Let's think about what can happen when one of your high-performance design alternatives reveals a noticeable reduction in the electric lighting energy consumption. This reduction would be a direct consequence of one of the following design improvements which can be reflected in one of your alternative simulation models:
Reduced Lighting Power Density (LPD) (W/m2) input for multiple building spaces (reducing LPDs signifies an assumption of a high efficacy lighting fixtures that would provide higher lumen output per watt of electricity or that would provide the same required lumen output by using less electric power. Maybe you start with a typical LPD of 11.84 W/m2 and reduced it down to 3.5 W/m2 because all electric lights are assumed to be upgraded from compact CFLs to LEDs). 11.84 W/2 is the baseline assumption according to ASHRAE 90.1-2016 Table G3.7 - LPD for Open Plan Offices. You can also assume occupancy sensors for all workstations in the office and decide to cut LPDs by 30% according to ASHRAE 90.1-2016 standard (Table G3.7), LPD levels can be reduced by 30% in case they are controlled by occupancy sensors (rather than fixed building automation schedules) at individual workstation level).
A change in the temporal profiles of the electric lighting systems such that the total full load equivalent times of operations are reduced. A change in the occupancy profile would cause such a change in favor of reduced operation of electric lights for certain intermittently used spaces or you are assuming a vacancy sensor which shuts of all the lights after a time delay (e.g., 20 minutes) or off-office hour lighting could be connected to an occupancy sensor such that they only become operational in case the cleaning crew is in the building. A more informed estimates about how building spaces will be occupied during the day and over the week will certainly help you to customize your (daily-weekly and annual) operational profiles of occupancy in your building energy model to represent the intended uses in the actual building. Customizing or fine tuning the occupancy profiles will necessarily change the profiles for the operation of electric lighting systems, domestic or office appliances and even some process equipment in the building.
Incorporation of a photosensor-based daylighting controls to electric lighting systems installed in the perimeter thermal zones (this will create a highly dynamic operation of electric lights based on the availability of daylight on the photosensor to collectively satisfy the required illuminance level of 300Lux on the desk-plane for typical office spaces. The power level of electric lights (controlled by smart ballasts) can be continuously (or in a step-wise fashion) adjusted from a max output level (100%) to a minimum output level (10%) which corresponds to variations in the luminous flux (lumen) output from the fixture. From another perspective, photosensor-based controls is another method to change the temporal profiles of the operation of electric lights not in a pre-defined manner but tied to the availability of an environmental resource (daylight) which is the result of climatic conditions and architectural design decisions (to shape the flow of incoming daylight).
The question is what would be the impact of reducing the electric lighting energy consumption on the building energy performance?
First of all, reduced electric lighting system capacity and related electricity consumption will increase the heating needs/loads and reduce the cooling needs/loads of the building spaces (at varying orders of magnitude. The relationship is pretty straightforward. Lighting fixtures (similar to all technical-electrical and electronic systems) cannot convert 100% of the input power source (of electricity) into useable output (light). Therefore, these fixtures emit heat to indoor spaces as a byproduct of their operation and consequently they happen to operate as small sized heaters in the spaces. It is no doubt that when we utilize less number of (or less power) of these little space heaters which are operated all year round (no matter how hot or cold the weather is), the cooling needs will go down while heating needs will rise up at the same time. However, this situation would be still beneficial because of the fact that using electricity is certainly an inefficient method of heating and most of the time savings in electric lighting energy consumption surpasses the increase in the heating demand.
You can argue that increases in the heating load (due to less support from electric lighting gains) can have the effect of increased heating system and component sizes to meet the increased demand. But it’d be important to remind that the capacity of heating systems are sized for when all lights are already turned off (i.e., zero internal gain to the spaces from electric lights). Turned-off means the fractional operational profile has an input value of zero (0.00) at all times during the sizing period (over a day or over a single hour). This will force all electric lighting power to level at zero watt of power output. So in a way there is no penalty or impact of efficient electric lighting systems on your heating equipment sizes which are already designed for the worst-case scenario.
From a totally different perspective, the peak capacity of the cooling systems are always sized for when all lights are turned on in the middle of summer (to experience the worst-case scenario similar to heating system sizing scenario). Turned-on means the fractional operational profile has an input value of one (1.00) at all times during the sizing period (over a month). This will force all electric lighting power level at maximum (100%) power output. So, if you assume reduced electric lighting power which equates to reduced internal heat gains (remember lights as little space heaters operating all year round) you’ll see that the cooling system size would be smaller (due to reduced cooling loads). There is a direct and net benefit of designing downsized and smaller cooling system for your building. The design would not only benefits from reduced electric lighting energy consumption, reduced cooling energy consumption in addition to reduced cooling system size and all of which again surpasses possible increases in the space heating energy levels. But there is one significant detail not to be missed here! To be able to get the benefit of reduced cooling system sizing, the electric lighting design should be specific before the system sizing is conducted.
After all of these discussions, I hope that you will have another look at the pie chart or the bar chart of the annual energy consumption by end-uses analysis and think about how things have changed from one iteration to the next when you focus is on electric lighting energy reductions. It is not just lights energy, keep an eye on your space heating and space cooling energy shares! LPD: Lighting Power Density CFL: Compact Fluorescent Light LED: Lighting Emitting Diode ASHRAE 90.1-2016 Energy Standards for Building Except Low-Rise Residential Buildings
Omer T. Karaguzel, PhD
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