Tuesday, 23 June 2009

More Hot Summers - More Air Conditioning?

(This article was originally published in issue number 80 (June 2009) of the IEMA journal the environmentalist.)

One of the main challenges in the fight against climate change is dealing with unexpected feedback effects. In many cases, a warming globe creates impacts that lead to even more warming. In this article, we explore the feedbacks between climate change and building heating and cooling systems, and discuss some of the options available to environment managers.

The Met Office has predicted a sweltering summer for 2009. According to the UK’s Chief Meteorologist, “….we can expect times when temperatures will be above 30°C, something we hardly saw at all last year.”

Hot summers are becoming more common as climate change takes hold. While summers in 2007 and 2008 were cooler in many northern latitude countries, the summer of 2003 was the hottest in Europe for at least five centuries and in the UK, six out of the seven warmest years since 1659 have occurred since 1990.

And it’s not just a European phenomenon - eight of the past ten summers in the USA have been warmer than the average for the 20th century.

Climate Change and Building Energy

These hot summers have energy implications: according to Government figures, the USA's residential energy demand was approximately 10 percent higher than what would have occurred under average climate conditions for the season, and it is likely that in the UK, electricity consumption will rise as a result of an increase in air conditioning. Since most of our electricity in both countries comes from fossil fuels, increasing air conditioner use makes it more difficult to meet challenging emissions reduction targets.

In the USA 65% of commercial buildings have air conditioning, compared to 27% in Europe, although a higher percentage of buildings constructed after 1991 rely on air conditioning. One rule of thumb is that a 2°C temperature increase translates into a 25% rise in air conditioning loads. If summers continue to get hotter, will the UK adopt the Continental tradition of afternoon siestas to deal with the heat, or follow the USA’s heavy reliance on round the clock air conditioning?

An indication of what might lie in store for the UK can be gained from looking at air conditioning trends in New England. Historically, electricity demand was greater during the region’s snowy winters due to heating demands and a greater reliance on electric heaters. In summer demand would drop as residents relied on windows and fans to keep cool. But around 2000, peak electric loads shifted to the summer due to the increased use of- and the perceived need for-air conditioning. Now, even in northern New England, peak load has shifted to the summer due to more regular use of air conditioning, and a switch away from electricity for winter heating.

Making matters worse are the unpredictable shoulder seasons of autumn and spring. Lag-times in heating and cooling mean gas-fired heating systems may be competing with air conditioners in those months where cool mornings transition into warm afternoons. Simultaneous heating and cooling is not uncommon, especially in small and mid-size buildings which do not have active management and may not have been properly commissioned. Increasingly variable weather during these seasons due to climate change may mean even greater energy consumption.

Can these trends in increased summer electricity demand be reversed, or will our hotter summers continue to be accompanied by a rise in air conditioning and the related emissions from electricity production? Can we take action to break this positive feedback loop?

Small buildings and air conditioning use

Historically, smaller buildings had a single boiler and thermostat. Now even modest buildings of 4,000 square feet (372 square meters) typically include heating, air conditioning and ventilation systems and automated controls with numerous control devices. These systems are generally design/build – meaning the same firm that designs them, installs them. This approach may result in a lack of independence and transparency in the set up and deployment of the building controls.

Typical problems in small retail and office premises can include:
  • Lack of documentation (i.e., no sequence of operation or controls wiring diagrams)
  • Comfort problems (intermittent overheating in the winter or overcooling in summer)
  • Loss of original intent as subsequent contractors modify the system with limited understanding of existing functionality (e.g., programmable thermostats not set properly for use)

This problem of proper commissioning and air conditioning use can be illustrated in an ongoing project evaluating a 4,200 square foot (380 sq meter) office building in northern New England. A review of the monthly consumption of purchased electricity showed that this building’s electricity usage was 40% higher in August than in January due to air conditioning use even though 2007 was not a particularly hot summer in New England The annual electricity usage amounted to 31,850 KWh causing almost one tonne of CO2e emissions . This indicated an average electricity energy intensity of 8.5 kWh per square foot. Regional best practice indicates an average electricity energy intensity of half this amount, 4.12 kWh per square foot. . Optimization of controls could reduce the building’s electricity usage by at least 15% overall - in this case, cutting annual greenhouse emissions by approximately 150 kg of CO2e.

The Heating Ventilation and Air Conditioning (HVAC) systems of small and mid-size commercial buildings typically do not work as effectively and as efficiently as they might. The deficiencies can result from a lack of expertise in control system diagnostics and operations in the staff and in contractors who typically are on site to perform routine maintenance. In particular, smaller buildings and companies often cannot afford to maintain a facilities manager or employee with facilities management expertise.

These results are not unique to the US. A pilot study in the UK evaluated 20 retail premises for temperature and relative humidity. The results showed that higher summer thermostat settings could improve both thermal comfort and the energy efficiency of air conditioning units. However, despite increased energy costs and the public’s mounting concern over climate change, few UK retail outlets have any plan for managing air conditioning use.

These deficiencies lead to on-going costs, lost personnel time due to comfort problems, increased operating costs as contractors are brought on site to address comfort issues, energy waste, and avoidable carbon emissions.

The building as a system

While proper operational control of energy use is often the starting point for making cost-effective improvements and reducing carbon emissions, it is also helpful to recognize a building as a dynamic system – with energy consumption influenced by its site and orientation, building envelope micro-climate, occupant behaviour and landscaping and the surrounding vegetation.

For example, ground soil and groundwater are both warmer in the winter and cooler in the summer than ambient air temperature. Ground source pumps use these temperature differentials to pre-cool incoming air and reduce the energy requirement of air conditioners in summer, and do the reverse in winter.

Construction materials can play an important role: masonry has a higher thermal mass than glass and steel, and therefore maintains a more even temperature. The lag time between heating and cooling can be used to maintain interior temperatures and reduce air conditioning loads.

Building occupants can be motivated to reduce internal heat gains in the summer by ensuring lights, computers, printers and other electrical equipment is turned off when not in use. Meanwhile staff can be encouraged not to overcool buildings simply because air conditioning is available – many companies are already encouraging casual wear on hotter days to reduce cooling requirements.

Landscaping can provide a shade canopy in the summer, lock up carbon through photosynthesis, and reduce ambient temperatures through evapo-transpiration. Broad-leaf deciduous trees in particular have canopies which reduce passive solar gain in the summer while allowing it when needed in the winter.

This type of holistic view is easier for new-builds, where such considerations can be factored in at the planning stage. Options for cost-effective improvements are more limited with existing buildings. However renovation does present real opportunities to improve the building envelope to manage heat flow. Natural ventilation can be improved by considering the placement of internal partition walls that do not impede cross ventilation, and windows can be retrofitted to make better use of nighttime cooling to lower cooling requirements during the day.
Nearly every human activity has an effect on the climate. Buildings occupy a critical role in modern society, and climate feedbacks threaten to amplify their impact. However, with careful planning, we may be able to break the link between buildings and global warming.

Suzy Hodgson AIEMA is a Principal Consultant and Jamal Gore AIEMA is Managing Director at specialist carbon management company Carbon Clear Limited.