measure of sustainability exergy

Measures of Sustainability
Overview / Embodied Energy / Operating Energy / Exergy / Durability / Externalities / Ecological Footprint / Eco-Labeling / Life Cycle Assessment

Exergy (Absolute Energy Efficiency)
The term "exergy" or absolute energy efficiency is used to define the combination of energy quantity (which is conserved according to the first law of thermodynamics) and energy quality (which is consumed according to the second law of thermodynamics).

Exergy = Energy Quantity X Energy Quality

Energy is efficiently used when the quality of the source is matched to the quality demanded by the task. Thus, electricity is a thermodynamically sound way to drive the motor that agitates the clothes in the washing machine. It is not a thermodynamically sound way to heat up the washing machine's water. By thermodynamically matching sources to tasks, we can avoid the enormous waste of using high quality energy for low quality tasks, and minimize the growing social and economic costs of energy production.

Ralph Torrie, Half Life: Nuclear Power and Future Society. A Report to the Royal Commission on Electric Power Planning, Ottawa: Ontario Coalition for Nuclear Responsibility, 1981.

The table reproduced below provides an interesting perspective on the "brute force" associated with conventional means of powering and conditioning buildings, which has led to the view that in most cases, we are using a "chainsaw to cut butter."

Energy Use and Absolute Efficiencies for Mechanical Devices
[Source: Energy and Power Needs and Availability in Housing, CMHC, Ottawa, 1993.]

Based on the data in the table, current lighting technology has a maximum exergy efficiency of 0.8%. This means that 99.2% of the electrical energy used by the most efficient lamp does not produce visible light, which is its intended purpose, but rather waste heat. If an ideal exergy efficiency for lighting could be attained, a current 100 Watt light bulb would be replaced with an ideal 1 Watt alternative. The truly inefficient means of energy use associated with our present way of life may appear discouraging for pessimists, but for optimists it signals a multitude of opportunities - some technological and others cultural.

For example, consider the mechanical cooling of buildings which has an exergy efficiency of 8.7% at best. By altering cultural expectations of air conditioned buildings, and coupling this to buildings which are less prone to overheating (proper fenestration and shading devices), acceptable comfort could be attained through the use of fans which have a 24% exergy efficiency. This translates into a two-thirds reduction in energy use for cooling, and possibly more if natural ventilation were emphasized. Add to this improvements in lighting and audio-visual equipment exergy efficiencies, and the waste heat associated with these devices would even further reduce the demand for cooling.

A critical examination of exergy as a measure of sustainability suggests that many of our so called labour saving devices are simply energy wasting gadgets that exacerbate the declining health and fitness of their users. There are some who claim the time modern man saves with these devices is subsequently squandered on home entertainment technologies with incredibly marginal exergy efficiencies, signaling our modern surrender to entropy. Such culturally based behaviour is beyond the control of the architect, however, it is important to recognize there remains tremendous potential to improve the absolute energy efficiency of many of today's equipment and appliances which account for much of the energy consumed by buildings. Since many of the needed and welcome improvements in exergy efficiency will be derived from outside of the architecture field, it is important that architects are aware of appropriate energy sources, appliance and equipment selection. Refer to the Related Resources + References page for further information on exergy efficiency.

From an exergy efficiency perspective, building technologies which rely on solar, wind and biomass energy sources, coupled to thermally efficient envelopes, appropriate fenestration strategies and natural ventilation/cooling are truly more elegant than "cutting butter with a chainsaw." Exergy efficiency remains to be fully considered by mainstream architectural science research and development initiatives, however it may be expected to gain importance as a critical measure of sustainability.

The next section deals with Durability as a measure of sustainability.