HVAC Savings?
HVAC Savings?
Do lighting retrofits also help save heating, ventilation, and cooling (HVAC) energy? This is a much-debated issue, and the answer is a complex one.
Simplified rules of thumb and engineering studies on individual buildings often conclude that energy saved in lighting translates into additional net savings in space-conditioning energy. A 30-40% increase in lighting energy savings is often assumed to account for reduced air-conditioning demand.
However, "waste" heat from lighting can represent useful energy during periods when space heating is required. This must be weighted against increased energy use when air conditioning is required. The net effect of interactions among lighting systems and heating, air-conditioning, and ventilation (HVAC) systems depends on a spectrum of technical and economic variables.
Technical factors include the extent to which lighting savings coincide with periods when the building requires space conditioning and the relative efficiencies with which heating and cooling are provided. For example, electric cooling can be three-times as efficient as direct-resistance electric heating; thus lighting savings during cooling hours would have to be more than three-times greater than those during heating periods if the net cooling-heating effect is to be positive. On the other hand, from an electric utility's perspective, system-wide peak-demand impacts may differ from energy impacts, depending on the coincidence of lighting energy use with the total load faced by the utility and on whether the utility peak occurs during the cooling season or the heating season. Thermostat setpoints and schedules are crucial in determining the outcome. If the indoor temperature is allowed to "float" upwards during the summer, then the true cooling energy savings that can be achieved are correspondingly reduced.
Relevant economic factors include the mix and costs of the energy sources involved, and the respective tariff structures and time-of-day or -year variations in energy prices. Because of the generally higher cost of electricity compared with fossil fuel, cooling savings can have relatively more weight than fuel-based heating in terms of financial impacts. Moreover, in summer-peaking regions, avoided demand charges typically associated with reduced cooling loads can be substantially greater than charges (if any) linked to electric heating loads. Reduced or increased HVAC loads can also significantly influence HVAC system sizing and thus the initial (or replacement) cost of the equipment.
NO SIMPLE TASK
Quantifying net heating-cooling impacts is no simple task. To accurately capture the interactive effects in a given building and location, it is necessary to employ a dynamic (e.g. hourly) whole-building energy simulation model. To evaluate national impacts, the analysis must correctly account for distinct building types and regional variations in weather, building envelope and equipment efficiencies, the times lighting is in use in various parts of the building stock, and assorted economic variables. For forecasting purposes, all technical and economic variables must be projected over time. Given the rapid changes possible for each variable, the analysis is fairly complicated and uncertain. For example, the net impact of HVAC interactions will diminish if it is anticipated that the energy efficiency of space conditioning increases over time.
EXAMPLES FROM BANGKOK AND CHICAGO
One recent simulation study looked at the effect of lighting savings in Chicago--a cold US city. The study accounted for the diversity of heating, cooling, and lighting loads throughout the office building stock. The illustration is based on converting from a standard fluorescent system with efficient magnetic ballasts to a T8/electronic ballast system. Beginning with a change in lighting power density of 4W/m^2 (17kWh/m^2-year savings), the net heating-cooling effect results in a slight (3%) reduction of net lighting savings. On the other hand, the utility peak demand savings is 1.1W/m^2, and there is a net downsizing of space conditioning equipment. (See table below)
In the case of Thailand, where no space heating is required, a detailed hourly simulation study found more dramatic results for various combinations of T8 lamps plus electronic ballasts, occupancy sensors, dimmable ballasts, specular reflectors, and compact fluorescent lamps. Indirect benefits increase the lighting savings by 23% in office buildings to 56% in hotels (where 24-hour operation means high cooling energy use). These indirect savings correspond to as much as 32% of total cooling and 25% of total ventilation energy use in retail stores. The savings resulting from downsizing cooling equipment reduce project payback times by 10 to 50%. Overall payback times range from three months to 2.5 years. (See figure below)
Evan Mills
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(Click on the image to enlarge it.)
In Bangkok buildings, lighting savings lead to significant percentage savings in cooling and ventilation systems (left), which contribute to up to a third of total combined lighting HVAC savings (right). Effects vary considerably, depending on the type of building. Combined savings equal 25-48% of total building energy use, with less than a three-year payback time. Source: J. Busch, et al., Energy-The International Journal, No.2. 1993 (Pergamon Press)
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IT IS SOMETIMES ARGUED that buildings in northern locations have nothing to gain from efficient lighting. This can be the case in certain circumstances, but is highly unlikely to apply broadly to all buildings in a city, state, or country.
Electric resistance heating is about as efficient as an electric lamp. However, compared with heating systems, lamps do not efficiently deliver heat to a space. Heat pumps are far more efficient than lamp filaments. Fuel-based heating uses far less energy than the fuel-equivalent required to provide the electricity for a lamp. Of course, the "no-savings" argument doesn't apply at all to lighting in unheated rooms or outdoors.
Even in the extreme case of Sweden, commercial buildings enjoy a net HVAC benefit from lighting savings. According to studies at Chalmers University, typical modern Swedish buildings require cooling even at an outdoor temperature of -10°C (this is because of considerable internal heat generated by people, lighting, and other energy-using equipment). In the far-northern city of Kiruna, the outside temperature falls below -10°C about 10% of the time.
See also: Moving the Thai Market: Promie and Perils (IAEEL 3/93)