Somfy – The energy effects of controlling solar shading

A paper produced by Somfy and Building Research Establishment (BRE) examines the energy and thermal comfort implications of installing solar shading. Two buildings were simulated under a range of different shading and control systems (both automatically and manually controlled) in three different UK locations. The study recorded the calculated delivered energy, carbon dioxide emissions, energy costs and internal temperatures for the same five cases of selected shading systems.

Key Findings:

  • “Additional CO2 savings with automatic control of shading, of around 3% compared to simple manually controlled of fixed systems”
  • “…an automatically controlled shading system gave building CO2 savings of 9% (with internal blinds) and 8% (with external blinds) in the London area compared to the no shading case.”
  • “The benefits of shading are latitude dependant; in Scotland installation of external shading gave an energy penalty of between 3% and 9%.”
  • “Thermal comfort is critically important too.”

Further Details:

In this simulation study, two types of buildings were modelled: an air-conditioned office, and a naturally ventilated hospital block. These were monitored and reviewed in three different locations in the UK: London, Manchester and Edinburgh. Each type of building in each of the above locations was modelled under the five different types of shading below:

  1. internal shading (roller blinds) that were manually controlled;
  2. internal as above that were automatically controlled with a manual override;
  3. fixed external shading (simple overhang);
  4. moveable external shading (venetian blind and closed when lowered) that was automatically controlled with a manual override; and
  5. no shading as a base case.

In the analysis, the addition of shading always produces a reduction of cooling demand and an increase in artificial lighting and, nearly always, heating. The only exception is in the hospital where adding internal blinds slightly reduces the heating load. This is because of heat loss through windows during cold nights.

The results for the office building show that substantial cooling savings are possible. External automatic shading control achieves the biggest savings in cooling energy compared to the no shading case. It gives a 66% reduction in London, 77% in Manchester and 85% in Edinburgh. Cooling savings are best achieved with external devices that prevent solar gains. When these savings are balanced against the increases in heating and lighting energy use, internal automatic shading produces the best results in all locations, with a total delivered energy reduction of up to 6.6% in London.

Comparison of the automatically controlled external shading with the simple fixed overhang shows that the controlled shading gives an additional 5% reduction in energy cost in London, 2% in Manchester but an energy penalty (increase in energy cost) of 6% in Edinburgh. This is due to the colder climate and the increase in space heating costs in winter because the modelling takes into account occupants adjusting the shading to reduce glare. A combination of automatically controlled internal shading (winter) and external shading (summer) could result in the best strategy for energy savings and comfort levels in an air conditioned building.

When reviewing the results from the naturally ventilated hospital (no cooling installed), it was observed that any form of shading increases the energy use either in terms of heating or lighting (except for the one case where internal blinds were installed in a London hospital and heating loads were reduced). Therefore, internal shading results in an energy penalty of 1-3% with and external shading a penalty of 7-12% compared to the no shading case.

However, energy is not the only important issue. Thermal comfort is critically important too. A typical thermal comfort criterion for naturally ventilated buildings is a temperature of 25°C over less than 5% of occupied hours. Temperatures of over 28°C are very uncomfortable and should be avoided. The hospital design is particularly prone to overheating, especially in Southern England. Taking the hospital as a whole, temperature of 29°C is exceeded 9% of the year with no shading. Internal shading reduces the temperature slightly, fixed external shading gives a further reduction, but the most effective system uses the moveable automatically controlled shading. Further north, moveable automatically controlled solar shading limits overheating to a few ‘hot spots’.

Download a copy of this report here.

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