Heating and cooling benefits of CO2

Westphalen et al (1999) prepared a report dealing with thermal distribution, auxiliary equipment and ventilation in the energy consumption characteristics of commercial building HVAC systems comprising 3,345 million m2 of cooled building floor space plus 4,459 million m2 of heated building floor space. The total annual HVAC primary energy consumption was 4.85 x 109 GJ. (see Fig. 1; Westphalen et al, 2001). Figure 2 (Westphalen et al, 1999) shows that, of the total parasitic primary energy use in HVAC systems in the healthcare category, the supply and return air fans consumed 43% of a total of 0.132 x 109 GJ, i.e. 56.8 x 106 GJ. The exhaust fans consumed a further 37% of parasitic primary energy, i.e. 48.8 x 106 GJ. In Table 1, seasonal efficiencies for different cooling equipment and the distribution of the equipment as applied in the healthcare building category (Westphalen et al, 1999) have permitted the evaluation of a weighted coefficient of performance (COP) for the entire healthcare building category.

Principles of reducing energy consumption

 Building heat load components The heat loads generated in a building are due to heat gains through the building structure, windows, power consumed by medical and office machines, lighting, air leakage into the building, ventilation air, and people. Once the heat load has been determined, an air conditioning plant usually comprising both heating and cooling functions is designed and installed. The compressor heat is rejected to atmosphere via a cooling tower providing cooling water to a condenser or via an air cooled condenser. Chilled and hot water is pumped around to air handling units (AHUs) for cooling and reheating, or heating only during winter. Fans circulate air through ducts and return air fans return air to the AHUs. Exhaust fans remove a minimum quantity of air, which is usually regulated by public health authorities Figures 1 and 2 show that the fans constitute a very significant energy consumption in buildings. It is noteworthy that the fans are installed in the AHUs and return air streams and, consequently, all the energy supplied to the fan motors, including losses, shows up as heat in the air stream, which thus increases the heat load to be handled by the compressor. Reducing energy consumption To reduce energy consumption in buildings it is necessary to reduce the amount of heat generated during hot weather by all constituent components, and ensure that the cooling plant runs at the Most Energy Efficient Operating Point (MEEOP). The supply and return fans constitute a large energy consumer, and add a significant heat load to the system as well. The fan laws state that a 25% reduction in air flow reduces the energy consumption to 0.753, i.e. a reduction in energy consumption of 1 – 0.753 = 57.8%. There is also a corresponding decrease in compressor power requirement equivalent to the fan power reduction divided by the COP. In existing buildings, a reduction in air flow needs to be compensated for by a lower air temperature, which in turn requires a lower chilled water and/or refrigerant evaporating temperature, To. A lower To reduces the compressor COP, which would tend to increase compressor-specific energy consumption. In new buildings, it would be possible to design ductwork for lower velocities, thereby saving considerable energy. Initially, the reduction in fan and resulting compressor energy consumption is greater than the increase in compressor energy consumption due to a reduction in the compressor COP. This constitutes an optimisation problem identical to that found by Visser (1976) when blast freezing export meat cartons (See Fig. 3). This shows that it is frequently more energy-efficient to select a compressor with a lower COP which needs to handle a much lower heat load. This principle is not widely understood, but universally applicable in cold air chilling and blast freezing systems, including air conditioning cooling, where the energy consumed by auxiliary equipment such as fans, chilled water pumps and refrigerant pumps is converted to heat to be removed by the compressor.