Intelligent Night Cooling

1. NatVent project,WP3/A3.4 A.H.C. van Paassen and S.H. Liem April 1998 Delft University of Technology Subfaculty:Mechanical Engineering and Marine Technology Section:Refrigeration Engineeringand Indoor Climate Technology Intelligent Night Cooling Intelligent Night Cooling Intelligent Night Cooling How to cool? How to desing? Comfort crit. Design rules Design equations How to control? Strategies Examples Benefits & limitations More information Back to “WP3” NatVent

2. How to cool? (1…) • Natural driving forces can be used for night cooling: • 1. Wind pressure around the building. • 2. Stack effect by temperature difference between the in- and outside air.

3. How to cool? (2…)single sided ventilation • Single sided ventilation: • outside air enters and leaves the room through openings on one side of the façade. • Main driving force is wind turbulence. High and low placed vent openings increase ventilation.

4. How to cool? (3…)cross ventilation • Cross ventilation: • Outside air enters the building from one side and leaves the building on the other side.It depends on wind direction and building orientation. • Cross ventilation gives higher ventilation rates than single sided.

5. How to cool? (4…)ventilation based on thermal stack • Stack ventilation: • The difference between indoor and outdoor temperature is the driving force. room room room room shaft room room Natural night ventilation with a chimney or a shaft

6. room room room room shaft room room Fan assisted night ventilation with a chimney or a shaft How to cool? (5|)fan assisted ventilation • Fan assisted ventilation: • Cooling effect can be increased with an chimney or fan.The fan will be switched at low wind speed. • Disadvantage is energy use of the fan.

7. How to design? (1...) The design of night ventilation: • The aim is to achieve an acceptable indoor summer comfort • SIMULATION PROGRAMS can be used to calculate the indoor summer climate for different: • night ventilation systems, • night ventilation strategies, • building characteristics: internal mass, glazing ratio, .. • ….. But…...TIME CONSUMING…... Need for simple design rules that can be applied for given conditions at the initial design stage

8. How to design? (2|) In the framework of this Work Package • design rules & • design equations have been developed based on the results of many dynamic simulations with the simulation program SIMULINK Remark: The design rules and design equations are only valid for the external climate of The Netherlands (De Bilt, NL) and similar climates. SIMULINK: dynamic simulation for different buildings and ventilation systems and strategies Design Rules Design Equations

9. Comfort criteria • Generally accepted comfort criteria: • No more than 100 hours in a year with the indoor temperature above 25.5 °C. • 28 °C must not be exceeded for more than 25 hours.

10. Design rules (1...)based on simulation program • Design rules are based on simulation of thermal and ventilation processes. • Input weather: Dutch Reference Year of De Bilt. • Simulation tool: SIMULINK.

11. Effect of Building Thermal Mass Internal load = 30 W/m² Cross Ventilation 250 High Medium Low 200 150 100 [hours > 25.5 °C] 50 0 0 1 2 3 4 5 Ratio Effective Vent Opening to Floor Area [%] Design rules (2...)effect of heat accumulation • Night ventilation is only effective when the building thermal mass is medium to high. • Use open ceiling and interior walls of bricks. • Low thermal mass buildings not appropriate.

12. Design rules (3...)area of effective vent opening • When using cross ventilation, comfort can be achieved with a ratio of 2.0 % effective opening to floor area when the internal heat gain < 30 W/m2). • 1% opening will lead at 30 W/m2 to discomfort (200 hours > 25°C).

13. Design rules (4...)single sided versus cross ventilation • Cross ventilation is better than single sided ventilation. • For single sided ventilation vent openings must be doubled (compared to cross ventilation).

14. Design rules - overview (5...) • Use an effective outdoor solar shading device. Without that, natural ventilation cooling is out of the question ! ! ! • Only appropriate in medium to high thermal mass buildings are appropriate. • Provide open ceilings and exposed interiors masonry walls to provide sufficient thermal mass. • An effective vent opening of at least 0.02 m2 per m2 floor area (2%) is needed for cross ventilation. For a standard room of 20 m2 an effective vent opening of 0.4 m2 is necessary. • Single sided ventilation requires 1.9 times larger openings. • The internal gains should not exceed 30 W per m2 floor area.

15. Design rules (5|)example Assumptions: • external climate: De Bilt, NL. • cross ventilation • high or medium internal mass • internal gains <= 30W/m² • effective solar shading !

16. Design equations for Aeff (1...)derived from simulations Simple equations are derived from simulation runs…. • Input: • Ventilation strategy: cross ventilation • Qi = internal heat gains from occupants, machinery and lighting [W/m2]. (20 is low, and 40 is high). • Comfort objective: N = the number of hours that the indoor temperature is greater than 25.5°C (100 hours is design criteria). • M = mass of the walls of a room divided by the total indoor area [kg/m2]. i = average density of wall;Ai = area of wall [m2];di = thickness of wall [m],  0.15 • Control strategy. • Output • The output is the effective area of vent openings for various ventilation systems. • Sensitivity • Example: • Effective opening as a function of mass. • Effective opening against number of temperature exceeding.

17. Design equations for Aeff (2...)from simulation to equation = g-value

18. Design equations for Aeff (3...)as function of internal gains • Effective opening in % floor area as function of internal heat gain Qi(20, 25 & 30W/m2) at various comfort: criteria: • Temperature exceeding 25.5°C for N = 50,100,150hours • Conditions: • Outside shading • 40% window • Medium M=75 • Equations can be used for fast analyses of cross ventilation

19. Design equations for Aeff (4...)as function of comfort criteria • Effective opening in % floor area as function of various comfort criteria. Temperatures exceeding 25.5°C for N = 50, 100, 150 hours at various internal heat gains Qi: (W/m2 ) = 20,25,30 • Nonlinear relation: Higher comfort requirements much larger openings. • Conditions: • Outside shading • 40% window • Medium M=75 • Cross ventilation

20. Design equations for Aeff (5...)sensitivity analysis - internal mass • Medium building (M=75) effective opening must be 2% of the floor area.(0.4 m2 in standardoffice room) • Conditions: • Outside shading • 40% window • Comfort 100 hours< 25.5 °C • Cross ventilation

21. Design equations for Aeff (5...)graphical design tool • Based on the same simulation outputs a graphical design tool is made. • No knowledge about building physics is required. • Choice of internal heat gain, window system, building mass and control lead to the night cooling system with required vent openings. • Design • Select: • Internal heat gain. • Shading • Mass • Control • Find system and effective opening in green area. • Output: …..

22. How to control? (1...)objective • The vents should be controlled such that during night the building is pre-cooled in a proper way. • The next morning the building air temperature should be sufficiently low so that no overheating will occur during the day. (However one has to take care that the temperatures are not too low in the morning.)

23. How to control? (2...)Effect of control strategies • Night cooling is very effective. • No difference between predictive control strategies proposed in literature. • Strategy with the lowest risk of undercooling is proposed. = no night ventilation

24. How to control? (3...)Cooling day control • Rule based predictive control: • If during the day vents were open for cooling the night setpoint is decreased with 2 K. The start value is 22 °C andthe minimum is 18 °C. • During office time the temperature setpoint is 22 °C.

25. Examples of controlled natural ventilation (1…)Field test controlled windows • Cooling Through Motorized Windows • Wind for fresh air supply and cooling. • A control system is setting cooling by motors fitted to vent windows. • The room controllers can be overridden manually. • Indoor temperatures monitored during summer 1993. • Automatic control of vent windows keeps temperatures comfortable. (internal gain 25 W/m2 ). • During extreme warm conditions in July 1994, temperatures were too high, but night cooling makes it acceptable.

26. Examples of controlled natural ventilation (2…)Demo of night cooling • Fresh air controlled with trickle ventilators and cooling with controlled vent windows. • In a normal summer comfort. • Energy consumption is dramatically lower than air conditioning.

27. Examples of controlled natural ventilation (3…)Components for automatic controlled natural ventilation • Control of night ventilation needs electronic devices. • Electric actuators for windows and electronic trickle vents are available commercially.

28. Examples of controlled natural ventilation (4…)The Franc-unit • Fresh Air Night Cooling-unit. • Filter • Attenuation • Not flexible. • Easy to apply. • 4 units / room (0.4 m²)

29. Examples of controlled natural ventilation (5…)The Franc-unit / the different settings Nobody in and no cooling Fresh air supply Night cooling • Dampers closed • Main damper turns to right in vertical position and filter stays horizontal • Filter in operation. • Position main damper is adjusted to deliver fresh air. It depends of wind speed and direction • No resistance of filter and max. opening. • airflow maximal for cooling

30. Examples of controlled natural ventilation (6…)The Franc-unit / the prototype • A first version is made • Control algorithm for the supply of constant fresh air is tested by simulation • Adaptive control is used to compensate the non linearity of the sensor signal. • A simple self made speed sensor is used, mounted on the edge of the damper

31. Examples of controlled natural ventilation (7|)Applications • Opening vents at the bottom and trickle ventilators at top. • A similar system is used in Urban Villa Amstelveen with flap windows instead of trickle vents.

32. Benefits & Limitations • Benefits • Natural ventilation is energy efficient, even fan assistance is more efficient than HVAC. • Natural ventilation systems are preferred by building occupants. • When natural ventilation is not sufficient , it can be supplemented by HVAC (first natural then mechanical). • There are limitations • natural ventilation may not be able remove all the heat gains from people, lighting and machine. • Comfort requirements may not be met in a building with internal heat gains higher then 30 W per m2 floor area. • In a well designed building: - windows glass area less then 40% of the facade - outside solar shading - (open ceiling and interior walls). • The comfort requirements of a building with internal heat gains of 30 W/m2 are equivalent to a room with 2 persons, 2 PC's and adequate electric lighting .

33. Intelligent Night Cooling More information... Intelligent Night Cooling You can find more information on: • the Simple Design rules and Equations • the Prototypes for controlled Night in the following documents: \Reports\ Technological solutions\ 3.4_night cooling1.pdf(Simple Design rules) 3.4_night cooling2.pdf (Prototypes) How to cool? How to desing? Comfort crit. Design rules Design equations How to control? Strategies Examples Benefits & limitations You can read and print pdf-files with the Acrobat® Reader ®3.0. Program. This program is free. Download it from the Acrobat web site: www.adobe.com OR run the installation file ar32e301(1).exe in the directory \Installation More information Back to “WP3” NatVent