Energy IEQ

Energy Consumption and CO₂ generation of some IEQ policies

Brief:

A good energy policy for the indoor environment should respond to both the needs of energy conservation and the needs for a desirable indoor healthy environment with a reduction of carbon dioxide (CO₂) generation.

Evaluation for the choice of environmental conditions attributed by air temperature and CO₂ concentration

Energy implications and the corresponding CO₂ generation of indoor environmental policies for air-conditioned office buildings in the subtropical climate is now a matter of concern. The thermal energy consumption in an air-conditioned office building was evaluated by the heat gains through the building fabric, the transport of outdoor fresh air for ventilation, and the heat generated by the occupant and equipment in the space respectively. With the Monte-Carlo sampling technique and the parameters from the existing office building stocks of Hong Kong, the energy consumption profiles of air-conditioned office buildings in Hong Kong were evaluated. Energy consumption profiles were simulated for certain indoor environmental quality (IEQ) policies on indoor air temperature and CO₂ concentration settings in the offices, with other building parameters remaining unchanged.

Figure: Per-floor-area yearly CO₂ generation at the power plant due to thermal energy consumption of air-conditioned offices in Hong Kong (Source: Energy Policy 2008:36, 714-721).

Treatment for outdoor air for air-conditioned office require energy

We studied the energy consumption of ventilation systems in air-conditioned offices. A benchmarking model was developed from the fundamental psychrometric analysis under probable office design conditions in Hong Kong. The results showed that the annual energy consumption of a ventilation system per floor area would be correlated closely with the carbon dioxide (CO₂) concentration in the space, but its correlation with the air temperature set point would be less significant. In some offices, significant energy saving potential was demonstrated to provide satisfactory indoor air quality (IAQ) without any comfort penalty to the occupants. This model would be useful for the energy performance evaluation and benchmarking of ventilation systems in air-conditioned offices.

Demand control ventilation

Ventilation to supply fresh air in an air-conditioned office consumes a considerable portion of energy in an air-conditioning system and affects the indoor air quality (IAQ). The ventilation demand is primarily related to the occupant load. The ventilation demands due to occupant load variations were examined against certain IAQ objectives using the mass balance of carbon dioxide (CO₂) volume fractions in an air-conditioned office. In particular, we proposed a transient ventilation demand model for occupant load, with the parameters determined from a year-round occupant load survey in Hong Kong. This model was applied to evaluate the performance of energy saving in different operating schedules of ventilation systems for typical office buildings in Hong Kong. The results showed that the energy consumption of a ventilation system would be correlated with the transient occupant load and its variations in the air-conditioned office. The ventilation system with schedules taking account of the transient occupant loads would offer a reduction in energy consumption up to 19% as compared to an operating schedule that assumed a steady occupant load in office during working hours. In both cases, the same IAQ objective was achieved.

Some applications for energy impact assessment