15 24, Fax +43 (1) 586 15 24 - 40 E-Mail:
[email protected], Internet: http://www.energyagency.at Editor in Chief: Dr. Fritz Unterpertinger Authors: Ralf Cavelius, IZES gGmbH, Charlotta Isaksson, AEE INTEC, Eugenijus Perednis, Lithuanian Energy Institute, Graham E. F. Read, NIFES Consulting Group Project management: Márton Varga Reviewing: Márton Varga Layout: Simone Biach Produced and published in Vienna The sole responsibility for the content of this report lies with the authors. It does not represent the opinion of the European Communities. The European Commission is not responsible for any use that may be made of the information contained therein.
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Content 1
Preamble................................................................................................................... 1
2
Technology description – „Evaporative Cooling“................................................. 3 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9
3
Technology description – “Radiative Cooling”................................................... 21 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10
4
Summary-Sheet – “Radiative Cooling”.............................................................21 Concept descriptions .........................................................................................23 Physical and technical descriptions .................................................................23 Calculation algorithms / tools............................................................................27 Energy performance and Economic figures/benefits .....................................29 Application...........................................................................................................31 Market situation...................................................................................................41 Operation and Maintenance ...............................................................................41 Further (non energy) benefits/synergies ..........................................................41 Possible obstacles..............................................................................................42
Technology description – “Night ventilation (mechanical and natural)” .......... 43 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10
5
Summary-Sheets...................................................................................................3 Concept descriptions ...........................................................................................5 Physical and technical descriptions .................................................................10 Calculation algorithms / tools............................................................................13 Energy performance and economic benefits ...................................................15 Application...........................................................................................................17 Market situation...................................................................................................19 Operation and Maintenance ...............................................................................20 Possible obstacles..............................................................................................20
Summary - Night ventilation (natural/mechanical) ..........................................43 Concept Descriptions .........................................................................................46 Physical and Technical Descriptions................................................................46 Calculation Algorithms /tools ............................................................................53 Energy and Economic Benefits .........................................................................54 Application...........................................................................................................54 Market Situation ..................................................................................................56 Operation and Maintenance ...............................................................................56 Further (non-energy benefits)............................................................................56 Possible Obstacles .............................................................................................57
Technology description – Earth to air underground heat exchanger ............... 58 5.1 5.2 5.3 5.4 5.5
Summary - Earth ground heat exchanger ........................................................58 Concept descriptions .........................................................................................61 Physical and technical descriptions .................................................................64 Calculation algorithms / tools............................................................................65 Energy and Economic Benefits .........................................................................66
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5.6 5.7 5.8 5.9 5.10 5.11
6
Deep Energy Sonds................................................................................................79 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10 6.11
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Summary-Sheets ................................................................................................ 90 Concept descriptions......................................................................................... 92 Physical and technical descriptions ................................................................ 95 Calculation algorithms / tools ........................................................................... 95 Energy and Economic benefits......................................................................... 96 Application .......................................................................................................... 98 Market situation ................................................................................................ 103 Operation and Maintenance ............................................................................ 104 Further (non energy) benefits/synergies ....................................................... 104 Possible obstacles ........................................................................................... 104 References ........................................................................................................ 105
Wet and dry cooling towers.................................................................................106 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9
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Summary Sheet .................................................................................................. 79 Concept descriptions......................................................................................... 81 Physical and technical descriptions ................................................................ 82 Calculation algorithms / tools ........................................................................... 83 Energy and economic benefits ......................................................................... 85 Application .......................................................................................................... 87 Market situation (Austrian market only)........................................................... 87 Operation and Maintenance .............................................................................. 88 Further (non energy) benefits/synergies ......................................................... 88 Possible obstacles ............................................................................................. 89 References .......................................................................................................... 89
Technology description – “Ground water, sea water, rivers” ............................90 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11
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Application .......................................................................................................... 69 Market situation .................................................................................................. 77 Operation and Maintenance .............................................................................. 77 Further (non energy) benefits/synergies ......................................................... 77 Possible obstacles ............................................................................................. 77 References .......................................................................................................... 78
Summary sheet – Cooling towers................................................................... 106 Concept descriptions....................................................................................... 108 Physical and technical descriptions .............................................................. 114 Performance and sizing and of cooling towers............................................. 117 Factors affecting cooling tower costs............................................................ 119 Application ........................................................................................................ 119 Operation and Maintenance ............................................................................ 120 Possible obstacles ........................................................................................... 121 References ........................................................................................................ 121
1 Preamble The following summary of passive cooling technologies is mainly based on the literature compilation below. Short abridgements are taken from the listed literature.
Angelotti, A. Summer cooling by earth-to-water heat exchangers: experimental results and optimisation by dynamic simulation. Politecnico di Milano
Argiriou, A. & Santamouris, M. Natural Cooling Techniques. Athens: CIENE, University of Athens; Bruxelles: European Commission, DG XVII for Energy.
Blümel, E. Luftdurchströmte Erdreichwärmetauscher - Handbuch zur Planung und Ausführung von luftdurchströmten Erdreichwärmetauschern für Heiz- und Kühlanwendungen, 4th EU - CRAFT-JOULE Framework. Gleisdorf: AEE INTEC
Barnard, N & Jaunzens, D. Low Energy Cooling Technology; Section and Early Design Guidance - Annex 28 – Low Energy Cooling – UK, January 2001. International Energy Agency.
Brown, G.Z. & DeKay, M. Sun, Wind & Light: Architectural Design Strategies, 2nd ed., 2000.
enercret - Nägele Energie- und Haustechnik GmbH, Thermoaktive Fundamente – Kühlund Heizenergiegewinnung aus Erde und Grundwasser über konstruktive Betonbauteile ohne Umweltbelastung
Feist, W. Energieeffiziente Raumkühlung, Passivhaus Institut, Protokollband Nr. 31 – Arbeitskreis kostengünstiger Passivhäuser, Phase III. Darmstadt, Juli 2005.
Fisenko, S.P., Brin, A.A., Petruchik, A.I., Evaporative cooling of water in a mechanical draft cooling tower. International Journal of Heat and Mass Transfer, 47, 165-177, 2004
Givoni, B. Energy and Buildings - Department of architecture, University of California Los Angeles (1991).
Givoni, B. Passive and Low Energy Cooling of Buildings, Wiley: 1994
International Energy Agency. Energy Conservation in Buildings and Community Systems Programme, Annex 28 – Low Energy Cooling: Review of Low Energy Cooling Technologies: Subtask 1 Report (December 1995), International Energy Agency.
International Energy Agency. Air Infiltration and Ventilation Centre – University of Warwick Science, England.
International Energy Agency. Low Energy Cooling - Technical Synthesis Report ECBCS - Annex 28, July 2000.
Kloppers, J. C. & Kröger, D. G. Influence of temperature inversions on wet-cooling tower performance, Applied thermal Engineering, 25, 1325-1336, 2005.
Koch-Nielsen, H. Stay Cool: A design guide for the built environment in hot climates, UK, 2002.
Koschenz, M. & Lehman, B. Thermoaktive Bauteilsysteme tabs, Juli 2000.
Ranft, F. & Frohn, B. Natürliche Klimatisierung; Energieagentur NRW - 2004.
Sanner, B. & Paksoy, H. Possibilities for Heating and Cooling through Underground Thermal Enerhy Storage in the Mediterranean Area, International Summer School on Di-
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rect Application of Geothermal Energy, Thessaloniki, Greece, 1-4 September 2002. pp 63-66.
Santamouris, M. & Asimakopoulos, D. Passive Cooling of Buildings; 1996.
Schweizerischer Ingenieur – und Architektenverein SIA, Energie aus dem Untergrund – Erdspeicher für moderne Gebäudetechnik, 2003
Su, M. D., Tang G. F. & Fu, S. Numerical simulation of fluid flow and thermal performance of a dry-cooling tower under cross wind condition. J Wind Eng. Ind. Aerodyn. 79, 289-306, 1999
Tan, K., Deng, S., A method for evaluating the heat and mass transfer characteristics in a reversibly used water cooling tower (RUWCT) for heat recovery. International Journal of Refrigeration, 25, 552-561, 2002
Wuppertal Institut für Klima-Umwelt-Energie & Planungs-Büro Schmitz Aachen, Energiegerechtes Bauen und modernisieren, 1996.
Zhai, Z. Q., Zhu, K. Q. & Fu, S. Experimental study of effects of cross wind on flow in natural draft dry cooling tower. Tsinghua Science & Technology, 3, 955-958, 1998
Zimmermann, M. & Anderson, J. : Low Energy Cooling, Case Study Buildings; Annex 28 – Low Energy Cooling – Switzerland, August 1998. International Energy Agency.
Zimmermann, M. Rationelle Energienutzung in Gebäuden – Handbuch der Passiven Kühlung, im Auftrag des Bundesamtes für Energie, EMPA, Dübendorf, Juni 1999.
2 Technology description – „Evaporative Cooling“ 2.1
Summary-Sheets
(from “Technology Selection and early design guidance”, Low Energy Cooling)
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Further applications of evaporative cooling can be: The use of natural vegetation for evapotranspiration Volume cooling technologies with cooling towers Roof ponds, roof sprinkling and moving water films Performance: As a thumb figure it can be stated that with hybrid indirect evaporative cooling concepts a cooling load of ~15 W/m2 (office area) can be covered. The daily cooling energy which can be delivered by this technology is < 150Wh/m2d. In case of higher cooling loads additional systems will be necessary.1
1
Ranft & Frohn, 2004
2.2
Concept descriptions
The process of evaporative cooling allows the cooling of air (incoming or exiting air) or of thermal masses (roofs, walls, ceilings). It uses the natural effect of evaporation to remove heat from the air. Sensible heat from the air is absorbed as latent heat necessary to evaporate water: arm dry air is changed to cool moist air - heat in the air is used to evaporate water. The amount of sensible heat absorbed depends on the amount of water that can be evaporated in the system. Evaporative cooling systems can be classified in two ways. They can be direct or indirect according to the contact of the cooled air with the evaporated water. In addition, evaporative cooling systems can be passive or hybrid, according to the energy required to produce evaporation. In direct evaporative cooling, the incoming air is in contact with the evaporated water and the water content of the cooled air increases. The indirect process lets water evaporate in the outlet air and transfers the cooling effect to the inlet air via a heat exchanger. This way, the water content of the cooled inlet air remains unchanged. In desert regions, where an increase of humidity is welcomed from the comfort point of view, the incoming airflow can be cooled down by direct evaporative cooling by natural airflows or even in combination with mechanical ventilation systems. In regions, where a rise of the relative humidity of inlet air is not acceptable, indirect evaporative cooling must be applied. Passive evaporation techniques use make use of the natural evaporation of water. For example, outdoor space can be cooled by passive evaporation, provided that there are surfaces of standing or moving water, such as basins or fountains. In hybrid evaporative systems, evaporation is controlled and/or promoted by means of some mechanical device. Air humidification and cooling by evapotranspiration of plants and the use of free water surfaces like pools and streams, is a passive direct technology. Passive indirect evaporative techniques are roof sprinkling and the use of a roof pond. 2.2.1
Passive direct evaporative cooling systems
This category includes the use of vegetation for evapotranspiration, the use of fountains, sprays, pools and ponds as well as the use of volume and tower cooling techniques. Use of vegetation for evapotranspiration Vegetation plays the role of a natural evaporative cooling device. Trees and other plants transpire moisture with a considerable cooling potential: On a sunny summer day, a normalsize deciduous tree can evaporate up to 1460 kg of water – the corresponding cooling effect is around 870 MJ or 240 kWh (the heat transferred by evapotranspiration is close to 2320 kJ per kg evaporated water). Of course, not all of this energy can be use to reduce cooling demand of adjacent buildings: Evapotranspiration from one tree can save between 250 and 650 kWh of electricity used for air conditioning per year. Similarly, the cooling potential of one acre of grassland is about 50 GJ (~13,8MWh) on a sunny day – this potential cools the ground surface temperature about 6-8°C below the
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average surface temperature of bare soil. Several studies and observations concerning the role of vegetation report from an average temperature reduction of about 2-3°C. Fountains, sprays, pools and ponds are particularly effective passive cooling techniques. The rate of evaporation from a wetted surface depends upon the air velocity and the difference between the water vapour pressure and the air pressure next to the moist surface. Calculations based on mean summer weather provide cooling potentials between 150-200 W/m2. Evaporative cooling in open spaces is particularly effective in areas that have wet bulb temperatures below 21°C. Volume cooling technologies Volume cooling techniques base on the use of a tower with water falling down, evaporating from wet pads, or being sprayed, inside. If supply air for a building is directed through this tower, it will be cooled down by evaporation. The towers work as a reverse chimney. The incoming air on the top of the tower is cooled by evaporation and since it becomes heavier, it falls to the bottom of the tower and will be directed into the building. In order to save water, the part of the water that does not evaporate can be recirculated. A variation of this concept is the combination of cooling tower on one side of the building and solar chimney on the other side of the building. The solar chimney further enhances ventilation through the building.
Figure 2.2-1: Cooling tower scheme (left side) - Building with attached cooling tower and solar chimney (ride side) Further passive cooling applications A simple passive evaporative cooling method is to use wetted pads made of fibres, placed in windows facing strong winds. This can be easily done in arid and desert regions, where on the one hand often strong winds blow from a constant direction – and on the other hand the dry and hot climate favours a humidification of the incoming air. As a drawback, such pads block the view through these windows.
2.2.2
Passive indirect evaporative cooling systems
These systems mainly include roof sprinkling, roof ponds and moving water films. Roof sprinkling Roof sprinkling can be an interesting idea for the cooling of a building, since in many buildings particularly in those with flat roofs, a main part of the external heat gains comes from the roof. Roof sprinkling is based on evaporation of a water mist layer created by misting sprayheads on roof of the building; when the water evaporates, it absorbs large amounts of heat. Roof ponds Roof pond systems are much simpler than roof sprinkling. Water ponds are constructed over flat-roofs. In order to avoid excessive water heating during the day, the water surface must be shaded. The evaporating water cools the building by conduction across the roof. Both indoor air and radiant temperatures decrease without rising indoor humidity. Since it requires the conduction through the roof, this technology should only be applied on not insulated roofs. In order to prevent high heat losses in wintertime, roof ponds are only recommeded for hot climates without cold winters. Ponds beneath the building Another variant of this physical method is to place ponds on the ground next to buildings, with a much larger depth of water. In this case it is possible to maintain the water temperature near or even below the average diurnal wet bulb temperature by fine spraying of the water over the insulation during the nights. The so produced cold can be used e.g. by tubes installed in the pond where warmer air of the building circulates during the day – the pond acts as a heat sink for the building. Moving water films The moving water film is based on the flow of a water film e.g. on the roof surface. The increase in the relative velocity between the air and the water surface enhances the evaporation process. The cooled water can cool the building by circulation inside the building or by being stored in the basement. The effect of moving water films can also be transferred as a direct cooling technique by the application of water falls inside the building. In this case water flows on a wall – the increase of the relative velocity between the air and the water surfaces enhances the evaporation effect decreasing directly the indoor temperature and increasing indoor humidity. 2.2.3
Hybrid direct evaporative cooling systems
Direct hybrid air coolers The main element of a direct air humidifier, or Single Stage Direct Evaporation Cooler, is a porous material that provides a large surface saturated with water. There are three main types: drip-type cooler, spray-type cooler and rotary pad cooler. The supply air for (mechanical) building ventilation loses a part of its heat by taking up a part of the water in the pad.
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The cooled and humid air is directed into the building. Because of its high humidity, the indoor air cannot be re-circulated through the evaporative cooler. Therefore, a very high rate of air flow is necessary, about 15 to 30 air changes per hour.
Figure 2.2-2: Principle of hybrid direct evaporative cooling This system is in general of low capital and operational cost. However, it has to be used carefully especially in spaces where latent heat gains are important since the increased water content of indoor air can easily cause discomfort. The main elements of this evaporative cooling device are:
Evaporative pads,
a water pump,
a ventilator,
a water injection system,
a collection system for the water droplets,
a water tank,
a water valve maintaining a constant water level,
a box containing the device.
Figure 2.2-3: Typical direct, hybrid evaporative cooler
2.2.4
Hybrid indirect evaporative cooling
In moderate climates, direct evaporative cooling cannot be used since comfort requirements could not be met with a rise of humidity in the incoming air. Nevertheless, it is still possible to evaporate water into the outgoing air of mechanical ventilation systems and to use a heat exchanger: The outgoing air, cooled down by evaporation, cools the heat exchanger, which in turn cools the supply air without modifying its water content. The potential for building cooling depends on the climatic conditions (temperature and humidity of ambient air). The scheme below illustrates the principle configuration of this system:
Figure 2.2-4: Principle of hybrid indirect evaporative cooling The main components in the system are:
the heat exchanger with humidifier, in which evaporation occurs,
two ventilators, usually centrifugal,
two filters one at fresh air one and a second one at the indoor building air inlet,
a water pump,
a water injection system,
a water tank,
The humidifier can be even an air washer, contact humidifier or a cold steam generator. Saturation is naturally limited to 100% rel. humidity. The cold steam generator enables an over saturation of the air, which allows higher cooling rates. The heat exchanger system can be constituted by a sensible (rotating) rotor, or a plate/tube heat exchanger.
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2.2.5
Combination of hybrid direct/indirect evaporative cooling
The two-stage evaporative cooler combines an indirect cooler in the first stage and a direct cooler in the second stage. Such systems are used when the required dry-bulb temperature of the supply air is lower than what could be achieved by a single-stage system. The better performance of the two-stage system reduces the working time of the equipment and therefore the energy consumed. In addition, the use of the two stage evaporative cooler results in lower relative humidity values and in lower temperature levels than a single system.
Figure 2.2-5: Two-stage-evaporative cooler
2.3
Physical and technical descriptions
Understanding evaporative cooling performance requires an understanding of psychrometrics. Evaporative cooling performance is dynamic due to changes in external temperature and humidity level. Dry air is obtained when all humidity and contaminants are removed from atmospheric air. Moist air is a mixture of dry air and water vapour. The amount of water varies from zero (dry air) to a maximum that depends on temperature and pressure. When this maximum is achieved this is called saturation. The thermodynamic properties of moist air are graphically represented on a chart called a psychrometric chart. It must be noted that the thermodynamic properties of moist air do not depend only upon the temperature and the water content, but also upon atmospheric pressure. For the following explanations normal pressure conditions are assumed. The x-axis of the psychrometric chart represents the temperature of the humid air, also called the dry bulb temperature. On the y-axis, the humidity ratio is reported: this is the ratio of the mass of the water vapour to the mass of dry air contained in an air sample. Relative humidity is defined as the ratio of the mole fraction of water vapour xw in a given moist air sample to the mole fraction xws in an air sample saturated at the same temperature and pressure. Practically, this means the higher the relative humidity is, the closer the saturation conditions are reached. The curved lines on the psychrometric chart are the relative humidity lines. On the graph below only the 100% relative humidity line is shown. The 0% relative humidity line coincides with the dry-bulb-temperature.
Figure 2.3-1: Principle of Psychrometric chart The temperature of a bulb that is covered by a thoroughly wetted wick, placed in an air stream with a certain relative humidity, so that evaporation can happen, depends on the water content of the air stream. In ideal cases the measured temperature would be the wetbulb temperature (100% saturation of air flow). Practically a 100% relative humidity is rarely reached so that the “wet-bulb temperature” is a few degrees lower than at 100% relative humidity. The oblique lines on the psychrometric chart show the constant wet-bulb temperature lines. Along this constant wet-bulb temperature lines the evaporative cooling potential of air can be determined. Some rough examples clarify this relationship:
At 26,7 °C (80°F) and 50% relative humidity, air may be cooled to nearly 19,4 °C (67 °F).
At 32 °C (90°F) and 15% relative humidity, air may be cooled to nearly 16 °C (60 °F).
At 32 °C (90 °F) and 50% relative humidity, air may be cooled to about 24 °C (75 °F).
At 40 °C (105 °F) and 15% relative humidity, air may be cooled to nearly 21 °C (70 °F).
Figure 2.3-2: Full psychrometric chart: Dry-bulb and wet-bulb temperatures of wet air.
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2.3.1
irect evaporating cooling process
When non-saturated air comes into contact with water, evaporation occurs. The necessary latent heat is provided by the air, which cools down. In addition, the moisture content of the air raises. Direct evaporative cooling is represented on the psychrometric graph below by a displacement along a constant wet-bulb temperature line AB:
Figure 2.3-3: Psychrometric chart of Direct Evaporative Cooling. The cooling process is represented by the line A-B. 2.3.2
Indirect evaporating cooling process
In indirect evaporative cooling, the evaporation occurs in a primary circuit of a heat exchanger. While the air to be cooled circulates in the secondary circuit, the air temperature decreases but its water content remains constant. It must be noted that, since the airtemperature drops, its relative humidity will still increase. Since the humidity content of the cooled air does not rise, indirect evaporation cooling is represented on the psychrometric chart by a displacement along a constant humidity ratio-line CD (as cooling at any other cool surface).
Figure 2.3-4: Psychrometric chart of Indirect Evaporative Cooling. The cooling process is represented by the line C-D. 2.3.3
Combined direct/indirect evaporating cooling process
The two-stage evaporative cooler combines an indirect cooler in the first stage and a direct cooler in the second stage. The psychrometric chart below shows the principle function of this system: Air to be cooled, initially at point A, is sensibly cooled by an indirect evaporative process until it reaches point B. Since the water content of the air has not changed, line AB
is parallel to the dry-bulb temperature axis. Then the air enters the second stage, where it becomes cooled down by a direct evaporative process to point C. Since this is a constant wet-bulb temperature process, line BC is parallel to the wet-bulb temperature lines.
Figure 2.3-5: Psychrometric chart of a two-stage evaporative cooling process. Line AB represents the indirect cooling in the first stage, whereas line BC represents the direct cooling in the second stage. 2.3.4
Performance of direct and indirect evaporative cooling
The performance of direct and indirect evaporation cooling systems can be assessed on the saturation efficiency (SE), defined as: Tdb,in Tdb,out SE Tdb,in Twb ,in with Tdb, in and Tdb, out the dry-bulb temperatures of the air at the inlet and outlet of the system as well as Twb, in as the wet-bulb temperature of air at the inlet of the system.
2.4
Calculation algorithms / tools
2.4.1
Calculation algorithms
In their book “Passive Cooling of buildings”, Santamouris and Asimakopoulos provide the following simplified scheme for the calculation of the performance of direct and indirect evaporative coolers (see figures below):
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Figure 2.4-1: Simplified methods to calculate the performance of evaporative cooling (Santamouris & Asimakopoulos, 1996)
Figure 2.4-2: Calculation examples using the simplified method (Santamouris & Asimakopoulos, 1996)
2.4.2
Design tools
There are several mathematical models for different types of direct/indirect evaporative coolers. In general the calculation algorithms are integrated in sophisticated building simulation software such as:
DOE2
TRNSYS
Some suppliers of evaporative cooling systems (e.g. MENERGA) developed proprietary design tools to promote their products.
2.5
Energy performance and economic benefits
2.5.1
Energy performance
The above described technologies can cover a certain amount of cooling demand. However, the effective cooling potential always depends on the relative humidity of the indoor and ambient air.
With hybrid indirect evaporative cooling concepts a cooling load of ~15 W/m2 (office area) can be covered. The daily cooling energy which can be delivered by this technology is < 150Wh/m2d.2 In case of higher cooling loads, additional systems will be necessary.3
A monitoring of a roof sprinkling system installed in Chicago showed that a 5°C average decrease in the indoor room temperature was obtained.
For direct evaporative cooling systems, no figures for cooling potential per office area could be found. However, as shown above, vegetation (natural passive direct evaporation) can lower air temperatures by 2-3 °C, and soil temperatures by 6-8 °C. Outdoor fountains, sprays, running water, or other wet surface (passive direct evaporation) have a cooling potential of around 150-200 W/m² wet surface. Wet cooling towers (Hybrid direct evaporation) using the principle of evaporative cooling achieve cooling potentials from 20 kW up to 1,5 GW.
The following table gives energy balance figures for green roofs (direct passive evaporative cooling) as average daily value for the months June-August:
2 3
Tables from Zimmermann, 1999. Ranft & Frohn, 2004
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Table 2.5-1: Energy balance of conventional and green roofs Bitumen-sheeted roof
Roof covered vegetation
Solar radiation
Wh/m²d
5320
5320
Radiation balance
Wh/m²d
1942
2050
Part of the rainwater evaporated
Wh/m²d
8%
77%
Evaporation
Wh/m²d
123
1185
Sensible heat
Wh/m²d
1827
with
872 4
Typical energy consumption values of hybrid evaporative systems : Direct evaporative cooler:
Typical electric fan power 250 Wel per 3600 m3/h
Electricity consumption for the pump ~60-100Wel
Cooling capacity in semi-dry climates: 250Wel for 1000W of cooling (COP=4)
Consumption of water: ~1,3 l / 0,277 kWh cooling load
Two-stage evaporative cooler:
Typical electric fan power 600 Wel per 3600 m3/h
Electricity consumption for the pump ~60-100Wel
Cooling capacity in semi-dry climates: 150Wel for 1000W of cooling (COP=6-7)
Consumption of water: ~1,5 l / 0,277 kWh cooling load
Further, the heat exchanger of an indirect evaporative cooling system can operate in wintertime as a heat recovery unit (with the evaporation system turned off), reducing the heating energy costs. 2.5.2
Economic figures
Investment costs are a bit higher than those of standard (vapour compression) airconditioning systems: A direct evaporative cooler costs about one third (1/3), a two-stage evaporative cooler about two thirds (2/3) more than comparable mechanical cooling equipment. The following table gives a rough overview of costs. Note that these figures count for residential applications.
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taken from International Enegy Agency, 1995 (Review of Low Energy cooling technologies IEA-BCS Annex 28)
Table 2.5-2: Costs for evaporative cooling systems and standard air-conditioning systems (table from International Energy Agency, 1995)
2.6
Application
Evaporative cooling is only efficient when the relative humidity is low. Evaporative cooling systems for air treatment can be designed as stand alone systems or as backup systems in conventional air-conditioning-systems. When designed as a stand-alone system, an evaporative system needs 3-4 times the air exchange rate of conventional airconditioning systems. In general, this requires larger ducts – on the other hand the indoor air quality may be improved. Beside the stand alone system, evaporative coolers can be integrated with conventional air-conditioning systems to pre-cool air using standard air-flow rates. Depending on the climate, evaporative cooling can also be used to boost night-time cooling by further reducing the air temperature. Evaporative cooling is most appropriate in buildings with relatively small cooling loads, such as commercial and residential buildings – or buildings that do not require tight humidity and temperature control, such as warehouses. Evaporative cooling can be used in retrofit applications, provided that ducting requirements can be met and potential conflicts with the existing HVAC-system are addressed. 2.6.1
Passive Direct evaporative cooling systems
Several case studies of volume cooling technologies have been realized – measurements have shown that incoming air with a dry bulb temperature of 35,6°C and a wet bulb temperature of 22,2°C left the tower close to 24°C, which shows a very good performance of this system. 2.6.2
Passive indirect direct evaporative cooling systems
In order to allow for evaporative cooling with roof ponds, roof temperature must be higher than the wet-bulb temperature of the air. According to Givoni5, the necessary condition for
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Givoni, 1994
17
applying this technology efficiently, is that the wet-bulb temperature of the air should be lower than 20°C. Natural cooling of roofs provides in general a cooling for the space below the roof. For the cooling of multi-store buildings additional cooling concepts have to be foreseen.
When the roof is made of materials with high thermal conductivity (e.g. concrete or metal), the combined water and roof structure serves as an integrated storage unit The water temperature of the roof ponds would be about 1-2K above the wet bulb temperature This technology can be applied in places where the wet bulb temperature is 25°C and the maximum dry bulb temperature is 46°C. in this case, no distinction is made to the climatic applicability.
In order to allow for evaporative cooling with moving water films, roof temperature must be higher than the wet-bulb temperature of the air. 2.6.3
Hybrid direct evaporative cooling systems
All direct evaporative coolers can be found on the market as room-sized units. The advantage of direct coolers is their low operating cost – their disadvantage is that humidity control is required and they cannot perform in locations with a high wet-bulb temperature. On the other hand, applied in locations with where the increase of relative humidity does not create significant problems, evaporative cooling can significantly lower energy consumption.
A saturation efficiency of >= 70% - generally the saturation efficiency ranges between 60-90%. A maximum indoor air velocity of 1 m/s (for hybrid systems) Air temperature of the indoor space should be around 2K higher than the discharged air temperature and its relative humidity should be below 70% The resulting temperature of the indoor space should be 4K below the outdoor dry-bulb temperature System depends on wet bulb depression – main climatic criterion for the applicability Indoor air temperature in residential buildings is about 2-4K above the temperature of the air exiting from a mechanical cooler This technology is advisable only if the Wet Bulb Temperature (WBT) in summer during daytime is at least 4K below the comfort limit (26-30°C) in well insulated buildings (so WBT ~ 22°C) – and at least 6K in poorly insulated buildings (so WBT~ 20°C)
2.6.4
Hybrid Indirect evaporative cooling systems
Performance of this device is strongly related to its saturation efficiency - typical saturation efficiency values for those systems are in the range of 60-80%. This technology can only be applied when there is no relevant humidity production in the building. Indirect evaporative coolers can operate only if the indoor wet-bulb temperature is lower than the outdoor dry-bulb temperature In practice the indoor wet-bulb temperature should be lower than 21°C. The treshold value for the use of this system is that the wet-bulb temperature should be lower than 24°C.
A further thumb figure says that the difference between outdoor temperature and the wet-bulb temperature on a typical day should be at least 60Kh A humidity control is not required, since they do not release any water vapour in the inlet air
2.7
Market situation
Table 2.7-1: Producers of Direct Evaporative Coolers Name
Address
Website
Port-A-Cool®
-
www.port-a-cool.com
Climat Controllers Take Control Summit House
40 Highgate West Hill, London, N6 6LS, England.
[email protected]
Essick Air Products
5800 Murray St. Little Rock AR 72209 USA
www.essickair.com
Bessam-Aire
P.O. Box 391525 Cleveland, Ohio 44139
www.bessamaire.com
Quietaire Corporation
505 North Hutcheson Houston, Texas 77003
www.quietaire.com
SPEC-AIR
Sales & Manufacturing 6850 McNutt Road Anthony, NM 88021
www.specair.net
MENERGA Apparatebau GmbH
Gutenbergstraße 51, 45473 Mülheim an der Ruhr Germany
www.menerga.com
S & S Manufacturing
Mesa, Arizona USA
www.swampy.net
Schaefer Fan Company Inc
Jim's Orchid Supplies 4157 Lebanon Rd. Fort Pierce, Florida - 34982
www.jimssupplies.com
Table 2.7-2: Producers of Indirect Evaporative Coolers Name
Adress
Website
SPEC-AIR
Sales & Manufacturing 6850 McNutt Road Anthony, NM 88021
www.specair.net
United Metal Products Inc
1920 E. Encanto Dr. Tempe, Arizona. 85281
www.unitedmetal.com
Al-Ko Therm GmbH
Hauptstrasse 248-250 89943, Jeltingen Scheppach
www.alko.de/lufttechnik/index.cfm
19
2.8
Operation and Maintenance
Evaporative cooling systems require more maintenance than conventional air- conditioning systems. However, maintenance costs are difficult to estimate. The extra cost is for preventive care needed to drain the systems and flush the wetted media to prevent accumulation of mineral deposits. This is particularly important when the evaporative cooler is turned off at the end of summer. Properly maintained, the mechanical reliability of these systems is on a par with conventional systems. With 20 years, the lifetime of this technology is comparable to conventional systems. However, as indirect coolers have a more complex technology, their investment maintenance, operation and repair costs are higher.
2.9
Possible obstacles
Evaporative coolers for air treatment are significantly larger than conventional HVAC units for similar cooling capacity. In addition, there must be sufficient space for the larger air ducts. Evaporative pads in windows block the view through the window and reduce the air flow in seasons and times when ventilation without evaporation cooling is desirable. A removable frame could be used instead a fixed one. Roof Sprinkling/Roof ponds: work only at non-insulated roofs in climates that allow for such roofs. 2.9.1
Hybrid Direct evaporative cooling
Possibility of legionella infection and hygiene requires regular maintenance.
Drawback of direct evaporative cooling systems are the rise of humidity ratio in the incoming air, raising the relative humidity of the indoor space. This works only with sufficiently ventilated spaces.
The use of a relative large quantity of high quality water limits the application of direct evaporative cooling in desert and arid regions, exactly where climatic conditions were preferable for these solutions.
2.9.2
Hybrid Indirect evaporative cooling
Indirect evaporative cooling systems do not create problems related to an increased humidity level. Neither adapted ventilation rates nor humidity control and dehumidification systems are necessary. However as these systems have a more complex technology, their investment maintenance, operation and repair costs are higher.
3 Technology description – “Radiative Cooling” 3.1
Summary-Sheet – “Radiative Cooling”
Description Radiative cooling is based on the heat loss by long-wave radiation emission from a body towards another body of lower temperature, which plays the role of a heat sink. In the case of buildings the cooled body is the building surface and the heat sink is the sky - since the sky temperature is lower, especially during night, than the temperatures of most of the objects upon earth. Sky temperature during summer nights can be
