Climate and Design Strategies for Beijing and Shanghai

Paul Rafiuly, Sean Kwok, Christoph Ospelt

Revised: 10/9/1998

• Climate of Beijing and Shanghai
• Comfort Zones and Building Bio-Climatic Charts
• Design Strategies

Climate of Beijing and Shanghai

The analysis of the climate is the starting point for a design that maximizes comfort and minimizes the energy consumption for heating and cooling. One possibility to show the distribution of temperature and humidity over a whole year is to plot hourly climatic data in a psychrometric chart. The horizontal axis of a psychrometric chart shows the dry-bulb-temperature. The vertical axis shows the absolute water content of air in g of water per kg of dry air. Air can take up water up to saturation. The 100% relative humidity line corresponds to saturation of air with water. The warmer air is, the more water it can take up.

The weather data generator "Medpha" from Tsinghua University creates hourly weather data for a full year. There is a statistical probability that a year is warmer or colder than an average year. We used a "high" and a "low" year to cover a wide range of possible weather situations for Beijing and Shanghai. However, an extreme year may have "very high" or "very low" climatic data, but this is statistically a small possibility, so it is not considered as a scenario in our studies.

For Beijing and Shanghai, we plot the climatic data into four graphs- Average Hourly Temperature categorized by months, Average Hourly Horizontal Total Solar Radiation, Annual Temperature Range and Annual Relative Humidity Range. From the graphs we are then able to determine the specific climatic characteristic of Beijing and Shanghai.

BJ-Daily.JPG - Figure 1 Average Hourly Temperature with Hourly Horizontal Total Solar Radiation for the months of January and July in Beijing

BJ-Annual.JPG - Figure 2 Annual Temperature Range and Annual Relative Humidity Range in Beijing

SH-Daily.JPG - Figure 3 Average Hourly Temperature with Hourly Horizontal Total Solar Radiation for the months of January and July in Shanghai

SH-Annual.JPG - Figure 4 Annual Temperature Range and Annual Relative Humidity Range in Beijing

To summarize the climatic data for the whole year, we made a scatter plot with hourly data into a psychrometric chart for each month. The zones with a high density of scatters are colored in figures 5 to 8. The rest of the scatters in each month are contained in the zone delimited by the solid line.

The determination of comfort zones (gray area) in figures 5 to 8 is discussed in detail in chapter Comfort Zones and Bioclimatic Charts.

Figure 5 Climate and comfort zones January to July in Beijing (Zoom)

Figure 6 Climate and comfort zones August to December in Beijing (Zoom)

Figure 7 Climate and comfort zones January to July in Shanghai (Zoom)

Figure 8 Climate and comfort zones August to December in Shanghai (Zoom)

It can be seen from the graphs that in both Beijing and Shanghai, the climates cover a wide range on the temperature scale. In winter Freezing occurs in both cities, however, Beijing shows lower temperatures, down to around -10ºC. Shanghai has only has a few hours of temperatures below freezing. The summers are hot and humid for both cities. The maximum temperatures are somehow higher in Shanghai. Shanghai has a very high humidity level during the summer. In Beijing humidity can also be quite high. As compared to Shanghai, Beijing has a higher diurnal temperature range ( about 10-15ºC ).

Prevailing wind directions and speed are another important issue for climate directed design strategies. Wind data will be available on this page soon.

Comfort Zones and Building Bio-Climatic Charts

At the state of thermal comfort there is no perceived discomfort due to neither heat nor cold. The range of comfortable combinations of air temperature and humidity can be plotted in a psychrometric chart. ASHARAE has published a standard for comfort zones. In Figures 5 to 8 AS represents the ASHRAE comfort zones for summer, AW represents the comfort zone for winter. The difference in summer and winter comfort zones is because of seasonal acclimatization and clothing habits.

Different authors pointed out that the ASHRAE comfort zones are too narrowly limited. Givoni gives a good overview on this issue (Givoni 1998). The principal findings are presented below.

There are two distinct and independent sources for heat discomfort: the thermal sensation of heat and discomfort resulting from skin wetness (sensible perspiration). Depending on the climate the two sources of thermal discomfort will be felt simultaneously or one of the discomfort sources is predominant. In Beijing and Shanghai, where the summer climate is hot and humid, both sources for discomfort will be felt.

Cold discomfort is felt when the skin temperature gets below 32-33ºC. Environmental factors that increase cold discomfort are low air temperatures, draft and low radiant temperatures. Both cities, Beijing and Shanghai have outside winter temperatures much lower than the comfort zone. Different measures for reducing the heat loss and maximizing the gains are necessary in order to reduce the energy consumption for heating purposes.

Acclimatization

The comfort standards published by ASHRAE have been established based on findings for American and European people living in a region of around 40 degrees latitude and who are used to mechanically conditioned buildings. People living in warmer regions generally prefer higher temperatures and often show no thermal discomfort at temperatures outside the ASHRAE comfort zone. This is due to physiological and psychological adaptation to the climate. A rule of thumb proposes to extend the comfort zone by 1ºC for being 12º latitude closer to the equator. The cultural background with intensive or little use of HVAC systems has an influence as well on the feeling for discomfort. This also explains the difference in comfort zones for developing and developed countries, as they are proposed by some authors. A recent study of at Tsinghua University in Beijing confirmed these principles. Temperature measurements in apartments were accompanied by a survey. The apartments did not have HVAC systems. Most of the people did not complain about thermal discomfort even at indoor temperatures of about 30ºC.

The inverse is true for people that grew up in colder regions. They will generally prefer a colder environment and might even feel discomfort at temperatures near the upper end of the ASHRAE comfort zone. Clothing and bedding habits also have a strong influence on the lower end of the comfort zone.

Zone E in Figures 5 to 8 shows the comfort zone taking into account acclimatization and standard of living. The zone for hot-developing countries is shown. For people that grew up in a temperate climate the comfort zone would be smaller.

Daytime ventilation

Increasing the airspeed is an effective means to increase the thermal comfort in summer, as long as the temperature is below 33ºC. It reduces the discomfort due to thermal sensation and in particular due to perspiration. ASHRAE guidelines recommend a maximal airspeed of 0.8m/s. In residential buildings, where no papers fly from desks, the airspeed can be increased up to around 2m/s. There are two ways to attain higher airspeeds:

The first one is natural ventilation by opening windows and doors. The indoor surfaces will then be at a temperature close to the outside temperature. Therefor it is only reasonable to use daytime natural ventilation if we also would feel comfortable outside with the same windspeed that we get in the apartment.

In the case of nocturnal convective cooling (see below), the building's openings should be closed during the day. In that case a simple fan can help increasing the airspeed during the day.

It has been shown that with airspeed of about 2m/s the comfort zone is extended to the area V=2 in Figures 5 to 8. Again, the comfort zone for hot developing countries is shown.

Passive and low energy cooling systems:

The indoor temperature can be kept below the outside temperature by passive and low energy cooling systems. This temperature difference between outside and inside temperature can also be added to the comfort zone graph, where the temperature scale represents the outside temperatures.

Nocturnal Ventilation

The Structural mass of the building can be used for thermal storage together with nocturnal ventilation (night flushing). The potential for lowering the indoor daytime temperature below the outside temperature is proportional to the outdoor diurnal temperature range. For a high-mass, well insulated and shaded building, closed during the daytime and ventilated only during the night, a drop of 45 to 55 percent of the outdoor range is possible (Givoni 1998). At night the indoor temperatures will be higher than the outside temperatures. Very good ventilation is necessary to keep this problem under control.

Beijing has a quite high daily temperature swing of nearly 10ºC during the hot summer months. There is a large potential for nocturnal ventilation. We can expect that it is possible to keep the indoor temperature about 5ºC below the outside temperature during daytime. This extends the comfort zone in Figures 5 to 8 to zone MB. Again, the comfort zone for hot developing countries is shown.

Shanghai on the other hand only has a moderate daily temperature swing of around 5ºC. In addition Shanghai is very humid. Further investigation is necessary to decide if a strategy with nocturnal ventilation or daytime ventilation is more promising. The inside temperature is expected to be about 2-3ºC below outside temperature with nocturnal ventilation, which extends the comfort zone to MS.

Direct and Indirect Evaporative Cooling

Evaporative cooling can be very efficient in hot arid climates, which is not the case for Beijing and Shanghai. With evaporative cooling the comfort range can be extended to temperatures as high as 45ºC (not shown in figures).

Design Strategies

From our study of the climatic data generated by "Medpha", we are able to suggest a few design criteria that aim to enhance the thermal comfort level in buildings for both Beijing and Shanghai. We divided these design strategies into four categories: Orientation, Building Envelope, Thermal Mass and Ventilation. Most of the design strategies suggested are in general applicable for both Beijing and Shanghai with a few that are more relevant to one of the cities according to its specific climatic characteristics (AIA 1978).

Orientation

Relevant to both Beijing and Shanghai

Relevant to both Beijing and Shanghai

Relevant to both Beijing and Shanghai

Relevant to both Beijing and Shanghai

Relevant to both Beijing and Shanghai

Building Envelope

Relevant to both Beijing and Shanghai

Relevant to both Beijing and Shanghai

Relevant to both Beijing and Shanghai

Relevant to both Beijing and Shanghai

Relevant to both Beijing and Shanghai

Thermal Mass

This is more particular for Beijing with its fairly large diurnal temperature range. The thermal mass created by building material can help to dampen indoor temperature fluxuation in comparison with outdoor temperature

Ventilation

Relevant to both Beijing and Shanghai

Relevant to both Beijing and Shanghai

Relevant to both Beijing and Shanghai, but may be particularly in Shanghai with its higher relative humidity all year round

Relevant to both Beijing and Shanghai

References:

Givoni, Baruch: Climate Considerations in Building and Urban Design. Van Nostrand Reinhold, 1998. 

AIA Research Corporation: Regional Guidelines for Building Passive Energy Conserving Homes. US Department of Housing and Urban Development, 1978.

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