Avoid overheating in mild climates: experiences
LACHAL, Bernard Marie, et al.
In Switzerland, even if the average daily temperature rarely exceeds 25 °C during the three summer months, more and more cooling plants are being installed in administrative buildings.
The CUEPE participated as experts in some realisations that showed (simulations and monitoring) that it is possible in most of the cases to stay within the comfort limit and to avoid air conditioning by using efficient solar protection, efficient envelopes and natural cooling systems
LACHAL, Bernard Marie, et al . Avoid overheating in mild climates: experiences. In: PLEA 2000, Passive and Low Energy Architecture . 2000.
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Avoid overheating in mild climates:
B. LACHAL, W. WEBER, P. GALLINELLI, P. HOLLMULLER, C. SOUTTER
University Center for the Study of Energy Problems, CUEPE Université de Genève
7 route de Drize CH - 1227 Carouge/Ge
In Switzerland, even if the average daily temperature rarely exceeds 25 °C during the three summer months, more and more cooling plants are being installed in administrative buildings. The CUEPE participated as experts in some realisations that showed (simulations and monitoring) that it is possible in most of the cases to stay within the comfort limit and to avoid air conditioning by using efficient solar protection, efficient envelopes and natural cooling systems.
Even in countries like Switzerland, where in cities the average daily temperature rarely exceeds 25°C during the three summer months, more and more cooling plants are being installed, principally in administrative buildings.
Nevertheless in this kind of climate, different natural cooling strategies are possible:
• Efficient solar protections
• Reduce internal gains
• Night cooling
• Evaporative cooling
In a few case studies around Geneva, the CUEPE experienced and applied some of these solutions and demonstrated the possibility to improve summer comfort without using mechanical air conditioning systems.
OFFICE BUILDING OF THE
"INTERNATIONALE METALWORKER FEDERATION”
Figure 1, south-east facade with detail of the window
In 1998, the “ International Metalworker Federation ” decided to install air-conditioning in their very large, seven storey administrative building (figure 1), after complaints from tenants. Different solutions of cooling plants were
proposed. But all these enhancements were very expensive to install and to run. Therefore the syndicate asked the CUEPE to evaluate other possibilities. As external expert, the CUEPE decided first to measure the summer comfort and to compare it to the law that requires indoors temperatures below 28°C as long as the outdoor temperature is less than 30°C.
Eight different spaces were monitored between July and August 1998 every fifteen minutes on the third floor as well as seven other spaces on the fifth floor.
The measures of this summer showed that the offices on the south-east and south-west facades were much too hot with temperatures over 30°C when the outside temperature did not exceed 30°C (figure 2). With an external temperature of 38°C on August, the internal temperature
reached 34 °C.
Figure 2, measured daily maximum temperatures during summer 1998, compared with law requirement.
There was no relevant difference between the third and fifth floor. So it became evident that the summer comfort had to be improved in these parts of the building.
Simulations were performed using the software SUNREL, a Swiss adaptation of the American software SERIRES. The simulations showed that the predominant heat gains were not due to internal gains, that were quite low, but to the inefficient solar screens locate in-between the double glasses of the windows. The solar protection was much lower than the 85% proposed by the Swiss norms.
The surface of the windows (4m2) was also very important for an office of 17 square meter. The panels under the windows did not bring much heat even if they were not well insulated.
indoor temperature °C
The proposed solutions were first to put solar protections on the outer side of the windows in order to ensure around 90% protection against solar radiation.
Simulations showed that without cooling plants, inner temperature could be limited less than 28°C (figure 3) excepted on very hot days (more than 32°C outside temperature).
Figure 3, maximum daily indoor temperatures with efficient solar protection
When favouring the night or morning ventilation by occupants or with automatic opening of windows the comfort could be further improved.
If needed, an office can then be cooled to around 23°C with small mechanical devices with low power.
The architects in charge of the project designed a solution with Venetian blinds leaving a small space between them and the glasses in order to evacuate overheating by ventilation and allowing slight openings of the tip windows, even with the blinds closed, in order to protect against rain and robbers.
Cost is expected to be less than half the price of a cooling plant.
Actually under construction, the building will be monitored again during summer to compare the result with the simulations.
BUILDING FOR THE ARCHIVES OF THE CANTON TICINO
In 1990, the architect Luca Ortelli won the competition for the “Archives cantonales du Tessin” (figure 4).
This building has offices, workshops and storage space.
The CUEPE was involved in the research of a good solution to maintain the storage at a temperature between 15 and 18°C with a humidity of around 50 to 60% in order to preserve precious documents and objects.
As the temperature could be quite high in summer in this part of Switzerland, it was necessary to find a cooling source.
Simulations with the software SERIRES, showed that with only two centimetres of insulation or, even better, with no insulation, this underground space could take advantage of the earth temperature around 14-15 °C during summer (figure 5).
Figure 5, the simulated temperature of the storage space without insulation during one year.
The floor of the storage space was realised without any insulation between ground floor and the earth. A very simple ventilation system was installed with possibility to dry the air.
The building was completed in 1998, and the measured temperatures in the storage space follow predicted simulated temperature.
We finally test small portable engines based on evaporating cooling for individual use in offices. Both lab and in situ measurements concerning one of them are presented.
Characteristics of the device
The manufacturer of the tested cooler (“Convair Cool- Master”, model 800B) recommends to “place the cooler near a open window or door so it can draw in 100% fresh air. Open another window or door on the opposite side of the room for flow-through ventilation”. A small pump wets a filter pad placed at the input side of the air. The output air is then fresh and humid.
Table 1 gives the characteristics of the cooler as measured in automn (T = 17 °C, H=58%).
The humidity efficiency is close to 70%, a typical value for such devices. The evaporated water flow (1.5 to 2.1 litres per second) corresponds to a nearly constant increase of water in air of 2 gr. of water per cubic meter of air passing through the cooler. If the air-change inside the room is not sufficient, it will result in a humidity accumulation and consequently in a decrease of cooler performance.
indoor temperature °C
Figure 4, section of the archives of the Canton Ticino
PARAMETERS FAN SPEED 1 FAN SPEED 2 FAN SPEED 3
Output air velocity, m/s 3.1 3.8 4.5
Humidity efficiency*, % 65 68 68
Output air flow, m³/h 815 1000 1180
Water evaporation, l/h 1.5 1.7 2.1
Water content increase, gwater/m³air 1.8 1.7 1.8
Cooling power, W 1040 1180 1460
Electricity consumption, W 40 56 81
Coefficient of performance 26 21 18
Noise level, qualitative acceptable high high
*Humidity efficiency = (Houtlet - Hinlet) / (100% - Hinlet), H : relative humidity.
Table 1. Measured air cooler characteristics.
Following the manufacturer’s recommendations, one can roughly estimate that the air flow through the cooler has to be put from the outside and then, as a first thumb rule, the condition of air after the cooler has to be computed with external conditions. High coefficients of performance (up to 26) are reached for these low priced devices. Finally, the noise level is only acceptable with the low fan speed when people are working.
Following  and direct observations, we can distinguish four zones in the air jet expansion:
• a first zone with a nearly constant air velocity, during some ten centimetres after the air outlet,
• a transition zone, extending to about one or two meters from the air outlet, depending on the fan speed and the room, where the air velocity decreases slowly,
• a turbulent zone, extending from the end of transition zone to about 5 – 8 meters, where the air velocity decreases more fast,
• a terminal zone, where the air velocity decreases below 0.25 m/s.
The observed air jet is an ellipsoid with an aperture angle close to 10°. At the terminal zone (typically 5 m with fan speed 1), the dimension of the jet is about 1.4 m high and 2.1 m wide; the lateral transition zone, where warm air aspiration takes place, being less and less clear when the turbulence increases.
Experiment in an office building
For a practical test of this device, the chosen office has the following characteristics:
• It is situated in an old building in the historical city of Geneva, with a high density situation. Urban heat island is very effective in the morning, (5 to 8 degrees more than the airport minimum, 3 degrees more for the monthly mean in summer, reaching 25°C in July instead of 22°C), giving spontaneously high temperatures inside this building. Night ventilation is insufficient to cool it.
• The office is occupied by one person, it’s area is 28 m² and only the south-east facade is in contact with the outside (courtyard). The solar protections are weak because of the historical character of this building.
• The evaporative cooler was installed from the end of July to mid-August 1999. No extreme climatic conditions were present during this rather fresh summer. The cooler was installed in front of the opened window, at 3 meters to the desk, the occupant is in the turbulent part of the air jet and rather in the
border, with a mean air speed of about 1m/s to 1.5m/s.
He switches on the device when he needs a better comfort. Only the lower fan speed was used owing to the noise.
The monitoring included :
• 4 temperatures and the humidity: external (ventilated), inlet and outlet sides of cooler, on the desk. The position of the occupant is between the two last thermometers,
• the quantity of water put inside the cooler,
• the quantitative comfort of the occupant.
Very good comfort conditions are obtained, the cooler being used only some hours during some days. A hotter summer would probably tend to more use. Only 24 litres of water were evaporated during some 20 hours, leading to a mean evaporation of 1.2 l/h, the same order of magnitude than the one obtained in lab (1.5l/h). Electrical consumption is also very low (less than 1 kWh for two weeks and for 28 m²).
Curve 1 shows the temperatures and humidity measured during two consecutive days (Monday and Tuesday) with typical summer weather conditions (not extreme) : clear days, minimal temperature : 22°C, maximal : 30°C, some wind during daytime ( less than 4m/s). During the first day, the occupant was absent. He was in his office on the second day. (figure 6)
From an initial temperature of 29°C, close to the external conditions, the cooler gives an outlet air jet at less than 24°C, increasing to about 26°C at the occupant position.
The inlet and office temperatures have also decreased by more than one degree: this shows that the input air in the cooler is a complex mixture of external and inside air. Air humidity in the office remains acceptable (60%), corresponding to an increase of 15% as compared with the situation without air cooler.
This experiment demonstrates that the comfort of the occupant is obtained only by the fresh airflow, the room air temperature regaining very fast the initial conditions after the cooler was stopped. Every change of the occupant’s position led to a change of air conditions (the further away from the cooler or the jet centre, the less air velocity as well as the air being warmer and dryer). This allows a fine- tuning for obtaining good conditions. This last point might be less true if such a device is used for more than one person.
Figure 6. Temperature (left) and relative humidity (right) during two consecutive days, without and with the air cooler.
1. ASHRAE, 1989 Handbook, Fundamentals
2. Norme SIA 180 "Isolation thermique des bâtiments", Edition 1988. .
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