Lighting vs. HVAC |
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Lighting vs. HVAC
Do lighting retrofits also help save heating, ventilation, and cooling (HVAC) energy? This is a much-debated issue, and the answer is a very complex one at that.
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 weighed against increased energy use when air conditioning is required. The net effect of interactions among lighting systems and 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 and 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 set points and schedules are crucial in determining the outcome. If the indoor temperature is allowed to float up 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. 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. Therefore in summer peaking regions, avoided demand charges are typically linked to electric heating loads. Reduced or increased HVAC loads can also significantly influence HVAC system sizing and thus the initial/replacement cost of equipment.
Quantifying net heating/cooling impact is no simple task. To accurately capture the interactive effects in a given building and location, it is necessary to employ a dynamic (hourly) whole building energy simulation model. 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 for 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 the space increases over time.
One recent simulation study of a Chicago building looked at the effect of lighting and HVAC savings. The study accounted for the diversity of HVAC and lighting loads througout the office space. The study was based on converting from a standard fluorescent system with efficient magnetic ballast to a T8/electronic ballast system. Beginning with a change in lighting power density of 4 watts per square yard, the net heating-cooling effect resulted in a 3% reduction of cost.
The basic fact remains that each iindividual building needs to be assessed per application. there is most likely some type of savings that can be accomplished through the cooperation of lighting retrofit and HVAC.